nondestructive testing handbook vol.8 (second edition)

NONDESTRUCTIVETESTING HANDBOOKSecond Edition

ik.VOLUME 8VISUAL AND OPTICAL TESTINGIMichael W. AllgaierStanley NessTechnical Editors

Paul McIntirePatrick 0. MooreEditors

AMERICAN SOCIETY FORNONDESTRUCTIVE TESTING

PRESIDENT'S FOREWORD

Visual and Optical Testing, the eighth volume in the sec-ond edition of the Nondestructive Testing Handbook series,demonstrates again AS NT's dedication to its missions of pro-viding technical information and instructional materials andpromoting nondestructive testing (NDT) technology' as aprofession.

The challenge of doing so for visual testing lies in the factthat this test method, plainly the oldest in practice, is still inthe process of becoming standardized. Visual testing is theninth test method that ASNT has supported through a LevelIII examination. In any new or newly defined technology,a major hurdle is establishing its scope and identifyingthe body of knowledge. Acoustic emission testing under-went the same process of definition a few years ago, and theNondestructive Testing Handbook volume on acoustic emis-sion was an important contribution to that process.

Building on the work of his predecessors, Michael Allgaierundertook the difficult task of identifying the scope of visualtesting in detail for ASNT. In 1992, he was joined by StanleyNess as technical co-editor.

Volunteers, as members of the Visual Committee, pre-pared this book under the direction of the Handbook Devel-opment Committee. Scores of volunteers—both authorsand reviewers—have shared generously with us their exper-tise and many thousands of hours of their valuable time.That alone is an eloquent testimonial to the importance ofthe Nondestructive. Testing Handbook to the technical com-munity. On behalf of ASNT, I wish to express our gratitudeto them all.

Sotirios J. VahaviolosASNT National President (1992-93)

FOREWORD

The Aims of a Handbook

The volume you are holding in your hand is the eighth inthe second edition of the Nondestructive Testing Handbook,a series that began publication eleven years ago. Two morevolumes are anticipated.

Handbooks exist in many disciplines of science and tech-nology, and certain features set them apart from other refer-ence works. A handbook should ideally contain the basicknowledge necessary for understanding the science and thetechnology of the subject, and as such should include boththe underlying scientific principles and examples of theirapplication, along with data for reference.

The typical reader may he assumed to have completedthree or four years of college in a science oriented discipline,typically mechanical or electrical engineering, physics ormaterials science. A knowledge of the mathematics thataccompanies these disciplines is essential for a thoroughunderstanding of the material presented in any volume ofthe Nondestructive Testing Handbook. Occasionally an engi-neer may be frustrated by the difficulty of the discourse, butthe mathematical basis of the technology must be presentedfor completeness.

Standards, specifications, recommended practices andinspection procedures are the domain of other technicalsocieties and NDT companies themselves and so may bediscussed in the Nondestructive Testing Handbook forinstructional purposes only, and then only at a level of gener-alization that is illustrative rather than comprehensive. Stan-dards-writing bodies take great pains to ensure that theirdocuments are definitive in wording and technical accuracy,and NDT personnel who write contracts or proceduresshould consult real standards when appropriate.

In the field of NDT, those who design qualifying exami-nations or study for them draw on volumes of the NDTHandbook series as a convenient way of approximating thebody of knowledge at the time the books were written. Com-mittees and individuals who write or anticipate questions

are, of course, selective in what they take from any source.The parts of a handbook that give scientific background, forinstance, may have little bearing on a practical examination.Other parts of a handbook are specific to a certain industry.Although a handbook does not pretend to offer a completetreatment of its subject, its value and convenience are not tobe denied.

A handbook offers a view of its subject at a certain periodin time. Even before it is published, it begins to becomeobsolete as knowledge advances. The authors and editors dotheir best to make each volume useful in the present and inthe future. The second edition supersedes the historic firstedition, and doubtless the seeds have been sown for a thirdedition.

In producing the present volume, ASNT volunteers dis-played great initiative and determination in pulling the infor-mation together from disparate sources. In no previousvolume of the second edition did editors have to determinewhat the subject actually was and then collect the relevantinformation. What makes this volume special is that theperimeter of visual testing had not been defined when theproject began. It is hoped that the example of this book willinspire people to treat other subjects in like manner.

The present volume is a worthy member of the secondedition. Containing most of what the visual inspector willneed to know, the hook is at home on both the academicbookshelf and the factory workbench.

ASNT owes a great deal to the volume's two technical edi-tors, Michael Allgaier and Stanley Ness, for the sterling workthey did in determining the bounds of the subject and get-ting volunteers to contribute to the text. A great deal is alsoowed to the volunteers who reviewed it and to ASNT head-quarters staff who keyed, edited, rekeyed and re-edited, anddisplayed infinite patience with all of us.

Roderic K. StanleyHandbook Director

PREFACE

Visual and Optical Testing presents, in a single cover,important information on such timely subjects as standardsand procedures, vision acuity, image processing, video tech-nology, machine vision, the physics of light and the physiol-ogy of sight, in addition to application-specific coverage ofvisual testing of car and plane components, oil field tubulars,welds and power plant components.

Enormous credit is due to the technical editors, MichaelW. Allgaier and Stanley Ness, for undertaking the difficultand time consuming tasks of volunteer coordination andtechnical review. Allgaier began work on this volume in 1989and for four years served as its primary guide. Ness becametechnical co-editor in 1992 and helped bring this importantproject to a successful close.

The use of metric units in the text was reviewed by Jan vanden Andel, as in all previous volumes of this edition. His

labors have greatly enhanced the value of this book and theentire series to posterity and to the world.

The volume was prepared under the auspices of two com-mittees in ASNT's Technical Council: the Visual Committeein the Methods Division and the Handbook DevelopmentCommittee in the Technical Publications Division. All themany volunteer contributors and reviewers deserve congrat-ulations for what they have accomplished.

Thanks are due to ASNT Publications Assistant HollisHumphries-Black, who made good things happen at everystage of production; Eugene Turner and the staff at Turner,Dixon and Lacey, Inc., for technical illustration; Steven A.Bravard and the staff at Harlan Type for typography and lay-out; and the people at Edwards Brothers for printing andbinding.

Paul McIntirePatrick 0. Moore

ACKNOWLEDGMENTS

The Scope of a TechnologyVisual testing is distinguished from the other methods of

nondestructive testing in the part of the electromagneticspectrum used for interrogation: visible light as opposed toX-rays, infrared radiation, microwaves, or ultraviolet radia-tion. Of these, visible light can be detected by an organicsense organ, the eye. Indeed, visual testing is the only non-destructive test method that can be—and often is—per-formed without any equipment.

Because the physical means of visual testing is available toeveryone and the process seems intuitive, some people maysuppose that any discussion of the method may be short andsimple. In fact, visual testing is as modern as robotics, ascomplex as particle physics, as quantitative as image pro-cessing, and as subtly pervasive in industrial operations asvision itself. For this reason, the present volume gives scien-tific background when appropriate, in addition to the discus-sion of technology and applications.

Thanks are due to the many people who worked on thevolume. Special thanks are due to Michael Allgaier, thetechnical editor who kept the project going. It has been apleasure working with him.

Stanley NessTechnical Editor

Volunteers Achieve Goals of Visual1 meNIIV LJIUI I IC

Visual and Optical Testing, Volume 8 of the Nondestruc-tive Testing Handbook's second edition, is the first effort totreat the visual testing method with the recognition it sorightly deserves as a separate nondestructive testing methodin the NDT Handbook series and in the industries that use it.Visual testing is the oldest nondestructive testing method buthas only recently been codified—in the 1988 edition ofASNT's Recommended Practice No. SNT-TC-1A.

One of the challenges in developing the visual method hasbeen the difficulty in defining the field. Many manufacturingand materials processes use visual testing to inspect, examineand evaluate their products. Additionally, the means bywhich we perform visual tests have become very sophisti-cated with the advent of fiber optics, computers and videotechnology. The many applications of visual and optical test-ing are reflected in this book.

Goals of This Volume

The goal of this volume is to provide a major single sourceof reference material on visual testing. Weld inspection isaddressed by material from the American Welding Society;nuclear component inspection, from the Electric PowerResearch Institute; metallurgy, from the ASM International;lighting, from the Illumination Engineering Society. Atten-tion is also paid to vision—both the physics of light and howthe eye works.

Also emphasized are the advances in visual equipment.Computers, fiber and geometrical optics, robotics, and elec-tronics have made great strides. The outline of subject mat-ter for visual testing has evolved to combine how we see andwhat we see. It is hoped this volume strikes a balancebetween the two approaches.

Volunteer Support

In 1983 the Visual Personnel and Qualification Commit-tee with 67 members drafted the first training outline and listof references for visual testing. William Bailey, RobertBaker, Richard Gaydos, Robert Loveless, Diana Nelson,Carl Shaw, Henry Stephens, Jr., Hank Swain and JosephWolf were especially active at this time. This outline wasused to develop the first visual testing supplement for certi-fying nondestructive testing personnel. By 1984 ASNT wasseriously considering what the visual method was and whatASNT's role in educating or qualifying visual testing person-nel should be.

In the spring of 1985 a Visual Committee was establishedfor the first time and chaired by Hank Swain. That year a vol-ume of the Nondestructive Testing Handbook series wasreserved for visual and optical testing. The Technical Editor,NDT Handbook editor, and visual Handbook Coordinatorsolicited authors in 1986 for subject matter matching theoutline developed by ASNT's Visual Personnel QualificationCommittee. Different methods of reporting and routingwere tried but few volunteer authors could be found.

A new approach was taken in the fall of 1989. We decidednot to write a book consisting entirely of new and uniquetext, but rather we would research reference material thatmatched our outline and with permission reprint previouslypublished material. In 1990 over 200 references had beenidentified that related to the visual method. About 60 bookswere substantively related to our outline.

vii

Volume 8 ContributorsHitoshi Aizawa, Kawasaki SteelDavid Alman, DuPont Color Operations GroupDonald Bailey, Air Logistics Center, McClellan Air

Force BaseWilliam BaileyRoman Baldur, Walsh AutomationBruce Bates, Douglas Aircraft CompanyThomas Cabe, DTS Inspection ServicesDavid Casasent, Carnegie Mellon UniversityYen Fwu Cheu, General Motors CorporationDonald Christina, Dougla:s Aircraft CompanyDavid Clark, Global Holonetics CorporationNewbold Cross, NDTechDayna DunnEugene Egger, CTS Power ServicesNat Faransso, Brown and RootFarshid Farrokhnia, InnovisionGreg Geiger, American Ceramics SocietyEdward R. Geierazio, NASA Lewis Research CenterLarry Goldberg, Sea Test ServicesJames Hedtke, Nuclear Energy ServicesDonald Hagemaier, Douglas Aircraft CompanyRichard Horth, Walsh AutomationAnil K. Jam, Michigan State UniversityJohn Kaufman, Illuminating Engineering SocietyEli Kimmel, Tempil Division, Big Three IndustriesWilliam Lang, Lenox Instrument CompanyJoseph L. Mackin, International Pipe Inspectors AssociationStephen L. Meiley, Champion InternationalKathy Mills, ASM InternationalKazuo Miyagawa, Kiyomidai MinamiRichard Nademus, GPU Nuclear CorporationStanley NessRobert L. O'Brien, American Welding SocietyRalph Olmsted, DTS Inspection ServicesDonald Parrish, Southern Company ServicesDavid Pasquazzi, Rhode Island Air National GuardDon J. Roth, NASA Lewis Research CenterLloyd A. Schaefer, Rockwell International CorporationAllen Schuele, Construction TechnologiesBarbara Sherison, Electric Power Research InstituteCharles Sherlock, Chicago Bridge and Iron CompanyPete Sigmund, Lindhult and JonesLawrence E. Smetana, Taussig AssociatesRoderic K. Stanley, Lone Star Steel

Henry SwainVirginia Torrey, Welch Allyn Video DivisionW. Russell Tweddell, Karta TechnologyMichael A. Urzendowski, DNV IndustryJan van den Andel, Westinghouse CanadaGeorge C. Wheeler, Wheeler NDTJohn Wilk, Coast-to-Coast Construction

Volume 8 ReviewersDavid Adler, Columbia Gas TransmissionDon BlanchetteWilmer Blankenship, MQS InspectionBruce Boris, Titanium Metals CorporationJohn Cavender, Duke Power CompanyGene Chemma, Bethlehem Steel CorporationDavid L. Culbertson, Tennessee Gas Pipeline CompanyClaude D. Davis, Professional Service IndustriesRobert D. Davis, United States Department of EnergyPaul Dick, General Electric Aerospace & DefenseR.E. DrewMahmoud Elshehry, Inspecta InternationalC.S. Ferguson, Huntington AlloysH. Franssen, Rockwell International, Rocketdyne DivisionRichard Gaydos, Richard Gaydos and AssociatesGerard K. Hacker, Teledyne Brown EngineeringFrank Herring, AnAid, IncorporatedJoe Kramer, Newport News ShipbuildingGlen Lacey, C.N. Flagg PowerJohn Lindberg, Pennsylvania Power and LightEd Macejak, Carpenter Technical CorporationM. McCreaty, Exxon Chemical AmericanS.O. McMillan, Charleston Naval ShipyardScott Miller, Aptech Engineering ServicesThomas B. Munson, Reynolds Metals CompanyW.E. NagelJohn Patsey, US Steel Tubular ProductsThomas Payne, NUC ServicesGeorge Pherigo, PH DiversifiedHarold Pinsch, NDT AssistanceSherri Pritchett, The Boeing CompanyNickolas Rohach, Jr., Raytheon Services NevadaStanislav I. Rokhlin, The Ohio State UniversityUjjal Sen, Spectronics CorporationHenry M. Stephens, Electric Power Research InstituteBernie Strauss, Army Materials Technology LaboratoryJoseph Zalewski, Carpenter Technology Corporation

ix

lengthmass

timeelectric currentthermodynamic temperatureamount of substanceluminous intensityplane anglesolid angle

meterkilogram kgsecondampere Akelvinmole molcandela cdradian radsteradian sr

MEASUREMENT UNITS FOR VISUAL AND OPTICAL TESTING

Origin and Use of the SI SystemIn 1961 the General Conference on Weights and Mea-

sures established the International System of Units. TheSystême Internationale (SI) was designed so that allbranches of science could use a single set of interrelatedmeasurement units. These established units are modified inspecified ways to make them adaptable to the needs of indi-vidual disciplines. Without the SI system, this Nondestruc-tive Testing Handbook volume could have contained aconfusing mix of Imperial units, old cgs metric units and theunits preferred by certain localities or scientific specialties.

SI is a version of the metric system and ends the divisionexisting between units used by scientists and those used byengineers. Scientists gave up their units based on centimeterand gram and engineers made a fundamental change inabandoning the kilogram-force in favor of the newton. Elec-trical engineers retained their amperes, volts and ohms butchanged all units related to magnetism. The main effect ofSI has been the reduction of conversion factors betweenunits to one (1)—in other words, to eliminate them entirely.

Table 1 lists seven base units followed by two supplemen-tary units. Table 2 lists all of the derived units with specialnames. In the SI system, time is officially given only in sec-onds (s) but hour (h) is used occasionally.

For more information, the reader is referred to the infor-mation available through national standards organizationsand the specialized information compiled by technical socie-ties (see ASTM E380, Standard Metric Practice Guide, forexample).

TABLE 1. Base SI UnitsQuantity

Unit Name Unit Symbol

TABLE 2. Derived SI Units

Relation

to Other

Quantity Name Symbol SI Units

frequency

hertz

Hzforce newton

N kg•mospressure (stress)

pascal

Pa N•menergy (work)

joule

J Nompower watt

VV J•s -1

electric charge coulomb

C A•selectric potential

volt

3 WA - 'capacitance

farad

F C•1/ 1-electric resistance ohm

SZ V•A-

conductance siemens

A•Vmagnetic flux weber

Wb V•s

magnetic fluxdensity tesla

T Wtyrn - 2inductance

henry

H Wb•Atemperature

degree Celsius

°C K – 273.15luminous flux

lumen

Im cd•srilluminance

lux

lx Im •mradioactivity

becquerel

Bq l os- 'radiation

absorbed dose gray

Gy J•kg 'radiation dose

equivalent sievert

Sv

Prefixes for Si Units

Very large or very small units are expressed using theSI multipliers, prefixes usually of 10 3 intervals (Table 3).The multiplier becomes a property of the SI unit. For exam-ple, a centimeter (cm) is 1/100 of a meter. The volume unit,cubic centimeter (cm3), is (1/100)3 or 10- 6 m3. Units suchas the centimeter, decimeter, decameter and hectometer areavoided in technical uses of SI because of their variance fromthe 103 interval.

SI Units for Visual and Optical TestingThis volume uses SI units for visual and optical applica-

tions in the visible range of electromagnetic radiation.Vision requires a source of illumination. The light source

is the candela (cd), defined as the luminous intensity in agiven direction of a source that emits monochromatic radia-tion of 540 x 10's hertz (Hz) at a radiant intensity of1/683 watt per steradian (W•sr-').

x

TABLE 3. SI Multipliers TABLE 4. Conversion of measurements to SI units

Measurement = MeasurementMultiplier

1024

102 '10 1810' 510 12

109

1031021010 '10 210- 310 -6

10- 910-' 210-' 510 -1810 -2110 24

Prefix Symbol

yotta*

Yzetta* z

exa E

peta P

tera T

gigamega M

kilo k

hector

hdeca tdadecit

dcenti tmilli m

micro P-nano npico pfemtoatto azepto*yocto*

* PROPOSEDt AVOID THESE UNITS FOR SCIENCE AND ENGINEERING.

The luminous flux in a steradian (sr) is measured inlumens (1m). The measurement in lumens is the product ofcandela and steradian (1 lm = 1 cd •sr).

A light flux of one lumen (1 Im) striking one square meter(1 m2 ) on the surface of the sphere around the source illumi-nates it with one lux (1 lx), the unit of illuminance. If thesource itself is scaled to one square meter (1 m 2) and emits

Quantity in old unit Multiply by in SI unit

length

angstrom (A)

0.1

nanometer (nm)

illuminance

footcandie (ftc)

10.76

lux (Ix)metercandle

1

lux (ix)phot

10,000

lux (1x)

luminance cd/ft2

10.76

cd•m 2cd/in.2

1,550

cd•rn - 2footlambert

3.426

cd•m- 2lambert

3,183 (I 0,000/a)

cd•m -2nit (nt)

1

cd•m -2stilb (sb)

10,000

cd•m-2

one candela (1 cd), the luminance (formerly called bright-ness) of the source is 1 cd•ril 2.

Some terms have been replaced. Illumination is now illu-minance; brightness is luminance; transmission factor istransmittance. Meter-candle is now lux and nit is candelaper square meter (cd•in 2).

Old units are to be converted (see Table 4). Footcandle(ftc) and phot now convert to lux (lx). Stilb (sb), footlambertand lambert convert to candela per square meter (cd •in 2).Nanometer (nm) replaces angstrom (A) for wavelength.

In SI, the distinction between upper and lower case lettersis meaningful and should be observed. It should be notedthat the symbols of all SI units for light are written in lowercase: they are derived from Latin, not named after scientists.

Jan van den AndelWestinghouse Canada

xi

CONTENTS

SECTION 1: FUNDAMENTALS OF VISUAL AND Reflectometers 42OPTICAL TESTING 1 Radiometers 43

PART 1: DESCRIPTION OF VISUAL ANDOPTICAL TESTS 2

S pectrophotometers Types of Photometers

4444

Luminous Energy Tests 2Geometrical Optics 2

PART 2: HISTORY OF THE BORESCOPE 4 SECTION 3: THE VISUAL AND OPTICALDevelopment of the Borescope 4 TESTING ENVIRONMENT 51Certification of Visual Inspectors 8

PART 3: VISION AND LIGHT 9 PART 1: EFFECT OF DESIGN CRITERIA ONThe Physiology of Sight 9 VISUAL AND OPTICAL TESTS 52Weber's Law 10 Visual Testing in Product Design 53Vision Acuity 10 Designing for Quality Assurance 53Vision Acuity Examinations 12 PART 2: ENVIRONMENTAL FACTORS 54Visual Angle 13 Cleanliness 54Color Vision 14 Texture and Reflectance 54Fluorescent Materials 16 Lighting for Visual Tests 54

PART 4: SAFETY FOR VISUAL AND Light Intensities 55OPTICAL TESTS 22 Vision in the Testing Environment 55

Need for Safety 22 PART 3: PHYSIOLOGICAL FACTORS 57Laser Hazards 22 The Lens 57Infrared Hazards 23 The Fovea 57Ultraviolet Hazards 23 Rods and Cones 57Photosensitizers Damage to the Retina Thermal Factor Blue Hazard Visual Safety Recommendations Eye Protection Filters

242425262626

Receptors Perception Physiology of Vision Mechanism of Vision Color and Color Vision Observer Differences

585860616162

SECTION 2: THE PHYSICS OF LIGHT 29 PART 4: VISUAL WELD TESTINGPERFORMANCE STANDARDS 63

PART 1: THE PHYSICS OF LIGHT 30 Near Vision Acuity 63Radiant Energy Theories 30 Color Perception 63Light and the Energy Spectrum .31 Target Detection 63Blackbody Radiation 31 Acuity Variables 64Atomic Structure and Radiation 33 Reserve Vision Acuity and VisualLuminous Efficiency of Radiant Energy 33 Efficiency 65Luminous Efficiency of Light Sources 34 Performance Standards for Visual Weld

PART 2: MEASUREMENT OF THE Testing 65PROPERTIES OF LIGHT 35 Use of Visual Reference Standards 66

Photovoltaic Cells Photoconductor Cells Photoelectric Tubes Photodiodes and Phototransistors Photometry Principles of Photometry Photometers

35353536363639

Knowledge of Crack Pattern RecognitionScanning Techniques Lighting Practical Qualification Requirements Remote Visual Tests Vision Hardware

676767686869

Photovoltaic Cell Meters 39 Other Factors Affecting Perception 69Meters Using Photomultiplier Tubes 41 Recommendations for Visual WeldEquivalent Sphere Illumination Testing 69

Photometers 41 Conclusion 70

xii

SECTION 4: BASIC AIDS AND ACCESSORIES Manual Systems 129FOR VISUAL TESTING 73 System Selection and Application 129

PART 2: VIDEO TECHNOLOGY 131PART 1: BASIC VISUAL AIDS 74 Photoelectric Devices 131

Effects of the Test Object 74 Photoemissive Devices 131PART 2: MAGNIFIERS 76 Photoconductive Cells or Photodiodes 131

Range of Characteristics 76 Photovoltaic Devices 132Low Power Microscopes 77 Uses of Photoelectric Detecting andMedium Power Systems 78 Measuring Devices 132High Power Systems 79 Photoelectric Imaging Devices 132

PART 3: BORE SCOPES 82 The Electron Microscope 133Fiber Optic Borescopes 82 Video Borescopes 134Rigid Borescopes 83 Video Borescope Applications 138Special Purpose Borescopes 85 Research in Video Technology 139Typical Industrial Borescope Remote Closed Circuit Television 139

Applications 86 Television Camera Tubes 139Borescope Optical Systems 87 Cathode Ray Viewing Tube 140Borescope Construction 88 Video Resolution 141Special Purpose Borescopes 89 Photographic Techniques for Recording

PART 4: MACHINE VISION TECHNOLOGY 92 Visual Test Results 142Lighting Techniques 92 Image Enhancement 144Optical Filtering ' 94 Novel Uses of Video Systems 144Image Sensors 94 PART 3: ACCESSORIES USED IN REMOTEImage Processing 96 INTERNAL VIDEO TESTS OF PIPES 148Mathematical Morphology 99 Basic System Designs 148Image Segmentation 100 Support Equipment 149Optical Feature Extraction for High Speed Field Applications 151

Optical Tests 101 Visual Testing with Remote Cameras 151Conclusion 107 Advantages of Using Remote Visual

PART 5: REPLICATION 108 Equipment 153Cellulose Acetate Replication 108Silicon Rubber Replicas 111Conclusion 113 SECTION 6: VISUAL AND OPTICAL

PART 6: TEMPERATURE INDICATING TESTING PROCEDURES 155MATERIALS 114

Other Temperature Indicators 114 PART 1: OBJECTIVES OF VISUAL WELDi'ertification of Temperature Indicators 114 TFSTC 156

Applications for Temperature Indicators ... 115 Visual Weld Testing Practices 156PART 7: CHEMICAL AIDS 118 Before Welding 156

Test Object Selection 118 During Welding 156Surface Preparation 118 After Welding 157Etching 119 Marking Repair Welds 158Using Etehants 121 Conclusion 158Conclusion 124 PART 2: SAMPLING PLANS FOR VISUAL

TESTS 160

SECTION 5: OTHER INSTRUMENTATION Procedural Sampling Methods 160

AND ELECTRONIC AIDS FOR VISUAL Control Charts for Attributes 161

TESTING 127 Conclusion 161PART 3: VISUAL TESTING ACCEPTANCE

PART 1: REMOTE POSITIONING AND CRITERIA FOR WELDS 163TRANSPORT SYSTEMS 128 Weld Discontinuities 163

Fixed Systems 128 Typical Power Boiler Visual TestingAutomated Systems 128 Acceptance Criteria 164

Typical Acceptance Criteria for Stress Corrosion Cracking 209Visual Testing of Pressure and Corrosion Fatigue Fracture 210Storage Vessels 166 Elevated Temperature Discontinuities 210

Acceptance Criteria for Pipe Welds 167 Tests of Welded Joints 214Typical Visual Testing Acceptance Criteria Microscopy 214

for Pipe Welds 169 Inspection for Discontinuities in Steel 222Alternative Visual Testing Acceptance PART 2: VISUAL AND OPTICAL TESTING IN

Criteria for Pipe Welds 171 THE STEEL INDUSTRY 228Typical Visual Testing Acceptance Criteria Surface Inspection Technologies 228

for Storage Tank Welds 172 Surface Discontinuity Inspection SystemsTypical Visual Testing Acceptance Criteria for Hot Slabs 230

for Pipelines 172 Inspection for Surface Discontinuities inLarge Section Steel Products 231

SECTION 7: CODES, STANDARDS AND Surface Inspection System for Hot Strip 232SPECIFICATIONS FOR VISUAL AND Surface Discontinuity Inspection of ColdOPTICAL TESTING 177 Strip Steel 232

Flatness Measurement of Hot RolledPART I: OVERVIEW OF CODES, Steel 234

STANDARDS AND SPECIFICATIONS Light Section Method Using Laser Beam 236FOR VISUAL TESTING , 178 Surface Measurement 236

Application to Welding and Allied Colorimeters 238Procedures 178 Strip Surface Purity Measuring Equipment 240

Visual Testing in Widely Used Standards 179 Distinctness of Image of Coated Surface .... 241PART 2: PERSONNEL QUALIFICATION AND Conclusion 241

CERTIFICATION 181Definition of Terms 181Value of Personnel Qualification and SECTION 9: APPLICATIONS OF VISUAL AND

Certification 181 OPTICAL TESTS IN THE ELECTRICOverview of SNT-TC-1A 181 POWER INDUSTRIES 245Details of SNT-TC-1A 182Application to Some Common INTRODUCTION 246

Specifications 185 PART 1: JOINING PROCESSES 247Nondestructive Testing Level III Metallurgical Joint Configurations 247

Certification by ASNT 187 Basic Welding Processes 248Conclusion 187 Fabrication Process 253

PART 3: OIL FIELD TUBULAR Visual Tests of Metal Joints 253SPECIFICATIONS 188 Visual Testing for Weld Discontinuities .... 256

Typical Visual Testing for New Pipe 189 Weld Joint Discontinuities 259Typical Visual Testing of Used Drill Pipe 190 Visual Testing of Brazed andVisual Testing of Oil Field Hoisting Soldered Joints 261

Equipment 191 Acceptance Standards 262Recording and Reporting Visual Test

SECTION 8: APPLICATIONS OF VISUAL AND Results 262

OPTICAL TESTS IN THE METALS PART 2: SPECIFIC VISUAL INSPECTIONINDUSTRIES 193 APPLICATIONS 263

Visual Testing of Reactor PressurePART 1: PHYSICAL PROPERTIES OF Vessels 263

METALS 194 Visual Testing of Pumps 269Mechanical Properties 194 Visual Testing of Valves 270Wear 197 Visual Tests of Bolting 272Contact Stress Fatigue 201 Visual Tests for Forging Discontinuities .... 274Cavitation Fatigue 205 Visual Tests for Rolled Stock 275Corrosion 206 Visual Tests for Casting Discontinuities .... 276

xiv

SECTION 10: APPLICATIONS OF VISUAL PART 3: VISUAL TESTS OF CERAMICS 320AND OPTICAL TESTS IN THE Visual Tests of Injection Molded TurbineTRANSPORTATION INDUSTRIES 277 Blades 320

Automatic Testing of Thin CeramicINTRODUCTION 278 Components 320PART 1: OPTICAL TESTS IN THE Laser Based and Microscopic Inspections

AUTOMOBILE INDUSTRIES 279 of Ceramics 320Texture Analysis of Automotive Finishes ... 279 PART 4: VISUAL TESTING OF THREADS INAutomotive Brake Shoe Sorting System .... 288 OIL COUNTRY TUBULAR GOODS 322

PART 2: OPTICALLY AIDED VISUAL TEST- Requirements 322ING OF AIRCRAFT STRUCTURE

Testing with Visual and Optical Aids 292292

Types of Seal Specifications

322323

Typical Applications 294 Visual Testing Procedures 323Conclusion 300 The Pin Threads 323

The Coupling Threads 325SECTION 11: OTHER APPLICATIONS OF Presence of Makeup Triangle 325

VISUAL AND OPTICAL TESTS 303 Makeup Connections 325Use of a Profile Gage 325

PART 1: INTERFACE OF VISUAL TESTING Arc Burns 326WITH OTHER NONDESTRUCTIVE Shoulders 326TESTING METHODS 304 PART 5: VISUAL TESTING OF COMPOSITE

Visual Aspects of Leak Testing 304 MATERIALS 328Visual Aspects of Liquid Penetrant Problem Areas 328

Testing 305 What to Look for in Composite Materials 328Visual Aspects of Radiography Visual Aspects of Electromagnetic

306 Applications Image Processing

330330

Testing 307 Summary 330Visual Aspects of Magnetic Particle PART 6: VISUAL TESTING OF MICRO-

Testing 307 ELECTRONIC COMPONENTS 331Visual Aspects of Ultrasonic Testing 309 Visual Testing of Solder Joints 331Conclusion 312 Visual Testing of Electronics 331

PART 2: APPLICATIONS OF PHOTOGRAPHYIN VISUAL TESTING 313

Photographs as a Permanent Record for SECTION 12: GLOSSARY OF VISUAL ANDVisual Testing 313 OPTICAL TESTING TERMS 339

Photogrammetry for Documenting theCondition of Petrochemical Furnaces ... 314

Conclusions 319 INDEX 357

SECTION

FUNDAMENTALS OF VISUAL ANDOPTICAL TESTING William Bailey, Westlake, Ohio (Part 3)William Lang, Lenox Instrument Company, Trevose, Pennsylvania (Part 2)Stanley Ness, Mission Viejo, California (Part 4)Pete Sigmund, Lindhult and Jones, Fort Washington, Pennsylvania (Part 2)Henry Swain, Augusta, Georgia (Part 2)

2 / VISUAL AND OPTICAL TESTING

PART 1 DESCRIPTION OF VISUAL AND OPTICALTESTS

Nondestructive tests typically are done by applying aprobing medium (such as acoustic or electromagneticenergy) to a material. After contact with the test material,certain properties of the probing medium are changed andcan be used to determine changes in the characteristics ofthe test material. Density differences in a radiograph orlocation and peak of an oscilloscope trace are examples ofmeans used to indicate probing media changes.

In a practical sense, most nondestructive tests ultimatelyinvolve visual tests—a properly exposed radiograph is usefulonly when the radiographic interpreter has the visionacuity required to interpret the image. Likewise, the accu-mulation of magnetic particles over a crack indicates to theinspector an otherwise invisible discontinuity. The interfaceof visual testing with other nondestructive testing methods isdiscussed in more detail in a later section of this volume.

For the purposes of this book, visual and optical tests arethose that use probing energy from the visible portion of theelectromagnetic spectrum. Changes in the light's propertiesafter contact with the test object may be detected by humanor machine vision. Detection may be enhanced or made pos-sible by mirrors, magnifiers, borescopes or other visionenhancing accessories.

Luminous Energy Tests

Visual testing was probably the first method of nonde-structive testing. It has developed from its ancient originsinto many complex and elaborate optical investigation tech-niques. Some visual tests are based on the simple laws ofgeometrical optics. Others depend on properties of light,such as its wave nature. A unique advantage of many visualtests is that they can yield quantitative data more readily thanother nondestructive tests.

Luminous energy tests are used primarily for two pur-poses: (1) testing of exposed or accessible surfaces of opaquetest objects (including a majority of partially assembled orfinished products) and (2) testing of the interior of transpar-ent test objects (such as glass, quartz, some plastics, liquidsand gases). For many types of objects, visual testing can beused to determine quantity, size, shape, surface finish,reflectivity, color characteristics, fit, functional characteris-tics and the presence of surface discontinuities.

Geometrical Optics

Image Formation

Most optical instruments are designed primarily to formimages. In many cases, the manner of image formation andthe proportion of the image can be determined by geometryand trigonometry without detailed consideration of thephysics of light rays.

This practical technique is called geometrical optics and itincludes the formation of images by lenses and mirrors. Theoperation of microscopes, telescopes and borescopes alsocan be partially explained with geometrical optics. In addi-tion, the most common limitations of optical instruments canbe similarly evaluated with this technique.

Light Sources

The light source for visual tests typically emits radiation ofa continuous or noncontinuous (line) spectrum. Monochro-matic light is produced by use of a device known as a mono-chromator, which separates or disperses the wavelengths ofthe spectrum by means of prisms or gratings.

Less costly and almost equally effective for routine testsare light sources emitting distinct spectral lines, Theseinclude mercury, sodium and other vapor discharge lamps.Such light sources may he used in combination with glass,liquid or gaseous filters or with highly efficient interferencefilters, for transmitting only radiation of a specificwavelength.

Stroboscopic Sources

The stroboscope is a device that uses synchronized pulsesof high intensity light to permit viewing of objects movingwith a rapid, periodic motion. A stroboscope can be usedfor direct viewing of the apparently stilled test object or forexposure of photographs.

The timing of the stroboscope also can be adjusted so thatthe moving test object is seen to move but at a much slowerapparent motion. The stroboscopic effect requires an accu-rately controlled, intermittent source of light or may beachieved with periodically interrupted vision.

FUNDAMENTALS OF VISUAL AND OPTICAL TESTING / 3

Light Detection and Recording

Once light has interacted with a test object (beenabsorbed, reflected or refracted), the resulting light wavesare considered test signals that may be recorded visually orphotoelectrically. Such signals may be detected by means ofphotoelectric cells, bolometers or thermopiles, photomulti-pliers or closed circuit television systems.

Electronic image conversion devices often are used for theinvisible ranges of the electromagnetic spectrum (infrared,ultraviolet or X-rays) but they also may he used to transmitvisual data from hazardous locations or around obstructions.

Occasionally, intermediary photographic recordings aremade. The processed photographic plate can subsequentlybe evaluated either visually or photoelectrically. Some

applications take advantage of the ability of photographicfilm to integrate low energy signals over long periods of time.Photographic film emulsions can be selected to meet specifictest conditions, sensitivities and speeds.

Fluorescence Detection

A material is said to fluoresce when exposure to radiationcauses the material to produce a secondary emission oflonger wavelength than the primary, exciting light. Visualtests based on fluorescence play a part in qualitative andquantitative inorganic and organic chemistry, as a means ofquality control of chemical compounds, for identifying coun-terfeit currency, tracing hidden water flow and for detectingdiscontinuities in metals and pavement.

4 I VISUAL AND OPTICAL TESTING

PART 2

HISTORY OF THE BORESCOPE

Development of the Borescope

The development of self illuminated telescopic devicescan be traced back to early interest in exploring the interiorhuman anatomy without operative procedures.

Devices for viewing the interior of objects are called endo-scopes, from the Creek words for "inside view." Today theterm endoscope in the United States is applied primarily tomedical instruments. Nearly all of the medical endoscopeshave an integral light source; some incorporate surgicaltweezers or other devices. Industrial endoscopes are calledhorescopes because they were 'originally used in machinedapertures and holes such as gun bores. There are both flexi-ble and rigid, fiber optic and geometric light borescopes.

Cystoscopes and Borescopes

In 1806 Philipp Bozzini of Frankfurt announced theinvention of his Lichtleiter (German for "light guide"). Hav-ing served as a surgeon in the Napoleonic wars, Bozzini envi-sioned using his device for medical research. It is consideredthe first endoscope.•

In 1876, Dr. Max Nitze, a urologist, developed the firstpractical cystoscope to view the human bladder.' A platinumloop in its tip furnished a bright light when heated with gal-vanic current. Two years later, Thomas Edison introduced anincandescent light in the United States. Within a short time,scientists in Austria made and used a minute electric bulb inNitze's cystoscope, even before the electric light was in usein America.

The early cystoscopes contained simple lenses but thesewere soon replaced by achromatic combinations. In 1900,Reinhold Wappler revolutionized the optical system of thecystoscope and produced the first American models. Theforward oblique viewing system was later introduced and hasproved very useful in both medical and industrial applica-tions. Direct vision and retrospective systems were also firstdeveloped for cystoscopic use.

Borescopes and related instruments for nondestructivetesting have followed the same basic design used in cysto-scopic devices. The range of borescope sizes has increased,sectionalized instruments have been introduced and other

special devices have been developed for industrialapplications.

Gastroscopes and Flexible BorescopesA flexible gastroscope, originally intended for observing

the interior of the stomach wall, was first developed byRudolph Schindler' and produced by Georg Wolf in 1932.The instrument consisted of a rigid section and a flexiblesection. Many lenses of small focal distance were used toallow bending of the instrument to an angle of 34 degrees inseveral planes. The tip of the device contained the objectiveand the prism causing the necessary axial deviation of thebundle of rays coming from the illuminated gastric wall. Thesize of the image depended on the distance of the objectivefrom the observed surface. It could be magnified, reducedor normal size but the image was sharp and erect with cor-rect sides. Flexible gastroscopes are now available, with rub-ber tubes over the flexible portion, in diameters ofapproximately 14 mm (0.55 in.) and 8 mm (0.31 in.).

Flexible borescopes for industrial use are more ruggedlyconstructed than gastroscopes, having flexible steel tubesinstead of rubber for the outer tube of the flexible portion. Atypical flexible borescope is 13 mm (0,5 in.) in diameter andhas a 1 m (3 ft) working length, with flexibility in about500 mm (20 in.) of the length. Extension sections are avail-able in 1, 2 or 3 m (3, 6 or 9 ft) lengths, permitting assemblyof borescopes up to 10 m (30 ft) in length. In such flexibleinstruments the image remains round and sharp when thetube is bent to an angle of about 34 degrees. Beyond thatlimit, the image becomes elliptical but remains clear untilobliterated at about 45 degrees of total bending.

American Development of BorescopesAfter the early medical developments, certain segments of

American industry needed visual testing equipment for spe-cial inspection applications. One of the first individuals tohelp fill this need was George Sumner Crampton.

George Crampton (Fig. 1) was born in Rock Island, Illi-nois in 1874. He was said to have set up a small machineshop by the age of 10 and his first ambition was to become anelectrical engineer. He chose instead to study medicine andreceived his M.D. from the University of Pennsylvania in1898.

While he was interning at Pennsylvania Hospital, Cramp-ton's mechanical and engineering abilities were recognizedand he was advised to become an oculist.' He returned to

FIGURE 1. George Crampton, developer of theborescope

FROM LENOX INSTRUMENT COMPANY. REPRINTED WITH PERMISSION.

FIGURE 2. Tests of forgings for a steam turbinegenerator shaft manufactured in the 1920s


FIGURE 3. Inspectors use early borescopes tovisually inspect piping at an Ohio oil refinery



the university, took a degree in ophthalmology and laterpracticed in Philadelphia, Pennsylvania and Princeton,New Jersey'

In 1921, the Westinghouse Company asked Crampton tomake a device that could be used to check for discontinuitiesinside the rotor of a steam turbine (Fig. 2). Crampton devel-oped the instrument in his Philadelphia shop and deliveredthe prototype within a week—it was the first borescope pro-duced by his company.

Crampton continued to supply custom borescopes fortesting inaccessible and often dark areas on power turbines,oil refinery piping, gas mains, soft drink tanks and other com-ponents (Fig. 3). Crampton soon was recognized for his abil-ity to design and manufacture borescopes, periscopes andother optical equipment for specific testing applications.

After retiring as emeritus professor of ophthalmology atthe university Crampton continued private practice indowntown Philadelphia. At the same time, he worked onborescopes and other instruments in a small shop he hadestablished in a remodeled nineteenth century coach house(Fig. 4).

Wartime Borescope Developments

After World War II began, Crampton devoted much of hisenergy to the war effort, filling defense orders for borescopes


FIGURE 4. Periscope built in the 1940s is checked before shipment to a Texas chemical plant


(Fig. 5). Crampton practiced medicine until noon, thenwent to the nearby workshop where he visually tested thebores of 37 mm antiaircraft guns and other weapons.'

During the war, borescopes were widely used for testingwarship steam turbines (particularly their rotating shafts).The United States Army also used borescopes for inspectingthe barrels of tank and antiaircraft weapons produced inPhiladelphia. An even more challenging assignment layahead.

The scientists working to develop a successful nuclearchain reaction in the top secret Manhattan Project askedCrampton to provide a borescope for inspecting tubes nearthe radioactive pile at its guarded location beneath thestadium seats at the University of Chicago's Stagg Field.Crampton devised an aluminum borescope tube 35 mm(1.4 in.) in diameter and 10 m (33 ft) long. The device con-sisted of 2 m (6 ft) sections of dual tubing joined by bronzecouplings which also carried an 8 V lighting circuit.

The inspector standing directly in front of the bore wassubject to radioactive emissions from the pile, so Cramptonmounted the borescope outside of a heavy concrete barrier.The operator stood at a right angle to the borescope, looking

through an eyepiece and revolving the instrument manually.The borescope contained a prism viewing head and had to berotated constantly. It was supported in a steel V trough rest-ing on supports whose height could be varied. Cramptonalso mounted a special photographic camera on theeyepiece.

The original Manhattan Project borescope was laterimproved with nondarkening optics and a swivel-joint eye-piece that permitted the operator to work from any angle(this newer instrument did not require the V trough). It alsowas capable of considerable bending to snake through thetubes in the reactor. A total of three borescopes were sup-plied fbr this epochal project and they are believed to be thefirst optical instruments to use glass resistant to radioactivity.

Borescopes and Aircraft Tests

Aircraft inspection soon became one of the mostimportant uses of borescope technology. In 1946, an ultravi-olet light borescope was developed for fluorescent testing ofthe interior of hollow steel propeller blades. The 100 W

FIGURE 5. Using a borescope, an inspector at anautomobile plant during World War H checks theinteriors of gun tubes for 90 mm antiaircraft guns



viewing instniment revealed interior surface discontinuitiesas glowing green lines.'

Later, in 1958, the entire United States' B-47 bomber fleetwas grounded because of metal fatigue cracks resulting fromlow level simulated bombing missions. Visual testing withborescopes proved to be the first step toward resolving theproblem. The program became known as Project Milkbottle,a reference to the bottle shaped pin that was a primary con-nection between the fuselage and wing (Fig. 6).

In the late 1950s, a system was developed for automatictesting of helicopter blades. The borescope, supported by along bench, could test the blades while the operator viewedresults on a television screen (Fig. 7). The system was usedextensively during the Vietnam conflict and helicopter man-ufacturers continue to use borescopes for such critical tests.

After a half century of pioneering work, George Cramptonsold his borescope business to John Lang of Cheltenham,Pennsylvania, in 1962.6 • 7 Lang had developed the radiationresistant optics used in the Manhattan Project borescope, aswell as a system for keeping it functional in high temperature

FIGURE 6. Inspector using a borescope to checkfor metal fatigue cracks in a B-47 bomber duringgrounding of the bomber fleet in 1958


FIGURE 7. Visual testing of the frame of a 10 m(32 ft) long helicopter blade using a 10 m (32 ftjborescope; the inspector could view magnifiedresults on the television screen at bottom left


environments. He also helped pioneer the use of closed cir-cuit television with borescopes for testing the inner surfacesof jet engines and wings, hollow helicopter blades andnuclear reactors. In 1965, the company received a patent ona borescope whose mirror could he very precisely controlled.This borescope could zoom to high magnification and could


intensely illuminate the walls of a chamber by means of aquartz incandescent lamp containing iodine vapor.

The basic design of the borescope has been in use formany decades and it continues to develop, accommodatingadvances in video, illumination, robotic and computertechnologies.

Certification of Visual Inspectors

The recognition of the visual testing technique and thedevelopment of formal procedures for educating and qualify-ing visual inspectors were important milestones in the his-tory of visual inspection. Because visual testing can beperformed without any intervening apparatus, it was cer-tainly one of the first forms of nondestructive testing. In itsearly industrial applications, visual tests were used simply toverify compliance to a drawing or specification. This wasbasically a dimensional check. The soundness of the objectwas determined by liquid penetrant, magnetic particle, radi-ography or ultrasonic testing.

Following World War II, few inspection standardsincluded visual testing. By the early 1960s, visual tests werean accepted addition to the American Welding Society's codehooks. In NAV SHIPS 250-1500-1, the US Navy includedvisual tests with its specifications for other nondestructivetesting techniques for welds.

By 1965, there were standards for testing, and criteria forcertifying the inspector had been established in five testmethods: liquid penetrant, magnetic particle, eddy current,radiographic and ultrasonic testing. These five were cited inASNT Recommended Practice No. SNT-TC-1A, introducedin the late 1960s. The broad use of visual testing hinderedits addition to this group as a specific method—there weretoo many different applications on too many test objects topermit the use of specific acceptance criteria. It also was

reasoned that visual testing would occur as a natural result ofapplying any other nondestructive test method.

Expanded Need for Visual Certification

In the early 1970s, the need for certified visual inspectorsbegan to increase. Nuclear power construction was at apeak, visual certification was becoming mandatory and non-destructive testing was being required. In 1976, the Ameri-can Society for Nondestructive Testing began consideringthe need for certified visual inspectors. ASNT had becomea leading force in nondestructive testing and Americanindustry had accepted its ASNT Recommended Practice No.SNT-TC-IA as a guide for certifying other NDT inspectors.

In the spring of 1976, ASNT began surveying industryabout their inspection needs and their position on visual test-ing. Because of the many and varied responses to the survey,a society task force was established to analyze the surveydata. In 1977, the task force recommended that visualinspectors be certified and that visual testing be made a sup-plement to ASNT Recommended Practice No. SNT-TC-IA(1975). At this time, the American Welding Society imple-mented a program that, following the US Navy, was the firstto certify inspectors whose sole function was visual weldtesting.

During 1978, ASNT subcommittees were formed for theeastern and western halves of the United States. Thesegroups verified the need for both visual standards andtrained, qualified and certified inspectors. In 1980, a VisualMethods Committee was formed in ASNT's Technical Coun-cil and the early meetings defined the scope and purpose ofvisual testing (dimensional testing was excluded). In 1984,the Visual Personnel Qualification Committee was formed inASNT's Education and Qualification Council. In 1986, atraining outline and a recommended reference list was final-ized and the Board of Directors approved incorporation ofvisual testing into ASNT Recommended Practice No.SN T-TC -1 A.

JJJJ T 1"),)‘/\)‘?',XT-F rT-C

'T TT_I TT T N)N

_I—I _ hi jTITT -P`)'). ,1)1—I T TT /%,

_1_1 1 _IT TT TT TTT

FIGURE 8. Pattern changes illustrating boundaryand edge detection

FUNDAMENTALS OF VISUAL AND OPTICAL TESTING 19

PART 3VISION AND LIGHT

The Physiology of SightVisual Data Collection

Human visual processing occurs in two steps. First theentire field of vision is processed. This is typically an auto-matic function of the brain, sometimes called preattentiveprocessing. Secondly, focus is localized to a specific object inthe processed field. Studies at the University of Pennsylvaniaindicate that segregating specific items from the general fieldis the foundation of the identification process.

Based on this concept, it is now theorized that various lightpatterns reaching the eyes are simplified and encoded, aslines, spots, edges, shadows, colors, orientations and refer-enced locations within the entire field of view. The first stepin the subsequent identification process is the comparison ofvisual data with the long-term memory of previously col-lected data. Some researchers have suggested that this com-parison procedure is a physiological cause of 'a vu, theuncanny feeling of having seen something before.

The accumulated data are then processed through a seriesof specific systems. Certain of our light sensors receive andrespond only to certain stimuli and transmit their data to par-ticular areas of the brain for translation. One kind of sensoraccepts data on lines and edges; other sensors process onlydirections of movement or color. Processing of these data dis-criminates between different complex views by analyzingtheir various components.'

Rv PYIIP ri mont it 11v1: it.tri.r. a mac nf

tivity have a kind of persistence. This can be illustrated bystaring at a lit candle, then diverting the eyes toward a blankwall. For a short time, the image of the candle is retained.The same persistence occurs with motion detection and canhe illustrated by staring at a moving object, such as a water-fall, then at a stationary object like the river bank. The bankwill seem to flow because the visual memory of motion is stillpresent.

Differentiation in the Field of View

Boundary and edge detection can be illustrated by the pat-tern changes in Fig. 8. When scanning the figure from leftto right, the block of reversed Ls is difficult to separate fromthe upright Ts in the center but the boundary between thenormal Ts and the tilted Ts is easily apparent. The difficultyin differentiation occurs because horizontal and vertical linescomprise the L and upright T groups, creating a similarity

that the brain momentarily retains as the eye moves from onegroup to the other. On the other hand, the tilted Ts share noedge orientations with the upright Ts, making them stand outin the figure.

Differentiation of colors is more difficult when the differ-ent colors are in similarly shaped objects in a pattern. Therecognition of geometric similarities tends to overpower thedifference in colors, even when colors are the object of inter-est. Additionally, in a grouping of different shapes of unlikecolors, where no one form is dominant, a particular form mayhide within the varied field of view. However, if the particu-lar form contains a major color variance, it is very apparent.Experiments have shown that such an object may bedetected with as much ease from a field of thirty as it is froma field of three.'"

Searching the Field of View

The obstacles to differentiation discussed above indicatethat similar objects are difficult to identify individually. Dur-ing preattentive processing, particular objects that sharecommon properties such as length, width, thickness or orien-tation are not different enough to stand out. If the differ-ences between a target object and the general field isdramatic, then a visual inspector requires little knowledge ofwhat is to be identified. When the target object is similar tothe general field, the inspector needs more specific detailabout the target. In addition, the time required to detect atarget increases linearly with the number of similar objectsin its general field.

When an unspecified target is being sought, the entirefield must be scrutinized. If the target is known, it has been

I

MEMORYELEMENTS

YREVIOUSLY

STORED ITEMS

AREA OF CONCERNELEMENTS

PROPERTIES2 RELATJONSHIPS3. NAME

FIGURE 9. Stages of visual perception

4.16

SIZE ELEMENT

A REA

OFCONCERN

COLOR ELEMENTORIENTATION

ELEMENT

GENERAL AREAOF VISION

DEPTH OF FIELDELEMENT


shown statistically that only about half of the field must besearched.

The differences between a search for simple features anda search for conjunctions or combinations of features canalso have implications in nondestructive testing environ-ments. For example, visual inspectors may be required totake more time to check a manufactured component whenthe possible errors in manufacturing are characterized bycombinations of undesired properties. Less time could betaken for a visual test if the manufacturing errors always pro-duced a change in a single property."

Another aspect of searching the field of view addresses theabsence of features. The presence of a feature is easier tolocate than its absence. For example, if a single letter 0 isintroduced to a field of many Qs, it is more difficult to detectthan a single Q in a field of Os. The same difficulty is appar-ent when searching for an open 0 in a field of closed Os. Inthis case statistics show that the apparent similarity in the tar-get objects is greater and even more search time is necessary.

Experimentation in the area of visual search tasks encom-passes several tests of many 'individuals. Such experimentsstart with studies of those features that should stand outreadily, displaying the basic elements of early vision recogni-tion. The experiments cover several categories, includingquantitative properties such as length or number. Alsoincluded are search tasks concentrating on single lines, ori-entation, curves, simple forms and ratios of sizes. All thesetests verify that visual systems respond more favorably to tar-gets that have something added (Q versus 0) rather thansomething missing.

In addition, it has been determined that the ability to dis-tinguish differences in intensity becomes more acute with adecreasing field intensity. This is the basis of Weber's law.The features it addresses are those involved in the earlyvisual processes: color, size, contrast, orientation, curvature,lines, borders, movement and stereoscopic depth.

Weber's LawWeber's law is widely used by psychophysicists and entails

the following tenets: (1) individual elements such as points orlines are more important singly than their relation to eachother and (2) closed forms appear to stand out more readilythan open forms. To view a complete picture, the visual sys-tem begins by encoding the basic properties that are pro-cessed within the brain, including their spatial relationships.Each item in a field of view is stored in a specific zone and iswithdrawn when required to form a complete picture. Occa-sionally, these items are withdrawn and positioned in error.This malfunction in the reassembly process allows the cre-ation of optical illusions, allowing a picture to be misin-terpreted.

The diagram in Fig. 9 represents a model of the earlystages of visual perception. The encoded properties aremaintained in their respective spatial relationships and com-pared to the general area of vision. The focused attentionselects and integrates these properties, forming a specificarea of observation. In some cases, as the area changes, thevarious elements comprising the observance are modified orupdated to represent present conditions. During this step,new data are compared to the stored information.

Vision AcuityVision acuity encompasses the ability to see and identify,

what is seen. Two forms of vision acuity are recognized andmust be considered when attempting to qualify visual ability.These are known as near vision and far vision.

FIGURE 10. Components of the human eye incross section

SCLERA (WHITE OUTER COVERING(

LAYER OF RODS AND CONES

THREADLIKE NERVES

EYE MUSCLE

CORNEA NNAk.

LIQUID Nbk

LENS

PUPIL

IRIS

EYE MUSCLEOPTIC NERVE

FIGURE 11. Magnified cross section showing theblind spot of the human eye

BIPOLAR CELLS

OPTIC NERVE

BLIND SPOT(WHERE FIBERS

LEAVE THE RETINATO FORM THEOPTIC NERVE)

LIGHT RECEPTORS


Components of the Human Eye

The components of the human eye (Fig. 10) are oftencompared to those of a camera. The lens is used to focuslight rays reflected by an object in the field of view. Thisresults in the convergence of the rays on the retina (film),located at the rear of the eyeball. The cornea covers the eyeand protects the lens. The quantity of light admitted to thelens is controlled by the contraction of the iris (aperture).The lens has the ability to become thicker or thinner, whichalters the magnification and the point of impingement of thelight rays, changing the focus. Eye muscles aid in the alteringof the lens shape as well as controlling the point of aim.

This configuration achieves the best and sharpest imagefor the entire system. The retina consists of rod and conenerve endings that lie beneath the surface. They are ingroups that represent specific color sensitivities and patternrecognition sections. These areas may be further subdividedinto areas that collect data from lines, edges, spots, positionsor orientations.

The light energy is received and converted to electricalsignals that are moved by way of the optic nerve system to thebrain where the data are processed. Because the light isbeing reflected from an object in a particular color or combi-nation of colors, the individual wavelengths representingeach hue also vary. Each wavelength is focused at differentdepths within the retina, stimulating specific groups of rodsand cones (see Figs. 10 and 11). The color sensors aregrouped in specific recognition patterns as discussed above.

To ensure reliable observation, the eye must have all therays of light in focus on the retina. When the point of focusis short or primarily near the inner surface of the retina clos-est to the lens, a condition known as nearsightedness exists.If the focal spot is deeper into the retina, farsightednessoccurs. These conditions are primarily the result of the eye-ball changing from nearly orb shaped to an elliptical or eggshape. In the case of the nearsighted person, the long ellip-tical diameter is horizontal, If the long diameter is in a verti-cal direction, farsightedness occurs. These clinicalconditions result from a very small shift of the focal spot, onthe order of micrometers (ten-thousandths of an inch).

Determining Vision Acuity

The method normally used to determine what the eye cansee is based on the average of many measurements. Theaverage eye views a sharp image when the object subtends anarc of five minutes, regardless of the distance the object isfrom the eye. The variables in this feature are the diameterof the eye lens at the time of observation and the distancefrom the lens to the retina.

When vision cannot he normally varied to create sharpclear images, then corrective lenses are required to make theadjustment. While the eye lens is about 17 mm (0.7 in.) fromthe retina, the ideal eyeglass plane is about 21 mm (0.8 in.)from the retina. Differences in facial features must thereforebe considered when fitting for eyeglasses. Under various

FIGURE 12. Letters used for acuity examinationcharts (measurements in stroke units)

37 DEGREES

KPH

HIV125 DEGREES

1.5

T M T5

0 ' 1-1

116 DEGREES DEGREES 7 1 3_

_t_


working conditions, the glass lenses may not stay at theirideal location. This can cause slight variations when evaluat-ing minute details and such situations must be individuallycorrected.

For the majority of visual testing applications, near visionacuity is required. Most visual inspections are performedwithin arm's length and the inspector's vision should beexamined at 400 mm (15.5 in.) distance. Examinations forfar vision are done at distances of 6 m (20 ft).

Vision Acuity ExaminationsVisual testing may occur once or more during the fabrica-

tion or manufacturing cycle to ensure product reliability. Forcritical products, visual testing may require qualified andcertified personnel.

Certification of the visual test itself may also be requiredto document the condition of the material at the time of test-ing. In such cases, testing personnel are required to success-fully complete vision acuity examinations covering specificareas necessary to ensure product acceptability. For certaincritical inspections, it may be required for the eyes of theinspector to be examined as often as twice per year.

Near Vision Examinations

The examination distance should be 400 mm (16 in.) fromthe eyeglasses or from the eye plane, for tests withoutglasses. When reading charts are used, they should he in thevertical plane at a height where the eye is on the horizontalplane of the center of the chart. Each eye should be testedindependently while the unexamined eye is shielded fromreading the chart but not shut off from ambient light.

The Jaeger" eye chart is widely used in the United Statesfor near vision acuity examinations. The chart is a 125 X200 mm (5 x 8 in.) off-white or grayish card with an Englishlanguage text arranged into groups of gradually increasingsize. Each group is a few lines long and the lettering is black.In a vision examination using this chart, visual testing per-sonnel may be required to read, for example, the smallest let-ters at a distance of 300 mm (12 in.). Near vision acuityexaminations that are more clinically precise are describedbelow.

Far Vision Examinations

Conditions are the same as those for near vision examina-tions, except that the chart is placed 6 m (20 ft) from the eyeplane. Again, each eye is tested independently.

Grading Vision Acuity

The criterion for grading vision acuity is the ability to seeand correctly identify 7 of 10 optotypes of a specific size at aspecific distance. The average individual should be able toread six words in four to five seconds, regardless of the lettersize being viewed.

The administration of a vision acuity examination does notnecessarily require medical personnel, provided the admin-istrator has been trained and qualified to standard andapproved methods. In some instances specifications mayrequire the use of medically approved personnel. In thesecases, the administrator of the examination may be trainedby medically approved personnel for this application. In noinstance should any of these administrators try to evaluatethe examinations.

If an applicant does not pass the examination (fails to givethe minimum number of correct answers required by speci-fication), the administrator should advise the applicant toseek a professional examination. If the professional respondswith corrective lenses or a written evaluation stating theapplicant can and does meet the minimum standards, theapplicant may be considered acceptable for performance ofthe job.

Vision Acuity Examination Requirements

There are some basic requirements to be followed whensetting up a vision acuity examination system. The distancesmentioned above are examples but there are also detailedrequirements for the vision chart.

The chart should consist of a white matte finish with blackcharacters or letters. The background should extend at leastthe width of one character beyond any line of characters.Sloan letters as shown in Fig. 12 were designed to be usedwhere letters must be easily recognizable. Each characteroccupies a five stroke by five stroke space.

FIGURE 13. Vision acuity of peripheral vision

VIEW ALIGNED WITH LENS

RETINA WITH IMAGE LOCATIONS _Le

AT DIFFERENT ANGLES


The background luminance of the chart should be 85 ±5 cd•m - 2 . The luminance is a reading of the light reflectedfrom the white matte finish toward the reader.

When projected images are used, the parameters for thesize of the characters, the background luminance and thecontrast ratio are the same as those specified for charts. Inno case should the contrast or illumination of the projectedimage be changed. A projection lamp of appropriate watt-age should be used. When projecting the image, room light-ing is subdued. This should not cause any change in theluminance of the projected background contrast ratio to thatof the characters.

The room lighting for examinations using charts should be800 lx (75 ftc). Incandescent lighting of the chart is recom-mended to bring the background luminance up to 85 -±5 cd•m - 2 . Fluorescent lighting should not be used for visionacuity examinations. Incandescent lamps emit more light inthe yellow portion of the visible spectrum. This makes read-ing more comfortable for the examinee. Fluorescent lamps,especially those listed as full spectrum, are good for colorvision examinations.

Many of the lighting conditions for vision acuity examina-tions can be met by using professional examination units.With one such piece of equipment, the examinee viewsslides under controlled, ideal light conditions.

Another common design is used both in industrial andmedical examinations. With this unit, the individual looksinto an ocular system and attempts to identify numbers, let-ters or geometric differences noted in illuminated slides.The examinee is isolated from ambient light.

The slides and their respective data were developed by theOccupational Research Center at Purdue University, basedon many individuals tested in many different occupations.Categories were developed for different vocations and areprovided as guides for examinations required by variousindustries. Such equipment is expensive and accordingly eyecharts are still very popular. Table 1 compares the results ofthese three vision acuity examination systems.

TABLE 1. Eye examination system conversion chart

EyeChart

SlideDisplay

SlideDisplay with

Ocular System360 mm 114 in.(

1 10 20/202 8 20/283 6 20/334 5 20/385 4.6 20/426 4.3 20/507 4 20/558 3 20/609 2.5 20/63

10 2 20/65

There are slight differences between the reading chartsand the slides. The reading chart distance for one popularletter card is 400 mm (16 in.). The simple slide viewer is setfor near vision testing at 330 min (13 in.).

There also are some differences between individual exam-ination charts. Most of the differences are the result of vari-ances in typeface, ink and the paper's ink absorption rate.Regardless of the examination system that is used, therequirements for the lighting and contrast remain the same.

Visual Angle

Posture

Posture affects the manner in which an object isobserved—appropriate posture and viewing angle areneeded to minimize fatigue, eyestrain and distraction. Theviewer should maintain a posture that makes it easy to main-tain the optimum view on the axis of the lens.

Peripheral Vision

Eye muscles may manipulate the eye to align the image onthe lens axis. The image is not the same unless it impingeson the same set of sensors in the retina (see Fig. 13). Asnoted above, different banks of sensors basically require dif-ferent stimuli to perform their functions with optimumresults. Also, light rays entering the lens at angles not paral-lel to the lens axis are refracted to a greater degree. Thischanges the quality and quantity of the light energy reachingthe retina. Even the color and contrast ratios vary and depthperception is altered.'

FIGURE 14. Shifting eye positions change apparentobject size and location

EYE POSITIONSI 2 3

RATIO I I

RATIO I :1 40

RATIO 1 1.65

RELATIVE IMAGE SIZE AND LOCATION I

IMAGE 2

IMAGE 3

AFME


The commonly quoted optimum, included angle of fiveminutes of arc is the average in which an individual enclosesa sharp image. There are other angles to be considered whendiscussing visual testing.

The angle of peripheral vision is not a primary consider-ation when performing detailed visual tests. It is of valueunder certain inspection conditions: (1) when surveyinglarge areas for a discontinuity indication that (2) has a highcontrast ratio with the background and (3) is observed to oneside of the normal lens axis. The inspector's attention isdrawn to this area and it can then he scrutinized by focusingthe eyes on the normal plane of the lens axis.

Visual Testing Viewing Angle

The angle of view is very important during visual testing.The viewer should in all cases attempt to observe the targeton the center axis of the eye. The angle of view should notvary more than 45 degrees from normal. Figure 14 showshow the eye perceives an object from several angles and howthe object appears to change or move with a change in view-ing angle.

The same principle applies to objects being viewedthrough accessories such as mirrors or borescopes. The fieldof view should be maintained much in the same way that it iswhen viewed directly.

On reflective backgrounds, the viewing angle should be offnormal but not beyond 45 degrees. This is done so that thelight reflected off the surface is not directed toward the eyes,reducing the contrast image of the surface itself. It alsoallows the evaluation of discontinuities without distortingtheir size, color or location. This is very important whenusing optical devices to view areas not available to direct lineof sight.

Color VisionThere are specific industries where accuracy of color

vision is important: paint, fabrics and photographic film areexamples. Surface inspections such as those made duringmetal finishing and in rolling mills are to determine manu-facturing discontinuities. Color changes are not indicative ofsuch discontinuities and therefore, for practical purposes,color is not as significant in these applications." However,heat tints are sometimes important and colors may be crucialin metallography and failure analysis.

When white light testing is performed, it must be remem-bered that white light is composed of all the colors (wave-lengths) in the spectrum. If the inspector has color visiondeficiencies, then the test object is being viewed differentlythan when viewed by an inspector with normal color vision.

Color deficiency may be as critical as the test itself. Dur-ing visual testing of a white or near white object, slight defi-ciencies in color vision may be unimportant. During visualtesting of black or near black objects, color vision deficien-cies make the test object appear darker."

Color Vision Examinations

Ten percent of the male population have some form ofcolor vision deficiency. The so-called color blind conditionaffects even fewer people truly color blind individuals areunable to distinguish red and green. But, there are manyvariations and levels of sensitivity between individuals withnormal vision and those with color deficiencies.

There are two causes of color deficiency: inherited andacquired. And each of these may be subdivided into specificmedical problems. Most such subdivisions are typically dis-covered during the first vision examination.

The most common color deficiencies are hereditary andoccur in the red-green range. About 0.5 percent of theaffected individuals are female, in the red-green range.Women constitute about 50 percent of those affected in theblue-yellow range. Most such deficiencies occur in both eyesand in rare instances in only one eye. About 0.001 percentof the affected groups in the hereditary portion have their


TABLE 2. Causes of acquired color vision deficiencies

Color Vision DeficiencyCause of Deficiency

Blue-yellow deficiencyGlaucomaMyopic retinal degenerationRetinal detachmentPigmentary degeneration of the retina

(including retinitis pigmentosa)Senile macular degenerationChorioretinitisRetinal vascular occlusionDiabetic retinopathyHypertensive retinopathyPapilledemaMethyl alcohol poisoningCentral serous retinopathy (accompanied

by luminosity loss in red)

Red-green deficiencyOptic neuritis (including retrobulbar

neuritis)Tobacco or toxic amblyopiaLeber's optic atrophyLesions of the optic nerve and pathwayPapillitisHereditary juvenile macular degeneration

{Stargardt's and Best's disease)

Blue-yellow deficiencyDominant hereditary optic atrophy

Red-green or blue-yellow deficiencyJuvenile macular degeneration

TABLE 3. Classification of color vision deficiencies andpercent of affected males

Color Vision Percent Males AffectedHereditary deficiencies

trichromatism(three colors: red, green. blue)

normal visionanomalous (defective)

6 or 7dichromatism (two colors)*

protanopia (red lacking)deureranopia (green lacking)

1trianopia {blue lacking) raretetratanopia (yellow lacking) very rare

Acquired deficienciestritan (blue yellow)

data not availableprotan-deutan (red-yellow)

data not available*DEFICIENCY MOST OFTEN REFERENCED WHEN DISCUSSING COLOR BLINDNESS

TABLE 4. Naval Submarine Medical ResearchLaboratory color vision classification system

Class Description

0 NormalMild anomalous trichromat

ll Unclassified anomalous trichromat(includes mild and moderate classes)

III Moderate anomalous trichromatIV Severely color deficient {includes severe

anomalous trichromats, dichromats andmonochromats)

deficiency in the blue-green range. Individuals in the red-green group may make misinterpretations of discontinuitiesin shades of red, browli, olive and gold.

Acquired color deficiency is a greater problem to goodcolor vision testing. The acquired deficiencies may affectonly one eye and a change from acceptable color vision to arecognizable problem may he very gradual. Various medicalconditions can cause such a change to occur (Table 2 listsconditions that produce color vision deficiencies in particularcolor ranges). Most acquired color vision problems vary inseverity and may be associated with ocular pathology. If thedisease continues for an extended period of time withouttreatment, the deficiencies may become erratic in intensityand may vary from the red-green or blue-yellow ranges.Aging can also affect color vision."

Color Vision Classifications

Two functions that determine an individual's sensationrange are their color perception and color discrimination.

When a primary color is mistaken for another primary color,this is an error in perception. An error in discrimination isan error oft-ss ,-: mapitude involving a mistake in hue selec-tion: 6 During a vision examination, these two functions aretested independently.

A color vision examination performed with an anomalo-scope allows the mixing of red and green lights to match ayellow light standard. Yellow and blue lights may be mixedto match a white light. An individual with normal visionrequires red, blue and green light to mix and match colorsof the entire color spectrum. A color deficient person mayrequire fewer than the three lights to satisfy the color sensa-tion.' Table 3 indicates the type of deficiencies and the per-cent of the male population known to be affected.'

For the practical purpose of classifying personnel affectedby hereditary color deficiencies, the Naval Submarine Medi-cal Research Laboratory has developed the classificationsshown in Table 4. about 50 percent of color deficient peoplecan be categorized in accordance with this table. Class I cov-ers 30 percent of the color deficient population and Class III

FIGURE 15. Electromagnetic spectrum and anenlargement of the ultraviolet region

VISIBLE

VOLE -1 4--RED

ULTRAVIOLET

INFRARED RADIO WAVESCOSMIC RAYS GAMMA RAYS X-RAYS

SCHUMANN -.- SHORT WAVE HONG

h•FAR ULTRAVIOLET-4 I--SIJN TAN 4

0_ I

I00 200 300 400

WAVELENGTH(nanometers)

VISIAL F


accounts for 20 percent. Individuals in Class I can judge col-ors used as standards for signaling, communication and iden-tification as fast and as accurately as zero class persons can.The limitation of Class I people is when good color discrimi-nation is necessary. Persons in Class III may be used in otherareas such as radio repair, chemistry, medicine and surgery,electrical manufacturing or general painting. Class IIencompasses staff members, managers or clerical help,whose need for color resolution is not critical. Individuals inClass IV must be restricted from occupations where colordifferentiation of any magnitude is required.

As with vision acuity examinations, there are many differ-ent examinations for color vision." Color vision is oftentested with pseudoisochromatic plates or cards on which thedetection of certain figures depends on red-green discrimi-nation. Unfortunately, most common vision acuity examina-tions were designed to identify hereditary red-greendeficiencies and ignore blue-yellow deficiencies.'

Agood, discriminating examination technique is illus-trated in color Plates 1 to 7. The diagrams show thesequence in which the colors are arranged in each photo-graph for each deficiency, differing from the sequenceaccording to normal vision illustrated in Plate 1. 21 (Caution:These plates are provided for educational purposes only.Photography, print reproduction and chemical changes allcause colors to vary from the original and fade with time.Under no circumstances should illustrations in this book beused for vision examinations.)

The exam consists of the examinee's arranging fifteen col-ored caps into a circle according to changes in hue pro-gressing from a reference cap. To help evaluate theoutcome, each cap is numbered on the back. A perfect scorehas the caps in numerical sequence. This test is used forthose known to have a color vision deficiency. The test allowsfor the evaluation of the individual's ability and determinesthe specific area of the deficiency. The arrangement of colorsallows confusion to exist across the quadrants of the circle.For instance, reds can be confused with blue-greens. Oneauthority has stated that anyone who can pass this test shouldhave no problem in any work requiring color vision acuity.

Two types of red-green deficient patterns can be noted.Individuals in these categories confuse green (4) with red-purple (13) and blue-green (3) with red (12). The sequencethen appears as 4, 13, 3 and 12. Persons with the blue-yellowdeficiency confuse yellow-green (7) with purple (15), creat-ing a sequence of 7, 15, 8, 14 and 9.

As in the normal vision acuity examinations, lightingrequirements and time must be controlled for color visionexaminations. The illumination intensity of full spectrumfluorescent lighting should be no less than 200 lx (20 ftc).The rating of the light source is known as the color tempera-ture. A low color temperature lamp such as an incandescentlamp makes it easier for persons with borderline color defi-ciencies to guess the colors correctly. A color temperature of

6,700 K is preferred. Too high a color temperature increasesthe number of reading errors. To eliminate glare, the lightsource should be 45 degrees to the surface while the patientis perpendicular to it. The reading distance should be about400 to 600 mm (15 to 24 in.) or arm's length. To performsuch an examination, two minutes should be allotted toarrange all fifteen caps in their appropriate positions.

In summary, color deficiency can be acquired or inherited.Some color deficiencies may be treated, alleviated or mini-mized. Pseudoisochromatic plates in conjunction with theprogressive hue color caps provide an adequate test for mostindustrial visual inspectors. Full spectrum lighting (6,700 K)is necessary for accurate test results.

It should be added that, because the visible spectrum ismade up of colors of varying wavelengths and the black andwhite colors consist of various combinations of colors, defi-ciencies in any part of the color spectrum has an impact oncertain black and white inspection methods, including X-rayfilm review

It is recommended that all nondestructive testing person-nel have their color vision tested annually, while taking theirvision acuity examination.

Fluorescent MaterialsFluorescence is a complex phenomenon that occurs in

gases, liquids and solids. It has also proved to be the greatestand most efficient source of the so-called cold light. For thepurpose of visual nondestructive testing, fluorescence is usedin conjunction with long wave ultraviolet radiation as an exci-tation source (see Fig. 15).

Text continued on page 21.

i5

7

9

1312

1 0

REFERENCECOLOR


Caps for Color Vision ExaminationsThe exam consists of the examinee's arranging fifteen

colored caps into a circle by a change in hue progressingfrom a reference cap. To help evaluate the outcome,each cap is numbered on the back. A perfect score hasthe caps in numerical sequence. The diagrams show thesequence in which the colors are arranged in eachphotograph for each deficiency, differing from thesequence according to normal vision illustrated inPlate 1.(Caution: These plates are provided for instructionalpurposes only. Photography, print reproduction andchemical changes all cause colors to vary from theOriginal and fade with time. Under no circumstancesshould illustrations in this book be used for visionexaminations.)

PLATE 1. Colored caps for normal color vision examination

435

6

7

815 (

914

131012 11

PLATE 2. Colored caps for normal color vision with minor errors

3 4

12 11

REFERENCECOLOR

PLATE 3. Colored caps for normal color vision with one error

3

43

15

REFERENCECOLOR

21

14

1312 II

5

6

7

8

9

10


PLATE 4. Colored caps for red blindness

12 11

2

REFERENCECOLOR

3

1 0

15

PLATE 5. Colored caps for green blindness

3 4

12 11

REFERENCECOLOR

15


432

PLATE 6. Colored caps for blue blindness

PLATE 7. Colored caps for anomalous trichromatic vision

REFERENCECOLOR

ANOMALOUSMICH ROMATIC

15

14

1311

12

10

5

61

7

8

9

1013

12

15

14


FIGURE 16. Human eye response at 1070 lx1100 ftc)

700600

lO z1 LU

100

60

40

20

500LL, z,2 I Low

(=0

WAVELENGTHInanometersi


Continued from page 16.

Visible light rays are made up of billions of photons, pack-ets of particle-like energy. Photons are so small they have nomass. They do however carry energy and this is what we seewhen a light bulb is energized—the photons have carriedenergy from the bulb to the eye.

Photons have different energies or wavelengths which wedistinguish as different colors. Red light photons are lessenergetic than blue light photons. Invisible ultraviolet pho-tons are more energetic than the most energetic violet lightthat our eyes can see.

Studies show that the intensity of fluorescence in most sit-uations is directly proportional to the intensity of the ultravi-olet radiation that excites it. Fluorescence is the absorptionof light at one wavelength and reemission of this light atanother wavelength. The whole absorption and emissionprocess occurs in about a nanosecond and because it keepshappening as long as there are ultraviolet radiation photonsto absorb, a glow is observed to begin and end with the turn-ing on and off of the ultraviolet radiation. Care must betaken when using short wave or wide bandwidth ultravioletsources. A safe, general operating principle is to always holdthe lamp so the light is directed away from you.

Long wave ultraviolet is generally considered safe. How-ever, individuals should use adequate protection if they arephotosensitive or subjected to long exposure times.

Commercially available fluorescent dyes span the visiblespectrum. Because the human eye is still the most com-monly used sensing device, most nondestructive testingapplications are designed to fluoresce as close as possible tothe eye's peak response. Figure 16 shows the spectralresponse of the human eye, with the colors at the ends of thespectrum (red, blue and violet) appearing much dimmerthan those in the center (orange, yellow and green).

While the fundamental aspects of fluorescence are stillincompletely understood, there is enough known to ensurethat nondestructive testing methods using fluorescence willcontinue to improve with the development of new dyesor new solvents to increase brightness or eye responsematching.


PART 4SAFETY FOR VISUAL AND OPTICAL TESTS

This information is presented solely for educational pur-poses and should not be consulted in place of current safetyregulations. Note that units of measure have been convertedto this book's format and are not those commonly used in allindustries. Human vision can be disrupted or destroyed byimproper use of any light source. Consult the most recentsafety documents and the manufacturer's literature beforeworking near any artificial light or radiation source.

Need for SafetyDevelopments in optical testing technology have created a

need for better understanding of the potential health hazardscaused by high intensity 'light sources or by artificial lightsources of any intensity in the work area. The human eyeoperates optimally in an environment illuminated directly orindirectly by sunlight, with characteristic spectral distribu-tion and range of intensities that are very different fromthose of most artificial sources. The eye can handle only alimited range of night vision tasks.

Over time, there has accumulated evidence that photo-chemical changes occur in eyes under the influence of nor-mal daylight illumination—short term and long term visualimpairment and exacerbation of retinal disease have beenobserved and it is important to understand why this occurs.Periodic fluctuations of visible and ultraviolet radiation occurwith the regular diurnal light-dark cycles and with thelengthening and shortening of the cycle as a result of sea-sonal changes. These fluctuations are known to affect all bio-logical systems critically. The majority of such light-darkeffects is based on circadian cycles and controlled by thepineal system, which can be affected directly by the trans-mission of light to the pineal gland or indirectly by effects onthe optic nerve pathway.

Also of concern are the results of work that has been donedemonstrating that light affects immunological reactions invitro and in vivo by influencing the antigenicity of molecules,antibody function and the reactivity of lymphocytes.

Given the variety of visual tasks and illumination that con-fronts the visual inspector, it is important to considerwhether failures in performance might be a result of exces-sive exposure to light or other radiation or even a result ofinsufficient light sources. A myth exists that 20/20 fovea'vision, in the absence of color blindness, is all that is neces-sary for optimal vision. In fact, this is not so: there may bevisual field loss in and beyond the fovea centralis for many

reasons; the inspector may have poor stereoscopic vision;visual ability may be impaired by glare or reflection; or actualvision may be affected by medical or psychologicalconditions.

Laser HazardsLoss of vision resulting from retinal burns following obser-

vation of the sun has been described throughout history.Now there is a common technological equivalent to thisproblem with laser light sources. In addition to the develop-ment of lasers, further improvement in other high radiancelight sources (a result of smaller, more efficient reflectors andmore compact, brighter sources) has presented the potentialfor chorioretinal injury. It is thought that chorioretinal burnsfrom artificial sources in industrial situations have been verymuch less frequent than similar burns from the sun.

Because of the publicity of the health hazard caused byexposure to laser radiation, awareness of such hazards isprobably much greater than the general awareness of thehazard from high intensity extended visible sources whichmay be as great or greater. Generally, lasers are used in spe-cialized environments by technicians familiar with the haz-ards and trained to avoid exposure by the use of protectiveeyewear and clothing. Laser standards of manufacture anduse have been well developed and probably have contributedmore than anything else to a heightened awareness of safelaser operation.

Laser hazard controls are common sense proceduresdesigned to (1) restrict personnel from entering the beampath and (2) limit the primary and reflected beams fromoccupied areas. Should an individual be exposed to exces-sive laser light, the probability of damage to the retina is highbecause of the high energy pulse capabilities of some lasers.However, the probability of visual impairment is relativelylow because of the small area of damage on the retina. Oncethe initial flash blindness and pain have subsided, theresulting scotomas (damaged unresponsive areas) can some-times be ignored by the accident victim.

The tissue surrounding the absorption site can much morereadily conduct away heat for small image sizes than it can forlarge image sizes. In fact, retinal injury thresholds (seeFig. 17) for less than 0.1 to 10 s exposure show a high depen-dence on the image size (0.01 to 0.1 W •mm - 2 for a 1,000 p.mimage up to about 0.01 klIV•mm - 2 for a 20 p.m image. To put

FIGURE 1 7. Typical retinal burn thresholds22.23

10'

10'

10' —

* LASER II W INTO EYE}

l0

III /0 kW XENON SHORT ARC SEARCHLIGHT44 4., 4:

IASER 4fat,1 mW} 4 f

kA,7

-,N) FOR RABBIT 110 s EXPOSUREJ

ELECTRIC e•WELDING ARC

WI F I s EXPOSURE

OR CARBONARC MAXIMUM PERMISSIBLE EXPOSURE FOR CONTINUOUS SOURCES

to - '

TUNGSTEN FILAMENT

FROSTEDI — INCANDESCENT

LAMP

io-

n§1 10-'co

1 0 -9

.20CANDLE

FLUORESCENT °MOM DAYLIGHTLAMP

TELEVISION• • • •

INTERIOR IIDAYJI 0- —

I 0 - 10' 05° I° 7 4° 1lEr

SOURCE ANGIE

10 -iz 1 11111111 1 11111111 1 i i111111

10

o = O- I T'

TYPICAL RETINAL IMAGE SIZE'micrometers'

z06

ZP

0

•


the scale into perspective, the sun produces a 160 pm diame-ter image on the retina.

High Luminance Visible Light Sources

The normal reaction to a high luminance light source is toblink and to direct the eyes away from the source. The prob-ability of overexposure to noncoherent light sources is higherthan the probability of exposure to lasers, yet extended (highluminance) sources are used in a more casual and possiblymore hazardous way. In the nondestructive testing industry,extended sources are used as general illumination and inmany specialized applications. Unfortunately, there arecomparatively few guidelines for the safe use of extendedsources of visible light.

Infrared HazardsInfrared radiation comprises that invisible radiation

beyond the red end of the visible spectrum up to about I mm

wavelength. Infrared is absorbed by many substances and itsprincipal biological effect is known as hyperthermia, heatingthat can be lethal to cells. Usually, the response to intenseinfrared radiation is pain and the natural reaction is to moveaway from the source so that burns do not develop.

Ultraviolet HazardsBefbre development of the laser, the principal hazard in

the use of intense light sources was the potential eye and skininjury from ultraviolet radiation. Ultraviolet radiation isinvisible radiation beyond the violet end of the visible spec-trum with wavelengths down to about 185 nm. It is stronglyabsorbed by the cornea and the lens of the eye. Ultravioletradiation at wavelengths shorter than 185 nm is absorbed byair, is often called vacuum ultraviolet and is rarely of concernto the visual inspector. Many useful high intensity arcsources and some lasers may emit associated, potentially haz-ardous, levels of ultraviolet radiation. With appropriate pre-cautions, such sources can serve very useful visual testingfunctions.

Studies have clarified the spectral radiant exposure dosesand relative spectral effectiveness of ultraviolet radiationrequired to elicit an adverse biological response. Theseresponses include keratoconjunctivitis (known as welder's

flash), possible generation of cataracts and erythema or red-dening of the skin. Longer wavelength ultraviolet radiationcan lead to fluorescence of the eye's lens and ocular media,eyestrain and headache. These conditions lead, in turn, tolow task performance resulting from the fatigue associatedwith increased effort. Chronic exposure to ultraviolet radia-tion accelerates skin aging and possibly increases the risk ofdeveloping certain forms of skin cancer.

It should also be mentioned that some individuals arehypersensitive to ultraviolet radiation and may develop areaction following, what would be for the average healthyhuman, suberythemal exposures. However, it is extremelyunusual for these symptoms of exceptional photosensitivityto be elicited solely by the limited emission spectrum of anindustrial light source. An inspector is typically aware ofsuch sensitivity because of earlier exposures to sunlight.

In industry, the visual inspector may encounter manysources of visible and invisible radiation: incandescentlamps, compact arc sources (solar simulators), quartz halo-gen lamps, metal vapor (sodium and mercury) and metalhalide discharge lamps, fluorescent lamps and flash lampsamong others. Because of the high ultraviolet attenuationafforded by many visually transparent materials, an empiricalapproach is sometimes taken for the problem of light sourcesassociated with ultraviolet: the source is enclosed and pro-vided with ultraviolet absorbing glass or plastic lenses. If


injurious effects continue to develop, the thickness of theprotective lens is increased.

The photochemical effects of ultraviolet radiation on theskin and eye are still not completely understood. Records ofultraviolet radiation's relative spectral effectiveness for elic-iting a particular biological effect (referred to by photohiolo-gists as action spectra) are generally available. Ultravioletirradiance may be measured at a point of interest with a por-table radiometer and compared with the ultraviolet radiationhazard criteria (Table 5).

For the near ultraviolet region (from 320 nm to the edgeof the visible spectrum), the total irradiance incident on theunprotected skin or eye should not exceed 1 mW.cm - 2 forperiods greater than 1,000 s. For exposure times less than1,000 s, incident irradiance on unprotected skin or eyeshould not exceed 1 km' within an eight hour period.These values do not apply to exposures of photosensitivepeople or those simultaneously exposed to photosensitizingagents,

For purposes of determining exposure levels, it isimportant to note that most inexpensive, portable radiome-ters are not equally responsive at all wavelengths throughoutthe ultraviolet spectrum and are usually only calibrated atone wavelength with no guarantees at any other wavelength.Such radiometers have been designed for a particular appli-cation using a particular lamp.

A common example in the nondestructive testing industryis the so-called blacklight radiometer used in fluorescent liq-uid penetrant and magnetic particle applications. These

TABLE 5. Threshold limit values for ultravioletradiation' within an eight hour period*

Wavelength(nanometers)

Threshold Limit Values

200 100210 40220 25230 16240 10250 7254 6260 4.6270 3280 3.4290 4.7300 10305 50310 200315 1,000

*THESE VALUES ARE PRESENTED FOR INSTRUCTIONAL PURPOSES AND NOT ASGUIDELINES. THEY DO NOT APPLY TO EXPOSURE OF PHOTOSENSITIVE PEOPLEOR THOSE SIMULTANEOUSLY EXPOSED TO PHOTOSENSITIZING AGENTS.CONSULT CURRENT SAFETY REGULATIONS, MANUFACTURERS DATA ANDINSPECTION CODES BEFORE ANY EXPOSURE TO ULTRAVIOLET RADIATION.

meters are usually calibrated at 365 nm, the predominantultraviolet output of the filtered 100 W medium pressuremercury vapor lamp commonly used in the industry. Use ofthe meter at any other wavelength in the ultraviolet spectrummay lead to significant errors. To minimize problems inassessing the hazard presented by industrial lighting, it isimportant to use a radiometer that has been calibrated withan ultraviolet spectral distribution as close as possible to thelamp of interest.

If the inspector is concerned about the safety of a given sit-uation, ultraviolet absorbing eye protection and facewear isreadily available from several sources. An additional benefitof such protection is that it prevents the annoyance of lensfluorescence and provides the wearer considerable protec-tion from all ultraviolet radiation. In certain applications,tinted lenses can also provide enhanced visibility of the testobject.

PhotosensitizersWhile ultraviolet radiation from most of the high intensity

visible light sources may be the principal concern, the poten-tial for chorioretinal injury from visible radiation should notbe overlooked.

Over the past few decades, a large number of commonlyused drugs, food additives, soaps and cosmetics have beenidentified as phototoxic or photoallergenic agents even at thelonger wavelengths of the visible spectrum. Colored drugsand food additives are possible photosensitizers for organsbelow the skin because longer wavelength visible radiationspenetrate deeply into the body.

Damage to the RetinaIt is possible to multiply the spectral absorption data of the

human retina by the spectral transmission data of the eye'soptical media at all wavelengths to arrive at an estimate of therelative absorbed spectral dose in the retina and the underly-ing choroid for a given spectral radiant exposure of the cor-nea. The computation should provide a relative spectraleffectiveness curve for chorioretinal burns. In practice, theevaluation of potential chorioretinal burn hazards may becomplicated or straightforward, depending on the maximumluminance and spectral distribution of the source; possibleretinal image sizes; the image quality; pupil size; spectralscattering and absorption by the cornea, aqueous humor, thelens and the vitreous humor; and absorption and scatteringin the various retinal layers. For convenience, the productof total transmittance of the ocular media Tx and the totalabsorptance of the retinal pigment epithelium and choroid

FIGURE 18. The extended source of length D Limaged on the retina with length d,

CHOROID

RETINA

LENS

7 mm


a, over all wavelengths may be defined as the relative retinalhazard factor R:

r2L — = E,=

SZ A AL ,(Eq. 3)

1'14 • a, • Lx • A

R=

IL, • dk

(Eq. 1)

where L, could be any other spectral quantity. Qualitatively,the ocular media transmission rises steeply from somewhatless than 400 nm and does not fall off again until about900 nm in the near infrared after which a peak at about1,100 nm is exhibited. These values finally fall off to virtuallyzero at about 1,400 nm thus defining the potential hazardouswavelength range.

For most extended visible sources, the retinal image sizecan be calculated by geometrical optics. As shown in Fig. 18,the angle subtended by an extended source defines theimage size. Knowing the effective focal length f of therelaxed normal eye (17 mm), the approximate retinal imagesize dr can be calculated if the viewing distance r and thedimensions of the light source Di, are known.

dr = DL (Eq. 2)

r

This analysis strictly holds only for small angles—correctionsmust be made at angles exceeding about 20 degrees.Because the solid angles Si subtended by the source and retinal image are clearly identical, the retinal illuminance areaAL and source luminance area A, are likewise proportional.The source luminance L is related to the illuminance at thecornea E, as follows:

Calculation of the permissible luminance from a permissi-ble retinal illuminance for a source breaks down for verysmall retinal image sizes or for very small hot spots in anextended image caused by diffraction of light at the pupil,aberrations introduced by the cornea and lens and scatteringfrom the cornea and the rest of the ocular media. Becausethe effects of aberration increase with increasing pupil size,greater blur and reduced peak retinal illuminance arenoticed for larger pupil sizes and for a given cornealillumination.

Thermal FactorVisible and near infrared radiation up to about 1,400 nm

(associated with most optical sources) is transmitted throughthe eye's ocular media and absorbed in significant doses prin-cipally in the retina. These radiations pass through the neu-ral layers of the retina. A small amount is absorbed by thevisual pigments in the rods and cones, to initiate the visualresponse, and the remaining energy is absorbed in the retinalpigment epithelium and choroid. The retinal pigment epi-thelium is optically the most dense absorbent layer (becauseof high concentrations of melanin granules) and the greatesttemperature changes arise in this layer.

For short (0.1 to 100 s) accidental exposures to the sun orartificial radiation sources, the mechanism of injury is gener-ally thought to be hyperthermia resulting in protein denatur-ation and enzyme inactivation. Because the large, complexorganic molecules absorbing the radiant energy have broadspectral absorption bands, the hazard potential for chorioret-inal injury is not erected to depend on the coherence ormonochromaticity of the source. Injury from a laser or anonlaser radiation source should not differ if image size,exposure time and wavelength are the same.

Because different regions of the retina play different rolesin vision, the functional loss of all or part of one of theseregions varies in significance. The greatest vision acuityexists only for central (foveal) vision, so that the loss of thisretinal area dramatically reduces visual capabilities. In com-parison, the loss of an area of similar size located in theperipheral retina could be subjectively unnoticed.

The human retina is normally subjected to irradiancesbelow 1 1.1.W•mm - 2, except for occasional momentary expo-sures to the sun, arc lamps, quartz halogen lamps, normalincandescent lamps, flash lamps and similar radiant sources.The natural aversion or pain response to bright lights nor-mally limits exposure to no more than 0.15 to 0.2 s. In someinstances, individuals can suppress this response with littledifficulty and stare at bright sources, as commonly occursduring solar eclipses.


Fortunately, few arc sources are sufficiently large and suf-ficiently bright enough to be a retinal burn hazard under nor-mal viewing conditions. Only when an arc or hot filament isgreatly magnified (in an optical projection system, for exam-

largecan hazardous irradiance be imaged on a sufficiently

large area of the retina to cause a burn. Visual inspectors donot normally step into a projected beam at close range orview a welding arc with binoculars or a telescope.

Nearly all conceivable accident situations require a haz-ardous exposure to be delivered within the period of a blinkreflex. If an arc is stnick while an inspector is located at avery close viewing range, it is possible that a retinal burncould occur. At lower exposures, an inspector experiences ashort term depression in photopic (daylight) sensitivity and amarked, longer term loss of scotopic (dark adapted) vision.That is why it is so important for visual inspectors in criticalfluorescent penetrant and magnetic particle test environ-ments to undergo dark adaptation before actually attemptingto find discontinuities. Not only does the pupil have to adaptto the reduced visible level in a booth but the actual retinalreceptors must attain maximum sensitivity. This effect maytake half an hour or more, depending on the preceding stateof the eye's adaptation.

Blue HazardThe so-called blue hazard function has been used in con-

junction with the thermal factor to calculate exposure dura-tions that do not damage the retina.

The blue hazard is based on the demonstration that theretina can be damaged by blue light at intensities that do notelevate retinal temperatures sufficiently to cause a thermalhazard. It has been found that blue light can produce 10 to100 times more retinal damage (permanent decrease inspectral sensitivity in this spectral range) than longer visiblewavelengths. Note that there are some common situations inwhich both thermal and blue hazards may be present.

Visual Safety RecommendationsThe American Conference of Governmental Industrial

Hygienists (ACGIH) has proposed two threshold limit values(TLVs) for noncoherent visible light, one covering damage tothe retina by a thermal mechanism and one covering retinaldamage by a photochemical mechanism. Threshold limitvalues for visible light, established by the American Confer-ence of Governmental Industrial Hygienists, are intendedonly to prevent excessive occupational exposure and are lim-ited to exposure durations of 8 h or less. They are notintended to cover photosensitive individuals.'

Eye Protection FiltersBecause continuous visible light sources elicit a normal

aversion or pain response that can protect the eye and skinfrom injury, visual comfort has often been used as an approxi-mate hazard index and eye protection and other hazard con-trols have been provided on this basis.

Eye protection filters for various workers were developedempirically but now are standardized as shades and specifiedfor particular applications.

Other protective techniques include use of high ambientlight levels and specialized filters to further attenuate intensespectral lines. Laser eye protection is designed to have anadequate optical density at the laser wavelengths along withthe greatest visual transmission at all other wavelengths.

Always bear in mind that hazard criteria must not be con-sidered to represent fine lines between safe and hazardousexposure conditions. To be properly applied, interpretationof hazard criteria must he based on practical knowledge ofpotential exposure conditions and the user, whether a profes-sional inspector or a general consumer. Accuracy of hazardcriteria is limited by biological uncertainties including diet,genetic photosensitivity and the large safety factors requiredto be built into the recommendations.


REFERENCES

1. Rathert, Peter, Wolfgang Lutzeyer and Willard E.Goddwin, "Philipp Bozzini (1773-1809) and theLichtleiter." Urology. Volume 3, No. 1. New York,NY: Cahners Publishing Company ( January 1974):p 113-118.

2. Bush, Ronnie Beth, Hanna Leonhardt, Irving Bush andRalph Landes. "Dr. Bozzini's Lichtleiter: A Translationof His Original Article (1806)." Urology. Volume 3,No. 1. New York, NY: Cahners Publishing Company( January 1974): p 119-123.

3. Young, H.A. A Surgeon's Autobiography. New York, NY:Harcourt, Brace and Company (1940).

4. Schindler, R. Gastroscopy. . Chicago, IL: University ofChicago Press (1937).

5. Publication marking forty-year class reunion. Philadel-phia, PA: University of Pennsylvania (1938)

6. "Crampton to Get Original of His Famous 'Scope." Phil-adelphia Inquirer. Philadelphia, PA (1946).

7. Spencer, Steven M. "Amiable Oculist Is Expert onDevice for U.S. Cannon." Philadelphia Bulletin. Phila-delphia, PA ( July 1941).

8. Frisby, John P Seeing: Illusion, Brain and Mind. NewYork, NY: Oxford University Press (1980).

9. Masland, Richard H. The Functional Architecture of theRetina. New York, NY: Scientific American Books(December 1986).

10. Bailey, William H. "Why Color Vision Tests?" MaterialsEvaluation. Vol. 42, No. 13. Columbus, OH: The Ameri-can Society for Nondestructive Testing (1984): p 1,546-1,550.

11. Treisman, A. Features and Objects in Visual Processing.New York, NY: Scientific American Books (November1986).

12. Bailey, William H. "The Case for Eye Test Standardiza-tion." Materials Evaluation. Vol. 40, No. 8. Columbus,OH: The American Society for Nondestructive Testing(1982): p 826.

13. Heginbotham, W.B. "Machine Vision: 'I See,' said theRobot." Engineering Research Association Journal.

Melton, Mowbrey, England: Engineering ResearchAssociation (October 1983): p 14-17.

14. Montville, Vicky L. "Color Control from Start to Finish."Quality. Vol. 22, No. 3. Carol Stream, IL: HitchcockPublishing Company (March 1983): p 36-39.

15. Bailey, William H. "Charts for Visual Testing of Inspec-tion Personnel." Materials Evaluation. Vol. 41, No. 7.Columbus, OH: The American Society for Nondestruc-tive Testing (1983): p 849.

16. Mossman, P.B. "Testing for Degrees of Color Blind-ness." Occupational Health and Safety. Washington,DC: Occupational Safety and Health Administration(August 1983): p 49-55.

17. Bailey, William H. "Surface (Temper Etch) Inspectionand Inspectors." Materials Evaluation. Vol. 41, No. 9.Columbus, OH: The American Society for Nondestruc-tive Testing (1983): p 1,000.

18. Adams, A.J. "Color Vision Testing in Optometric Prac-tice." Journal of the American Optometric Association.Vol. 45, No. 1. St. Louis, MO: American OptometricAssociation ( January 1974): p 35-42.

19. Miller, S.C. Private correspondence. St. Louis, MO:American Optometric Association.

20. Frisby, John P. Seeing: Illusion, Brain and Mind. NewYork, NY: Oxford University Press (1980).

21. Masland, Richard H. The Functional Architecture of theRetina. New York, NY: Scientific American Books(December 1986).

22. Clarke, A.M., W.J. Geeraets and W.T. Ham, Jr. AppliedOptics. Vol. 8. New York,New York: Optical Society ofAmerica (1969): p 1,051.

23. Sliney, D.H. and B.C. Freasier. Applied Optics. Vol. 12.New York, New York: Optical Society of America (1973):

1.24. Threshold Limit Values for Chemical Substances and

Physical Agents in the Workroom Environment withIntended Changes for 1980. Cincinatti, Ohio: AmericanConference of Governmental Industrial Hygienists,Threshold Limit Values Committee (1980).

SECTION 2

THE PHYSICS OF LIGHT

PARTS I AND 2 ADAPTED FROM IES LIGHTING HANDBOOK REFERENCE VOLUME, © THE ILLUMINATING ENGINEERING SOCIETY OF NORTH AMERICAREPRINTED WITH PERMISSION.


PART I

THE PHYSICS OF LIGHT

Light can be defined as radiant energy capable of excitingthe human retina and creating a visual sensation. From theviewpoint of physics, light is defined as that portion of theelectromagnetic spectrum with wavelengths between 380and 770 nm. Visually, there is some variation in these limitsamong individuals.

Radiant energy at the proper wavelength makes visibleanything from which it is emitted or reflected in sufficientquantity to activate the receptors in the eye. The quantity ofsuch radiant energy may be evaluated in many ways, includ-ing: radiant flux (measured in joules per second or in watts)and luminous flux (measured in lumens).

Radiant Energy TheoriesSeveral theories describing radiant energy have been pro-

posed and the text below briefly discusses the primary theo-ries."

Corpuscular Theory

The corpuscular theory was advocated by Sir Issac New-ton and is based on the following premises.

1. Luminous bodies emit radiant energy in particles.2. These particles are intermittently ejected in straight

lines.3. The particles act on the retina of the eye, stimulating

the optic nerves to produce the sensation of light.

Wave Theory

The wave theory of radiant energy was advocated byChristian Huygens and is based on these premises.

1. Light results from the molecular vibration in luminousmaterial.

2. The vibrations are transmitted through the ether inwavelike movements (comparable to ripples in water).

3. The vibrations act on the retina of the eye, stimulatingthe optic nerves to produce visual sensation.

The velocity of a wave is the product of its wavelength andits frequency.

Electromagnetic Theorys

The electromagnetic theory was advanced by James ClerkMaxwell and is based on these premises.

1. Luminous bodies emit light in the form of radiantenergy.

2. This radiant energy is propagated in the form of elec-tromagnetic waves.

3. The electromagnetic waves act on the retina of the eye,stimulating the optic nerves to produce the sensation oflight.

Quantum Theory

The quantum theory is an updated version of the cor-puscular theory. It was advanced by Planck and is based onthese premises.

1. Energy is emitted and absorbed in discrete quanta(photons).

2. The energy in each quantum is hv.

The term h is known as Planck's constant and is equal to6.626 x 10- 34 joule•second. The term v is the frequency inhertz.

Unified Theory

The unified theory of radiant energy was proposed by DeBroglie and Heisenberg and is based on the premise thatevery moving element of mass is associated with a wavewhose length is given by:

= — (Eq. 1)my

Where:

X= wavelength of the wave motion (meters);h = Planck's constant or 6.626 x 10' joule•second;m=v = velocity of the particle (meters per second).

mass of the particle (kilograms); and

It is impossible to determine all of the properties that aredistinctive of a wave or a particle simultaneously, since theenergy to do so changes one of the properties beingdetermined.

FIGURE 1. Small aperture in an enclosure exhibitsblackbody characteristics

ABSORB! ION BY WALLS

....- INCIDENT RAY

FROM ILLUMINATING ENGINEERING SOCIETY. REPRINTED WITHPERMISSION.

THE PHYSICS OF LIGHT / 31

The quantum theory and the electromagnetic wave theoryprovide an explanation of radiant energy that is appropriatefor the purposes of nondestructive testing. Whether itbehaves like a wave or like a particle, light is radiation pro-duced by atomic or molecular processes. That is, in an incan-descent body, a gas discharge or a solid state device, light isproduced when excited electrons have just reverted to morestable positions in their respective atoms, thereby releasingenergy.

TABLE 1. Speed of light for a wavelength of589 nanometers (sodium D-lines1

SpeedMedium 1106 meters

per second)

Vacuum 299.793Air f100 kilopascals [760 mm Ng] at 0° C; 299.724Crown glass 198.223Water 224.915

Light and the Energy SpectrumThe wave theory permits a convenient representation of

radiant energy in an arrangement based on the energy'swavelength or frequency. This arrangement is called a spec-trum and is useful for indicating the relationship betweenvarious radiant energy wavelength regions. Such a represen-tation should not be taken to mean that each region of thespectrum is physically divided from the others—actuallythere is a small but discrete transition from one region to thenext.

The general limits of the radiant energy spectrum extendover a range of wavelengths varying from 10- 16 to over HP m.Radiant energy in the visible spectrum has wavelengthsbetween 380 x 10 - 9 and 770 x 10 - 9 m.

In the SI system, the nanometer nm (10- 9 m) and themicrometer p.m (10- 6 m) are the commonly used units ofwavelength in the visible region. In the cgs system, the ang-strom A (10' m) was used to denote wavelength.

All forms of radiant energy are transmitted at the samespeed in a vacuum: 299,793 km•s - 1 (186,282 miss '). How-ever, each form of energy differs in wavelength and thereforein frequency. The wavelength and velocity may be altered bythe medium through which it passes but the frequency isfixed independently of the medium. Equation 2 shows therelationship between radiation velocity, frequency andwavelength.

XI'v = — (Eq. 2)

Where:

v = velocity of waves in the medium (meters per second);n = the medium's index of refraction;

= wavelength in a vacuum (meters); andv = frequency (hertz).

Table 1 gives the speed of light in different media for a fre-quency corresponding to a wavelength of 589 nm in air.

Blackbody RadiationThe light from common sources, particularly light from

incandescent lamps, is often compared with light from a the-oretical source known as a blackbody. For equal area, ablackbody radiates more total power and more power at anywavelength than any other source operating at the sametemperature.

For experimental purposes, laboratory radiation sourceshave been built to approximate closely a blackbody. Designsof these sources are based on the fact that a hole in the wallof a closed chamber, small in size compared with the size ofthe enclosure, exhibits blackbody characteristics. This canbe understood with the help of Fig. 1. At each reflection,some energy is absorbed. In time, all incoming energy isabsorbed by the walls. Conversely, a blackbody can be a per-fect radiator. If the interior walls of the blackbody are uni-formly heated, the radiation which leaves the small openingwill he that of a perfect radiator for a specific temperatureand its emission energy and wavelength spectrum will beindependent of the nature of the enclosure.

From 1948 to 1979, the international reference standardfor the unit of luminous intensity was taken to be the lumi-nance of a blackbody operating at the temperature of freez-ing platinum.

FIGURE 2. Blackbody radiation curves showingWien displacement of peaks for operatingtemperatures between 500 and 20,000 K

100.00010 100 1.000 lam]

WAVELENGTHinanometers)

IVISIBLE REGION

I IA



Planck Radiation Law

Data describing blackbody radiation curves have beenobtained using a specially constructed and uniformly heatedtube as the source. Planck, introducing the concept of dis-crete quanta of energy, developed an equation depictingthese curves. It gives the spectral radiance of a blackbody asa function of the wavelength and temperature.

Figure 2 shows the spectral radiance of a blackbody as afunction of wavelength for several values of absolute temper-ature, plotted on a logarithmic scale.

Wien Radiation Law

In the temperature range of incandescent filament lamps(2,000 to 3,400 K) and in the visible wavelength region (380to 770 nm), a simplification of the Planck equation, knownas the Wien radiation law, gives a good representation of theblackbody distribution of spectral radiance.

The Wien displacement law gives the relationshipbetween the wavelength at which a blackbody at tempera-ture T in degrees Kelvin emits maximum power per unitwavelength and the temperature T. In fact the productof absolute temperature T and the peak wavelengthis a constant. It gives the relationship between blackbody

distributions at various temperatures only with thisimportant limitation.

Stefan-Boltzmann Law

The Stefan-Boltzmann law is obtained by integratingPlanck's expression for the spectral radiant excitance fromzero to infinite wavelength. The law states that the total radi-ant power per unit area of a blackbody varies as the fourthpower of the absolute temperature. The Stefan-Boltzmannlaw is explained in introductory physics texts.

Note that this law applies to the total power (the wholespectrum) and cannot be used to estimate the power in thevisible portion of the spectrum alone.

Spectral Emissivity

No known radiator has the same emissive power as a black-body. The ratio of a radiator's output at any wavelength tothat of a blackbody at the same temperature and the samewavelength is known as the radiator's spectral emissivity e(K).

Graybody Radiation

When the spectral emissivity is uniform for all wave-lengths, the radiator is known as a graybody. No known radi-ator has a uniform spectral emissivity for all visible, infraredand ultraviolet wavelengths. In the visible region, a carbonfilament exhibits very nearly uniform emissivity and is nearlya graybody.

Selective Radiators

The emissivity of all known materials varies with wave-length. In Fig. 3, the radiation curves for a blackbody, a gray-body and a selective radiator (tungsten), all operating at3,000 K, are plotted on the same logarithmic scale to showthe characteristic differences in output.

Color Temperature

The radiation characteristics of a blackbody of unknownarea may be specified with the aid of the radiation equationsby modifying two quantities: the magnitude of the radiationat any given wavelength and the absolute temperature. Thesame type of specification may be used with reasonable accu-racy in the visible region of the spectrum for tungsten fila-ments and other incandescent sources. However, thetemperature used in the case of selective radiators is not thatof the filament but a value called the color temperature.

The color temperature of a selective radiator is that tem-perature at which a blackbody must be operated so that itsoutput is the closest approximation to a perfect color matchwith the output of the selective radiator. While the match is

FIGURE 3. Radiation curves for blackbody,graybody and selective radiators operating at3,000 K

100 400 1.000 4 000 10,000



- BLACKBODY

illtRAYBODY

- ilLi=MAi SELECTIVE''' MIRAD

i (TL1N CASTE NR 1 I.• xX

1.000

400

200

100

40

20

I0

4

2


never actually perfect, the small deviations that occur in thecase of incandescent filaments are not of practicalimportance.

The apertures between coils of the filaments used in manytungsten lamps act something like a blackbody because ofthe interreflections that occur at the inner surfaces of thehelix formed by the coil. For this reason, the distributionfrom coiled filaments exhibits a combination of the charac-teristics of the straight filament and of a blackbody operatingat the same temperature.

The use of the color temperature method to deduce thespectral distribution from other than incandescent sources,even in the visible region, usually results in appreciable error.Color temperature values associated with light sources otherthan incandescent are known as correlated color tempera-tures and are not true color temperatures.

Atomic Structure and RadiationThe atomic theories first proposed in 1913 have been

expanded and confirmed by much experimental evidence.The atom consists of a central positively charged nucleus

about which revolve negatively charged electrons. In thenormal state, these electrons remain in specific orbits orenergy levels and radiation is not emitted by the atom.

The orbit described by a specific electron revolving aboutthe nucleus is determined by the energy of the electron

(there is a particular energy associated with each orbit). Anatom's system of orbits or energy levels is characteristic ofeach element and remains stable until disturbed by externalforces.

The electrons of an atom can he divided into two classes.The first includes the inner shell electrons which areremoved or excited only by high energy radiation. The sec-ond class includes the outer shell or valence electrons whichcause chemical bonding into molecules. Valence electronsare readily excited by ultraviolet or visible radiation or byelectron impact and can be removed completely with relativeease. The valence electrons of an atom in a solid whenremoved from their associated nucleus enter the so-calledconduction band and give the material the property of elec-trical conductivity.

After absorption of sufficient energy by an atom, thevalence electron is pushed to a higher energy level furtherfrom the nucleus. Eventually, the electron returns to thenormal orbit or to an intermediate orbit. In so doing, theenergy that the atom loses is emitted as a quantum of radia-tion and this is the source of light. The wavelength of theradiation is determined by Planck's equation:

E, E, = by (Eq. 3)

Where:

E, = energy associated with the excited orbit;E2 = energy associated with the normal orbit;h = Planck's constant; andv = frequency of the emitted radiation.

This equation can he converted to a more practical form:

X = 1 , 239.76

(Eq. 4)

Where:

x = wavelength (nanometers); and= the potential difference between two energy levels

through which the displaced electron has fallen inone transition (electron volts).

Luminous Efficiency of Radiant EnergyMany apparent differences in intensity between radiant

energy of various wavelengths are in fact differences in theability of various sensing devices to detect them. 6

The reception characteristics of the human eye have beensubject to extensive investigations and the results may besummarized as follows.


1. The spectral response characteristic of the human eyevaries between individuals, with time and with the ageand health of an individual, to the extent that the selec-tion of any individual to act as a standard observer is notscientifically feasible.

2. However, from the available data, a luminous efficiencycurve has been selected to represent a typical humanobserver. This curve may be applied mathematically tothe solution of photometric problems.

The standard spectral luminous efficiency curve for pho-topic (light adapted) vision represents a typical characteris-tic, adopted arbitrarily to give unique solutions tophotometric problems, from which the characteristics of anyindividual may be expected to vary. Some data indicate thatmost human observers are capable of experiencing a visualsensation on exposure to radiant energy of wavelengthslonger than 770 urn, called infrared under most circum-stances, provided the radiant energy reaches the eye at a suf-ficiently high rate. It also is known that ultraviolet radiation(wavelengths less than 380 nm) under most circumstancescan be seen if it reaches the retina even at a moderate rate.

Most observers yield only a slight response to ultraviolet radi-ation at the nearly visible wavelengths because the lens of theeye absorbs nearly all of it.

Typically, human observers have a response that undernormal lighting conditions extends from 380 to 770 manome-ters but some individuals have greater sensitivity at the blueand/or red ends of this range. Of course, at lower lightinglevels even the average observer experiences a shift of thevisible spectrum to shorter wavelengths and vice versa athigher lighting levels. The spectral range of visible responseis therefore not static but greatly dependent on the lightingconditions.

Luminous Efficiency of Light SourcesThe luminous efficiency of a light source is defined as the

ratio of the total luminous flux (lumens) to the total powerinput (watts or equivalent).

The maximum luminous efficiency of an ideal whitesource (defined as a radiator with constant output over thevisible spectrum and no radiation in other parts of the spec-trum) is about 220 lin•W

FIGURE 4. Cross section of a barrier layerphotovoltaic cell showing motion ofphotoelectrons through a microammeter circuit

SEMITRANSPARENT CATHODE

INCIDENT LIGHT

1.1 SURFACE RESISTANCE MICROAMMETER

LIBERATEDELECTRON

'71

EXTREMELY THIN FILM OF METAL


FIGURE 5. By the photoelectric effect, electronsmay be liberated from an Illuminated metal surface,flowing to an anode and creating an electriccurrent that may be detected by a galvanometer(see Eqs. 1 and 2)

LIGHT QUANTUMx (ENERGY = hv)

ANODE

CATHODEMETAL PLATE).

ENERGY TO RELEASEELECTRON = Eo

ELECTRONENERGY = I!2 my'

hv -

GALVANOMETER



PART 2

MEASUREMENT OF THE PROPERTIES OFLIGHT

The most widely used detector of light is the human eye.Other common, mechanical detectors are photovoltaic cells,photoconductive cells, photoelectric tubes, photodiodes,phototransistors and photographic film.

Photovoltaic CellsPhotovoltaic cells typically include selenium barrier layer

cells and silicon or gallium arsenide photodiodes operated inthe photovoltaic or unbiased mode. These devices dependon the generation of a current resulting from the absorptionof a photon. The cell is .comprised of (1) a p-type material,typically a metal plate coated with a semiconductor, such asselenium on iron; and (2) a semitransparent n-type materialsuch as cadmium oxide.

Unless there is an external circuit, electrons liberated fromthe semiconductor are trapped at the p-n junction afterexposure to light. The device thereby converts radiantenergy to electric energy, which can be used directly oramplified to drive a microammeter (see Fig. 4). Photovoltaiccells can be filtered to correct their spectral response so thatthe microammeter can be calibrated in units of illuminance.Factors such as response time, fatigue, temperature effects,linearity stability, noise and magnitude of current influencethe choice of cell and circuit for a given application.

Photoconductor CellsPhotoconductor cells depend on the resistance of the cell

changing directly as a result of photon absorption. Thesedetectors use materials such as cadmium sulfide, cadmiumselenide and selenium. Cadmium sulfide and cadmium sele-nide are available in transparent resin or glass envelopesand are suitable for low illuminance levels less than 10- 4 lx(10 - 5 ftc).

Photoelectric TubesThe photoelectric effect is the emission of electrons from a

surface bombarded by sufficiently energetic photons. If thesurface is connected as a cathode in an electric field (seeFig. 5), the liberated electrons flow to the anode, creating aphotoelectric current. An arrangement of this sort may beused as an illuminance meter and can be calibrated in lux orfootcandles.

The photoelectric current in vacuum varies directly withthe illuminance level over a wide range (spectral distribution,polarization and cathode potential remain the same). In gasfilled tubes, the response is linear only over a limited range.If the radiant energy is polarized, the photoelectric current


varies as the orientation of the polarization is changed(except at normal incidence).

Photodiodes and PhototransistorsPhotodiodes or junction photocells are based on solid state

p-n junctions that react to external stimuli such as light.Conversely, if properly constructed, they can emit radiantenergy (light emitting diodes). In a photosensitive diode, thereverse saturation current of the junction increases in pro-portion to the illuminance. Such a diode can therefore beused as a sensitive detector of light and is particularly suit-able for indicating extremely short pulses of radiationbecause of its very fast response time.

Phototransistors operate in a manner similar to photodi-odes but, because they provide an additional amplifier effect,they are many times more sensitive than simple photodiodes.

PhotometryProgress in the sciences is often dependent on our ability

to measure the physical quantities associated with the tech-nology—each advance in measurement ability or accuracyprovides a broadening of the science's knowledge base.

The measurement of light and its properties is called pho-tometry. The basic measuring instrument is known as a pho-tometer. The earliest photometers depended on visualappraisal by the operator as the means of measurement andsuch meters are rarely used now. They have been replacedby nonvisual meters that are sensitive to light's physicalproperties, measuring radiant energy incident on a receiver,producing measurable electrical quantities. Physicalphotometers are more accurate and simpler to operate thantheir earlier counterparts.

Observer Response Curves

Light measurements by physical photometers are usefulonly if they indicate reliably how the eye reacts to a certainstimulus. In other words, the photometer should be sensi-tive to the spectral power distribution of light in the sameway that the eye is.

Because of the substantial differences between individualeyes, standard observer response curves or eye sensitivitycurves have been established. The sensitivity characteristicsof a physical photometer should be equivalent to the stan-dard observer. The required match is typically achieved byadding filters between the sensitive elements of the meterand the light source.

Photopic and Scotopic Vision

The human eye contains two basic types of retinal recep-tors known as rods and cones. They differ not only in relativespectral response and other properties but by orders of mag-nitude in responsivity. The rods are the most sensitive andspectrally respond more to the blue and less to the red endof the spectrum. However, they do not actually give the sen-sation of color as the cones do.

Luminance is measured in candelas (cd). When the eyehas been subjected to a field luminance of more than3.0 cd.m - 2 (0.27 ed•ft - 2) for more than a few minutes, theeye is said to he in a light adapted state in which only thecones are responsible for vision; the state is also known asphotopic vision or fovea/ vision.

At light levels five or more orders of magnitude below this,at or below 3.0 x 10- 5 cel.m - 2 (2.7 x 10- 6 cd-ft - 2), thecones no longer function and the responsivity is that of therods. This is known as dark adapted, or scotopic vision orparafoveal vision. After being light adapted, the eye usuallyrequires a considerable time to become dark adapted whenthe light level is lowered. The time needed depends on theinitial luminance of the starting condition but is usuallyachieved in 30 to 45 minutes.

Between the levels at which the eye exhibits photopic andscotopic responses the spectral and other responses of theeye are continuously variable; this is known as the mesopicstate, in which properties of both cone and rod receptorscontribute. Many visual tests are made under photopic con-ditions but most measurements of fluorescent and phospho-rescent materials are made under scotopic and mesopicconditions. Because of the changes in the eye's spectralresponse at these levels it is necessary to take luminance intoaccount when evaluating the results of such measurements.'

Measurable Quantities

As indicated in Table 2, many characteristics of light, lightsources, lighting materials and lighting installations maybe measured, including (1) illuminance, (2) luminance,(3) luminous intensity, (4) luminous flux, (5) contrast,(6) color (appearance and rendering), (7) spectral distri-bution, (8) electrical characteristics and (9) radiant energy.

Principles of Photometry"Photometric measurements frequently involve a consider-

ation of the cosine law and the inverse square law (strictlyapplicable only for point sources).


TABLE 2. Measurable characteristics of light, light sources and lighting materials

Characteristic Dimensional Unit Equipment

LightWavelength'Color'Illuminance (flux density)'Orientation of polarization'Degree of polarization'

Light SourcesEnergy radiated'Color temperature2Luminous intensity'Luminance'Spectral power distribution'Power consumption'

Luminous flux (light output('Zonal distribution'

Lighting MaterialsReflectance'Transmittance2 •Spectral reflectance and transmittance'Optical density

meternoneluxdegree (angle)percent (dimensionless ratio)

joule per square meterkelvincandelacandela per square meterwatts per nanometerwatt

lumenlumen or candela

percent (dimensionless ratios)percent (dimensionless ratios)percent (at specific wavelengths)dimensionless number

spectrometerspectrophotometer and colorimeterphotometeranalyzing prismpolarization photometer

calibrated radiometercolorimeter or filtered photometerphotometerphotometer or luminance meterspectroradiometerwattmeter, or voltmeter and ammeter (fordirect current and for unity power factoralternating current circuits)integrating sphere photometerdistribution meter or goniometer

refiectometerphotometerspectrophotometerdensitometer

1. CAN BE MEASURED IN THE LABORATORY.2. CAN BE MEASURED IN THE FIELD OR THE LABORATORY.

Inverse Square Law

The inverse square law (see Fig. 6a) states that the illumi-nation E (in lux) at a point on a surface varies directly withthe luminous intensity I of the source and inversely as thesquare of the distance d between the source and the point.If the surface at the point is normal to the direction of theincident light, the law may be expressed as:

E = (1-

2 cos 0

(Eq. 6)

Cosine Cubed Law

An extension of the cosine law is the cosine cubed law,which may be a convenient alternative in certain calcula-tions. By substituting a/cos 0 for d (see Fig. 6c), the aboveequation may be written as:

I cos' 0E =

a'

E= d2 (Eq. 5)

(Eq. 7)

This equation is accurate within 0.5 percent when d is at leastfive times the maximum dimension of the source, as viewedfrom the point on the surface.

Lambert Cosine Law

The Lambert cosine law (see Fig. 6b) states that the illu-minance of any surface varies as the cosine of the angle ofincidence. The angle of incidence 0 is the angle between thenormal to the surface and the direction of the incident light.The inverse square law and the cosine law can be combinedto yield the following relationship (in lux):

Photometric Reference Standards m

Reference standards for candlepower, luminous flux andcolor are established by national standard laboratories. Aprimary reference standard is reproducible from specifica-tions and is typically found only in a national laboratory. Sec-ondary reference standards are usually derived directly fromprimary standards and must be prepared using precise elec-trical and photometric equipment.

Preservation of the reference standard's rating is veryimportant. Accordingly, a standard is used as seldom aspossible and is handled and stored with care. Photometric

FIGURE 6. Principles of photometry: (a) the inversesquare law, illustrating how the same quantity oflight flux is distributed over a greater area as thedistance from source to surface is increased; (b) theLambert cosine law, showing that light fluxstriking a surface at angles other than normal isdistributed over a greater area; and (c) the cosinecubed law, explaining the transformation of theinverse square formula

(a)

(b)

( c J

E



reference lamps are used when accuracy warrants thehighest attainable precision.

Because of the cost of reference standards, so-calledworking standards are prepared for frequent use A labora-tory can prepare its own working standards for use in cali-brating photometers. The working standard is not used toconduct a test, except where a direct comparison isnecessary.

Photometric Applications

Photometric measurements make use of the basic laws ofphotometry, applied to readings from visual photometriccomparison or photoelectric instruments. Various proce-dures are discussed below

Direct photometry is the simultaneous comparison of astandard lamp and an unknown light source.

Substitution photometry is the sequential evaluation ofthe desired photometric characteristics of a standard lampand an unknown light source in terms of an arbitraryreference.

To avoid the use of standard lamps, relative photometry isoften used. Relative photometry is the evaluation of adesired photometric characteristic based on an assumedlumen output of the test lamp. Alternately, relative photom-etry refers to the measurement of one uncalibrated lightsource to another uncalibrated light source.

It is sometimes necessary to measure the output of sourcesthat are nonsteady or cyclic and, in such cases, extreme careshould be taken."'

Means of Achieving Attenuation

During photometric measurement, it often becomes nec-essary to reduce the luminous intensity of a source in aknown ratio to bring it within the range of the measuringinstrument. A rotating sector disk with one or more angularapertures is one means of doing this. If such a disk is placedbetween a source and a surface and is rotated at such speedthat the eye perceives no flicker, the effective luminance ofthe surface is reduced in the ratio of the time of exposure tothe total time (Talbot's law). The reduction is by the factor0/360 degrees. The sector disk has advantages over manyfilters: (1) it is not affected by a change of characteristics overtime and (2) it reduces luminous flux without changing itsspectral composition. Sector disks should not be used withlight sources having cyclical variation in output.`920

Various types of neutral filters of known transmittance arealso used for attenuation. Wire mesh or perforated metalfilters are perfectly neutral but have a limited range. Par-tially silvered mirrors have high reflectance but the reflectedlight must be controlled to avoid errors in the photometer.When a mirror filter is perpendicular to the light source pho-tometer axis, serious errors may be caused by multiplereflections between the filter and receiver surface. This canbe avoided by mounting the filter at a small angle (not over3 degrees) from perpendicular at a sufficient distance fromthe receiver surface to throw reflections away from the pho-tometric axis. In this canted position, care must be taken notto reflect light from adjacent surfaces on to the receiver.Also, it is difficult to secure completely uniform transmissionover all parts of the surface.

So-called neutral glass filters are seldom neutral and trans-mission characteristics should be checked before use. Ingeneral, they have a characteristic high transmittance in thered and low transmittance in the blue, so that spectral cor-rection filters may be required. However, this type of filtervaries in transmittance with ambient temperature, as domany other optical filters.

700 800300 400 500 600

WAVELENGTHInanometersl


FIGURE 7. Average spectral sensitivitycharacteristics of selenium photovoltaic cells,compared with relative eye response (luminousefficiency curve)

I2

1.0

0 5

0.6

0.4

0.2

0

UNCORRECTEDSELENIUM

CELL

MECORRECTED

toeCELL,

I.-MOM LUMINOUS

CUBEm

I X— 111111110MEgoim•

O'•


Neutral gelatin filters are satisfactory, although notentirely neutral. Some have a small seasoning effect (losingneutrality over a period of time). They must be protected bymounting between two pieces of glass and must be watchedcarefully for loss of contact between the glass and gelatin.Filters should not be stacked unless cemented, because oferrors that may be created by interference between surfaces.

With modem metering techniques, electronic alterationscan be accomplished to keep the output of a receiver andamplifier combination in range of linearity and readability.

PhotometersA photometer is a device for measuring radiant energy in

the visible spectrum. Various types of physical instrumentsconsist of an element sensitive to radiant energy and appro-priate measuring equipment and are used to measure ultravi-olet and infrared energy. When used with a filter to correcttheir response to the standard observer, such devices canmeasure visible light.

In general, photometers may be divided into two types:(1) laboratory photometers are usually fixed in position andyield results of highest accuracy and (2) portable photome-ters are of lower accuracy for making measurements in thefield.

Both types of meters may be grouped according to func-tion, such as the photometers used to measure luminousintensity (candlepower), luminous flux, illuminance, lumi-nance, light distribution, light reflectance and transmittance,color, spectral distribution and visibility

Illuminance PhotometersVisual photometric methods have largely been supplanted

by physical methods. Because of their simplicity, visionbased photometry methods are still used in educational labo-ratories for demonstrating photometric principles and forless routine types of photometry." .'

Photoelectric photometers' may be divided into twoclasses: (1) those employing solid state devices such as pho-tovoltaic and photoconductive cells and (2) those employingphotoemissive tubes.

Photovoltaic Cell Meters32-33

A photovoltaic cell is one that directly converts radiantenergy into electrical energy. It provides a small current thatis about proportional to the incident illumination and alsoproduces a small electromotive force capable of forcing thiscurrent throtigh a low resistance circuit. Photovoltaic cellsprovide much larger currents than photoemissive cells and

can directly operate a sensitive instrument such as a micro-ammeter or galvanometer. However, as the resistance oftheir circuit increases, photovoltaic cells depart from linear-ity of response at higher levels of incident illumination.Therefore, for precise results, the external circuitry andmetering should apply nearly zero impedance across thephotocell.

Some portable illuminance meters consist of a photovol-taic cell or cells, connected to a meter calibrated directly inlux or footcandles. However, with solid state electronicdevices, operational amplifiers have been used to amplify theoutput of photovoltaic cells. The condition that produces themost favorable linearity between cell output and incidentlight is automatically achieved by reducing the potential dif-ference across the cell to zero. The amplifier power require-ments are small and easily supplied by small batteries. Inaddition, digital readouts may be conveniently used to elimi-nate the ambiguities inherent in deflection instruments."- 36

Spectral Response

The spectral response of photovoltaic cells is quite differ-ent from that of the human eye and color correcting filtersare usually needed.."-" As an example, Fig. 7 illustrates thedegree to which a typical commercially corrected seleniumphotovoltaic cell commonly used in illuminance metersapproximates the standard spectral luminous efficiencycurve. Cells vary considerably in this respect and for preciselaboratory photometry each cell should be individually colorcorrected.

The importance of color correction can be illustratedby comparing the human eye match under illumination


generated by a monochromatic source. For example, if apredominant blue light source is used, the majority of thevisible energy is concentrated near 465 nm. It can be seen inFig. 7 that the relative eye response and the filtered receptorresponse are about 10 and 15 percent. This represents a50 percent differential and indicates that the photoreceptorcould read as much as 50 percent high under the blue lightsource. Care should be taken to correct for this difference.

Transient Effects

When exposed to constant illumination, the output of pho-tovoltaic cells requires a short finite rise time to reach a sta-ble output. The output may decrease slightly over timebecause of fatigue. 4U-42 Rise times for silicon cells often areconsiderably shorter than for selenium cells.

Effect of Incidence Angle

At high incidence angles, part of the light reaching a pho-tovoltaic cell is reflected by the cell surface and the coverglass and some may be obstructed by the rim of the case.The resulting error increases with angle of incidence. Whenan appreciable portion of the flux comes at wide angles, anuncorrected meter may read illuminance as much as 25 per-cent below the true value.

The cells used in most illuminance meters are providedwith diffusing covers or some other means of correcting thelight sensitive surface to approximate the true cosineresponse. The component of illuminance contributed by sin-gle sources at wide angles of incidence may be determinedby positioning the cell perpendicular to the direction of thelight and multiplying the reading by the cosine of the inci-dence angle.

The possibility of cosine error must be taken into consid-eration for some laboratory applications of photovoltaic cells.One satisfactory solution to the problem consists of placing anonfluorescent opal diffusing acrylic plastic disk with a mattesurface over the cell. At high angles of incidence, the diskreflects the light specularly so that the readings are too low.This can be compensated by allowing light to reach the cellthrough the edges of the plastic. The readings at very highangles are then too high but can be corrected using a screen-ing ring.43-48

In general it is important that the opal diffusing plasticdisk with a matte surface should be nonfluorescent or erro-neous values of illuminance may be obtained in the presenceof blue-violet and ultraviolet radiations; such a situation iscommon in fluorescent penetrant and magnetic particletesting applications in which measurements of the ambientvisible light, in he presence of the blacklight are requiredby certain industrial and military specifications. Certain

photometers are actually provided with fluorescent diffusersand should be avoided in such situations.

Effect of Temperature" ."

Wide temperature variations affect the performance ofphotovoltaic cells, particularly when the external resistanceof the circuit is high. Prolonged exposure to temperaturesabove 50 °C (120 °F) permanently damages selenium cells.Silicon cells are considerably less affected by temperature.

Measurements at high temperatures and at high illumi-nance levels should therefore be made rapidly to avoid over-heating the cell. Hermetically sealed cells provide greaterprotection from the effects of temperature and humidity.When using photovoltaic cells at other than their calibratedtemperature, conversion factors may be used or means maybe provided to maintain cell temperatures near 25 °C(77 °F).

Effect of Cyclical Variation of Light

When electric discharge sources are operated on alternat-ing current power supplies, precautions should be taken withregard to the effect of frequency on photocell response.' 4'In some cases, these light sources may be modulated at sev-eral kilohertz. Consideration should then be given towhether the response of the cell is exactly equivalent to theTalbot's law response of the eye for cyclic varying light.

Because of the internal capacitance of the cell, it cannotalways be assumed that its dynamic response exactly corre-sponds to the mean value of the illuminance. It has beenfound that a low or zero resistance circuit is the most satisfac-tory for determining the average intensity of modulated orsteady state light sources with which photovoltaic cell instru-ments are generally calibrated.

Although a microammeter or galvanometer appears toregister a steady photocell current, it may not be receivingsuch a current. The meter actually may be receiving a pulsat-ing current which it integrates because its natural period ofoscillation is long compared to the pulses. Meters are avail-able that can average over a period of time, eliminating theeffect of cyclic variation.

Photometer Zeroing

It is important to check photometer zeroing before takingmeasurements. If an analog meter is used, this requiresmanual positioning of the indicator to zero. For any type ofequipment using an amplifier, it may be necessary to zeroboth the amplifier and the dark current (current flowingthrough the device while it is in absolute darkness). Whenpossible, it should be verified that the meter remains cor-rectly zeroed when the photometer scale is changed. Alter-nately, any deviation from zero under dark current


conditions may be measured and subtracted from the lightreadings.

Electrical interference

With electronic meters, care should be taken to eliminateinterference induced in the leads between the cell and theinstrumentation. This can be achieved by filter networks,shielding, grounding or combinations of the above.

Meters Using Photomultiplier TubesPhotoelectric tubes produce current when radiant energy

is received on a photoemissive surface and then amplified bya phenomenon known as secondary emission. These tubesrequire a high voltage to operate (2 to 5 kV) and an amplifierto provide a measurable signal. The resulting current maybe measured by a deflection meter, oscillograph or a digitaloutput device. Dark current (current flowing through thedevice while it is in absolute darkness) must be compensatedfor in the circuitry or subtracted from the lighted tube out-put. Meters using this device are often extremely sensitive.

Photomultiplier tubes can he damaged by shock and thecalibration of the meter can be altered by strong magneticfields. In addition, the device is temperature sensitive andshould be operated at or below room temperature. As withother photoelectric devices, the photomultiplier spectralresponse curve does not match the human eye and color cor-recting filters are required. Because of the large number ofphotomultiplier types available, manufacturers commonlysupply the proper optical filter for their design.

When a photomultiplier tube is used in conjunction withan optical lens system, the resulting luminance meter can beof high sensitivity and broad range.

Luminance Photometers

The basic principles for the measurement of illuminanceapply equally well for the measurement of luminance. Lumi-nance meters are essentially a photoreceptor in front of afocusing mechanism. By suitable optics, the luminance of acertain size spot, when cast onto the receptor, generates anelectrical signal that is dependent on the object luminance.This signal can be measured and, assuming that the neces-sary calibration has been performed, a reading is producedthat directly measures luminance. Usually an eyepiece isprovided so that the user is able to see the general field ofview through the instrument.

By changing the lens system in front of the photoreceptor,different fields of view and therefore different sizes of mea-surement area may be achieved. This can vary from areassubtending a few minutes of arc up to several degrees.

Photoreceptors may be selenium but are usually siliconcells or photomultipliers. The meter reading may be analogor digital and either built into the meter or remote. Ampli-fier controls for zeroing and scale selection are usually pro-vided. Other options include optical filters for color work orscale selection by means of neutral density filters.

Brightness Spot Meter55- 56

The brightness spot meter is a photoelectric device formeasuring the luminance of small areas, typically 0.25, 0.5 or1 degree field of view. A beam splitter allows a portion of thelight from the objective lens to reach a reticule viewed by theeyepiece.

The remainder of the light is reflected in front of the pho-tomultiplier tube. The output of the tube after amplificationis read on a microammeter with a'scale calibrated in candelasper square meter or footlamberts. One of the filters pro-vided for such instruments corrects the response of thephotomultiplier to the standard spectral luminous effi-ciency curve. Full scale deflection is produced by 10- 1 to

cd•m

High Sensitivity Photometer

One version of the photomultiplier photometer has inter-changeable field apertures covering fields from arc minutesto 3 degrees in diameter. Full scale sensitivity ranges arefrom 10' to 108 cd•m ".

In this meter, readings of the measured light are free fromthe effects of polarization because there are no internalreflections of the beam. The spectral response of each pho-tometer is individually measured. The filters to match it bestto the standard spectral luminous efficiency curve are theninserted. Filters are also included to permit evaluation ofpolarization and color factors.'

Equivalent Sphere IlluminationPhotometers

Equivalent sphere illumination (ESI) may be used as a toolas part of the evaluation of lighting systems. The equivalentsphere illumination of a visual task at a specific location in aroom illuminated with a specific lighting system is defined asthat level of perfectly diffuse (spherical) illuminance thatmakes the visual task as visible in the sphere as it is in the reallighting environment.

Measurements may be made visually and/or physically. Inthe visual method the measurement is made by comparisonbetween a task viewed in the measured (actual) environmentand the task viewed in a luminous sphere by using a visibilitymeter. The physical method, however, is based on certain


algorithms and requires only measurements in situ of back-ground illuminance Lb and task illuminance L,. All physicalequivalent sphere illumination devices measure Lb and L, inone way or another.

Measuring devices are discussed below in chronologicalorder of development.

Visual Task Photometer

The visual task photometer is a basic, reference instru-ment against which others are often compared." Equivalentsphere illumination is not measured directly: Lb and L, aremeasured and equivalent sphere illumination is subse-quently calculated. The task to be evaluated is mounted ona target shifter and a telephotometer is aimed at it from thedesired viewing angle.

The task and telephotometer (usually mounted on a cart)are then positioned so that (1) the task is in the locationwhere the measurement is to he made and (2) the telepho-tometer is facing the proper direction of view. The standardbody shadow (attached to the telephotometer) shades thetask in a manner similar to an actual observer. The Lb valueis measured, then the shifter is activated to bring the targetinto view of the telephotometer. The L, value is thenmeasured.

Visual Equivalent Sphere Illumination Meter

The visual equivalent sphere illumination meter' consistsof an optical system, variable luminous veil, target carrier,luminous sphere, illuminance meter (inside the sphere) anda body shadow. A task is placed on the target carrier andviewed through the optical system. The contrast of the taskis then reduced to threshold by adjusting a variable luminousveil. Field luminance is automatically kept constant so as notto alter the adaptation luminance of the observer's eye.

The task is then carried inside the sphere and the opticalsystem is adjusted until the target is again at threshold (taskvisibility is the same inside the sphere as it was outside). Theilluminance in the sphere is measured directly to determineequivalent sphere illumination.

Physical Equivalent Sphere Illumination Meters

Two devices are available that do not rely on the actualpresence of a task for their precision. Instead, they usenumerical data that represents the task's reflectance charac-teristics: bidirectional reflectance distribution functions.

One meter uses cylinders that represent an optical analogof the visual task photometer." There are two cylinders usedper measurement: one representing a task's Lb is called thebackground cylinder and one representing Lb — 4 is calledthe difference cylinder. These two parameters can be used

to calculate equivalent sphere illumination in place of L 1, andL, alone.

Each cylinder has its own body shadow. A cosine cor-rected illuminance probe is placed where the measurementis desired. The background cylinder is placed atop the probeand oriented in the appropriate viewing direction. Back-ground illuminance is then recorded. The background cylin-der is replaced by the difference cylinder, oriented in thesame direction and the difference illuminance is recorded.Equivalent sphere illumination is then calculated from thebackground and difference illuminance readings.

A second meter using bidirectional reflectance distribu-tion functions is a scanning luminance meter." This instru-ment contains a narrow field luminance probe attached to amotorized scanning apparatus and a minicomputer to con-trol scanning, store the distribution function data and per-form calculations.

To use this device for measuring equivalent sphere illumi-nation, the probe is positioned at the desired location and theminicomputer is instructed to begin scanning. Luminancesare multiplied by their appropriate bidirectional reflectancedistribution functions to determine Lb and L,. The minicom-puter then calculates equivalent sphere illumination directly.The scanning luminance meter has the capabilities of rotat-ing the task in any viewing direction and of determining theequivalent sphere illumination on different tasks with onlyone set of scanning measurements.

Visual Task Photometers

The bidirectional reflectance distribution functions usedwith physical meters are obtained by illuminating a task froma particular direction and by viewing the task from someother unique direction. A visual task photometer is used toperform these measurements. The visual task photometer isthe same as that used for equivalent sphere illuminationmeasurements except that it includes a collimated lightsource that can be positioned anywhere on a hemisphereover the task.

The task is illuminated from each azimuth and declinationangle (usually in 5 degree increments) and the reflectance ismeasured at each angle. The collection of bidirectionalreflectance data for the task and its background form the dis-tribution function,'

ReflectometersReflectometers are photometers used to measure

reflectance of materials or surfaces in specialized ways. Thereflectometer measures diffuse, specular and totalreflectance."' Those instruments designed to determinespecular reflectance are known as glossmeters.

FIGURE 8. A light cell reflectometer In anarrangement for transmittance measurement

MICROAMMETERPROJECTOR

RHEOSTAT

LIGHTSENSITIVE

CELL

•FLOATING

PRESSURE PLATE

PROJECTION LAMP

ILLUMINATOR


THE PHYSICS OF LIGHT 1 43

One popular reflectometer uses a collimated beam and aphotovoltaic cell. The beam source and cell are mounted ina fixed relationship in the same housing. The housing has anaperture through which the beam travels. This head or sen-sor is set on a standard reflectance reference with the aper-ture against the standard. The collimated beam strikes thestandard at a 45 degree angle. The photovoltaic cell is con-structed so that it measures the light reflected at 0 degreesfrom the standard. The instrument is then adjusted to readthe value stated on the standard. The sensor is placed on thetest surface and the reading is recorded.

Two cautions are recommended for use of reflectometers.The reference standard should be in the range of the valueexpected for the surface to be measured. Also, if the area tobe considered is large, several measurements should hetaken and averaged to obtain a representative value.

Another type of reflectometer (see Fig. 8) measures bothtotal reflectance and diffuse transmittance."' The instru-ment consists of two spheres, two light sources and two pho-tovoltaic cells. The upper sphere is used alone for themeasurement of reflectance. The test object is placed overan opening at the bottom of the sphere and a collimatedbeam of light is directed on it at about 30 degrees from nor-mal. The total reflected light, integrated by the sphere, ismeasured by two cells mounted in the sphere wall. The tubecarrying the light source and the collimating lenses is thenrotated so that the light is incident on the sphere wall and asecond reading is taken. The test object is in place duringboth measurements, so that the effect on both readings ofthe small area of the sphere surface it occupies is the same. 6sThe ratio of the first reading to the second reading is thereflectance of the object for the conditions of the test. Testobjects made of translucent materials should be backed by anonreflecting diffuse material.

Transmittance for diffuse incident light is measured byusing the light source in the lower sphere and taking readingswith and without the test object in the opening between thetwo spheres. The introduction of the test object changes thecharacteristics of the upper sphere. Correction must bemade to compensate for the introduced error.'

Various instruments are available for measuring suchproperties as specular reflectance and the gloss characteris-tics of materials.' For example, an instrument similar tothat described above for the measurement of diffuse reflec-tance may be used, except that the cell is fixed at 45 degreeson the side of the test object opposite to the light source,thus measuring the specularly reflected beam. The anglesubtended by the photocell to the test object affects thereading and appropriate compensations are recommended.

RadiometersRadiometers are sometimes called radiometric photome-

ters and are used to measure radiant power over a wide range

of wavelengths, including the ultraviolet, visible or infraredspectral regions. Radiometers may use detectors that arenonselective in wavelength response or that give adequateresponse in the desired wavelength band. Nonselectivedetectors (response varies little with wavelength) includethermocouples, thermopiles, bolometers and pyroelectricdetectors. One class of wavelength selective detectors isphotoelectric and includes photoconductors, photoemissivetubes, photovoltaic cells and solid state sensors such as pho-todiodes, phototransistors and other junction devices.

The overall response of such detectors can he modified byusing appropriate filters to approximate some desired func-tion. For example, these detectors can be color corrected bymeans of a filter to duplicate the standard luminous effi-ciency curve in the visible range or to level a detector'sresponse to radiant power over some hand of wavelengths.The corrections must compensate for any selectivity in thespectral response of the optical system. Care must be exer-cised to eliminate a detector's response to radiation lyingoutside the range of interest.

When a monochromator is used to disperse the incomingradiation, the radiant power can be determined in a verysmall band of wavelengths. Such an instrument is called aspectroradiometer and is used to determine the spectralpower distribution (the radiant power per unit wavelength asa function of wavelength) of the radiation in question. Thespectral power distribution is fundamental; from it radiomet-ric, photometric and colorimetric properties of the radiation


can be determined. The use of digital processing has greatlyfacilitated both the measurement and the use of the spectralpower distribution!'

The range of spectral response generally depends on thenature of the detector. Photomultiplier tubes extend wave-length sensitivity from 125 to 1,100 nm.' Various types ofsilicon photodiodes cover the range from 200 to 1,200 nm. 82

In the infrared range are intrinsic germanium (0.9 to1.5 p.m), lead sulfide (1.0 to 4.0 p.m), indium arsenide (1.0to 3.6 p,m), indium antimonide (2.0 to 5.4 gm), mercury cad-mium telluride (1.0 to 13 p.m) and germanium doped withvarious substances such as zinc (2.0 to 40 µm). .84 Theresponse of nonselective detectors ranges from near ultravio-let to 301.1,m (300,000 A) and beyond. 83."

The electrical output of detectors (voltage, current orcharge) is very small and special precautions are oftenrequired to achieve acceptable signal levels, signal-to-noiseratios and response times (for rapidly varying signals). Pho-ton counting and charge integration techniques are used forextremely low radiation

In all radiometric work, it is important to avoid stray radia-tion and care must be taken to ensure its exclusion. This isdifficult because stray radiation is not visible and a surfaceseen as black may actually be an excellent reflector of radiantenergy outside the visible spectrum. Often, unwanted radia-tion can be absorbed by an appropriate filter. Sometimessuch a high flux must be removed to avoid the absorption fil-ter's heating to the point of breaking or its transmittance forother desired wavelengths is altering.

Because radiated flux of some wavelengths is dispersed orabsorbed by a layer of air between the radiator and the detec-tor, consideration must be given to the placement of thesource and the detector and to the medium surroundingthem.

SpectrophotometersPhotometry is the measurement of power in the visible

spectrum, weighted according to the visual response curve ofthe eye. When the power is measured as a function of wave-length, the measurement is referred to as spectrophotome-try. Its applications extend from precise quantitativechemical analysis to the exact determination of the physicalproperties of matter. Spectrophotometry is important forthe determination of spectral transmittance and spectralreflectance. It is also applied to the measurement of thespectral emittance of lamps, in which case it is known asspectroradiometry. This form of measurement commonlycovers the visible portion of the spectrum, the ultraviolet87and near infrared wavelengths.

Instruments used for performing such measurements arecalled spectrophotometers and spectroradiometers. These

devices consist basically of a monochromator (separates ordisperses the wavelengths of the spectrum using prisms orgratings) and a receptor (measures the power containedwithin a certain wavelength range of the dispersed light). Ifthe spectrum is examined visually rather than by a photore-ceptor, the instrument is known to as a spectroscope.

In the visible spectrum, the only fundamental means ofexamining a color for analysis, standardization and specifica-tion is by spectrophotometry. In addition, this is the onlymeans of color standardization that is independent of mate-rial color standards (always of questionable permanence) andindependent of the abnormalities of color vision existingamong so-called normal observers.

Commercial development of spectrophotometers hasextended the wavelength range from about 200 to 2,500 nm,made them automatically record and added tristimulus inte-gration. Self scanned silicon photodiode arrays providenearly instantaneous determination of spectral powerdistributions.

Types of Photometers

Optical Bench Photometers

Optical bench photometers are used for the calibration ofinstruments for illumination measurement. They provide ameans for mounting light sources and photocells in properalignment and a means for easily determining the distancesbetween them. If the source is of known luminous intensity(candlepower), the inverse square law is used to computeilluminance, provided that the source-to-detector distance isat least five times the maximum source dimension.

Distribution Photometers

Luminous intensity (candlepower) measurements aremade on a distribution photometer which may be one of thefollowing types: (1) goniometer and single cell, (2) fixed mul-tiple cell, (3) moving cell and (4) moving mirror.

All types of photometers have advantages and disadvan-tages. The significance attached to each advantage or disad-vantage depends on factors such as available space andfacilities, polarization requirements and economic consid-erations.

Goniometer and Single Cell

The light source is mounted on a goniometer, which allowsthe source to rotate about horizontal and vertical axes. Thecandlepower is measured by a single fixed cell.

FIGURE 9. Goniometer variations: (a J the projectorturns about a fixed horizontal axis and about asecond axis which, in the position of rest, is verticaland, on rotation, follows the movement of thehorizontal axis; and (b) the light source turns abouta fixed vertical axis and also about a horizontalaxis following the movement of the vertical axis;the grid lines shown represent the loci traced bythe photocell as the goniometer axes are rotated

90

fbf

(a)30

30

30

30

FROM ILLUMINATING ENGINEERING SOCIETY. REPRINTED WITHPERM rssJoiv.

FIGURE 10. Schematic side elevation of a fixedmultiple cell photometer


FIGURE 11. Schematic side elevation of a movingcell photometer

PIVOT LIGHT SOURCE

BOOM TRACK

MOVING PHOTOCELL



There are several kinds of goniometers, each related to thetype of source being photometered and the facilities in whichit is located. With the use of computers, the coordinate sys-tem of one goniometer system can be easily changed toanother coordinate system and the compatibility of datareporting becomes practical. Figure 9 shows two types ofgoniometer systems.

Fixed Multiple Cell Photometer

In a multiple cell photometer, many individual photocellsare positioned at various angles around the light sourceunder test. Readings are taken on each photocell to deter-mine the light intensity or candlepower distribution (seeFig. 10).

Moving Cell Photometer

The moving cell photometer (Fig. 11) has a photocell thatrides on a rotating boom or an arc shaped track. The lightsource is centered in the arc traced by the cell. Readings arecollected with the cell positioned at the desired angular set-tings. Sometimes a mirror is placed on a boom to extend thetest distance.

Moving Mirror Photometer

In the moving mirror photometer, a mirror rotates aroundthe light source, reflecting the candlepower to a single pho-


tocell. Readings are taken at each angle as the mirror movesto that location.

Integrating Sphere Photometer

The total luminous flux from a source can be measured bya form of integrator sphere.'" Other geometric forms aresometimes used.°4 The theory of the integrating sphereassumes an empty sphere whose inner surface is perfectlydiffusing and of uniform nonselective reflectance. Everypoint on the inner surface reflects to every other point andthe illuminance at any point is made up of two components:the flux coming directly from the source and that reflectedfrom other parts of the sphere wall. With these assumptions,it follows that, for any part of the wall, the illuminance and

the luminance from reflected light only is proportional to thetotal flux from the source, regardless of its distribution.

The luminance of a small area of the wall or the luminanceof the outer surface of a diffusely transmitting window in thewall, carefully screened from direct light from the source butreceiving light from other portions of the sphere, is thereforea relative measurement of the flux output of the source. Thepresence of a finite source, its supports, electrical connec-tions, the necessary shield and the aperture or window, areall departures from the assumptions of the integratingsphere theory. The various elements entering into the con-siderations of a sphere, as an integrator, make it undesirableto use a sphere for absolute measurement of flux unless vari-ous correction factors are applied. This does not detractfrom its use when a substitution method is employed.'"6


REFERENCES

1. Richtmyer, EK., E.H. Kennard and T Lauritsen. Intro-duction to Modern Physics. New York, NY: McGraw-HillBook Company (1969).

2. Born, M. Atomic Physics. R.J. Blin-Stoyle and J.M. Rad-cliffe, eds. New York, NY Hafner Publishing Company(1969).

3. Born, M. and E. Wolf. Principles of Optics: Electromag-netic Theory of Propagation, Interference and Diffrac-tion of Light. New York, NY: Pergamon Press (1970).

4. Elenbaas, W. Light Sources. New York, NY: Crane,Rusaak and Company (1973).

5. Maxwell, J.C. A Treatise on Electricity and Magnetism.New York, NY: Dover Publications (1960).

6. Forsythe, W.E. Measurement of Radiant Energy. NewYork, NY McGraw-Hill Book Company (1937).

7. Wright, W.D. Photometry and the Eye. London,England: Hatton Press Limited (1949): p 31, 80, 123,124.

8. Walsh, J.VV.T. Photometry, third edition. London,England: Constable and Company (1958): p 120-173.

9. "General Guide to Photometry." Illuminating Engi-neering. Vol. 50. New York, NY Illuminating Engi-neering Society of North America (March 1955): p 147.

10. Walsh, J.W.T. Photometry, third edition. London,England: Constable and Company (1958): p 174.

11. Walsh, J.W.T. Photometry, third edition. London,England: Constable and Company (1958): p 235-237.

12. Projector, T.E. "Effective Intensity of Flashing Lights."Illuminating Engineering. Vol. 52. New York, NY Illumi-nating Engineering Society of North America (Decem-ber 1957): p 630.

13. Douglas, C.A. "Computation of the Effective Intensityof Flashing Lights." Illuminating Engineering. Vol. 52.New York, NY: Illuminating Engineering Society ofNorth America (December 1967): p 641.

14. Lash, J.D. and G.F. Prideaux. "Visibility of SignalLights." Illuminating Engineering. Vol. 38. New York,NY Illuminating Engineering Society of North America(November 1943): p 481.

15. Preston, J.S. "Note on the Photoelectric Measurementof the Average Intensity of Fluctuating Light Sources."Journal of Scientific Instruments. Vol. 18. Bristol,England: IOP Publishing (April 1941): p 57.

16. Schuil, A.E. "The Effect of Flash Frequency on theApparent Intensity of Flashing Lights Having ConstantFlash Duration." Transactions of the IES. Vol. 35. Lon-

don, England: Illuminating Engineering Society (Sep-tember 1940): p 117.

17. Neeland, G.K., M.K. Laufer and W.R. Schaub. "Mea-surement of the Equivalent Luminous Intensity ofRotating Beacons. Journal of the Optical Society ofAmerica. Vol. 28. Washington, DC: Optical Society ofAmerica (August 1938): p 280.

18. Blondel, A. and J. Rey. "The Perception of Lights ofShort Duration at their Range Limits." Transactions ofthe IES. Vol. 7. New York, NY: Illuminating EngineeringSociety of North America (November 1912): p 625.

19. Walsh, J.W.T. Photometry, third edition. London,England: Constable and Company (1958): p 223.

20. Lewin, I., G.A. Baker and M.T. Baker. "Developmentsin High Speed Photometry and Spectroradiometry."Journal of the IES. Vol. 8. New York, NY IlluminatingEngineering Society of North America ( July 1979):p 214.

21. Kaufman, J.E. and J.E Christensen. IES Lighting Hand-book, fifth edition. New York, NY: Illuminating Engi-neering Society (1972).

22. Kingsbury, E.F. "A Flicker Photometer Attachment forthe Lummer-Brodhun Contrast Photometer." journal ofthe Franklin Institute. Philadelphia, PA: Franklin Insti-tute {August 1915).

23. Guild, J. "A New Flicker Photometer for Heterochro-matic Photometry." Journal of Scientific Instruments.Bristol, England: 10P Publishing (March 1924).

24. Feree, C.E. and G. Rand. "Flicker Photometry." Trans-actions of the IES. Vol. 18. New York, NY: IlluminatingEngineering Society of North America (February 1923):p 151.

25. Little, WE. and R.S. Estay. "The Use of Color Filters inVisual Photometry" Transactions of the IES. Vol. 32.New York, NY: Illuminating Engineering Society ofNorth America ( June 1937): p 628.

26. Johnson, L.B. "Photometry of Gaseous-ConductorLamps." Transactions of the IES. Vol. 32. New York, NYIlluminating Engineering Society of North America( June 1937): p 646.

27. Sharp, C.H. and WF. Little. "Compensated Test Platefor Illumination Photometers." Transactions of the IES.Vol. 10. New York, NY Illuminating Engineering Societyof North America (November 1915): p 727.

28. Little, WE "Practical Hints on the Use of Portable Pho-tometers." Transactions of the IES. Vol. 10. New York,

48 VISUAL AND OPTICAL TESTING

NY: Illuminating Engineering Society of North America(November 1915): p 766.

29. Morris, A., F.L. McGuire and H.P. Van Cott. "Accuracyof Macbeth Illuminometer as a Function of OperatorVariability, Calibration and Sensitivity" Journal of theOptical Society of America. Vol. 45. Washington, DC:Optical Society of America ( July 1955): p 525.

30. Taylor, A.H. "A Portable Reflectance Meter." Illuminat-ing Engineering. Vol. 55. New York, NY: IlluminatingEngineering Society of North America (November1960): p 614.

31. Weibel, W.A. "Portable Electric Photometers-A Sur-vey." Light Des. Applications. Vol. 5. New York, NY Illu-minating Engineering Society of North America (August1975): p5.

32. "Report of Committee on Portable Photoelectric Pho-tometers." Transactions of the IES. Vol. 32. New York,NY: Illuminating Engineering Society of North America(April 1937): p 379.

33. Lange, B. Photoelements and their Application. NewYork, NY Reinhold Publishing Corporation (May 1938).

34. Projector, TH., M.K. Laufer and C.A. Douglas. "AnImproved 'Zero-Resistance' Circuit for Photo-Cell Pho-tometry." Review of Scientific Instruments. Bristol,England: IOP Publishing (April 1944).

35. Barbrow, L.E. "A Photometric Procedure Using Barrier-Layer Photocells." Journal of Research of the NBS. Vol.25. Washington, DC: National Bureau of Standards(December 1940): p 703.

36. Horton, G.A. "Electronic Instrumentation in LightMeasurements." Illuminating Engineering. Vol. 64. NewYork, NY Illuminating Engineering Society of NorthAmerica (December 1969): p 701.

37. Fogle, M.E. "New Color Corrected Photronic Cells forAccurate Light Measurements." Transactions of the IES.Vol. XXXI. New York, NY: Illuminating EngineeringSociety of North America (September 1936): p 773.

38. Parker, A.E. "Measurement of Illumination from Gas-eous Discharge Lamps." Illuminating Engineering.Vol. 35. New York, NY Illuminating Engineering Societyof North America (November 1940): p 883.

39. Preston, J.S. "The Relative Spectral Response of theSelenium Rectifier Photocell in Relation to Photometryand the Design of Spectral Correction Filters." Journalof Scientific Instruments. Vol. 27. Bristol, England: IOPPublishing (May 1950): p 135.

40. Elvegard, E., S. Lindroth and E. Larson. "The DriftEffect in Selenium Photovoltaic Cells." Journal of theOptical Society of America. Vol. 28. Washington, DC:Optical Society of America (February 1938): p 33.

41. Houston, R.A. "The Drift of the Selenium Barrier-LayerPhoto-Cell." Philosophical Magazine. Basingstoke,Hampshire, England: Taylor and Francis Limited( June 1941).

42. Preston, J.S. "Fatigue in Selenium Rectifier Photocells."Nature. Vol. 153 ( June 1944): p 680.

43. "Report of the Committee on Portable PhotoelectricPhotometers." Transactions of the IES. Vol. 32. NewYork, NY Illuminating Engineering Society of NorthAmerica (April 1937): p 379.

44. Goodbar, I. "New Procedure to Measure AccuratelyIllumination at Large Angles of Incidence with a Barri-er-Layer Cell." Illuminating Engineering. Vol. 40. NewYork, NY: Illuminating Engineering Society of NorthAmerica (November 1945): p 830.

45. Morton, C.A. "Cosine Response of Photocells and thePhotometry of Linear Light Sources." Light and Light-ing. Vol. 28. London, England (November 1945): p 157.

46. Buck, G.B., II. "Correction of Light Sensitive Cells forAngle of Incidence and Spectral Quality of Light." Illu-minating Engineering. Vol. 44. New York, NY Illuminat-ing Engineering Society of North America (May 1949):p 293.

47. Dows, C.L. "Illumination Measurements with LightSensitive Cells." Illuminating Engineering. Vol. 37. NewYork, NY: Illuminating Engineering Society of NorthAmerica (February 1942): p 103.

48. Pleijel, G. and J. Longmore. "A Method of Correctingthe Cosine Error of Selenium Rectifier Photocells."Journal of Scientific Instruments. Vol. 29. Bristol,England: TOP Publishing (May 1952): p 137.

49. Atkinson, J.R., N.R. Campbell, E.H. Palmer and G.T.Winch. "The Accuracy of Rectifier-Photoelectric Cells."Proceedings of the Royal Society of London: Mathemati-cal and Physical Sciences. London, England: Royal Soci-ety of London (November 1936).

50. MacGregor-Morris, J.T. and R.M. Billington. "The Sele-nium Rectifier Photocell: Its Characteristics andResponse to Intermittent Illumination." Journal of theInstitution of Electrical Engineering. Vol. 78. London,England: Institution of Electrical Engineering (October1936): p 435.

51. Zworkin, V.K. and E.G. Ramberg. Photoelectricity andIts Application. New York, NY: John Wiley and Sons(1949): p 211.

52. Gleason, P.R. "Failure of Talbot's Law for Barrier-LayerPhotocells." Physical Review. Vol. 45 (1934): p 745.

53. Walsh, J.W.T. Photometry, third edition. London,England: Constable and Company (1958): p 98, 107.

54. Lange, B. Photoelements and Their Application. NewYork, NY Reinhold Publishing Company (1938): p 151.

55. RCA Photomultiplier Manual. Booklet PT-61. Harrison,NJ: RCA Electronic Components (1970).

56. Freund, K. "Design Characteristics of a PhotoelectricBrightness Meter" Illuminating Engineering. Vol. 48.New York, NY: Illuminating Engineering Society ofNorth America (October 1953): p 524.


57. Horton, G.A. "Evaluation of Capabilities and Limita-tions of Various Luminance Measuring Instruments."Illuminating Engineering. Vol. 60. New York, NY: Illu-minating Engineering Society of North America(April 1965): p 179.

58. Eastman, A.A. "Contrast Determination with the Pritch-ard Telephotometer." Illuminating Engineering. Vol. 60.New York, NY: Illuminating Engineering Society ofNorth America (April 1965): p 217.

59. Spencer, D.E. "Out of Focus Photometry." Journal ofthe Optical Society of America. Vol. 55. Washington,DC: Optical Society of America (April 1965): p 396.

60. Spencer, D.E. and R.E. Levin. "On the Significance ofPhotometric Measurements." Illuminating Engineering.Vol. 61. New York, NY: Illuminating Engineering Societyof North America (April 1966) p 196.

61. Blackwell, H.R., R.N. Helms and D.L. DiLaura. "Appli-cation Procedures for Evaluation of Veiling Reflectionsin Terms of ESI: III. Validation of a Prediction Methodfor Luminaire Installations." Journal of the IES. Vol. 2.New York, NY: Illuminating Engineering Society ofNorth America (April 1973): p 284-298.

62. Ngai, P.Y., R.D. Zeller and J.W. Griffith. "The ESIMeter-Theory and Practical Embodiment." Journal ofthe IES. Vol. 5. New York, NY Illuminating EngineeringSociety of North America (October 1975): p 58.

63. DiLaura, D.L. and S.M. Stannard. "An Instrument forthe Measurement of Equivalent Sphere Illumination."Journal of the IES. Vol. 7. New York, NY: IlluminatingEngineering Society of North America (April 1978):p 183.

64. Green, J.D. "A Practical Direct Reading ESI Meter forField Use." Journal of the IES. Vol. 9. New York, NYIlluminating Engineering Society of North America( jury 1980): p 247.

65. Blackwell, H.R. and D.L. DiLaura. "Application Proce-dures for Evaluation of Veiling Reflections in Terms ofESI: II. Gonio Data for the Standard Pencil Task."Jour-nal of the IES. Vol. 2 (April 1973): p 254-283.

66. "IES Approved Method of Reflectometry" Journal ofthe IES. Vol. 3. New York, NY Illuminating EngineeringSociety of North America ( January 1974): p 167.

67. Baumgartner, G.R. "A Light-Sensitive Cell Reflectome-ter." General Electric Review Vol. 40 (November 1937):p 525.

68. Taylor, A.H. "Errors in Reflectometry." Journal of theOptical Society of America. Vol. 43. Washington, DC:Optical Society of America (February 1953): p 51.

69. McNichols, H.J. "Absolute Methods in Reflectometry."Journal of Research of the NBS. Washington, DC:National Bureau of Standards (1928): p 29.

70. Hunter, R.S. "A Multipurpose Photoelectric Reflec-tometer." Journal of Research of the NBS. RP 1345.

Washington, DC: National Bureau of Standards(November 1940): p 581.

71. McNichols, H.J. "Equipment for Measuring the Reflec-tive and Transmissive Properties of Diffusing Media."Journal of Research of the NBS. RP 704. Washington,DC: National Bureau of Standards (August 1934):p 211.

72. Hunter, R.S. A New Goniophotometer and Its Applica-tions. Bulletin 106. Bethesda, MD: Henry A. GardnerLaboratory (December 1951).

73. Sharp, C.H. and W.E. Little. "Measurement of Reflec-tion Factors." Transactions of the IES. Vol. XV, NewYork, NY: Illuminating Engineering Society of NorthAmerica (December 1920): p 802.

74. Taylor, A.H. "A Simple Portable Instrument for theAbsolute Measurement of Reflection and TransmissionFactors." Bulletin of the NBS. Paper 405. Washington,DC: National Bureau of Standards ( July 1920): p 421.

75. Dows, C.L. and G.R. Baumgartner. "Two Photo-VoltaicCell Photometers for Measurement of Light Distribu-tion." Transactions of the IES. Vol. 30. New York, NYIlluminating Engineering Society of North America( June 1935): p 476.

76. "Permanent Gloss Standards." Illuminating Engi-neering. Vol. 45. New York, NY Illuminating Engi-neering Society of North America (February 1950):p 101.

77. Hunter, R.S. A New Goniophotometer and Its Applica-tions. Bulletin 106. Bethesda, MD: Henry A. GardnerLaboratory (December 1951).

78. Spencer, D.E. and S.M. Gray. "On the Foundations ofGoniophotometry." Illuminating Engineering. Vol. 55.New York, NY: Illuminating Engineering Society ofNorth America (April 1960): p 228-229.

79. Nimeroff, I. "Analysis of Goniophotometric Curves."Journal of Research of the NBS. RP2335, Vol. 48. Wash-ington, DC: National Bureau of Standards ( June 1952):p 441-447.

80. Standard Recommended Practice for Goniophotometryof Objects and Materials. ANSUASTM E167. Philadel-phia, PA: American Society for Testing and Materials(1987).

81. Field, H.P. "Digital Processing in Optical Radiometry"Electro-Optical Systems Design. Vol. 7. New York, NY:McGraw-Hill Publishing Company (November 1975):p 78.

82. Cunningham, R.C. "Silicon Photodiode or Photomulti-plier Tube?• Electro-Optical Systems Descriptions.Vol. 6 (August 1974): p 21.

83. Bode, D.E. "Optical Detectors." Handbook of Lasers.Cleveland, OH: Chemical Rubber Company (1971):p 171.

SECTION 3

THE VISUAL AND OPTICAL TESTINGENVIRONMENTDayna Dunn, San Jose, California (Part 2)Charles Sherlock, Chicago Bridge and Iron Company, Houston, Texas (Part 2)Stanley Ness, Mission Viejo, California (Part 3)Larry Goldberg, Sea Test Services, Merritt Island, Florida (Part 4)


PART I EFFECT OF DESIGN CRITERIA ON VISUALAND OPTICAL TESTS

To use visual testing effectively for quality control of amanufactured component, the test method's capabilitiesmust be considered early in the product's design phase (seeTable 1). Realistic accept and reject criteria must be estab-lished as a first step in designing for process control but theserealistic criteria are not always obvious. For example, whatis the distribution of voids in nonstructural composite honey-comb that can be tolerated for satisfactory service life? Whatquality of surface finish must be achieved to make a product

acceptable? Or to make a product marketable? What leveland type of material anomalies can be reliably detected byvisual testing? How must the product design be changed toaccommodate visual testing procedures? If correct controlsare to be established, these and similar questions must beconsidered and answered as early as possible.

One of the most complex problems is determining when,during a fabrication or assembly process, visual testing ismost effective and least expensive.

TABLE 1. Summary of the visual and optical testing method

Direct visual and optically aided testing is applied to object surfaces for indications ofunacceptable conditions

Visible natural or artificial lightReflected or transmitted photonsEyes, optical aids, magnifiers, borescopes, video and film camerasVisual image, video and filmDirect, used with other methods for direct interpretation (liquid penetrant, magnetic particle)

Cracks, voids, pores and inclusionsRoughness, grain and filmMechanically aided measurementsNoneNoneVisible responses to stressNone

AllSurfaces, layers, films, coatings, entire objectsOn-line and off-line monitoring or controlAll forms of nondestructive testingMachined parts, internal surfaces, indefinite range of test objects, materials, components,

assemblies

Visual accessSpecialized optical aids often requiredVarious degrees of magnificationMay require supplementation with other nondestructive test methods for discontinuity

detection and measurementBorescopy, refractometry, diffractometry, interferometry, microscopy. telescopy, light

radiometry, phase-contrast and Schlieren techniques

MethodKey process and basic result

PrinciplesProbe medium or energy sourceNature of signal or signatureDetection or sensing methodIndication or recording methodInterpretation basis

ObjectivesDiscontinuities and separationsStructureDimensions and metrologyPhysical and mechanical propertiesComposition and chemical analysesStress and dynamic responsesSignature analysis

ApplicationsApplicable materialsApplicable features and formsProcess control applicationsin situ or diagnostic applicationsTypical structures and components

LimitationsAccess, contact or preparationProbe and object limitsSensitivity or resolutionInterpretation limits

Related techniques

THE VISUAL AND OPTICAL TESTING ENVIRONMENT / 53

Visual Testing in Product DesignProduct design typically comprises four steps: conceptual,

preliminary, layout and detail. During the concept phase,compatibility with visual and other nondestructive testingprocedures must be ensured.

In the preliminary design phase, performance criteria andmaterial selection should be made compatible with nonde-structive testing. During layout, inspectability of the productmust be determined. It is important that the efforts of quali-fied engineering, manufacturing and nondestructive testingpersonnel he closely coordinated during this determination.Producibility and quality should receive the greatest atten-tion in the detail design phase but all disciplines must beconsidered.

Complex structures may not be inspectable because ofgeometric constraints or accessibility. It is necessary eitherfor (1) such components must be redesigned or (2) for theapproval of the design to take uninspectability into account.Nondestructive testing is an added cost but, when properlyapplied, it can substantially reduce total life-cycle costs.

The visual testing specialist participates in the design pro-cess by providing knowledge of the visual testing function.This can best be accomplished by (1) providing qualifiedNDT support during design, (2) revising design handbookdata to cover nondestructive testing and (3) establishing non-destructive testing guidelines to govern testing as part ofoverall quality procedures.

Designing for Quality AssuranceQuality assurance is the establishment of a program to

guarantee the desired quality level of a product from raw

materials through fabrication, assembly and delivery. Qual-ity control is the physical and administrative actions requiredto ensure compliance with the quality assurance program.Quality control includes nondestructive testing at appro-priate points in the manufacturing cycle.

A quality assurance program consists of five basicelements.

I. Prevention: a formalized plan for designing, forinspectability and cost-effectiveness.

2. Control: documented workmanship standards and com-patible procedures for training of and use by productionand quality control personnel.

3. Assurance: establishing quality control check points anda rapid information feedback system.

4. Corrective action: implementation of the feedback sys-tem and necessary corrective action.

5. Audit: unbiased third party review of all aspects of theprogram, including vendor materials.

Management must decide what quality level it will pro-duce and support. Once this is established, production andtesting personnel aim to maintain this level and not to departfrom it either toward lower or higher quality.

For example, when drawing a component, the designersets tolerances on dimensions and finish. If a drawing spe-cifies a certain dimension as 32 mm (1.25 in.) but fails tospecify the tolerance, the machine shop supervisor could(1) reject the drawing as incomplete or (2) assume a standardtolerance.

In nondestructive testing, a quality tolerance (the accept-able limits on the characteristic of interest) must also bespecified. For example, no defects is an unworkable qualityacceptance criteria. The lack of this single requirement hascaused much misunderstanding of nondestructive testing ingeneral and visual tests in particular.


PART 2 ENVIRONMENTAL FACTORS

An important environmental factor affecting visual tests islighting. Often, emphasis is placed on equipment variablessuch as borescope view angle or degree of magnification.But if the lighting is incorrect, no magnification is going toimprove the image. Other working conditions are alsoimportant including factors causing operator discomfort andfatigue.

CleanlinessThe act of seeing depends on the amount of light reaching

the eye. In visual tests, the amount of light may be affectedby distance, reflectance, brightness, contrast or the cleanli-ness, texture, size and shape of the test object.

Cleanliness is a basic requirement for a good visual test—it is impossible to gather visual data through layers of opaquedirt unless cleanliness itself is being examined. In additionto obstructing vision, dirt on the test surface can mask actualdiscontinuities with false indications. Cleaning typically maybe done by mechanical or chemical means or both. Cleaningavoids the hazards of undetected discontinuities andimproves customer product satisfaction.

Texture and ReflectanceVision is dependent on reflected light entering the eye.

The easiest way to ensure adequate lighting is by placing thelight source and eye as close to the test surface as the focaldistance allows. Similarly, a magnifier should be held as closeto the eye as possible, ensuring that the maximum amount oflight from the target area reaches the eye.

Reflectance and surface texture are related characteristics.It is important for lighting to enhance a target area, but glareshould not be allowed to mask the test surface. A highlyreflective surface or a roughly textured surface may requirespecial lighting to illuminate without masking. Supplemen-tary lighting must be shielded to prevent glare from interfer-ing with the inspector's view.

Reflected or direct glare can be a major problem that isnot easily corrected. Glare can be minimized by decreasingthe amount of light reaching the eye. This is done by increas-ing the angle between the glare source and line of vision byincreasing the background light in the area surrounding the

glare source or by dimming the light source. Such solutionsare simple to implement for direct glare from a supplementallight or the reflected glare from a small test object. Glarefrom permanent lighting fixtures is more difficult to control.

Ceiling fixtures should be mounted as far above the line ofsight as possible and must be shielded to eliminate light at anangle greater than 45 degrees to the field of vision. Tasklighting should be shielded to at least 25 degrees from hori-zontal. Such shielding must allow a sufficient amount of lightto reach the test area.

Lighting for Visual TestsThe amount of light required for a visual test is dependent

on several factors, including the type of test, the importanceof speed or accuracy, reflections from backgrounds andinspector variables. Physiological processes, psychologicalstate, experience, health and fatigue all contribute to theaccuracy of a visual inspection.

The reflections and shadows from walls, ceiling, furnitureand equipment must also be considered. Some reflectancefrom the environment must occur or the room will be toodark to be practical. Recommended reflectance values are:ceiling, 80 to 90 percent; walls, 40 to 60 percent; floors, notless than 20 percent; desks, benches and equipment, 25 to 45percent.

For visual and other nondestructive testing applications, aratio of 3:1 between the test object and darker background isrecommended. A 1:3 ratio is recommended for a test objectand lighter surroundings.

Certain psychological factors can also affect a visualinspector's performance. Wall colors and patterns have beenshown to have a measurable effect on attitude and this isespecially important when visually inspecting critical orsmall components. In general, a visual inspector's optimumattitude is relaxed but not bored, alert but not restless. Tocomplement the illumination needed for visual testing, allcolors in a room should be light tones. Otherwise, up to 50percent of the available light can be absorbed by dark wallsand flooring. A strong contrast of pattern or color can causerestlessness and eventually fatigue. Cool (blue) colors arerecommended for work areas with high noise levels andheavy physical exertion.

FIGURE 1. Minimum angle for typical visual testing

VIEWING ANGLE RANGE

-EYE

ca

TEST SURFACE30 1^.DEGREES'', .

NO CLOSER THAN150 ram I6 in I

30 DEGREES

TEST SITE


Light IntensitiesThe nanometer (tip), equal to 10- 9 meters, has replaced

the angstrom unit (A) as the preferred unit for measuringradiation wavelengths. There are ten angstrom units in ananometer.

To perform a visual test, there must be a source of naturalor artificial light adequate in both intensity and spectral dis-tribution. Even under optimum conditions the human eyecan be stimulated by only a small part of the electromagneticspectrum. The limits of this visible portion are ill defined,depending on the amount of energy available, its wavelengthand the health of the eye. For most practical purposes, thevisible spectrum may be considered to be between about380 nm at the beginning of the violet and 770 nm at the endof the red, However, with especially intense sources andwith a completely dark adapted eye, the shorter wavelengthboundary may be extended down to 350 nm or shorter, witha corresponding reduction in the longest wavelength per-ceived. Similarly, with an especially intense longer wave-length source and an eye adapted to a higher level of light,the longer wavelength boundary may extend up to 900 nm.These ranges together are only a small part of the electro-magnetic spectrum.

Brightness is an important factor in visual test environ-ments. The brightness of a test surface depends on itsreflectivity and the intensity of the incident light. Excessiveor insufficient brightness interferes with the ability to seeclearly and so obstructs critical observation and judgment.For this reason, light intensity must be tightly controlled.

A minimum intensity of 160 lx (15 ftc) of illuminationshould be used for general visual testing. A minimum of500 lx (50 ftc) should be used for critical or finely detailedtests.

According to the Illuminating Engineering Society, visualtesting requires light at 1,100 to 3,200 lx (100 to 300 ftc) forcritical work.' A commercially available light meter can beused to determine if the working environment meets thisstandard.

To ensure

Vision in the Testing EnvironmentThe eye is a critical variable in visual tests because of varia-

tions in the eye itself as well as variations in the brain andnervous system. For this reason, visual inspectors must beexamined to ensure natural or corrected vision acuity. The

TABLE 2. Distances for minimum 500 Ix (50 ftc)illumination

Maximum Source-to-Object DistanceLight Source millimeters (inchesl

2 D cell flashlight 250 (10)60 W incandescent bulb

250 (10)75 W incandescent bulb

380 (15)100 W incandescent bulb

460 (18)

frequency of such examinations is determined by code, stan-dard specification, recommended practice or company pol-icy and yearly examinations are common.

The Jaeger'" eye chart is widely used in the United Statesfor near vision acuity examinations. The chart is a 125 x 200mm (5 x 8 in.) off-white or grayish card with an English lan-guage text arranged into groups of gradually increasing size.Each group is a few lines long and the lettering is black. Ina vision examination using this chart, visual testing personnelmay be required to read, for example, the smallest letters ata distance of 30 cm (12 in.).

More clinically precise ways of measuring vision acuityinvolve recognition of Roman capital letters of various sizesfrom controlled distances. More on the determination ofvision acuity may be found in the discussion of the physiologyof sight.

The exact requirements for near vision acuity examinationare specified by the employer. If prescription lenses arerequired to pass a vision examination, then the subject mustwear them during subsequent visual testing. Photograyinglenses can be a problem where ultraviolet light is high, e.g.,under some fluorescent lights.

compliance with the minimum intensityrequirement, a known light source held within a specifiedmaximum distance must be used. Alternatively, a light mea-suring device such as a photocell or phototube must be used.Examples of known light sources are shown in Table 2.


Visual Angle and Distance

The angle of vision and the distance of the eye from thetest surface determine the minimum angular separation oftwo points resolvable by the eye. This is known as the eye'sresolving power.

For the average eye, the minimum resolvable angular sep-aration of two points on an object is about one minute of arc(or 0.0167 degrees). This means that at 300 mm (12 in.)from a test surface, the best resolution to be expected is

about 0.09 mm (3.5 mil). At 600 mm (24 in.), the best antici-pated resolution is about 0.18 mm (0.007 in.).

To complete a visual test, the eye is brought close to thetest object to obtain a large visual angle. However, the eyecannot sharply focus on an object if it is nearer than 250 mm(10 in.). Therefore, a direct visual test should be performedat a distance of 250 to 600 mm (10 to 24 in.).

Also of importance is the angle the eye makes with the testsurface. For most indications, this should not be less than 30degrees (see Fig. 1).

FIGURE 2. Cross section of the eye, showing foveacentralis (see also figures in discussion of thephysiology of eyesightl

ORA SERRATA

CILIARY MUSCLE

CONJUNCTIVAIRIS

AQUEOUS HUMOR \CORNEA

SCLERACHOROID

RETINA

,FOVEACENTRALIS

FROM THE AMERICAN OPTOMETRIC ASSOCIATION. REPRINTED WITHPERMISSION.


PART 3PHYSIOLOGICAL FACTORS

The LensThe human eye is a roughly spherical organ, set in a socket

where it is free to rotate in all forward directions. (Refer tofigure showing eye elsewhere in this book.) At the front, acompound lens (including the cornea) is set into an openingthrough which light enters the eye. This lens is of variablefocal length and changes without conscious effort to focusobjects at varying distances, forming images at the back ofthe eye. With aging, focusing becomes sluggish. Immedi-ately in front of the lens is the iris, a circular pigmentedmembrane, perforated by an aperture known as the pupil.The iris, analogous to the diaphragm of a camera, adjustsspontaneously the area of the pupil to change the amount oflight entering the eye by a maximum factor of about 16:1.The pupil tends to be wider at low light intensities andsmaller at higher intensities. It plays little part in colorperception.

The lens does not pass light of the shortest wavelengthsand is largely responsible for the termination of response atthe low end of the spectrum. As age increases, the lens yel-lows, increasing the absorption in the blue region and tend-ing to increase the shortest wavelength that can be seen.This can be a factor in color differences reported betweenobservers of different age, especially for tasks involvingshorter wavelength perceptions.

The FoveaThe photographic plate used in the camera is represented

in the eye by the retina, which contains the end plates of theoptic nerve. These receptors are extremely complicatedstructures called rods and cones. Figures showing theapproximate location of these microscopic receptors are inthe introductory discussion on the physiology of sight. Nerveimpulses stimulated by light arise in these structures and areconducted along the visual pathways to the occipital regionof the brain.

When the eye looks directly at a small area in the field ofview, the images impinge on a region called the foven cen-trails (see Fig. 2). This is the region of sharpest vision andthe retina component most important for visual testing. It isconvenient to consider the cone and rod distributions andtheir dependence on increasing distance from the fovea cen-tralis. The central part of the fovea consists almost entirely

of color sensitive cones, nearly all of which are connectedindividually to optic nerve fibers. The foveal cones arepacked more tightly together and the structure above themis much thinner, forming a depression in the retina in thisregion.

There is a sound physiological basis for the superiority ofdetail perception in the fovea centralis. This rod-free areaextends outward to around 2 or 3 degrees as measured by thearea's angular subtense in the external field; it is notablyinsensitive to shorter visible wavelengths. This may aid detailvision by offsetting chromatic aberrations of the eye's lens.Broadly speaking, no other part of the eye is used to perceivea momentary object of interest. At a distance of 500 mm(20 in.), the 2 degrees of rod-free surface corresponds to anobject size of about 19 mm (0.75 in.). Visual testing of a com-ponent larger than 19 mm (0.75 in.) then becomes a series ofquick and successive fixations along the object with occa-sional returns for verification.

Rods and ConesProceeding outward from the fovea centralis, rods are

found mingled with cones. With distance from the fovea,the percentage of cones decreases exponentially and the


percentage of rods increases exponentially. At the sametime, both rods and cones show a tendency to connect ingroups to single nerve fibers. This tendency is muchstronger for rods and these groups become larger withincreased distance from the fovea. Vision for detail thereforedecreases steadily but color perception persists, at normallight intensity levels.

Partly overlapping the fovea and surrounding it out toaround 10 degrees in the visual field is an irregular, diffusering of yellow pigment known as the macular lutae. Itsimportance in perception comes from its absorption of bluelight, thus changing the spectral energy distribution of thelight reaching receptors that are under it.

Rods and cones differ in the minimum intensity of light towhich they can respond. This difference is caused in rods bythe presence of a photosensitive pigment called rhodopsin.This material is very easily bleached by light at low levels andis assumed to produce an electrochemical response in therods. This visual response is essentially without color sensa-tion and the sensitivity of the eye as a function of wavelengthat these intensity levels corresponds to the wavelengthabsorption curve of rhodopsin. It is distinctly different fromthe wavelength response curve of the whole eye at higherintensity levels, which is representative of the sensitivity ofthe cones.

Three classes of human cones have been identified, with asensitivity peaking at 445, 535 and 570 nm. It is known thatthe blue absorbing cones are relatively sparse in the foveacentralis, thereby explaining its insensitivity to shorter wave-lengths. Because of the absence of rods in the fovea, there isDO response for low level light, even if the chromatic sensitiv-ity is at its highest level and the iris is fully dilated. It is thelevel of the stimulus that is inadequate to elicit a chromaticresponse. To obtain any response at all, it is necessary to lookoff to one side of the stimulus so that at least some rods par-ticipate in the perception. However, any sufficient stimulusin the central field of view (a distant light source, for exam-ple, or a discontinuity filled with fluorescent penetrant irra-diated with ultraviolet radiation) produces a chromaticresponse while everything else remains colorless.

ReceptorsBecause receptors are grouped and the size of the groups

increases rapidly with increasing distance from the fovea,peripheral vision is very indistinct and largely serves pur-poses of orientation and the detection of motion. The mech-anism appears to play little if any part in perception ofstationary objects at normal room and daylight intensities.

The fibers from the various receptors cross the inner(vitreous humor) side of the retina and pass through ittogether in the optic nerve bundle. This transitional area is

called the optic disk and is completely blind. Its surface areais comparable to that of the fovea. The optic disk lies about16 degrees toward the nose from the fovea (outward in thevisual field) so that corresponding parts of the visual fieldcannot fall on both disks simultaneously. An observer is notaware of this blind spot except when consciously arrangingfor an image to fall wholly within the optic disk.

As mentioned above, the retina does not detect light uni-formly over its area. The importance of this for perception isnot so much the details overlooked because of the nonunifor-mity as the fact that even a rather keen observer is not nor-mally aware of the nonuniformity unless an instance ispointed out.

As we look about a scene (rather than at a fixed point), theimage in the eye moves across the region of sharpest visionas well as all the other regions. This voluntary, though notusually conscious, movement corresponds to shifting focus ofattention on details. During each pause, there is also a fairlyrapid tremor of the eyes called saccadic movement. Bothmovements encourage contours in the image to cross thereceptor elements of the retina. It is believed that this effectplays a role in contour perception and even appears to be anecessary condition for vision. If the center of such a fieldis rigidly fixated and viewed without blinking (both difficult),there is a gradual loss of both brightness and saturation overthe whole area and this can eventually make the stimulus dis-appear. The progressive change can be interrupted at anypoint by either blinking or moving the eye quickly (changingthe fixation point) from side to side. Together with otherdata, it is apparent that there has been a loss in sensitivity inthe area of the retina covered by the image.

PerceptionIn terms of its visible response, the sensitivity of the eye

to light is not constant. The eye tends to respond more todifferences in the field of view than to absolute values. Itappears to do this automatically by adjusting sensitivities tosomething approaching the average of the stimuli. Sensitiv-ity is also affected laterally by stimuli lying near the primaryobject. These are time-dependent factors, with the timescale being determined largely by the magnitude of changefrom the previous stimulation.

Adaptation is essentially independent in the two eyes sothat they may have quite different sensitivity levels at thesame time. For gross light level changes, adaptation occursas (1) the familiar and painful glare of a bright light after along period in relative darkness, with adjustment sometimestaking as long as a minute and (2) the blindness after enteringa normally lighted room from full sunlight, with adjustmenttaking as long as thirty minutes. Adaptation time increaseswith age.

FIGURE 3. In the MUller-Lyer illusion, the shafts oftwo arrows are the same length—contrary toappearances


Influences on Perception

The science of perception is the study of (1) how ideas andother mental events become organized to yield impressionsof objects and (2) the influence of the observer's mental andphysical states. It is known that the perceived qualities of aviewed object may change with the state of the observer,based on knowledge of or assumptions about the causes of astimulus. For example, under certain conditions, two linesof the same length can be perceived as different lengths, asin the well known Muller-Lyer illusion with double endedarrows (see Fig. 3).

For the purposes of visual and optical testing, it isimportant to know why physical reality may differ from per-ception and what are the effects of the observer's knowledge,fatigue, health and attitude. Perception is an active processin which the observer uses vision in combination with experi-ence to maximize the wanted details and minimize theunwanted details. Most visual inspectors recognize that testobjects exist, that they emit or reflect light, that this lightcauses neural activity and that the brain then synthesizessome representation of the original object. Other assump-tions are specific to the application and may even be errone-ous for example, what sort of defects are likely to occur orare of concern to the employer. The following discussionemphasizes somatic conditions that can impair the inspec-tor's judgment.

Sluggishness of the iris or of the muscles adjusting the lenscan be caused by age, fatigue, drugs, disease or emotions.

Such sluggishness in turn can affect what the observer seesand does not see.

Effects of Fatigue

Seeing is not the passive formation of an image. It is anactive process in which the observer keeps track of personalactions through a kind of feedback loop in which the per-ceived thing may be altered by the observer's actions.

As one of the first steps in this complex feedback system,the image is formed by the lens of the eye on 100 million orso rods and cones in the retina. There are only about 1 mil-lion fibers that can carry the responses of these elements outof the eye through the optic nerve. Clearly, there must begrouping of these sensitive elements into single channels.Both this grouping and the distribution of rods and coneschange systematically over the retina. In common with otherpsychological subjects, but unlike the physical sciences, theend result of seeing cannot be measured. It can only bedescribed or compared to the effect of a previous, similarexperience. In common with all other processes that requireactive participation, fatigue reduces the observer's efficiencyfor accurately interpreting visual data.

Effect of Observer's Health

There are many somatic conditions that can directly orindirectly affect an individual's ability to see. Glaucoma isone such disease, characterized by increased intraocular ten-sion which can cause vision impairments ranging from slightabnormalities to absolute blindness. In many cases, thecause of visual impairment is not known and not easilydiscovered. Some problems of perception are secondaryeffects supplemented by predispositions of heredity, emo-tional state or circulatory factors. In other cases, impairmentcan result directly from disease of the ocular structures,including intraocular tumors, enlarged cataracts or intraocu-lar hemorrhage.

Presbyopia is a condition in which the lens stiffens withage and so loses its ability to focus.

Diabetic retinopathy is another condition that impairsnormal vision. It can occur eight years after the onset of dia-betes, with effects ranging from minor to severe. Diabetescan also lead to degenerative changes in a normally devel-oped lens, characterized by gradual loss of transparency.Well developed, diffuse cataracts sometimes result from dia-betes as well as other causes. The condition can reducevision until only light perception remains. Sometimes myo-pia develops in the early stages of nuclear cataracts so thatsomeone whose vision is presbyopic may be able to readwithout corrective lenses. Gradual loss of vision in middleage is characteristic of both cataracts and glaucoma.

Prolonged use of the eyes with defective illumination anda strained position should always he avoided. It is also


important to avoid fatigue of the eye muscles particularlywhen caused by errors of refraction. Inability to concentrateon the subject and a rhythmic oscillation of the eye and eye-lids may occur as a result of eye muscle fatigue, leading toineffective visual tests. Corrective lenses and rest oftenrelieve simple forms of eye strain. Because of physiologicalchanges in the lens with age, the lens is rendered less respon-sive to the process of accommodation and the resulting pres-byopic individual is unable to focus well for near vision.Blurring, increasing awareness of photophobia, too-wateryeyes, throbbing pains in the eyeballs, burning, eyeball ten-derness, a feeling of discomfort in the eyes and sluggish reac-tion of the iris are some of the signs that a thorough eyeexamination is needed. Color blindness is discussed in thepart of this book on the physiology of eyesight.

It is the initial stage of impairment that commonly causesthe most problems for the unknowing visual inspector.Because vision impairment typically progresses slowly, indi-viduals may not be aware of a problem until it impairs perfor-mance. Any individual who needs frequent changes ofcorrective lenses, who notes diminished vision acuity, hasmild headaches, sees halos around light sources or hasimpaired dark adaptation should have an eye examination assoon as the condition is discerned. This is especially true forindividuals over the age of 40.

Effect of Observer's Attitude

A complete representation of the visual field probably isnot present in the brain at any one time. The brain must con-tain electrochemical activity representing some majoraspects of a scene but such a picture typically does not corre-spond to how the observer describes the scene. This occursbecause the observer adds experience and prejudices thatare not themselves part of the visual field. Such sensoryexperience may reflect physical reality or may not.

Sensory data entering through the eye are irretrievablytransformed by their contexts—an image on the retina is per-ceived differently if its background or context changes. Per-ceptually, the image might be a dark patch in a brightbackground that can, in turn, appear to be a white patchif displayed against a dark background. No single sensa-tion corresponds uniquely to the original retinal area ofexcitation.

The context of a viewed object can affect perception and,in addition, the intention of the viewer may also affect per-ception. The number of visible objects in a scene far exceedsthe typical description of the scene. And a great deal ofinformation is potentially available to the observer immedi-ately after viewing. If an observer has the intention of look-ing for certain aspects of a scene, only certain visualinformation enters the awareness, yet the total picture is cer-tainly imaged on the retina. if a scene or an object is vieweda second time, many new characteristics can be discerned.

This new information directly influences perception of theobject, yet such information might not be available to theviewer without a second viewing.

The selective nature of vision is apparent in many commonsituations. An individual can walk into a room full of peopleand effectively see only the face of an expected friend. Thesame individual can walk right by another friend without rec-ognition because of the unexpected nature of the encounter.Vision is strongly selective and guided almost entirely bywhat the observer wants and does not want to see. Any addi-tional details beyond the very broadest have been built up bysuccessive viewing. Both the details and the broad image areretained for as long as they are needed and then they arequickly erased.

The optical image on the retina is constantly changing andmoving as the eye moves rapidly from one point to another—the sensing rods and cones are stimulated in ways that varywidely from one moment to the next. The mental image isstationary for stationary objects regardless of the motion ofthe optical image or, for that matter, the motion of theobserver's head. It is very difficult to determine how aunique configuration of brain activity can be the result of aparticular set of sensory experiences. A unique visual con-figuration must be a many-to-one relationship requiringcomplex interpretation. If an observer does not apply experi-ence and the intellect, it is likely that a nondestructive visualtest will be inadequate.

Physiology of VisionVisual Functions

Vision comprises a number of factors, including percep-tion of light, form, color, depth and distance. Form percep-tion occurs when light from an object is focused in the eye.This visual image is affected by the lens system in almost thesame way that any inorganic lens brings rays of light to a focusand forms an image. The focus of the lens system in the eyecan be changed like that of a camera. A diaphragm (the iris)regulates the quantity of light admitted. The retina is a lightsensitive plate on which the image is formed. Adjustmentsof focus are made by changing the thickness and curvature(the focusing power) of the lens. Increasing the lens thick-ness is called accommodation. This is done by the action oftiny muscles attached to the lens.

Refractivity and Binocular Vision

In the normal eye, the length of the eyeball and the refrac-tive power of the cornea and lens are such that images ofobjects at a distance of 6 m (20 ft) or more are sharplyfocused on the retina when the muscles of accommodation


are relaxed. Errors in these relationships require correctionwith specially prepared lenses. In a farsighted individual, thesituation can be corrected with convex lenses. These bringlight from distant objects to a focus without contracting theaccommodation muscles which make the lens more convex.In the nearsighted person, light rays from distant objectscome to a focus in front of the retina. This causes blurring ofall objects located beyond a critical distance from the eye. Byuse of concave lenses, distant objects can be seen clearly bythe nearsighted individual.

Distance Judgment

Binocular vision is an important aid in accurate judgmentof distance. Distance judgment is the basis for depth percep-tion or stereoscopic vision. Stereoscopic vision depends, atleast in part, on the fact that each eye gets a slightly differentview of close objects. The right eye sees a little more of theright hand surface of the object. The left eye sees a little lessof this surface but more of the left hand surface. When theimages on the two retinas differ in this way, the object is per-ceived as three-dimensional.

Mechanism of VisionThe photographic plate used in the camera is represented

in the eye by the retina, which contains the end plates of theoptic nerve. These receptors are the rods and cones. Nerveimpulses stimulated by light arise in these structures and areconducted along the visual pathways to the occipital regionof the brain.

Photochemical Processes

The mechanism of converting light energy into nerveimpulses is a photochemical process in the retina. The so-called visual purple, a chromoprotein called rhodopsin, is thephotosensitive pigment of rod vision. It is transformed bythe action of radiant energy into a succession of products,finally yielding the protein called opsin plus the carotenoidknown as retinene.

This process occurs by the action on the visual purple of asmall number of quanta of radiant energy in the visible rangeof wavelengths. It has been shown that the peak and slope ofthe curve of scotopic (night vision) luminosity sensation arealmost identical with the absorption curve of rhodopsin.

Light Receptors

The two kinds of light receptors in the retina, the rods andthe cones, differ in shape as well as function. At the pointwhere the optic nerve enters the retina, there are no rods

and cones. This portion of the retina, called the blind spot,is insensitive to light. At the other extreme, the maximumvision acuity at high brightness levels exists only for that smallportion of the image formed on the center of the retina. Thisis the fovea centralis discussed in detail earlier. Here, thelayer of blood vessels, nerve fibers and cells above the rodsand cones is far thinner than in peripheral regions of theretina.

Daylight Vision

Daylight vision, which gives color and detail, is performedby the cones, mainly in the fovea centralis. There are at leastthree different kinds of cones, each of which is in some wayactivated by one of the three fundamental colors, as dis-cussed earlier in this section.

Color and Color VisionColor vision is one of the most interesting aspects of the

function of the human eye. Color vision occurs only in thelight-adapted eye and is dependent on the acuity of thecones. Light is the specific stimulus for the eye but the eyeis sensitive only to rays of certain wavelengths. Within thosewavelengths, the stimulus must have a certain minimumintensity. The sensation of color varies according to theintensity of the light, the wavelength of the different radia-tions and the combinations of different wavelengths. In day-light vision, yellow is the brightest color.

Color Characteristics

Every color has three characteristics: (1) tone or hue,(2) saturation or purity and (3) brightness or luminosity.

Hue is associated with a range of wavelengths in the spec-trum and is usually what an observer means when describinga color (red or blue, for instance). An estimated seven mil-lion or more colors can be discriminated but, because thetransition from one hue to the next is gradual, the demarca-tions are ill defined and to some extent a matter of opinion.For practical purposes only a few main colors are commonlydistinguished, with the following approximate wavelengths:violet, 380 to 450 nm; blue, 450 to 480 nm; blue-green, 480to 510 nm; green, 510 to 550 nm; yellow-green, 550 to 570nm; yellow, 570 to 590 nm; orange, 590 to 630 nm; and red,630 to 730 nm. Light from a limited part of the spectrum iscalled monochromatic.

A hue may also vary in brightness, according to the inten-sity of its predominant radiation.

Indigo, with wavelengths approximately from 425 to 455nm, is sometimes included between violet and blue, perhaps


because of the name Roy G. Biv, a mnemonic comprisinginitials of the colors of the rainbow.

Another color characteristic is saturation. This is a relativeor comparative characteristic and may be described as ahue's dilution with white light.

Color ChangesThe critical evaluation of color and color change is one of

the basic principles of most visual tests. Corrosion or oxida-tion of metals or deterioration of organic materials is oftenaccompanied by a change in color imperceptible to the eyeitself. For example, minute color changes on the surface ofmeat may not be detectible by the human eye but can bedetected with photoelectric devices designed for the auto-matic inspection of meat before canning.

Brightness CharacteristicsBrightness contrast is generally considered the most

important factor in seeing. The brightness of a diffuselyreflecting colored surface depends on its reflection factorand the quantity of incident light (lux or footcandles of

illumination). Excessive brightness (or brightness within thefield of view varying by more than 10:1) causes an unpleasantsensation called glare. Glare interferes with the ability ofclear vision, critical observation and judgment. Glare can beavoided by using polarized light or other polarizing devices.

Observer DifferencesThe visibility of an object is never independent of the

human observer. Human beings differ inherently in thespeed, accuracy and certainty of seeing, even though theymay possess average or normal vision. Individuals vary par-ticularly in threshold measurements and in their interpreta-tions of visual sensations. Their psychological state, tensionsand emotions influence their appraisals of the visibility ofobjects and influence their performance of visual tasks undermany conditions.

The importance of an inspector's attitude cannot be over-emphasized. Because many visual testing decisions mayinvolve marginal material, all interpretations must be impar-tial and consistent. A defined policy of test procedure andstandards should be adopted and followed faithfully.


PART 4 VISUAL WELD TESTING PERFORMANCESTANDARDS

The text below focuses on direct and remote visual weldtesting with emphasis on crack detection. Visual weld testingfor cracks is one of the most prevalent nondestructive tests.However, of all the nondestructive methods, visual testinghas the least defined performance procedures for qualifyingor quantifying minimum test performance.

Three procedures are used to verify a visual inspector'sperformance or sensitivity: (1) near vision acuity, (2) colorrecognition and (3) target detection. The validity of the pro-cedure for verifying field reliability can be improved byunderstanding the use and limitations of performance ori-ented tests.

Near Vision AcuityThe majority of recommended practices or standards

require 20/30 uncorrected or corrected vision in one eye.'Section V of AS ME's Boiler and Pressure Vessel Coderequires 20/30 near vision acuity and Section XI requires20/20. 5 While this provides a baseline for vision perfor-mance, it does not measure stereo vision or other visionproblems such as astigmatism that can significantly affectdetection reliability. Measurement of physiological visioncapability involves several tests that can have complex inter-active variables. As a screening standard, 20/20 or 20/30 nearvision acuity in both eyes is a reasonable beginning for thevision requirements of weld testing. However, near visionacuity measurement alone is not sufficient for predictingprobability of detection for fine discontinuities.

Color PerceptionAlthough color recognition is not part of typical visual test-

ing specifications, it is a part of the requirements for inspec-tor qualification. Color recognition screening is usually arequirement for nondestructive tests that are distinctly colorbased. For example, magnetic particle testing in manycases requires the ability to see red or green (fluorescent)indications.

Individuals with good vision acuity and red/green colordeficiency can often pass practical tests based only on con-

trast recognition. Color can be a significant factor for patternrecognition of color based information during weld testing.However, it is difficult to qualify or quantify visual weld test-ing performance criteria. The fundamental question of whatdegree of color deficiency disqualifies a visual weld inspectoris not well defined. The American Welding Society's Certi-

fied Welding Inspector Programs states that color vision acu-ity is desirable in some specific applications but is notconsidered essential for all inspections.

Target DetectionA critical performance standard for some visual tests is the

detection of a line to verify a system's sensitivity. This proce-dure is often called a resolution test. Detection may bedefined as the task of perceiving the absence or presence ofan object. In vision physiology and psychology,' resolution isthe ability of a vision system to discriminate between the crit-ical elements of a stimulus pattern. Detection of a single linedoes not fulfill the standard definition of resolution.

Single line detection for a direct visual examination is usu-ally performed using a 750 p.,m (30 mil) width black line onan 18 percent neutral gray flat uniform background. Someperformance criteria" require detection of a 25 p.m (1 mil)black line for remote visual tests in critical applications.

Because simple line detection is a relatively gross task, itcan be a poor performance standard, allowing detection of ahighly blurred image. This does not emulate sharpness qual-ity recognition for evaluation of weld discontinuities. A750 p,m (30 mil) black line can be reliably detected by indi-viduals classified as legally blind (20/200 corrected botheyes). The 750 p.m (30 mil) and even the smaller 25 jim(1 mil) widths should not be used as performance standardsbecause they do not determine image sharpness.

Image sharpness is critical to discontinuity recognitionand is a key feature for pattern recognition of welding discon-tinuities. Figures 4 to 6 show a 750 p.m (30 mil) line detec-tion in which detection occurs with both 20/20 and 20/200near vision acuity One means of simulating the effect of20/200 near vision acuity is to observe objects underwaterwith the naked eye (assuming 20/20 near vision acuity as abaseline).

FIGURE 4. General view of 20/20 near visual acuitycard and line card of 18 percent neutral graybackground

2

REMOTE VISUAL INSPECTIONEQUIVALENT NEAR VISUAL ACUITY CARD

:11,449-tPOW aFf .R PIM

2 6 3 5 8 1 972 45 83 61 59

3 4 6 5 1 7 219 40 42 33 61

9 7 3 1 4 6 228 79 53 46 12

6 5 4 0 4 879 56 44 37 21205 835 440 147

• 4 6 9 4 I

Sit 03 WI R ,9170 MI 741 43:9

• a •••a a

NO n••n

i64968515,90 to rneasont MOWN now Awe 00641 acuity 05199 54

ay6i45 4. Eq01961940 new 094051- 50507y i6 45706187E 777 Om minImuin 5446543s140 r45.0984545411 4, it Ono, apooftic 44,

r RAP,

AY*


Acuity VariablesSeveral variables affect vision acuity including target

movement, lighting, target angle, target knowledge and psy-chophysics. Information about these variables is helpful forquantifying visual performance standards for measuring testsystem sensitivity.

Kinetic Vision AcuityNear vision acuity examinations are performed with the

eye chart (target) in a stable position. Performing visualtesting tasks that require the object to be scanned resultsin a dynamic observer—or video camera— to target move-ment. The term kinetic vision acuity' is used for acuity

FIGURE 5. Photograph of 0.75 mm (0.033 in.) lineon 18 percent neutral gray card taken with anequivalent 20/20 near vision acuity

0,

20.(4,

3 •-4

—0•

361 w11. 70,50

0ca

b

3 '905 C0

5520 , 30

8444

FIGURE 6. Photograph of 0.75 mm (0.033 in.) lineon 18 percent neutral gray card taken with anequivalent 20/200 near vision acuity (line isdetectable but blurred)

measurements with a moving target. Studies indicate that 10to 20 percent of visual efficiency can be lost by targetmovement.'

Lighting and Target AngleNear vision acuity tests are performed under uniform

lighting and on targets that do not cast shadows. Because thetarget characters have a uniform luminance contrast for boththe figures and the background, near vision acuity tests are


not designed to measure detection of detail in rough surfacetopographies such as welds. While a visual testing specifica-tion may specify a viewing angle, near vision acuity charts aremade with the eye or video camera perpendicular to the tar-get, resulting in optimum vision acuity

Target Knowledge

Target knowledge is the key feature for detection and rec-ognition. Targets such as letters, numbers and straight linesare simple for human recognition, especially on a uniformbackground. Such targets have little transferability for thediscontinuities of interest during a visual weld test.

In fact, there are several different near vision acuity testsbased on varying targets. One of the problems with usingwell known patterns such as letters is that the individual maybe responding to visual clues and filling in a partially visiblepattern deduced from letter shape. This is known as closure.

Optometrists score and measure vision acuity based onboth the number of errors and response time. Additionally,vision acuity is a function of the observer's acuity for a giventime. For example, acuity may be diminished if the observerhas been performing strenuous detailed vision tasks.Because of these variables, near vision acuity is not a precisequantified measurement but one having the accuracyrequired to fit a high probability of eyesight correction to the20/20 standard. Variability in eyesight measurement is suchthat a 10 percent difference in measurement is possiblebased on the type of acuity test and the individual's perfor-mance for that time.

Psychophysics

Psychophysics is the interaction between vision perfor-mance and physical or psychological factors. One example isthe so-called vigilance decrement, the degradation of relia-bility based on performing visual tasks over a period of time.If not identified as a significant variable and controlled,vigilance decrement can result in diminishing visualperformance.'

Reserve Vision Acuity and VisualEfficiency

The 20/20 standard for near vision acuity is a baselinedesigned in the late 1800s as a means for standardizing eye-sight relative to the ability to read fine print and to provide ameans for prescribing corrective lenses. The standard wasnot intended to identify vision acuity relative to the detect-ability of fine lines such as cracks but measurements of nearvision acuity are transferable to the ability to detect cracks.Visual systems with a near vision acuity of 20/20 can detect

cracks with widths of 10 p.m (0.4 mil) on polished surfaces.Such systems can detect hairline weld fractures with widthsnear 25 pan (1 mil) in the toe of a weld.

The term reserve vision acuity refers to the ability of anindividual to maintain acuity under poor viewing condi-tions. u An individual with 20/20 near vision acuity observingunder degraded viewing conditions has considerable reservevision acuity compared to an individual with 20/70 nearvision acuity.

The term visual efficiency uses 20/20 near vision acuity asa baseline for 100 percent visual efficiency.' The concept isuseful for defining the reliability of a visual system based ondetection relative to visual efficiency.

Performance Standards for Visual WeldTesting

In addition to specific calibration or verification standards,the majority of nondestructive testing specifications includeuse of test objects with known discontinuities. These refer-ence standards have intentionally fabricated discontinuitiesor discontinuities from production cutouts. Reference stan-dards with known discontinuities have three disadvantages:(1) procurement of the test object, (2) validity of the testobject and (3) standardization of discontinuity sizes.

Fabrication of tight visual cracks is controllable and suchstandards can be manufactured or purchased for a reason-able cost. The simplest method for creating a tight visualcrack is to butt two highly machined plates together with asurface weld head minimally joining the faying surface. Theweld is then broken and the plates reassembled mechanicallyor by tack welding (see Figs. 7 to 9). Another method for cre-ating toe cracks is to fabricate a highly restrained weldedobject with an invisible crack that becomes visible as theplate cools. These two methods produce toe weld cracks thatare representative of the most prevalent inservice weldingdiscontinuity.

Transverse cracks can be fabricated by grinding a notchtransverse to the weld and then filling the notch with copper.When a stringer bead is run over the copper, a tight visualtransverse crack is produced. Transverse cracks can also beproduced by restraining high tensile low alloy steel such asA514 or A517 and welding with 10018, 11018 or 12018 weldelectrode. The amount of restraint and other variables, suchas the moisture content of the weld electrode flux and inade-quate preheat or postheat, will determine the size of thecracks created.

The use of the weld discontinuity reference standards hastwo significant advantages: (1) they represent actual condi-tions that cannot be accurately simulated by vision acuity eyetests or line detection and (2) reference standards are criticalfor training inspectors in pattern recognition as well asproper detection and evaluation methodology

FIGURE 8. Same plate as in Figure 8 with 0.025 mm(0.001 in.) width separation achieved by spacingwith a feeler gage

FIGURE 9. Same plate as in Figure 8 with apurposely cracked surface weld bead at the fayingsurface; can be used to show the effects of line ofsight and width opening; separation here is 0.5mm (0.02 in.) to show fabrication method


1

FIGURE 7. Two plates machined to a 4 (100 nm)rms finish and bolted together to appear as oneplate

Plastic reference standards replicated from actual cracksare sometimes used in training programs. These plastic stan-dards are convenient and transportable but they often lackthe realism essential for effective training.

Use of Visual Reference StandardsVisual testing inspectors with 20/20 or 20/30 near vision

acuity in one eye should reliably pass the 8 t.t.m (0.32 mil)detection test (based on transferring predicted acuity to

detection). Therefore, for sensitivity verification, the linetest does not provide a means for determining which visualinspectors cannot detect actual cracks.

In other nondestructive testing techniques, verificationand practical tests are designed to determine sensitivity andcan result in some personnel failures (the pass/fail rate isdependent on the testing technique and the application).For example, there is typically a greater pass rate for ultra-sonic discontinuity detection. Likewise, the pass rate for anintergranular stress corrosion crack detection program canproduce a lower pass rate than typical ultrasonic detectionmethods. A greater pass rate can be expected for genericmagnetic particle testing than for ultrasonic tests.

Therefore, a reliable method must be established for qual-ifying visual inspectors for crack detection—one solution isto use valid visual reference standards containing the typesof cracks predicted and required to be detected.

Since 1985, major oil and gas companies have used perfor-mance demonstration programs to test underwater inspec-tors for nondestructive testing qualification." Theseprograms are strongly based on practical demonstration forproficiency. The reference standards typically contain nonvi-sual magnetic particle testing indications. In 1990, oil andgas company operators placed additional emphasis on visualtesting and instituted a program using visual reference stan-dards for performance demonstration. Magnetic particleand visual testing trials were carried out concurrentlybecause the two methods are complementary for underwaterweld tests.


Before visual and magnetic particle testing, personnelwere given basic near vision acuity and color recognitionscreening tests. Near vision measurements were recordedfor each eye and for both eyes. The majority of diving per-sonnel fell into the 20/20 to 20/30 range with a small percent-age in the 20/40 to 20/50 category. In some cases, divingpersonnel with minor near vision acuity deficiencies do notwear corrective lenses (wearing bifocals in the diving helmetis somewhat tedious and uncomfortable). Contact lenses arenot recommended for diving.

The initial qualification program indicated that personnelwith near vision efficiency less than 20/20 did not performdetection of cracks as well as those near 20/20. The initialobservation was based on a small sample population but wasnoted as a potential problem. Subsequent testing of a largerpopulation indicates that individuals with less than near20/20 have significantly poorer detection ability.

There are several integrated visual testing variablesbeyond equating near vision acuity to performance, includ-ing (1) knowledge of crack pattern recognition, (2) knowl-edge of scanning techniques, (3) lighting, (4) orientation ofthe test objects, (5) test instructions, (6) feedback from thetest administrator and (7) psychophysical factors.

Knowledge of Crack PatternRecognition

The tour main weld crack categories are weld toe, trans-verse, face and heat-affected zone cracks. Most nondestruc-tive tests require the observer to focus attention onreasonably well defined targets and patterns. In visual testsfor weld discontinuities, detection is constrained by a lack ofknowledge about the patterns to be detected. Tight hairlineweld cracks are not well defined targets and can be discrete,based on their position in the weld.

If reference standards or photographic examples are notused, the inspectors' reliability of detection is determinedalmost solely by experience. For inservice weld tests, the fre-quency of fine cracks is small and does not provide a highdegree of pattern recognition based on experience. Detec-tion of relatively gross cracks with tight ends may supply theinspector with some knowledge of hairline crack recognition.

Training programs should include discontinuities thathave crack like appearances but cannot be fully evaluatedwithout supplemental nondestructive testing. This results inlowering the false positive alarm rate (there are some weldconditions that have suspect crack like appearances).Although visual testing is often considered a stand alone test,knowledge of penetrant testing or magnetic particle testingis essential.

Scanning TechniquesDetection is a function of both scanning coverage and

speed. Other nondestructive testing techniques define theseparameters by physically positioning a probe or the measure-ment material. Visual testing is primarily noncontact andcontrols for scanning and coverage are difficult to quantify.Coverage and scanning rate are determined by the type ofdiscontinuity to be detected. Fine crack detection requiresconsiderably more control than detection of gross cracks, toensure 100 percent area coverage at a reasonable speed.While the human eye and machine vision have required sen-sitivity to detect extremely fine cracks, the vision systemmust be positioned in the proper orientation to detect thediscontinuity. If a hairline crack is present in the weld toeopposite from the primary viewing angle, there is a highprobability that the crack will be missed.

Because most visual tasks do not require a high degree ofangular probing, there is a tendency for inspectors to viewthe weld from a single position, resulting in a measurable lossof test sensitivity. This problem can be minimized by speci-fying that the inspector view the weld from several differentangles. Visual testing is analogous to ultrasonic crack detec-tion, in which the probability of detection is increased byusing different angle probes and defining maximum scan-ning speeds.

Scanning coverage requires a visual mapping plan to com-pensate for the fact that the human memory is not optimallyequipped for scanning tasks. While the brain is processingthe area under test, little data can he retained about past cov-erage and no hard copy documentation is produced to showthe coverage area.

In some qualification tests, crack reference standards areusually placed with the cracks opposite the observer's initialviewing angle. If the inspector does not visually scan the testobject from more than one angle, the crack is usually missed.When the reference standard is placed so that the crack is indirect line of sight, crack detection significantly increases.

LightingLighting for visual testing has two functions: (1) providing

luminance contrast for discontinuity detection and (2) illumi-nating the object to assist in scanning guidance.

There are subtle lighting thresholds at which cracksbecome detectable and it may accurately be said that optimallighting conditions increase visual sensitivity. Some visualtesting specifications require 500 Ix (50 ftc) at the test site butlight characteristics (hue, for example) are not given. Manyspecifications refer only to light levels adequate for testinspection.


Other nondestructive testing specifications require visualdetection of physical indications, as in magnetic particle andliquid penetrant testing." Such techniques typically require1,000 to 2,000 lx (100 to 200 ftc) on the viewing area. Inaddition, these surface techniques often make use of highcontrast backgrounds to maximize indication detection. The500 lx (50 ftc) minimum does not give optimum visualdetection.

The other key feature of artificial lighting is that thelighted area provides a guidance system for the visual testingand aids the mapping required for coverage. With video sys-tems, use of side lighting can further optimize visual testingwhen component geometry creates shadows that degradevisual performance.

Simple empirical tests on the effects of lighting can beperformed using reference standards. In qualification tests,lighting is an important variable that has a measurable influ-ence on performance.

Practical Qualification RequirementsPerformance verification can be achieved using three dis-

tinct methodologies.

1. Use a completely quantified test regime in which perfor-mance is measured with no prior cues regarding testobject information (sometimes referred to as a blindtest).

2. Use a written review to provide guidelines on means tooptimize visual testing techniques and basic test objectcharacteristics without giving knowledge of the discon-tinuities.

3. Provide the test candidate with some degree of real-timeperformance feedback.

The blind test regime represents the most severe environ-ment for visual testing. For quality assurance reasons, theblind test may be required but there are drawbacks to exclu-sive use of blind testing programs. Many visual testing quali-fication programs benefit by providing test candidates withsome knowledge of required detection criteria. The real-time feedback process allows the test administrator to deter-mine if lack of detection is an eyesight acuity problem or amatter of poor technique. In some cases, when the candi-date is given the exact location of a hairline crack, the indi-vidual still cannot trace the crack. This strongly indicatesthat lack of detection is a vision acuity problem that can heremedied with corrective lenses and retesting.

If the missed crack can be traced once the candidate hasknowledge of the location, it can be possible that impropertechnique was a factor. Detection may be such that the can-didate's recognition is at a threshold level and a range of

crack sizes should be used. The larger crack widths are firstused to separate technique problems from vision acuitydeficiencies.

One unique feature of underwater testing is the presenceof an oral nasal mask in the diving helmet. This oral nasalmask is in the field of view when performing close visual tasksand can create potential problems with visual disturbanceand binocular vision. This is especially true when visual tasksmust he performed for long periods of time.

Remote Visual TestsRemote visual testing is used in hostile environments

unsafe for human intervention or in areas of inaccessibility.All of the variables that apply to direct visual testing can beapplied to remote visual testing. The main differences are:(1) some loss of depth cues caused by the two-dimensionalmedium, (2) more difficulty in scanning the test site with fullcoverage line of sight and (3) inability to easily implementsupplemental nondestructive tests.

Of these, the inability to use supplemental nondestructivetesting is the most severe constraint. This is critical becausea percentage of visual targets that appear as crack-like dis-continuities cannot be separated into nonrelevant or relevantindications without additional nondestructive testing.Although the number of suspect crack targets may be low,the inability to provide evaluation with a high confidencelevel is a significant limitation of the remote visual testingmethod. The potential for false positive alarms must be criti-cally evaluated before effecting a remote visual testing planover a direct visual testing plan.

The line detection procedure is often used to qualify videosystems. This is a poor test because the line can be detectedat acuity levels much lower than the optimal 20/20. Imagesize should be consistent with magnification allowed only toaid in acuity. Magnification levels should be set so that fea-tures necessary for recognition are still identifiable.

Other tests cited in the use of video quantification are avariety of resolution tests.' 5 Video cameras are often statedto be high resolution. The term high resolution can be mis-leading if thought to be based on actual image quality usinga specific system. Resolution is a function of the completesystem's ability to resolve minimum line pairs. As a visualstandard, line pair resolution can be quantified with greaterprecision than simple vision acuity tests. However, line pairresolution is a relatively cumbersome concept. It must bedetermined if 400 horizontal television lines are adequate fordetection and with what ease the visual inspector can identifyresolution with crack detection.

The few visual standards that reference remote visual test-ing state that remote visual tests must be equivalent to directvisual requirements. This implies that the video system


should have a near vision acuity equivalent to that of directvisual testing. In 1990, the term equivalent 20/20 near visionacuity was introduced to resolve this inconsistency forremote video testing systems. 16 Most near vision acuity cardsare designed to be read at a defined distance. Using theequivalent 20/20 near vision acuity criterion, the observerreads the near vision acuity 20/20 characters on a video mon-itor. Camera distance from the object is not critical but fieldof view and depth of field are set according to the needs ofthe test. For example, a specification may require equivalent20/20 near vision acuity for a 100 cm 2 (16 in. 2 ) viewing areawith a depth of field of 25 mm (1 in.). If the depth of field isextremely shallow, constant focusing is required and this pro-duces operator fatigue. Medium wide angle lenses with highf-stops (achievable with high intensity lights) usually produceequivalent 20/20 near vision acuity. The 20/20 standard isbetter for remote than 20/30 because of the inherent loss ofvisual sensitivity caused by some lost depth cues. In fact,20/10 is a preferable acuity but magnification should not beso great as to remove key pattern recognition features.

As stated, 20/20 near vision acuity is a good guideline forhairline crack detectability. However, use of actual crackedtest objects is preferred for performance testing and training.In remote visual testing, both 20/20 near vision acuity char-acters and reference standards with specified discontinuitysizes can often be mounted on the robotic system for perfor-mance checks. On steel structures, where remote video isperformed using manipulators, 20/20 near vision acuity char-acters can be magnetically positioned at test sites to verifyvisual performance.

Vision HardwareThe link between the eyes and brain poses unique prob-

lems of visual perception and pattern recognition. While theeyes can be equated to hardware, the brain must be consid-ered the processor for tasks which require specific patternrecognition, such as hairline crack detection. Using thisconcept, high vision acuity alone does not guarantee detec-tion of hairline cracks. The term neural acuity is often usedto define the ability of the eye and brain together to discrimi-nate patterns from the background. Discrimination ability isstrongly driven by knowledge of the target pattern, by thescanning technique and by the figure/ground relationship ofthe discontinuity. The figure/ground relationship can bereferred to as having a level of visual background noise.

In detection of radar targets, clutter is often used to definea background containing a high degree of spurious (nonrele-vant) targets. In a given population of inspectors with 20/20near vision acuity and no other vision problems, there is a

measurable difference of hairline crack detection based onneural vision performance.

Other Factors Affecting PerceptionMost nondestructive testing techniques require specific

learning regimes while visual testing is wrongly assumed tobe an innate human process. There are several factors thatmake individuals good observers and these factors are some-times counterintuitive. In one case, for example, the bestobserver had poor near vision acuity but was able to findobjects more rapidly than observers with good eyesight. Aplausible explanation lies in the fact that the test objects weremore discernible from the background when slightlyblurred. This is not the case for detection of hairline cracksbut it does confirm the complexity of using the human visionsystem.

Visual inspectors are highly encouraged to discuss visionconsiderations and problems with an optometrist. However,it should be recognized that optometrists are trained toexamine eyes and not welded test objects. Interesting resultsare sometimes obtained when optometrists are given nearvision acuity tests using unfamiliar crack reference stan-dards. In one experiment, an optometrist with 20/20 nearvision acuity was unable to detect a hairline crack and unableto trace the crack after given its location. The crack refer-ence standard was validated as detectable when the optome-trist's six year old daughter was able to detect the crackwithout prior knowledge of its location.

Recommendations for Visual WeldTesting

There is a need to improve training and standards forvisual weld testing. Personnel qualification and certificationin visual testing were first formalized by ASNT in 1988 withcompletion of a training outline but there is no ASNT certi-fication as such for visual inspectors. The American WeldingSociety's Certified Welding Inspector Certification is a broadprogram of which visual testing for cracks is a small part.

The majority of visual testing specifications are typicallyshort (one or two pages) and provide minimum performancecriteria that can be applied to practical conditions. The fol-lowing recommendations have proved to be effective.

1. Adopt a policy of using valid crack reference standardsfor training and verification of a system's performancesensitivity.

2. Use a 1,070 lx (100 ftc) minimum for lighting on the testsite.


3. Require scanning based on predicted line of sight for thediscontinuity

4. Require yearly eye tests with a minimum of 20/30 nearvision acuity in both eyes. The eye tests should be per-formed by an optometrist.

5. Remove the detection of a 8µm (0.32 mil) wide line as avisual testing performance standard.

6. Develop courses and specifications especially designedfor weld testing.

ConclusionImproved training and specifications for visual weld test-

ing which address vision acuity, lighting and line of sight

requirements will measurably increase visual testing sensitiv-ity. More attention must be placed on detection methodol-ogy— present training tends to focus on categorizing discon-tinuity types. Because the training issues are academicallyfundamental, visual weld testing methods will be easy toimplement.

Enhancement and miniaturization of video hardware willallow equivalent 20/20 near vision acuity testing usingremote techniques.

Career testing personnel should have yearly check-ups byoptometrists to ensure there are no vision problems whichcan affect visual testing. Without yearly testing, vision degra-dation may go unnoticed and may be more difficult tocorrect.


REFERENCES

1. Kaufman, J.E. and J.F. Christensen. IES Lighting Hand-book, fifth edition. New York, NY Illuminating Engi-neering Society (1972).

2. Visual Examination Technology. Charlotte, NC: EPRINDE Center (1982).

3. American Society for Nondestructive Testing Recom-mended Practice, No. SNT-TC-1A. Columbus, OH:American Society for NondestructiveTesting ( June 1980).

4. Boiler and Pressure Vessel Code: Visual Examination.Section V, Article 9, T-923. New York, NY: AmericanSociety of Mechanical Engineers (1977).

5. Boiler and Pressure Vessel Code: Visual Examination.Section XI, IWA-2211. New York, NY: American Societyof Mechanical Engineers (1980).

6. Guide to AWS Welding'Inspector and Qualification andCertification. Miami, FL: American Welding Society(1991).

7. Haber, Ronald and Maurice Hershenson. The Psychol-ogy of Visual Perception. New York, NY: Holt, Reinhartand Winston (1973): p 113.

8. Vickerman, David. Private communication. Palo Alto,CA: General Electric Company ( January 1989).

9. Borish, Ivan. Clinical Refraction. Chicago, IL: The Pro-fessional Press (1985): p 362.

10. Magnetic Particle Inspection Using Remote OperatedVehicles. Merritt Island, FL: Sea Test Services (1990): p8-88.Borish, Ivan. Clinical Refraction. Chicago, IL: The Pro-fessional Press (1985): p 392.

12. Borish, Ivan. Clinical Refraction. Chicago, IL: The Pro-fessional Press (1985): p 378.

13. Recommended Practice for Underwater Magnetic Parti-cle Weld Inspection, Including Qualification and Certi-fication of Inspector Divers. Dallas, TX: AmericanPetroleum Institute (1993).

14. Requirements for Nondestructive Testing Methods.M IL-STD-271F( S H). Washington, DC: Department ofDefense ( June 1986).

15. Electronic Industries Standard, Electrical PerformanceStandards - Monochrome Television Studio Facilities.RS-170 (Revision TR 132. Washington, DC: ElectronicIndustries Association (November 1957).

16. Nondestructive Evaluation of Component Interiors:Technology Assessment. NP-6832. Palo Alto, CA: Elec-tric Power Research Institute ( June 1990).

11.

SECTION 4

BASIC AIDS AND ACCESSORIES FORVISUAL TESTINGDavid Casasent, Carnegie Mellon University, Pittsburgh, Pennsylvania (Part 4)Yen Fwu Cheu, General Motors Corporation, Detroit, Michigan (Part 4)David Clark, Global Holonetics Corporation, Fairfield, Iowa (Part 4)Eli Kimmel, Tempil Division, Big Three Industries, Incorporated, South Plainfield, New Jersey (Part 6)Stephen Meiley, Champion International, West Nyack, New York (Part 5)Donald Parrish, Southern Company Services, Birmingham, Alabama (Part 7)Michael Urzendowski, DNV Industry, Incorporated, Houston, Texas (Part 5)


PART 1 BASIC VISUAL AIDS

The human eye is an important component for performingvisual nondestructive tests. However, there are situationswhere the eye is not sensitive enough or cannot access thetest site. In these cases mechanical and optical devices canbe used to supplement the eye to achieve a complete visualtest.

Visual tests comprise five basic elements: the inspector,the test object, an optical instrument, illumination and arecording method. Each of these elements interacts with theothers and affects the test results.

Training and vision acuity are the two most important fac-tors affecting the visual inspector. According to the Ameri-can Society of Mechanical Engineers' Boiler and PressureVessel Code, Section XI, visual inspectors must be qualifiedthrough formal training programs for certification to ensurecompetency.

Levels of vision acuity are determined by eye examination.Approximately 50 percent of Americans over the age oftwenty need corrective eyeglasses. In early stages of eyesightdeficiency, many people are unaware of their condition—some simply do not want to wear glasses.

It is important that borescopes be designed to allow diop-ter adjustments on the eyepiece. Frequently, wearing glassesis an inconvenience when using a borescope it is difficult toplace the eye at the ideal distance from the eyepiece and theview is distorted by external glare and reflections. Rubbereyeshields on borescopes are designed to shut out externallight but are not as effective when glasses are worn. Forthese reasons, it is critical that the inspector be able to adjustthe instrument without wearing glasses to compensate forvariations in vision acuity.

Effects of the Test ObjectThe test object determines the specifications for (1) the

instrument used during the visual test and (2) the requiredillumination. Objective distance, object size, discontinuitysize, reflectivity, entry port size, object depth and directionof view are all critical aspects of the test object that affect thevisual test.

Objective distance (see Fig. 1) is important in determiningthe illumination source, as well as the required objectivefocal distance for the maximum power and magnification.

Object size, combined with distance, determines whatlens angle or field of view is required to observe an entire testsurface (see Fig. 2).

Discontinuity size determines the magnification and reso-lution required for visual testing. For example, greater reso-lution is required to detect hairline cracks than to detectundercut (see Fig. 3).

Reflectivity is another factor affecting illumination. Darksurfaces such as those coated with carbon deposits requirehigher levels of illumination than light surfaces do (seeFig. 4).

FIGURE 1. Objective distance (arrows, for directand side viewing borescopes

DIRECT VIEW BORESCOPE kla;

FROM ELECTRIC POWER RESEARCH INSTITUTE. REPRINTED WITHPERMISSION.

FIGURE 2. Arrows indicate portion of object failingwithin the field of view for side viewing borescope

DIRECT VIEW BORESCOPE (o)

ENTRY PORT


FIGURE 5. Entry port size (arrows) limits the sizeof the borescope

DIRECT VIEWING1f 11 BORESCOPE

ENTRY PORT


FIGURE 3. Discontinuity size affects resolutionlimits and magnification requirements

D SCONTINUITY

DIRECT VIEWINGBORESCOPE

ENTRY PORT


FIGURE 4. Reflectivity helps determine levels ofillumination

ENTRY PORT LIGHT SURFACE

DARK SURFACE

DIRECT VIEWINGBORESCOPE

(IC)


BASIC AIDS AND ACCESSORIES FOR VISUAL TESTING / 75

Entry port size determines the maximum diameter of theinstrument that can he used for the visual test (see Fig. 5).

Object depth affects focusing. If portions of the object arein different planes, then the borescope must have sufficient

focus adjustment or depth of field to visualize these differentplanes sharply (see Fig. 6).

Direction of view determines positioning of theborescope, especially with rigid borescopes. Viewing direc-tion also contributes to the required length of the borescope.

Some of the factors affecting visual tests with borescopesare in conflict and compromise is often needed. For exam-ple, a wide field of view reduces magnification but hasgreater depth of field (see Fig. 7). A narrow field of view pro-duces higher magnification but results in shallow depth offield. Interaction of these effects must be considered indetermining the optimum setup for detection and evaluationof discontinuities in the test object.

FIGURE 6. Object depth (arrows) is a critical factoraffecting focus


FIGURE 7. Effects of viewing angle on other testparameters: (a) narrow angle with highmagnification and shorter depth of field and(b) wide angle with low magnification andgreater depth of field

(a)

lb)



PART 2 MAGNIFIERS

Range of CharacteristicsMagnification as an aid to vision ranges in magnifying

power from 1.5 x to 2,000 x . Field coverage of conventionalmagnifiers ranges from 90 mm (3.5 in.) down to 0.15 mm(0.006 in.) wide. Resolving powers range from 0.05 mm(0.002 in.) to 0.2 p,m (0.008 mil). Powers of magnificationrefer to enlargement in one dimension only. A two-dimen-sional image magnified x 2, for example, doubles in widthand in height though its area quadruples.

The microscope is a typical magnifier. In its simplest form,it is a single biconvex lens in a housing adjustable for focus.Many forms of illumination are available, including brightfield, dark field, oblique, polarized, phase contrast andinterference.

Conventional Magnifiers and Readers

The major considerations for choosing a magnifier are:(1) power or magnification, (2) working distance, (3) field ofview, (4) chromatic correction and (5) binocular or monocu-lar vision.

These magnifier attributes are interrelated. A high powermagnifier, for example, has a short working distance, a smallfield of view and cannot he used for binocular observation. Alow power magnifier, such as a rectangular reader lens, has along working distance, a large field of view and can he usedfor binocular vision. To attain chromatic correction (to elimi-nate color fringing), the high power lens must be complex. Ittypically contains a cemented doublet or triplet of differentoptical glasses. By comparison, the low power reader lens issufficiently achromatic as a simple lens.

Table 1 shows the characteristics of a few typical magnifiers.These values are approximations because eye accommodation

can cause each of the values to vary. Except for the readerlens, all magnifiers are used with the eye fairly close to themagnifier, giving the largest field of view. The reader lens isused binocularly and is normally held some distance awayfrom the eyes.

Because of its large diameter, the 3.5 x doublet magnifierhas as large a field as the 2 x loupe. The double convex lensof the doublet magnifier with its central iris has a compara-tively small field. The triplet is a three-element design hav-ing excellent optical correction for field coverage andreduction of color fringing. Its resolving power is the limitof detection for fine structures. In comparison, the doubletmagnifier can barely differentiate two points 0.025 mm(0.001 in.) apart.

There are many variations of these characteristics. Com-mercial magnifiers can be as high as 30 x in power and thereare many special mountings for particular applications.

Surface Comparators

The surface comparator is a magnifier that provides ameans for comparing a test surface against a standard surfacefinish. The observer views the two surfaces side by side, asshown in Fig. 8. The surface comparator uses a small batterypowered light source, a semitransparent beam divider and a10 x triplet.

The light is divided between the reference surface and thestandard surface. Flat and shiny surfaces reflect the filamentimage directly into the pupil of the eye so that these partslook bright. Sloping or rough surfaces reflect the light awayfrom the pupil and such areas appear dark. This form of illu-mination sharply delineates surface pattern characteristics.The resolving power is about 7.5 p.m (0.3 mil). The field ofview is about 1 mm (0.4 in.) diameter.

TABLE 1. Characteristics of typical magnifiers

Working ResolvingField of View Distance Powermillimeters millimeters micrometers

Magnifier Type (inches) Power (inches) (mils)

Reader lens 90 x 40 (3.5 x 1.5) . 5 x 100 (4) 50 )2)Eyeglass loupe 60 (2.375) 2x 90 (3.5) 40 (1.5)Doublet magnifier 60 (2.375) 3.5 x 75 (3) 25 (1)Coddington magnifier 19 (0.75) 7x 25 (I) 10 (0.4)Triplet magnifier 22 (0.875) 10 x 20 (0.75) 7.5 (0.3)

FIGURE 8. The surface comparator: (a) two surfacesmagnified for comparison and (131 test setup

COMPARJSONSTANDARD

(b)

TESTSURFACE

10x MAGNIFIER

1,, BEAM DIVIDERii

LIGHTSOURCE

FROM BAUSCH AND LOMB OPTICAL COMPANY. REPRINTED WITHPERMISSION.

FIGURE 9. Measuring magnifier in transparentsleeve mount

FROM BAUSCH AND LOMB OPTICAL COMPANY. REPRINTED WITHPERMISSION.

a pencil. Some illuminated magnifiers can be obtained ineither a battery powered model or equipped for 115 V lineoperation. Such triplet magnifiers give about a 50 mm (2 in.)field of view. Resolving power is about 1.5 (0.06 mil).

BASIC AIDS AND ACCESSORIES FOR VISUAL TESTING 177

Measuring Magnifier

A measuring magnifier incorporates a measuring scale thatis positioned against the test object to measure tiny details onits flat surfaces (see Fig. 9). A transparent housing permitslight to fall on the measured surface. Scales are available formeasurements in inches, millimeters and other units (seeFig. 10). The magnifier uses a 7 x triplet lens. The resolvingpower is about 1 pLm (0.04 mil). The diameter of the field ofview is about 25 mm (1 in.).

Illuminated Magnifiers

Illuminated magnifiers range from large circular readerlenses, equipped with fluorescent lighting and an adjustablestand, to a small battery powered 10 x magnifier shaped like

Low Power MicroscopesWhen magnifications above 10 x are required, the short

working distance of the magnifier becomes a problem and alow power compound microscope is preferred. Two suchmagnifiers are described below. Their resolving powers areabout 7.5 p.m (0.3 mil).

Wide Field Tubes

The simplest form of compound microscope is a wide fieldtube, comprising an objective lens mounted in one end of atube and an eyepiece in the other. This design is typicallysupplied either in a tripod sleeve mount or in a simplifiedmicroscope stand. Focusing is accomplished by a frictionslide fit in a sleeve. The 10 x wide field tube covers a fieldof 25 mm (1 in.) and has a working distance (the clearance

FIGURE 11. Wide field stereoscopic microscope

EYEPIECES

ACUITY BALANCING ADJUSTMENT

0 'PRISM HOUSINGS

ti EYE SPACING ADJUSTMENT GEAREY

FOR ILLUMINATOR

. 5' ..- .41,--- liptATTACHMENTWITH LOCKING SCREW

1 ........ I .1:411010P NOSEPIECE

1 4 NorA 11114

.IIIIIi,..‘,.

HOLD 07

cups

GLASS STAGE

.-- - 7 6 - '-'-'4 —. —A I "I- "

11:

REMOVABLEFRONT STRIP

C111;k

CONTRAST PLATE

ATTACHMENT HOLEILLUMINATORMIRROR OR

SUBSTAGE MIRROR

HAND REST

ATTACHMENT SCREW

OBJECTIVES

FOCUSINGKNOB

FOCUSINGMECHANISMS

THUMB SCREW(FOR DETACHINGSUBSTAGE BASEI

SUBSTAGE BASE

ARM


FIGURE 10. Typical measuring scales and reticules(in inches) for the measuring magnifier

between the objective and test object) of about 80 mm(3.25 in.). The 40 x version has a field of about 6 mm(0.25 in.) and a working distance of about 40 mm (1.625 in.).

The image from such a simple microscope is inverted andreversed and is not convenient for hand manipulation of thetest object during observation. Wide field tubes are fre-quently equipped with eyepiece scales to permit measure-ments in the test object plane.

Wide Field Macroscope

The wide field macroscope is similar to a wide field tube,with the same magnification range (10 x to 40 x) and thesame mounting and focusing devices. Unlike the wide fieldtube, the macroscope produces an image that is upright andnot reversed, so that manipulation of the test object can beconveniently done during observation. The prism systemthat corrects the image also provides an inclined observationtube for more convenient prolonged viewing. The mac-roscope is often supplied with measuring scales for sizedeterminations.

Medium Power SystemsTypical medium_power magnifiers range from 20 x to

100 x in a variety of designs.

Wide Field Stereoscopic Microscopes

As can be seen in Fig. 11, the wide field stereoscopicmicroscope is very complex. It is basically two erect imagemicroscopes, one for each eye, comprising two objectives,two erecting prisms, two inclination prisms and two eye-pieces. Furthermore, as shown in the figure, the stereo-scopic microscope is usually supplied with several pairs ofobjectives in a nosepiece so that the power can be changedrapidly. It may also be provided with a glass stage and a sub-stage mirror for transmitted illumination.

The power range of the stereomicroscope is typically 7 xto 150 x , although its usefulness beyond 60 x is limited.The resolving power is about 5 p.m (0.2 mil). Field coverageis approximately inverse to the power: at 10 x field coverageis about 25 mm (1 in.). The instrument provides binocularvision, which makes possible its prolonged use for visual test-ing. Like the macroscopes, manual manipulation duringobservation is practical. The stereoscopic microscope pro-vides a true view of depth, so that test objects may beinspected in three dimensions.

There are many variations in the construction of the ste-reoscopic microscope. They are sometimes built on standshaving long universal joint arms, permitting vertical as wellas lateral, horizontal and angular movements, for scanningextended regions of the test object. A single pair or severalpaired objectives may he supplied and stereoscopic


microscopes may be inclined or vertical. Various forms ofillumination are available for top or bottom lighting,depending on the test object. Measuring scales for the eye-pieces are available for direct measurements of microscopicobjects without touching the objects.

Shop Microscope

The shop microscope is similar to a wide field tube. It is asimple tube with an objective near one end and an eyepieceat the other. It has a power of 40 x and contains a built-inlight source that may be operated from a battery or 115 V linecurrent. The shop microscope contains a scale permittingdirect measurement on the object plane to 0.025 mm (0.001in.) or estimates to 6 (0.25 mil), over a scale length of4 mm (0.15 in.). The field of view is 5 mm (0.22 in.) and theresolving power is about 3.3 p.m (0.13 mil). The instrumentis extremely lightweight, only 500 g (18 oz) with dry cells.

Applications of the shop microscope include on-site testsof plated, painted or polished surfaces; detection of cracks,blowholes and other discontinuities; and measurement ofsmall holes in heading dies, 'gages and other machined com-ponents. It also provides a quick method for checking wearin mechanical components. Welding of machine tool frames,piping, structural members, pressure vessels, jigs and fix-tures, can be quickly inspected.

In finishing and electroplating operations, surface testswith the shop microscope can detect cracks, blister, irregulardeposits, pitting and poor quality buffing or polishing. It canreveal slag inclusions and poor surfacing of base metalsbefore plating. On painted surfaces it permits quick andaccurate evaluation of quality, uniformity and pigment distri-bution. In the graphic arts, it is used to check halftones forsize, shape and distribution of dots.

Textile mills use shop microscopes for identification offiber textures, distribution of coloring matter and test ofweave, twist and other general characteristics. Fabric fin-ishes, markings, lusters and dye transfers can be inspectedfor penetration and quality. In the paper industry, the shopmicroscope is used to check fiber uniformity, evenness ofcoating and wear of Fourdrinier wires.

Brinell Microscope

The Brinell microscope is similar to the shop microscope.It is specifically designed for measuring the diameter of animpression made by the ball of a Brinell hardness testingmachine. Its magnification is 20 x the field of view is 8 mm(0.32 in.) and its resolving power is 3.5 p.m (0.14 mil). Thescale is calibrated to read (in tenths of millimeters) the actualsize of the impression over a range of 6 mm.

Focusing is accomplished by rotating the eyepiece in itsspiral mount. Adequate illumination of the Brinell depression,

regardless of the color of the test object, is ensured by anannular mirror in the base of the microscope. The mirrorreflects light on the viewing area and the outline of the Bri-nell impression stands out in contrast. Three types of illumi-nation are available: integral battery in a side tube; 0.3 A,3.8 V, with 115 V alternating current transformer; and day-light or ordinary room illumination.

High Power SystemsHigh power optical systems are used in laboratory, metal-

lurgical, metallographic, polarizing, interference and phasecontrast microscopes. The power of such systems rangesfrom 100 x to 2,000 x

Laboratory Microscope

The conventional compound microscope is often called alaboratory microscope. Inclined binocular eyepieces pro-vide ease of vision over prolonged periods of use. Complex-ity of design for this type of microscope ranges from a simplestraight monocular model for student use to elaborate sys-tems for combined visual and photomicrographic use. Agreat range of magnification, resolution and field coverage isavailable, depending on the objective design (see Table 2).The field coverage, magnification and resolving power givenfor the laboratory microscope may be roughly applied toother types of high power microscopes.

The laboratory microscope is designed principally fortransmitted light, so that it is largely useful on transparent orsemitransparent materials. It is normally supplied withmeans for illuminating the test object under controlled con-ditions to provide the optimum balance between contrastand resolution. Among many available accessories are grad-uated mechanical stages, eyepiece and stage micrometerscales, filar micrometer eyepieces, comparison eyepieces forviewing two objects under separate microscopes, crosslineeyepieces and various cross-ruled slides for particlecounting.

Metallurgical Microscope

The metallurgical microscope is similar to a laboratorymicroscope with the addition of top or vertical illuminationto permit viewing of opaque materials. The vertical illumina-tor, located directly above the objective, is a semireflecting,thin, transparent plate. It directs light down through theobjective onto the test object. The microscope is normallyequipped with a built-in light source and has field and aper-ture iris controls in the illuminating arm. Because thickpreparations are common in opaque test objects, the stagemay be focused. This also permits the use of an intense

FIGURE 12. Arrangement of elements in a phasecontrast microscope

IMAGE PLANE

r - REAR FOCA

PLANE OF OBJECTrVE,

OBJECTIVE

UNDIFFRACTED

PHASE

I

ORDER

„

SHIFTINGELEMENT

DIFFRACTED ORDERS

TEST OBJECTPLANE

OCONDENSER

ANNULARDIAPHRAGM

FROM LIGHT SOURCE

BO / VISUAL AND OPTICAL TESTING

TABLE 2. Ranges of magnification, resolution and field coverage based on objective design

Resolving Approximate RealObjective Power Field Diameter(Numerical micrometers millimeters ApproximateAperture] !mills)

(Inches) Useful Power Range

3.5 x (0.09 3 (0.12) 4.3 (0.17) 20 to 50 x10.0 x (0 09 I (0.044) 1.5 (0.06) 50 to 100x21.0 x (0.50 0.6 (0.022) 0.75 (0.029) 100 to 250 x43.0 x (0.65 0.4 (0.017) 0.35 (0.014) 250 to 750 x97.0x (1.25 0.2 (0 009) 0.15 (0.006) 750 to 1,500x90.0 x (1.40 0.2 (0.008) 0.15 (0.006) 1,000 to 2,000 x

external light source, so that focusing can be carried outwithout upsetting the illumination centering.

Although this microscope finds its principal applications inmetallurgy, it can he used on almost any opaque materialhaving a reasonably high reflectivity. When test objects aredark by nature (dark plastics, paints, minerals) or have exces-sive light scattering (fabrics, paper, wood, or biological speci-mens), a form of incident dark field illumination is superiorto regular vertical illumination.

Metallographic MicroscopeWhen a camera is built into a metallurgical microscope, it

is called a metallographic microscope or a metallograph. Ingeneral, the increase in design complexity for a typical metal-lograph goes far beyond the simple addition of a camera.Most metallographic microscopes also have the followingfeatures.

1. They are built on a stand with concealed shockabsorbers.

2. They use an intense light source, often an automaticcarbon arc.

3. They use an inverted stand so that the test object neednot be plane parallel (the test object is face down on thestage).

4. They have viewing screens for prolonged visual taskssuch as dirt count or grain size measurements.

5. They have bright field, dark field and polarized lightillumination for diverse applications.

Polarizing MicroscopeThe addition of two polarizing elements and a circular

stage converts a laboratory microscope into an elementarypolarizing microscope. A polarizing element is a device thatrestricts light vibration to a single plane. This form of lightis useful for studying most materials with directional opticalproperties, including fibers, crystals, sheet plastic and mate-rials under strain. As such materials are rotated between

crossed polarizers on the microscope stage, they changecolor and intensity in a way that is related to their directionalproperties.

The polarizing microscope normally has other added fea-tures, beyond the polarizing elements and circular stage.Much work, for example, requires study of crystal propertiesor minerals in three dimensions. The simplest of these


accessories is the Bertrand lens, which focuses an image ofthe objective aperture in the eyepiece. In the aperture is achart of crystal properties in many directions. For morequantitative work, a universal stage is used, on which thecrystal can be rotated around one of five axes through its cen-ter. The amount of rotation is then measured.

Interference Microscope

The interference microscope is a tool using the wave-length of light as a unit of measure for surface contour andother characteristics. In one form of interference micro-scope, the stage is inverted and the test object is placed facedownward. The image appears as a contour map, with a sep-aration of one half-wave or about 0.25 (0.01 mil) betweencontour lines. Extremely precise measurements can bemade with such equipment.

Applications of the interference microscope include themeasurement, testing and control of very fine finishes,including highly polished or glossy finished surfaces, wherethe degree of surface roughness is within a few wavelengthsof light. With coarser surfaces, the contour lines are closetogether and interpretation is difficult. An advantage of theinterference microscope is that the test object is not movedmanually during inspection.

A considerably less elaborate device called an interferenceobjective is also available as an accessory to the metallurgicalmicroscope. This objective has a small, metallized glassmounted in contact with the test object and adjustable for tiltto control fringe spacing. The disadvantage of the interfer-ence objective is that the test surface must be moved manuallyduring inspection. Otherwise, its test results are virtually thesame as those from an interference microscope.

Phase Contrast MicroscopeCompletely transparent materials with refractive index

discontinuities can be only faintly seen in a normal micro-scope. Such index discontinuities are readily visible in aphase contrast microscope. Figure 12 shows the two addi-tional optical elements needed to convert a normal micro-scope to a phase contrast microscope. An annular diaphragmlocated below the condenser is imaged into an annular phaseshifting element in the objective. The combined effect of thediffracted and undiffracted light transmitted by this phaseshifting element produces contrast in a completely transpar-ent object.

The phase contrast microscope is limited to uses withtransparent materials having very small index discontinuities.If the index discontinuities are gross, a normal microscope isused for visual inspection. Extensive work with living tissuesand cells has been done with phase contrast devices.

FIGURE 13. Internal reflection of light in an opticfiber can be used to move the light path in a curve

FIGURE 14. Light paths in fiber bundles:(a) uncoated fibers allow light to travel laterallythrough the bundle and (b) coated fibers restrictthe light's path to its original fiber

(a)

(b)COATING CORE


FIGURE 15. Optical fiber bundle used as an imageguide

FIBER BUNDLE

EYEPIECE

OBJECTIVE


PART 3BORESCOPES

Fiber Optic BorescopesThe industrial fiber optic borescope is a flexible, layered

sheath protecting two fiber optic bundles, each comprisingthousands of glass fibers. One bundle serves as the imageguide and the other bundle helps illuminate the test object.

Light travels only in straight lines but optical glass fibersbend light by internal reflection and so can carry light aroundcorners (see Fig. 13). Such fibers are 9 to 30 (0.4 to1.2 mil) in diameter or roughly one-tenth the thickness of ahuman hair.

A single fiber transmits very little light, but thousands offibers may be bundled for transmission of light and images.To prevent the light from diffusing, each fiber consists of acentral core of high quality optical glass coated with a thinlayer of another glass with a different refractive index(Fig. 14). This cladding acts as a mirror—all light enteringthe end of the fiber is reflected internally as it travels(Fig. 13) and cannot escape by passing through the sides toan adjacent fiber in the bundle.

Although the light is effectively trapped within each fiber,not all of it emerges from the opposite end. Some of the lightis absorbed by the fiber itself and the amount of absorptiondepends on the length of the fiber and its optical quality. Forexample, plastic fiber can transmit light and is less expensiveto produce than optical glass but plastic is less efficient in itstransmission and unsuitable for use in fiber optic borescopes.

Fiber Image Guides

The fiber bundle used as an image guide (see Fig. 15) car-ries the image formed by the objective lens at the distal endor tip of the borescope hack to the eyepiece. The image

guide must be a coherent bundle: the individual fibers mustbe precisely aligned so that they are in identical relative posi-tions at their terminations.

Image guide fibers range from 9 to 17 Rrn (0.35 to0.67 mil) in diameter. Their size is one of the factors affect-ing resolution, although the preciseness of alignment is farmore important.



FIGURE 16. Diagram of a typical fiber opticborescope

EYEPIECE LENS FOCUSING RING

IMAGE OBJECTIVEGUIDE LENS

DIOPTER RING LIGHTGUIDE PROTECTIVE

SHEATH LIGHT GUIDE EXITLIGHT SOURCE

PROJECTION LAMP

FIGURE 17. Typical lens system in a rigid borescope


FIGURE 18. Borescope images for a variety ofdistances with fixed focus (see Fig. 19): (a) at 75mm (3 in.), (b) at 200 mm (8 in.) and (c) at 300 mm(12 in.)

(a)

(b)

(c)


Note that a real image is formed on both highly polishedfaces of the image guide. Therefore, to focus a fiber opticborescope for different distances, the objective lens at the tipmust be moved in or out, usually by remote control at theeyepiece section. A separate diopter adjustment at the eye-piece is necessary to compensate for differences in eyesight.

Fiber Light GuidesAnother fiber bundle carries light from the an external

high intensity source to illuminate the test object. This iscalled the light guide bundle and is noncoherent (seeFig. 16). These fibers are about 30 u,m (1.2 mil) in diameterand the size of the bundle is determined by the diameter ofthe scope.

Fiber optic borescopes usually have a controllable bend-ing section near the tip so that the inspector can direct theborescope during testing and can scan an area inside the testobject. Fiber optic borescopes are made in a variety of diam-eters, some as small as 3.7 mm (0.15 in.), in lengths up to10 m (30 ft), and with a choice of viewing directions at thetip.

Rigid BorescopesThe rigid borescope (see Fig. 17) was invented to inspect

the bore of rifles and cannons. It was a thin telescope with asmall lamp at the top for illumination. Most rigid borescopesnow use a fiber optic light guide system as an illuminationsource.

The image is brought to the eyepiece by an optical trainconsisting of an objective lens, sometimes a prism, relaylenses and an eyepiece lens. The image is not a real imagebut an aerial image: it is formed in the air between the lenses.This means that it is possible to both provide diopter

correction for the observer and to control the objective focuswith a single adjustment to the focusing ring at the eyepiece.

Focusing a Rigid BorescopeThe focus control in a rigid borescope greatly expands the

depth of field over nonfocusing or fixed focus designs. At thesame time, focusing can help compensate for the wide varia-tions in eyesight among inspectors.

Figures 18 and 19 emphasize the importance of focusadjustment for expanding the depth of field. Figure 18 wastaken at a variety of distances with fixed focus. Figure 19 was



FIGURE 19. Borescope images with variable focus(see Fig. 18): (a) 75 mm (3 in.), (b) 200 mm (8 in.)and (c) 300 mm (12 in.)

(a)

(b)

( C )


FIGURE 20. Borescope direction of view: (a) direct,(b) side, (c) forward oblique and (di retrospective

(a) VIEW

VIEW

lb(LIGHT

(diLIGHT


LIGHT


taken at the same distances as in Fig. 18 but with a variablefocus, producing much sharper images.

Need for Specifications

Because rigid borescopes lack flexibility and the ability toscan areas, specifications regarding length, direction of viewand field of view become more critical for achieving a validvisual test. For example, the direction of view should alwaysbe specified in degrees rather than in letters or words suchas north, up, forward, or left. Tolerances should also bespecified.

Some manufacturers consider the eyepiece to be zerodegrees and therefore a direct view rigid borescope(Fig. 20a) is 180 degrees. Other manufacturers start with theborescope tip as zero degrees and then count back towardthe eyepiece, making a direct-view 0 degrees.

Setup of a Rigid Borescope

To find the direction and field of view during visual testingwith a rigid borescope, place a protractor scale on a board orworktable. Position the borescope carefully so it is parallel tothe zero line, with the lens directly over the center mark on

the protractor. Remember that the optical center of aborescope is usually 25 to 50 mm (1 to 2 in.) behind the lenswindow.

By sighting through the borescope, stick pins into theboard at the edge of the protractor to mark the center andboth the left and right edges of the view field. This simpleprocedure gives both the direction of view and the field ofview (see Figs. 21 and 22).

M iniborescope

One variation of the rigid borescope is called the minibor-eseope (see Fig. 23). In this design, the relay lens train isreplaced with a single, solid fiber. The fiber diffuses ions ina parabola from the center to the periphery of the housing,giving a graded index of refraction. Light passes through thefiber and at specific intervals an image is formed.

The solid fiber is about 1 mm (0.4 in.) in diameter, makingit possible to produce high quality and thin rigid borescopesfrom 1.7 to 2.7 mm (0.07 to 0.11 in.) in diameter. The lensaperture is so small that the lens has an infinite depth of field(like a pinhole camera) and no focusing mechanism isneeded.

FIGURE 21. Field of view for a rigid borescope

60n /

9° ea60 r"


FIGURE 22. Field of view width for varyingdistances

STANDARD FIELDS OF VIEW(degrees)

80 70 60 55504540 30 20MIROILVMS111111NIMMINMEI1=1EnnRNMMEMInn ENIII111111nSEMMINMENMEE=111•111111EVEMIIIIIAMEINEMENEEMOINEFAMMENN•111IIIIMMEMMIIIIWAN•••••••••••••EIFFSIMENNE111IF

1t

225 200 75 150 125 100 75 50 25 25 50 75 100 125 150 175 200 2251911 81 71 16 1 1 51 {4 1 131 .1211 111 111 i2 13 141 151 161 171 181 191

FIELD OF VIEW WIDTHmillimeters inches

20E

30

WAPA EM4045505560 70 80

110j

/AM MINE 3 m1/1 101NEFAM 1 7 1

2 161

IS;141

1 131

121

1 1 1

rI

1C.11

50 mm12 in.)

LENS WP100WDEPTH


FIGURE 23. Miniborescope wide angle lens:(a) general shape and (la) lens detail

(a)

(b J

.11111F "Mb

STAINLESS STEELLIGHT GUIDE FOR

ILLUMINATION



Accessories

Many accessories are available for rigid borescopes.Instant cameras, 35 mm cameras, and video cameras can beadded to provide a permanent record of a visual test. Closedcircuit television displays, with or without video tape, arecommon as well. Also available are attachments at the eye-piece permitting dual viewing or right angle viewing forincreased accessibility.

Special Purpose BorescopesAngulated borescopes are available with forward oblique,

right angle or retrospective visual systems. These instru-ments usually consist of an objective section with provisionfor attaching an eyepiece at right angles to the objective sec-tion's axis. This permits inspection of shoulders or recessesin areas not accessible with standard borescopes.

Calibrated borescopes are designed to meet specific testrequirements. The external tubes of these instruments canbe calibrated to indicate the depth of insertion during a test.Borescopes with calibrated reticles are used to determineangles or sizes of objects in the field when held at a predeter-mined working distance.

Panoramic borescopes are built with special optical sys-tems to permit rapid panoramic scanning of internal cylindri-cal surfaces of tubes or pipes.

Wide field borescopes have rotating objective prisms toprovide fields of view up to 120 degrees. One application ofwide field borescopes is the observation of models in windtunnels under difficult operating conditions.

Ultraviolet borescopes are used during fluorescent mag-netic particle and fluorescent penetrant tests. Theseborescopes are equipped with ultraviolet lamps, filters andspecial transformers to provide the necessary wavelengths.

Waterproof and vaporproof borescopes are used forinternal tests of liquid, gas or vapor environments. They arecompletely sealed and impervious to water or other types ofliquid.


Water cooled or gas cooled borescopes are used for tests offurnace cavities, jet engine test cells and for other high tern-perature applications.

Typical Industrial BorescopeApplicationsAviation Industry

The use of borescopes for tests of airplane engines andother components without disassembly has resulted in sub-stantial savings in costs and time. A borescope of 11 mm(0.44 in.) diameter by 380 mm (15 in.) working length canbe used by maintenance and service departments for visualtesting of engines through spark plug openings, without dis-mantling the engines. An excellent view of the cylinder wall,piston head, valves and valve seats is possible and severalhundred hours of labor are saved for each engine test. Spareengines in storage can also be inspected for corrosion of cyl-inder wall surfaces.

Aircraft propeller blades are visually tested during manu-facture. The entire welded seam of a blade can be inspectedinternally for cracks and other discontinuities. Propellerhubs, reverse pitch gearing mechanisms, hydraulic cylinders,landing gear mechanisms and electrical components also canbe inspected with borescopes. Aircraft wing spars and strutsare inspected for evidence of fatigue cracks and rivets andwing sections cam be tested visually for corrosion.Borescopes used for tests of internal wing tank surfaces andwing corrugations subject to corrosion have saved airlineslarge sums of money by reducing the time aircraft are out ofservice.

Automotive Industry

Borescopes are widely used in the manufacturing andmaintenance divisions of the automotive industry. Enginecylinders can be examined through spark plug holes withoutremoving the cylinder head. The cylinder wall, valves andpiston head can be visually tested for excess wear, carbondeposits and surface discontinuities. Crankcases and crank-shafts are examined through wall plug openings withoutremoving the crankcase. Transmissions and differentials aresimilarly inspected.

Borescopes are also useful for locating discontinuities suchas cracks or blowholes in castings and forgings. Machinedcomponents such as cross bored holes can be examined forinternal discontinuities. Borescopes are used to inspect cyl-inders for internal surface finish after honing. Tapped holes,shoulders or recesses also can be observed. Inaccessibleareas of hydraulic systems, small pumps, motors and

mechanical or electrical assemblies can be visually testedwithout dismantling the engine.

Machine Shops

Borescopes find applications in production machineshops, tool and die departments and in ferrous, nonferrousand alloy foundries. In production machine operations,horescopes of various sizes and angles of view are used toexamine internal holes, cross bored holes, threads, internalsurface finishes and various inaccessible areas encounteredin machine and mechanical assembly operations. Specificexamples are visual tests of machine gun barrels, rifle bores,cannon bores, machine equipment and hydraulic cylinders.

In tool and die shops, borescopes are used to examineinternal finishes, threads, shoulders, recesses, dies, jigs, fix-tures, fittings and the internal mating of mechanical parts. Infoundries, horescopes are widely used for internal inspec-tions to locate discontinuities, cracks, porosity and blow-holes. Borescopes are also used for tests of many types ofdefense materials, including the internal surface finish ofrocket heads, rocket head seats and guided missilecomponents.

Power Plants

In steam power plants, borescopes are used for visual testsof boiler tubes for pitting, corrosion, scaling or other discon-tinuities. Borescopes used for this type of work are usuallymade in 2 or 3 m (6 or 9 ft) sections. Each section is designedso that it can be attached to the preceding section, providingan instrument of any required length.

Other borescopes are used to examine turbine blades,generators, motors, pumps, condensers, control panels andother electrical or mechanical components without disman-tling. In nuclear plants, horescopes offer the advantage thatthe inspector can be in a low radiation field while the distal,or sensor, end is in a high radiation field.

Chemical Industry

Visual tests of high pressure distillation units are used todetermine the internal condition of tubes or headers. Evap-oration tubes, fractionation units, reaction chambers, cylin-ders, retorts, fUrnaces, combustion chambers, heatexchangers, pressure vessels and many other types of chemi-cal process equipment are inspected with borescopes orextension borescopes

Tank cars are inspected for internal rust, corrosion and thecondition of outlet valves. Cylinders and drums can beexamined for internal conditions such as corrosion, rust orother discontinuities.

FIGURE 24. Sectional view of a typical borescope,showing relationship of parts in its optical system

OBJECTIVE LENSES

SPACERS

LAMP WIREMIDDLE LENSES(ACHROMATIC)

OCULAR

IIAM. 11ir mom. pir.

CORRECTINGPRISM

DIFFUSER


TABLE 3. Comparison of vision types and angles ofobliquity

Type of Vision

Angle ofObliquity(degrees)

AngularField

(degrees)

Direct 0 45Forward oblique 25 50Forward vision 45 45Right angle 90 50Retrospective 135 45Circumferential 0 45

90 15

Petroleum Industry

Borescopes are used for visual tests of high pressure cata-lytic cracking units, distillation equipment, fractionationunits, hydrogenation equipment, pressure vessels, retorts,pumps and similar process equipment. Use of the borescopein the examination of such structures is doubly significant.Not only does it allow the examination of inaccessible areaswithout the lost time and expense incurred in dismantling, itavoids breakdown and the ensuing costly repair.

Borescope Optical SystemsBorescopes are precise optical devices containing a com-

plex system of prisms, achromatic lenses and plain lensesthat pass light to the observer with high efficiency. An inte-gral light source is usually located at the objective end of theborescope to provide illumination for the test object.

Angles of Vision

To meet a wide range of visual testing applications,borescopes are available in various diameters and workinglengths to provide various angles of vision for special require-ments. The most common types of vision are: (1) right angle,(2) forward oblique, (3) direct and (4) retrospective (seeFig. 20).

These types of vision are characterized by different anglesof obliquity for the central ray of the visual field, with respectto the forward direction of the borescope axis (see Table 3).

General Characteristics

Desirable properties of borescopic systems are large fieldof vision, no image distortion, accurate transmission of colorvalues and adequate illumination.

The brightest images are obtained with borescopes oflarge diameter and short length. As the length of theborescope is increased, the image becomes less brilliantbecause of light losses from additional lenses required totransmit the image. To minimize such losses, lenses are typi-cally coated with antireflecting layers to provide maximumlight transmission.

Optical Components

The optical system of a borescope consists of an objective,a middle lens system, correcting prisms and an ocular section(see Fig. 24). The objective is an arrangement of prisms andlenses mounted closely together. Its design determines theangle of vision, the field of view and the amount of light gath-ered by the system.

The middle lenses conserve the light entering the systemand conduct it through the borescope tube to the eye with aminimum loss in transmission. Design of the middle lenseshas an important effect on the character of the image. Forthis reason, the middle lenses are achromatic, each lensbeing composed of two elements with specific curvaturesand indexes of refraction. This design preserves sharpness ofthe image and true color values.

Depending on the length of the borescope, the image mayneed reversal or inversion or both, at the ocular. This isaccomplished by a correcting prism within the ocular forborescopes of small diameter and by erecting lenses forlarger designs.

Depth of Focus, Field of View and Magnification

The depth of focus for a borescopic system is inverselyrelated to the numerical aperture N.

N = n sin a (Eq. 1)

Where:

n = the refractive index of the object space; anda = the angle subtended by the half diameter of the

entrance pupil of the optical system.

FIGURE 25. Components of typical borescopesystem (case not shown)

CURRENTCONTROLLER_

CURRENT -REGULATOR

INLET CABLE

CONDUCTING

CORD TIPS CONTACT RINGSpoi

LAMP CAP

OBJECTIVELENSeg". '"-

ROTATING CONTACT

CORDCONNECTOR

INDICATINGBUTTON

EYEPIECE

FIGURE 26. A typical right angle borescope

UGHT RIGHT ANGLE INDICATING BUTTONSOURCE OBJECTIVE LENS

TELESCOPE TUBE _NROTATING CONTACT

\- 2,1

= -1

DIAMETER LAPLAM P CHAMBER LENGTH CONTACT RINGSWORKING

EYEPIECE


The entrance pupil is that image of any of the lens apertures,imaged in the object space, which subtends the smallestangle at the object plane. Because the numerical aperture ofborescope systems is usually very small compared with thatof a microscope, the corresponding depth of focus is exceed-ingly large. This permits the use of fixed focus eyepieces inmany small and moderately sized instruments.

Field of view, on the other hand, is relatively large, gener-ally on the order of 50 degrees of angular field. This corre-sponds to a visual working field of about 25 mm (1 in.)diameter at 25 mm (1 in.) from the objective lens. At differ-ent working distances, the diameter of the field of view variesalmost directly with the working distance (see Fig. 22).

Magnification of a borescope 's optical system is given bythe relation:

M = m, x m, x ma (Eq. 2)

where m,, m, and m, are the magnifications of the objective,middle lenses and ocular. The total magnification ofborescopes varies with diameter and length but generallyranges from about 2 x to 8 x in use. Note that the linearmagnification of a given borescope changes with working dis-tance and is about inversely proportional to the object dis-tance. A borescope with 2 x magnification at 25 mm (1 in.)working distance therefore will magnify 4 x at 13 mm(0.5 in.) distance.

Borescope ConstructionA borescopic system usually consists of one or more

borescopes having integral or attached illumination, addi-tional sections or extensions, a battery handle, battery box ortransformer power supply and extra lamps, all designed to fitin a portable case (see Fig. 25). The parts of a fixed lengthborescope for right angle vision are shown in Fig. 26. Alsoshown is a lamp at the objective end of the device. In thisconfiguration, insulated wires are located between the innerand outer tubes of the borescope and serve as electrical con-nections between the lamp and the contacts at the ocularend. A contact ring permits rotation of the borescopethrough 360 degrees for scanning the object space withoutentangling the electrical cord. In other models, a fixed con-tact post is provided for attachment to a battery or a trans-former, or the illumination is provided by fiber optic lightguides (see Fig. 16).

Borescopes with diameters under 37 mm (1.5 in.) are usu-ally made in sections, with focusing eyepieces, interchange-able objectives and high power integral lamps. This kind ofborescope typically consists of an eyepiece or ocular section,a 1 or 2 in (3 or 6 ft) objective section, with I, 2 or 3 m (3, 6or 9 ft) extension sections. The extensions are threaded for

fitting and ring contacts are incorporated in the junctions forelectrical connections. Special optics can be added toincrease magnification when the object is viewed at adistance.

Eyepiece extensions at right angles to the axis of theborescope can be supplied, with provision to rotate theborescope with respect to the eyepiece extension, for scan-ning the object field.

Right Angle BorescopesThe right angle borescope is usually furnished with the

light source positioned ahead of the objective lens (seeFig. 26). The optical system provides vision at right angles tothe axis of the borescope and covers a working field of about25 mm (1 in.) diameter at 25 mm (1 in.) from the objectivelens.

Applications of the right angle borescope are widespread.The instrument permits testing of inaccessible corners andinternal surfaces. It is available in a wide range of lengths, inlarge diameters or for insertion into apertures as small as2.3 mm (0.09 in.). It is the ideal instrument for visual testsof rifle and pistol barrels, walls of cylindrical or recessedholes and similar components.


Another application of the right angle borescope is inspec-tion of the internal entrance of cross holes, where it may becritical to detect and remove burrs and similar irregularitiesthat interfere with correct service. Drilled oil leads in cast-ings can be visually inspected, immediately following thedrilling operation, for blowholes or other discontinuities thatcause rejection of the component. Right angle borescopescan be equipped with fixtures to provide fast routine tests ofparts in production. The device's portability allows occa-sional tests to be made at any point in a machining cycle

Forward Oblique Borescopes

The forward oblique system is a design that permits themounting of a light source at the end of the borescope yetalso allows forward and oblique vision extending to an angleof about 55 degrees from the axis of the borescope.

A unique feature of this optical system is that, by rotatingthe borescope, the working area of the visual field is greatlyenlarged.

Retrospective Borescope

The retrospective borescope has an integral light sourcemounted slightly to the rear of the objective lens. For a borewith an internal shoulder whose surfaces must be accuratelytooled, the retrospective borescope provides a uniquemethod of accurate visual inspection.

Direct Vision Borescope

The direct vision instrument provides a view directly for-ward with a typical visual area of about 19 mm (0.75 in.) at25 ram (1 in.) distance from the objective lens. The light car-rier is removable so that the two parts can be passed succes-sively through a small opening.

LI 1/4/1 IC- LI LKJI C.34- VIJC.3

Borescopes under 38 mm (1.5 in.) diameter are oftenmade in pieces, with the objective section 1 or 2 m (3 or 6 ft)in length. The additional sections are 1, 2 or 3 m (3, 6 or9 ft) long with threaded connections. These sections may headded to form borescopes with lengths up to 15 m (45 ft) fordiameters under 37 mm (1.5 in.).

Tables 4 through 7 list the diameters and working lengthsof typical borescopes. For special applications, custom madesizes and designs are available.

Special Purpose BorescopesBorescopes can be built to meet many special visual

testing requirements. The factors affecting the need for cus-tom designs include: (1) the length and position of test area,(2) its distance from the entry port, (3) the diameter andlocation of the entry port and (4) inspector distance from theentry port.

TABLE 4. Specifications of right angle borescopes

BorescopeDiameter

millimeters(inches;

Typical WorkingLength Ranges

millimeters f inches)Lamp

Voltage1.75 (0.07) 38 to 100 )1.5 to 4) 1.52.25 (0.09) 50 to 313 {2 to 12.5) 1.52.75 (0 11) 113 to 300 (4.5 to 12) 2.53.5 (0.14) 100 to 750 (4 to 30) 2.54 (0.16) 150 to 750 (6 to 30) 2.54.65 (0.186) 88 to 700 (3.5 to 28) 2.55.25 (0.21) 188 to 450 (7.5 to 18) 2.55.8 (0.232) 225 to 1,500 (9 to 60) 2.56.5 (0.26) 450 to 1,550 (8 to 62) 2.56.5 (0.26) 200 to 375 (8 to 15) 6.06.9 (0.276) 75 to 1,550 (3 to 62) 2.56.9 (0.276) 250 to 400 (10 to 16) 6.09.75 (0.39) 138 to 1,175 {5.5 to 47) 4.59.75 (0.39) 1.500 to 4,500 )60 to 180) 4.59.75 (0.39) 500 to 750120 to 30 1 12.0

10.9 (0.436; 200 to 300 (8 to 121 4.510.9 (0.436; 375 (15) 12.012.25 (0.49) 625 (25) 4.5

Environmental conditions such as temperature, pressure,water immersion, chemical vapors or ionizing radiation areimportant design factors. The range of special applicationsis partly illustrated by the examples given below

Miniature Borescopes

Miniature borescopes are made in diameters as small as1.75 mm (0.07 in.), including the light source. They are use-ful because they can go into small holes. Inspection ofmicrowave guide tubing is a typical application.

Periscr,pes

A large periscopic instrument with a right angle eyepieceand a scanning prism at the objective end is shown in Fig. 27.This instrument is 125 mm (5 in.) in diameter and 9 m (27 ft)long. It is sectioned and provides for visual or photographicstudy of models in wind tunnels. A field of view 70 degreesin azimuth by 115 degrees in elevation is covered by thisdesign.

The cave borescope is a multiangulated, periscopic instru-ment used for remote observation of otherwise inaccessibleareas.

Indexing Borescope

Butt welds in pipes or tubing 200 mm (8 in.) in diameter orlarger can be visually tested with a special 90 degree indexingborescope. The instrument is inserted in extended formthrough a small hole drilled next to the weld seam and is thenindexed to the 90 degree position by rotation of a knob at theeyepiece.

FIGURE 27. Eyepiece end of large wind tunnelperiscope


TABLE 5. Specifications of section borescopes with working lengths of 1, 2 and 3 m (3, 6 and 9 ft) and extensionsections of 1, 2 and 3 m (3, 6 and 9 ft)

Type of Vision

Right angle, forwardoblique or flexible right angleRight angle, direct vision,forward oblique orcircumferentialRight angle, direct visionor forward obliqueRight angle, direct visionor retrospective

BorescopeDiameter

millimeters (inches)

MaximumLength

meters (feet)

MaximumLamp

Voltage

13 (0.5) 10 (30) 24

19 (0.75) 12 (36) 24

25 (1) 13 (39) 24

34 (1,375) 15 (45) 48

The objective head is then centered within the tube forviewing the weld. A second knob at the eyepiece rotates theobjective head through 360 degrees for scanning the weldseam. Another application of this instrument is forinspecting the inside surface of cathode ray tubes.

Panoramic Borescopes

The panoramic borescope has a scanning mirror mountedin front of the objective lens system. Rotation of the mirroris accomplished by means of an adjusting knob at the ocularend of the instrument. This permits scanning in one planeto cover the ranges of forward oblique, right angle and retro-spective vision (see Fig. 28).

Another form of panoramic borescope permits rapid scan-ning of the internal cylindrical surfaces of tubes or pipes.This instrument has a unique objective system that simulta-neously covers a cylindrical strip 30 degrees wide around theentire 360 degrees with respect to the axis of the borescope.The diameter of this instrument is 25 mm (1 in.) and theworking length is 1 m (3 ft) or larger.

TABLE 6. Specifications of forward oblique borescopes

BorescopeDiameter

millimeters (inches)

WorkingLengths

millimeters (inches)Lamp

Voltage

2.25 (0.09) 50 (2) 1.52.75 (0.11) 138 to 600 (5.5 to 24) 2.03.5 (0.14) 113 to 550 (4.5 to 22) 2.54 (0.16) 250 (10) 2.54.65 (0. I 86) 88 to 750 (3.5 to 30) 2.55.25 (0.21) 178, 432 (7, 17) 2.55.8 (0.232) 275 to 300 (11, 12) 2.56.5 (0.26) 150 to 1,500 (6 to 60) 2.59.75 (0.39) 900, 1,800 (36. 72) 3.0

10.9 (0.436) 250, 375 (10, 15) 12.0

Reading Borescopes

Low power reading borescopes are used in plant or labora-tory setups for viewing the scales of instruments such ascathetometers at moderately remote locations. The magni-fication is about 3 X at 1 m (3 ft) distance.

FIGURE 28. Panoramic borescope: (a) comparativeranges of vision and (b) panoramic systemcomponents

(al

56 DEGREES / 70 DEGREES

FOROBLIOUE TELESCOPE RIGHT ANGLE RETROGRADE TELESCOPETELESCOPE

(b)60 DEGREES

EYEPIECE

56 DEGREES 70 DEGREES

IL

LIGHT SOURCE LIGHT SOURCE

DEFLECTING MIRROR ADJUSTING MECHANISM FOR MIRROR DEFLECTION

UGHT POST


TABLE 7. Specifications of borescopes with separate light carriers

Type

Diametersmillimeters (inches) Working Lengths

millimeters (inches) VoltageCombined Light Carriers Borescope3.5 0.17) 2 {0.08) 2.25 (0.09) 119 14.75) 2.04.7 0.186) 88 {3.5) 2.56.4 0.255) 119, 188 (4.75, 7.5) 2.59.75 0.39) 5.3 (0.212) 4.7 (0.186) 188 to 1,250 17.5 to 50) 2.5

11 0.436) 3, 4 (12. 16) 2.513 0.5) 131 15.25) 2.5

2 3.5 0.17) 2 (0.08) 2.25 {0.09) 119 /4.75) 2.02 9.75 0.39) 5.3 (0.212) 4.7 {0.186) 188 to 600 17.5 to 24) 2.53 4.9 0.195) 1.90 (0.0751 2.9 {0.117) 113 (4.5) 2.0

2.53 6 0.24) 206 (8.25)3 7.2 0.286) 2.8 (0.113) 4 10.161 213 (8.5) 2.53 9.75 0.39) 5.3 (0.212) 4.7 (0.186) 188, 600 (7.5, 24) 2.54 6 0.24) 188, 463 (7.5 to 18.5) 2.54 7.3 0.29) 207, 219 (8.25, 8.75) 2.54 93 0371) 250110) 2.55 6 0.24) 375, 900 1i 5, 36) 12.05 9.5 0.38) 207 (8.25) 2.55 ' 9.75 0.39) 5.3 (021) (0.186) 188 (7.5) 2.5

PERMANENTLY MOUNTED

LEGENDI. RIGHT ANGLE2. FORWARD OBLIQUE3. DIRECT4. RETROSPECTIVE5. FORWARD

Photographic AdaptationsMany borescopes also include the ability to record with

still photography, motion picture or video tape. For example,still pictures on 35 mm film can be taken with a borescopefitted with an adapter designed for the purpose. A telescopicsystem with a movable prism built into the adapter operateson the reflex principle, permitting observation of the visualfield of the horescope up to the instant of photographic expo-sure. High intensity light sources incorporated into theborescope provide illumination for 16 mm circular pictureson 35 mm film. Motion pictures are possible with a fiberoptic light source or a rod illuminator that eliminates electri-cal connections and the heat of a lamp from the objectiveend of the borescope. This is especially valuable whereexplosive vapors are present.

Photography of the interiors of large power plant furnacesduring operation has been done since the 1940s using a unitpower periscope and camera.' The periscope extendsthrough the furnace wall and relays the optical image to thecamera. A water cooled jacket protects the optical systemand the camera from the furnace's high temperatures. Withthis equipment, still and motion picture studies have beenmade of the movement of the fuel bed and the action of thepowdered fuel burner in furnaces operating at full load.

FIGURE 30. A digital image is made fromcombinations of pixels with varying gray levels

-255WHITE

GRAY -127

BLACK-0

8-BIT GRAY LEVEL

ROW

PIXELCOLUMN

FIGURE 29. A basic machine vision system

DISPLAY DIGITIZER COMPUTER

TERMINAL CAMERA

LEI]

UGHTSOURCE

FIGURE 31. Setup for front lighting with machinevision

SENSOR

LIGHTSOURCE

OBJECT

BACKGROUND


PART 4 MACHINE VISION TECHNOLOGY

Machine vision acquires, processes and analyzes an imageto reach conclusions automatically. A machine vision systemconsists of a light source, a video camera, a video digitizer, acomputer and an image display (see Fig. 29).

The light source illuminates the test object for the camerato form a video image. The video digitizer converts theimage into digital form and the digital image is then stored ina two-dimensional memory. The image is divided into rowsand columns, which are subdivided into picture elements orpixels. Each pixel has an integer number which representsthe brightness or darkness of the image at that point. Thisinteger value is called the gray level (see Fig. 30).

Once an image is in the form of an array of gray level pix-els, it is ready for computer processing. The computer firstenhances the contrast of the image with a procedure knownas image enhancement. Following image enhancement, thecomputer simplifies the image with image segmentation.The next step is known as feature extraction and finally, at the

classification stage, the computer identifies and groupsobjects in the image.

Machine vision encompasses all the steps of image acqui-sition, image conversion, image enhancement, image seg-mentation, feature extraction and classification.

Lighting TechniquesThe success or failure of a machine vision system is largely

dependent on the quality of the image acquired by the sys-tem. And proper lighting techniques are essential forobtaining high quality images.

For example, a picture of a fast moving object taken witha video camera under continuous lighting results in a blurredimage. A clear image could be obtained with a strobe lamp.The following text describes lighting techniques that arecommonly used in machine vision systems.

Front LightingWith front lighting, the light source and the image sensor

are placed on the same side of the test object (see Fig. 31).Front lighting is the most convenient method of illuminationfor machine vision systems. It is used when the features ofinterest contrast sharply with the background—for example,when scanning black characters on a white label.

SENSOR

FIGURE 32. Setup for back lighting with machinevision

OBJECT

V

FIGURE 33. Forming a line of light through asemicylindrical lens

LENSLINE OF LIGHT

LIGHT SOURCE

4 4 4 4 4 4 4

LIGHT BOX

LIGHT SOURCE

CONVEYOR

FIGURE 34. Forming a line of light using a scanningmirror

SCANNINGMIRROR

LASERUNE OF LIGHT

FIGURE 35. Height and silhouette detection usingline of light

SENSOR


Back Lighting

With back lighting, the light source and the image sensorare placed on opposite sides of the test object (see Fig. 32).Back lighting is used when the silhouette of a feature isimportant.

Structured Lighting

Combining a light source with optical elements to form aline of light is called structured lighting. There are two waysto form a line of light: (1) place a semicylindrical lens in frontof the light source (see Fig. 33) and (2) use a scanning mirror

to deflect a laser beam (see Fig. 34). A line of light may beused to measure the height or to detect the silhouette of atest object (see Fig. 35). 2

Strobe Lighting

Strobe lighting is sometimes called flash lighting. Theelectronic circuit for a strobe light is shown in Fig. 36. Whenthe circuit is triggered by a video camera, a flash of light illu-minates the test object momentarily. Strobe lighting is usedto image moving objects or still objects with potentialmovement.3

Ultraviolet Lighting

Ultraviolet light causes fluorescent material to glow and isused in magnetic particle and liquid penetrant testing todetect discontinuities.'

FIGURE 36. Strobe lighting circuit FIGURE 38. Characteristics of bandpass filters

PEAK WAVELENGTHPEAK TRANSMISSION 'PERCENTAGE)

BLOCKED TOWAVELENGTH

MAXIMUMTRANSMISSIONOUTSIDEBANDPASS'PERCENTAGE'

WAVELENGTH

65

SO

10

LUUz<

zap

7,s T'so0

00

FIGURE 39. Characteristics of short pass filters

WAVELENGTHinanometersi

100

80

CUT OFF WAVELENGTH

CUT ON WAVELENGTH

<1.2 x CUT OFF WAVELENGTH


Optical FilteringImage sensors used in machine vision systems detect the

intensity of electromagnetic waves in the visible range asshown in Fig. 37. If only a portion of the visible spectrum isof interest, a filter in front of the sensor produces a higherquality image.

Bandpass filters transmit a band of electromagnetic wavesand rejects the rest as shown in Fig. 38. Short pass filterstransmit electromagnetic waves below a cut off wavelengthas shown in Fig. 39. Long pass filters transmit electromag-netic waves above a cut off wavelength as shown in Fig. 40.

Neutral density filters attenuate the light level incident onthe image sensor when the light source itself is difficult tocontrol.

Image SensorsThe two primary types of image sensors are image tubes

and solid state imaging devices.

Image Tubes

Image tubes are used to generate a train of electricalpulses that represent light intensities present in an opticalimage focused on the tube. The most widely used imagetube is the vidicon (see Fig. 41).

FIGURE 37. The spectrum of visible, ultraviolet and infrared radiation

WAVELENGTH lo200 300 380 450 4910 570 590 630

(nanometers)730 I 53 x 103 6 x 1& 4 x 10 6

EXTREME FAR NEAR I VIOLET BLUE GREEN YELLOW ORANGE RED

NEAR MIDDLE FAR FAR FAR

ULTRAVIOLET

VISIBLE LIGHT

INFRARED

FIGURE 40. Characteristics of long pass filters

100

PEAK T

50 PERCENT OF PEAK T

80 PERCENT OF PEAK T

CUT OFF WAVELENGTH

CUT ON WAVELENGTH

AVERAGE 7

85 PERCENT OF WAVELENGTH

415 to 1,000

2,200

80

1 0

5

WAVELENGTH{nanometers;

FIGURE 42. Spectral response of video cameratubes

200 300 400 500 600 700 800 900 1 ,000

1,109

1100

ULTRAVIOLET IVIOLETI.L

GREEN' °F1 RED I INFRARED


rialmanagoli SILICON DIODE

INTENSIFIERS ispdigni giA ARRAY

1.1511MNEWIAWN MI SPECTRAL DISTRIBUTIO_LinginamElmo...0knrig.g..

STANDARD

V10100N

.nk.

2,854

EK TUNGSTEN

n..•n•EMIL! ivy No n.

n4 PHOTOPIC wenal a.

M AnMEM

lEYE1 ONINnalnM

M MNNNE

LC

09

0.8

0.7

0.6

0.5

04

0.3

02

01


FIGURE 41. Schematic diagram of a vidicon tube

LIGHTELECTRONBEAM

PHOTOCONDUCTIVETARGET CATHODE

SIGNAL OUTPUT

The vidicon tube uses an electron beam to scan a photo-conductive target known as the light sensor. A conductivelayer applied to the front of the photoconductor serves as thesignal electrode. The signal electrode is operated at a posi-tive voltage with respect to the back of the photoconductorwhich operates at the cathode voltage.

The scanning beam initially charges the back of the photo-conductor to cathode potential. When a light pattern isfocused on the photoconductor, its conductivity increases inthe illuminated areas and the back of the photoconductorcharges to more positive values. The scanning electronbeam deposits electrons on the positively charged areas,resulting in current pulses that are read out as video signals.

In addition to the standard vidicon, variations such as sili-con target vidicons and intensifiers are useful for some appli-cations (see Fig. 42).

Solid State Imaging Devices

The principle of the solid state imaging device is based onthe photoelectric effect and the fact that free electrons arecreated in a region of silicon illuminated by photons. Thenumber of free electrons is linearly proportional to the inci-dent photons. If a silicon device is made with a repetitivepattern of small but finite photo sensing sites, the number ofelectrons generated in each site (charge packet) is directlyproportional to the incident light on that specific site. If thepattern of incident intensity is an optical image of an objectfocused on the surface of the silicon array, the charge packetsgenerated in the array form an electronic image of theobject.

There are two basic classes of solid state imaging devices:charge coupled devices (CCD) and charge injected devices(CID).

Charge Coupled Devices

The operation of a charge coupled device is shown inFig. 43. When two out-of-phase clock strings are applied tothe gate electrodes, the charge packets underneath the elec-trodes move from one storage element to the next. Becauseeach charge packet may be of different size, the line of ele-ments becomes a simple analog shift register.

The transfer of charges from one element to the next isvery efficient. The amount of charge in each packet stayssubstantially the same, even after it has been passed througha thousand sequential elements. When the string of chargepackets is read through the output register line by line(Fig. 44), a video image is obtained.

FIGURE 43. Operating principles of a chargecoupled device array

CLOCK HIGHVOLTAGE

LOW

GATE ELECTRODES

. Ip.,410,IM.—A4,11

t = 0 t = I/ I = 1 TIME

ON SURFACE n REGIONS TO PROVIDE ASATO POTENTIAL WELLS

wi m Fin rm E IKEA i ii ag NI 6

I 1 I 1 I -..- 1 1 I 1 ' • -

,.."/"-%_ POT. 1,---,_ POT WEL E--•,.._,POT gil •

1 FITE - ' - I-... i

W

.__.• 1

I 7FICEM IOPE Sn SUBSTRATE

1 lait = 0 CYCLE 1

1

1•

I I I 1 I I I -... I I". .1.40T Cie., POT WEL ,--„__, 1 POT VII ,--,_I L_/.---s1—••••.--/ —'r 1J 1

_.,

J I _J, Olt = 1/2DCLE 1

1i---1 1-----11.--1 4 r I

II- I I I .... 1

...- I I 4 ••••.IPOT WELL ' I— LL r -

—

ROT l , PO/ WELL ,n\--• ----—

WE

Y I '---N---`. iii 1 1

I CIPRACIF Fl FMFNT I r,. _ 1 rel i I

METRY

INSLILAIINGLAYER

n TOT

Si LAYER

FIGURE 45. A charge injected device array

-,.-..Fikr,cc-.-.10‘z

PHOTOSENSITIVEELEMENT

CHARGING TRANSFERHOLDING ELEMENTS

HORIZONTAL REGISTER

VIDEO

OUT

32 x 44 ELEMENTBUCKET BRIGADE

PHOTOSENSITIVE ARRAY

FIGURE 44. A charge coupled device array

OLITPUT14___AMPLIFIER

HORIZONTALCLOCK

VERTICALCLOCK

VERTICAL SYNC VIDEO OUT •


Charge Injected Devices

The charge injected device resembles the charge coupleddevice except that during sensing the charge is confined tothe image site where it was generated (see Fig. 45). Thecharges are read using an XY addressing technique similar tothat used in computer memory. Basically, the stored chargeis injected into the substrate and the resulting current issensed as the video signal.

Image ProcessingAs illustrated in Fig. 30, a digital image is a two-dimen-

sional array of pixels that represent the gray levels of animage. Image processing is a way of analyzing these pixels toreach conclusions about the image and typically involves foursteps: image enhancement, image segmentation, featureextraction and classification. 5.63

Image Enhancement

A digital image contains low spatial frequency compo-nents, high spatial frequency components and noise. Imageenhancement is used to increase the visibility of specificcomponents. An image may be enhanced by spatial filters,gradient operators or mathematical morphology. The mostcommonly used filters are high pass, low pass and medianfilters. Four of the most common gradient operators areRoberts, Prewitt, Sobel and Laplacian operators. The basicmorphological operations are dilation, erosion, opening andclosing.

High Pass Spatial Filter

The high pass filter is an n x n square matrix with differ-ent values. The matrix is referred to as the mask and the val-ues are called coefficients of the filter. A specific 3 x 3 maskis shown in Fig. 46 with the original image on top and the fil-tered image on the bottom. Each pixel in the filtered image

FIGURE 46. A high pass spatial filter: (a) originalimage, (b) mask and RI filtered image

1 I 0 0 0

I 0 0 0

I 1 0 0 0

I I 0 0 0

I 1 I 0 0 0

(a)

(b) -1 - I

-I 8 - I

- I -1 -

0 0 3 -3 0 0

0 0 3 -3 0 0

0 0 3 3 0 0

0 0 3 -3 0 0

0 0 3 -3 0 0

( C )

1/9 1/91/9

1/9 1/9 1/9

1/9 1/9 1/9

FIGURE 47. A low pass spatial filter: (a) originalimage, (b) mask and (c) filtered image

1 1 I 0 0 0

1 1 I 0 0 0

1 I I 0 0 0

1 1 1 0 0 0

1 I I 0 0 0

(a)

(b)

1 067 0.33 0 0

1 1 0_67 0.33 0 0

1 1 0.67033 0 0

1 10 67 0.33 0 0

1 I 067 033 0 0

(t)


is the arithmetic sum of the nine pixels (the center pixel andits eight neighbors), multiplying each pixel by the corre-sponding coefficients.

In Fig. 46, the center pixel in the 3 x 3 mask is multipliedby 8 and the eight neighboring pixels are multiplied by —1.The sum of these nine weighted pixel values becomes thecentral pixel in the filtered image. This process is also calledimage convolution. The high pass filter enhances edges andattenuates smooth transitions.

Low Pass Spatial Filter

The low pass filter uses a linear combination of pixel val-ues, as does the high pass filter, but all the coefficients arepositive. An example of low pass filtering is shown in Fig. 47.Low pass filtering smoothens abrupt transitions in an imageand is also called smoothing.

Median Spatial Filter

Median filtering takes the median value (not the mean) ofn x n neighbor pixels in the original image and uses themedian value as the central pixel in the filtered image. Fig-ure 48 shows the operation of a 3 x 3 median filter. Medianfiltering reduces background noise without smoothingedges; low pass filtering smooths both noise and edges.

Gradient

A digital image can be described by a two-dimensionalvector function f(X,Y), where (X,Y) is the coordinate of thepixel and the amplitude off(X,Y) is the gray level (brightness)of the pixel. The gradient is defined as the first derivative off(X,Y), denoted by g(X,Y). The gradient function g(X,Y) has

FIGURE 48. A median spatial filter: (a) originalimage and (b) filtered image

28 I 31 29 .28

.27 32 1 29 27

32 .3 1 29 1 32

28 27 .30 28 I

.29_26 .31

.

32 .31

(a)

31 .32 .29

30 .30 .30

30 31 31

(b)


two components: amplitude go and phase gp . The amplitudeis given by Eq. 3 and the phase is given by Eq. 4.

g.(X,Y) = {[PX,Y)] 2 + (x, y 21 1 (Eq. 3)

g p(X,Y) = arc tan [-f.(X'17)1F(X, Y) (Eq. 4)

Because the square root operation is computationallyexpensive, Eq. 3 is often approximated by the maximum ofthe absolute vertical and horizontal neighboring pixeldifference:

g a(X,Y) = max [1f(X,Y) – f(X + 1,Y)1, (Eq. 5)

1f(X,Y) – f(X,Y + 1)I]

The gradient amplitude is high at edges and low at uniformregions of an image. Therefore, gradient operators are highpass spatial filters.

Roberts Gradient

A 2 x 2 gradient operator was proposed by L.G. Robertsto enhance edges. The Roberts gradient is given by Eq. 6:

g a(X,Y) = maxl1 f(X,Y) – f(X + 1,Y – 1)1, (Eq. 6)

1f(X + 1,17) – f(X,Y – 1)1]

Prewitt Gradient

A 3 x 3 gradient operator was proposed by J.M.S. Prewitt.The coefficient of the Prewitt operator is shown in Fig. 49.Mathematically, the Prewitt gradient amplitude is given by:

(Eq. 7)

g„(X,Y) = {[f(X + 1,Y + 1) – f(X – 1,17 + 1)

+ f(X + 1,17) – f(X – 1,Y)

+ f(X + 1,Y – 1) – f(X – 1,Y – 1)1 2

+ [f(X – 1,Y + 1) – f(X – 1,Y – 1)

+ f(X,Y + 1) – f(X,Y – 1)

+ f(X + 1,Y + 1) – f(X + 1,Y – 1)121"2

Sobel Gradient

I. Sobel proposed a 3 x 3 gradient operator as shown inFig. 50. The Sobel gradient is given by:

(Eq. 8)

ga(X,Y) = {[f(X + 1,Y + 1) – f(X – 1,Y + 1)

+ 2f(X + 1,Y) – 2f(X – 1,Y)

+ f(X + 1,Y – 1) – f(X – 1,Y – 1)12

+ [f(X – LY + 1) – f(X – 1,Y – 1)

+ 2f(X,Y + 1) – 2f(X,Y – 1)

+ f(X + 1,Y + 1) – f(X + 1,Y – 1)1 2 1"2

Note that the Sobel operator is weighted. The Sobel opera-tor enhances edges and also reduces the effects of noise andis probably the most widely used edge enhancementmethod.

Laplacian Operator

The Laplacian operator is an approximation to the mathe-matical Laplacian 6x 2 + 82f/ 2y2 which is the second

FIGURE 49. The coefficients of the Prewitt operator

0

0

0

0

FIGURE 50. The coefficients of the Sobel operator

-1 0 2

-2 0 0

-1 0 -1 —

U

d e

9

FIGURE 51. Dilation function


derivative of the image f(X,Y). The discrete Laplacian isgiven by:

(Eq. 9)

L(X,Y) = f(X,Y) — [f(X,Y + 1) + f(X,Y — I)

+ f(X + 1,Y) — f(X — 1,Y)]

The Laplacian operator enhances edges but because it is anapproximation to the second derivative, it also enhancesnoise. Phase information is lost after Laplacian operation.

Mathematical MorphologyThe image processing technique of expanding and shrink-

ing is called mathematical morphology. The basic operatorsin mathematical morphology are dilation (expanding), ero-sion (shrinking), opening and closing."

Dilation

Dilation of a binary image means the pixel in the outputimage is a 1 if any of its eight closest neighbors is a 1 in theinput image.

Dilation can be written as a function using the notation inFig. 51.

(Eq. 10)

e0 =a+b+c+d+e+f+g+h+ i

Equation 10 is the logical or of all eight neighbors. Otherdilations are possible. For example: e0 = b + e + h + d +f (the is grow out in X and Y axes), e, = g + e + c (the isgrow out in the 45 degree direction) and so on.

Erosion

Erosion of a binary image means the pixel in the outputimage is a 1 if all of its eight neighbors are is in the inputimage, as given by Eq. 11.

e, = ahcdefghi (Eq. 11)

Equation 11 is the logical and of all eight neighbors.Other erosions are possible. For example: e„ = b-d-el-h(the is shrink in X and Y direction), e„ = c •e •g (the is shrinkin the 45 degree direction) and so on.

Opening and Closing

Opening is the operation of erosion followed by dilation.Closing is the operation of dilation followed by erosion.

ning and closing are powerful image processing tools.For example, a single pixel opening eliminates all isolatedsingle pixels. A single pixel closing connects a broken featureseparated by one pixel.

Arithmetic and Logical Operation

Most commercially available image processors andmachine vision systems are capable of performing imageaddition, subtraction, multiplication, division and logicaloperations of and, or, x or.

The arithmetic operations are applied to gray level imagesand logical operations are applied to binary images. Addition

FIGURE 52. The bimodal histogram threshold


of two images is essentially averaging. Subtraction of twoimages brings out the differences in the two images. Multi-plication enhances features where either image is fairlybright. Division brings out features where the denominatoris dark (denominator cannot be 0). The and of two binaryimages brings out the bright areas common in both images.The or of two images shows the areas that are bright in eitherimage. The X or of two images shows the areas that arebright in either image but not in both.

Image SegmentationImage segmentation is a process that partitions an image

into disjointed (nonoverlapping) regions called blobs. Themost common technique for image segmentation isthresholding.

Thresholding

Thresholding is a technique that reduces a gray levelimage into a binary image. Thresholding can be definedmathematically as:

Px,Y) = 1 if f(X,Y) > t (Eq. 12)

and

f(X,Y) 0 otherwise

Where:

f(X,Y) = the original image;f(X,Y) = the threshold image; andt = the value of a gray level.

Bimodal Histogram Thresholding

An image containing an object on a contrasting back-ground has a bimodal gray level histogram as shown inFig. 52. The two peaks correspond to the background andthe object. The value at the valley is the threshold gray level.

Global Thresholding

Global thresholding occurs when an image is thresholdedby a constant value throughout the entire image. The tech-nique works well if the background gray level is reasonablyconstant throughout the image and if the objects all haveapproximately equal contrast above the background.

Adaptive Thresholding

In most images, the background gray level is not constant.In such cases, a varying threshold value produces betterresults. This is called adaptive thresholding.

Subtraction

Subtraction of two gray level images reveals the differ-ences between the two. This is a very fast and efficient pro-cess for segmenting images and is widely used inradiographic nondestructive testing.

Feature Extraction

Feature extraction is the process of deriving some featurevalues from the enhanced image. These values are usuallyparameters that may be used to distinguish objects in theimage.

Pixel Counting

Pixel counting is a process that counts the number of pix-els above a threshold within a window. For simple applica-tions, pixel counting provides enough information to makean accept or reject decision. For example, pixel countingmay be used to determine the presence or the absence of avoid in the first quadrant of a binary image.

Edge Finding

The location of edges is a parameter that is frequentlyrequired in machine vision applications. The X,Y location fora binary edge is the transition of black to white or white toblack. The X,Y location for a gray level edge is the point ofmaximum gradient. Gray level edge finding is usually associ-ated with edge enhancement techniques.

FIGURE 53. Determination of blob perimeter:fa) direction numbers for chain code elements and(131 chain code for the boundary shown

(a)

4 • o

(b)

CHAIN CODE: 1 110101030333032212322


Sizes

The measurement of area, length, width, perimeter andcentroid are the most commonly required parameters forimage classification in machine vision applications. The areaof a blob is the total number of pixels inside the blobmultiplied by the area of a single pixel.

The length and width of a blob are measured relative to itsmajor axis. One way to establish the axis is to use what isknown as the minimum enclosing rectangle method.' Withthis method, the boundary of the blob is rotated through 90degrees in steps of 3 degrees or more. After each incremen-tal rotation, a horizontally oriented minimum enclosing rect-angle is fit to the boundary. Computationally, this involveskeeping track of the minimum and maximum X and Y valuesof the rotated boundary points. At some angle of rotation,the area of the minimum enclosing rectangle goes through aminimum. The dimensions of the minimum enclosing rect-angle at that point can be taken as the length and width ofthe blob.

The perimeter of the blob can be obtained from its bound-ary chain code (see Fig. 53). Basically, the chain code con-sists of line segments that must lie on a fixed grid with a fixedset of possible orientations. The perimeter is the sum of thetotal steps multiplied by the length of a single step.

The centroid is the coordinates of the center of gravity ofthe blob given by:

X= (Eq. 13)`'. 00

and

y = jtt01

MOO

where Moo, M10, M01 are the zero order and the two first ordermoments as defined by the following equations:

M00 = f f(X,Y)dXdY (Eq. 14)

Mio = f Xf(X,Y)dXdY (Eq. 15)

Mo, = Yf(X,Y)dXdY (Eq. 16)

Classification

Classification is the process of interpreting blobs (objects)using features extracted to make decisions. For example, atest object may be classified as having a crack if a magneticparticle indication is present with a length much greater thanits width. To classify complex blobs, decision theory and arti-ficial intelligence are usually needed."'

Optical Feature Extraction for HighSpeed Optical Tests

Optical processing has much to offer robotics and auto-mated testing.' The initial motivation for using optical pro-cessing is high speed—operations such as the Fouriertransform" and other feature spaces'' are easily producedoptically in real time.' This allows real time rates of thirtytest objects per second or faster. The major disadvantage ofsuch systems has been their high cost, which occurs becauseof the need to produce online transparencies of the testobject. This requires real-time and reusable spatial lightmodulators.' Such devices are expensive and difficult toacquire. With the introduction of liquid crystal televisions,the availability and costs have become much more reasonable.

FIGURE 54. General diagram of the optical featureextraction product inspection system

VIDEOCAMERA

LIGHT

OPTICALFEATURE

EXTRACTOR

IMAGE OF OBJECT

LASER

„....-PROGRAMMABLETRANSPARENCY

eurainjrIMERWES

FEATURESIGNATURE

STANDCDUPE

MUCCOMPUTER

(TO FACTORY DEVICES(

OPTICSNTERFACE

ELECTRONICS

STATISTICALANALYSIS

SOFTWARE


Many researchers have suggested the use of liquid crystalmatrix devices or liquid crystal television for electricallyaddressed transmissive spatial light modulators.

However, there are some disadvantages in using liquidcrystal devices, including: poor contrast, lack of shift invari-ance (caused by phase distortions), poor light efficiency, lowresolution and slow response time. One solution is to workdirectly with component vendors to obtain more suitabledevices. Some spatial light modulators have better than 30:1contrast ratio, about 16 levels of gray scale (the gray scale isactually continuous over the range of contrast), more than70,000 pixels and are driven for this application at a 15 Hzframe rate, These specifications are expected to improve.

The remaining shortcomings are addressed in some sys-tems using system-level compensation techniques, both inoptics and electronic interfaces to achieve an adequate levelof performance. The true two-dimensional Fourier trans-form, rather than an approximation, is now possible, as it wasin most earlier systems.

A problem with many testing techniques has been thecomplex training they demand. Training is simplified by theuse of two-dimensional parallel' optical image processing forautomated test applications,

The hardware discussed below is suitable for discontinu-ities that are 3 percent of the input field of view.

System Overview

Figure 54 shows the general optical feature extraction sys-tem. The test object is strobe lighted and the imageis detected by the television camera. Synchronization isprovided by a photocell and position sensor, to denote whenthe object enters the camera's field of view. The electronic

output from the television camera is written onto the spatiallight modulator. The input image on the spatial light modu-lator is illuminated with coherent light from a laser diode.The pattern image in the back focal plane of a lens behindthe spatial light modulator is known to be the two-dimen-sional Fourier transform of the input image (the testobject)."

A two-dimensional Fourier space contains information onthe spatial frequencies and their orientation in the inputobject. The two-dimensional fast Fourier transform of a 512x 512 x 8 bit image is calculated at video rates. This infor-mation is most useful for general product testing. Thefeature space has a high dimensionality but it can be signifi-cantly compressed (much more so and much more easilythan is possible on the input image directly) as discussedbelow. A dimensionally compressed Fourier space descrip-tion of the input image constitutes the feature signal. This isa list of numbers (values of specific elements) in a reduceddimensionality Fourier feature space. The values (featurevector) are fed to a personal computer (with a processor anda math coprocessor), from which the qualitative (accept orreject) nature of the test object is determined online.

Use of the Fourier Transform

A Fourier transform feature space was chosen for the fol-lowing reasons.

1. It is easily and automatically produced optically inparallel.

2. When only the intensity of the Fourier transform isdetected, the output is shift invariant to the location ofthe input test object within the camera's field of view

3. The speed of the test is independent of the complexityof the test object.

There are many ways to reduce the dimensionality of theFourier space. The best example of this is to sample the two-dimensional Fourier transform using wedge and ring shapeddetectors placed in different hemispheres of the two-dimen-sional Fourier transform space. The original concept forthisa) and its optical realization using special detectors' 1 ' 22 (aswell as many examples of its use) are available in the litera-ture. The wedge ring detector feature space is attractive forseveral reasons, including its (1) dimensionality reduction,(2) shift invariance, (3) scale invariant wedge outputs and(4) rotational invariant ring outputs.

Another attractive feature space results from the magni-tude Fourier transform of the coordinate transformedversion of the input image. An attractive coordinate(that has been achieved online with a computer generatedhologram and a liquid crystal television) is the polar logcoordinate transform.' The magnitude Fourier transform ofthis centered pattern is shift, scale and rotation invariant and

FIGURE 55. Frozen dinner test using Fourierfeature signatures

17 3 4 5 6 7 8 9 10 II 12 13 14 15 16

LEGEND4 = MEAN OF THE SAMPLED TRAINING SET DATA

= FOURIER SAMPLES= MEASURED DISCONTINUITY DATA


and represents an attractive feature space for optical nonde-structive testing.

The system of Fig. 54 is suitable for a factory environment,simple to use and inexpensive to fabricate. The same hard-ware is suitable for a number of applications, allowing oneoptical unit with a limited range of digital software to satisfymany diverse testing problems. The number of featuresused in the feature space is 32 (16 features and a comple-mentary set of another 16).

Training Procedures and Automated LinearDiscriminant Functions

To train this system, the user inputs acceptance standardsthrough the video camera. The optical system calculates the32 reduced dimensionality feature vectors. The mean andvariance of each feature are recorded for the training setdata.

Figure 55 shows data for half of a typical feature vector.The value of each feature is shown by the location of thecross and the standard deviation of each feature is indicatedby the vertical extent of the bars for each feature. As seen inthe figure, a significant difference in the standard deviationoccurs for different feature vectors over the training set ofacceptable objects.

This indicates which Fourier features are expected toexhibit a larger variation. Optionally, for a fully automatedsetup, the user may elect to show sample defective objects.

This allows the system to automatically suggest what isknown as an initial goodness of fit threshold. This completesthe training. The solid line in Fig. 55 shows data for a testobject.

The training exercise noted above has chosen which fea-tures are the most reliable (those with the smallest variance)and which are the least reliable (those with the largest vari-ance). It is not yet known which features are best for dis-crimination of good and bad products (the system describedhere has not been shown rejectable test objects). This canbe achieved by methods detailed in the literature. 24

Testing Procedure

In the sample case below, no false class objects are used intraining. For test data, the measured energy in each Fourierfeature is divided by the energy of all Fourier features (this isperformed separately for the first set of features and for thecomplement set). This normalizes the Fourier feature datawith respect to energy. The central region (about 9 pixels)of the Fourier transform is blocked in all cases (it does notcontain useful discriminatory information).

The mean and standard deviation o for each training setfeature are known. For each test set feature, the training setmean for that feature is subtracted and the difference isdivided by the standard deviation cr of that feature from thetraining set. The square root of the sum of the squares of theresulting normalized inspection set features is then formedand yields the final (standard deviation o- normalized)Euclidean distance difference measure or the performancemeasure, used in the test evaluation.

Assuming that the training set is statistically representa-tive, the cr normalized value of each feature reduces theweight or contribution of the least reliable features (with theassumr— '-hat features with a larger spread or variation inthe training set are less useful or reliable). Each object isviewed as a 32 dimensionality feature vector in a hyperspace.The Euclidean distance difference measures describe theseparation of new test objects from the training set (smallvalues of this performance measure indicate closeagreement or good objects and large values indicated largeseparations or objects with various product discontinuities).This performance measure thus provides an indication of thesimilarity of new test input data to the original training setdata (in normalized difference units of standard deviation ofeach feature).

Denoting training and test feature vector elements by x,and yi, the mean and standard deviation of the training setfeatures (for N training set objects) are:

(Eq. 17)

d =[


and

where x," denotes the feature for then`'' training set object.The performance measure or goodness of fit measure is:

(Eq. 19)

This training and testing technique is simple and results inone scalar output performance measure in Eq. 19 for thequality of the input object. This allows for: (1) simplifiedtraining; (2) easy decision making (accept or reject) on theproduct; (3) a simplified digital portion of the hybrid optical/digital generalized architecture; and (4) if the user manuallyselects the goodness of fit reject threshold, there is no needfor false class (product error) data in the training set. Suchdata are not easily available or easily quantified in most test-ing environments.

This procedure has proved successful in many producttests. The a normalization assumes that features with morevariation in the training set are less useful for discriminationof good from bad objects (and for identification of goodobjects). The Euclidean distance difference measure pro-vides a simple scalar goodness of fit measure (performancemeasure) for the similarity of the training set objects and thetest object.

Quantitative Test Results

To summarize performance, tests are considered below todemonstrate and quantify system performance. For eachtest problem, only acceptable test objects were used as thetraining set. The performance measures calculated for thetraining and the test data sets are provided. From the aver-age and range of the distortion measures for the training set,a threshold at a slightly higher distortion measure was manu-ally selected to evaluate new test objects. Test objects belowthis threshold are considered acceptable and those above itare considered rejects. The average value of the distortionmeasure is given for the training set and for good test objects(these are often combined in the data, because they are gen-erally close) as well as the average for the discontinuity (insome of cases, the full range of values is provided to give ameasure of confidence). Small values of the distortion mea-sure indicate items close to the good training set and largevalues indicate items different from the training set andassumed to be defective.

In some cases, an overlap occurs in the range of trainingset distance measures and the distance measures for some ofthe discontinuities. This generally occurs for small disconti-nuities that represent a small percentage of the object's fieldof view. In other cases, when the range of variation of goodtraining set objects is large, the threshold is increased and inthese cases detection of small discontinuities is not expectedto be reliable. However, a large range of discontinuities on awide variety of products can be reliably detected.

In these tests, training is typically done with 5 to 10 objectsand testing is done on 10 to 16 defective objects per testproblem (depending on the number and types of discontinu-ities included). The training set objects consist of differentitems in each set and include slight differences in positionand orientation (no attempt was made to position thecentroids of each object exactly). Conventional automatedsystems cannot achieve such shift invariant tests without sig-nificant losses in speed and increases in system complexity.

Simulation Results

Table 8 summarizes the simulated results to demonstratethe wide range of test problems the reduced Fourier featurespace can accommodate. Each test problem is assigned atest number and the average distance measures for the train-ing set and good items are given, as well as the average forthe discontinuities.

Test 1 concerns the quality of the outer wrappings of achocolate bar. The test set gives an average distance of 2.2.This parameter provides a measure of the acceptable datavariations. The possible discontinuities include tears andfolds in the wrapper (these are the dominant discontinuityand gave distance measures of 5 to 8). These values are closeto those of the training set but are sufficiently separated toallow their detection. As long as the tear or fold was suffi-ciently long, it could be detected. The wrappers that wereskewed or misaligned by 50 percent gave distance measuresof 16. Missing wrappers were easily detected by their verylarge difference distance measures of 21 to 27.

The global nature of the feature space allows the disconti-nuities (such as tears and folds in the wrapper) to be detectedregardless of their location on the product. The set of dis-continuities included many in different locations. In thiscase, the distance measure for the range of discontinuitieswas quite large (the range, not the average was entered inTable 8). It should be possible to determine the nature ofthe discontinuity in this case from the distance measurevalue: values from 5 to 8 denote tears and folds, values near16 denote wrappers rotated halfway around (a function of thevisible text or patterns and their locations) and values inexcess of 20 denote missing or partially missing wrappers.Similar tests were performed for soup can labels and otherproducts with comparable results.

,) 2

11/2

(Eq. 18)


TABLE 8. Selected Fourier space tests (simulated'

TestNumber Product

Trainingand Good

TestVarious Test

Discontinuities

1 Candy bar wrapper 2.2 5-27

2 Capsules 3 73 Pill pack 6 10.7

4 Soda bottle 6 295 Aspirin seal 1.7 7-16

Discontinuity Remarks

tears and folds (5-8)skewed and missing (16-27)broken7 not 8 pills

cap missingseal missing {1 1 ) or partial (7)

Remarks

small tears not detected

half capsules easily detectedindependent of location ofmissing pillfactor of 5 differencelarge factor between good

and discontinuities

Test 2 concerns capsules and whether they are whole.Reject discontinuities include broken capsules and half cap-sules. Only the broken capsule data are given in Table 8 (halfcapsules gave very large difference measures in excess of 90).

Test 3 considers a pill pack to determine if all eight pillsare present. The larger good object test data distance mea-sure was probably caused by the range of lighting, illumina-tion and reflectance from these objects. However, theseparation between accepted and rejected packs wassufficient to allow detection of a single missing pill in anylocation.

Test 4 considers detection of missing caps on soda bottles.Here the difference between the averages of the acceptedand rejected bottles is quite large (a factor of 5).

In test 5, missing or partial aspirin safety seals aredetected. The range of distance measures corresponds tohalf seals (performance measure 7), no seals (performancemeasure 11), no seals or lid (performance measure 15).

Optical and Simulation Test Data

Table 9 includes quantitative data on the results of simula-tions and tests on the same product using the optical test sys-tem. The same image frame from memory through thecamera was used in both the optical and digital tests. Thereare several sources of differences expected in the optical

system. These include: (1) the modulation characteristics ofthe spatial light modulator, which produces its own imagedifferent from the digital one; (2) a nonuniform spatial lightmodulator response; and (3) a nonuniform spatial light mod-ulator illumination, causing an additional weighting of theoptical Fourier transform pattern. These effects have beenreduced to small levels.

Proper sampling of the optical Fourier transform patternin time and synchronization was used to reduce temporaldecay effects of the input light modulator pattern and hasyielded reproducible results. The present data do notinclude phase correction of the light modulator. The tempo-ral decay and phase correction, together with the fact thatthe optical system automatically performs interpolation (notincluded in the digital simulations) are the major sources ofthe differences in the absolute distance measures obtainedoptically and in simulation. The separation betweenaccepted and rejected products remains good in all cases,with the ratio of the average distortion measures for rejectedand accepted products being better with the optical system(in some cases) and better for the simulations (in othercases).

Test 6 in Table 9 concerns inspection of frozen dinners forerrors such as spillage (of one item into another compart-ment), missing items and partially filled compartments. Therange of distance measures for the defects was larger with

TABLE 9. Fourier space tests: average and range of performance measure for optical and simulation tests

Remarks

partially full item hard to detectpartially full item hard to detectwell suited to Fourier analysiswell suited to Fourier analysis

LEGEND5: SIMULATED0: OPTICAL

TrainingTest and Good Various Test Discontinuity

Number Product Test Discontinuities Remarks

6 Frozen dinners S. 1.83 6.3 (2.7-7.81 spillage, missing011.8 3.6(2.2-5.0) partially

7 Toilet paper S: 2.0 7.6 creases0: 3.6 15. 1 bad embossing

TestNumber

9

Training Good TestParameters Set (5) Objects (2)

Average 3.4 4.83Range 2.2 to 5.0 4.75 to 5.0

Thin CrackDiscontinuities (7)

4.73.4 to 6.5

Cracks f 14f

105.5 to 1411 to 17

Breaks (7) Remarks

13.7 thin cracksnot easily detected


TABLE 10. Testing corn on the cob: average and range of performance measure for optical and simulation tests

Test Training Test Set Discontinuity: Discontinuity:Number Set Good

One Bad Kernel 7-8 Bad Kernels

8

5:2 (1.4 to 2.7)

2.2, 2.7

5.1 (3.4 to 7.8)

21 5(18 to 25)0: 4.2(2.9 to 4.5)

4, 8

6.4 (4.2 to 9.3)

18.8(11 to 25)

LEGEND5: SIMULATED0: OPTICAL

TABLE 11. Optical testing of candy bars for cracks (number of test objects in parentheses)

simulations than optically, with the simulated values beingmore separated from the training set data than were the opti-cal values. However, the agreement in both cases is good.Detecting partially filled compartments was the most diffi-cult discontinuity to locate.

Test 7 is an inspection of toilet paper for creases and badembossing. This is well suited for Fourier analysis and herethe ratio of the average measures were comparable for theoptical (15.1 / 3.6 = 4.2) and the simulated (7.6 / 2.0 3.8)data, although the absolute values are different.

In Fig. 55, sample data for the frozen dinner tests areshown: (1) the crosses represent the sampled training setdata mean; (2) the vertical extent of the crosses denotes thestandard deviation of these data and (3) the solid line repre-sents the measured data for the discontinuity. As seen, thevalues for the fifth, sixth and thirteenth Fourier samples(denoted by arrows) are greater than one standard deviationfrom the training set values. This is sufficient to increase thedistance measure and allow detection of this discontinuity.

Case Studies

Table 10 shows the inspection of corn on the cob for thepresence of bad (discolored) kernels. 15' 26 The training dataset was a small sample (six or seven items). Here, the datafor accepted corn are listed separately. In general, these val-ues are consistent with the average and range of training setvalues, except for the second optical sample. Its large dis-tance measure (8 or twice the training set average) may bedue to the irregularity and variability of the sizes and patternsexpected of kernels on different ears of corn. This good testear would be classified as an error (with the normally lowthreshold of 5 to 6 used). Such a problem should be elimi-nated by use of a larger training set size, which is expectedfor Fourier analysis of such objects with large anticipated

differences. Still, there is sufficient indication that in manycases single defective kernels can often be detected andwhen good ears are rejected, they are often ones withdeformed or very irregular kernel patterns. When more badkernels are present, detection is easier (as shown in the tablefor the case of samples with 7 to 8 bad kernels). The averagedistance measure for the simulated and optical data in thiscase are similar.

Table 11 details test 9 and the inspection of candy bars forcracks and breaks (optical data only). A small sample set (35candy bars) was used to obtain the initial results and to indi-cate trends. The training set data gave an average measureof 3.4 with the largest value being 5.0 (differences werecaused by shading and coloration). The test set (two sam-ples) gave a large average (4.9) with no value above 5.0. Testson thin cracks gave a comparable average (4.7), with a rangeof measures from 3.4 to 6.5. In general, such thin cracks givemeasures too close to and overlapping those of the trainingset to allow easy detection. This may require higher sam-pling and contrast ratio to detect such discontinuities.Larger cracks and breaks gave larger distance measures,allowing their easy detection. As before, global discontinu-ities (cracks in any location) can be detected even when theyare a small percent of the field of view.

Table 12 presents a test problem in which the presenceof a prize in a box of candy is determined. This problem

TABLE 12. Optical testing for prizes in candy boxes

Test Training Test DataNumber Prizes Prize Remarks

Absent (7) Present (4)

10 2.5 to 5.9 3.9 to 57 train on discontinuity9 of 14 detected

rmaa rumu rut VI3UAL It5IiI\

TABLE 13. Optical inspection of syrup bottles (5 - 7 samples per set)

TestNumber

TrainingSet

Small AmountMissing Liquid

Medium AmountMissing Liquid

Large AmountMissing Liquid

No LabiTest f 7

11 1.6 to 3.8 4 7 to 7 5.6 to 1 1.4 14.7 to 22.6 14.3 to i ;

represented considerable variability in the measure for thetraining set data (because the prize can be in many positions,sometimes with print showing on the prize envelope and atother times without any text present). The training proce-dure was reversed in this case (training performed on a boxwith no toy) because this datum point was much more con-sistent, with distance measures varying from 2.5 to 5.9. Thetest data then involved evaluation of boxes with the prize indifferent locations. The distance measure obtained rangedfrom 3.9 to 5.7. With a threshold of 7, 9 of 14 discontinuitiestested were detected.

Such performance is expected from most vision systemsbecause, when the prize lies against right or left walls of thebox, it is not visible. When it lies at an angle across the box,no text is visible and the box appears empty However, per-formance was best when the training procedure was reversed(training on a discontinuity) with large measures indicatinggood products (that differ from the no prize or empty boxtraining set).

Table 13 represents the optical test of syrup bottles for theproper amount of liquid and for the presence of labels.These tests were successful in determining various amountsof missing liquid and missing labels. In other data (notshown), in the good test set (only two samples), one bottlegave a large (7.5) distance measure. Tests for discontinuitiessuch as missing caps and the spout up on the syrup gave sur-prisingly low (2.3 to 3.8) distance measures. Further analysis

is needed to determine the origin of these values.sample sizes and the need to include discontinuitiestraining set, may be required for reliable detection. Ha number of different discontinuities were succdetected in initial tests.

ConclusionA Fourier transform feature space has been consith

nondestructive optical testing. The dimensionalitytion of this feature space makes its analysis sim feffective. The use of an optical processor to prodifeature space makes the system fast and inexpensive.ple true class training set concept was described asthe system easy to set up and use (with only a singlelated output performance measure required). A wideof test problems were provided with various discontin each,

The results were promising and indicate the sigpotential for this technique. Such a system allows deof discontinuities located anywhere in the input fieldIt also allows detection of irregular and unpredicted dnuities. The area sampling nature of optical processortors allows sufficient tolerance to rotations of the in jobject. The intensity nature of the optically detecterier transform provides shift invariance.

FIGURE 56. Principles of acetate tape replicationproducing a negative image of the surface:(a) microstructure cross section, IN softenedacetate tape applied, (cJ replica curing and(d) replica removal

(a) p"--r—j—'---"1-1

(b)

(C)

(d)


PART 5 REPLICATION

Replication is a valuable tool for the analysis of fracturesurfaces and microstructures and for documentation of cor-rosion damage and wear. There is also potential for uses ofreplication in other forms of surface testing.

Replication is a method used for copying the topographyof a surface that cannot be moved or one that would be dam-aged in transferal. A police officer making a plaster cast of atire print at an accident scene or a scientist malting a cast ofa fossilized footprint are common examples of replication.These replicas produce a negative topographic image of thesubject known as a single stage replica. A positive replicamade from the first cast to produce a duplicate of the originalsurface is called a second stage replica.

Many replicating mediums are commercially available.The types commonly used in nondestructive testing typicallyfall into one of two categories: cellulose acetate replicas andsilicone rubber replicas. Both have advantages and limita-tions but both can also provide valuable information withoutaltering the test object.

Cellulose Acetate ReplicationAcetate replicating material is used for surface cleaning,

removal and evaluation of surface debris, fracture surfacemicroanalysis and for microstructural evaluation. Singlestage replicas are typically made, creating a negative imageof the test surface. A schematic diagram of microstructuralreplication is shown in Fig. 56.

Cleaning and Debris Analysis

Fracture surfaces should be cleaned only when necessary.Cleaning is required when the test surface holds loose debristhat could hinder analysis and that cannot be removed with athy air blast.

Cleaning debris from fracture surfaces is useful when thetest object is the debris itself or the fracture surface. Debrisremoved from a fracture can be coated with carbon and ana-lyzed using energy dispersive spectroscopy. This provides asemiquantitative analysis when a particular element is sus-pected of contributing to the fracture.

Removal of loose surface particles is usually done by wet-ting a piece of acetate tape on one side with acetone,allowing a short period for softening and applying the wetside of the tape to the area of interest. Thicker tapes of0.013 mm (0.005 in.) work best for such cleaning applications

(thin tapes tend to tear). Following a short period, the tapehardens and is removed. This procedure is normallyrepeated several times until a final tape removes no debrisfrom the surface.

Fracture Surface Analysis

The topography of fracture surfaces can be replicated andanalyzed using an optical microscope, scanning electronmicroscope or transmission electron microscope. The maxi-mum useful magnification obtained using optical micro-scopes depends on the roughness of the fracture but seldomexceeds 100 x . The scanning electron microscope has gooddepth of field at high magnifications and is typically used formagnification of 10,000 x or less. The transmission electronmicroscope has been used to document microstructuraldetails up to 50,000 x .

In general, scanning electron microscope analysis of a rep-lica provides information regarding mode of failure and, in

FIGURE 57. Fracture surface documentation usingreplication shows fatigue striations on the surfaceat magnifications originally of (a) 2,000 x and(b) 10,000 x

(a)

(b)


most instances, is sufficient for completion of this kind ofanalysis. An example of a replicated fracture surface isshown in Fig. 57. The transmission electron microscope isused in instances where information regarding dislocationsand crystallographic planes is needed. Both single stage(negative) and second stage (positive) replicas can be usedfor failure analysis. Some scanning electron microscopemanufacturers offer a reverse imaging module that providespositive images from a negative replica. This eliminates theneed to think and interpret in reverse. This feature has alsoproven valuable for evaluating microstructures throughreplication.

As with the removal of surface debris, it has been foundthat the thicker replicas provide better results, for the samereasons. The procedure for replication of fracture surfacesis identical to that for debris removal. On rough surfaces,however, difficulty may be encountered when trying to

remove the replica. This can cause replication material toremain on the fracture surface but this can easily be removedwith acetone.

Replicas, in the as-stripped condition, typically do notexhibit the contrast needed for resolution of fine microscopicfeatures such as fatigue striations. To improve contrast, shad-owing or vapor deposition of a metal is performed. Themetal is deposited at an acute angle to the replica surface andcollects at different thicknesses at different areas dependingon the surface topography. This produces a shadowing thatallows greater resolution at higher magnifications.

Shadowing with gold or other high atomic number metalsenhances the electron beam interaction with the sample andgreatly improves the image in the scanning electron micro-scope by reducing the signal-to-noise ratio.

Microstructural InterpretationTo date, the greatest advances in the use of acetate replicas

for nondestructive testing have come from their use in micro-structural testing and interpretation. Replication is an inte-gral part of visual tests in the power generation industries aswell as in refining, chemical processing and pulp and paperplants. Replication, in conjunction with microstructuralanalysis, is used to quantify microstrain over time and to pre-dict the remaining useful life of a component. Future appli-cations are not limited by material type.

In industry, tests are carried out at preselected intervals toassess the structural integrity of components in their sys-tems. These components can be pressure vessels, piping sys-tems or rotating equipment. Typically these components areexposed to stresses or an environment that limits their ser-vice life. Replication is used to evaluate such systems and toprovide data regarding their metallurgical condition.

Microstructural replication is done in two steps: surfacepreparation followed by the replication procedure. Surfacepreparation involves progressive grinding and polishing untilthe test surface is relatively free of scratches (metallurgicalquality). Depending on the material type and hardness, thiscan be obtained by using a I to 0.05 p.m (0.04 to 0.002 mil)polishing compound as the final step. Electrolytic polishingcan increase efficiency if many areas are being tested. Sur-faces can be electropolished with a 320-400 grit finish.

The disadvantages of electropolishing are that (1) theequipment is costly, (2) with most systems only a small areacan he polished at one time and (3) pitting has been knownto occur with some alloy systems containing large amounts ofcarbides.

Next, the polished surface is etched to provide microstruc-tura' topographic contrast which may be necessary for evalu-ation. Etchants vary with material type and can be appliedelectrolytically, by swabbing or spraying the etchant onto thesurface. With some materials, a combination of etch-polish-etch intervals yields the most favorable results.

FIGURE 58. Comparison of optical microscopyto scanning electron microscopy in thedocumentation of a replicated microstructure;evidence of creep damage is visible in the grainboundaries; etchant is aqua regia; 100 x originalmagnification: (a) optical microscope image and(b) scanning electron microscope image

(14

(a)


To replicate the surface microstructure, an area is wettedwith acetone and a piece of acetate tape is laid on the surface.The tape is drawn by capillary action to the metal surface,producing an accurate negative image of the surface micro-structure. Thin acetate tape at 0.025 mm (0.001 in.) providesexcellent results and gives the best resolution at high magni-fications. Thicker tapes must be pressed onto the test sur-face and, depending on the expertise of the inspector,smearing can result. Thicker tapes are more costly and theresolution of microscopic detail does not match thinnertapes. Studies of carbide morphology and creep damagemechanisms have been performed at magnifications as highas 10,000 x with thin tape replicas.

Before removal of the tape from the test object, the backis coated with paint to provide a reflective surface thatenhances microscopic viewing. The replica is removed andcan be stored for future analysis.

If analysis with the scanning electron microscope isneeded, replicas should be coated to prevent electron charg-ing. This is accomplished by evaporating or sputter coatinga thin conductive film onto the replica surface. Carbon, gold,gold-palladium and other metals are used for coating. Thereare differences in the sputtering yield from different ele-ments and this should he remembered when choosing anelement or when attempting to calculate the thickness of thecoating.

The main advantage of sputter coating over evaporationtechniques is that it provides a continuous coating layer.Complete coating is accomplished without rotating or tiltingthe replica. With evaporation, only line of sight areas arecoated and certain areas typically are coated more thanothers.

Some examples of replicated microstructures, docu-mented with both a scanning electron microscope and withconventional optical microscopy, are shown in Figs. 58 to 61.Replication is used for detection of high temperature creepdamage, stress corrosion cracking, hydrogen cracking mech-anisms, as well as the precipitation of carbides, nitrides andsecond phase precipitates such as sigma or gamma prime.Replication is also used for distinguishing fabrication discon-tinuities from operational discontinuities.

Strain ReplicationThe replication technique can be used to evaluate and

quantify the occurrence of localized strain in materialsexposed to elevated temperatures and stresses over time(materials susceptible to high temperature creep), Replica-tion allows monitoring for accumulated strain before detect-able microstructural changes occur. Strain replicationinvolves inscribing a grid pattern onto a previously polishedsurface. A reference grid pattern is replicated using materialwith a shrinkage factor that has been quantified through

analysis. This known shrinkage factor is included in futurenumerical analysis of strain. The grid is then coated to pre-vent surface oxidation during use.

After a predetermined period of operation, the coating isremoved and the area is again replicated. The grid intersec-tion points on the two replicas are compared for dimensionalchanges and the changes are then correlated to units ofstrain. This technique does not yield absolute values of strainbut does provide the change in strain calculated over time.This gives the operator information that can help approxi-mate where the component is in its service life.

FIGURE 59. Documentation of creep damage:(al a weld viewed originally at 500 x in an opticalmicroscope; the microstructure consists of anaustenitic matrix, precipitated nitrides andcarbides; linked creep voids can be observed; andlb) the alloy in Fig. 58 viewed originally at 1,000 xin a scanning electron microscope; grain boundarycarbides, creep voids and particles believed to benitrides can be observed in the matrix

(a)

BASIC AIDS AND ACCESSORIES FOR VISUAL TESTING / I I 1

Strain replication is especially useful for materials that donot exhibit creep void formation until late in their servicelife. As long as the strain calculations indicate a linear rela-tionship between strain and time, the material is still said tobe in the second stage region on the creep curve (see Fig. 62and Table 14). When the relationship deviates from linearity,the material has begun third stage or tertiary creep, wherethe strain rate can become unstable.

FIGURE 60. Documentation of stress corrosioncracking found in the welds of an anhydrousammonia sphere; 3 percent nital etch at 200 xoriginal magnification

FIGURE 61. Documentation of heat affected zonecracking in A516 grade 70 steel; crackingassociated with a nonstress relieved repair weld;the presence of this repair weld was not knownuntil in-field metallography and replication wereperformed; 3 percent nital etch at 100 x originalmagnification

Silicone Rubber ReplicasSilicone impression materials have been used extensively

in medicine, dentistry and in the science of anthropology. Innondestructive testing, silicone materials are used as tools fordocumenting macroscopic and microscopic material detail.

FIGURE 62. Creep damage curve showing thetypical relationship of strain to time for a materialunder stress in a high temperature atmosphere;note that development of creep related voids inthis alloy occurs early in service life; their eventuallinkage is shown schematically on the curve [seereference 27): fa) isolated cavities, (b) orientedcavities, (c) microcracks and (d) macrocracks (seeTable 14)

EXPOSURE TIME

FIGURE 63. Silicon replicas used to determine wearvariance on a failed pinion gear


Quantitative measurements can be obtained for depth of pit-ting, wear, surface finish and fracture surface evaluation.

Silicone material is made with varying viscosities, settingtimes and resolution capabilities. Compared to an acetatereplica, the resolution characteristics of a silicone replica islimited. With a medium viscosity compound, fine featuresvisible at 50 x can be resolved but difficulties are encoun-tered at higher magnifications. With a low viscosity com-pound, slightly better resolution is obtained but curing timesare long and not suited to field applications. The lower vis-cosity medium is also known to creep with time and is notrecommended for applications where very accurate dimen-sional studies are needed.

Use of Silicone Replicating Materials

Silicone replicating materials are supplied in two parts: abase material and an accelerator. Although it is best to followthe recommended mixing ratios, these can be altered slightlyto change the working time of the material. The two partsare mixed thoroughly and spread over the subject area.Additional material can he added to thicken the replica.Molding clay can also be used to build a dam around a repli-cated area. The dam supports the replica as its sets andallows thicker replicas to be made.

Measurements of pit depth and surface finish can beobtained easily because of the silicone's ability to flow into

TABLE 14. Action required for creep damage in typicalstressed material (see Figure 62)

Damage Parameter Action Required

isolated cavities no action until next majorscheduled maintenance outage

Oriented cavities replica test at specihed intervalsMicrocracks limited service until repairMacrocracks immediate repair

crevices on the test object. To evaluate pit depth and surfacefinish, the replica is cut and the cross section is examinedwith a microscope or a macroscopic measuring device (amicrometer or an optical comparator).

Wear can be determined in a similar manner by replicat-ing and comparing a worn surface to an unworn surface (seeFig. 63). Fracture surfaces with rough contours can be easily


replicated with silicone (taking an acetate replica of such sur-faces is difficult). However, the resolution characteristics ofa silicone replica are not as good as acetate replicas and thislimits the amount of interpretation that can be performed.Macroscopic details such as chevron markings can be easilylocated with the silicone technique to determine crack prop-agation direction or to trace a fracture path visually to itsorigin.

ConclusionCellulose acetate tape and silicone impression materials

are commonly used for nondestructive visual tests of surface

phenomena such as corrosion, wear, cracking and micro-structures."' Both types of replicating material have advan-tages and limitations but when used in the correctapplication, can provide valuable information.

In terms of resolution, the silicone replica typically doesnot have the capability to copy fine detail above 50 x . Theacetate replica can reveal detail up to 50,000 x on a trans-mission electron microscope. The acetate replica is limited,however, by the roughness of the topography it can copy. Onrough fracture surfaces, difficulty is encountered in bothapplying and removing an acetate replica. The siliconematerial is not as restrictive in terms of the surface featuresit can cop_y. The need for fine, resolvable detail versus mac-roscopic features normally indicates whether acetate or sili-cone replicas are best for the application.

FIGURE 64, Temperature indicating pellets

f g


PART 6TEMPERATURE INDICATING MATERIALS

A temperature indicating stick (chalk or crayon) is typi-cally made of materials with calibrated melting points andtemperature measuring accuracies to ± 1 percent. Indica-tors are available in closely spaced increments over a rangefrom 38 °C (100 °F) to 1,370 °C (2,500 °F).

The workpiece to be tested is marked with the stick.When the workpiece attains the predetermined meltingpoint of the indicator mark, the mark instantly liquefies, noti-fying the observer that the workpiece has reached thattemperature.

Premarking with a stick is not practical under certain cir-cumstances—when a heating period is prolonged a highlypolished surface does not readily accept a mark or themarked material gradually absorbs the liquid phase of theindicator. In such instances, the operator frequently marksthe workpiece with the stick. The desired temperature isnoted when one ceases to make dry marks and begins toleave a liquid smear. A similar procedure can be employedto indicate temperature during a cooling cycle. But a meltedmark, on cooling, will not solidify at the exact same tempera-ture at which it melted, so solidification of a melted indicatormark cannot be relied on for temperature indication.

Temperature ratings are in increments as small as 3.4 °C(6 °F) but increments ranging from 14 to 28 °C (25 to 50 °F)are typically used for welding applications. For most appli-cations, a jump of 28 to 56 °C (50 to 100 °F) and a range ofsticks up to 650 °C (1,200 °F) are usually adequate.

Temperature indicating sticks were developed in Americaby a metallurgist working on submarine hulls in the 1930s.At the time, preheat was measured with so-called meltingpoint standards, granules of substances with known meltingpoints used to calibrate heat sensing instruments. The engi-neer used the granules directly, spreading them on the pre-heated metal and using their melt as a signal to proceed withwelding.

The melting point granules were next formed into sticksheld together with organic hinders. Different temperatureratings were added and some refinements have been madebut the principle of indicators has remained unchanged. Thesticks make physical contact with the heated test object,reach thermal equilibrium rapidly and do not conduct heataway from the test surface.

For temperature ratings less than 340 °C (650 °F), indica-tor marks can usually be removed with water or alcohol. Forratings above 340 °C (650 °F), water is preferred. If the markhas been heated well above the rated temperature and has

become charred, abrasion may be needed for completeremoval.

Other Temperature IndicatorsIn addition to the stick, temperature indicating pellets and

liquids are available. The liquid indicator is brushed onbefore welding starts and is useful on highly polished sur-faces or for making large marks viewed at a distance.

Heat indicating pellets, about the size and shape of anaspirin, have greater mass than stick or lacquer marks (seeFig. 64). Pellets are sometimes selected for use with large,heavy pieces requiring prolonged heating—applicationswhere stick or lacquer marks could fade with time.

Certification of Temperature IndicatorsTemperature indicating sticks are mixtures of organic and

inorganic compounds. The purity of the source materialsdirectly affects the accuracy of the predicted melting point.There is the possibility of contamination with trace quanti-ties of other elements, which may be detrimental to the accu-racy of the indicator. In some cases, low melting point

FIGURE 65. Pellets used to verify oventemperatures over 450 °C (850 °C1


materials (lead, tin, sulfur, halogenated compounds) may beundesirable for the welding procedure.

Most manufacturers can provide certification supportedby analyses of typical batches. Documentation indicateswhich temperature ratings may contain contaminants thatcan be avoided by the user.

In some critical applications (nuclear fabrication, aircraftassembly), actual chemical analysis of the specific lot numberof the temperature indicators may be required. If the cus-tomer supplies a written certification requirement listing thecompounds to be tested for, most manufacturers will send lotnumbered samples for laboratory analysis. The customer isusually expected to pay lab charges for such specializedrequirements.

Marking materials used on austenitic stainless steels typi-cally have a certified analysis that meets the following speci-fied maximum amounts of detrimental contaminants:

1. inorganic halogen content less than 200 ppm by weight;2. halogen (inorganic and organic) content less than

1 percent by weight;3. sulfur content less than 1 percent by weight (measured

in accordance with ASTM D129); and4. total content of low melting point metal (lead, bismuth,

zinc, mercury, antimony and tin) less than 200 ppm byweight and no individual metal content greater than50 ppm by weight.

The certification typically indicates the methods and accu-racy of analysis and the name of the testing laboratoiy.

Applications for Temperature IndicatorsTemperature indicators can be used for preheat tempera-

ture tests and in annealing and stress relieving procedures,hardfacing, overlaying for corrosion resistance, flame cut-ting, flame conditioning, heat treating, pipe bending, shear-ing of bar steel, straightening hardened parts, shrink fitting,brazing, soldering and nonferrous fabrication. The indica-tors can help find hot spots in insulation and engines, helpmonitor temperatures in curing and bonding operations andhelp check pyrometric calibration.

Tests of Railway Bearings

Bearing breakdown can be detected by using fluorescenttemperature indicating pellets as heat sensors for inboardjournal boxes. The pellets are inserted in a specially fabri-cated stainless steel holder that contains two pellets. Theholder is inserted into the hollow axle of each rail car with aninsertion tool. The tool has a mechanical stop to ensure that

the holder is located at a predetermined depth. This permitsproper monitoring of journal box operating temperatures.

Once a specified temperature is exceeded, in this case100 °C (212 °F), the pellets melt and flow completely out ofthe holder. The fluorescent material is easy to detect andclearly indicates that excessive heat has been conductedfrom the bearing to the axle.

Verifying Oven Temperatures

Technicians can determine if self cleaning ovens reach theproper cleaning temperature using pellets with precalibratedmelting points at 450 °C (850 °F). The pellets are placed ona flat piece of aluminum foil situated on the oven's centerrack (see Fig. 65). The cleaning cycle is activated and as thetemperature reaches 450 °C (850 °F), the pellets begin tomelt.

When the cleaning cycle is completed and the oven hascooled, the pellets are inspected—complete melting of thetablet verifies that the nominal cleaning temperature hasbeen achieved.

Process Control Applications

A gas tight seal is needed to prevent leakage of combustiongases through the glass portion of a spark plug. To obtainoptimum fusion properties, it is important to know and con-trol the temperature inside the ceramic insulator and thiscan be done using a temperature indicating pellet. Sample

FIGURE 66. Laminate postforming around aradiused core


insulators are loaded with pellets and processed with pro-duction parts. Information obtained from analyzing the sam-ples is used to adjust furnace conveyor speed andtemperature.

Monitoring Fabric Seam Temperature

In the making of specialized cloth (protective clothing,aerostat balloons), seam integrity is an important manufac-turing function. A good radiofrequency seal can be achievedon a given fabric substrate only within a specific temperaturerange, determined by the minimum temperature needed toensure a complete seal and the maximum temperature possi-ble before material degradation. Constant temperature con-trol and verification are required.

This can be achieved using temperature sensitive strips(one for the upper limit, one for the lower limit) applied tothe sealing tape used in production. A visual test of eachseam after sealing indicates whether the seam temperaturewas within the required range, allowing visual verification ofconditions for all dielectric seams.

Precise Postforming Heat Control

Temperature indicating materials are incorporated intomany industrial applications where an indication is neededto show that a critical temperature has or has not beenreached. A phase changing fusible liquid is used to indicateoptimum postforming temperatures when bending decora-tive laminate for the contoured edges of countertops, desks,tables and other surfaces (see Fig. 66).

Postforming is the process of bending a flat sheet of lami-nate around a radiused core material (particle board, ply-wood or fiber board). The process is typically done aftercontrolled heating monitored with temperature sensitive liq-uids. Postforming can be a manual or mechanical operation.Hand postforming is used for unusual configurations or lim-ited quantity production and mechanical postforming is usedfor high quantity production. Both methods have the needfor a heat source, prepared cores, postforming grades of dec-orative laminate, pressuring guides and evenly appliedpressure.

A core is prepared by first shaping the edges to be lami-nated. The core and laminate are evenly coated with acontact adhesive, preferably a spray. The laminate is posi-tioned and registered with the core, allowing the laminate tooverhang the radius. Postforming grades of decorative lami-nate are formable between temperatures of 156 and 163 °C(313 and 325 °F).

A popular example of hand postforming is the 180 degreeedge wrap. In this example, radiant heat is applied to thedecorative surface of the laminate with the work supportedover the heat. To determine the proper postforming temper-ature, the temperature indicating liquid is painted in stripes

onto the laminate. When the liquid changes from a dry(matte) to a wet (melted) appearance, the assembly is wipedinto the cavity of a fixture to form the 180 degree radius. Thefixture is a U channel made by two boards attached to a base.The dimension of the U channel is the thickness of the coreplus the thickness of the laminate, allowing about 0.5 mm(0.02 in.) clearance.

Another example of handforrning is known as a full wrap.In this application, the core is positioned over radiant heaterswith temperature indicating stripes painted on the adhesivein the area of the radius. When the melt indicates formingtemperature has been reached, the assembly is moved backonto a flat supporting surface. The wrapping action uses theflat surface as a pressure point.

An example of mechanical postforming is the roll formingmachine. Radiant heaters are located above an assemblysupported by a moving carrier. When the forming tempera-ture has been reached, slanted forming bars wipe the lami-nate over the radius. After the laminate has been formed, asuccession of rollers maintains pressure until the assemblyhas cooled. In this application, temperature sensitive liquidis painted onto the laminate in order to verify that the dwell

FIGURE 67. Temperature indicating stick


time under heat has been sufficient for reaching formingtemperature.

Pipeline CoatingsEpoxy powders are specially formulated to enhance corro-

sion proof resistance of utility pipe: that is, pipe usually bur-ied underground, where it is subject to widely varyingpipeline operating conditions. Intimately bonded to thepipe, the bonded epoxy is unaffected by widely varying soilcompaction, moisture penetration, fungus attack, soil acidsand chemical degradation.

To achieve a long lasting bond of epoxy coating to metalpipe, the pipe must be preheated very carefully to the rec-ommended preheat of 230 °C (450 °F). A spot on the pipeneeds to be touched with the stick; its melting shows that thecorrect temperature for coating has been reached.

Preheating before WeldingHeating to the proper temperature before welding lessens

the danger of crack formation and shrinkage stresses in manymetals. Hard zones near the weld are reduced and lessen thepossibility of distortion. Preheating also helps diffuse hydro-gen from steel and helps reduce the likelihood of subsequenthydrogen inclusions.

The need for preheating increases with the mass of thematerial being welded. It is most useful for the thick, heavyweldments used in bridge construction, shipbuilding, pipe-lines and pressure vessels. Preheating is also recommendedfor (1) welding done at or below – 18 °C (0 °F); (2) when theelectrode is a small diameter; (3) when the joined pieces areof different masses; (4) when the joined pieces are of com-plex cross section; and (5) for welding of high carbon or man-ganese steels.

The most common use for temperature indicators is themeasurement of preheat, postheat and interpass tempera-tures for welding. In a typical application, the welder marksthe test surface with an indicating stick of a specific tempera-ture rating (see Fig. 67). When the mark changes phase(melts), the material has reached the correct temperatureand is ready for welding. It is important for the user tounderstand that change of color has no significance; only theactual melting of the mark should be considered.

Oxyacetylene equipment cannot be used for welding orcutting of high strength steels used in automotive compo-nents because too much heat can reduce their structuralstrength. However, in some instances an oxyacetylene torchmay be used if the critical temperature of 760 °C (1,400 °F)for high strength steel is not exceeded.

When preheat temperatures are 370 °C (700 °F) or whenheating is prolonged, an indicating mark could evaporate orcould be absorbed by the test material, Under these condi-tions, marks should be added periodically during heating.When the rated temperature is reached, the stick leaves a liq-uid streak instead of a dry mark and welding can begin.

To ensure accurate temperature indication with no over-ride, two or more indicators can be used to alert the operatorthat the test object is approaching the correct temperature.When a range of recommended preheat temperatures isgiven, use of several indicators might be appropriate. Forexample, carbon-molybdenum steel should he preheated tobetween 95 and 205 °C (between 200 and 400 °F). A bundleof indicators with ratings at 95, 120, 150, 175 and 205 °C(200, 250, 300, 350 and 400 °F) might be useful fordetermining how much of the test object is within the pre-heat temperature range.

FIGURE 69. Improper surface preparation; thegrind marks mask indications and even a severeetchant does not give good test results


PART 7

CHEMICAL AIDS

The information contained in this text is simplified andprovided only for general instruction. Local health (OSHA)and environmental (EPA) authorities should be consultedabout the proper use and disposal of chemical agents. Forreasons of safety, all chemicals must he handled with care,particularly the concentrated chemicals used as aids to visualand optical tests.

In visual nondestructive testing, chemical techniques areused to clean and enhance test object surfaces. Cleaningprocesses remove dirt, grease, oil, rust and mill scale. Con-trast is enhanced by chemical etching.

Macroetching is the use of chemical solutions to attackmaterial surfaces to improve the visibility of discontinuitiesfor visual inspection at normal and low power magnifications.Caution is required in the use of these chemicals—the useof protective clothing and safety devices is imperative. Testobject preparation and the choice of etchant must be appro-priate for the inspection objectives. Once the desired etch isachieved, the metal surface must be flushed with water toavoid over etching.

Test Object SelectionFigure 68 shows typical test objects removed from their

service environment. Governing codes, standards or speci-fications may determine the number and location of visualtests. Specific areas may contain discontinuities from form-ing operations such as casting, rolling, forging or extruding.Weld tests may be full length or random spots and typicallycover the weld metal, fusion line and heat-affected zone.The service of a component may also indicate problem areasrequiring inspection.

Location of the test site directly affects surface prepara-tion. The test site may he prepared and nondestructivelyinspected in situ. Removal of a sample for laboratory exami-nation is a destructive alternative test method that typicallyrequires a repair weld.

Surface PreparationPreparation of the test object before etching may require

only cleaning or a process including cleaning, grinding andfine polishing (improper grinding is shown in Fig. 69). The

FIGURE 68. Components removed from service forvisual testing

extent of these operations depends on the etchant, the mate-rial and the type of discontinuity being sought.

Solvent Cleaning

Solvent cleaning can be useful at two stages in test objectpreparation. An initial cleaning with a suitable solventremoves dirt, grease and oil and may make rust and mill scaleeasier to remove.

One of the most effective cleaning solvents is a solution ofdetergent and water. However, if water is detrimental to thetest object, organic solvents such as ethyl alcohol, acetone ornaphthas have been used. These materials generally havelow flash points and their use may be prohibited by safetyregulations. Safety solvents such as the chlorinated


TABLE 15. Chemicals for etchants

ChemicalType

AcidAcidAcidAcidAcidBaseBaseSaltSaltSaltSaltSaltSaltElementAlcoholAlcoholAqueous

NameConcentration'percentage) Formula

hydrochloric (muriatic)hydrofluoricnitricphosphoricsulfuricammonium hydroxidesodium hydroxideammonium persulfateammonium sulfatecopper ammonium chloridecupric chlorideferric chloridepotassium iodideiodineethanol (ethyl alcohol)methanol (methyl alcohol)hydrogen peroxide

374870

30

HCIHFHNO3H,PO4H 250,NH 4OHNaOH(NH4 ) 2S20,(NH4) 25042NH4CI •CuC1 2.2H 20CuCl zFeCl 3KII.

C,1-150HCH3OHH 202

hydrocarbons and high flash point naphthas may be requiredto meet safety standards.

Removing Rust and Scale

Rust and mill scale are normally removed by mechanicalmethods such as wire brushing or grinding. If appropriatefor a particular test, the use of a severe etchant requires onlythe removal of loose rust and mill scale. Rust may also beremoved chemically. Commercially available rust removersare generally inhibited mineral acid solutions and are notoften used for test object preparation.

Most surface tests require complete removal of rust andmill scale but a coarsely ground surface is often adequatepreparation before etching.

Grinding may be done manually or by belt, disk or surfacegrinding tools. Surface grinders are usually found only inmachine shops. Hand grinding requires a hard flat surfaceto support the abrasive sheet. Coolant is needed duringgrinding and water is the preferred coolant but kerosene maybe used if the test material is not compatible with water.

Grinding and Polishing

Fine grinding and polishing are needed for visual tests ofsmall structural details, welds and the effects of heat treat-ment. Finer grinding usually is done with 80 to 150 abrasivegrit followed by 150 to 180 grit and finally 400 grit (an Ameri-can indication of grit size, 400 being the finest). At eachstage, marks from previous grinding must be completely

removed. Changing the grinding direction between succes-sive stages of the process aids the visibility of previouscoarser grinding marks. Coolant is required for grinding andtypical abrasives include emery, silicon carbide, aluminumoxide and diamond.

If the required finish cannot be achieved by fine grindingwith 400 grit abrasive, the test surface must be polished. Pol-ishing is generally done with a cloth-covered disk and abra-sive particles suspended in paste or water. Commonpolishing media include aluminum oxide, magnesium oxide,chromium oxide, iron oxide and diamond with particle sizesranging from 0.5 to 15 p.111.

During polishing, it is critical that all marks from the previ-ous step he completely removed. If coarser marks do notclear, it may be necessary to repeat a previous step usinglighter pressure before continuing. Failure to do so can yieldfalse indications.

Etching

Choice of Etchant

The etchant, its strength, the material and the discontinu-ity all combine to determine surface finish requirements (seeTable 15). Properly selected etchants chemically attack thetest material and reveal welds (Fig. 70), pitting (Fig 71),grain boundaries, segregation, laps, seams, cracks aria heataffected zones. The indications are highlighted or con-trasted with the surrounding base material.

FIGURE 71. Effect of etching: (a) unetchedcomponent with shiny appearance at rolled areaand (t)) pits are visible in the dulled area afteretching with ammonium persulfate

fa/

FIGURE 70. Example of contrast revealing a weldin stainless steel

11111110mmiiii


Safety Precautions

Etchants are solutions of acids, bases or salts in water oralcohol. Etchants for macroetching are water based. Etch-ing solutions need to be fresh and the primary concerns dur-ing mixing are safety concentration and purity

Safety precautions are necessary during the mixing anduse of chemical etchants. Chemical fumes are potentiallytoxic and corrosive. Mixing, handling or using etchantsshould be done only in well ventilated areas, preferably in anexhaust or fume hood. Use of an exhaust hood is mandatorywhen mixing large quantities of etchants. Etching largeareas requires the use of ventilation fans in an open area oruse of an exhaust hood.

Contact of etchants with skin, eyes or clothes should beavoided. When pouring, mixing or handling such chemicals,

protective equipment and clothing should be used, includingbut not limited to glasses, face shields, gloves, apron or labo-ratory jacket. A face-and-eye wash fountain is recommendedwhere chemicals and etchants are sorted and handled. Asafety shower is recommended when large quantities ofchemicals or etchants are in use.

Should contact occur, certain safety steps must be fol-lowed, depending on the kind of contact and the chemicalsinvolved. Skin should be washed with soap and water.Chemical burns should have immediate medical attention.Eyes should be flushed at once with large amounts of waterand immediate medical attention is mandatory. Hydroflu-oric and fluorosilic acids cause painful burns and seriousulcers that are slow to heal. Immediately after exposure, theaffected area must be flooded with water and emergencymedical attention sought.

Other materials that are especially harmful in contact withskin are concentrated nitric acid, sulfuric acid, chromic acid,30 and 50 percent hydrogen peroxide, sodium hydroxide,potassium hydroxide, bromine and anhydrous aluminumchloride, These materials also produce vapors that causerespiratory irritation and damage.

Containers

Containers used with etchants must be rated for mixing,storing and handling of chemicals. Glass is resistant to mostchemicals and is most often used for containment and stir-ring rods. Hydrofluoric acid, other fluorine based materials,strong alkali and strong phosphoric acids can attack glass,requiring the use of inert plastics.

Generation of Heat

Heat may be generated when chemicals are mixedtogether or added to water. Mixing chemicals must be doneusing accepted laboratory procedures and caution. Strongacids, alkalis or their concentrated solutions incorrectlyadil.ed to water, alcohols or other solutions, cause violentchemical reactions. To be safe, never add water to concen-trated acids or alkalis.

In general, the addition of acidic materials to alkalinematerials will generate heat. Sulfuric acid, sodium hydroxideor potassium hydroxide in any concentration generate largeamounts of heat when mixed or diluted and an ice bath maybe necessary to provide cooling. Three precautions in mix-ing can reduce or prevent a violent reaction:

1. add the acid or alkali to the water or a weaker solution;2. slowly introduce acids, alkali or salts to water or solu-

tion; and3. stir the solution continuously to prevent layering and a

delayed violent reaction.


Concentration

The strength of an acid, base or salt in solution isexpressed by its concentration or composition. Etchants aretypically mixtures of liquids or solids in liquids. The concen-tration of liquid mixtures is expressed as parts or percent byvolume.

For solids in liquids, units of concentration are parts orpercent by weight. Generally, etchants are mixed in smallquantities. Table 15 lists the reagents and quantities neededto make a variety of etchants.

Chemical Purity

Chemicals are available in various grades of purity rangingfrom technical to very pure reagent grades. For etchants, thetechnical grade is used unless a purer grade is specified. Formacroetchants, the technical grade is generally adequate.

Water is the solvent used for most macroetching solutionsand water purity can affect the etchant. Potable tap watermay contain some impurities that could affect the etchant.Distilled water has a significantly higher purity than tapwater. For macroetchants using technical grade chemicals,potable tap water is usually acceptable. For etchants inwhich high purity is required, distilled water isrecommended.

Disposal

Before disposing of chemical solutions, check environ-mental regulations (federal, state and local) and safetydepartment procedures. The steps listed here are used onlyif there are no other regulations for disposal. Spent etchantsare discarded and must be discarded separately—mixing ofetchant materials can produce violent chemical reactions.

Using a chemical resistant drain under an exhaust hood,slowly pour the spent etchant while running a heavy flow oftap water down the drain. The drain is flushed with a largevolume of water.

Using EtchantsAfter proper surface preparation and safe mixing of

etchants, the application of etchants to the test object may bedone with immersion or swabbing. The technique is deter-mined by the characteristics of the etchant being used.

immersion

During immersion, a test object is completely covered byan etchant contained in a safe and suitable material—glass

can be used for most etchants except hydrofluoric acid, fluo-rine materials, strong alkali and strong phosphoric acid.

A glass heat resistant dish on a hot plate may be used forheated solutions. The solution should be brought to temper-ature before the test object is immersed. Tongs or other han-dling tools are used and the test object is positioned so thatthe test surface is face up or vertical to allow gas to escape.The solution is gently agitated to keep fresh etchant in con-tact with the test object.

Swabbing

Etching may also be done by swabbing the test surfacewith a cotton ball, cotton tipped wooden swab, bristled acidbrush, medicine dropper or a glass rod. The cotton ball andthe cotton tipped wooden swab generally are saturated withetchant and then rubbed over the test surface.

Tongs and gloves should be used for protection and theetchant applicator must be inert to the etchant. For exam-ple, strong nitric acid and alkali solutions attack cotton andthese etchants must be applied using a fine bristle acidbnish. A glass or plastic medicine dropper may be used toplace etchants on the test object surface and a suitable stir-ring rod can be used to rub the surface. The test object maybe immersed in etchant and swabbed while in the solution.

Etching Time

Etching time is determined by (1) the concentration of theetchant, (2) the surface condition and temperature of the testobject and (3) the type of test material (see Tables 16 and17). During etching, the material surface loses its brightappearance and the degree of dullness is used to determinewhen to stop etching. Approximate dwell times are given inthe table procedures but experience is important as well.

Test Object Preservation

Rinsing, drying, desmutting and coating may be requiredfor preservation of the test object. Rinsing removes theetchant by flushing the surface thoroughly under runningwater. Cold water rinsing usually produces better surfaceappearance than hot water rinsing. Hot water rinsing doesaid in drying.

If smutting is a problem, the test object can be scrubbedwith a stiff bristled brush or dipped in a suitable desmuttingsolution. The test object should be dried with warm dry air.Shop air may be used if it is filtered and dried. After visualinspection, the test surface may be coated with a clear acrylicor lacquer but such coatings must be removed before subse-quent tests. If the component is returned to service, a photo-graphic record of the macroetched area should be made.


TABLE 16. Etchant characteristics and uses*

Comments, EtchantMaterials Surface Finish

Type of Discontinuity Composition Procedures

Carbon and alloy steelsHigh speed and tool

steelsCutlery (12 to 14

percent Cr)Stainless steelsIron base high

temperature alloys

saw-cut, machined oraverage groundsurface

general purpose HO (concentrate)H2O

50 mL50 mL

1

Carbon steelsLow alloy steels

fine ground or polishedsurface


average ground surfaceor polished forprocedure 4

saw-cut, machined oraverage groundsurface forprocedure 5

grain size, welds

produces contrast

carburization,decarburization, cracks,

segregation, welds

(NH4 ) 25208log 2H2O

100 mL

10g 3KI

20 gH 2O

100mL

HNO325 mL 4, 5H2O

75 mL

Aluminum best results obtained

HCI (concentrate)

15 mL 6

Aluminum alloys with ground surface HF (48 percent)

10 mL

(180 grit)

H10

85 mL

CopperCopper alloys

machined or groundsurface

removes cold workedlayer

brings out grain contrast;pits if not agitated;Al bronzes may smut(remove by immersionin HNO3 concentrate)

Si bronzes may depositSiO2 (remove byswabbing)

HNO3 (concentrate)

HNO, (concentrate)H2O

50 mL50 mL

7

8

Nickel grinding produces HNO3 (concentrate)

9

Low carbon nickel

best resultsNickel copper

Nickel(Ni-Cr-Fe 600 and800)

Iron base hightemperature alloys

Cobalt base hightemperature alloys


HNC) ) (concentrate)HCI {concentrate)

25 mL50 mL

10

(morel


Table 16. Etchant characteristics and uses* (continued)

Materials

NickelNickel base alloysStainless steels

Surface FinishComments,

Type of DiscontinuityEtchant

Composition Procedures

average to fine groundsurface

general surfacecondition and weldstructure

I.(NH,), SO,H,0

II.FeCI,HCI (concentrate)HNO 3 (concentrate)

15 g75 mL

250 g100 mL30 mL

1-3 mL2-6 mLi 00 mL

1 mL5 mL3 mL

90 mL

6 mL45 mL45 mL

Titanium alloys fine ground or polishedsurface

SiC papers, run wetand give finefinish

HF (48 percent)HNO, (concentrate)1-1 10

12

Titanium alloysAluminum alloys

best results fromgrinding

general structure, welds;SiC papers, run wet

HF (48 percent)HNO, (concentrate)HCI (concentrate)H 2O

13

Zirconium best results fromgrinding

SiC papers, run wetand give fine

finish

HF (48 percent)HNO3 (concentrate)H2O

14

PROCEDURES1.IMMERSE TEST OBJECT IN SOLUTION AT 70 TO 80 °C 1160 TO 180 °F) FOR 15 TO 30 MINUTES. DESMUT BY SCRUBBING WITH BRUSH UNDER RUNNING WATER.

DESMUT STAINLESS STEEL BY DIPPING IN WARM 20 PERCENT HNO3.2.SWAB SOLUTION AT ROOM TEMPERATURE OVER TEST OBJECT OR WHILE IT IS IMMERSED. RINSE AND DRY.3. BRUSH ON SURFACE AT ROOM TEMPERATURE. RINSE AND DRY.4.APPLY TO SURFACE WITH GLASS STIRRING ROD OR MEDICINE DROPPER AT ROOM TEMPERATURE 15 TO 30 MINUTES). RINSE AND DRY.5. HEAT SOLUTION TO BOILING. IMMERSE TEST OBJECT UNTIL ETCHED. RINSE AND DRY. CAUTION: USE ADEQUATE VENTILATION.6.SWAB OVER SURFACE OR IMMERSE TEST OBJECTAT ROOM TEMPERATURE. WHEN DESIRED ETCH IS OBTAINED, RINSE IN WATER AND DESMUT IN HNO3

CONCENTRATE. RINSE IN WARM WATER AND DRY.7. IMMERSE FOR SEVERAL SECONDS UNDER FUME HOOD. RINSE AND DRY. REGRIND LIGHTLY TO REMOVE ROUGH SURFACE.8.AFTER PROCEDURE 7, RE-ETCH BY IMMERSION FOR A FEW MINUTES AT ROOM TEMPERATURE. AGITATE. RINSE AND DRY. CAUTION: USE ADEQUATE

VENTILATION.9. IMMERSE TEST OBJECT AT ROOM TEMPERATURE FOR 3 TO 5 MINUTES, RINSE AND DRY.

10.IMMERSE TEST OBJECT IN SOLUTION AT ROOM TEMPERATURE. WARMING TEST OBJECT HASTENS THE PROCESS. MAY ALSO BE SWABBED.I I. COMBINE I AND II THEN ADD III. IMMERSE TEST OBJECT AT ROOM TEMPERATURE UNTIL DESIRED CONTRAST IS REACHED 10.5 TO 2 MINUTES).12.SWAB SURFACE (3 TO 10 SECONDS) OR IMMERSE 110 TO 30 SECONDS). HF ATTACKS SURFACE AND HNO3 BRIGHTENS SURFACE. SELECT CONCENTRATIONS

ACCORDINGLY.13.IMMERSE 10 TO 20 SECONDS. WASH IN STREAM OF WARM WATER. BLOW DRY. MAY ALSO SWAB.14. SWAB TEST OBJECT WITH SOLUTION AT ROOM TEMPERATURE. RINSE 10 SECONDS AFTER YELLOW FUMES FORM. DRY.

'ADAPTED FROM REFERENCE 31. ADDITIONAL ETCHANTS MAY BE FOUND IN REFERENCES 29, 30 AND 32.


TABLE 17. Etchants for welds

Comments, Application Etchant Composition Surface Finish Procedures

Welds in steel I.FeC13.6H10II.HNO, (concentrate)

H2O

50 g75 mL25 mL

best results with sample atroom temperature; grindor sand surface smooth;highly polished surfacenot required.

Stainless steel FeCI 310g fine ground or polished

2structures HCI (concentrate)

30 mL surface

H,0 90 mL

Welds in aluminum alloys NaOH 10 g general purpose, used on 3 or 4

H2O 100 mL almost all alloys; fine

grinding not required

Dissimilar metal welds(carbon or low alloyto stainless steels)

I.CuCI,HCI (concentrate)Ethanol

II.2 NH4CICuC1 2.2H20FeCl2HOH2O

10 g200 mL500 mL

4g15g

50 mL25 mL


5

PROCEDURESI. DISSOLVE I IN III THEN ADD II. WET SURFACE USING A MEDICINE DROPPER, ETCH 1 TO 5 MINUTES. RINSE UNDER RUNNING WATER. DRAIN AND SWAB WITH

ALCOHOL2. SWAB SURFACE WITH SOLUTION AT ROOM TEMPERATURE TO DESIRED CONTRAST 120 TO 30 SECONDS). RINSE AND DRY.3. IMMERSE TEST OBJECT 5 TO 15 MINUTES IN SOLUTION HEATED TO 60 TO 70 °C 1140 TO 160 °F), RINSE. DESMUT IN CONCENTRATED NITRIC ACID. RINSE. REPEAT

IF NECESSARY.4. WET SURFACE USING DROPPER OR STIRRING ROD. RINSE AND DRY.5. SWAB WITH EITHER I OR II UNTIL DESIRED CONTRAST IS OBTAINED. RINSE AND DRY.

ConclusionVisual testing is performed in accordance with applicable

codes, standards, specifications and procedures. Chemicalaids enhance the contrast of discontinuities making themeasier to interpret and evaluate. This enhancement is

attained by macroetching—a controlled chemical pro-cessing of the surface. Macroetching gives the optimumresults on a properly cleaned and prepared surface. Chemi-cals for etching must be mixed, stored, handled and appliedin strict accordance with safety regulations.''''


REFERENCES

1. Duncan, J.A. and A.A. Levin. "The Pyroscope." PhotoTechnique. Vol. 2. New York, NY: McGraw-Hill BookCompany (1940): p 16.

2. Ward, M.R., L. Rossol, S.W. Holland and R. Dewar."CONSIGHT: A Practical Vision-Based Robot Guid-ance System." Ninth International Symposium onIndustrial Robots. Dearborn, MI: Society of Manufac-turing Engineers (1979).

3. Cheu, Y.F. "Automatic Crack Detection with ComputerVision and Pattern Recognition of Magnetic ParticleIndications." Materials Evaluation. Vol. 42, No. 12.Columbus, OH: The American Society for Nondestruc-tive Testing (November 1984): p 1,506-1,510.

4. The Nondestructive Testing Handbook. Vols. 2 and 6.Liquid Penetrant Tests and Magnetic Particle Testing.Columbus, OH: The American Society for Nondestruc-tive Testing (1982, 1989).

5. Rosenfeld, A. and A.C. Kak. Digital Picture Processing.New York, NY: Academic Press (1976).

6. Ballard, D.H. and C.M. Brown. Computer Vision.Englewood Cliffs, NJ: Prentice-Hall Publishing (1982).

7. Castleman, K.R. Digital Image Processing. EnglewoodCliffs, NJ: Prentice-Hall Publishing (1979).

8. Serra, J. Image Analysis and Mathematical Morphology.London, England: Academic Press (1982).

9. Sternberg, S.R. "Esoteric Iterative Algorithms." SecondInternational Conference on Image Analysis and Pro-cessing. New York, NY: New York Academy of Science(1982).

10. Hildith, J. and D. Rutovitz. "Chromosome Recogni-tion." Annals of the New York Academy of Science. NewYork, NY: New York Academy of Science (1969): p 157,339-364.

11. Fu, K.S. Sequential Methods in Pattern Recognition andMachine Learning. New York, NY: Academic Press(1968).

12. Duda, R.O. and P.E. Hart. Pattern Classification andScene Analysis. New York, NY John Wiley and Sons(1973).

13. Fukunaga, K. Introduction to Statistical Pattern Recog-nition. New York, NY: Academic Press (1972).

14. Patrick, E.A. Fundamentals of Pattern Recognition.Englewood Cliffs, NJ: Prentice-Hall Publishing (1972).

15. Winston, P.H. Artificial Intelligence. Reading, MA:Addison-Wesley Publishing Company (1984).

16. Casasent, D. "Optical Processing Techniques forAdvanced Intelligent Robots and Computer Vision."

Proceedings of the SPIE. Vol. 579. Bellingham, WA:Society of Photo-Optical Instrumentation Engineers(September 1985): p 208-214.

17. Goodman, J.W. Introduction to Fourier Optics. NewYork, NY: McGraw-Hill Book Company (1968).

18. Casasent, D. "Hybrid Optical/Digital Image PatternRecognition: A Review." Proceedings of the SPIE.Vol. 528. Bellingham, WA: Society of Photo-OpticalInstrumentation Engineers ( January 1985): p 64-82.

19. "Spatial Light Modulators and Applications." Proceed-ings of the SPIE. U. Efon, ed. Vol. 465. Bellingham, WA:Society of Photo-Optical Instrumentation Engineers(1984). See also: "Optical Computing/Optical Informa-tion Processing Components." Optical Engineering.Vol. 24, No. 1. Bellingham, WA: Society of Photo-Opti-cal Instrumentation Engineers ( January/Febniary1985).

20. Lendaris, G.G. and G.L. Stanley. "Diffraction-PatternSampling for Automatic Pattern Recognition." Proceed-ings of the IEEE. Vol. 58, No. 2. New York, NY: Instituteof Electrical and Electronics Engineers (February1979): p 198.

21. Kasdan, H. and D. Mead. "Out of the Laboratory andinto the Factory- Optical Computing Comes of Age."Proceedings of the Electro-Optical Systems Design Con-ferencelInternational Laser Exposition. Chicago, IL;Industrial and Science Conference Managment Inc.(1975): p 248-258.

22. Kasdan, H. "Industrial Applications of Diffraction Pat-tern Sampling." Optical Engineering. Vol. 18, No. 5.Bellingham, WA: Society of Photo-Optical Instrumenta-tion Engineers (September/October 1979): p 496-503.

23. Casasent, D., S.F. Xia, A. Lee and J.Z. Song. "Real-TimeDeformation Invariant Optical Pattern RecognitionUsing Coordinate Transformations." Applied Optics.Vol. 26. Woodbury, NY: American Institute of Physics(March 1987): p 938-942.

24. Casasent, D. and V Sharma. "Feature Extractors forDistortion-Invariant Robot Vision." Optical Engi-neering. Vol. 23. Bellingham, WA: Society of Photo-Optical Instrumentation Engineers (September/Octo-ber 1984): p 492-498.

25. Clark, D. "An Optical Feature Extractor for MachineVision Inspection." Proceedings of the Vision 87 Confer-ence. Dearborn, MI: Society of Manufacturing Engi-neers ( June 1987).


29.

30.

31.

32.

33.

26. Belilove, J. "Optical Processing Feature Extraction; ASurvey in the Consumable Goods Industry." Proceed-ings of the Vision 87 Conference. Dearborn, MI: Societyof Manufacturing Engineers ( June 1987).

27. Neubauer, B. and U. Wedel. "Restlife Estimation ofCreeping Components by Means of Replicas."Advances in Life Prediction. D.A. Woodford and J.R.Whitehead, eds. New York, NY: American Society ofMechanical Engineers (1983): p 307-314.

28. "Metallography and Microstructures." Metals Hand-book, ninth edition. Vol. 9. Materials Park, OH: ASMInternational (1985).

Metals Handbook, eighth edition. Vol. 8. Materials Park:ASM International (1973): p 70, 71-72, 121, 130, 137.Annual Book of ASTM Standards. Philadelphia, PA:American Society for Testing and Materials.Morgan, T.R. Macroetch Inspection of Welds. Bir-mingham, AL: Southern Company Services (April1985)."Etching Processes and Reagents." Welding and Braz-ing Qualifications. QW-470, Section IX. New York, NY:American Society of Mechanical Engineers (1983).Dangerous Properties of Industrial Materials, sixth edi-tion. Newton Irving Sax and Benjamin Feiner, eds. NewYork, NY Van Nostrand Reinhold Company (1984).

SECTION 5

OTHER INSTRUMENTATION ANDELECTRONIC AIDS FORVISUAL TESTING Thomas Cabe, DTS Inspection Services, Houston, Texas (Part 3)Eugene Egger, CTS Power Services, Liverpool, New York (Part 1)Edward R. Generazio, Lewis Research Center, Cleveland, Ohio (Part 2)Ralph Olmsted, DTS Inspection Services, Houston, Texas (Part 3)Don J. Roth, Lewis Research Center, Cleveland, Ohio (Part 2)Virginia Torrey, Welch Allyn Video Division, Skaneateles Falls, New York (Part 2)


PART 1REMOTE POSITIONING AND TRANSPORTSYSTEMS

There are many types of remote positioning and transportsystems currently in use. Regardless of its specific form andfunction, each system shares with the others a common ele-ment: the controlled manipulation of a video camera forremote visual examination of system components. Inte-grating video camera technology with the latest in position-ing and transport systems results in the creation of newinspection tools that can be used to safely complete the test-ing process in situations where radiation, heat or chemicalenvironments present serious health hazards to the visualinspector or where physical configurations prevent inspectoraccess.

The term positioning and transport systems as used in thistext refers to any apparatus that puts a video camera andlighting in proper spatial relationship to visually test a com-ponent so that the camera can detect discontinuities. Thisdefinition is general to include as many configurations aspossible.

Equipment used in positioning and transport can beloosely divided into three categories: fixed, automated andmanual.

Fixed SystemsFixed systems, as the name suggests, hold the cameras and

lighting in fixed spatial orientation. A transport subsystemin this system can carry components into the camera'sinspection envelope and position components to detectdiscontinuities.

Fixed systems range in complexity from simple wallmounts to complex materials handling equipment. Fixedmounts can he used to provide monitoring capabilities oncomponents that either operate in a hazardous environmentor create such an environment as a result of operation.

Sophisticated systems can manipulate the workpiece forinspection from multiple angles.

Automated SystemsFully Autonomous—Robotic

A fully autonomous system will transport a camera to agiven location without operator intervention. These systems

have closed loop control logic and respond to the environ-ment in which they operate. In theory, a system such as thiscan transport a camera between two points negotiating allobstacles encountered en route. Consequently, they requirehighly developed sensors and sophisticated feedbackprocessing.

With technology limitations, most of the equipment of the1990s has been truly effective only in structured environ-ments. An environment can be structured in many ways.Mechanically structured environments can employ rails ortracks. Autonomous submarines are being developed thatcan calculate their position on the basis of known sonar buoylocations. Vehicles that optically follow painted lines havebeen operating in warehouses since the 1980s.

Research has continued on systems of this type. However,unless the visual test requires simple pattern recognition, thevisual data need a visual examiner to evaluate them. Thisrequires operator remote control over vehicle movement andopens the vehicle control logic loop, destroying the system'sfully autonomous character. Because the vehicle must per-form semiautonomously at the inspection location, mostautomated vehicles are constructed as semiautonomousfrom the onset, chiefly for economic reasons.

Semiautonomous

Semiautonomous positioning and transport systems oper-ate on open loop control logic. These systems respond toinput from an outside source, typically an operator, ratherthan from acquired sensory input and internally processedfeedback.

A good example of this type of device is a stepping pipecrawler. The feet of a stepping pipe crawler extend to andwithdraw from the pipe wall as the vehicle body expands andcontracts, respectively. This design provides the vehicle withthe ability to climb within vertical piping. Each step involvesa series of cylinder pressurizations and depressurizations. Anoperator oversees the overall motion (i.e., forward, reverse,stop, scan) and the computer determines the proper cylindersequencing to produce the motion. The operator-computerrelationship is necessary particularly in vertical piping wherethe computer ensures leg engagement. This preventsan operator from erroneously disengaging the fixture and

OTHER INSTRUMENTATION AND ELECTRONIC AIDS FOR VISUAL TESTING / 129

letting it fall. In turn, the operator prevents the vehicle fromwalking into thermowells or flow orifices.

Manual Systems

Physical

Physical positioning and transporting of cameras havebeen the most common functions of manual systems in usein the early 1990s. Physical positioning includes pushingcameras with rod or stiff cable and lowering cameras withropes, as well as many other simple yet effective methods.These systems usually provide adequate results and areeconomical.

In some applications the simple methods produce videofootage superior to that of more complex automated equip-ment designed to replace them. For example, a flying rig is athree point suspension camera rig used in boiling water reac-tor vessel examination. It is simply a cleverly bent length ofstainless tube suspended by ropes into the vessel. This rig hasbeen used for years and still outperforms the latest technol-ogy in submarines and telescoping masts.

Physically positioned cameras are heavily dependent onthe skill and experience of the operator. Experience is essen-tial in selecting the proper technique to be used for a giventask. Many inspections are performed by inserting videoprobes into pipes and headers. More often than not, inspec-tors are disappointed with the results. Typically, an off-the-shelf camera will have only a small field application withoutmodifications such as the use of sleds, carts, cable rigs or taskspecific lighting. Of these, the most important modificationtypically overlooked is lighting. The more versatile the light-ing capabilities, the more versatile the camera assembly.

Mechanical

Another common group of manually operated positioningand transport systems are mechanical systems, normally pro-viding better results or more versatility than physically posi-tioned devices. The costs involved are often correspondinglygreater. These range from simple designs such as remotecontrolled cars and remotely operated booms to more com-plex remotely controlled submarines and aircraft.

Mechanical systems are typically created by the combina-tion of off-the-shelf cameras and transport vehicles customdesigned per specific industry requirements. Most industrialresearch and development budgets are spent in this categoryrather than for automated transport systems becausemechanical control systems are the least expensive. Unfortu-nately, highly specialized rigs are limited in their range ofpossible applications.

System Selection and ApplicationThe criteria in developing the proper positioning and

transport system for a particular task include proper selec-tion of both the video system (cameras and lighting) and thetransport system.

Video Systems (Cameras and Lighting)All automated or robotic visual examination systems con-

sist of a fixture or vehicle carrying a video camera and aremote lighting system. The system operator normally uses amonitor for real time viewing of the examination and a videocassette recorder for a permanent record of the examination.

A wide array of video cameras suitable for visual testing areavailable. These cameras usually employ tubes or semicon-ductor chips.

The tube camera uses a vacuum tube similar to that foundin a television to receive and convert video images into elec-trical signals. Tube type cameras commonly available includevidicon and image orthicon video tubes. The primary differ-ence between the video tubes is the chemical composition ofthe materials inside the tubes. Each type has its particularadvantage but they all produce good video images.

The chip camera employs a photosensitive semiconductorchip to receive and convert video images into electrical sig-nals. Chip-type cameras commonly available use charge cou-pled device (CCD) chips, charge injection device (CID)chips, or metal oxide semiconductor (MOS) chips.

Each type of chip is constructed of different materials andprocesses and converts video images in different ways. Likethe various types of video tubes, the different chips havetheir characteristic advantages. The charge injection devicechip can best tolerate radiation exposure whereas the metaloxide semiconductor chip consumes the least power.

Typically, tube cameras can withstand higher radiationdoses and higher temperatures than chip cameras. Chipcameras have proven successful for use in medium and lowradiation dose rate inspection work, and are typically morereliable and less expensive than tube cameras.

The inherent compactness of the photosensitive chipsenables chip cameras to be manufactured in much smallerpackages. This has made possible remote video inspection ofsmall diameter piping and other limited access components.As video borescopes, miniature video cameras are challeng-ing fiber optic borescopes in performing internal inspectionof small diameter piping. Video borescopes can usually beinserted for a much greater distance than fiber opticborescopes and may give a better video image as well.

All camera systems discussed here are available in color aswell as black and white versions. In some cases, black andwhite cameras offer higher resolution than color cameras.For video footage recorded by a video cassette recorder


letting it fall. In turn, the operator prevents the vehicle fromwalking into thermowells or flow orifices.

Manual Systems

Physical

Physical positioning and transporting of cameras havebeen the most common functions of manual systems in usein the early 1990s. Physical positioning includes pushingcameras with rod or stiff cable and lowering cameras withropes, as well as many other simple yet effective methods.These systems usually provide adequate results and areeconomical.

In some applications the simple methods produce videofootage superior to that of more complex automated equip-ment designed to replace them. For example, a flying rig is athree point suspension camera rig used in boiling water reac-tor vessel examination. It is simply a cleverly bent length ofstainless tube suspended by ropes into the vessel. This rig hasbeen used for years and still outperforms the latest technol-ogy in submarines and telescoping masts.

Physically positioned cameras are heavily dependent onthe skill and experience of the operator. Experience is essen-tial in selecting the proper technique to be used for a giventask. Many inspections are performed by inserting videoprobes into pipes and headers. More often than not, inspec-tors are disappointed with the results. Typically, an off-the-shelf camera will have only a small field application withoutmodifications such as the use of sleds, carts, cable rigs or taskspecific lighting. Of these, the most important modificationtypically overlooked is lighting. The more versatile the light-ing capabilities, the more versatile the camera assembly.

Mechanical

Another common group of manually operated positioningand transport systems are mechanical systems, normally pro-viding better results or more versatility than physically posi-tioned devices. The costs involved are often correspondinglygreater. These range from simple designs such as remotecontrolled cars and remotely operated booms to more com-plex remotely controlled submarines and aircraft.

Mechanical systems are typically created by the combina-tion of off-the-shelf cameras and transport vehicles customdesigned per specific industry requirements. Most industrialresearch and development budgets are spent in this categoryrather than for automated transport systems becausemechanical control systems are the least expensive. Unfortu-nately, highly specialized rigs are limited in their range ofpossible applications.

System Selection and ApplicationThe criteria in developing the proper positioning and

transport system for a particular task include proper selec-tion of both the video system (cameras and lighting) and thetransport system.

Video Systems (Cameras and Lighting)

All automated or robotic visual examination systems con-sist of a fixture or vehicle carrying a video camera and aremote lighting system. The system operator normally uses amonitor for real time viewing of the examination and a videocassette recorder for a permanent record of the examination.

A wide array of video cameras suitable for visual testing areavailable. These cameras usually employ tubes or semicon-ductor chips.

The tube camera uses a vacuum tube similar to that foundin a television to receive and convert video images into elec-trical signals. Tube type cameras commonly available includevidicon and image orthicon video tubes. The primary differ-ence between the video tubes is the chemical composition ofthe materials inside the tubes. Each type has its particularadvantage but they all produce good video images.

The chip camera employs a photosensitive semiconductorchip to receive and convert video images into electrical sig-nals. Chip-type cameras commonly available use charge cou-pled device (CCD) chips, charge injection device (CID)chips, or metal oxide semiconductor (MOS) chips.

Each type of chip is constructed of different materials andprocesses and converts video images in different ways. Likethe various types of video tubes, the different chips havetheir characteristic advantages. The charge injection devicechip can best tolerate radiation exposure whereas the metaloxide semiconductor chip consumes the least power.

Typically, tube cameras can withstand higher radiationdoses and higher temperatures than chip cameras. Chipcameras have proven successful for use in medium and lowradiation dose rate inspection work, and are typically morereliable and less expensive than tube cameras.

The inherent compactness of the photosensitive chipsenables chip cameras to be manufactured in much smallerpackages. This has made possible remote video inspection ofsmall diameter piping and other limited access components.As video borescopes, miniature video cameras are challeng-ing fiber optic borescopes in performing internal inspectionof small diameter piping. Video borescopes can usually beinserted for a much greater distance than fiber opticborescopes and may give a better video image as well.

All camera systems discussed here are available in color aswell as black and white versions. In some cases, black andwhite cameras offer higher resolution than color cameras.For video footage recorded by a video cassette recorder


(VCR), the picture resolution is no better than that of therecorder, which is usually lower than the resolution of nearlyany color or black and white cameras. For example, a700 line camera will only produce a 200 line video tape on a200 line video recorder.

In reference to resolution, it is important to keep in mindthe actual required resolution. High resolution equipmentwill cost thousands of dollars more than commercial gradeequipment; however, most lower resolution equipment willresolve a 2.5 I.Lni (1 mil) wire without incurring additionalcost. The camera and lighting should be selected for theirability to detect discontinuities of interest and their ability tofunction within the environment of the inspection area.

Poor resolution is frequently a result of poor lighting.Lighting is often poor because, as the least sophisticated partof the system, it is given the least technical consideration.Operator attention to the range of lighting styles and featuresavailable can help improve resolution. A significant develop-ment in lighting systems has been an increase in their relia-bility and durability. Many can withstand thermal andmechanical shock.

Transport Systems

There currently exists a great variety of transport vehiclesavailable, ranging from submersible vehicles to remotelycontrolled aircraft with all manner of walking and rollingvehicles in between. With no shortage of vehicles to choosefrom, the difficulty arises in finding an appropriate transportvehicle for a given task.

The most effective method for the selection of transportsystems is sintering: networking between similar industrialfacilities to obtain the most current information regardingvehicle application to specific tasks. The aim of this commu-nication is to locate a vehicle that will most closely meet theoperator's immediate needs. Many companies employ per-sonnel whose sole function is to network in this fashion andmaintain current files for a variety of applications. Vehicleslocated by this method may not meet the operator's specificneeds; however, it is most likely that reasonable modifica-tions can be made to the available vehicle to fit inspectionrequirements.

OTHER INSTRUMENTATION AND ELECTRONIC AIDS FOR VISUAL TESTING 1 131

PART 2 VIDEO TECHNOLOGY

Photoelectric DevicesElectronic aids to vision are based primarily on photoelec-

tric devices. These devices convert information in the formof light into electrical signals that may then be amplified orprocessed to perform some function that increases the abilityof the observer to gather and interpret the test data. Theinformation may be in the visible region or in a form invisibleto the eye, such as ultraviolet or infrared radiation.

Classifications of Photoelectric Devices

There are two broad classifications of photoelectricdevices; (1) detectors or measuring devices and (2) a two-dimensional image of visual data over an area. The first cate-gory includes photoemissive cells and photomultipliers, pho-toconductive cells, photovoltaic cells and various devices thatmeasure radiant energy directly, such as bolometers (thermalradiation detectors) and thermocouples. In the second cate-gory are various image converter tubes, image amplifierscreens and television pickup tubes.

Photoemissive DevicesPhotoemissive devices, whether simple phototubes or

multiplier phototubes, are characterized by use of materialsthat emit electrons under the influence of light. These elec-trons are then drawn away from the emitting surface by anelectric field and used as the signal current, which may actu-ate relays to be amplified electronically.

Photoelectric Emission

The emission of electrons in response to light is based onthe quantum theory of radiation. The radiation behaves asthough it were composed of photons or quanta, each with anenergy of hv, where v is the frequency of the radiation and his Planck's constant.

Within the material, electrons have kinetic energies up toa maximum value W, W. A potential barrier W„ exists at thesurface. This is the total energy an electron must have toescape. When hv exceeds W„, an electron is emitted.The term W,, – W. is known as the work function and is theminimum value of hv which must be added to the energy ofan electron to cause emission.

Photocathode Materials

Many materials (primarily metals) have sufficiently highvalues ofWem^and low values of W, to permit photoemission.By far the most important such materials are compounds andalloys of the alkali metals, principally cesium. Of these, themost widely used are cesium antimony alloys and cesiumoxygen silver compounds. The cesium antimony emitters aretrue alloys, while the cesium oxygen silver emitters are com-plex surfaces made from layers of cesium, cesium oxide andsilver oxide in varying proportions on a silver base. Cesiumantimony surfaces may have a quantum efficiency of 10 to30 percent. Photoemitters of this type have responses thatare selective to wavelength of illumination. This selectivityvaries greatly with processing.

Photoconductive Cells or PhotodiodesIn the class of solid materials known as semiconductors,

the energy levels of electrons in the atoms are so arrangedthat electrons can be excited into conduction bands by ther-mal or other means. Without such excitation, these materialsexhibit very little electrical conductivity. Photoconductorsare a class of semiconductors in which the absorption of lightenergy excites some of the electrons into conduction bandsso that conductivity is increased under the influence of light.

Selenium was the earliest known photoconductor. In thepure or combined form, it is still the basis for many photo-conductive cells. More recently, many photoconductivematerials have been discovered with efficiencies higher thanselenium. Among them are lead sulfide, thallous sulfide, cad-mium sulfide and cadmium selenide. Some of these materi-als are now used in commercial cells.

All photoconductive cells require an external source ofcurrent because their electrical resistance varies in responseto illumination. They are extremely sensitive but, because ofa lag inherent in photoconduction, they are slower inresponse than photoemissive devices. Consequently, photo-conductive cells are generally more useful in light measuringdevices and control equipment than in high speed applica-tions such as sound reproduction. Some photoconductivecells are useful at audio frequencies and the speed ofresponse of some photoconductors may be increased by biaslighting.


Photovoltaic DevicesPhotovoltaic devices are similar in some respects to photo-

conductive cells. They differ in one important way: photovol-taic cells are true energy converters rather than controldevices. Light falling on such cells produces a potential dif-ference across the terminals and a continuous current can bedrawn from them (the energy in the light is converteddirectly into electric energy). This is particularly useful inlight measuring devices such as photographic exposuremeters because no external electrical source is required.

Construction

Photovoltaic cells are made in sandwich form: a base con-ductor coated with active material, over which is applied atransparent conductor. In most cells, the active material iseither copper oxide on copper or selenium on iron. The lat-ter is often preferred because of its high stability.

The selenium barrier layer cell is a reasonably efficientconverter. Under optimum conditions it delivers about 2 per-cent of the light energy falling on it as electric energy. Recentwork with silicon and other materials in solar batteries hasyielded efficiencies of 10 percent or greater. Power output ofseveral watts has been obtained in large area cells.

While the inherent response of the photovoltaic or barrierlayer cell is very fast, its high capacitance reduces itsresponse at higher frequencies.

Uses of Photoelectric Detecting andMeasuring Devices

The photoelectric cell or multiplier phototube is used inmany ways in industrial nondestructive testing. One of themost common uses is the measurement of radiant flux. Inmany respects, the phototube exceeds the capabilities of thehuman eye. It can detect not only radiation invisible to theeye but can also accurately measure quantities of light with-out reference to a standard, as required in a visual photome-ter. The familiar photoelectric exposure meter is no morethan a barrier layer photovoltaic cell and a sensitive meter.

In addition to measuring light flux, photoelectric devicespermit measurements of reflectance and transmission ofmaterials, comparisons of two or more sources of light and(with the aid of filters or some type of spectrometer) colori-metric measurements. Phototubes find a natural applicationin spectrophotometry. Next to the measurement of radiantflux, perhaps the most widespread use of photoelectricdevices is in monitoring and control applications. Streetlamps may be turned on by phototube circuits when daylightdecreases to a certain level. In power plants, phototubes

watch furnace flames. They monitor the amount of smokeemitted from the stack. They control door openers and safetyinterlocks on punch presses, inspect bottles after washingand detect foreign matter in filled soft drink bottles. Theysort products such as beans, peas and coffee and actuatemechanisms for rejecting off-color products.

The electrical circuitry for these operations is usually sim-ple and can be made extremely rugged and reliable.

Photoelectric Imaging DevicesSolid State Image Amplifier

The solid state image amplifier' is a sandwich of a photo-conductive layer and an electroluminescent layer. The elec-troluminescent layer is composed of a material that emitslight in response to an applied voltage. The photoconductorand the electroluminescent material are essentially in seriesacross a suitable alternating current voltage supply. In dark-ness, the photoconductor is highly resistive and passes nocurrent. When light falls on it, it becomes conductive andcurrent flows through the sandwich, causing the luminescentmaterial to emit light in the illuminated regions. Light ampli-fications up to 1,000 x have been obtained.

A modification of this light amplifier is the amplifyingfluoroscope. In this device, the photoconductive materialresponds directly to X-ray radiation and by the same princi-ple as the light amplifier converts it to visible light. An ampli-fication of 100 x over the output of a conventionalfluoroscope has been obtained.

Television Systems

Television pickup systems make use of a scanning processto convert a two-dimensional spatial distribution of light val-ues into a time sequence. In this way, a three-dimensionalsystem of information (two spatial dimensions and intensity)is converted to a two-dimensional signal (time and ampli-tude). It is therefore possible to collect considerably moreinformation with a television system than with a simple pho-tocell circuit.

Two types of pickup tubes are commonly used: (1) thephotoemissive tube and (2) the photoconductive tube. Thephotoemissive type is represented by the iconoscope, theorthicon and the image orthicon. The image orthicon hasalmost completely replaced the earlier forms for commercialtelevision in the United States. The photoconductive tube isexemplified by the vidicon.

The image orthicon has high sensitivity and produces ahigh quality image. However, its relatively high cost and com-plexity limit its use to the larger and more specialized closedcircuit applications. One use of the image orthicon is the

O

FIGURE 1. Response of typical ultraviolet vidicon

0 0a- 200 300 400 500

(2.000) (3,000) )4,000) )5.000)

WAVELENGTHnanometers (angstroms)


lI

pickup of images at very low light levels' By means of a spe-cially constructed image orthicon, the sensitivity atextremely low light levels has been made to exceed that ofthe human eye.

For most uses in visual testing instrumentation, the needfor a simpler and less expensive system has led to the devel-opment of the vidicon. 3 The vidicon is a photoconductivetube available in several sizes. The standard size is 25 mm(1 in.) in diameter and 150 mm (6 in.) long. The sensitivearea is about equal to a frame of 16 mm motion picture film.Because of the high quantum efficiency of the photoconduc-tive process, the vidicon compares favorably in sensitivitywith the image orthicon. It is difficult to make a direct com-parison because of differences such as amplitude responsecharacteristics (gamma) and lag effects but at ordinary levelsof illumination the factor is between 5 and 10 times in favorof the image orthicon.

The smooth gamma characteristic (m 0.7 power) of thevidicon gives it an excellent halftone range. This, coupledwith its high signal-to-noise ratio, enables it to reproduce avery high quality picture under good lighting conditions.Photoconductive lag, Which limits the response speed of allphotoconductive devices, is present in the vidicon and maylimit its usefulness with rapidly moving objects at low lightlevels. The lag varies inversely with light level. At typicallight levels, the response speed is adequate for meetingnearly all test requirements.

Many types of pickup equipment have been built aroundthe vidicon for industrial purposes. In general, there are twobasic designs: (I) a very compact and inexpensive type of selfcontained unit designed for use with a standard televisionreceiver as a monitor and (2) elaborate and more versatileunits complete with integral circuitry and monitoring facili-ties, used where the requirements of performance are morestrict.

There are many advantages of television for industrialvisual and optical testing. Television can be used in restrictedspace or hazardous environments. Televised informationfrom several points can be brought to a central location forcoordination. Conversely, the same information may be dis-tributed to a number of monitors in several locationssimultaneously.

Television is used for visual inspection of shock absorberson automobiles by mounting a camera underneath the carand observing the spring action. Television observation offurnace flames has greatly aided the monitoring of burnertests. For tests of radioactive materials, television gives asafe, close view. Television may also be used in conjunctionwith the light microscope,' particularly in biology andmetallurgy.

In addition to the advantage of viewing convenience, vid-icons can also be made sensitive to ultraviolet (see Fig. 1).This permits visual testing of materials under illumination

invisible to the eye but in a spectral region where manymaterials have distinct absorption characteristics.' Likewise,vidicons can be made sensitive to the near infrared regionand to X-rays.'

With the wide variety of industrial television equipmentavailable commercially, it is a relatively simple matter toselect suitable units to set up a visual information link. Vari-ous accessories are available, including weatherproof hous-ings and a wide variety of auxiliary monitors and switchingunits.

The Electron MicroscopeThe electron microscope extends the limit of visual resolu-

tion from hundreds of nanometers (thousands of angstroms),the usually accepted value for the light microscope, toless than 1 nm (10 A). Many other imaging systems, usingsubatomic particles, electric resistance or computerenhancement techniques, can produce images of materialsat the atomic and molecular levels.

The applications of the electron microscope are normallyassociated with destructive testing, that is, the test objectmust be destructively removed from a larger object for exam-ination. Notable exceptions are shadow casting (describedbelow) and the visual testing of microelectronic components.Electronic microscopy also serves to supplement the data ofother nondestructive methods.

Electron Beam Paths

In principle, the electron microscope is almost a perfectanalog of the conventional compound light microscope.'However, the instrument itself takes quite a different formbecause the electron beams must remain entirely in a

FIGURE 2. Paths of rays in a light microscope andin an electron microscope: (a) light microscope(inverted to illustrate the analogy more directly)and (b) electron microscope

(a) (1))

LIGHT SOURCE ELECTRON SOURCE

IMAGNETIC OBJECTIVE r4

CONDENSER LENS

TEST OBJECT

OBJECTIVE LENS

INTERMEDIATEIMAGE ffPROJECTOR

MAGNETICCONDENSER

TEST OBJECT

PROJECTOR LENS'EYEPIECE)

OBSERVATION SCREENT/PHOTOGRAPHIC PLATE) SECOND STAGE

MAGNIFIED IMAGE


vacuum. Test objects must be introduced into the vacuumchamber and into the path of high velocity electrons.Figure 2 compares rays of light and electron beams in thetwo instruments.

Components

The electron microscope consists of a long evacuated tubewith (1) a source of electrons at one end, (2) provisions foraccelerating the electrons to about 50 kV (3) a series of mag-netic lenses and (4) a fluorescent screen and plate chamberfor recording the image at the other end.

The test object is usually mounted on a thin film such ascollodion supported by a fine mesh. The specimen must bethin enough to be penetrated by the electron beam. At 60 kV,a maximum thickness of 1 (0.04 mil) can be penetrated.Required thicknesses vary with the density of the material.

Images and Replication

After passing through the test object, electrons are sub-jected to the action of twd or more magnetic lenses and thenfall on a photographic plate or a fluorescent screen that isalso in the vacuum. Here, a greatly enlarged electron imageis formed showing the pattern of electron absorption by thetest material.

For opaque specimens such as metallurgical samples, it ispossible to obtain a micrograph of the test surface. This isdone by forming a thin film of plastic, such as polyvinyl or

polystyrene, on the surface. When hardened, the film ispeeled off and inserted into the microscope. The contrast inthe observed surface structure may be enhanced by evapo-rating a thin metal film onto the replica at an oblique angle.This procedure is known as shadow casting and isnondestructive.

Applications

The electron microscope can be used for studies of stnic-ture in the 1 to 100 nm (10 to 1,000 A) range (well below thewavelength of visible light). The test material must be pre-pared in a film of the necessary thickness by slicing or, in thecase of particulates, by embedding in a plastic film. The testobject must also be in such a form that it can be introducedinto a vacuum (this excludes liquids, suspensions and wetsolids).

Video BorescopesThe coupling of video and borescope technologies has

solved some of the long standing problems experienced byoperators of conventional borescopes. In some cases, videoequipment has simply been adapted to an existingborescope, transmitting images to a monitor as they appearin the eyepiece. More sophisticated systems transmit imagesto a monitor electronically by means of a tiny camera locatedat the distal tip of the borescope. This camera is typically asolid state silicon chip or light sensor known as a charge cou-pled device.

Development of Charge Coupled Device Technology

The charge coupled device was introduced by Bell Labo-ratories in 1970 and was originally used for low cost, highdensity semiconductor memories for computers and micro-processors. Five years later, RCA introduced the first blackand white solid state television camera using charge coupleddevices as imaging devices.8

The charge coupled device is widely used for digital mem-ory and analog signal processing, as well as optical imaging.The device may be defined as a monolithic array of closelyspaced metal oxide semiconductor capacitors that transfersan analog signal charge from one capacitor to the next. It isthis analog capability that makes possible the device's use inself scanned optical imaging . 9

In the early 1980s, video borescope technology wasenhanced by the ability to create small, high resolutioncharge coupled devices—a rectangular chip measuring5 mm (0.2 in.) diagonally can hold an array of 195 by 162 ormore than 31,000 light sensitive capacitors. These capacitorsare the familiar pixels or picture elements.

FIGURE 3. Components of a video borescope

VUXOPROCESSOR

FIGURE 4. Distal tip of a video borescope

CHARGE COUPLED DEVICE CHIPWIRE CARRYING THE

IMAGE SIGNAL

LIGHT OUTLETLENS


The first chips were intrinsically sensitive only to lightintensity and not to light color, so they could be used onlyfor black and white imaging. Today, however, techniqueshave been developed that achieve color imaging from themas well.

One of these techniques uses color sequential lighting totimeshare each pixel among the three additive primary col-ors of light. Red, green and blue images are capturedsequentially and then resolved into a single image either atthe monitor or in a simple processor prior to the monitor.This technique is completely external to the chip.

Another technique involves the placement of tiny colorfilters on each chip detector, yielding what the industryrefers to as the color chip. By grouping detectors of differentcolor filters together and using them for a single pixel ofinformation, one can capture the three primary colors ineach pixel simultaneously rather than sequentially. This tech-nique simplifies the image processing but cannot achieve thesame resolution without doubling the size of the chip.

Video Borescope Components

A video borescope has four main components (see Fig. 3):(1) a probe, with a charge coupled device embedded in thedistal tip; (2) a video processor, to communicate signals to themonitor; (3) a monitor, black and white or color; and (4) analphanumeric keyboard, for entering identification refer-ences onto the display or into a permanent record.

Probe features vary to include lengths over 30 m (100 ft),diameters as small as 6 mm (0.25 in.), remote four way steer-ing control, right angle adapters for 90 degree viewing andthe ability to measure accurately what is seen in theborescope image.

Video borescopes are easily coupled with such accessoriesas standard video recorders, telephone modems, electronichardcopy devices or computer enhancement equipment.

Video Borescope Operation

Video borescopes transmit images in the following way.First, light is sent to the test area by fiber optic light guidesor by light emitting diodes. Fiber optic lighting can be usedfor either color or black and white imaging. Light emittingdiodes are used only for black and white imaging, becausethey cannot transmit multicolored light. While several colorsare available, light emitting diodes transmitting red lightoffer two advantages: red has the brightest output in the lightemitting diode family and charge coupled devices are moreresponsive to red than to any other color.

Once the light has reached the test area, a fixed focus lensin the tip of the probe gathers reflected light and directs it tothe surface of the charge coupled device (see Fig. 4). On thechip, the pixels convert light into analog electrical signals.The signals travel down the length of the probe through aseries of amplifiers and filters.

The processes for black and white and color imagingdiverge at this point. Black and white imaging with a mono-chrome charge coupled device (or color imaging with a colorcharge coupled device) requires only one frame of informa-tion, 1/60 s exposure to light per image. Color imaging witha monochrome device requires three frames per image, onefor each of the primary colors. This type of color imagenecessitates the use of one or two sequential lighting meth-ods, strobing or light chopping, both of which render highlyaccurate color.

Using the strobe lighting technique, a video borescoperecords each of the three colors separately at full bandwidthfor maximum resolution. A strobe lamp in the processor

FIGURE S. Sequential lighting with the strobemethod: fa) light transmitted !red, green or bluefor color imaging) and (b) no light transmitted

COLOR WHEELGREEN FILTER

BLUE FILTERRED FILTERLENS ASSEMBLY(INCLUDESINFRARED FILTER)

LAMP ON

LAMP OFF

(a)

FIBER OPTICBUNDLE

(b)

LIGHTDEFLECTED

FIGURE 6. Sequential lighting with the lightchopping method: (a) light transmitted (red,green, or blue for color imaging) and (b) nolight transmitted

(a) GREEN FILTERCOLOR WHEEL(PIVOTS OUT OF LIGHTPATH FOR BLACK ANDWHITE IMAGING)

BLUE FILTERAPERTURE WHEEL

FIBER OPTIC BUNDLE

RED FILTERLENSLAMP (ONCONSTANTLY)COLOR WHEEL

AND INFRAREDFILTER INPOS LTION FORCOLOR IMAGING INFRARED

FILTER

FIGURE 7. Processing video image signals

aihki CatED DEICECLOCK COVEZ.

AMURAND FILTERS

ONTOSYSTEM

MP AND COLCB

COMPOSITE NDEO

VIDEOROCBSOR

CB VIDEO OUT RUS

LEGEND

A/D ANALOG TO DIGITALD/A DIGITAL TO ANALOG

UAW' YJCONTEt

NBCLOCK

ARALOGan

PROCESSING

At

CrIOD N MEIMCCfirRa 3 IIi BLUE MEMORY JO

I^ I caquw MI

I 1 WARY 131


illuminates the test area in intervals of 1/60 s (60 Hz). Theclock driver circuitry synchronizes these frames with therotation of the processor's red, green and blue color wheel.Each full color image requires a combined time of 1/20 s (20Hz), three times as long as a black and white image or a colorimage created from a color charge coupled device (seeFig. 5) but at full chip resolution.

Light chopping works similarly, except that the light itselfremains steady. Blanks on the color wheel produce a strobeeffect without sacrificing brightness (see Fig. 6).

Once the clock drivers receive the red, green and blueinformation, they send it in the form of electronic signals tothe video processor. Each of the three frames, stored sepa-rately in processor memory, retains its full resolution. Theprocessor digitizes and assembles the signals, then transmitsthem either in parallel for high quality red, green and bluemonitors (four signals: red, green, blue and sync) or as a sin-gle composite signal for standard video taping and modemdevices (see Fig. 7).

Images are magnified when displayed, making it easier foroperators to evaluate details but understanding the dimen-sions of the viewed objects has been limited to very roughestimates. The magnification formulas could not be used,because the object's distance from the probe tip was typicallyunknown. However, a technique has been developed thatmeasures a shadow projected onto the inspected surface inorder to calculate the object distance and magnification.

The magnification factor depends on (1) the screen size,(2) the lens field of view and (3) the unit of measure for thedistance. For example, for a screen with a 33 cm (13 in.)

FIGURE 8. Interior of a jet engine burner showingseams and apertures: (a) through a fiber opticborescope and (b) through a video borescope

(a)

(b)


diagonal and distance to object measured in centimeters (orinches), the factor is about 11.9 (or 4.7) for a 90 degree fieldof view; about 20.1 (7.9) for a 60 degree field of view; and42.9 (16.9) for a 30 degree field of view.

m = –d

s,s„ = —

m

Where:

m = magnification;x = factor;d = object distance;sc, = object size; and

= screen image size.

Diameter and Length Limitations

Diameter and length limitations are of critical interest tovideo borescope operators. Monochrome charge coupleddevices are made as small as 5 mm (0.2 in.) diagonally.Because it also contains light guides, a monochrome probecan be no smaller than 6 mm (0.24 in.) in diameter, slightlylarger than the smallest monochrome charge coupled device.The smallest color chips are about 13 mm (0.5 in.) in diame-ter, just over twice the size of their monochromecounterparts.

Color probes, with color or monochrome chips, can be nolonger than 30 m (100 ft). Otherwise, too much light dissi-pates and the image quality degrades. Black and whiteprobes, which require less light than color probes and whosedistal light emitting diodes eliminate the problem of lightdissipation, can exceed 30 m (100 ft) in length.

Advantages of Video Borescopes

Standard borescopes can cause eyestrain and oftenrequire the operator to adopt awkward positions to seethrough the eyepiece. Over time, fatigue and discomfort caninterfere with the inspector's ability to interpret images cor-rectly. Video borescopes eliminate these problems byallowing the inspector to sit comfortably in front of a monitor.

Video borescopes also allow multiple views of the sameimage, making evaluations more reliable and facilitatingtraining. In fact, a given image can be transmitted simultane-ously to any number of monitors at the site or, by modem orsatellite, to remote locations.

Image quality can be low in certain borescopes—onecomplaint of some fiber optic borescopes is that they conveyan inherently fuzzy honeycomb pattern caused by the tiny

spaces between the optical fibers. Also, individual imageguide fibers degrade over time, causing density inconsisten-cies in the image. In contrast, breakage of fibers in videoborescopes has no effect on the transmitted image unless it isso severe as to significantly decrease the amount of deliveredlight. A third image quality problem is color inaccuracycaused by the glass fibers' absorption of blue light, resultingin reddish images.

By replacing fiber optic image guides with an electronicsignal, video borescopes solve such image quality problems(see Fig. 8). Images are further enhanced because a videoborescope magnifies them and increases their resolution. Asa side benefit of replacing the breakable fiber optic imageguides, a video borescope's durability is increased and its ser-vice life is extended.

One of the standard borescope's most significant limita-tions is that it requires frequent refocusing. Primary focusingoccurs when the lens focuses the image onto the fiber opticbundle at the tip of the probe. Secondary focusing takesplace at the eyepiece. When changing the viewing distanceeven slightly, the operator must manually refocus the lens.

A video borescope's depth of field (the range of distancein focus) is so much expanded that focusing becomes unnec-essary (see Fig. 9). This feature is sometimes called auto-matic focusing, although no mechanism is actively adjusted.The expanded depth of field is attributable to both the prox-imity of the lens to the charge coupled device and the smalldiameter of the lens aperture. By eliminating the time con-suming task of refocusing, video borescopes make remotevisual testing more efficient and less fatiguing.

(Eq. 1)

(Eq. 2)

FIGURE 9. Depths of field for video borescopes with(a) 90 degree (b) 60 degree and (c) 30 degree fieldsof view

LEGEND

SHARP FOCUS

If IttiI 4 1 420 40 60 80 100

OBJECT DISTANCE(millimeters)

1 I20 40 60 80 100


-

—

1 I r t I 4 t l i20 40 60 80 1 00


(alz

. EF- 7-

0 aia ci)

16i41210064

400

300

200

100UJ

oe 20

(b) —z _cE 1E)

16141210

400

300

-J . a 86

200

OVI 0_ 4 100w C 2CC CD

C 0

(c) 16 400

zO E= c

— '

141210

300

E ,/r, 8 2006

0 4 100LL . 0

—2

c 0

US IL! E


Another major advantage of video borescopes is the abilityto use the shadow projection technique to calculate magni-fication and measure objects and indications seen in thevideo screen. Obviating optical comparators, this techniqueis practical in probes that use imaging chips in their distal orobjective tips.

Finally, video taping with a standard borescope is an awk-ward procedure but with a video borescope it becomes asimple and convenient way to document tests for future ref-erence or training.

Some video borescopes offer other advantages, includingillumination level adjustment, detail amplification in darkareas, freeze frame capability and interchangeable probes.

Disadvantages of Video Borescopes

Video borescopes are typically less portable and morecostly than other designs. A system can weigh up to 25 kg(55 lb) and cannot be easily carried from site to site. The ini-tial cost of a video horescope can be greater than that of anstandard borescope, although the price difference decreasesor disappears when a closed circuit television camera isadded to the borescope.

Video Borescope ApplicationsVideo borescopes lend themselves to any application

requiring remote visual testing, from the aerospace andpower generation industries to engine manufacturing andmarine operations. Video horescope technology is especiallyuseful for interpreting and confirming questionable indica-tions from other nondestructive testing techniques. Forexample, a video borescope might be instrumental in identi-fying the nature of a discontinuity revealed by an X-ray orultrasonic test.

Video borescopes are well suited to applications wheremultiobserver viewing of inside surfaces is desirable. Forexample, in the aerospace industries, the inspection of anengine's turbopump or a plane's wing cell are absolutely crit-ical to the safe operation of the craft. The video borescope'sability to display an area evaluation to several viewers simul-taneously, while minimizing fatigue, is of value in such testenvironments. The freeze frame ability is particularly usefulfor evaluation.

For applications requiring critical assessment of detail,video borescopes provide high image quality in terms ofmagnification, resolution and color accuracy as well as accu-rate measurement. Image quality and measurement arehelpful when checking coatings and seals, analyzing chemi-cal reactions, identifying corrosion and pitting or locatingweld area burn through. Steam plant operators, for example,need to evaluate the inside surfaces of boiler tubes accu-rately. Using video borescopes, they can detect and identifychemical deposits and oxygen pits (depressions formed byoxygen attacking the metal) early enough to prevent theboiler tubes from developing serious discontinuities.

Video borescopes are also ideal for applications requiringvideo tape documentation. For instance, pharmaceuticalprocessing plants subject to exacting federal regulationscan meet documentation requirements conveniently byattaching a standard video recorder to a video borescope todocument processes of interest.

Finally, video borescopes are particularly advantageouswhen test time and proximity must be kept to a minimum,as with procedures involving exposure to radiation, heat orharmful chemicals. For example, nuclear power plants thatregularly inspect generator tube sheets for corrosion product

FIGURE 10. Typical electron beam scan path as seenon a television screen

PATH OF SCANNINGELECTRON BEAM

2345

67B9

1011

1213



sludge deposits may discover that by switching to a videoborescope they can reduce test time significantly. Less timewill he spent retrieving debris because the operator can feeda recovery tool through the video probe's operating channelrather than removing the probe and using a recovery toollater. Given the limitation on the radiation a technician canreceive, increased efficiency means that video borescopetests are safer and can involve fewer workers.

Research in Video TechnologyAs a result of continuing miniaturization of electronic

components, video borescope systems will become moreportable as they decrease in size and weight. Another areawith promise is the use of satellites for transmitting videoborescope images. The technology is in place and, as indus-tries using nondestructive testing become more global, satel-lite communications will he used more often for longdistance consultation and training,

Video borescope costs are not expected to drop becausethe devices' specialized and customized uses eliminate thepossibility of mass production and the benefits of economiesof scale.

Remote Closed Circuit TelevisionPrinciples of Scanning

The picture on a television screen is created by moving anelectron beam back and forth across the inside of the screen,varying the intensity of the beam as it scans (see Fig. 10). Theelectron scan initiates at the top left and moves from left toright across the screen, forming the path shown in Fig. 10between point 1 on the left and point A on the right.

After reading the right side, the electron beam is rapidlyreturned from point A on the right to point 2 on the left. Asthe electron beam returns to the left, no picture informationis transmitted. The line 1-A is called a trace and the dashedline A-2 is called the return trace. This process of electronbeam scanning is repeated throughout the generation ofthe picture.

The electron beam continues to scan back and forth acrossthe screen even in the absence of a picture. In this situation,the beam traces a white rectangle on the screen and thislighted rectangle is called a raster.

After the electron beam reaches the bottom of the raster,it is rapidly moved back to the top of the screen where itrepeats the sequence. The image appearing on the screen isformed by a number of complete rasters each second.

Taking into consideration brightness, transmission band-width and the effect of continuous motion, an optimum

number of thirty rasters each second was chosen for standardtelevision. These complete rasters are sometimes referred toas frames. Because a frame frequency of thirty pictures persecond produces a flicker discernible to the eye, each pictureis divided into two parts calledfie/ds. Two fields are requiredto produce one complete picture or frame. The field fre-quency is sixty fields per second and the frame frequency isthirty frames per second. Each field contains one half of thetotal picture elements.

The picture appearing on the television screen is dividedinto its two parts by a process called interlaced scanning. Thepurpose of interlaced scanning is to eliminate flicker and it isdone by increasing the electron beam's downward rate oftravel so that every other line is sent. When the bottom isreached, the beam is returned to the top and the alternatelines are sent. The odd and even line scans are each transmit-ted at 1/60 s, totaling 1/30 s per frame and retaining the stan-dard rate of 30 frames per second. The eye's persistence ofvision allows the odd and even lines to appear as a singleimage without flicker (see Fig. 11).

Television Camera TubesThe television camera tube is a critical component in the

closed circuit television system. The images received by thetube are converted into electrical impulses and this deter-mines the frame rate and the quality of the final image repro-duced at the receiver. For high resolution at the viewer, thecamera tube must separate the object image into as manypicture elements as possible. The higher the number of pix-els, the greater the resolution for detail at the receiver.

FIGURE 11. Complete electron beam scanning pathwith two fields per frame using interlacedscanning; beam path for second field, not shown,travels between scans of first field

START OF FIRST FIELD START OF SECOND FIELD

END OF FIRST FIELDEND OF SECOND FIELD


FIGURE 12. Principles of vidicon tube operation


GLASS FACEPLATE

FOCUSEDREFLECTED LIGHTFROM THE SCENE

TRANSPARENT CONDUCTING FILM(SIGNAL ELECTRODE)PHOTOCONDUCTIVE LAYER -1 + I

CANNING BEAM

VIDEO SIGNAL

I IIl i t


Television camera tubes are divided into two classifica-tions, based on the way they produce an electrical imagewithin the tube. The first method is called photoemission, inwhich electrons are emitted by a photosensitive surfacewhen light reflected from the object is focused on the sur-face. Television tubes that use the photoemission method arecalled image orthicon tubes.

The second method is called photoconduction. In this pro-cess, the conductivity of the photosensitive surface changesin relation to the intensity of the light reflected from thescene focused onto the surface. Tubes using the photocon-duction process are called vidicon tubes. Vidicon tubes arethe ones used most in industrial applications and the discus-sion below concentrates on their use.

Vidicon tubes can operate in direct sunlight to nearly totaldarkness. The principles of operation are as follows. The tar-get (see Fig. 12) is composed of a transparent conducting

film (called the signal electrode) on the inner surface of thefaceplate and a thin photoconductive layer deposited on thefilm. Light reflected from the object is focused on the surfaceof the photoconductive layer next to the faceplate and eachilluminated element conducts current, depending on theintensity of light striking it. The resulting effect is to causethe potential of its opposite surface to rise toward the signalelectrode potential. On the gun side of the photoconductivelayer, there appears a positive potential replica of the objectcomposed of various element potentials corresponding tothe light focused on the photoconductive layer.

As the gun side of the photoconductive layer (with its posi-tive potential replica) is scanned by the electron beam, elec-trons are deposited from the beam until the surface potentialis reduced to that of the cathode in the gun. This produces achange in the difference of potential between the two sur-faces of the photoconductive layers. When these two surfacesare connected through the external target circuit and a scan-ning beam, a current is produced which constitutes the videosignal. The signal is decoded, amplified, and projected ontoa viewing screen.

Cathode Ray Viewing TubeThe two aspects of a cathode ray tube most important to

visual interpretation are brightness and contrast. As the elec-tron beam scans the back side of the fluorescent screen, notall of the emitted light is useful. About 50 percent of the lighttravels back into the tube, 20 percent is lost in the glass of thetube screen by internal refraction, leaving only 30 percent toreach the observer.

Image contrast is reduced by light returned to the screenreflecting from some other point. There are four mainsources of this interference: (1) halitation, (2) reflectionsfrom screen curvature, (3) reflections at the surface of thescreen face and (4) reflections from inside the tube.

HalitationIf the electron scanning beam is held in one spot, the visi-

ble spot on the screen is surrounded by rings of light. Theserings are caused by a phenomenon known as halitation (seeFig. 13). Light rays leaving the fluorescent crystals at theinner surface of the glass are refracted as they travel intothe glass.

In Fig. 13, certain rays are reflected back into the glass bythe outside surface of the glass. Where these reflected raysstrike the fluorescent crystals, they produce visible rings onthe screen causing a hazy glow surrounding the beam spot.Halitation reduces the maximum detail contrast.

FIGURE 13. Diagram of reflected rays causinghaiitation

AIR SIDE

GLASS FACE

FLUORESCENTSCREEN

VACUUM SIDE

STRIKING ELECTRON BEAM


FIGURE 14. Reflections caused by screen curvature

USEFUL DIAMETER

EMITTING AREA


FIGURE 15. Relative units used to figure verticalscreen dimension from diagonal dimension


zLU

LU

FOUR RELATIVE UNITS


Reflections Caused by Screen Curvature

Reflections caused by curvature of the screen (see Fig. 14)produce a loss of contrast. The interface between glass andair provides an angle of incidence that refracts the light strik-ing it. Flatter surfaces retain more contrast.

Reflections at the Surface of the Screen Face

A portion of incident light is reflected when it reaches theoutside surface of the glass (the glass/air interface). Theserays are reflected back and forth between the inner and outersurface of the glass with some of the light being emitted andthe balance being absorbed.

Reflections from Inside the Tube

Reflections from the inside surface of the tube candecrease the field contrast of the image. Adding an extremelythin film of aluminum to the back of the fluorescent screencan virtually eliminate these reflections.

Video ResolutionDetermining the Minimum Visible Discontinuity Size

The resolution of a television system is the number of linesin the picture. As shown in Fig. 11, the electron beam

produces a picture by drawing repeated lines of varyingbrightness across the tube. In a video broadcast picture, asignal with 525 lines is used. About 480 lines actually formthe picture and the balance are used to return the beam fromthe bottom to the top of the screen. There is also a kind ofresolution in the horizontal direction, because televisionmonitors are designed to have equivalent horizontal and ver-tical resolution. Closed circuit television systems used forvisual nondestructive tests may have resolution of 500 lines,higher than consumer broadcast systems of about 200 lines.

Usually, a video system cannot resolve detail smaller thanone line. if the system has a 53 cm (21 in.) monitor and a 900active line display, then the smallest detail that can beresolved is predicted as follows. First, the vertical dimensionof the screen is determined—note that a nominal 53 cm pic-ture tube is 53 cm (21 in.) on the diagonal with a standardheight-to-width ratio of 3:4 (forming the right triangle shownin Fig. 15). For every 5 units on the diagonal (hypotenuseof the right triangle), there are 3 units on the vertical; forevery 1 unit on the diagonal, there are 3/5 or 0.6 units on thevertical. One might suppose that the smallest detail thatcan be resolved with 1:1 magnification and a 530 mm(21 in.) monitor then is (530 x 0.6) / 900 = 0.35 mm or(21 x 0.6) / 900 = 0.014 in.

In practice, however, there are other variables to consider.It is better to magnify by moving the camera closer to the testobject than merely to plug the video output into a largerscreen. Also, because of considerations such as orientationand lighting, indications may be detected that are narrowerthan a resolution calculation would predict. Moreover, lineresolution must be considered to be a characteristic of anentire system—the recording medium (video tape), thevideo camera and the monitor. The best way to quantify the

FIGURE 16, Typical underwater lamp for visual testswith a television system

HANGING SHROUD

PROTECTIVE BAR

WATERPROOF DOME PORT

BULB

\ BODY/HANDLE

CONNECTOR

CABLE



resolution of a system is to use resolution charts. Such chartsare expensive and may be replaced with visual acuity chartsand test pieces with known defects.

In addition to the line spacing requirements, the fre-quency response of the system must be high enough to allowsmall details to be electronically processed. The necessaryfrequency response can be calculated from systemcharacteristics,

If the vertical frame frequency of a high resolution televi-sion system is 30 frames per second and each frame consistsof 103 lines, the line frequency is 3 x 10 4 lines per second.To show a detail of the size figured above, the system uses1.8 X 10' line and requires a frequency response of about1.7 x 107 cycles per second. This is considerably more thancan be expected of the best closed circuit television equip-ment. A good system with a processing rate up to 5 x 10 6cycles per second should be capable of displaying 1.2 mm(0.05 in.) details.

These resolutions are computed without magnification. Ifa lens with 2:1 magnification is used, the requirements onthe electronic system are na so high and the size of the detailthat can be resolved is reduced, as expected.

Effect of Magnification

The benefits of magnification come with a price—not onlydoes magnification increase the size of the image, it alsoincreases the effect of camera motion and noise. A camerathat has 1:1 magnification or greater is difficult to positionaccurately. In order to minimize the effect of motion, it isimportant to have as many adjustments as possible within thecamera itself. For scanning where no specific fixturing isinvolved, a zoom lens is very useful. The camera can he posi-tioned with the lens in the wide angle mode and then focusedon the area of interest.

Some underwater cameras have built-in panning systems.Such units allow the lens to scan over a 180 degree segment,looking forward and to each side of the camera—a usefulfeature in restricted locations.

If a camera with internal focusing is not available, it isespecially important to provide a stable operating platform.The camera mount should allow the camera to freely rotateso that accessible areas can he viewed from all angles. Innuclear reactor applications, the camera is mounted onthe refueling bridge and may be working at depths up to30 m (100 ft). At these depths, water pressure increases atroughly 10 kPa.•m ' (about 0.5 psi.ft ).

Proper performance of a television system depends onlighting. Many types of lights are available; Fig. 16 shows atypical light source used with underwater television equip-ment. Adjustment of light intensity with a rheostat isdesirable.

Photographic Techniques for RecordingVisual Test ResultsDepth of Field

Depth of field may be defined as the range of distance overwhich a camera gives satisfactory definition, when its lens isin the best focus for a certain specific distance. It determinesthe overall sharpness of focus throughout a photograph.When a photograph is made, a single plane through the sub-ject is actually in focus and this is called the principal plane offocus. In a typical 35 mm camera, the lens aperture (f-stop)controls the thickness of the principal plane of focus or whatis known as the image's depth of field (Fig. 17). When mak-ing a photographic record of a visual test, focusing is nor-mally done with the lens diaphragm fully closed (highf number) for best image quality.

Using a standard 35 mm camera and a 55 mm lens, thebest control over depth of field can usually be obtained byfocusing one-third into the area of interest. This is donebecause the depth of field with a standard 55 mm lens

FIGURE 17. Effect of aperture on depth of field instill photography: (a) large aperture and (b) smallaperture

fa)LENS DIAPHRAGM

)-PRINCIPAL PLANE OF FOCUS

DEPTH OF HELD A A

(b)LENS DIAPHRAGM[

-

PRINCIPAL PLANE OF FOCUS

EXPANDED DEPTH OF FIELD


FIGURE 18. Principal plane of focus for measuringdimension A-B approximately off a photographicprint

LENS AXIS

SUBJECTA

PRINCIPAL PLANEOF FOCUS

FIGURE 19. Setup for use of a bounce flash to helpreduce subject reflection


extends farther behind than in front of the principal plane offocus. As magnification is increased (with longer lenses), thedepth of field extends farther in front of the principal plane.

Because most discontinuities are three-dimensional, thereis another factor to consider. Magnification is exact only atthe principal plane of focus. Where measurements of overalldiscontinuity size are made directly off a photograph, theprincipal plane of focus must be at the widest part of the sub-ject (Fig. 18).

Lighting

In general, orientation of the light source is an importantfirst consideration. Where possible, lighting should originatefrom the top of the subject. Lighting should originate fromone direction on most three-dimensional objects to avoidambiguity in relief—if more light is required, it should beslightly weaker and more diffuse than the main light source.

The photographer should exercise care to ensure that theillumination is sufficient for purposes of the inspection. Ifsurfaces of interest are obscured in shadow, another lightsource may be added. A single light source may createpatches of glare, a problem that may be solved by side light-ing. Too much light, on the other hand, may reduce contrastand make it difficult to see indications.

When photographing certain test objects (pipe welds, forexample), unwanted reflections from the flash unit are acommon problem. These can usually he eliminated by mov-ing the flash unit to direct the specularly reflected light awayfrom the lens. Another effective method of eliminatingsubject reflection is to bounce the flash off a white surface(see Fig. 19).

Film Choice

The size of a photographic negative is another importantconsideration. Negative size directly affects the quality ofenlargements—larger negatives produce better enlarge-ments.

The film speed is equally critical. Several factors influencethe choice, including the amount of light available on thesubject and the size of the final photographic print. High




speed film (high ASA number) requires less light but canproduce grain in the final print. This effect is increased withincreasing enlargement.

Slow speed films are used when fine detail is required. Thedisadvantage is that more light is required for proper expo-sure with a slow speed film.

Film speed is rated with two different systems: ASA num-ber (in America) and DIN number (Europe). Table 1 showssome ASA and MN numbers available for black and whiteand color photographic films.

Image EnhancementVisual images are a valuable tool in nondestructive testing

of many kinds. The primary advantage of a visual record isthat it can be reviewed and evaluated more than once.

Digital image processing can be a powerful tool in theinterpretation of many types of visual images. Frequently,such images contain more information than the human eyecan see because of the eye's limited ability to detect edgesand gray level differences. For example, radiographic filmcontains sufficient sensitivity to detect density differences of0.05 to 0.01 percent (1,000 to 2,000 gray levels). The humaneye can only resolve gray levels that differ by at least 2 per-cent (between 32 and 64 gray levels). A boundary or edgecondition can be distinguished by the eye only when twoadjoining areas of an image differ in density by 12 percentOr more.

Enhancement systems digitize an image in order toprovide data in a format acceptable to standard computers.

TABLE 1. Comparative photographic film speeds

Color Black and White

ASA DIN ASA DIN

Slow 25 15

32 16

Medium 64

19

80

20

100

21

125

22

Fast 160

23

200

24

400

27

1,000

31

FROM ELECTRIC POWER RESEARCH INSTITUTE. REPRINTED WITH PERMISSION.

The original picture information can be generated from avariety of sources: X-rays, gamma rays, ultrasonics and visi-ble or infrared light. After the image information is trans-ferred by appropriate mathematical models, the resultingimage enhancement is then displayed for analysis by theuser.

The digital image from the computer is an array of over256,000 elements. Each element of the array is called a pixeland each pixel has a numerical value attached to it. Thehigher the number associated with the pixel, the brighter isits appearance. The enhanced image is a result of the trans-fer of these numbers from the host computer through ananalog-to-digital processor and onto video tape.

Digital Matrix

A standard 8 x 8 matrix contains 64 gray levels. Movingfrom left to right along a row of the matrix, each box is 1/64brighter than the box preceding it. The boxes along each roware 1/8 brighter than the boxes in the row above. Each boxcontains pixels of identical value, but because of an opticalphenomenon called the match bend effect, each box appearsto be lighter on the top and darker on the bottom.

It is more difficult to distinguish gray level differencesbetween the boxes on the top and bottom of the 8 x 8matrix, as the intensity of the image is very high or very low.Again, it is a property of human vision that the eye can distin-guish smaller gray level differences at medium intensity thanat high or low intensity. Once the information from a pictureis digitized, the relative gray level differences between any ofthe 256,000 elements can be increased to improve visibility.

Digital computer enhancement can be used to correct avariety of image problems. A limited contrast range can bemathematically expanded over the full visual range of theviewer, permitting observation of details missed by the lim-ited gray scale resolution of the human eye. Edges and con-tours can be digitally enhanced for easier identification ofdiscontinuities or features in an image.

Mathematical models of the photographic process can beused to correct exposures. Errors in focus and blurring ofvideo tapes can be corrected with digital techniques.Selected portions of an image can be expanded by digitalmagnification to create enlargements for viewing and inter-pretation. Existing enhancement techniques may be usedwith archival images or new data produced from varioussources.

Novel Uses of Video SystemsAdvanced high temperature materials are being devel-

oped for use in aerospace systems," with considerable atten-tion given to monolithic silicon carbide and silicon nitride

32

1664

1980

20

100

21125

22160

23

200

24400

271,000

31

FIGURE 20. Digitized image of grain structure


ceramic materials." Research on monolithic ceramics subse-quently has led to research on advanced high temperaturecomposites consisting of particles, whiskers or fibers in metalor ceramic matrices.

Plasma spraying, reaction bonding, slurry pressing andsintering are typical techniques used to produce high tem-perature ceramics. These processes often produce materialswith widely variable microstructures. Typical microstruc-tural variations (porosity, agglomerates, grain size, interfacialstructure between phases and orientation of phases andgrains) all play a role in determining material properties. Theimportance of microstructural variations on a compositematerial's thermal and mechanical properties is beingaggressively researched.''''

Conventional X-ray and ultrasonic testing cannot identifydiscontinuities that lead to failure in ceramics. There are twoproblems that prevent obtaining this crucial information.The first is that ceramic materials, even monolithic ones, arecomplex systems that yield X-ray and ultrasonic data that aredifficult to interpret. For example, the standard techniquesfor making ultrasonic attenuation measurements for metals(single point measurements) are inadequate for monolithicceramies," . -19 requiring the use of imaging techniques. Thesecond problem is that the complex structure of ceramics hashindered the modification or development of nondestructivetesting methods appropriate for ceramics.

In spite of these problems, nondestructive testing per-formed during material processing can be used to monitorand track processes that lead to the creation of critically sizedpores and the matrix second phase interface in composites.

The text below gives an overview of an image intensivetechnique developed specifically for assisting in productionof materials: tone pulse encoding of microstructural images.

Microstructure Size Distributions

Material characteristics such as tensile strength, hardness,yield stress, fracture stress, impact resistance and fracturetoughness are directly related to grain, porosity and whiskersize distributions. Prediction of these properties requiresdetailed knowledge of the related size distributions. The the-oretical determination of size distributions has received con-siderable attention.' Several accepted techniques determinethe mean grain or pore size without measuring the size distri-bution function. However, these techniques are not applica-ble to any arbitrary system and the researcher mustdetermine which method yields the most accurate data. Theguidelines for making this decision are general and can leadto error in the results.'

In this discussion, a technique is described for determin-ing the grain and pore size distribution functions from ametallographically prepared sample. The resulting relation-ship is two-dimensional and yields the grain and pore size

distribution functions from which mean shape, size and ori-entation can be obtained.

Tone Pulse Encoding

In tone pulse encoding, a microstructural image (Fig. 20)is digitally recorded into a 512 x 512 pixel array by a video-camera attached to a computer controlled video digitizer.This image may reveal grain and pore boundaries. The pre-ferred method is to work with images that reveal only onefeature (grains or pores).

A two-dimensional gradient of the image (Fig. 21) is usedto enhance the boundaries of the feature. Optical noise inthe gradient image is removed using a two-level gray scale tohighlight the boundaries. Next, an image containing the fun-damental harmonics of each grain is generated. The tonepulse begins and ends at a grain boundary and has a widthequal to the width of the grain. The amplitude of the tonepulse is inversely proportional to the width. A new tone isstarted at an adjacent grain boundary, which typically has adifferent width. This is done digitally, starting from the cen-ter of the image and proceeding outward along the radius tothe perimeter of the image. The process is repeated for allangles (0 to 217 radians) so that a frequency encoded imageappears as shown in Fig. 22.

The density of length components IF,,(8,441s is obtained(see Fig. 23) by multiplying the two-dimensional Fouriertransform IF„,(s 4)1 of the tone pulse encoded image bythe magnitude s of the radial vector. Here 4 is the polar angleand s is the reciprocal vector of the spatial vector r'. Byreplacing s with 1/r' in IF,n(s A)) i s , the density of length com-ponents P(r',(13)) is obtained as a function of length r' (see

FIGURE 22. Tone pulse encoded image of materialshown in Figure 21

FIGURE 23. Density of length components as afunction of reciprocal vector s

FIGURE 24. Density of length components as afunction of length

FIGURE 21. Two-dimensional gradient of Figure 20exhibits enhanced grain boundaries

r ENHANCED/ GRAIN

BOUNDARY

70 PIXELS


Fig. 24). The mean grain length DA)) along any direction isobtained from P(r',)) and is given by:

/11)(r',(1))irr

D((I)) =

(Eq. 3)

(r',43)1

Equation 3 yields the mean grain shape from which the ori-entation can be obtained.

The tone pulse encoding technique can be modified fordetermining porosity (Fig. 25) size distributions. The areathat is tone pulse encoded between the pores is masked sothat only the pores are encoded. Subsequent processingyields the porosity size distribution function. The theory oftone pulse encoding may be studied in detail in theliterature.'

0 0

•

•a

FIGURE 25. Photomicrograph of pores, densityof length components and mean pore; image ofpores is enhanced by clipping data above apredetermined level to a constant value

1.-1 1-4 20 PIXELS

20 PIXELS ad 1-4


FIGURE 26. Front viewing camera set up forhorizontal underground pipe tests

r PIPE

SWAP 4,7//e/M4414////e/ 47,1407 OPYWN0q//0/1:65SW 741/PMASR/VAWIRMglibl/NReir/ WN9' 4/LIGHT

IEW ).>CAMERA 0 DEGREE ANGLE

TAGLINE

myyzgy, Ave a", ww,,,,,4,04407/01,W71.1,957...M6W/14141./ AFINIAIN/I/A64 7.711.1q7,14,

n LINEAMPLIFIER

CABLE CONNECTOR

FIGURE 27. Pipe inspection camera withsimultaneous axial and side view capability

PIPE„ „

MMIVUIVAXIAL

1:01WDZIO

LEGEND1. MULTICONDUCTOR ELECTRICAL CABLE2 MULTIPIN ELECTRICAL CONNECTOR3.REAR CONNECTOR WITH CENTRALIZER MOUNT4.CABLE EQUALIZATION AND DRIVE AMPLIFIER5_ CAMERA ELECTRONICS6.WATERPROOF HOUSING7.FRONT CENTRALIZER MOUNT8.LENS9.SIDE VIEWING MIRROR

IC. GLASS MIRROR COVERI I_ ROTATION MOTOR AND WIRING HARNESS12. LAMPIT SUPPORT RODS FOR MIRROR ASSEMBLY14.END COVER WITH ATTACHMENT FOR TAG LINE15.CENTRALIZING SPRING FOR VERTICAL PIPES

0 1 3

14MINIMUM

IS SIDE VIEW!,-ViEWANGLE

.1 NAO APPOPWW • ng 11.,17.01. HP .1 H:1,1

I 48 / VISUAL AND OPTICAL TESTING

PART 3

ACCESSORIES USED IN REMOTE INTERNALVIDEO TESTS OF PIPES

Visual testing of pipe and vessels can be performed using avariety of accessories, including a mobile, remote controlledtelevision camera.

Although common in some industries, remote video test-ing is new to chemical and petrochemical plant applications.Because of the wide range of pipe sizes and the variety ofpiping layouts, small cameras with sophisticated lightingsystems and a wide variety of support equipment weredeveloped.

Basic System DesignsThe most important accessory for this application is the

video horescope. A television camera is built into a cable thathouses multiple signal wires. A fiber optic bundle is includedfor conducting light up to 15 m (50 ft) and light emittingdiodes are provided for longer distances. One end of thecable has a charge coupled device imager chip and lens. Theother end has an electronics module containing the rest ofthe camera electronics and a light source. Commercial unitsof this type are designed to inspect bores as small as 13 mm(0.5 in.) inside diameter—a pipe inspection camera is typi-cally designed for 50 mm (2 in.) inside diameters up to thesize of a holding tank.

Pipe inspection cameras are divided into two basicdesigns: a one-piece unit that can send signals more than60+ m (200 + ft) and a much smaller two-piece camera thatcan transmit about 30 m (100 ft).

One-Piece Camera Designs

The one-piece camera is self-contained and needs onlyelectrical power to operate. The output is a standard videosignal, either RS-170 black and white or NTSC color. Thesecameras are connected to a specialized cable reel and themonitor console connects to the cable reel with a patch cord.

There are two basic configurations of the one-piece pipecamera. Both designs are usually 50 to 75 mm (2 to 3 in.)diameter and are enclosed in a watertight metal housing withintegral lighting systems. The more common design is a frontviewing camera with a wide angle lens and lights positionedon either side of the lens (Fig. 26). This design provides anaxial view of the pipe and is useful for visual tests of vesselsor tests that must cover 1 km (three thousand feet) of pipe in

a limited time. Cameras of this type are usually 30 to 50 cm(12 to 20 in.) long.

A second type of one-piece camera uses a rotating scan-ning head for viewing the sidewalls of the pipe (see Figs. 27and 28). This design allows a direct view of the sidewall andusually provides a simultaneous axial view of the pipe. Rota-tion is controlled, stopped and started by the operator. Thescanning head also contains the system's light source.

OTHER INSTRUMENTATION AND ELECTRONIC AIDS FOR VISUAL TESTING / !49

FIGURE 28. Combined view camera for visual testsof pipe

Two-Piece Camera Designs

The two-piece camera uses an electronics modulemounted remotely from a small camera head. The camerahead contains lights, a lens and a charge coupled deviceimager chip or a miniature vidicon tube. The head sends asignal to the electronics module through a specially designedcable. The electronics module processes the signal into astandard RS-170 or NTSC signal.

There are three types of two-piece cameras. The first typeuses a vidicon tube instead of an imager chip. This type hasevolved into a specialized design that is highly developed forspecial service but uncommon in general service. Camerasof this type have been incorporated into high performanceunits with special capabilities and requirements.

The second type is a very small camera, often using fiberoptics to transmit light from a distance. The head assembly isfixed to the insertion cable and can be very small, 6 to 13 mm(0.25 to 0.5 in.) in diameter. Cable lengths vary from 7.5 to30 m (25 to 100 ft) and are rarely lengthened beyond that dis-tance because of electronic timer problems. This type ofcamera is useful only in bores under 50 mm (2 in.). indiameter.

The third type has a small camera head, measuring 13 to25 mm (0.5 to 1.0 in.) in diameter and separated from thehead by a distance of up to 30 in (100 ft). The head is acharge coupled device imager, usually color. These are thesmallest color cameras that can be fitted with a variety oflenses. With lights and protective housing, these camerasrange from 25 to 38 mm (1 to 1.5 in.) in diameter and 50 to100 mm (2 to 4 in.) in length. Many two-piece cameras of thisthird type were replaced in the early 1990s by one-piececameras, which are compact and have lower light levelrequirements than two-piece cameras. The monochromecameras have light requirements low enough so that high

output light emitting diodes can provide enough light forvisual testing of pipe and small vessels.

Color Transmission

Camera systems can be built for black and white or colorimage transmission. Black and white systems have higherresolution and require lower light levels than color systems.Black and white cameras are typically smaller than color sys-tems, allowing a smaller package. Black and white reproduc-tion uses the ability of the human eye to see detail inmonochrome with resolution practically as good as thatachieved with color viewing.

Color cameras provide a more vivid image. Eventually,color cameras will be introduced with reduced size and light-ing requirements. Color imaging is important in inspectionswhere a change in color is one of the indicators of a problem,but black and white systems are superior for tests where thefeature of interest is the same color as the parent material.Electronic enhancement of the image is far easier in blackand white than in color.

Support EquipmentThe support equipment for video pipe testing systems can

be as important as the television camera. This equipmentconsists of a means of transport, video control console, cable,cable reel and centralizers for moving and positioning thecamera inside the pipe.

Means of Transport

Three means of transport are (1) pushing, where the oper-ator pushes a cable or rod from one end of the pipe, (2) pull-ing, where a tag line is first sent pneumatically orhydraulically through the pipeline and (3) carrying, wherea crawler serves as a robotic vehicle to carry the cameraor probe.

1. Most inspections require access to only one end of thepipe. Cables are stiff enough to push through pipelengths of up to 30 m (100 ft). For distances up to 60 in(200 ft), fiberglass rods about 13 mm (0.5 in.) in diame-ter may be used to push the probe.

2. For lengths over 30 m (100 ft) a tag line may be blownthrough the pipe and the camera pulled through. Thefluid moving the tag line may be water or air with pres-sure provided by any of the following three means: asuction truck of the sort common in sewer mainte-nance; an air compressor, used with a parachute con-sisting of a metal mandrill and one or two cups; and a


conventional pump for liquids. Pumping water throughthe pipeline has the additional benefit of cleaning it.

3. Crawlers or pigs have been used for years for radio-graphic testing. The same sorts of devices are easilyconfigured for visual testing.

Control Console

The monitor or control console consists of a portable elec-trical cabinet that contains the camera controls and powersupplies for the camera and light sources. Console stylesrange from simple electronics cases designed for desktop use(with a separate monitor and video recorder) to rugged,purged cabinets designed for use in hazardous areas housingthe monitor, power supply and controls.

The video monitor is typically a 23 cm (9 in.) diagonal or a13 cm (5 in.) portable console. Larger units are more suitedto truck-mounted systems or desktop applications. Systemmonitors are industrial video monitors with a direct videoinput. Direct current restoration is normal, while selectableunderscan (a video enlargement feature) is common. Moni-tors may be color or black and white: depending on the cam-era. Color monitors typically measure horizontal resolutionsof 350 to 400 lines. Black and white monitors often measure700 to 800 lines of resolution. Both are available in modelsthat offer 1,000 horizontal lines.

Video recorders typically consist of a consumer grade unitused with desktop consoles or a professional grade portable(12 V DC) video cassette recorder used when mountedinside a cabinet. Super VHS recorders with their super-ior resolution (400 line horizontal resolution compared to225 line resolution for standard VHS) are becoming morepopular.

System Video Cable

In a pipe testing system, cables carry signals and power forthe camera in three ways. Cameras used for distances over30 m (100 ft) are usually lowered or pulled through the pipe,primarily in a straight line or through bends of 45 degrees orless. The cable must be flexible enough to move around cablesheaves. Typical sizes run from 11 mm (0.44 in.) to 16 mm(0.62 in.) diameter with four to eight conductors plus a coax-ial conductor. Jacket materials are usually nitrite, neopreneor urethane. Bend radii are typically 200 to 250 mm (8 to10 in.).

The second type of cable is similar to the first but is stifferfor purposes of pushing the camera through the pipe. Typi-cally a 16 mm (0.62 in.) outside diameter urethane cable canmove a camera in a straight 100 mm (4 in.) inside diameterpipe for 9 m (30 ft) before buckling. A significantly stiffer19 mm (0.75 in.) outside diameter jacketed cable can push acamera through a longer distance and multiple bends,including 90 degree and multiple offset bends. The stiffer

cable cannot easily run through a 250 mm (10 in.) diametercable sheave.

The third type is an armored coaxial cable used for longerdistance transmission. A multiplexer combines the video sig-nal, power and control signals for simultaneous transmissionon a single conductor. This cable is used for specialized sys-tems at distances exceeding 600 m (2,000 ft). Such systemsare expensive because of the cost of the cable, special elec-tronics and mandatory powered cable reel.

Cable Reels

Portable cable reels for handling multiconductor cable areavailable in two styles. Both use a cable drum, electrical slipring, a frame with casters and drum support, hand crank andbrake. The larger styles, holding about 90 m (300 ft) of cable,are used with cameras designed to be pulled through straightruns of pipe. This type has a drum 40 to 60 cm (16 to 24 in.)in diameter and 20 to 30 cm (8 to 12 in.) wide. The reel isusually hand powered with a chain driven crank or a crankmounted on the drum. The width of the drum requires thata level wind unit be added if a cable measuring head is used.A hand crank cable reel with 90 m (300 ft) of 16 mm(0.62 in.) outside diameter cable typically weighs 80 kg(180 lb). This design must be stable to handle vertical runs ofcable that may have up to 45 kg (100 lb) of camera and cablehanging inside a pipe or well.

The second reel design is used with cameras that arepushed through pipe by the cable, frequently through multi-ple bends. This cable is stiffer and its length is usually about30 m (100 ft). Basic components are the same, but the cabledrums are typically narrower and taller: 8 to 15 cm (3 to 6 in.)wide and 50 to 90 cm (20 to 36 in.) diameter. This designallows for use of a cable measuring head without a level windattachment. This reel is usually lighter, with a simple handcrank mounted on the drum and a screw-type friction brake.

Camera Centralizers

Camera centralizers may be solid rings or nylon blades,wheels, brushes, flat springs or sleds. Centralizer designs arebased on the size and type of pipe being inspected, whetherthe pipe is horizontal or vertical and whether the camera isto move through bends.

Long, horizontal runs of concrete pipe are commonlyinspected with a camera mounted on a sled made of tubularrunners. This design is simple and reliable but is unsuited forvertical pipes. Sleds are adjustable so that one size can cen-tralize a camera in a number of pipe sizes.

Wheeled centralizers are used in pipe where low drag isneeded or for inspecting coated pipe without damage to thecoating. There are two basic designs: fixed wheels andwheels with suspension. Fixed wheels are suitable for push-ing a camera through one size of pipe. Suspended wheels can

FIGURE 29. Typical test setup for inspection ofvertical pipe, well or stack

LEGENDMONJTOR CONSOLE

2.CABLE REEL3.CAMERA CABLE4.CAMERA5.VERTICAL TEST OBJECT6 CABLE SHEAVE

OTHER INSTRUMENTATION AND ELECTRONIC AIDS FOR VISUAL TESTING / T$T

flex for moving through a reducer or through dented pipe.Both designs use three or four wheels at each end of the cam-era, with the wheels arranged symmetrically around thecamera body. Suspended systems can be made adjustable fora range of pipe sizes and are especially useful in large pipesof 75 to 200 cm (30 to 84 in.).

Solid centralizers use solid rings or blades made of nylon.These are simple to maintain, effective in traversing badwelds and will not damage coated pipe. A ring centralizer isplaced at each end of the camera, with the ring about 5 mm(0.2 in.) smaller in diameter than the pipe's drift diameter.This design is effective in 50 to 100 mm (2 to 4 in.) sizes.Above that, long blades or runners on adjustable spacers areused.

Cameras for inspection of vertical pipe frequently use a setof half-elliptic springs for centralizers. These are fixed to thecamera body at one end and to a sleeve that slides over thecamera body at the other end. A combination of spring sizesand spacer blocks can accommodate many pipe sizes. In ver-tical pipes, the springs centralize but do not support the cam-era. This allows for relatively soft springs and easy movementthrough swages and reducers. Centralizing half-ellipticsprings work well in horizontal pipes if stiffer springs areused. The stiffer springs cause extra drag going through areducer into a smaller pipe and this must be allowed forwhen selecting the centralizer to be used in a line.

Field ApplicationsThe openings to pipes and vessels are often above ground

or in an awkward location. The camera cable must be guidedfrom this opening to the cable reel. Cable sheaves and guidesare used, whenever the cable must change directions, toavoid cutting on sharp edges and to avoid bending at less thansafe-bend radius. Sheaves may be hung from a beam with atensioning chain pulling from below, clamped to a beam, ormounted on a baseplate. In confined areas, a curved guidewith the same bend radius as the sheave is used (Fig. 29).

Cameras cannot see through sand, mud, scale or soot. Thepipe must be purged and cleaned for practical reasons as wellas for safety, according to ANSI/AWS Z49.1, Safety in Weld-ing and Cutting. Cameras with lights are not as hazardous aswelding operations but size restraints have inhibited thedevelopment of fully explosion-proof lights. If necessary, acamera may be operated in a line filled with water, eliminat-ing problems with heat from the lamps.

Tag lines are frequently blown through the pipe in order topull a camera through. Tag lines are commonly used in pipecleaning and the same tag line can be used in pulling thecamera. A drift is frequently run through smaller sizes ofpipe in advance of the camera.

Video monitor consoles should be protected from theweather. The preferable setup is to run sufficient patch cordfrom the cable reel to the console in order to place it indoors.The console may be rain-proof but glare on the monitor isstill a problem. Viewing through a hood avoids the glare, butis tiring.

A video visual inspection crew normally consists of a cam-era operator, an assistant and an inspector or engineer. Thecamera operator is supplied by the video inspection firm; theassistant and inspector are often employees of the customer.The video inspection firm usually provides analysis of thevideo tapes with an in-house Level III inspector or metallur-gist writing the report.

It is the camera operator's responsibility to ensure that allfeatures (discontinuities, welds and so forth) in the pipe orvessel are recorded on video tape. Tested areas are identifiedfor future examination and editing. The second duty of thecamera operator is to set up the cable reel, cable guides andsheaves for moving the camera through the pipe.

Visual Testing with Remote Cameras

Tests of Pipe Wall

Inspection of the pipe wall provides information on distri-bution and severity of erosion, cracks, dents or pitting fromcorrosion. With additional equipment, the diameter of pitscan be measured. Direct measurement of pit-depth withelectronic imaging is a recent development. A direct view ofthe pipe wall can be compared to an image of a reference

FIGURE 30. Four types of weld discontinuitiesin 100 mm (4 in.) diameter pipe: (a) lack ofpenetration and small burn-through, (b) undercut,(c) intermittent lack of penetration and (d) largeburn-through bridged over and unfilled

(a)

(c)

(d)


standard of pits with a known depth and an estimate of pitdepth is then made.

Tests of Equipment

Inspection of valves and other inline equipment with acamera reveals information not available by any other type ofvisual testing. Using a sidescan camera, valves may beinspected for erosion, corrosion or damage from foreignobjects. A front viewing camera may be used to view a valveunder pressure or a valve closing to determine if an actuatoris operating properly. Bubble trays, weirs and other vesselcomponents are routinely tested with a remote videocamera.

Tests of Welds

Weld inspection is an important use of video testing tech-nology. A camera with a rotating scanning head can show thecomplete weld root for documentation on video tape. Dis-continuities such as incomplete penetration, excessive pro-trusion, cracking and internal undeicutting are clearly visible(Fig. 30). With a good lighting system, a camera can provideimages equal in quality and detail to many published refer-ence photographs.

If the weld is to be video taped and analyzed at a latertime, such procedures can occur rapidly. Scanning of theweld by means of a rotating mirror can be done at a rate of10 to 20 cm (4 to 8 in.) per minute, depending on motorspeed and pipe diameter.

There are three basic techniques for remote video testingof welds. The first method is to move the camera through apipe at 3 to 6 m (10 to 20 ft) per minute. The pipe is viewedusing the axial image and the camera is stopped at problemareas where the side wall is scanned for 360 degrees. Assum-ing that a sidewall image covers a distance of 1 mm per 8 mmof pipe diameter (1 in. per 8 inches). In 100 mm (4 in.) ofpipe, the sidewall image covers a width of 12.5 mm (0.5 in.).If the scanning head is rotated at a speed of 2 rpm, a weldand an area of 25 mm (1 in.) on either side of the weld canbe scanned in three minutes. A more detailed examinationtakes longer.

The second method for remote testing of welds is to scan100 percent of the pipe wall, note problem areas and per-form a detailed post inspection analysis of the video tapes.Using the speeds listed above, 30 cm (12 in.) of 10 cm (4 in.)pipe can be scanned in six minutes. This video tape methodis especially useful for crack detection and corrosion inspec-tion. Careful selection of scanning (rotation) speeds and therate of camera movement are needed to achieve a completescan of the pipe wall. Depending on pipe conditions, the rateof camera movement may be doubled but the rate does notchange for larger (or smaller) pipe sizes (the angle of cover-age is constant for all sizes).


The third method for remote testing of welds is to rapidlypull a front viewing camera through the pipe. This allows upto 800 m (0.5 mi) of pipe to be visually tested in eight hourswith an axial view. Large cracks can be detected with thismethod and a general assessment of the pipe wall can hemade. Except for determination of excess penetration, noother weld inspection can be done. The technique has theadvantage of speed and ease. Test results have been good,especially when looking for cracks in concrete pipe, leakageinto underground pipe and foreign object damage.

Tests of Vessels

Vessel inspection may be performed using a front viewingvideo camera. This is useful for visual tests of bubble trays,tank bottoms and other vessel internals. Cameras with sidescanning capability are useful for inspecting vessel wallswhen a considerable area can be covered by rotation of thescanning head. With the camera mounted horizontally, theside scanning head can be used to inspect vessel internals.

Advantages of Using Remote VisualEquipment

Remote video testing is the most cost effective way toinspect piping and vessels. The cost of such tests is deter-mined by three factors: (1) setup time, (2) time and laborneeded to move the camera through the pipe and (3) timespent interpreting the test image. In all cases, these remotevideo costs are less than alternative means of gaining thesame information.

With its ability to produce images available in no otherway, remote video tests are invaluable for the inspection ofpipes and vessels. For example, some piping underground orin confined areas cannot be examined with radiography.

Video tests require access to the line without majorexpense and assumes the line can be shut down and cleaned.Insulation does not need to be removed.


REFERENCES

1. Kazan, B. and F.H. Nicoll. "An ElectroluminescentLight-Amplifying Picture Panel." Proceedings of theIRE. Vol. 43. New York, NY Institute of Radio Engi-neers (1955): p 1,888.

2. Rotow, A.A. "An Image Orthicon for Pickup at LowLight Levels." RCA Review. Vol. 17. Princeton, NJ: RCALaboratories (1956): p 425. See also: "TV AmplifiesLight 40,000 Times." Electronics. Vol. 29, No. 3. NewYork, NY McGraw-Hill Book Company (1956): p 16.

3. Weimer, P.K., S.V. Forgue and R.R. Goodrich. "The Vid-icon Photoconductive Camera Tube." Electronics.Vol. 23, No. 5. New York, NY McGraw-Hill Book Com-pany (1950): p 70. See also: Vine, B.H., R.B. Janes andF.S. Veith. "Performance of the Vidicon, a SmallDevelopmental Television Camera Tube." RCA ReviewVol. 13. Princeton, NJ: RCA Laboratories (1952): p 3.

4. Flory, L.E. "The Television Microscope." Cold SpringHarbor Symposia for Quantitative Biology. Vol. 16. ColdSpring Harbor, NY: Cold Spring Harbor Laboratory(1951): p 505.

5. Zworykin, VK., L.E. Flory and R.E. Shrader. "Ultravio-let Television Microscopy ' Electronics. Vol. 25, No. 9.New York, NY McGraw-Hill Book Company (1952):p 150.

6. Cope, A.D. and A. Rose. "X-Ray Noise ObservationUsing a Photoconductive Pickup Tube." Journal ofApplied Physics. Vol. 25. Woodbury NY American Insti-tute of Physics (1954): p 240.

7. Zworylcin, V.K., G.A. Morton, E.G. Ramberg, J. Hillierand A.W. Vance. Electron Optics and Electron Micro-scope. New York, NY John Wiley and Sons (1945).

8. "Whatever Happened to CCDs?" IEEE Spectrum. NewYork, NY: Institute of Electrical and Electronics Engi-neers (October 1981): p 26.

9. "Charge-Coupled Devices: Technology and Applica-tions." IEEE Press. R. Melen and D. Buss, eds. NewYork, NY: Institute of Electrical and Electronics Engi-neers (1977).

10. HITEMP Review 1989: Advanced High Tempera-ture Engine Materials Technology Program. NASACP-10039. Washington, DC: National Aeronautics andSpace Administration (1989).

11. Structural Ceramics. NASA CP-2427. Washington, DC:National Aeronautics and Space Administration (1986).

12. Greil, P., G. Petzow and H. Tanaka. Ceramics Interna-tional. Vol. 13, No. 1. Barking, Essex, United Kingdom:Elsevier Applied Science Publishers Limited (1987):p 19-25.

13. Nickel, K.G., M.J. Hoffman, P Greil and G. Petzow.Advanced Ceramic Materials. Vol. 3, No. 6. Westerville,OH: American Ceramic Society (November 1988):p 557-562.

14. Freedman, M.R., J.D. Kiser and WA. Sanders. ASintering Model for SiC 4 Composites. NASATM-101336. Washington, DC: National Aeronautics andSpace Administration (1988).

15. Wei, C.C. and P.E Becher. American Ceramic SocietyBulletin. Vol. 64, No. 2. Westerville, OH: AmericanCeramic Society (February 1985): p 298-304.

16. Kobayashi, S., T. Kandori and S. Wada. Journal of theCeramic Society of Japan. Vol. 94, No. 8. Tokyo, Japan:Yogyo Kyokai (1986): p 903-905.

17. Ishigaki, H., R. Nagata, M. Iwasa, N. Tamari andI. Kondo. Journal of Tribology (Transactions of theASME). Vol. 110, No. 3. New York, NY American Soci-ety of Mechanical Engineers ( July 1988): p 434-438.

18. Generazio, E.E., D.J. Roth and D.B. Stang. Journal ofAmerican Ceramic Society. Vol. 72, No. 7. Westerville,OH: American Ceramic Society ( July 1989): p 1,282-1,285.

19. Cenerazio, E.R., D.J. Roth and G.Y. Baaklini. MaterialsEvaluation. Vol. 46, No. 10. Columbus, OH: The Ameri-can Society for Nondestructive Testing (September1988): p 1,338-1,343.

20. Quantitative Microscopy. R.T. DeHoff and F.N. Rhines,eds. New York, NY: McGraw-Hill Book Company(1968).

21. Generazio, E.R. "Determination of Grain-Size Distribu-tion Function Using Two-Dimensional Fourier Trans-forms of Tone-Pulse-Encoded Images." MaterialsEvaluation. Vol. 46, No. 4. Columbus, OH: The Ameri-can Society for Nondestructive Testing (March 1988):p 528-534.

SECTION 6

VISUAL AND OPTICAL TESTINGPROCEDURES

PARTS I AND 2 AND PORTION OF PART 3 ADAPTED FROM WELDING INSPECTION, AMERICAN WELDING SOCIETY. REPRINTED WITH PERMISSION.PORTIONS OF PART 3 ADAPTED FROM ASME BOILER AND PRESSURE VESSEL CODE 989] AND ASME CODE FOR PRESSURE PIPING (ANSI/ASME B31: 1 982, 1 984, 1989, 1990J,

0 AMERICAN SOCIETY OF MECHANICAL ENGINEERS. REPRINTED WITH PERMISSION.PORTIONS OF PART 3 ADAPTED FROM DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS [API 620, 1982 and 1990( AND WELDED STEEL

TANKS FOR OIL STORAGE (API 650, 1988), © THE AMERICAN PETROLEUM INSTITUTE. REPRINTED WITH PERMISSION.


PART 1 OBJECTIVES OF VISUAL WELD TESTS

Weld integrity is often verified with visual testing tech-niques. Compared to other nondestructive methods, visualtests are easy to implement and relatively inexpensive (seeTable 1). Visual tests have proven to be reliable sources ofaccurate information about a weldment's conformity tospecifications.

Visual Weld Testing PracticesA weld inspector must be familiar with applicable docu-

ments, workmanship standards and all phases of good shoppractice. During a visual test, the weld must be well lighted(an extension lamp or flashlight may be needed). Scales andgages are used for checking the adequacy of the weld. Inac-cessible areas can be viewed with a borescope and, whenrequired, a low power magnifier can be used. Magnifiersshould be used with caution because they accentuate surfacecharacteristics. Visual testing of welds is generally donewithout the aid of magnifiers, unless specified.

Welds that are inaccessible in a finished product shouldbe visually inspected during construction or assembly.Although visual testing is the most basic of the nondestruc-tive methods, a specific procedure should be established andused to ensure adequate coverage of the test surface.

Before WeldingVisual testing begins with inspection of the material before

fabrication, to detect and eliminate conditions that tend tocause weld discontinuities. Scabs, seams, scale or otherharmful surface conditions can be detected visually. Platelaminations may be observed on cut edges (particularly dur-ing edge preparation burning operations) and plate dimen-sions may be determined by measurement. Identification ofmaterial type and grade also must be made.

After the components are assembled in position for weld-ing, the inspector must check root openings, edge prepara-tion and other features of joint preparation that might affectthe quality of the welded joint. The following conditionsmust be checked for conformity to specifications: (1) jointpreparation, dimensions and finish; (2) clearance dimensionsof backing strips, rings or backing filler metal; (3) alignmentand fit-up of the welded pieces; (4) cleanliness; and (5) met-allurgical characteristics.

During WeldingWhile fabrication is in progress, visual testing can be used

to check details of the work, including: (1) welding process

TABLE 1. Comparison of visual and radiographic testing of welds

Visual equipment magnifiers, color enhancement, projectors, measurement equipment (rulers, micrometers, opticalcomparators), light source

applications welds with surface breaking discontinuitiesadvantages economical, expedient, requires relatively little equipment, applicable at all stages of fabrication and

weldinglimitations for surface conditions only; dependent on the visual acuity of the inspector

Radiography equipment gamma ray sources, gamma ray camera projectors, X-ray machines, film holders, film, lead screens, filmprocessing equipment, accessories for film, radiation monitoring equipment

applications most weld discontinuities, dimensional evaluationsadvantages permanent record for later review; radiation sources may be positioned inside certain test objects;

no external energy sources needed for gamma rayslimitations radiation safety hazard requires special facilities and monitoring of exposure levels; gamma sources decay

and must be periodically replaced; gamma radiation output (wavelength) of certain isotopes (iridium-I92or cobalt-60) cannot be adjusted; related licensing requirements are expensive; radiography requireshighly skilled personnel

FIGURE 1. Visual testing reference standards for(a) groove welds and 03) fillet welds

(alPOLISH AND ETCHTHIS SURFACE

MACRO SAMPLE

25 mm(1 in.)

, 747/

DUPLICATE FILLET WELD ylON THIS SIDE ALSO

TACK MACRO SAMPLETO PLATE WITH

ETCHED SURFACE UP(13J

25 mm(I in.)"),

7,947

POLISH AND ETCHTHIS SURFACE50 mm

)2 in,)

MACRO SAMPLE

TACK MACRO SAMPLEUPRIGHT TO PLATE WITHETCHED SURFACE OUT

FROM AMERICAN WELDING SOCIETY. REPRINTED WITH PERMISSION.

VISUAL AND OPTICAL TESTING PROCEDURES / 157

and conditions; (2) filler metal; (3) flux or shielding gas;(4) preheat and interpass temperature; (5) distortion control;and (6) interpass chipping, grinding or gouging.

Testing of successive layers of the weld deposit is some-times carried out with the assistance of a workmanship orreference standard. The typical reference standard is a sec-tion of a joint similar to the one in manufacture, revealingportions of successive weld layers. Each layer of the produc-tion weld may be compared with corresponding layers of thereference standard (see Fig. 1). Allowances must be madefor production tolerances.

From the point of view of final soundness the first weldlayer or root pass is the most critical. The root pass freezesquickly because of: the geometry of the joint, the relativelylarge volume of base metal compared to the root pass weldmetal, the fact that the plate may be cold and the possibilitythat the arc may not strike into the root. By freezing quickly,the root pass tends to trap slag or gas that resists removal dur-ing subsequent passes. The metal melted during this passis particularly susceptible to cracking. Such cracks may notonly remain but may extend to subsequent passes. Visualtesting of the root pass should be thorough and shouldinclude careful reference to a workmanship standard. Othernondestructive tests may give evidence of nonvisual

conditions in the root pass and can be supplemental to thevisual testing procedure.

Inspection of the root pass offers another opportunity tolook for plate laminations, which tend to open up as a resultof the heat present during welding. In the case of doublegroove welds, slag from the root pass on one side of the platemay cause slag deposits on the other side. Such deposits areusually chipped, ground or gouged out before welding theopposite side. If this is done improperly, slag can remain inthe root of the finished weld.

The root opening should be monitored as root pass weld-ing progresses. Special emphasis is given to the adequacy oftack welds, clamps or braces designed to maintain the rootopening to ensure penetration and alignment. The impor-tance of this root opening is not limited to butt joints but alsoapplies to branch and angle connections that are more diffi-cult to inspect after the weld has been completed.

After WeldingVisual testing is useful for verifying critical characteristics

of finished products, including:

1. dimensional conformance of the finished weldment(including distortion) to specification;

2. determination of weld completion and conformation ofwelds to required location, size and contour;

3. acceptability of weld appearance (including surfaceroughness, ripple and weld spatter);

4. presence of unfilled craters, porosity, undercuts, over-laps and cracks;

5. surface imperfections from punch marks, gouges,other test markings or excessive grinding; and

6. postweid heat treatment time and temperature.

Dimensional conformance of finished weldments is usu-ally determined by conventional measuring methods. Theconformity of weld size and contour is normally determinedwith a weld gage. The designer generally defines the size ofany fillet weld, right angle or skewed, in terms of the lengthof the leg measured at a right angle to the member fromwhich the weld size is being determined. As examples ofboth right angle and skewed fillet welds being sized withweld gages are shown in Figs. 2 and 3. A gage is used todetermine whether the weld size is within allowable limitsand whether there is excessive concavity or convexity. Whennecessary, special gages may be made for use where surfacesare at acute or obtuse angles.

The width of finished groove welds fluctuates with therequired groove angle, root face, root opening and permissi-ble tolerances. The height of reinforcement should be con-sistent with specified requirements. Where not specified,

FIGURE 2. Fillet weld gaging: (a) right angle filletweld and (b) skewed fillet weld

WELD SIZE

(a)

(b)

AFTER MENSA LABORATORIES LTD.

FIGURE 3. Fillet weld gaging: (a) right angle filletweld and (b) skewed fillet weld

—1 rWELD SIZE

AFTER FIBREMETAL PRODUCTS CO.

(a)

(b)


the inspector may have to rely on judgment and a knowledgeof good welding practice. Requirements for surface appear-ance differ widely and should be specified in a code or cus-tomer specifications. Visual reference standards or sampleweldments submitted by the fabricator and agreed to by thepurchaser can be used as guides to appearance. Sometimes asmooth weld, strictly uniform in size and control, is requiredbecause the weld forms part of the exposed surface of theproduct and good appearance is required.

The presence of discontinuities that affect service is, inmost instances, more objectionable than those that affectappearance. A few examples of such discontinuities includecracks, suckback, thinning, undercut, overlap and dimen-sional nonconformance.

For reliable detection of such discontinuities, the weldsurface should be thoroughly cleaned of oxide and slag. Thecleaning operation must be carried out carefully. For exam-ple, if a chipping hammer is used to remove slag, hammermarks could mask fine cracks. Shot blasting and wire brush-ing may peen the surface of relatively soft materials andcould hide discontinuities.

Marking Repair WeldsOne of the most important details of nondestructive test-

ing is proper marking of the areas to be repaired. Such mark-ings should be:

1. in accordance with a method of marking establishedand understood by all inspectors, welders and othershop personnel involved in the repair;

2. of a distinctive color not easily confused with othermarkings;

3. permanent enough to be visible after the repair hasbeen made and inspected;

4. selected so that the marking agent does not damage thematerial; and

5. removable if not acceptable in service conditions.

Because marks often disappear in the repair process, a linerepresenting the weld with all comments and marks could bemade precisely 100 mm (4 in.) beside the weld. Originaldefect indications remain intact and the defect position canbe exactly relocated.

After a repair has been made and inspected, it should bemarked to indicate if the repair is satisfactory


ConclusionVisual testing is valuable for inspection of welds but

caution must be used during interpretation. For example,good surface appearance is sometimes considered indicativeof careful workmanship and high weld quality. However,

surface appearance alone does not prove careful workman-ship and is not a reliable indication of subsurface conditions.Judgment of weld quality must be based on evidence addi-tional tional to surface conditions. Such evidence is availablefrom observations made before and during welding and fromother nondestructive tests.


PART 2SAMPLING PLANS FOR VISUAL TESTS

In some cases, 100 percent testing of a product may berequired. Otherwise, sampling procedures are specified.Sampling is the selection of a representative portion of a pro-duction run for purposes of testing. Conclusions can bedrawn about the overall quality of an entire production runbased on (1) the outcome of sample tests and (2) the statisti-cal basis for the sampling procedure. Test specifications orprocedures usually detail sampling methods for the visualinspector.

Procedural Sampling MethodsSome of the terms found An visual testing procedures for

sampling are complete, partial, specified and random.

Complete InspectionComplete inspection is the testing of an entire production

lot in a prescribed manner. Sometimes, complete inspectiondesignates the inspection of only the critical joints in eachweldment. One hundred percent testing requires theinspection of all welded joints by prescribed methods. Com-plete sampling is used where the highest quality is required.It is costly and time consuming compared to partialsampling.

Partial Sampling

Partial sampling is the testing of a certain number lessthan the total in a production run. The method of selectionfor inspection and the type of testing are prescribed. Therejection criteria and disposition routine for any substandardweldment should also be specified on the procedure.

Specified Partial Sampling

For specified partial sampling, a particular frequency orsequence of sample selection is prescribed. An example ofspecified partial sampling is the selection of every fifth unit,starting with the fifth unit. Because a producer knows inadvance which units will be tested, more care could bedevoted to the production of those tested units and the over-all quality level of the production run may be lower than

indicated by the inspected units. This would invalidate thesampling.

Random Partial Sampling

Random partial sampling occurs when test object selec-tion is fully random. For example, one out of every five unitsfrom a production run is to he inspected and the selection ofeach of the candidate units from the subgroups of five ismade by the inspector in a random manner. Then each ofthe five units has an equal chance of being chosen and ran-dom partial sampling occurs.

Because the producer does not know in advance whichunits will be tested, equal care must be given to the produc-tion of all units and a more uniform product quality results.

Sampling Plans for Testing by Attributes

Visual testing by attributes (or characteristics) involvesclassifying the test object as defective or nondefective. Thetest object may also be classified by the number of disconti-nuities associated with a given requirement (such as disconti-nuities per hundred units) or a given set of requirements.

The final result of weldment tests is the identification ofdiscontinuities—certain discontinuities may or may not beacceptable within code or specification requirements.Therefore, typical visual tests of welds can be classified astesting by attributes.

MIL-STD-105D contains procedures and tables withsampling plans for testing by attributes.' The sampling plansare relatively easy to use and interpret.

Operating Characteristic Curve

A sampling plan is specified by a lot size N, a sample sizen and an acceptance number c. For each sampling plan,there is an operating characteristic curve indicating the per-centage of lots to be accepted for a given process qualitylevel. Put another way, the operating characteristic (OC)curve (see Fig. 4) presents the probability of acceptance asopposed to the true quality level of the process.

In most sampling plans, the producer runs a 10 percentrisk (1.00 —0.90) of having a lot rejected in which the qualitylevel is actually as good as the acceptable quality level (AQL).The acceptable quality level is defined as the maximum per-cent defective (or the maximum number of discontinuities

FIGURE 4. Operating characteristic curve1.00

LOT TOLERANCEPERCENT DEFECTIVE

05 1.0 1.5 2.0 25 30 3.5

PERCENT DEFECTIVE


0.90 --- ACCEPTABLE (DUALITY LEVEL

0.80 -

0.70 -

0.60 -

0.50 -

0 40 -

a3o --

0 20 -

0.10 -

0

0


per hundred units) that, for the purposes of sampling tests,can be considered satisfactory as a process average.

In most sampling plans, the consumer runs a 10 percentrisk (0.10) of accepting lots in which the quality level is actu-ally as low or lower than the lot tolerance percent defective(LTPD). The lot tolerance percent defective is usuallydefined as the percent defective at which there is a 10 per-cent probability of acceptance.

From Fig. 4, a typical operating characteristic curve, theacceptable quality level and the lot tolerance percent defec-tive can be determined. A 0.10 probability of rejection(1.00 –0.90), the producer's risk, is a 0.90 probability ofacceptance (1.00 – 0.10). With a quality level of 0.4 percentdefective, the producer runs a 10 percent risk of having lotsrejected that have as few as 0.4 percent defective units. Theconsumer's risk of 0.10 leads to a lot tolerance percent defec-tive of 2.2 percent, which means that the consumer runs a10 percent risk of accepting lots that have as many as 2.2 per-cent defective units.

In plans where defective units found in sampling arerepaired or replaced, the average outgoing quality (AOQ) isgreater than the process quality. This is true because defec-tive units are replaced with good units, thereby increasingthe average outgoing quality over that which would haveoccurred had the defective units not been replaced. In suchplans, the average outgoing quality at the lot tolerance per-cent defective point is greater than that indicated by the

operating characteristic curve. The average outgoing qualitylimit (AOQL) can be determined for any given quality level(percent defective).

The sampling plans in MIL-STD-105D are indexed byacceptable quality level and sample size code letter. Thesample size code letter is determined by lot size from a table.The sample plans cover a wide range of acceptable qualitylevels and lot sizes from 2 to infinite. Operating characteris-tic curves and average outgoing quality limit curves are givenfor each sampling plan. For any given lot size and acceptablequality level, the user can find a plan that satisfies a givenconsumer's risk.

Control Charts for AttributesControl charts for attributes can be used effectively for

production line products to indicate the product averagequality level and the acceptable variation about the average.As the name implies, the chart shows whether or not thequality level is in control (within prescribed limits). Controlcharts are constructed by taking samples of a given size fromeach subgroup of product as it comes off the line. The aver-age percent defective is calculated from each sample, as wellas the sample standard deviation (a measure of varianceabout the average). Once the process average has beenestablished, the sample percent defectives are plotted on thechart against the sequence (time, lot) in which the samplesare taken. The sample standard deviations are used to calcu-late control limits.

A process that is out of control is shown in Fig. 5a and aprocess that is in control is shown in Fig. 5b. When a processgoes out of control, the manufacturing processes are investi-gated to determine the cause. Control charts can also beused to detect when the process average is shifting. Z

ConclusionSampling is used when 100 percent testing is uneconomi-

cal or impractical. When sampling procedures are used,there are always risks to both the producer and theconsumer.

When the sampling plan is given in the procedure or spec-ification, the visual inspector need only follow the procedure.Sampling plans may be chosen for specific risks fromMIL-STD-105D.

Complete inspection is used when weldments of the high-est quality are required for critical services. One or moreother methods of nondestructive testing, along with visualtesting, may be specified for critical points.

FIGURE 5. Typical control charts for percentdefective: (al out of control process and (b) incontrol process

(a)

LCL

UCL

70

6.0 -

5.0 -

4.0

3.0

2.0

1-0

i 2 3 4 5 6 7 Ei 9 1 0DATE

7.0

6.0

5.0

4.0

3,0

20

1.0

UCL

----------LCL

0 - 2 3 4 5 6 7 8 9 1 0DATE

LEGENDUCL = UPPER CONTROL LIMITLCL = LOWER CONTROL LIMIT

PROCESS AVERAGE PERCENT DEFECTIVE



For a typical welding job, visual testing generally involvesa combination of complete inspection and random partialsampling. All of the welds are inspected visually. Randompartial sampling is then used to select test objects for theother nondestructive testing methods.

If a lot is rejected, any of several courses of action can betaken, depending on considerations of production and qual-ity: (1) the lot could be scrapped: (2) each part in the lotcould be inspected individually; (3) a larger sample could betaken; or (4) more sophisticated test methods such as ultra-sonics or radiography could be applied.

FIGURE 6. Weld alignment: (a) incorrect angularalignment and (b) correct alignment obtainedusing proper control methods


(b)

FIGURE 7. Desirable fillet weld profiles: (a) concaveprofile and (b) convex profile, where C is not toexceed 0.1 x size + 0.75 mm (0.03 in.)

(a)

(b)



PART 3VISUAL TESTING ACCEPTANCE CRITERIAFOR WELDS

The text below is provided for instructional purposes andpresents weld testing as typical of all visual tests. Thoughseveral published standards are referenced, their wordinghas been adapted to the style of the NDT Handbook. Inaddition, the wording of the original documents has beenabstracted and generalized to facilitate a broad instructionalbase. Do not we this text in place of any specification, codeor standard.

Weld DiscontinuitiesA discontinuity is, an interruption of a typical structure,

such as inhomogeneity in the mechanical, metallurgical orphysical characteristics of a material. A discontinuity is notnecessarily a defect, but all defects are discontinuities.

Discontinuities associated with welds may be divided asfollows: (1) dimensional, (2) process, (3) mechanical orchemical and (4) base metal properties. Dimensional dis-continuities include distortion; incorrect weld size, profile orproportions; and excess weld reinforcement. Process discon-tinuities include porosity, inclusions, incomplete fusion,inadequate joint penetration, undercut, cracks and surfaceirregularities.

Dimensional DiscontinuitiesAlthough dimensional checks are usually considered to be

distinct from nondestructive tests, in practice there is often aneed to consider them together. Dimensional characteristicsare directly related to the material and mechanical quality ofa test object and to the very sorts of performance, such asservice life, that nondestructive testing is used to evaluate.For these reasons, some welding situations call for dimen-sional gaging, subjective dimensional visual tests and conven-tional nondestructive tests—all to be performed by the sameinspector.

Welding typically involves the application of heat and thefusion of metal. Stresses of high magnitude result from ther-mal expansion and contraction during weld metal solidifica-tion. These stresses remain in the weldment after coolingand can cause discontinuities such as distortion (see Fig. 6).

The size of a normal equal leg fillet weld is expressed asthe leg length of the largest isosceles right triangle that can

be inscribed within the fillet weld cross section (see Figs. 7to 9). The size of a groove weld is the joint penetration(depth of chamfering plus the root penetration when speci-fied). Welds that are not adequate in size may he detected

FIGURE 8. Acceptable fillet weld profiles

(a) -

(b)


FIGURE 9. Unacceptable fillet weld profiles:(a) insufficient throat, (b) excess convexity,(c) excess undercut, (d) overlap, (e) insufficient legand (f) inadequate penetration

(a)

(b)

(c)

(d)

(e)



visually using a weld gage or by comparison with approvedreference standards.

The profile of a finished weld may have considerable effecton its performance under load. In addition, the profile ofone pass in a multipass weld may increase the tendencytoward certain discontinuities (incomplete fusion or slaginclusions) when subsequent layers are deposited. Specificrequirements concerning the acceptability of such disconti-onuities are usually included in the working specifications (seeFigs. 7 to 11).

Overlap is another dimensional discontinuity— the weldmetal protrudes beyond the fusion line at the weld toe (seeFig. 12). Overlap tends to produce notches that serve asstress concentrators under load. Overlap can also occur atthe toe of a completed weld's reinforcement.

Typical Power Boiler Visual TestingAcceptance CriteriaCleaning and Alignment

Surfaces to be welded should be free of paint, oil, rust,scale or other foreign materials before welding. Cleanlinessis verified by visual inspection before welding.

Offset tolerances of butt welded edges are given inTable 2. Any offset within the allowable tolerance is faired at

(a J

(bj

(cl

Id)

FIGURE 10, Acceptable butt weld profile; Rreinforcement (see Table 3)

FRAM AMERICAN wrimiNG SOCIETY. REPRINTED WITH pERMitsiON

FIGURE 11. Unacceptable butt weld profiles:fa) excess convexity, (13) Insufficient throat,(c) excess undercut and (di overlap

FROM AMERICAN WELDING SOCIETY. REPRINTED WITH FERmiSsIoN.

FIGURE 12. Weld overlap

FROM AMERICAN wELDiNG SOCIET y, rrrekii9rE0 turn,' PERMISSION.

VISUAL AND OPTICAL TESTING PROCEDURES f 165

a 3 tot tapyr over the width of the finiAied weld or, if times-

saty by adding additional weld metal beyond the edge of theweld.

Reinforcement and Spacing

The thickness of reinforcement on each face tithe weld in

TABLE 2. Typical offset tolerances of butt weldededges ft Is the nominal thickness of the thinner sectionat the Joint)

Section Thickness Maximum Permissible Offsetmillimeters {inches' Longitudinal' Circumferential'

Up to 13 10.51 inclusive

0.25i 0,251Over 13 to 19 (0.5 to 0.751

IElcluCtVe

3 10.1251 0.25tOver 19 to 38 (0.75 to 1.51

roclwNe

3 (0.1251 4.7 (0.18751Over 38 to 5011.5 to 2)

lusive

3 10.1251

0. i 1St

Over 50 12]

see tied ow'

see below'

I. GWEN M A MULTIPLE OF t OR IN miU.IMETERS {INCHES'1. LESSER OF 6. 6254 or 9...r 10.3753. LESSER OF 0.12St or 19 rnm 10.75 InI.

FROM AMERICAN SOCIETY OF MECHANICAL ENGINEERS. REPRINTED WITHPERMISSION.

1'ig. 10 should Got exceed the vanes shown in Table 3. Theroot opening of joints is done according to weld procedurespecifications.

Discontinuities Found by Visual Testing

The dIA•miltirkilit.tes build by ►•isnal testing of power boilerwelds include those shown in Table 4 for pressure vessels.The acceptance limits are also listed.

TABLE 3. Typical maximum thicknesses forreinforcements on each face of a weld

Nominal Thicknessmillimeters {Inches}

Maximum ReinforcementOtherWelds

millimeters'inches)

CircumferentialJoints

millimeters(inches)

Up to 310.1251 2.3 10 093/51 23 /0.093/51Over 3 to 4.7 (0.125 to 0.1875)

inclusive 3.0'0.1251 2.3 10.093751Over 4.7 to 1310.1875 to 0.51

inclusive 3.9 (0.156) 2.3 10.093751Over 13 to 2510.5 tO 11 i nciir5ive 4.7 (0.18 15) 2.3 10.09375)Over 25 to 50 (I to 21 inclusive 6.3 P.251 310.1251Over 50 to 75 12 to 31 inclusive 3.9 (0.1561Over 75 to 10013 to 41 inclusive 5.47 (0.219)Over 1 00 to 125 14 to 5' inclusive 6.3 10.251Over 125 (51 7.8 10.3125)

• cREATER Of 6.)ThAt10.251r. I of 0.125 TIMES THE WIDTH OF THE WELD

FROM AMERICAN SOCIETY OF MECHANICAL ENGINEERS. RERRrNYED WITHPERMISSION,


TABLE 4. Typical disposition of weld discontinuitiesdetected by visual testing of pressure vessel welds

Cracking none permittedIncomplete penetration none permittedLack of fusion none permittedUndercut

0.75 mm (0.03 in.) or 10 percentExternal porosity none permittedExternal inclusions none permitted


Typical Acceptance Criteria for VisualTesting of Pressure and Storage VesselsAlignment

The edges of butt joints are restrained during welding sothat the maximum offset shown in Table 5 is not exceeded inthe completed joint (see also Figs. 13 and 14). When fittedgirth joints have deviations exceeding permitted tolerances,the head or shell ring, whichever is out-of-true alignment, isrefit, reworked or reformed until the misalignment is withinthe specified limits.

When fillet welds are used, the lapped plates must fitclosely and are kept in contact during welding. The size of afillet weld is determined as shown in Fig. 15.

Attachments such as lugs, brackets, saddle nozzles, man-hole frames and reinforcements around openings must fitreasonably well to the curvature of the shell or surface towhich they are attached. When pressure components such

TABLE 5. Typical maximum allowed offset for weldjoint categories; section thickness t is the nominalthickness of the thinner section at the joint (seeFigs. 13 and 14)

Section Thicknessmillimeters (inches!

JointCategory

1*

JointCategories

2,3,4*

Up to 13 (0.5) inclusive 0.25t 0.25tOver 13 to 19 (0.5 to 0.75) inclusive 3.0 (0.125) 0.25tOver 19 to 38 (0.75 1.5 ) inclusive 3.0 (0.125) 4.7 (0.1875)Over 38 to 50 11.5 to 2} inclusive 3.0 (0.125) 0.125tOver 50 {2) 0.06.3r or O. i 25t or

9.4 (0.375) 18.8 (0.75}

GIVEN ASA MULTIPLE OF t OR IN MILLIMETERS (INCHES)


as saddle nozzles, manhole frames and reinforcementaround openings, extend over pressure retaining welds, theportion of the weld to be covered is ground flush with thebase metal and should be visually tested before being cov-ered. When nonpressure components such as lugs, bracketsand supports, extend over pressure retaining welds, thewelds are ground flush or the components may be notchedor coped to clear the welds.

For circumferential head-to-shell butt welds, the length ofthe taper does not extend beyond the tangent line of thehead and the misalignment of the centerlines of the shell andhead should be no greater than half the difference in thethicknesses of the two. (See Fig. 13 and Table 5.)

FIGURE 13. Welded joint locations in a pressure vessel typical of categories 1, 2, 3 and 4 (see Table 5)

FROM AMERICAN SOCIETY OF MECHANICAL ENGINEERS. REPRINTED WITH PERMISSION.

FIGURE 14. Minimum welding dimensions requiredfor socket welding components other than flanges

11-]LLI

LU

0w

E

0CO

LEGENDto = NOMINAL PIPE WALL THICKNESSCx = MINIMUM IS I .09tn OR THE THICKNESS OF THE SOCKET WALL,WHICHEVER i5 SMALLER

FROM AMERICAN SOCIETY OF MECHANICAL ENGINEERS. REPRINTEDWITH PERMISSION.

FIGURE 15. Fillet weld size: (a] convex equal legfillet weld, (131 concave equal leg fillet weld, (c)convex unequal leg fillet weld and (d) concaveunequal leg fillet weld

(alTHEORETICAL THROAT

(b)

THEORETICAL THROAT

( C )THEORETICAL THROAT

LEG LENGTH

HIE-LEG LENGTH–Nod

THEORETICAL THROAT

LEG LENGTH

NOTESI THE SIZE O F AN EQUAL LEG FILLET WELD IS THE LEG LENGTH OF THE

LARGEST INSCRIBED RIGHT ISOSCELES TRIANGLE. THEORETICAL THROAT =0.7 x SIZE

2 FOR UNEQUAL LEG FILLET WELDS, THE SIZE OF THE WELD iS DESCRIBEDUSING BOTH LEG LENGTHS AND THEIR LOCATION ON THE JOINEDMEMBERS.

3. FILLET WELDS BETWEEN MEMBERS AT ANGLES OTHER THAN 90 DEGREESARE DESCRIBED AS IN NOTES 1 AND 2.

4. FOR ALL FILLET WELDS, THE THEORETICAL THROAT IS DETERMINED BYCALCULATIONS BASED ON THE ANGLE BETWEEN THE WELDED SURFACESAND SPECIFIED LEG LENGTHS.

5 FOR ALL FILLET WELDS, LEG DIMENSIONS AND THEORETICAL THROATDIMENSION LIE WITHIN THE CROSS SECTION OF THE DEPOSITED WELDMETAL AS SHOWN ABOVE


(d)

LEG LENGTH


tr

Reinforcement on Welds

The thickness of the weld reinforcement on each face ofthe weld should not exceed the specified values.


Typical discontinuities found by visual testing of pressureand storage vessel welds are shown in Table 6.

Acceptance Criteria for Pipe WeldsEnd Prep for Socket Welds

For assembly of a joint before welding, a pipe or tube maybe inserted into a socket to the maximum then withdrawnabout 1.5 mm (0.06 in.) away from contact between the endof the pipe and the shoulder of the socket. In sleeve jointswithout internal shoulders, there is typically a distance ofabout 1.5 mm (0.06 in.) between the butting ends of the pipeor tube. (See Fig. 16.).

Alignment

The inside diameters of piping components should bealigned as accurately as practical within existing commercialtolerances on diameters, wall thicknesses and out-of-roundness.

I

CrackingIncomplete penetrationLack of fusionUndercut in vertical jointsUndercut in horizontal jointsExternal porosityExternal slag inclusionsConcave root surface

see below'none permitted2none permitted'0.4 mm (0.016 in.)0.75 mm (0.033 in.)see below'see below'see below3

1. WHEN THE DISCONTINUITY IS FOUND BY A HYDROSTATIC LEAK TEST, ITSHALL BE REJECTED AND REPAIRED.

2. THE DEPTH OF INCOMPLETE PENETRATION OF A GIRTH AND MITER BUTTWELD SHALL NOT EXCEED THE LESSER OF 0.75 mm 10.033 In.} OR 20 PER-CENT OF WALL THICKNESS. THE TOTAL LENGTH OF SUCH IMPERFECTIONSSHALL NOT EXCEED 38 mm 11.5 In.} OF WELD LENGTH.

3. FOR SINGLE SIDED WELDED JOINTS, CONCAVITY OF THE ROOT SURFACESHALL NOT REDUCE THE TOTAL THICKNESS OF THE JOINT, INCLUDINGREINFORCEMENT, TO LESS THAN THE THICKNESS OF THE THINNER OFTHE COMPONENTS BEING JOINED.


(al

t„, tm

FIGURE 16. Joint trimming and permittedmisalignment in butt welds: (a) thicker pipe taperbored to align and (b) thicker pipe bored foralignment

PERMUTED MISALIGNMENTPER WELDING SPECIFICATION

30 DEGREEMAXIMUM

(b)

fPERMITTED MISALIGNMENTPER WELDING SPECIFICATION

ROUND "-CORNER

30 DEGREEMAXIMUM

FIGURE 17. Typical details for double welded slip-on and socket welding flange attachment welds(X„,,„ is the lesser of 1.4T or the thickness of thehub): (a) front and back welds, (b) face and backwelds and (c) socket welding flange

(al ror

1025 in.)

11111.1111. 11•\1V

mmLESSER OF I OR 6.3

(13 )

X„,„

'Mk\ `V\''%• A. NIL.

LESSER OF T OR6 3 mm 10-25 in.)

(c)

ABOUT I5 mm 10.063 I n. )BEFORE WELDING



TABLE 6. Typical discontinuities located by visualtesting of storage vessels or tanks

Discontinuity Limitations

such trimming should not result in a piping component wallthickness less than the minimum design thickness and thechange in contour should not exceed 30 degrees.

Fillet Welds

Fillet pipe welds may vary from convex to concave. Thesize of a fillet weld is determined as shown in Fig. 15. Typicalminimum fillet weld details for visual testing of slip-onflanges and socket welding components are shown in Figs. 14and 17.

Welded Branch Connections

Figures 18 and 19 show details of typical branch connec-tions with and without added reinforcement. Figure 20shows basic types of weld attachments used in the fabricationof branch connections. Figure 21 shows branch connectionsmade by welding half couplings or adapters directly to therun pipe. The cover fillet welds have minimum throatdimensions not less than that shown in Fig. 21a.

Where ends are to be joined and the internal misalign-ment exceeds 1.5 mm (0.06 in.), the component with the wallextending internally may be internally trimmed (see Fig. 16)so that adjoining internal surfaces are nearly flush. However,


(a)

(b)

FIGURE 18. Typical welded branch connection:(a) without additional reinforcement and (b) withadditional reinforcement


FIGURE 19. Typical welded angular branchconnection without additional reinforcement

WOLIONIIIMILVINNIMININIMILVONWOMMI011


Table 7 lists typical reinforcements ofOrth and longitudi-nal butt welds. Discontinuities detected with visual testsinclude cracking, undercut, lack of fusion, external slag,porosity, incomplete penetration and concave root surface.

FIGURE 20. Some acceptable types of weldedbranch attachment details showing minimumacceptable welds {note that weld dimensions maybe larger than the minimum values shown here)

(a) tnb YI IF

(b) trib

(c)

(d)

(e)

LEGENDtc — SMALLER OF 6.3 mm 10.25 in./ OR 0 7 robi nr — NOMINAL THICKNESS OF REINFORCING ELEMENT (RING OR SADDLE((min = SMALLER OF tniD OR tnrtrip = NOMINAL THICKNESS OF BRANCH WALLrnh = NOMINAL THICKNESS OF HEADER WALL



Typical Visual Testing AcceptanceCriteria for Pipe WeldsEnd Prep for Girth and Miter Joint Butt Welds

If component ends are trimmed for fitting backing rings or

FIGURE 21. Typical full penetration weld branchconnections for 75 mm (3 in.) and smaller halfcouplings or adapters: (a) branch connection usingforged steel socket welding or threaded halfcoupling and (b) branch connection using forgedsteel socket welding or threaded adapter forpressure and temperature conditions greater thanpermitted for forged steel fittings

SOCKET WELDING ORTHREADED HALF COUPLING

1.5 mm (0.063 in.)

FULL PENETRATION

AE/WELD AND FILLET WELD

A* HEADER OR RUN PIPE

q..*Vb.4.8 mm

(0.19 in.)

SOCKET WELDING ORTHREADED ADAPTER

4.8 mm (0.19 in.)MINIMUM

NW- FULL PENETRATION/ WELD AND FILLET WELD

/ HEADER OR RUN PIPE

BORE AFTER WELDING


faj

(b)

/ 70 / VISUAL AND OPTICAL TESTING

consumable inserts or as shown in Figs. 16 and 22 forcorrect-ing internal misalignment, such trimming may not result ina finished wall thickness before welding that is less than therequired minimum wall thickness t„,. Where necessary, weldmetal may be deposited on the inside or outside of the com-ponent to provide alignment or sufficient material formachining to ensure satisfactory seating of rings or inserts.It is also permissible to size pipe ends of the same nominalsize to improve alignment, if the above wall thicknessrequirements are maintained.

AlignmentFor girth and miter joint butt welds, inside diameters

of components at the ends to be joined are aligned withinthe dimensional limits in the welding procedure and theengineering design. If the external surfaces of the twocomponents are not aligned, the weld is tapered between thetwo surfaces.Preparation for longitudinal butt welds shall conform to therequirements of the welding specification. Branch connec-tion welds that abut the outside surface of the run wall arecontoured to meet the welding specification requirements(see Fig. 23).

Fillet Socket WeldsFillet welds (including socket welds) may vary from convex

to concave. The size of a fillet weld is determined as shownin Fig. 15. Typical weld details for slip-on flanges and socketwelding components are shown in Figs. 17 and 24. If slip-onflanges are single welded, the weld is at the hub.

TABLE 7. Typical reinforcement of girth and longitudinal butt welds

Maximum Thickness ofReinforcement for Design Temperature

Thickness of Base Metalmillimeters (inches)

400 °C1> 750 °F)

Trim in.

175 to 400°C(350 to 750 °F)

mm in.

175 °CI> 350 °F)

mm in.

Up to 3 (0.125) inclusive 7 0.063 2.5 0.10 5 0.19Over 3 to 5 (0.125 to 0.019) inclusive 2 0.063 3 0.125 5 0.19Over 5 to 13 (0.019 to 0.5) inclusive 2 0.063 4 0.16 5 0.19Over 13 to 25 (0.5 to 1) inclusive 2.5 0.083 5 0.19 5 0.19Over 25 to 50 (1 to 2) inclusive 3 0.125 6 0.25 6 0.25Over 50 (21 4 0.16 *

• THE GREATER OF 6 mm (0.25 In.) OR 0.125 TIMES THE WIDTH OF THE WELD IN MILLIMETERS 'INCHES/.1. FOR DOUBLE WELDED BUTT JOINTS, THE LIMITATION ON REINFORCEMENT GIVEN ABOVE APPLY SEPARATELY TO BOTH INSIDE AND OUTSIDE SURFACES

FOR THE JOINT.2. FOR SINGLE WELDED BUTT JOINTS, THE REINFORCEMENT LIMITS GIVEN ABOVE APPLY TO THE OUTSIDE SURFACE OF THE JOINT ONLY.3. THE THICKNESS OR WELD REINFORCEMENT IS BASED ON THE THICKNESS OF THE THINNER OF THE MATERIALS BEING JOINED.4. THE WELD REINFORCEMENT THICKNESSES IS DETERMINED FROM THE HIGHER OF THE ABUTTING SURFACES INVOLVED.5. WELD REINFORCEMENT MAY BE REMOVED IF DESIRED.

FROM AMERICAN SOCIETY OF MECHANICAL ENGINEERS. REPRINTED WITH PERMISSION.

FIGURE 22. Typical backing rings and consumableinserts: (a) butt joint with bored pipe ends and solidor split backing ring and (b) butt joint with taperbored ends and solid backing ring

4.8 mm (0.19 in.)(a)

3 to 4.8 mm(0.125 to 0.19 in.) 19 mm (0.75 in_)

(14 4.8 mm (0.19 in.)

• 7;7 .oz.•

3 to 4.8 mm(0.125 to 0.19 in.)

19 mm (0.75 in.)


FIGURE 23. Preparation for branch connections

B

4% A

B

0 mm (0 in.) MINIMUM

hh-LEGENDg = ROOT GAP PER WELDING SPECIFICATIONm = THE LESSER OF 3 mm P.I25 tn.) OR 0.5Tb


FIGURE 24. Minimum welding dimensions forsocket welding components other than flanges

ABOUT 1.5 mm (0.063 in.)BEFORE WELDING

LEGENDt = PRESSURE DESIGN THICKNESSCx = MINIMUM IS 1 251 BUT NOT LESS THAN 3 mm (0.125 in )



Pipe weld discontinuities found by visual testing are shownin Table 8.

Alternative Visual Testing AcceptanceCriteria for Pipe WeldsEnd Preparation

Some acceptable end preparations are shown in Figs. 25and 26.

Figures 27 to 29 show acceptable end preparation for buttwelding of pieces having unequal thickness or unequal yieldstrength or both.

Flange weld configurations are shown in Fig 15 and Figs.30 to 32.

Fillet Welds

Minimum dimensions for fillet welds used in branch con-nections are shown in Figs. 33 and 34.

Reinforcement of Welds

Branch connections are attached by a weld for the fullthickness of the branch or header wall plus a fillet weld W,shown in Figs. 33 and 34. The use of concave fillet welds ispreferred to further minimize corner stress concentration.

FIGURE 25. Standard end preparations:(a) optional end preparation of pipe; (b) standardend preparation of pipe and butt welding fittings22 mm (0.875 in.) and thinner; and (c) suggestedend preparation, pipe and fittings over 22 mm(0.875 in.) thickness

(a) 30 1+5, -0) DEGREES

4= 1.5 + 0 75 mmT (0.063 ± 0.033 in.)

(b)

22 mm (0.875 rt-71-MAXIMUM

( C )10 ± 1 DEGREES

37.5 ± 2.5 DEGREES

( 5 ± 0.75 mmT (0.063 ± 0.033 in.)

-1--› 22 mm (0.875 in.)

19 mm 10.75 In 11 t

i RADIUS

°'. 37.5 ± 2.5 DEGREES1

_L_ IS ±0.75 0.75 mill(0.063 ± 0.033 in.1



TABLE 8. Typical pipe weld discontinuities located byvisual testing methods

Discontinuity Limitations

Cracking none permittedIncomplete penetration none permitted'Lack of fusion none permittedUndercut see below2External porosity none permittedExternal slag inclusions none permittedConcave root surface see be low3Weld reinforcement

(groove and fillet} see below45

I. FOR NORMAL FLUID SERVICE, THE DEPTH OF INCOMPLETE PENETRATIONOF A GIRTH AND MITER BUTT WELD CANNOT EXCEED THE LESSER OF 0.75mm (0.033 In.) OR 0.2 Tw. THE TOTAL LENGTH OF SUCH DISCONTINUITIESDOES NOT EXCEED 38 mm 41.5 In.) OF WELD LENGTH.

2. FOR NORMAL FLUID SERVICE, LESSER OF 0.25 Tw OR 0.75 mm 10.033 In.),EXCEPT NONE PERMITTED IN STRAIGHT OR LONGITUDINAL BUTT WELDS.GIRTH AND MITER GROOVE WELD LIMIT FOR CATEGORY 4 FLUID SERVICE,IS LESSER OF 0.25 Tw OR 1.5 mm 10.063 In. ), EXCEPT THAT UNDERCUT LESSTHAN 0.75 mm (0.033 In.) IS ACCEPTABLE.

3. FOR SINGLE SIDED WELDED JOINTS, CONCAVITY OF THE ROOT SURFACECANNOT REDUCE THE TOTAL THICKNESS OF THE JOINT, INCLUDING REIN-FORCEMENT, TO LESS THAN THE THICKNESS OF THE THINNER OF THECOMPONENTS BEING JOINED.

4. EXTERNAL WELD REINFORCEMENT AND INTERNAL WELD PROTRUSION(WHEN BACKING RINGS ARE NOT USED) IS FUSED WITH AND MERGESSMOOTHLY INTO THE COMPONENT SURFACES. THE HEIGHT OF THEEXTERNAL WELD REINFORCEMENT OR INTERNAL WELD PROTRUSIONFROM THE ADJACENT BASE MATERIAL SURFACE CANNOT EXCEED THEFOLLOWING LIMITS.

WELD REINFORCEMENT ORWALL THICKNESS INTERNAL WELD PROTRUSION

millimeters (inches) millimeters (Inches)

6.3 10.25) AND UNDER

1.5 (0.063)OVER 6.3 (0.251 TO 13 (05)

3 10.125)OVER 13 10.5) TO I 4 (0 16)

OVER 1 4.810.19)

NOTE: FOR GROOVE WELDS, USE LESSER OF THE MEASUREMENTS MADEFROM EACH SIDE OF WELD

5. FOR CATEGORY 4 FLUID SERVICE THE ACCEPTABLE VALUE LIMITS ARE TWOTIMES MAXIMUM LIMITS LISTED UNDER NOTE 4. EXAMPLE: 6.310.25J ANDUNDER WOULD BE 2 x 1.5 (0.063) = 3 (0.125).


Ring or saddle reinforcements are attached as shown inFig. 34. When a full fillet is not used, it is recommended thatthe edge of the reinforcement be relieved or chamfered atabout 45 degrees to merge with the edge of the fillet.

Discontinuities found by visual testing of pipe welds arelisted in Table 8.

Typical Visual Testing AcceptanceCriteria for Storage Tank WeldsReinforcement on Welds

The thickness of the weld reinforcement on each face ofthe weld does not exceed the values shown in Table 9.

The thickness of the reinforcement on each side of awelded storage tank plate may not exceed the values inTable 9. Discontinuities found by visual testing of storagetank welds are shown in Table 6.


The maximum out ofplumbness of the top of the shell rel-ative to the bottom of the shell does not exceed 0.5 percentof the total tank height. For roundness, radii measured at0.3 m (1.0 ft) above the bottom corner weld do not exceedspecified tolerances. With a horizontal sweep board 0.9 m(3.0 ft) long, peaking does not exceed 13 mm (0.5 in.). Witha vertical sweep board 0.9 m (3.0 ft) long, banding does notexceed 13 mm (0.5 in.).

Typical Visual Testing AcceptanceCriteria for PipelinesDiscontinuities Found by Visual Testing

Typically a weld must be free of cracks. However, shallow

FIGURE 26. Acceptable combinations of pipe endpreparations

(a)60 TO BO DEGREES

30 1+5, —01 DEGREES 37.5 ± 25 DEGREES

301+5, — Cl DEGREESTO 37.5 ± 2.5 DEGREES

37.5 ± 2.5 DEGREES

/170 I DEGREES


FIGURE 27. Internal weld offset (t = nominalthickness of the thinner section at the joint)

L 2.4 mm 10.094 in.)MAXIMUM

(al

fc)

051 MAXIMUM

(b)la = 1.5)

0 5t MAXIMUM30 DEGREE MAXIMUM14 DEGREE MINIMUM )1:4)* A

30 DEGREE MAXIMUM

(dit„

30 DEGREE MAXIMUM 0.51. MAXIMUM

30 DEGREE MAXIMUM14 DEGREE MINIMUM I I 4)* i

* NO MINIMUM WHEN JOINED MATERIALSHAVE EQUAL YIELD STRENGTHS


+1/4\

‘N.


crater or star cracks located at the stopping point of weldbeads resulting from weld metal contraction during solidifi-cation are not considered injurious discontinuities unlesstheir length exceeds 4 mm (0.16 in.).

At no point should the crown surface be below the outsidesurface of the pipe, nor should it be raised above the parentmetal by more than 1.5 mm (0.06 in.). Two beads are notstarted at the same location. The face of the completed weldis about 3 mm (0.125 in.) greater than the width of the origi-nal groove.

When visual and mechanical means are used to determinedepth, undercutting adjacent to the cover or root bead can-not exceed the values shown in Table 10.

Note that when both mechanical and radiographic mea-surements are available, the mechanical measurements areused to determine if the undercut is acceptable or rejectable.

Inadequate penetration of the weld root should notexceed 25 mm (1 in.). The total length of such a condition inany continuous 300 mm (12 in.) length of weld cannot exceed25 mm (1 in.). If the weld is less than 300 mm (12 in.) long,

then the total length of such a condition cannot exceed 8 per-cent of the weld length.

Inadequate penetration from high-low, when one edge ofthe root is exposed (or unbonded), cannot exceed 50 mm(2 in.) at individual locations or 75 mm (3 in.) in any continu-ous 300 mm (12 in.) length of weld.

Incomplete fusion cannot exceed 25 mm (1 in.) in lengthat individual locations. The total length of such a conditionin any 300 mm (12 in.) length of weld metal cannot exceed25 mm (1 in.). If the weld is less than 300 mm (12 in.) long,then the total length of such a condition cannot exceed 8 per-cent of the weld length.

Internal concavity, incomplete fusion from cold lap, slag'inclusion, internal porosity or gas pockets and burn-throughare inspected by radiography. When automatic or semiauto-matic welding is used, clusters of surface porosity, bead startsand high points are removed by grinding before depositingweld metal over them.

FIGURE 31. Piping weld flange attachments:(a) front and back weld and (b) face and back weld

0.25t

0;5t

13 mm 0.5 in.iMAXIMUM

(b)

0.251 •

0 25t

0.707tOR t IF PREFERRED)


(a)

FIGURE 32. Socket weld details: (al socket weldingonly and (b) socket welding flange

( a )

NOT LESS THAN t

1.5 mm I0.063 in.)

0.25t(b)

LEGENDt NOMINAL PIPE WALL THICKNESSAmin = .25t BUT NOT LESS THAN 4 mm !0.16 in J


(b)

FIGURE 30. Flange weld attachment details:(a) lap joint flange and (b) butt weld flange

(a)

FROM AMERICAN SOCIETY OF MECHANICAL ENGINEERS. REPRINTEDWITH PERMISSION

FIGURE 28. External weld offset0.51 MAXIMUM

(a)4

30 DEGREES MAXIMUM

30 DEGREES MAXIMUM14 DEGREES MINIMUM(b)

* NO MINIMUM WHEN JOINED MATERIALSHAVE EQUAL YIELD STRENGTHS


FIGURE 29. Combination internal and externalweld offset

30 DEGREE MAXIMUM14 DEGREE MINIMUM 11-41''

1 10.5t MAXIMUM

ti,30 DEGREE MAXIMUM

14 DEGREES MINIMUM 41* 30 DEGREES MAXIMUM

` NO MINIMUM WHEN JOINED MATERIALSHAVE EQUAL YIELD STRENGTHS



FIGURE 33. Welding details for openings withoutreinforcement other than in header and branchwalls

NHEADER

NOTESI. WHEN A WELDING SADDLE IS USED, IT IS INSERTED OVER THIS TYPE OF

CONNECTION2. WI = 38/8 BUT NOT LESS THAN 6.3 mm 10.25 in.)3. N = I 5 mm 10.063 in.) MINIMUM AND 85 mm (0.33 in.) MAXIMUM UNLESS

BACK WELDING OR BACKING STRIP IS USED


1n 1 BRANCH

45 DEGREE MINIMUM

1.\\C/ Fb1 45 DEGREE MINIMUM

FIGURE 34. Welding details for openings withlocalized reinforcement: (a) saddle and (b) pad

(al

( b J

NOTES1 W I min = 38/8, BUT NOT LESS THAN 6.3 mm (0.25 in.)2. W2min = M/2, BUT NOT LESS THAN 6.3 mm 10.25 in.)3. W3min M, BUT NOT GREATER THAN H4. N = 1.5 mm 10.063 in.) MINIMUM UNLESS BACK WELDING OR BACKING

STRIP IS USED5. ALL WELDS TO HAVE EQUAL LEG DIMENSIONS AND MINIMUM THROAT =

0.707 x LEG DIMENSION6. IF M IS THICKER THAN H, THE REINFORCING MEMBER IS TAPERED DOWN

TO THE HEADER WALL THICKNESS7 PROVIDE HOLE IN REINFORCEMENT TO REVEAL LEAKAGE IN BURIED WELDS

AND TO PROVIDE VENTING DURING WELDING AND HEAT TREATMENT


Depth

Length

Over 0.75 mm (0.033 in.) or over not acceptable12.5 percent of the pipe wallthickness (whichever is smaller)

Over 0.4 mm (0.016 in.) through0.75 mm (0.033in.) or over 6 to12 percent of the pipe wall thick-ness (whichever is smaller)

0.4 mm (0.016 in.J or 6 percent ofpipe wall thickness (whicheveris smaller)

50 mm (2 in.) in a continuousweld length of 300 mm (12 in.)or 0.17 time the length of theweld, whichever is smaller

acceptable regardless of length


TABLE 9. Thickness of reinforcement on each face of thebutt welded joints of vessel or tank plate

Maximum Thicknessof Reinforcement

Plate Thickness millimeters (inches)millimeters (inches)

Vertical Joints Horizontal Joints

Up to 13 (0.5) 2.4 (0.094) 3 (0.125)Over 13 to 25 (0.5 to 1) 3 (0.125) 4.8 (0.19)Over 25111 4.8 (0.19) 6.3 (0.25)

NOTE: MAXIMUM REINFORCEMENT IN AREAS TO BE RADIOGRAPHED IS 1.5 mm10.063 in.) FOR THICKNESS 13 mm 10.5 In.) AND LESS; 2.4 mm (0.094 In.) FORTHICKNESS 13 mm 10.5 In.) TO 25 mm 11 In.); AND 3 mm (0. T25 in.) FORTHICKNESS OVER 25 mm (1 in.)


TABLE 10. Undercutting adjacent to the cover or rootbead


REFERENCES

1. Sampling Procedures and Tables for Inspection by Attri-butes. MIL-STD-105D. Washington, DC: Department ofDefense ( July 1961).

2. Grant, E.L. Statistical Quality Control. New York, NY.McGraw-Hill Book Company (1952).


PART 1 OVERVIEW OF CODES, STANDARDS ANDSPECIFICATIONS FOR VISUAL TESTING

Many phases of design and construction are governed bydocuments known as specifications, codes and standards oras guides, practices, regulations or rules. These terms areoften used interchangeably.

Many purchasing organizations develop and issue specifi-cations designed to meet their individual requirements.Whenever such specifications are cited in purchase docu-ments, they influence the product quality and the work of thevisual inspector. Often, such specifications reflect the needsof a specific situation.

Common interest in subjects like safety and reliabilitystarted the development of nationwide or industry-widecodes and standards. Committees of engineering or techni-cal societies are organized to evaluate such problems and topropose specific guidelines. Membership in these commit-tees is usually balanced among technical experts represent-ing all interested parties. In addition, organizationsrepresenting the public interest, such as government agen-cies and insurance companies, have employees who belongto many committees and participate in their deliberations.

After the document produced by a specific committee isendorsed by one or more review boards, it is published in thename of the engineering society as a standard. When incor-porated into purchase orders, such a document becomes acontractual document whether it is called a code, a specifica-tion or an industrial practice.

Documents having significant influence on public healthand safety are sometimes accepted by legislative bodies or byfederal regulating agencies. In those jurisdictions, such doc-uments become law and are often referred to as codes or reg-ulations. Because each state in the United States enacts itsown laws, the acceptance and scope of such codes differwidely throughout the United States. A visual inspector mustunderstand what codes and regulations have been mandatedfor a component, for its intended service and for its final des-tination. Neither the supplier nor the purchaser can waivethe requirements of law Well-justified requests for modifi-cation can sometimes be obtained from governing bodies ortheir technical representatives although the process is timeconsuming.

Codes and standards are periodically revised by the issu-ing organizations. When contracts for services are signed, itis typically assumed that the current reference document isin effect unless a particular revision is specified. Also, whena new revision is published, it is customary for it to stipulate

a grace period (six months, for example) during which thecontractor may adjust practices to meet new requirements.

The following organizations in the United States writestandards that commonly apply to visual testing: (1) Aero-space Industries Association (AIA), (2) American Bureau ofShipping (ABS), (3) American National Standards Institute(ANSI), (4) American Society for Nondestructive Testing(ASNT), (5) American Society for Testing and Materials(ASTM), (6) American Society of Mechanical Engineers(ASME), (7) American Welding Society (AWS), (8) Depart-ment of Defense (DOD), (9) Ship Structure Committee(SSC), (10) Society of Automotive Engineers (SAE),(11) American Petroleum Institute (API).

Organizations outside the United States that issue stan-dards include the French Association for Standardization(AFNOR), the British Standards Institute (BSI), the Euro-pean Committee for Standardization (CEN), the GermanInstitute for Standardization (DIN), the International Orga-nization for Standardization (ISO), and the Japanese Indus-trial Standards Committee ( JISC). It is usual for standards tocover vision acuity and it seems likely that revisions of stan-dards in the 1990s will treat visual testing as a nondestructivetesting method.

Application to Welding and AlliedProcedures

To a large extent, the quality of welding and the quality ofvisual inspection depend on the application of correct proce-dures. A welding inspector should verify that proceduresspecified for a job are used. A checklist may be prepared foreach procedure to guide visual inspections.

In general, testing processes should be performed insequence with the manufacturing operations, as establishedby the fabricator. There are good reasons for doing this:

1. interaction between testing and production is mademore efficient;

2. visual testing operations required at certain stages offabrication can be completed without obstruction; and

3. in-process testing provides an opportunity for mean-ingful corrective action, permits earlier correction ofdeficiencies, improves efficiency, speeds delivery andresults in a product of better quality.

CODES, STANDARDS AND SPECIFICATIONS FOR VISUAL AND OPTICAL TESTING / 179

Actual welding operations (see Table 1) and the order oftheir application depend on the type of weldment, themethod of manufacture and the requirements of the govern-ing contract.

Visual Testing in Widely UsedStandards

As of 1993, visual testing has not yet been treated as a sep-arate method in International Organization for Standardiza-tion (ISO) ISO/DIS-9712, Nondestructive Testing—Qualification and Certification of Personnel; in ANSI/ASNTCP-189-1991, Standard for Qualification and Certificationof Nondestructive Testing Personnel; or in standards issuedby the American Society for Testing and Materials.

As of 1993, however, visual testing is cited in major stan-dard and practices issued by four professional engineeringsocieties in the United States: ASME, API, AWS and ASNT.The importance of each to visual testing is described in thefollowing discussion. (Visual acceptance criteria, especiallyfor the ASME Code, can be found in the discussion of testprocedures in another section of this book.)

American Society of Mechanical Engineers (ASME)Boiler and Pressure Vessel Code

According to Section III (Rules for Construction ofNuclear Power Plant Components) of the ASME Boiler andPressure Vessel Code, the results of the Section III visualtesting shall be evaluated in accordance with the require-ments and acceptance standards of the referencing codesubsection. The general requirements of Section III, Sub-sections NB and NF, specify that visual testing shall be per-formed in accordance with Section V (NondestructiveExamination), Article 9,

Article 9 (Visual Examination) of Section V (Nondestruc-tive Examination) of the ASME Code gives details for theneeded level of quality for inspection circumstances such asaccess and illumination. The proper application of opticaldevices is also described.

ANSI B31.1 (Power Piping)—developed to parallelSec. 1 (Power Boilers) of the ASME Boiler and Pressure Ves-sel Code—is generally used for fossil fuel plant applicationsand for piping systems in nuclear power plants not under thescope of other codes. Visual testing in these documentsincludes dimensional checks of welds.

IWA-2200 of Section XI (Rules for Inservice Inspection ofNuclear Power Plant Components) of the ASME Codedescribes three kinds of visual testing and calls them VT-1,VT-2 and VT-3. VT-1 is for general service inspection, con-ducted to detect discontinuities and imperfections on the

TABLE 1. Typical weld inspection plan incorporatingvisual testing

Before welding material identificationchemical analysismechanical properties

Base metal laminations, laminar discontinuities,scabs and cracks

surface irregularitiesflatness

Joint geometry' edge preparation (including root faceand beveling)

dimensionscleanlinessroot openingtackingbacking (where required)

Special setups2jigging and bracingprestressing or precambering

During welding preheat and interpass temperaturescontrolsmeasurement methods

filler metalidentificationcontrolhandling

root pass contour and soundnessroot preparation before welding the

second sidecleaning between passesappearance of passes'in-process NDT as required or specifiedconformance to approved welding

procedure

After welding postheat treatment requirements

Acceptance testing method of cleaning for inspectionconformity of welds with drawingsnondestructive testing

visual testingsurface contour and finish of weldsmagnetic particle testingliquid penetrant testingradiographyultrasonic testingother methods

proof testingdestructive testing

chemicalmechanicalmetallographic

Repairs margin for acceptance or rejectiontesting after repair

1. FIT-UP2. USED FOR ASSEMBLY AND FABRICATION TO VERIFY USE OF UNIFORM

PRACTICES3. SOMETIMES IN COMPARISON WITH WORKMANSHIP STANDARD


wear, corrosion and erosion. VT-2 is conducted to detect evi-dence of leakage from pressure retaining components. VT-3is conducted to detect discontinuities and imperfectionssuch as loss of integrity at bolted or welded connections,loose or missing parts, debris, corrosion, wear or erosion.

IWA-2300 of Section XI of the ASME Code has beenaddressing visual testing since its 1977 edition. The 1989 edi-tion of ASME Section XI requires visual testing to be con-ducted in accordance with Article 9 of Section 5. Section XIlists requirements for near vision test charts, remote visualtesting and illumination level requirements.

Visual testing in the ASME Code is also discussed in thishandbook's sections on test procedures and on the electricpower industry.

American Petroleum Institute (API) Standard forWelding Pipelines and Related Facilities

Section 6 (Standards of Acceptability—NondestructiveTesting) of API 1104 (Standard for Welding Pipelines andRelated Facilities) explicitly addresses radiographic andvisual testing. The Section describes a number of welddefects that can be detected or measured through visualtesting. Visual testing in this standard and in other API docu-ments is discussed more fully later in this section.

American Welding Society (AWS) Structural WeldingCode

Many buildins with structural steel have been designedand built according to requirements of AWS D1.1 (Struc-tural Welding Code—Steel). D1.1 contains references tovisual testing both in workmanship requirements and in spe-cific visual testing acceptance standards. It should be notedthat AWS D1.1 uses the term inspection where the ASMEdocuments would use examination and where ASNT docu-ments would use testing.

Visual testing of welds is discussed in the section on testprocedures and elsewhere in this volume.

ASNT

The 1988 edition of ASNT Recommended Practice No.SNT-TC-IA, discussed in the following part of this Section,gives recommended training and experience levels for visualtesting. A recommended training outline is given for Level Iand Level II in visual testing.

Visual testing is likely to have a place in revisions of ANSI/ASNT CP-189-1991, Standard for Qualification and Certi-fication of Nondestructive Testing Personnel.


PART 2 PERSONNEL QUALIFICATION ANDCERTIFICATION

Definition of TermsNDT, NDE and NDI are initialisms for nondestructive

testing, nondestructive examination or evaluation and non-destructive inspection, respectively. These terms all relate tothe same processes and differ only in connotation. However,certain segments of industry in the United States prefer oneterm over another. In other countries, the English term non-destructive testing (NDT) is widely used.

Nondestructive testing is any or all of the processes thatuse various physical principles to determine the condition ofan object without affecting the object's ability to fulfill itsintended function. The most common kinds of nondestruc-tive testing use visible light, X-rays or gamma rays, ultra-sound, electricity magnetism, liquid penetrants or tracergases to detect discontinuities in or through the test object.Certain nondestructive testing methods can also measure orcompare some physical, mechanical or chemical properties.

The terms qualification and certification are often usedinterchangeably but they mean different things as definedbelow. Qualification is the process of demonstrating that anindividual has the required amount and the required type oftraining, experience, knowledge and capabilities. An individ-ual is qualified when such a demonstration has been satisfac-torily completed. Certification is the process of providingwritten testimony that an individual is qualified. An individ-ual is certified when written testimony of qualification hasbeen provided. In other words, an individual can be qualifiedwithout being certified but should not be certified withoutbeing qualified.

Value of Personnel Qualification andCertification

It is important to conduct personnel qualification and cer-tification in order to (1) enhance understanding of the tech-nology, principles and practices of the nondestructive testingmethod being used and (2) ensure proper use of the tech-niques. This understanding is essential to good nonde-structive testing performance because (1) most nondestruc-tive testing methods do not inherently create their ownrecords; (2) nondestructive testing results are stronglydependent on the nondestructive testing process and the

individual performing the process; and (3) nondestructivetesting results can seldom be cross checked without conduct-ing another test.

Nondestructive testing personnel must know and believein the importance of performing the tests properly and mustbe dedicated to performing the tests properly. They mustunderstand the potential seriousness of, and their responsi-bility for, the consequences if the tests are not performedproperly.

To ensure that nondestructive testing personnel possessthese qualifications, the following checklist may be used:(1) training to impart the necessary knowledge; (2) supervi-sion by knowledgeable people; (3) qualification tests to dem-onstrate that the individual has acquired sufficientknowledge and capabilities; and (4) certification to docu-ment successful demonstration of competence.

In most nations, a central agency (usually the governmentor an organization sanctioned by the government) conductsthe tests and certifications. While some central certifica-tion systems exist in the United States, such as those forNAVSEA-250-1500-1 and MIL-STD-410, most US certifica-tions today are conducted by employers in accordance withASNT Recommended Practice No. SNT-TC-1A. For this rea-son, it is important to understand how SNT-TC-1A is appliedin widely used specifications that require nondestructivetesting personnel certification.

There is also an optional nondestructive testing Level IIIcertification offered by the American Society for Nonde-structive Testing to qualified personnel who pass ASNT'srequirements. It is important to be familiar with this pro-gram because: (1) there are many individuals who have beencertified under this program; (2) this certification is recog-nized around the world; and (3) it is required by some con-tracting organizations.

Overview of SNT-TC- 1 AASNT Recommended Practice No. SNT-TC- IA is not a

specification or standard. It is a set of guidelines or recom-mendations for employers to establish and conduct a nonde-structive testing personnel qualification and certificationprogram.


Certification in accordance with SNT-TC-1A means certi-fication of an individual by the employer. Because it can onlybe given by the employer, certification in accordance withSNT-TC-1A terminates if the individual leaves the employer.

Basically, SNT-TC-IA has only two requirements or condi-tions which must be met.

The first condition is that a written practice or procedurefor qualifying and certifying company nondestructive testingpersonnel must be prepared by the employer. The followingrecommendations about preparing a written practice areincluded in SNT-TC-IA:

1. establish three levels of nondestructive testing person-nel qualification (Levels 1, II and HI—Level III beingfor the method specialist and administrator);

2. specify the activities or functions that personnel shouldbe able to perform at each level of nondestructive test-ing qualification;

3. specify the training, education and experience that per-sonnel should have at each level of qualification;

4. specify the subjects that the training should cover; and5. specify degrees of acceptability for each Level.

The second condition is that the employer must (shall)modify the suggestions of SNT-TC-1A if necessary to makethe qualification and certification requirements appropriatefor the needs of the employer, customer or both. In otherwords, the employer is responsible for assuring that the quali-fication requirements are satisfactory for the nondestructivetesting work that the company employees perform.

Recommended Practice SNT-TC-IA is needed also if non-destructive testing is done by outside laboratories or suppli-ers. It provides a yardstick against which to evaluate theirqualification programs and the capabilities of their nonde-structive testing personnel.

Recommended Practice SNT-TC-IA also includes the fol-lowing recommendations:

1. what types of examinations should be given;2. how examinations should be graded;3. who should perform the training and prepare the

examinations;4. what records of the qualification process should be

kept;5. how long the certifications should remain valid; and6. how personnel should be recertified.

It is important to remember that SNT-TC-1A does notrequire an employer to follow all its recommendations to theletter. In fact, paragraph 1.4 of all SNT-TC-1A editions statesthat the employer shall modify them as necessary to meetparticular needs.

However, standards and specifications of some regulatorybodies require that employers adhere rigidly to SNT-TC-IA.

If the contract contains such requirements, SNT-TC-1Amust be treated as minimum requirements. In such cases,inquiring about those parts that seem inappropriate is a goodidea.

Details of SNT-TC-1AThe text below discusses the 1988 edition of SNT-TC-1A

in more detail. The exact wording of SNT-TC-IA is notreiterated.

Scope

SNT-TC-IA provides guidelines intended to assist employ-ers developing their own written practices or procedures forqualifying and certifying their own nondestructive testingpersonnel. That does not mean SNT-TC-IA should beignored if other companies perform the nondestructive test-ing. The recommendations of SNT-TC-1A can still be a use-ful guide for determining whether vendors are usingwell-trained and well-qualified personnel to perform non-destructive testing.

Paragraph 1.4 requires modification of the recommenda-tions as needed to make them properly fit particular needs.Such modifications should only be made by individualsexpert in the field of nondestructive testing. Such help couldcome from several sources, including customers' experts,nondestructive testing schools, government agencies such asthe Department of Defense, nondestructive testing labora-tories, independent nondestructive testing consultants orother sources. ASNT's journal Materials Evaluation pub-lishes an annual directory of nondestructive testing labora-tories and consultants. That journal's advertisers are anothereood source for the names and addresses of nondestructivetesting consultants.

DefinitionsTo understand the recommendations of SNT-TC-IA, it is

important to also understand the definitions of some wordsas they are used in the document. In particular, the defini-tions of should and shall in the 1988 edition are differentfrom the usual dictionary definitions. Shall is used for mini-mum recommended guidelines that are not mandatory;should is used for desired guidelines. The document is only aguide, unless the customer has imposed other restrictions.

Nondestructive Testing MethodsThe general recommendations of SNT-TC-1A are applica-

ble to any method, whether or not it is listed in Section 3 ofSNT-TC-1A. However, ASNT has training course outlines


and sample examination questions only for the methodslisted.

Levels of Qualification

There are several important points to the section on levelsof qualification. The first is that the three basic levels of qual-ification may be subdivided. This is important especially ifonly certain segments of a nondestructive test are per-formed, such as using a digital ultrasonic thickness gage, pro-cessing radiographic film or only precleaning or developingin a liquid penetrant test. Individuals who are certified forsuch specific operations are usually called limited Level (I orII) or are designated as having limited certification becausethey are not qualified to perform the full range of activitiesexpected of personnel at that level of qualification.

Notice that trainees may do useful nondestructive testingwork, contrary to what is sometimes asserted. A crucial partof the training process is physically performing the necessaryactivities. This is the only way to obtain the experience thatis required to become certified. The limitation on trainees'activities is that they should work with, and not just under thesupervision of, a qualified person.

Note carefully the use of should and shall in the descrip-tions of Levels I, II and III. Shall means this is strongly rec-ommended unless there is a strong reason why it does not fitspecific needs. For example, the recommendations state thata Level III ... shall be capable of interpreting and evaluatingresults ... If there is a Level HI who cannot keep current inevaluation skills, that individual's status as a Level III shouldbe re-evaluated and perhaps limited. However, it is desirablefor all individuals at the same level to be capable of meetingall the criteria for that level.

Written Practice

This section is one of the two mandatory sections inSNT-TC-M. The written practice is a procedure that definesin detail how the personnel qualification and certificationoperations are conducted. Prepare it with the same care andthoroughness used when preparing specifications for pur-chasing products and equipment. Seriously consider engag-ing expert assistance in preparing a written practice if thereare no nondestructive testing experts available on staff.

Requirements of Initial Qualification

The requirements detailed below are for initial qualifica-tion and certification, not for recertification. The answer tohow much education, training and experience is enoughfor specific purposes depends on the specific nondestruc-tive testing applications. If a new program is being estab-lished, seek expert help or use the recommendations inSNT-TC-IA's Table 6.3.1.

The phrases equivalent activities and at least comparableto are used frequently here and can cause concern. Neitherphrase has been officially defined, partly because the mean-ings of equivalent and comparable depend on each employ-er's requirements for each level. As a suggestion, positionsand activities that satisfy all of the recommendations for agiven level should be accepted as equivalent to that level,even if no certification was given. If the positions and activi-ties do not satisfy all of the recommendations, exercise judg-ment and recognize that the intent is to ensure that thecandidate is competent in nondestructive testing.

Despite its title (Recommended Initial Training and Expe-rience Levels), it is often assumed that Table 6.3.1 applies tocertified individuals. However it applies only to initial quali-fication. When looking at the table's footnotes, rememberthat shall indicates a strong recommendation. The actualrequirements of a written practice may be different, asrequired.

The SNT-TC-1A Questions and Answers: Inquiries to andResponses from ASNT's SNT-TC-1A Interpretation Panel(also called SNT-TC-1A Interpretations) contains the officialanswers to many of the questions about various parts ofSNT-TC-IA. It must be decided (and included in a writtenpractice) which activities constitute nondestructive testingrelated activities. Some choices others have made includedestructive testing, metallography, welder or welding proce-dure qualification testing and the performance of nonde-structive testing methods other than the one for whichcertification is sought.

Training Programs

Training includes classroom training and on-the-job train-ing. A combination of classroom and on-the-job training ispreferable to memorization without comprehension. Alltraining should be organized, formal and suited to theemployer's needs. It should use the actual processes, use theactual product and relate specifically to the appropriate levelof certification. The Recommended Training Course Outlinesappended to SNT-TC-IA can be used as a guide but someexpertise is needed to adapt these publications to specialcases.

The 1988 edition of ASNT Recommended Practice No.SNT-TC-1A, discussed in the following part of this Section,gives recommended training and experience levels for visualtesting. A recommended training outline is given for Level Iand Level II in visual testing.

Administering Examinations

If someone other than a Level III administers the exami-nations, that individual should (1) have documented qualifi-cations for the work and (2) be designated in writing. Forexample, an experienced and qualified Level II might


administer Level I examinations, if appropriately docu-mented to do so.

The procedure should state what vision examinations willbe used and whether any modifications to the standards willapply. For example, some specifications require Jaeger I"rather than Jaeger 2" eye examinations. Some specify a dis-tance other than 300 mm (12 in.). Color blindness can bedetected by tests described elsewhere in this book. Colorvision can also he evaluated by having the candidate conducta nondestructive test. Some specifications require a densitydiscrimination test for radiographers, using special radio-graphs containing known faint images. If there is such arequirement, suitable radiographs must be bought or made.

The general examination should deal with the physicalprinciples of the nondestructive testing method, generalknowledge of basic equipment used in the method, familiar-ity with materials and manufacturing processes and otherknowledge that can be imparted in the classroom or labora-tory. It should not deal with knowledge of specific equip-ment, codes, standards and procedures applicable to theparticular application—these are best left to the specificexamination.

The specific examination should deal with the details ofthe method that are applicable to the application. Becausethese examinations may be given with the applicable specifi-cation or procedure as reference material, it is important thatthe questions be phrased so that the examinee cannot answerthem merely by finding the right paragraph in the document.The questions should discern whether the candidates canapply the material to realistic, practical situations that mayoccur during nondestructive testing.

There may be a need for more or fewer questions thanSNT-TC-IA recommends, depending on, for example, howmany specifications and procedures must be covered andhow simple or complex the specifications are. In some cases,it is difficult to write twenty questions, for example, for lim-ited certifications. In other cases, twenty questions do notbegin to cover the knowledge a candidate needs to do thejob properly.

The practical examination requires the use of sample testobjects and these must be carefully considered. The sampletest objects may he actual production parts with known flawsor may be parts with intentionally fabricated discontinuitiestested and approved by a Level III. While most employersprefer to make their own test samples, one sample per testitem is not enough to guarantee test security and efficacy ofthe examination. Not only do examination results circulatequickly but also the test samples must be safeguardedbetween examinations. A security system and a follow-upprocess are needed to ensure that the system is used. It mustalso be decided what checkpoints are covered by the exami-nation and how to grade the results. Usually, the checkpointsinclude (1) critical variables that must be controlled in theprocess and (2) details of the test procedure.

Two opinions exist about the questions in the Method-Specific Supplement (Question and Answer Books) toSNT-TC-IA. One opinion is that these questions should onlybe used as examples, because the candidates can memorizethe answers. The other opinion is that these questions shouldbe used verbatim because several experts have reviewedthem and they are likely to be correct and free from ambigu-ities. Both opinions are valid.

When reference material is provided, it is important to usethe proper type of questions. The answer must be reasonedfrom the reference material, not copied verbatim.

No examination should be prepared by the person who istaking the examination this caveat applies most of all toLevel III personnel. The Level III examinations must beprepared by a nondestructive testing expert. Holding asupervisory position over a nondestructive testing technicianis not enough of a qualification to prepare the examination.The objective of examination is to demonstrate and docu-ment in a credible way that the candidate is qualified. TheASNT Level III certification is one way to document the can-didate's qualifications, as are examinations by other qualifiedsources. Regardless of who prepares the examinations, theemployer is still responsible for certifying the individual(issuing and signing the certificate) according to the com-pany's written practice.

It is particularly useful for a Level III to be involved ingrading the examinations to deal with ambiguous questionsor incorrect answers. This is most important if the examina-tion questions are essay or fill in the blank questions—thecorrectness of answers for such questions is often a matter ofinterpretation.

Although all three parts of the examinations are weightedequally, if it is felt that some parts should receive more or lessweight than others for the application, the weighting may bemodified to suit the application (note the definition of shall).For example, some employers weight heavily the specific andpractical examinations for Level I, weight all parts equally forLevel II and weight the basic examination more heavily forLevel III. The weighting depends on where the emphasisshould be placed. This weighting is best done by an expert inthe nondestructive testing method.

If a candidate fails an examination, SNT-TC-1A addressesthe follow-up. The intent here is that the candidate shouldgain further experience or training to correct deficiencies.Under no circumstances should a candidate receive the sameexamination twice in a short period of time. The objective ofeach examination is to demonstrate the candidate's knowl-edge of the subject, not the ability to memorize the answers.

CertificationThe employer is responsible for certification of all levels

of nondestructive testing employees. As a corollary underSNT-TC-1A, an employer can only certify its employees but


an organization can examine nonemployees to be sure theyare qualified. The recommendations in SNT-TC-1A shouldbe modified to fit the employer's needs.

An outside agency may be hired to provide Level IIIservices or training, examinations or other nondestructivetesting services. Remember—when using any outsidenondestructive testing service, the employer is responsiblefor all certifications issued to the employer's personnel underSNT-TC-1A. Therefore, even if an outside agency conductstraining and examination, the employer must still issue thecertification.

With regard to records, consider that their purpose is todocument the beginning and the continuity of qualificationsof people who perform nondestructive testing. Recordsshould be sufficient to prove that the people were qualifiedat the time they performed the work. After the work is done,the records should be retained for as long as the customerrequires or as long as may be legally desirable to prove thatnondestructive testing performed on a given part was doneby personnel qualified at that time.

Interrupted service seems to be a frequent problem, so aclarification of paragraph 9.7(4) may be useful. Someemployers define interrupted service as a break of six monthsor some other fixed interval during which no nondestructivetesting was done, but this practice has weaknesses. Anemployer might decide to require full recertification only ifthe duration of the interruption exceeds the duration of theemployee's nondestructive testing experience or five years,whichever is less. If the interruption is shorter than this butlonger than a month, a specific examination might berequired and, if the interruption is longer than a year, a prac-tical examination might be added. This procedure or a simi-lar one is more cumbersome than having examinations atfixed intervals but it acknowledges that recent knowledge ismore easily forgotten than old habits.

Termination

With regard to paragraph 10.2 of SNT-TC-1A, the termproof is usually understood to mean written evidence fromthe previous employer. It is regrettable that in recent yearsparagraph 10.2 has become increasingly difficult to satisfywith documentation. However, with SNT-TC-1A as a guide,the employer can adopt different requirements. Even ifSNT-TC-1A is used as a guide, the employer is responsiblefor the certifications that result.

Application to Some CommonSpecifications

The text below details ways that SNT-TC-IA can beapplied to some of the more frequently used industrial speci-fications. Each specification is discussed separately.

The American Society for Mechanical Engineers(ASME) Boiler and Pressure Vessel Code

The ASME Boiler and Pressure Vessel Code has elevensections, each with different requirements. The personnelqualification and certification requirements of different sec-tions of the Code differ from each other. Some, such asSection XI, are quite detailed. The 1983 version is used as anexample here—earlier and later versions of the code maydiffer from the requirements reviewed in this text.

Section XI, IWA-2300 specifies that the personnel quali-fication and certification procedure shall comply withSNT-TC-1A, with the following exceptions.

1. Level Ills can only be qualified by examination andbasic method and specific examinations must be given.This means that if an ASNT Level III certification isused to fulfill the basic and method examinationsrequired in SNT-TC-1A, paragraph 8.8(4), proof mustbe given that examinations were taken and passed. Italso means that paragraph 8.8(5) of SNT-TC-1A can-not be used to avoid giving specific examinations forLevel III. Although new ASNT certification can onlybe obtained by examination, recertification can beobtained either by examination or by documentation ofcontinued Level III activities plus increasing Level IIIknowledge. ASNT certificates do not identify whichrecertification method was used. However, the ASNTletter advising that the candidate has been recertifieddoes state whether or not an examination was passedand may be used as the necessary proof.

2. Recertification is required every three years, instead ofthe five year interval recommended by SNT-TC-1Aand required by the ASNT Level III Program.

3. All references to SNT-TC-1A paragraph numbers inthe 1983 version of the Boiler and Pressure Vessel Coderefer to the 1980 edition. The 1988 addenda to theCode referred to the 1984 edition of SNT-TC-1A. Thevisual testing recommendations, new to the 1988 edi-tion of SNT-TC-1A, are available to guide employerswho must set up a visual testing program. However,unless future code rulings decree otherwise, there is noreason not to use SNT-TC-1A recommendations, mod-ified to suit the needs of the application.

4. When the ASNT certification is used to satisfy the basicand method examination requirements of Section XI,the minimum passing grade on the employer's specificexamination must be 80 percent, because ASNT doesnot report grades for its examinations.

5. Section XI allows limited certifications and these limi-tations must be described in the employer's writtenpractice. Note that the limitations must also bedescribed on the certificate.


6. The vision examination requires Jaeger 1" , not Jaeger2". Also, the code does not specify a distance. There-fore, this examination must be conducted at 400 mm(15.75 in.), the standard Jaeger T" distance, rather thanat the 300 mm (12 in.) distance quoted in SNT-TC-1A.A far-distance vision examination is also required forVT-2 and VT-3 work, as defined in the code.

7. A difference that may have an important impact onsome users is that Level I personnel may not indepen-dently evaluate results, contrary to paragraph 4.3 ofSNT-TC-1A. Thus, for Section XI work, it may be nec-essary to train and qualify additional Level IIpersonnel.

8. Another difference that may have significant impact isthat IWA-2300 requires that Level I and II practicalexaminations he demonstrated to the Level HI. Thereis no provision for delegation of this Level III function.

American Petroleum Institute (API)

API Standard 1104 (17th edition, 1988) covers therequired nondestructive testing for pipeline welding. It spec-ifies that nondestructive testing personnel shall be qualifiedin accordance with SNT-TC-1A for the test method used. Itallows but does not require ultrasonic, penetrant, magneticparticle, and radiographic testing. There is ambiguity aboutthe duties of Level I personnel but the only limitation is thatonly a Level II or Level III shall interpret the test results.Requalification is required every three years, includingLevel III.

API Specification 5CT for inspection of casing and tubingpassed a letter ballot for adoption in 1992. This specificationrequires that the minimal level of qualification of inspectorshe in accordance with SNT-TC-1A.

American Welding Society (AWS) D1.1

This specification covers the design, fabrication and exam-ination of welded buildings, bridges and tubular structures.It requires that nondestructive testing personnel, exceptfor visual testing, be qualified to the current edition ofSNT-TC-1A. Only a Level II or a Level I working under aLevel II is permitted to perform nondestructive testing. Thiscan hinder training by precluding Trainees from doing thehands-on work needed to thoroughly learn the nuances ofmethods such as ultrasonic testing.

Ultrasonic testing personnel are expressly required to takea specific and practical examination on AWS D1.1. Theseexaminations must be taken because the ultrasonic testingrequirements are very detailed and unique. There is no men-tion of examinations for any other nondestructive testingmethod and these examinations may be omitted at theemployer's discretion insofar as they are only recommended

in SNT-TC-1A. Such a practice is discouraged if high qualitynondestructive testing is the goal.

Military Standard MIL-STD-271

This widely used military standard is written primarily tospecify testing requirements but it also includes qualificationrequirements based on the 1980 edition of SNT-TC-1A. Therecommendations of SNT-TC-1A are invoked as minimumrequirements except for limited certifications. Training andexperience for limited certifications may be less than those inSNT-TC-1A, provided that the amounts are described in thewritten practice and in certification records.

All nondestructive testing methods, whether covered inSNT-TC-1A or not, must be treated similarly with regard topersonnel certification. This can be a problem with methodsnot covered in SNT-TC-1A because it requires the employerto devise training course outlines, training and experiencetimes for each level, numbers of questions required on exam-inations and the many other details recommended inSNT-TC-1A.

Recertification of all levels is required every three years orless and the procedure must be of the same complexity andthoroughness as the original examinations. In addition,recertification is required for anyone who has not performednondestructive testing within the past six months. This maybe an operational examination instead of the full battery ofinitial examinations.

The vision examinations require Jaeger 1" rather thanJaeger 2" , as in SNT-TC-1A. A brightness discriminationexamination is required for radiographic testing personnel.For this examination, the employer must prepare and main-tain several radiographs with a variety of densities and sizesof penetrameter images and must establish credible pass/failcriteria.

Military Standard MIL-STD-410

This standard is a stand-alone specification for nonde-structive testing personnel qualification in five of the majormethods: radiographic, ultrasonic, eddy current, magneticparticle and liquid penetrant testing. It is based on theSNT-TC-1A system of three levels and employer certifica-tion but it differs in many details and it provides require-ments, not recommendations. One difference is that Level Iis not permitted to evaluate results for acceptance. Anotheris that vision examinations must be given by medically quali-fied personnel. A third difference is that Level III personnel,regardless of educational background, must have a specifiedamount of experience which varies with the nondestructivetesting method. Many other differences may also be found.

Level III certification requires examination, with theexception that Level III certifications granted by ASNT withor without examination satisfy the requirements for the gen-eral examination.

FIGURE 1. Certification using RecommendedPractice SNT-TC-1A—note that the employeris responsible for all aspects of certification,regardless of who performs the activity

ACTIVITY PERFORMED BY

PREPARE WRITTEN PRACTICE EMPLOYER OROUTSIDE AGENCY

QUALIFY EMPLOYEES

!RAINING

EXPERIENCE

VERIFY AND DOCUMENT THETRAINING AND EXPERIENCE

EMPLOYER OROUTSIDE AGENCY

EMPLOYER ORPREVIOUS EMPLOYER

EMPLOYER

PREPARE AND ADMINISTEREXAMINATIONS

LEVEL I AND LEVEL IIVISION. GENERAL. SPECIFICAND PRACTICALEXAMINATIONS


LEVEL IIIBASIC AND METHODEXAMINATIONS

VISION AND SPECIFICEXAMINATIONS

CERTIFICATION ISSUED

ACCEPTANCE OF CERTIFICATION

ASNT EMPLOYER OR OTHEROUTSIDE AGENCY


EMPLOYER

CUSTOMER

CODES, STANDARDS AND SPECIFICATIONS FOR VISUAL AND OPTICAL TESTING I 187

Nondestructive Testing Level 111Certification by ASNT

Because employers may vary in their requirements for cer-tification under SNT-TC-1A, nondestructive testing person-nel at a given certification level may differ markedly incapabilities from employer to employer. Audits by variousorganizations have found certified personnel who wereincompetent. While employer ignorance is one reason forincompetent personnel, there have also been cases of indi-vidual Level Ills who knowingly aided certification of incom-petent personnel.

In order to provide a firm minimum standard of compe-tence for Level HI personnel and to provide a means ofensuring unbiased judgment of which individuals possesssuch competence, ASNT established its own Level III certi-fication program in 1976.

For an initial period of six months, individuals with exten-sive, documented nondestructive testing experience (mini-mum of fifteen years) and high competence were certified byASNT without examination. Since February 1977, individu-als must pass examinations administered by ASNT to obtainLevel III certification by ASNT. Note that this is not thesame as certification to SNT-TC-1A because it is ASNT, notthe employer, who grants and signs the certificate of ASNTcertification.

The ASNT examinations are difficult for most candidatesbecause they require Level II knowledge of all the commonmethods of nondestructive testing, a thorough knowledge ofSNT-TC-1A and of materials and processes technology and abroad Level III knowledge in the method for which certifi-cation is sought. The breadth of knowledge required in themethod examinations may cover any application of themethod, as well as the ability to understand and applydetailed specifications. The examinations are not slantedtoward any industries or applications.

In addition to passing the examinations, ASNT requiresthat applicants for examinations agree in writing to adhere toa strict Code of Ethics. Those who are certified by AS NT areassigned a unique certificate number, given a certificate fromASNT and listed in the register published annually by ASNT.

Recertification is required every five years and may beaccomplished either by examination or by documentation ofcontinued Level III activity and continued growth in appro-priate Level III knowledge.

Although ASNT Level III certification is not required bySNT-TC-1A, it is one of the options for satisfying the basicand method examination requirements and it is required bya growing number of purchasers and regulatory bodies. Also,ANSI/ASNT CP-189-1991 (ASNT Standard for Qualificationand Certification of Nondestructive Testing Personnel),approved by ASNT in 1989 and by the American NationalStandards Institute (ANSI) in 1991, requires that everyLevel III working to ANSI/ASNT CP-189-1991 possess an

ASNT nondestructive testing Level III certificate in a givenmethod before receiving employer certification as a Level IIIin that method. The 1991 edition, however, does not includevisual testing as a method.

ConclusionA graphic summary of the various ways to achieve certifi-

cation according to SNT-TC-1A is shown in Fig. 1.As of 1993, there are seven editions of SNT-TC-IA that

may be encountered, depending on the industry or the con-tract being used. These editions are dated 1966, 1968, 1971,1975, 1980, 1984 and 1988. Care should be used to exercisethe one that applies to the contract in force, especially if ver-batim compliance with SNT-TC-1A is required.


PART 3 OIL FIELD TUBULAR SPECIFICATIONS

and completed with P-110 or Q-125 casing and tubing. Manyproprietary grades of pipe are also available.

The level of testing in a manufacturing plant must meet orexceed relevant specifications. Lower grade tubulars mayreceive only hydrostatic, dimensional, drift and visual testingfor relatively gross discontinuities. The end user may, how-ever, require other nondestructive tests, such as magneticparticle, electromagnetic, ultrasonic and, in the case of linepipe (pipe for pipelines), radiographic testing. Higher gradetubulars receive different levels of automated nondestructivetests and it is common for them to be inspected as many asfour times before use.

Field tests for casing, tubing, plain end drill pipe and linepipe are very well defined (see Table 4) and include visualtesting as one of the six techniques used.

Definition of Full Length Visual Testing for Line Pipe

Full length visual testing of line pipe is an inspection of thetotal length, including bevel and root face, to detect gouges,cuts, flats, dents, ground areas, mechanical damage, lack ofstraightness or other visually detectable discontinuities. Spe-cial attention is given to the weld line for undercut and off-joint weld. Rolling each length and viewing the entire exter-nal surface is required. The entire inside surface is inspectedusing a high intensity light source or borescope on smalldiameter pipe.

The testing of oil field pipes and tubes, referred to as tubu-lar goods or tubulars, is performed to the specifications andrecommended practices of the American Petroleum Insti-tute (API) and to test specifications that are written by oiland gas companies. Visual testing organizations are expectedto have written practices for the tests they perform. Table 2lists relevant API documents that cover tests of tubularmaterials.

Pipe grades and their minimum yield strengths are shownin Table 3. Generally, the lower thegrade, the less critical isthe pipe's service. For example, shallow wells might bedrilled with grade E-75 drill pipe and be completed withH-40, J-55 or K-55 casing and tubing. Deep wells, 4.5 to6 km (15,0(X) to 20,000 ft), are drilled with S-135 drill pipe

TABLE 2. American Petroleum Institute documentscovering tests of tubular materials

API 5CT

Specification for Casing and TubingAPI 50

Specification for Drill PipeAPI 5L

Specification for Line PipeAPI 7

Specification for Rotary Drilling EquipmentAPI 5T1

Bulletin on Imperfection TerminologyAPI RP-5A5

Recommended Practice for FieldInspection of New Casing, Tubing andPlain End Drill Pipe

API RP-5L8

Recommended Practice for FieldInspection of New Line Pipe

API RP-7G

Recommended Practice for Drill StemDesign and Operating Limits

TABLE 3. Pipe grades and their minimum yield strengths

Casing and TubingGrade MPa (ksi) Grade

Drill PipeMPa Grade

Line PipeMPa (ksi)

H-40 280 (40) D-55' 380 (55) AA-25 170 (25J-55 380 (55) E-75 520 (75) B 240 (35K-55 380 (55) X-95 650 (94)C-75 520 (75) G- 1 05 720 (104) X-42 290 (42L-80 550 (80) S-135 930 (135) X-46 320 (46N-80 550 (80) X-52 360 (52C-90 620 (90) X-56 390 (57C-95 650 (95) X-60 410 (60T-95 650 (95) X-65 450 (65P-110 760 (110) X-70 480 (700-125 860 (125) X-80 550 (80

1. D-55 WAS DROPPED BYAPI IN 1981.


TABLE 4. Field testing for oil field casing, tubing and pipe

Test Procedure CW (LPO)Pipe Type

SMLS ERW AW (LPOJ

FLVT all all all allDBE (LPO) all all all allHardness all all all allFLMPIW N NA all allFLMPOW N NA all allFLMPI N all all allFLMPO N all all allEAT all all all NEMT EQ EQ EQ NResidual magnetism all all all allGamma wall thickness N all all NGrade comparison N all all NUTBL N all all NUTBLTO N EQ EQ NUTW N NA all allUTLE (LPO) N all all allHandheld ultrasonic gaging all all all all

LEGENDallEQNNA

= TESTING MAY BE APPUCABLE THROUGHOUT THE DIAMETER RANGE= TESTING MAY BE APPLICABLE THROUGHOUT THE DIAMETER RANGE SUBJECT TO EQUIPMENT LIMITATIONS= TESTING USUALLY NOT APPLICABLE FOR THIS TYPE OF PIPE= NOT APPLICABLE INO WELDS IN SEAMLESS PIPE)

TESTING ABBREVIATIONSDBE = DIAMETER AND BEVEL CHECK (PIPE ENDS)EMT = ELECTROMAGNETIC TESTINGEAT = END AREA TESTINGFLVT = FULL LENGTH VISUAL TESTINGFLMPIW = FULL LENGTH MAGNETIC PARTICLE (INSIDE WELD)FLMPOW = FULL LENGTH MAGNETIC PARTICLE (OUTSIDE WELD)FLMPI = FULL LENGTH MAGNETIC PARTICLE (INSIDE SURFACE)FLMPO = FULL LENGTH MAGNETIC PARTICLE (OUTSIDE SURFACE)LPO = UNE PIPE ONLYUTBL = ULTRASONIC BODY LAMINATIONS AND WALL THICKNESSUTBLTO = ULTRASONIC BODY LONGITUDINAL, TRANSVERSE, OBLIQUEUTV/ = ULTRASONIC TESTING (WELD ONLY)UTLE = ULTRASONIC LAMINATION CHECK (PIPE ENDS)

PIPE ABBREVIATIONSAW = ARC WELDED PIPECW = CONTINUOUS WELDED PIPE (BUTT WELDED)ERW = ELECTRIC RESISTANCE WELDED PIPEPALS = SEAMLESS PIPE

Typical Visual Testing for New PipeSpecifications typically require that all new pipe be visu-

ally tested. In so doing, the following conditions might bedetected:

1. out-of-roundness;2. excessive height of an inside flash weld or excessive

trim of the inside surface weld on electric resistancewelded pipe;

3. out-of-straightness;4. three-dimensional discontinuities such as slugs,

gouges and pits;5. excessive mill ripple or roll marks, which can lead to

out-of-tolerance pipe walls;

6. stretch indentations in tubing, which may lead to out-of-tolerance pipe walls;

7. unacceptable threads;8. seams, cracks, porosity, pits, gouges and grip marks on

couplings;9. workmanship problems in belled ends of line pipe;

10. dents;11. offset plate edges in line pipe;12. out-of-line weld beads;13. arc burns;14. undercuts of welded pipe;15. blisters;16. scabs and slivers; and17. upset underfill or materials.

FIGURE 2. The sealing shoulders on drill pipe occurat the left hand end of section 2 (flat faces at thesepoints are essential); note that the threads on drillpipe and drill collars do not seal

r- 2 3

LEGENDi LENGTH COVERED UNDER DRILL PIPE CLASSIFICATION SYSTEM2 LENGTH COVERED UNDER TOOL JOINT TESTING STANDARD3 CAUTION LENGTH NOT COVERED BY TESTING STANDARDS

FROM AMERICAN PETROLEUM INSTITUTE. REPRINTED WITHPERMISSION.

FIGURE 3. Stretched pin end threads on drill pipe(the sealing face is just to the left of the gage)

FROM EXXON PRODUCTION RESEARCH. REPRINTED WITH PERMISSION.


Detection of such conditions can initiate further testing orevaluation and may lead to rejection of the tube, pipe or itscoupling.

Straightness

At one time, oil field pipe was only required to be reason-ably straight. However, later specifications define the degreeof permissible bend in pipe with an outer diameter of114 mm (4.5 in.) and larger. Comparison with a taut stringor a 1.5 in (5 ft) straight edge is required such that devia-tion from chord height or straight does not exceed either0.2 percent of the total pipe length or 3 mm (0.125 in.) in the1.5 m (5 ft) length at each end. (Note: chord height appliesto string and straight applies to straight edge.)

Couplings

Couplings require a visual test for the detection of discon-tinuities such as visible seams and cracks. Certain depths ofpits and round bottom gouges are permitted, requiring theuse of a mechanical pit depth gage. Sharp bottom imperfec-tions have separate criteria for acceptance and also requiredepth measurement.

Borescope Testing

Where borescope testing is required, the recommenda-tion is not made for surface brightness but the lamp wattagefor the pipe inside diameter is stated (see Table 5). The reso-lution of the borescope's optical system should be such thatJaeger J_4TM letters are distinguishable when placed 100 mm(4 in.) from the objective lens. Alternatively, sufficient resolu-tion may be indicated when the borescope allows reading thedate on a penny or dime.

Typical Visual Testing of Used Drill PipeThe document API RP-7G (see Table 2) outlines the test-

ing of drill pipe, much of which is performed with rulers and

TABLE 5. Incandescent lamp wattage for borescopetests of various pipe diameters

Pipe Inner Diameter Minimum Lamp Wattagemillimeters (inches) (watts)

Oto 25 (O to 11 1025 to 75)1 1o3) 3075 to 125 (3 to 5) 100

> 125 (> 5) 250

mechanical gages. Magnetic flux leakage and wall gaging,either by ultrasonic or radiation absorption, is also commonlyperformed. Because many flaws found in used drill pipe areservice induced they can usually be visually detected. Excep-tions are wear and fatigue cracking.

External Surface Visual Testing

Visual testing of the sealing shoulders (Fig. 2) often leadsto dimensional measurement and flatness gaging.

Visual testing of threads may detect indications of insuffi-cient torque or overtorque, lapped threads, galling andstretching. A profile gage is often used to help detect threaddiscontinuities. Pin stretch is shown in Fig. 3. Box swell isalso detected during visual inspection of the connections.

Visual testing of the pipe body may reveal such conditionsas washes, mashes, string shot, necking, crushing, gouges,slip marks and outside diameter pitting. Washes or wash-outs


occur when a fatigue crack travels through the pipe wall dur-ing drilling and high pressure drilling mud is forced throughthe opening to rapidly abrade the crack walls into a three-dimensional hole. Wash-outs are often detected by loss ofpressure while drilling. Necking and crushing arise fromexcessive inward force on the pipe while in the slips of thedrilling rig.

String shot is an expansion of the pipe wall during a down-hole controlled explosion performed to break out a pin boxconnection (the pipe material is taken past its elastic limit).Stretching indicates excessive pulling on a stuck pipe. Slipmarks are circumferential grooves cut into the pipe's outsidediameter when the pipe is rotated in the slips of the rig. Slipmarks act as stress risers and encourage formation of fatiguecracks. Such cracks in the bases of slip marks (Fig. 4) are verydifficult to detect with visual or conventional wet fluorescentmagnetic particle testing but are often detected by shearwave ultrasonic testing.

Internal Surface Visual Testing

One of the oldest forms of inservice drill pipe testing is theuse of an internal borescope to study magnetic particle indi-cations. In this test, the inspector is searching for transversefatigue cracks on the inside diameter wall, often in the pres-ence of pitting.

Visual Testing of Oil Field HoistingEquipment

The American Petroleum Institute's Recommended Prac-tice for Hoisting Tool Inspection and Maintenance Procedures

FIGURE 4. Compared to visual tests, ultrasonic testshave advantages for the detection of subsurfacefatigue cracking at the base of slip marks in thewalls of drill pipe

SLIP MARK

FATIGUE CRACK

ULTRASOUND

(API RP-8B) covers testing of crown and traveling blocks,block-to-hook adapters, connectors, link adapters, drillinghooks, tubing and sucker rod hooks, elevator links, casing,tubing and drill pipe elevators, sucker rod elevators, swivelbail adapters, rotary swivels, spiders, deadline tie downs,kelly spinners, rotary tables and slips, heave compensatorsand tension members of underwater handling equipment.The document recommends routine field testing, periodicfield tests, critical load testing and disassembly tests forcracks, loose fits, elongation of parts, wear, corrosion andoverloading. Much of the recommended testing is visual.

SECTION 8

APPLICATIONS OF VISUAL ANDOPTICAL TESTS IN THE METALSINDUSTRIESHitoshi Aizawa, Kawasaki Steel, Mizushima, Kurashiki, Japan (Part 2)Kazuo Miyagawa, Kiyomidai Minami, Kisarazu City, Japan (Part 2)

PORTIONS OF PART I ADAPTED FROM DONALD MARL UNDERSTANDING HOW COMPONENTS FAIL, ASIA INTERNATIONAL. REPRINTED WITH PERMISSION.PORTIONS OF PART I ADAPTED FROM WELDING HANDBOOK, 7TH ED . VOL I , FUNDAMENTALS OF WELDING, C THE AMERICAN WELDING SOCIETY. REPRINTED

WITH PERMISSION.PORTIONS OF PART I ADAPTED FROM STEEL PRODUCTS SHEET STEEL-CARBON, HIGH STRENGTH LOW ALLOY, AND ALLOY COILS AND CUT LENGTHS, © THE IRON 8 STEEL

INSTITUTE. REPRINTED WITH PERMISSION

FIGURE 1. General stress-strain curve showingelastic and plastic portions of a typical curve; yieldis the area of transition from elastic to plasticdeformation [yield strength, yield point, elasticlimit and proportional limits are all in this area)

YIELDFRACTURE

IPLASTIC DEFORMATION {PERMANENT)

ELASTIC DEFORMATION (TEMPORARY)

STRAIN(ELONGATION)

FROM ASM INTERNATIONAL. REPRINTED WITH PERMISSION.

FIGURE 2. Typical stress-strain curve for a lowstrength, easily deformable metal such as solderwire or household aluminum foil

FRACTURE

STRAIN(elongation)



PART I

PHYSICAL PROPERTIES OF METALS

The properties of metals are often categorized intomechanical and other physical properties. Mechanical prop-erties are associated with the strength or load bearing abilityof materials. The other characteristics pertaining to thephysics of materials include electrical, thermal, magneticand mechanical properties.

Mechanical PropertiesIn metals, fracture and wear are closely related to mechan-

ical properties, and it is important to understand these gen-eral relationships in order to perform accurate visual tests.Mechanical properties of a material may be described interms of its elastic and inelastic behavior when force isapplied. Mechanical properties are used to indicate the suit-ability for mechanical applications and include modulus ofelasticity, tensile strength, elongation, hardness and fatiguelimit.' Other mechanical properties include yield strength,yield point, impact strength and reduction of area. In gen-eral, any property relating to the strength of a metal can beconsidered a mechanical property.

Elastic and Plastic Deformation

Elastic deformation refers to a metal's ability to return toits original size and shape after being loaded and unloaded.As shown in Fig. 1, elastic deformation occurs on the straightpart of a typical stress-strain curve. Hooke's law states thatstress and strain are linearly proportional within the elasticrange. Elastic deformation is the state in which most metalcomponents are used in service.

If higher loads are applied, the range of elasticity or elasticdeformation is exceeded and the metal is permanentlydeformed (is in the plastic deformation state) as shown inFig. 1. Metals with a low elastic range but a high capacity forplastic deformation include aluminum foil, solder wire andhot rolled steel. Both are readily deformed and, in fact, alu-minum foil must have high ductility (plastic delimitation) forit to be useful. If aluminum foil is too strong (low ductility), itdoes not wrap properly and is considered defective. Figure 2shows the stress-strain curve for a low strength, highly duc-tile metal.

An important feature of the stress-strain curve is the slopeof the straight line or elastic portion of the curve. This slope,called the nwdulus of elasticity, measures the stiffness of themetal in the elastic range. Changing the hardness or

•

FIGURE 3. Stress-strain curves for steels of differentstrength levels, ranging from very hard, strong,brittle steel 1 to relatively soft, ductile steel 5

5

STRAIN(elongation(


FIGURE 4. Relationship of stiffness tor modulus ofelasticity) to temperature for four common alloysystems

TEMPERATURE°C (°F)


207 (3)

69 (1) '1"-i"AlLim

0 I

- 30(-200)

----GIL......z,............=.44,1 . ALLOYS'" AL LOYS

1 1 1 I ] 1 1 1 1 1 1 1 I

-20(0)

95200)

205(400)

315(600)

425(800)

540 6501,000)(I,200)

COPPER A

LLOYS

STEEL ALLOYS

APPLICATIONS OF VISUAL AND OPTICAL TESTING IN THE METALS INDUSTRIES 1 195

strength of the same base alloy does not change the stiffnessof the metal.

As it applies to steel, the modulus of elasticity can beexplained as follows: elastic deflection under load is a func-tion of the component's section, not the composition, heattreatment or hardness of the steel. In other words, the mod-ulus of elasticity of all the commercial steels, both plain car-bon and alloy, is the same so far as practical design isconcerned. Consequently, if a component deflects exces-sively within the elastic range, the remedy is found in designnot in metallurgy. Either a heavier section must be used, thepoints of support must be increased or some similar changeis made—under the same conditions of loading, all steelsdeflect the same amount within the elastic limit.

The same point may be made with the diagram in Fig. 3,showing the stress-strain curves for steels of differentstrength levels, all branching from the same straight line(elastic portion). A hard, brittle metal 1 (very strong steel,for example) goes straight up the elastic line with no devia-tion, then fractures. A slightly less strong steel 2 has slightplastic deformation (ductility). Steel 3 and 4 are of interme-diate strengths. Steel 5 is the relatively tow strength, highductility type desired for deep drawing and severe forming.Note that the straight line (elastic) portion of the curve isidentical for all steels.

Effect of Temperature

Temperature is the only condition that changes the stiff-ness of a metal. Metal stiffness varies inversely with its

temperature (as temperature increases, stiffness decreasesand vice versa). This relationship is illustrated for four com-mon alloys in Fig. 4.

Steel alloys typically have a modulus of elasticity of 200 to210 GPa (2.9 X 104 to 3.0 x 104 ksi) at room temperature.A spring deflects more at an elevated temperature than atroom temperature and must be designed accordingly (aspring at low temperature deflects less). Typical room tem-perature values are 100 to 130 GPa (1.5 x 104 to 1.9 x 104ksi) for copper alloys; 70 to 75 GPa (1.0 x 10 4 to 1.1 x 104ksi) for aluminum alloys; and 40 to 50 GPa (6 x 10 3 to 7 x103 ksi) for magnesium alloys. These values decrease withincreasing temperature.

Nonlinear Behavior

There are a few metals that do not conform to Hooke's law,which states that stress and strain are linearly proportionalwithin the elastic range. Figure 5 shows the nonlinear stress-strain curves for three classes of gray cast iron.' This nonlin-ear behavior is caused by the embedded graphite flakes inthe steel matrix that give gray cast iron its unique properties.The flakes form internal stress concentrations when themetal is loaded in tension. They tend to cause microscopicand irreversible yielding at the sides or ends of the flakes.The elastic properties of gray cast iron are determined, inpart, by the size, shape and distribution of graphite flakes.

Sintered metals also have nonlinear stress-strain curves forthe same reason. Their internal pores, like the flakes in graycast iron, act as stress concentrators. However, as the densityof a sintered metal approaches the maximum theoreticaldensity for the alloy, the curves tend to approach linearity. 3

FIGURE 5. Typical stress-strain curves for threeclasses of gray cast iron

STRAIN(elongation)


FIGURE 6. Engineering stress-strain curve showingthe tensile region (upper right) and thecompression region (lower left)

STRAIN (elongation)

YIELD


FIGURE 7. Effect of elevated temperatures Ti andT2 on tensile (upper right) and compressive (lowerleft) properties of a typical metal

STRAIN (ELONGATION)



Cold drawn steel bars also have slight curves in their elasticregions because of the high residual (internal) stressescaused by cold drawing. However, heating at temperaturesof 370 to 480 °C (700 to 900 °F) relieves the stresses andrestores linearity in the elastic region. It also simultaneouslyincreases the yield strength. Consequently, cold drawn,stress relieved bars are commonly specified and readilyavailable.'

Bidirectional Stresses

Stress-strain curves also apply to bidirectional stresses.Normally, only the tensile part of the curve is shown, as inFigs. 1 to 3. However, the straight line portion also extendsinto the compression region, as can be seen in Fig. 6. In met-als with yield strengths, the compressive yield strength isusually about equal to the tensile yield strength. With duc-tile metals in compression, there is no definite end point.The end point must be an arbitrarily selected valuedepending on the degree of distortion that indicates com-plete failure of the material.' Certain metals fail in compres-sion by a shattering fracture. These are normally brittlematerials that do not deform plastically. Gray cast iron,which is relatively weak in tension because of the mass ofinternal graphite flakes, has a compressive strength that isseveral times its tensile strength.'

By consolidating the information given above, it followsthat the modulus of elasticity is reduced when a metal is atelevated temperature. In Fig. 7, temperature T representsan arbitrarily selected base temperature, while T, and T2

FIGURE 8. Relationship between hardness andtensile strength of metals in the absence of stressconcentrations

HARDNESS


FIGURE 9. Relationship between fatigue limit andtensile strength In polished and severely notchedsteel reference standards

x

_c,

828 (120)

690 (100) -

POLISHED REFERENCESTANDARDS

k")552 (80) -

414 (60)-1.1.1

cu_ a

276 (40).

:q1o_

138 (20) _

cnSEVERELY NOTCHED

REFERENCE STANDARDSE

0345 690 1,035 1,380 1,725

(0)

(50) (100) ( i 50) (200) (250)TENSILE STRENGTH

megapascals (pounds per square inch x 103)


APPLICATIONS OF VISUAL AND OPTICAL TESTING IN THE METALS INDUSTRIES / 197

represent elevated temperatures. Note not only thedecrease in the modulus of elasticity (the slope of the straightline portion) but also the decrease in yield strength and ten-sile (compressive) strength with increasing temperature.

Effect of Stress Concentrations

In general, the tensile strength of a metal changes in pro-portion to hardness, as shown in Fig. 8. However, this rela-tionship does not always hold true at high hardness levels orwith brittle materials, for such materials are more sensitiveto stress concentrations or notches and may fracture prema-turely when stressed in tension.

The harder and stronger the metal, the more sensitive it isto stress concentrations. Therefore, high hardness, highstrength metals must be treated carefully. They cannot flowor deform plastically at stress concentrations as well as canmore ductile metals of somewhat lower hardness. However,high hardness, high strength metals are useful for their highstatic and fatigue strength and their high wear resistance.

Fatigue strength is greatly reduced by severe notches thathave not been mechanically prestressed by compressiveresidual stresses. Figure 9 illustrates the effect of suchnotches on nominal fatigue properties as derived by testingpolished bars (reference standards) without stress concen-trations.' In typical visual testing environments, polishedcomponents without stress concentrations are very rare andgreat care must be taken when nominal or tabulated strengthvalues are used for design purposes. It is for this reason thatcomponents and assemblies are laboratory tested and fieldtested to get data on actual performance of the component.

WearWear can he defined as the undesired removal of material

from contacting surfaces by mechanical action.' Although itis not typically as serious as fracture, wear is an expensive andoften predictable problem.

Contacting surfaces are expected to wear in any machine.In many cases the deterioration can be minimized by lubrica-tion, oil filtering, materials engineering and proper design,among other measures.

In many respects, wear is similar to corrosion. Both havemany types, of which two are usually occurring simultane-ously. Both are somewhat predictable in stable environ-ments. Both are extremely difficult to evaluate inaccelerated laboratory or service tests, with rankings ofmaterials subject to change depending on seemingly minorchanges in the test conditions. Finally, wear and corrosionboth are of enormous economic importance.

When visual inspections are done, it is necessary to under-stand the history and operation of the mechanism involved.

FIGURE 10. Idealized representations of the twotypes of force applied to abrasive wear particles:(a) cutting or plowing action of a containedparticle under pressure and (b) cutting or plowingaction of a loose particle flowing across the metalsurface after impinging on the surface

(a )

LOAD IWI

DIRECTION OF VAL REMOVED BY

TRAVEL ABRASIVE PARTICLE

DIAMETER OFABRASIVE PARTICLE --

DISTANCE ABRASIVEPARTICLE MOVED

ABRADED SURFACE

IMPINGEMENT ANGLEDIRECTION OF APPROACH

i.DIAMETER OF

ABRASIVE PARTICLE —

METAL REMOVED BY ,ASIVr JED SURFACE



In many cases, it is not possible to conduct a complete inves-tigation simply by visually testing the worn component.Wear is a surface phenomenon that results from the interac-tion of other components and materials that also must bestudied.

The definition of wear is broad and the subject may beorganized into the following two nonconventional categories.The first category includes abrasive wear (erosive wear,grinding wear and gouging wear), adhesive wear and frettingwear. The second category consists of contact stress fatigue(subsurface origin fatigue, surface origin fatigue, subcase ori-gin fatigue and cavitation fatigue).

Fretting wear and cavitation fatigue are typically includedin discussions of corrosion because both result from chemi-cal change. However, because both are primarily the resultof undesired removal of material from contacting surfaces bymechanical action, they may also be included with otherwear phenomena.

Abrasive Wear

Abrasive wear is characterized by cutting. It occurs whenone surface rolls or slides under pressure against anothersurface (see Fig. 10). Machining could be considered abra-sive wear except that it is intentional. Another veryimportant characteristic of abrasive wear is the heat gener-ated by friction between the two materials.

One method of reducing abrasive wear is to increase sur-face hardness but this is not the answer to all abrasion prob-lems.' In cutting tools such as knives, increasing thehardness may make the cutting tool more resistant to dullingof the sharp edge but it also increases the chance of brittlefracture, a much more serious problem than dulling fromabrasive wear. A dull tool can always be sharpened andreused—a fractured blade prohibits subsequent repair andmay cause injury at the time of failure.

Erosive We&

Erosive wear (or erosion) occurs when particles in a fluidor other carrier slide and roll at relatively high velocityagainst a surface. Each moving particle contacting the sur-face cuts a minute particle from the surface. Individually,each particle is insignificant but a large number of particlesremoved over a long period of time can produce high degreesof erosion.

Erosive wear can be expected in metal assemblies such aspumps and impellers, fans, steam lines and nozzles, insidesharp bends in tubes and pipes, sand and shot blasting equip-ment and similar areas where there is considerable relativemotion between the metal and the particles.

Erosive wear can be recognized during visual tests by anyor all of the following conditions, depending on the materialsinvolved.

The first condition is a general removal of soft surfacecoatings or material. This is a common form of wear for fanand propeller blades. In automotive applications, for exam-ple, the paint on the trailing end of the concave side of theblade is usually removed by the scouring or cutting action ofdust and dirt particles in the air.

A second visible condition is grooving or channeling of thematerial. This type of erosive wear is common in assembliesthat move liquids or gases where the design of the compo-nent is such that the fluid flows faster or in a different direc-tion at certain locations. Examples include impellers withvanes that push particle-laden fluids into various passages.The inside of tubes or pipes is often damaged at curvesbecause the inertia of the particles and the fluid forces themagainst the outside of the curve. Sudden, sharp curves orbends cause more erosion problems than gentle curves. Intextile machinery, high velocity thread or yarn can cause ero-sion when a sudden change in direction causes grooving inan eyelet.

Grooving and channeling are also quite common in vari-ous types of nozzles where high speed or high pressure fluidsscour through the metal. Drops of liquid can lead to erosivewear, as is frequently seen on the leading edges of high speedaircraft.

A third visible condition caused by erosive wear isrounding of corners. Erosive wear can change the shape of

FIGURE 11. Diagram of self sharpeningcomponents in a rodent tooth (see reference 9)

CONTINUOUS GROWTH

SOFT DENTINELOW STRESS SIDE

HIGH STRESS SIDE

WORN AWAY HARD ENAMEL


FIGURE 12. Components of a self sharpeningplowshare

SOFTER STEEL

DIRECTION OF TRAVEL -

HIGH STRESS SIDE

LOW STRESS SIDEWORN, SOFTER STEEL

HARD FACING ALLOY



impellers, turbine blades and vanes in ways that substantiallyimpair operating efficiency.

Grinding Wear

Grinding wear occurs primarily when particles under highpressure cut very small grooves at relatively low speed acrossa metal surface. High pressure, low speed operation is char-acteristic of tillage tools (plows, cultivators, rakes) and otherground contact components such as bulldozer track shoesand the cutting edges of blades. In many other industries,similar effects are produced on metals, tending to changetheir shape and to dull cutting edges, consequently loweringefficiency of operation.

Grinding wear can be recognized in visual tests if the ser-vice environment is known and if the wear occurs at highstress locations (particularly points and edges), causing ageneral change in shape. When two hard metal surfacesslide against each other, frequently in the presence of a lubri-cant, each may tend to smooth the other, particularly if fineabrasives are present. When properly controlled, this pro-cess may be useful for lapping dr polishing.

Hard facing (by welding, metal spraying or other means ofdeposition) is frequently used to improve grinding wearresistance. Such deposits typically contain large quantities ofalloy carbides such as those of tungsten, titanium, chro-mium, molybdenum and vanadium. In certain applications,oxides, borides or nitrides may be more satisfactory—theservice conditions and the environment are important forselecting the best alternative."

In many cases, diffusion treatments such as carburizing,carbonitriding, nitriding, chromizing or boronizing are use-ful for hard facing.

Evaluating the best type of coating or diffusion treatmentfor a given application can be difficult because simulated lab-oratory tests (typically accelerated to reduce test time) oftengive misleading results. Actual service testing is usually thebest way to evaluate wear resistance of various materials orprocesses. Eyen this can be misleading, because the combi-nation that is best in one type of abrasive environment mayperform poorly in another environment. The same abrasivematerial used in different environments may produce vary-ing results because of differences in packing and thermalcharacteristics. Sand, for example, tends to cool a cuttingedge more when wet than it does when dry. A cutting edgewith greater hardness when hot may be more efficient,depending on frictional temperatures at the cutting edge.

Slight, controlled grinding wear may sometimes be anadvantage for self sharpening of certain cutting tools. Byjudicious use of the so-called rat's tooth principle, hardenedand soft surfaces may be used together to keep an edge. Thefront teeth of all rodents have a hard, brittle enamel on thefront convex surfaces but relatively soft dentine on the rearconcave surfaces (see Fig. 11)8" When the animal uses its

teeth, the dentine on the rear of the teeth wears away, leavinga thin, sharp edge of enamel. The tip of the brittle enameleventually breaks off, keeping the teeth the proper length.

This same principle can be applied to certain cutting tools.For example, the cutting edges of plows (plowshares) can bemade self sharpening if the front surface is soft and the rearsurface is faced with a hard material. As the plowshare cutsthrough the soil, the relatively soft steel on the forward, highstress side is slowly worn away. The hard facing applied tothe rear, low stress side is continually exposed at the tip anda sharp edge is retained (see Fig. 12).

Electric carving knives with two blades sliding againsteach other are sometimes hard surfaced on the outer, lowstress sides. As the blades slide rub each other during cut-ting, slight metal wear occurs on the soft inner surface, keep-ing the blades sharp.

In the mining industry, digging tools are sometimes hardfaced on one side to maintain this self sharpening ability(Fig. 13).'"

FIGURE 13. Selective hard facing helps controlwear by letting a digging tooth on ground contactequipment sharpen itself: a) blunt tooth with bothsides hard faced and (b) sharp tooth with one sidehard faced and the other side worn away

(a)

(b)


FIGURE 14. Illustration of one process by whicha particle of debris is detached during adhesivewear: (a) a bonded junction forms; (a) junction istorn from one peak or asperity; and (c) the asperitythen is sheared off by impact with a larger,adjacent peak

BONDED JUNCTION

(a)

(b)

On'SHEARED ASPERITY

(c)

WEAR DEBRIS PARTICLE



Gouging Wear

Gouging wear is caused by high pressure impact that liftslarge fragments from a metal surface. Gouging is encoun-tered in the fields of earthmoving, mining, quarrying, oil welldrilling, steelmaking, cement and clay product manufacture,railroading, dredging and lumbering. When hard, abrasiveproducts are crushed, battered or pounded under high pres-sure, rapid deterioration of the contact surfaces can beexpected unless specific steps are taken to prevent theproblem.

In certain cases, it may be more economical to usereplaceable components (the teeth on backhoe buckets, forexample). In other cases, components are not easy to replaceand must be made from a more resistant material. A type ofsteel invented in England in the 1800s by Sir Robert Had-field has been used successfully in applications such as rail-way crossing frogs and switches, rock crushers, grindingmills, dredge buckets, power shovel buckets, bucket teethand pumps for handling gravel and rocks s'

This specialty steel, typically called Hadfield's steel, is anaustenitic manganese steel used in the form of castings orwelded to a steel base. 6.7 It is machinable only with difficulty,for the machining operation work hardens the austeniticsteel, as does battering during high stress service. In somecases, it is possible to preharden this material by submittingit to heavy hammering before service. This type of steel isnot intended for resistance to erosive wear or most kinds ofgrinding wear.

Remedies for gouging, as with other types of wear, usuallyare chosen on the basis of a combination of economics,

availability, accessibility and design. Frequently, there areseveral ways to improve a product's wear resistance; the oneis chosen that provides optimum properties at lowest cost.

Adhesive Wear

Adhesive wear can be characterized as microwelding. Theactual wear mechanism is described by the term adhesivewear. Other terms are sometimes used, including scoring,scuffing, galling and seizing, but adhesive wear is preferredbecause it accurately characterizes the phenomenon.

Figure 14 is an exaggerated view of two surfaces slidingagainst each other. They may or may not he separated bylubricant. When a rough peak (or asperity) from one surfacecomes in contact with a peak from the other surface, theremay be instantaneous microwelding caused by frictional heat(see Fig. 14a). Continued sliding fractures one side of thewelded junction (see Fig. 14h), making the asperity on oneside higher and the asperity on the other side lower than theoriginal height. The higher peak is now available to contactthe peak on the opposite side (see Fig. 14c).

The tip may either be fractured by the new contact orrewelded on the opposite side and the cycle repeated. Ineither case, adhesive wear frequently starts out on a smallscale but rapidly escalates as the two sides alternately weldand tear metal from each other's surfaces. Also, if a lubricantis present, the debris may be carried to other components of


the mechanism. In severe adhesive wear, the debris is com-posed of free metallic particles. In mild cases, much finerparticles may react with the environment to form debris thatis largely free metal oxide particles.

The interface between two sliding surfaces is an extremelycomplex system, consisting of two metal surfaces (each withits own metallurgical, mechanical, chemical and topographi-cal characteristics) and often a lubricant (itself a complexblend of physical and chemical characteristics that changewith the temperature). Various modes of lubrication are dis-cussed in the literature' and are covered here only as neededto point out the ideal situation when the lubricant achievescomplete separation between every part of the two metalsurfaces.

The heat generated by friction is locally high enough tocause microwelding. This means that the temperature also ishigh enough to cause localized heat treatment of the surfacemetal. Adhesive wear is similar to grinding burn in that bothcan cause tempering of the subsurface metal and actualrehardening of steel microstructures." This produces white,untempered martensite, which is extremely susceptible tocracking because of its brittlenesS. Such cracks can lead tobrittle fracture or fatigue fracture, depending on the testobject and the application.

A practical way of visually inspecting for the presence ofadhesive wear in hardened steels is to use the Tarasov etch-ing technique, where two etching solutions give a high con-trast, nondestructive means of checking for adhesive wear(and grinding burn). Of course, other etchants such as theconventional nital (a weak solution of nitric acid in alcohol)also may be used but the contrast and sensitivity are usuallynot as good as with the Tarasov etching solutions. Metallo-graphic examination on a cross section of the surface also canreveal evidence of adhesive wear when studied at moderateto high magnification, depending on the amount of thermaldamage.

Fretting Wear

Fretting wear is similar to adhesive wear in that micro-welding occurs on mating surfaces. The difference is thatadhesive wear is related to moving surfaces and fretting wearis related to stationary surfaces. However, when minute elas-tic deflections or slight motion does occur, a cyclic motion ofextremely small amplitude is enough to cause microweldingon both surfaces. Fretting wear is also known as fretting cor-rosion, false brinnelling, friction oxidation, chafing fatigueand wear oxidation.'

Fretting frequently occurs in stationary joints that arefixed from shrinking or pressing by interference fits or bybolts, pins, rivets or other mechanisms and also at variouscontact points in antifriction or rolling elements. This meansthat nonrotating antifriction bearings are subject to vibrationover a period of time and may have fretting wear wherever a

ball or roller contacts a raceway under load. If the bearingssubsequently rotate in normal service, they may be noisybecause of the wear patterns and small indentations presentin the raceways and the corresponding flat spots on the roll-ing elements.

The term false brinelling is sometimes used to describefretting wear indentations. Fretting also is a serious problemon components such as shafts, where it can initiate fatiguecracking on the contacting surfaces. In fact, many fatiguefractures of shafts are caused directly by fretting. Becausefretting is extremely difficult to prevent, special means mustbe taken to prevent fracture resulting from fretting, whichcan occur in unexpected locations.

Because fretting wear is essentially a stationary phenome-non, its debris (usually oxides of the contacting metals) isretained at or near the site of its formation, a condition espe-cially helpful during visual tests. With ferrous metals, thedebris is brown, red or black, depending on the type of ironoxide formed. For this reason, ferrous debris is called cocoaor, when mixed with oil or grease, red mud. Aluminum alloysform a black powder when fretting wear is present.

Contact Stress FatigueThe text below considers various causes of metal removal,

including fatigue that produces cavities or pits. The cavitiesthemselves are serious because they frequently act as stressconcentrators that can lead to fracture, particularly in com-ponents such as gear teeth. The metal removed from cavitiesis typically hard and brittle and is readily fragmented intosmaller particles. Such particles can cause abrasive wear, inaddition to pitting, and can he carried by lubricants to otherareas of the mechanism. Rolling bearings, sliding bearings,gears and components such as pumps, impellers and propel-lers are subject to fatigue wear.

Types of Contact Fatigue

Fatigue, as discussed here, is the same phenomenon thatresults from cyclic slip under repetitive load. The only differ-ence is that, instead of causing gross fractures, fragments ofthe surface are removed, causing cavities. Contact stressfatigue frequently is the limiting factor in load carryingability.

There are three types of pitting typically associated withcontact stress fatigue. Some cavities start as microscopic pitsand remain microscopic through the life of the component.During visual tests, these cavities cause a dull or frostedappearance on an otherwise bright surface. The second typebegins as microscopic but gradually enlarges in service, espe-cially rolling or sliding service motions under load. The thirdtype of pitting begins large and rapidly becomes larger. The

FIGURE 15. Sketch of surfaces in contact:(a) counterformal and (13) conformal surfaces

(a)

(b)


FIGURE 16. Subsurface origin pitting fatigueoriginates slightly below the surface where theshear stress is high



two types of growing cavities are destructive of hardenedsteel gears, rolling element hearings, roller cams and otherassemblies exposed to a combination of rolling and slidingmotions.

Components subject to contact stress fatigue generallyhave two convex or counterformal surfaces in contact underload (for example, gear teeth and various antifriction bear-ings). The same type of discontinuity can occur where a con-vex shape fits within a concave shape (a shaft within a slidingbearing or balls in a hall bearing race).

Pitting fatigue occurs on mating surfaces under compres-sive load because the contact is concentrated into either abroadened line or an elliptical point, depending on thegeometries involved (see Fig. 15). Because the instantaneouscontact areas may be small under heavy loads, the compres-sive and shear stresses may locally be very high.'

Types of Component Contact

When testing for contact stress fatigue, it is necessary tounderstand the difference between pure rolling contact androlling plus sliding contact. Pure rolling of metals undercompressive load is difficult and perhaps impossible toachieve because of the physical properties of metals.

All metals are elastic and deform elastically under load. Infact, harder and stronger metals, such as those typically usedin gears and rolling contact bearings, deform elastically to agreater degree than the softer metals.

Elasticity under heavy compressive loads produces slidingor shearing forces at the interface between the contactingmetals. This sliding under load generates heat that must bedissipated, usually by a lubricant. Elasticity under load alsocauses internal friction within the metals, generating evenmore heat. In fact, heat dissipation is one of the primaryfunctions of a lubricant, in addition to reduction of friction atthe interface.

Because of the many variations in rolling and sliding con-tact, as well as in metallurgical and geometrical variables,there are several types of fatigue that can occur in rolling orsliding components. In a real sense, there is a competitionbetween the different kinds of fatigue, with all modes prog-ressing simultaneously.

Convex surfaces under pressure deform elastically to forma bulge at the ends of contact. Surfaces in contact have themaximum shear stress a short distance below the surfaceswhen the members are either stationary or rolling withrespect to each other. Because shear stresses cause fatiguefractures, this location is of primary concern in rolling ele-ment components.

Subsurface Origin Fatigue

Pitting in hardened steel as a result of subsurface fatigueoccurs during nearly pure rolling of one element across oraround another. This is most common in antifriction or roll-ing element bearings such as ball bearings, roller bearings,needle bearings and roller cams.

The maximum shear stress is located a relatively short dis-tance below the surface and this is the normal location forfatigue fracture to originate. The most common geometricalstress concentrations in steel are at inclusions, especiallyhard, brittle inclusions that are angular in shape. Becauseinclusions are distributed randomly within steel, only thosewithin the high shear stress region are likely to cause subsur-face pitting fatigue.

Figure 16 shows a rolling element under pressure. Thedepth of the maximum shear stress may differ, depending onthe geometry of the components. Inclusions within the highshear stress region can cause fatigue cracks to originate par-allel to the surface. However, continued rolling across thedamaged area eventually causes cracks to reach the surface.The same thing may be occurring at several locations in thesteel until eventually a volume of metal is surrounded bycracks. When this happens, a particle of metal is lost fromthe surface and an irregularly shaped cavity is left visible onthe surface.

FIGURE 17. Schematic diagram of pure rolling,ignoring elastic deformation: (al when two rollersof different sizes but the same surface velocitycontact in pure roiling, point 1 contacts point 1A,then point 2 contacts point 2A and so on; (12) if thelower roller stops, the mating roller rolls aroundthe lower roller In a planetary motion and againpoints contact successively the correspondingpoints on the other roller and the relative motionis the same

lap

lb)


FIGURE 18. Schematic diagram of roiling andsliding contact: (a) the upper roller is driven at ahigher surface velocity than the lower roller, whichintroduces sliding into the Interface; lb) afterheavy compression forces are applied to the tworollers, the faster upper roller tends to drag thesurface of the slower lower roller to the left,increasing the bulge on that side of the slowerroller !see Table 1)

(a)

FAS I LR ROLL

SI °WU ROLL



Surface Origin Fatigue

Surface origin pitting of hardened steel surfaces may alsocause bending fatigue fracture. When sliding is added torolling contact, an entirely different and complicated set ofcircumstances arises. The maximum shear stress is no longerlocated below the surface of the steel but is brought to thesurface under the influence of sliding friction and the associ-ated traction forces.

In order to understand the state of stress at the surface ofa rolling and sliding interface, it is necessary to analyze therelative motions of rolling and sliding.

Bolling is shown in Fig. 17, where a small roller rotatescounterclockwise against a larger roller rotating clockwise.There is pure rolling (ignoring elastic deformation) if the sur-face velocities (not revolutions per minute) of the two rollersare identical. Points 1, 2 and 3 on each roller successivelycome into contact (Fig. 17a). However, the same motion rel-ative to the small roller can be achieved if it is held stationaryand the large roller rotates and rolls around it with a clock-wise planetary motion (Fig. 17b).

If the larger roller rotates with higher surface velocity thanthe smaller roller (see Fig. 18 and Table 1), then the slidingtends to drag the surface of the small roller to the left (coun-terclockwise). The surface of the larger roller is dragged tothe right, also counterclockwise on that roller. Note that thesliding direction is opposite to the rolling direction of thesmall roller, while the rolling and sliding directions are thesame on the larger and faster roller.

The term negative sliding is used when rolling and slidingare in opposite directions (the small roller in this example).Positive sliding occurs when the rolling and sliding directionsare the same (the larger roller). Because the surface of thesmall negative sliding roller is, in effect, rolling in one direc-tion and simultaneously being dragged in the opposite direc-tion, the frictional, thermal and shear stresses tend to behigher in this member than in the larger positive slidingroller. In addition, the smaller roller has more load applica-tions on each surface point than does the larger roller.Therefore, the small roller is far more likely to undergodestructive surface origin pitting than the larger roller in thisexample.

For components that undergo combined rolling and slid-ing, knowledge of the relative velocities and directions ofrolling and sliding is necessary for visual inspection and sub-sequent definition of the wear mechanism. The direction ofrolling is the direction in which the point of contact moves.The direction of rolling is always opposite the direction ofrotation of a rolling element. On a given surface, a conditionof positive sliding exists if the direction of sliding is the same

TABLE 1. Comparison of contact roller (see Fig. 181

Lower Roller Upper RollerSurface Motion Slower Faster

Rotation counterclockwise clockwiseRolling clockwise counterclockwiseSliding clockwise counterclockwiseP/N sliding negative (NI) positive (1= )

FROM THE AMERICAN WELDING SOCIETY. REPRINTED WITH PERMISSION.

FIGURE 19. Diagram of rolling and sliding actionin gear teeth: (a) beginning of contact and(b) end of contact

(a) DRIVEN GEAR

DRIVING GEAR

(b) DRIVEN GEAR

LEGENDR - ROLLING STRESSS SLIDING STRESS


DRIVING GEAR


as the direction of rolling. Negative sliding occurs on themating surface, where the directions of rolling and slidingare opposite each other. Most surface fatigue originates inregions of negative sliding because shear stresses there aremore severe than in regions of positive sliding. Negative slid-ing occurs on the dedenda (the regions below the pitch line)of gear teeth, on the earn follower riding on a cam and indevices that have lower surface velocity in a rolling and slid-ing system.'

The reasons for severe stress in negative sliding are shownby Fig. 18b. Note that the lower, slower roller is beingdragged to the left, making a bulge on the exit side of thecontact. At the same time, the point of contact on the upperroller is moving to the right, making a bulge on the entranceside of contact on the right. The complex stress conditionsfrom this negative sliding are the reasons why the maximumcompressive stress that can be carried is less in a rolling andsliding situation than in a pure rolling situation (the additionof sliding reduces the load carrying capacity). At about 65percent rolling and 35 percent sliding, hardened steels com-monly pit at 20 to 30 million stress cycles when the calcu-lated contact stress is about 2.4 GPa (350 ksi). 5 When rollingalone is involved, the calculated contact stress must be 4.1 to4.5 GPa (600 to 650 ksi) to cause pitting in the same numberof stress cycles.

Gear teeth have complex rolling and sliding motions andare the principal hardened steel components subject to sur-face origin pitting (see Fig. 19 and Table 2). It is significantthat the dedenda (the regions below the pitch line) of all gearteeth are in negative sliding and this is where surface origin

pitting is most likely to occur. Pure rolling occurs only on thepitch line of spur, bevel and helical gears but not at all onwonns, spiral bevel gears, hypoid gears and pinions. In fact,the direction of sliding undergoes a reversal at the pitch line,as shown in Table 2.

The driving gear is usually the smaller of the pair of gearsand receives many more load applications per tooth,depending on the ratio between the gears. The dedendumof the smaller gear is where pitting fatigue originates if thecomponents have the same metallurgical properties. Thesmaller gear often is made slightly harder than the largergear to compensate for the difference in the number of loadapplications.

Because sliding is always present in the operation of gears,the lubricant is extremely important for the survival of heav-ily loaded gears. In addition to an adhesive wear problem,the reduction of surface friction is critical in the effort toresist pitting fatigue. Because of the difficulty in increasingthe life or load carrying capacity of gear systems, the devel-opment of pits usually represents the limit of gear service.Extensive research with rollers'" has indicated that opti-mum conditions for long term resistance to pitting fatigue ongears are provided with a surface hardness near Rockwell C(Be) 60, smooth surface finishes and at least 10 percentretained austenite on the surface of case hardened steel.Apparently, the function of the retained austenite is to per-mit some plastic deformation by increasing the contact areaand reducing the actual compressive stress. Bending fatiguestrength, however, is reduced by excessive retainedaustenite.

Another type of surface origin pitting fatigue occurs onconformal surfaces such as a shaft rotating within a slidingbearing. Fatigue in sliding bearings is usually the result oflong, hard service under severe repetitive, compressiveforces, such as occurs on the upper half of engine connectingrod bearings (which transmit the explosive forces to thecrankshaft) or the lower half of main crankshaft bearings(which resist bending of the crankshaft because of the explo-sive forces). Locations of highest stress are about 35 degreeson each side of top center of the upper halves and about 35degrees on each side of the bottom center of the lowerhalves. From these origins, the fatigue pits usually spreadout to wider locations on the bearings until the bearings arecompletely destroyed. In addition to the explosive forcesimposed on the bearings, there are also centrifugal and iner-tial forces that contribute to the total load on the individualbearings.

Fatigue of soft, nonferrous bearing metals—such as tinbase or lead base babbitts, copper lead alloys, bronzes, alu-minum alloys, certain powder metal alloys and trimetal bear-ings—normally originates at the surface. This occursbecause shearing stresses are highest as a result of slidingactions on the surface. The lubricant system and geometricalcharacteristics of the bearing and of the mating surface are

FIGURE 20. Subcase origin fatigue cracks spreadlaterally, parallel to the surface, and then they joinand cause cracks that break the surface

CASE DEPTH

F A I IGUE CRACKS



TABLE 2. Movements in mating gears (see reference 5)

RollingDirection'

Slidingon Dedendurre

Slidingon Addendum'

Sliding inrelation toPitch Line

awaytoward

Driving gearDriven gear

updown

downup

updown

LEGENDI. UP, TOWARD TIP; DOWN, TOWARD ROOT.2. BELOW PITCH LINE. NEGATIVE SLIDING (ROLLING AND SLIDING IN OPPOSITE DIRECTIONS).3. ABOVE PITCH UNE. POSITIVE SLIDING (ROLLING AND SLIDING IN SAME DIRECTION).


critical. If an oil film can prevent surface contact, neitherfatigue nor other problems associated with sliding contactcan occur. 5

Subcase Origin Fatigue

Subcase origin fatigue damage to case hardened rollingand sliding surfaces such as gear teeth and certain rollermechanisms can destroy contacting surfaces when largepieces are suddenly lost from the surface of the component.Subcase fatigue is relatively easy to prevent once it is identi-fied by visual inspection.

Note that fatigue at the base of the hardened ease is possi-ble if the stress exceeds the strength at that location. Thesame principle holds for contact stress fatigue.

Subcase origin fatigue is known as spalling fatigue or casecrushing.' However, the term subcase fatigue is moredescriptive of the mechanism involved. Case crushingimplies static fracture, which may be accurate in the instanceof severe overloading but is not a fatigue mechanism.

As shown in Fig. 20, subcase fatigue is similar co subsur-face fatigue, with the difference being in the magnitude.Subsurface fatigue produces pits tens of micrometers (thou-sandths of an inch) from the surface. Subcase fatigue origi-nates below the case depth, 1.0 mm (0.04 in.) or more fromthe surface, depending on the heat treatment of thecomponent.

Fatigue damage can also originate deep within the mate-rial as a result of contact stress fatigue, causing cracks paralleland perpendicular to the surface. These cracks tend to belong (see Fig. 20). Surface cracking is the first indication ofsubsurface fatigue, although hidden cracks could bedetected by ultrasonic testing. Continuing service rapidlyleads to severe damage in which long, undermined pieces arelost.

Correction of subcase fatigue is relatively simple: increas-ing the case depth or increasing the core hardness andstrength with steel of higher carbon or alloy content. Metal-lurgical changes must be made carefully. For example, ifcase depth or core hardness is increased too much, gearteeth could be through-hardened, leading to brittle fracture.Table 3 summarizes the characteristics of contact stressfatigue on mating metal surfaces.

Cavitation FatigueCavitation fatigue is a fonn of pitting caused by vibration

and movement in liquid environments. Because many liq-uids are corrosive to most metals, the problem of environ-mental reaction is closely linked to the problem of cavitationfatigue.

Cavitation can be a serious problem in marine propellersof all sizes, diesel engine cylinder liners, pump impellers,hydraulic equipment, turbines, torque converters and othercomponents that contact or vibrate in The pitscan be as small as a pinhead or much larger. They can com-pletely penetrate metals hundreds of millimeters (severalinches) of metal thickness.

Cavitation pitting occurs in low pressure regions at rapidlyvibrating liquid/metal interfaces (see Fig. 21). The motionsthat cause cavitation actually occur in microseconds,depending on the vibration frequency of the componentsand the vapor pressure of the liquid.

In Fig. 21a, the metal wall is moving to the right, againstthe inertia of the liquid. In Fig. 21b the metal has reachedthe end of its travel to the right but the liquid is still moving

ib

FIGURE 21. Mechanism of cavitation pittingfatigue in a metal wall vibrating against a liquid tothe right of the wall: (a) the metal moves to theright against the stationary liquid; (b) the metalreaches the end of its travel but the inertia of theliquid causes it to continue to move; (t) the metalstarts moving toward the left, away from theliquid, which cannot catch up (cavities or voids areformed; (d) the metal reaches the end of Its travelto the left and the liquid tries to catch up to themetal and cavities collapse; then the cycle startsagain

(aJ

(b)



TABLE 3. Characteristics of contact stress fatigue (see reference 5)

SurfacePitting

SubsurfacePitting

SubcaseFatigue

Location of origin often at micropits usually at anonmetallic inclusion

near case core boundaryin case hardened parts

Initial size small small largeInitial area/depth ratio small small largeInitial shape arrowhead irregular gouged and ridgedCrack angle to surface acute sides perpendicular sides perpendicularApparent occurrence gradual sudden sudden


to the right. As the metal moves back to the left in Fig. 21c,the liquid is still moving to the right because of its inertia.Small cavities or negative pressure bubbles form in the liquidat the interface with the metal as the two materials momen-tarily move away from each other. The metal pulls the cavit-ies with it. In Fig. 21d, the metal has reached the end of itstravel to the left and the liquid is still moving to the left.When the metal again moves to the right, it collides with theliquid moving to the left. The cavities collapse because of theinertia of the metal and the liquid as they move toward eachother.

The collapsing cavities implode on the metal with com-pressive stresses estimated at tens of thousands of kilopascals(thousands of pounds per square inch). Because the geome-try of the vibrating system and the properties of the liquidare relatively constant, the cavities form in clusters at certainpreferred locations. With constant repetition of the pound-ing, the fatigue mechanism progresses until pits form in themetal surface at these locations.

Corrosion may enter the picture if surface films areformed on the virgin metal in the pit when the system is atrest. Then, when the motion resumes, surface film is rapidlydestroyed by the high compressive forces. Because glass andceramics can have cavitation pitting in inert liquids, corro-sion is not considered necessary for cavitation pitting fatigue.

CorrosionCorrosion can be defined as the deterioration of a metal

resulting from electrochemical reactions with its environ-ment`' The National Institute of Standards and Technologyhas estimated the cost of corrosion and corrosion preventionin the United States at about 4.2 percent of the gross nationalproduct." Similar percentages have been estimated by othercountries. Applying this figure to recent gross national prod-ucts shows that corrosion and corrosion prevention cost hun-dreds of billions of dollars annually in the United Statesalone. Although corrosion typically is not catastrophic, it canbe dangerous when it leads to fracture.

It is commonly known that slight changes in metals, theirdesign or their environment can make significant differencesin their corrosive behavior. For this reason, it is important toobtain direct information about the circumstances of a corro-sion problem, particularly corrosive effects complicating afracture or wear condition.

There are many types of corrosion and usually at least twotypes are progressing simultaneously.

FIGURE 22. Life cycle of a typical metal

NO PRODUCT

REFINED METAL

CORROSIOF>'RECYCLING

\ CORROSION PRODUCTORE

TIME


FIGURE 23. Galvanic cell showing the basicprinciples of the electrochemical nature ofcorrosion

AMMETER



Life Cycle of a Metal

Corrosion is a natural process that reverses the chemicalactions of refining. In their natural stable state, metals arefound primarily either as oxides or as sulfides in ore. Duringrefining, the addition of large amounts of energy strips theoxides or sulfides and produces relatively pure metals in amore unstable state. These refined metals are used singly orby alloying with other metals.

To illustrate for a common metal, iron is found as ironoxide in the ore. it is refined to iron or steel for most usesbut eventually reverts to iron oxide (rust). Figure 22 showsthe life cycle of a typical metal. The ore at the lower left is inthe natural state at a low energy level. The thermal or elec-trical energy added during the refining process moves themetal to a chemically unstable condition at a higher energylevel. The refined metal may be remelted and east, hotformed, cold formed or machined into useful shapes in themanufacture of an end product. However, the refined metaltends to deteriorate and revert to the original, chemically sta-ble condition, releasing energy as heat in the process. (Theenergy released by corrosion is sometimes converted intoelectrical energy, as it is in a dry, cell battery.)

Galvanic Corrosion

Galvanic corrosion is caused by the physical differencesbetween contacting metals or a metal and its environment.Figure 23 shows the reactions found in a simple battery asan illustration of the principles of galvanic corrosion. Threecomponents are needed for this electrochemical reaction:two materials (different metals or a metal and graphite) inphysical or electrical contact. Electrical connection isachieved with an electrolyte, an electrically conductive liquidor paste. Under these conditions, one of the materials is cor-roded, hydrogen is released and energy is released in theMill of an el--tric current. The materi-al that corrodes iscalled the anode. The other material is known as the cathodeand does not corrode.

Metals and graphite (an electrically conductive nonmetal)are often listed in a sequence that has the most anodic (easilycorroded) at one end and the most cathodic (least easily cor-roded) at the other end. For practical purposes, thissequence is compiled using seawater as the electrolyte-3 to5 percent sodium chloride and other salts dissolved in water.The list is called the galvanic series and is shown in Table 4for some pure metals and certain alloys.

The position of a metal in the galvanic series is veryimportant for determining corrosion properties. The noblemetals—so called because they include gold, platinum andsilver—strongly resist corrosion.

When two metals make electrical contact in an electrolyte,the farther apart they are in the galvanic series, the more rap-idly corrosion will occur. For example, aluminum in contactwith gold in seawater rapidly corrodes, while aluminum incontact with iron (steel) corrodes less rapidly. An ordinarydry cell battery consists of a zinc anode separated by an elec-trolytic paste from a manganese dioxide cathode. When thecircuit is closed the generated electric current corrodes thezinc anode. When the zinc is depleted, the electric currentceases and the battery is dead.

The standard galvanic series may be limited by several fac-tors. A given metal may be either high or low in the seriesdepending on the nature of the surface film. For example,many stainless steels may be active (anodic) or passive(cathodic), depending on whether the chromium in the sur-face metal is alloyed with the iron or is in the form of a com-plex metal oxide. The latter compound is the actualcorrosion resisting material and is formed in air andenhanced by a nitric acid passivating treatment, which alsoremoves foreign metal particles that could impair corrosionresistance. (Nitric acid is a strong oxidizing agent.)

The sequence of two metals may be reversed by theaffected area. For example, if a large anode metal is con-nected to a small cathode metal in an electrolyte, there maybe little or no observable corrosion of the anode because the


TABLE 4. Galvanic series in seawater (see reference 1)

Anodic end (most easily corroded)

MagnesiumMagnesium alloysZincGalvanized steel or galvanized wrought ironAluminum alloys 5052, 3004, 3003, 1100, 6053 (in this order)CadmiumAluminum alloys 2117, 2017, 2024 (in this order)Low carbon steelWrought ironCast ironHigh nickel cast ironType 410 stainless steel (active)50-50 lead tin solderType 304 stainless steel (active)Type 316 stainless steel (active)LeadTinCopper alloy 280Copper alloy 675 (manganese bronze A)Copper alloys 464, 465, 466, 467 (naval brass)Nicker 200 (active)Inconel® alloy 600 (active)Hastelloy® BChlorimet® 2Copper alloy 270 (yellow brass, 65 percent)Copper alloys 443, 444, 445 (admiralty brass)Copper alloys 608, 614 (aluminum bronze)Copper alloy 230 {red brass, 85 percent)Copper 110 (ETP copper)Copper alloys 651, 655 (silicon bronze)Copper alloy 715 (copper nickel, 30 percent)Copper alloy 923, cast (leaded tin bronze G)Copper alloy 922, cast (leaded tin bronze M)Nickel 200 (passive)Inconel ® alloy 600 {passive)Mond® alloy 400Type 410 stainless steel (passive)Type 304 stainless steel (passive)Type 316 stainless steel (passive)Inc°loy® alloy 825Inconel® alloy 625Hastelloy® CChlorimet® 3SilverTitaniumGraphiteGoldPlatinum

Cathodic end (least easily corroded)


electrical effect is dissipated over a large area. However, if asmall anode metal contacts a large cathode metal in an elec-trolyte, the reverse is true: the anode metal may corrode rap-idly, essentially becoming an electrical stress concentration.

The galvanic series should be used as a starting place in aparticular corrosion study but there are practical variationsin the metals, their relative sizes and in the electrolyte.

Uniform Corrosion

Many metals corrode uniformly, without obvious galvaniccouples. The most common uniform corrosion is rust on ironor steel. Uniform corrosion is a result of microscopic gal-vanic cells in the surface of the metal. Because of localchemical differences, impurities and alloying intermetallicsin the metal, there are microscopic anodes and cathodesready to corrode if an electrolyte is introduced—the corro-sion is uniform only on a macroscopic scale.

Because uniform corrosion is the most common corrosion,it is the most significant economically and damages the great-est tonnage of metal. From a technical standpoint, uniformcorrosion is fairly predictable and is relatively easy to control,provided other types of corrosion are not present.

One way to combat uniform corrosion is to use a morenoble metal or stainless steel. Surface corrosion is alsoslowed by protective coatings, such as paint. The choice ofmeans to combat corrosion is influenced by considerationssuch as the physical properties and cost of availablematerials.

Crevice Corrosion

Crevice corrosion is a kind of galvanic corrosion that is dif-ficult to combat without careful control of design, materials,engineering and quality Crevice corrosion is the commonlyused term for differential oxygen concentration cell corro-sion, or what is also called poultice corrosion.

Because most corrosion is caused by oxidation of reactivemetals, areas of high oxygen concentration might beexpected to corrode more readily than areas of lower oxygenconcentration. However, a crevice between two surfaces, ormetal under a poultice of moist debris, is more likely to cor-rode the more exposed metal. This occurs because there islittle oxygen within the crevice or under the poultice. Themetal there is anodic and corrodes. Areas exposed to higheroxygen content are cathodic and do not corrode. Concealedmetal at the edge of a joint or under debris tends to pit andeventually perforate the metal thickness.

Crevice corrosion is the primary cause of automobile bodycorrosion, which originates in crevices, joints or under debrisin the presence of moisture—frequently laden with electro-lytic salts from road deicing.

Crevice corrosion also occurs under fasteners, such asbolted or riveted joints, if moisture can penetrate and

Aluminum alloys

Copper alloys

Gold alloys

LeadMagnesium alloys

Monet ®

NickelCarbon and alloy steels

Stainless steels

Titanium

NaCI-H 101 solutionsNaCI solutionsseawaterair, water vaporammonia vapors and

solutionsamineswater, water vaporFeCI 3 solutionsacetic acid-salt solutionscaustic soda solutionslead acetate solutionsNaCI-K,Cr04 solutionsrural and coastal

atmospheresdistilled waterfused caustic sodahydrofluoric acidhydrofluosilic acidfused caustic sodaNaOH solutionsNaOH-Na,SiO, solutionscalcium, ammonium and

sodium nitride solutionsmixed acids (H2S0a-FiNO 3 )HCN solutionsacidic H,S solutionsmoist H,S gasseawatermolten Na-Pb alloysacid chloride solutions such as

MgCI, and BaCl 2NaCI-H,02 solutionsseawaterH,SNaOH-H,S solutionscondensing steam from

chloride watersred fuming nitric acid



remain. The condition can occur even if both metals are thesame but it is aggravated by dissimilar metals in contact, par-ticularly with a small anode and large cathode. The Statue ofLiberty in New York Harbor is subject to crevice corrosionbecause the copper exterior (cathode) is held in place with asteel framework and bolts (anode) which have corroded atthe joints after many years of exposure to sea air.

Personnel should visually inspect exposed surfaces andremove deposits frequently—if materials such as dirt, rust orsand are not present, they cannot contribute to crevice cor-rosion. Filters, traps or settling tanks can help remove parti-cles from a system but also require periodic maintenance toremove accumulations.

cracking from chlorides (and other halides) in the presenceof tensile stress (see Table 5).

Because stress corrosion cracking is the result of static ten-sile stress and a particular environment, it is important toexamine some of the ways that tensile stresses are generated.It is usually felt that residual tensile stresses (includingassembly stresses) are more often the cause of stress corro-sion cracking than are tensile stresses from applied force."'Residual stress is frequently the result of welding.

TABLE 5. Environments that can cause stress corrosioncracking under certain conditions (see reference 19)

Material Environment

Stress Corrosion CrackingStress corrosion cracking is defined as cracking under the

combined action of corrosion and tensile stresses. Thestresses may result from applied (external) forces or may beresidual (internal). Stress corrosion cracks may be transgran-ular or intergranular, depending on the metal and the corro-ding agent. As in all brittle fractures, the cracks areperpendicular to the tensile stress. Usually there is little orno visual evidence of corrosion.

The classic example of stress corrosion cracking is the so-called season cracking—a term that arose during the Indiancampaigns of the British army in the 1800s. During the mon-soon seasons, spontaneous cracking of the thin walled necksof cartridge cases became a recurrent problem. High tem-perature and high humidity, plus traces of ammonia in theair, caused stress corrosion cracking to occur in the car-tridges' severely deformed thin sections. These same loca-tions were subjected to high tensile hoop stress duringloading. It is now known that zinc and copper alloys are sus-ceptible to stress corrosion cracking when their surfacesundergo tensile stress in the presence of chemicals such asammonia and mercurous nitrate.

Stress corrosion cracking is a progressive fracture similarto fatigue. The cracks grow gradually until a critical size isreached. Stress concentration then may cause sudden brittlefracture of the remaining metal. In other instances, thecrack grows away from the high stressed origin, then stopswhen it is no longer highly stressed in tension.

Tensile Stress

Most metals are susceptible to stress corrosion cracking inthe presence of tensile stresses in specific environments. Forexample, ordinary carbon and alloy steels are subject to caus-tic embrittlement when exposed to sodium hydroxide at rela-tively low tensile stresses. Austenitic stainless steels, such asthose in the 200 to 300 series, are subject to stress corrosion


Tensile stresses are also generated by shrink fits, bendingor torsion during assembly and crimping. The only require-ment for stress corrosion cracking is tensile stress on the sur-face of a metal in a critical environment. The stress need notexceed the yield strength of the metal but the higher thestress, the less critical the environment and vice versa.

Effects of Welding

In certain metals, particularly austenitic stainless steels,the heat of welding causes depletion of chromium by form-ing complex chromium carbides in the grain boundaries.Because chromium is the element that makes stainless steelscorrosion resistant, stress corrosion cracking can occuralongside the carbides in the grain boundaries, where thereis little or no chromium.

There are two ways to solve this problem: (1) use a stain-less steel with carbon content below 0.03 percent to reducethe carbide sites preferential to corrosion or (2) use a stain-less steel containing an element that forms carbides morereadily than chromium. Such elements include titanium (asused in type 321 stainless steel) or niobium (columbium)plus tantalum (as used in type 347 stainless steel).

Identifying Stress Corrosion Cracking

Identification of stress corrosion cracking is not alwayseasy and may he confused with other types of fracture. If thetest object has no cyclical stress, then fatigue can be elimi-nated." The texture of the fracture surface is sometimeshelpful in differentiating a stress corrosion fracture from afatigue fracture. The surface texture in a fatigue fractureusually is smooth near the origin and gradually becomesrougher toward the final rupture. This change in roughnessis not usually seen in a stress corrosion fracture.

Stress corrosion cracking is also frequently confused withhydrogen embrittlement cracking. In fact, it is sometimesimpossible to distinguish with certainty between hydrogeninduced and stress corrosion cracking, particularly if the dis-continuities occurred in service after exposure to hydrogengas, hydrogen sulfide and water or dilute aqueous solutions.Several things may be considered during visual testing forstress corrosion cracking, including (1) the history of the testobject, (2) crack origin, (3) crack pattern, (4) evidence of lit-tle corrosion on the fracture surfaces and (5) microstnicturalfeatures.'

Corrosion Fatigue FractureStress corrosion cracking is a complex phenomenon

caused by static stress with many contributing factors,including alloy, heat treatment, microstructure, stress

Elevated Temperature DiscontinuitiesElevated temperature discontinuities are the most com-

plex kind of material anomaly because most other disconti-nuities can occur at elevated temperatures (low temperaturebrittle fracture is an obvious exception). Elevated tempera-tures greatly complicate the analysis of the problem and thepossible solutions.

Normally, the useful static strength of a metal is limited byits yield strength. However, as temperature increases, theuseful static strength of a metal is limited by the factor ofcreep, a time dependent strain occurring under stress.'Each metal or alloy must be considered individually because

system, test object geometry, time, environmental conditionsand temperature.

Corrosion fatigue may be seen as the effect of fluctuatingstresses in a corrosive environment, characterized by shorterlife than would result from either the fluctuating stressesalone or the corrosive environment alone.'

In general, all corrosion that occurs during cyclic stressingis thought to reduce fatigue life. Even air can affect thebehavior of fatigue fracture in certain alloys compared to thealloy's behavior in vacuum. For example, there is a tendencyof 2024-T3 aluminum to form normal fatigue striations whentested in air but the material has a relatively flat and feature-less fracture region when tested in vacuum." This impliesthat air was necessary for the formation of fatigue striations,at least under the conditions of the test.

In real life, the type of cyclic stressing strongly influencesthe metal under aggressive corrosion conditions." Thelonger and more frequently a fatigue crack is opened to thecorrosive environment, the more severe is the effect of theenvironment on fatigue life. In many cases, fatigue crackingis initiated from small pits on a corroded surface. In othercases, it appears that the fatigue crack initiates and then ismade to grow more rapidly by moisture or another corrodentthat enters the crack by capillary action. A combination ofthese two mechanisms is probably the most common.

Identification of corrosion fatigue fracture is complicatedby the environmental effects. The origin of a fatigue fractureis likely to be the most severely corroded because of its longexposure to the environment. Note that the corrosive filmcan obscure the fracture origin and make detailed studymore difficult. A complicating factor, on this or any othertype of fracture surface, is that corrosion occurs over all thefracture surface if the broken component is not removedfrom the corrosive environment, immediately cleaned andprotected from further corrosive or mechanical damage.Comparison of the two mating fracture surfaces, if both areavailable, may give some insight into when the corrosionoccurred.

FRACTURE

FIRST STAGE

SECOND STAGEINITIAL LOAD STRAIN

FR

zTRUEELONGATION

THIRD STAGE

FIGURE 24. Schematic tension creep curve,showing the three stages of creep (seereference 5)

TIMEOM ASM ASM INTERNATIONAL. REPRINTED WITH PERMISSION.

171

FIGURE 25. Creep curves for a molybdenumvanadium low alloy steel at 600 °C (1,110 °F) undertension at four stress levels (see reference 51

12

1 0

7 — 0.8(1"01,.L.z 0 6

1§i Q 0.1-u-

0.2


500 1,000TIME

(hours]

1,500 2,000

125 MPa 118 ksil


I

of differences in their properties. Approximate thresholds ofelevated temperature behavior for several metals and alloysystems are shown in Table 6.

In service at elevated temperature, the life of a metal com-ponent is predictably limited, whether subject to static or todynamic loads. In contrast, at lower temperatures and in theabsence of a corrosive environment, the life of a componentin static service is unlimited, if the operational loads do notexceed the yield strength of the metal.

The principal types of elevated temperature discontinuityare creep, low cycle fatigue, high cycle fatigue, thermalfatigue, overload failure and combinations of these, as modi-fied by the service environment. Generally, the type of dis-continuity is established by (I) visual testing of fracturesurfaces and (2} comparison of operating conditions withdata on creep, stress rupture, tension, elevated temperaturefatigue and thermal fatigue properties. More thorough anal-ysis may he required when stress, time, temperature andenvironment act to change the metallurgical microstnictureof the component.

CreepBy definition, creep means the gradual change of shape in

a metal under stress. It is a result of tensile stress but creepcan and does occur under all types of stress. Gradual changeof shape under compressive, torsion bending and internalpressure stresses may or may not lead to fracture. In the fol-lowing discussion, creep is assumed to be caused by tensilestress.

Creep occurs in three stages, as shown in Fig. 24, whichplots strain or elongation caused by the stress against time atfixed values of temperature and stress. Forming initial elas-tic strain resulting from the immediate effects of appliedload, a metal undergoes increasing plastic strain at increasing

TABLE 6. Approximate values for the lower limit ofelevated temperature behavior (see reference 5)

TemperatureMetal I nFl (°C]

Aluminum alloys 400 205Titanium alloys 600 315Carbon steels 700 370Low alloy steels 700 370Aristenitic, iron base high

temperature alloys1,000 540

Nickel base high temperature alloys 1,200 650Cobalt base high temperature alloys 1,200 650Refractory materials 1,800 to 2,800 980 to 1,540FROM THE AMERICAN WELDING SOCIETY. REPRINTED WITH PERMISSION.

strain rate. This primary or first stage of creep occurs withinthe metal during the first few moments after the Load isapplied. The creep rate usually slows as crystallographicimperfections within the metal undergo realignment, lead-ing to secondary creep.

Stage two or secondary creep is essentially an equilibriumcondition between the mechanisms of work hardening andrecovery when metal is still stretching under tension but notstretching as rapidly as in the primary stage. The duration ofsecondary creep depends on the temperature and stress levelof the metal (see Fig. 25). In the figure, a steel componentwas tested at a specific temperature under four stress levelsthat caused different behaviors. The lowest stress levelcaused little change in shape, while successively higher stressrapidly led to fracture.

FIGURE 26. Creep curves showing no primarycreep and no tertiary creep (see reference 5)

ND PRIMARY CREEP

NO TERTIARY CREEP

TIM E

FROM ASPA INTERNATIONAL. REPRINTED WITH PERMISSION.

2 2 / VISUAL AND OPTICAL TESTING

Stage three or tertiary creep is the gradual increase instress to fracture. It may result from metallurgical changeswithin the material. These changes permit rapid increases indeformation, accompanied by hardening that is insufficientto retard the increased flow rate. In tension, tertiary creepmay be accelerated by a reduction in sectional area resultingfrom cracking or localized necking. Effects such as oxidationor corrosion may also increase tertiary creep rate.

Under certain conditions, some metals may not exhibit allstages of plastic deformation. For example, at high stresstemperatures, the absence of primary creep is not uncom-mon, with secondary creep or, in extreme cases, tertiarycreep following immediately on loading. At the otherextreme, notably in cast alloy, tertiary creep may be observedand fracture may occur with minimum extension (seeFig. 26).

Depending on the alloy, creep fracture may be macroscop-ically brittle or ductile. Brittle fracture is intergranular andoccurs with little or no elongation. Ductile fracture is trans-granular and typically is accompanied by discernible elonga-tion and necldng. The extent of fracture depends not only ontemperature but also on stress. At constant temperature, theoccurrence of either transgranular or intergranular fracturedepends on strain rate. Conversely, at constant strain rate,the type of fracture depends on temperature. In general,lower creep rates, longer rupture times or higher tempera-tures promote intergranular fracture.

Stress rupture is identical to creep fracture, except that ina test condition, only stress, temperature, time to fractureand total elongation are recorded—insufficient data for plot-ting the complete curve.

Elevated Temperature Fatigue

Fatigue strength decreases with increasing temperature.The precise relationship between temperature and fatigue

strength varies widely, depending on the alloy and the tem-perature to which it is subjected.

In some cases, a component may operate at elevated tem-perature with alternate steady state and fatigue (cyclic)stress. In this case, the combined creep fatigue loads resultin substantially decreased life at elevated temperatures com-pared with that anticipated in simple creep loading. If oxida-tion on the fracture appears to be maximum near the surfaceof the component, the fracture occurs primarily because offatigue. However, if oxidation appears to be relatively uni-form on the fracture surface, steady state or static loads mayhave been more significant.

Thermal Fatigue

Fatigue may be caused either by cyclic mechanical stress-ing or by cyclic thermal stressing. Thermal fatigue cracks arethe result of repeated heating and cooling cycles, producingalternate expansion and contraction. When a metal cools, itcontracts, causing residual tensile stresses if restrained fromfree motion. If this alternate expansion and contraction con-tinues, fatigue cracks form and propagate each time themetal is cooled.

Thermal cycles may be caused by friction, as in brakedrums and clutch plates. Here the surface is frequentlyheated and expanded by friction but is prevented fromexpanding freely by the colder, stronger metal below the sur-face. Compressive yielding occurs in the hot surface layer,causing tensile residual stresses when the metal contractsduring cooling. This condition frequently causes thermalfatigue cracks called heat checking. This network of crackson the friction surface may be harmless unless the crackswear the mating surface or unless the cracks progress tocomplete fracture.

Engine exhaust manifolds also are subject to thermalfatigue, particularly on heavy duty engines. They maybecome very hot under certain conditions, then cool whenthe engine is stopped. If the manifold is not permitted tofloat or move freely in an axial direction, tensile residualstress may be generated when it cools, eventually causingfatigue fracture.

Thermal fatigue may be prevented in many componentsby designing curves rather than straight lines into the system.When this is done, heating and cooling cycles simply distortthe curves, rather than forming tensile residual stresses oncooling. Expansion loops, bellows in elevated temperaturepiping and tubing systems operate on this principle.

Metallurgical instabilities

Stress, time, temperature and environment may act tochange metallurgical structures during service, resulting inreduced strength. These microstructural changes arereferred to as metallurgical instabilities. Sources of


instabilities include transgranular or intergranular fracturetransition, recrystallization, aging or over aging, intermetallicphase precipitation, delayed transformation to equilibriumphases, order to disorder transitions, general oxidation,intergranular corrosion, stress corrosion cracking, slagenhanced corrosion and contamination by trace elements.'

Environmentally Induced Discontinuities

The most important source of elevated temperature dis-continuities is environmental degradation. Control of envi-ronment or protection of materials is essential to mostelevated temperature applications.

General oxidation can lead to premature failure. Grainboundary oxidation may produce a notch effect that also canlimit service life. Some environments may be more harmfulthan others. For example, attack of fireside surfaces of steamboiler tubes by ash from vanadium bearing fuel oils can bequite severe. Vanadium ash attack and hot corrosion in gen-eral are equally harmful in gas turbines.

In all elevated temperature discontinuities, the character-istics of the environment must be carefully considered.These include not only the temperature itself but alsowhether the elevated temperature is steady or fluctuating,the rate of temperature change (which affects differentialexpansion and contraction), the thermal conductivity of themetals involved, the characteristics of the fluids (both liquidand gases) in contact with the surfaces and the way in whichthe fluids contact the metal surfaces. Fluid contact is mostimportant in components that have high gas or liquid flowrates at elevated temperatures, causing erosion problems.

Corrosion and Corrosion Erosion

Certain components function in environments where highrates of fluid flow at high temperature are normal. Typical ofthose in gaseous environments include engine exhaustvalves, blades and vanes in the hot sections of gas and steamturbine engines or generators, certain locations (particularlyinlets and outlets) in various furnaces and ducts or pipes thatconduct hot gases. Typical components in high temperatureliquid environments are piping systems, pumps, rotors, pro-pellers and nozzles.

The problem with these components is that the combina-tion of high temperature and high velocity fluid flow oftenresults in erosive wear at critical locations. Such wear fre-quently destroys the efficiency of the components and theirassemblies. Erosive wear is caused by high speed, low stressparticles that tend to cut or erode materials in their path. Ingeneral, elevated temperatures reduce metal strength andhardness. Any component that changes the direction of hightemperature, high velocity fluids is subject both to increasederosion from mechanical action and to increased corrosionfrom the chemical action of the fluid.

General Oxidation

In certain applications, the primary elevated temperatureproblem is general oxidation or scaling (formation of metaloxide layers). This is particularly true when the metal is sub-jected to repetitive heating and cooling cycles in an oxidizingatmosphere. The oxide scale flakes off when the metal coolsbecause of differences in the thermal expansion characteris-tici of the scale and the base metal.

As a group, ferritic stainless steels are usually superior inoxidation resistance when compared to iron base alloys. Infact, the main advantage of ferritic stainless steels for hightemperature use is their good oxidation resistance, compara-ble to austenitic grades. In view of their lower alloy contentand lower cost, ferritic steels should be used in preferenceto austenitic steels when stress conditions permit. Oxidationresistance of stainless steel is affected by many factors,including temperature, time, service (cyclic or continuous)and atmosphere. For this reason, selection of a material fora specific application should he based on tests that duplicateanticipated conditions as closely as possible.'

Because of the need for good oxidation resistance in auto-motive exhaust systems and catalytic converters, the stan-dard ferritic stainless steels, particularly type 409, are widelyused. Under favorable conditions, these steels form a tightlyadhering oxide scale which expands and contracts with thebase metal and are suitable when there is no need for highstrength and elevated temperatures.

Carburization

The problem of steel carburization is common to manyindustrial applications—especially stainless steels in furnaceenvironments. Simultaneous carburization and oxidation ofstainless heating elements results in a form of attack some-times referred to as green rot. This discontinuity is commonto nickel chromium and nickel chromium iron alloys.

Carburization occurs when carbon rapidly combines withchromium to form chromium carbides. Carburizationdepletes chromium from the grain boundaries, sometimesresulting in intergranular fracture. In addition, the carbonchanges the density and the expansion characteristics of themetal and tends to cause residual stress.

Carburization of austenitic stainless steels and elevatedtemperature alloys may be easily detected by changes intheir magnetic properties. Austenitic stainless steels arenonmagnetic. When chromium is depleted by reaction withcarbon after diffusion at elevated temperature, the alloybecomes ferritic and hence magnetic.

Liquid Metal Contact

Liquid metal contact is another problem encountered inhigh and low temperature service environments. High


temperature alloys often cannot tolerate contact with liquidmetals because high temperatures cause the precipitation ofchromium carbides in the grain boundaries. This condition,called sensitization, is the depletion of the surrounding areasof chromium, permitting grain boundary corrosion, crackingand fracture.

Liquid mercury can cause severe stress corrosion crackingby contact with high strength steels and with cobalt, alumi-num, nickel, titanium and their alloys.

Many high temperature alloys frequently cannot be usedwith liquid or molten metals. Molten lead, for example, ishighly corrosive to most high temperature alloys. 5 Moltenzinc, used in hot dip galvanizing of fabricated components, iscommonly contained in tanks or vats made from plain carbonplate steel. Aside from strength, the principal requirementof galvanizing tank material is the ability to resist the corro-sive attack of molten zinc. (Some alloying elements dissolvein the liquid metal, changing the base metal alloy.)

A common problem with molds used to die cast zinc, alu-minum, magnesium and copper is heat checking or thermalfatigue cracking of surfaces in contact with the cast, liquidmetal. This condition produces ridges in the casting whenmolten metal flows into the mold cracks. It is necessary tokeep the die at high temperature so that there is little differ-ential expansion and contraction that can cause tensile resid-ual stresses and cracking on its surfaces.

Cooling MethodsIn gaseous flow mechanisms, it is possible to use air or

other gases to cool components. This is commonly done inthe hot sections of gas turbines where tremendous air flow isavailable. Some of the incoming air is routed through holesin the blades and vanes.

Internal combustion engines may he cooled by liquid orair. However, no cooling system can fiinction effectively ifits heat transfer properties are impaired. An effective cool-ing system is critical to engine operation.

Certain mechanisms that operate in very high tempera-tures can survive only with the aid of their cooling systems.Two spectacular examples are the oxygen lances used insteelmaking basic oxygen furnaces, (BOFs) and water cooledcupolas used in making various types of cast iron. The oxy-gen lances are essentially double walled tubes inserteddirectly above the liquid metal in the furnace. The oxygenblast into the molten metal causes extremely high tempera-tures at the lance. Only the recirculating cooling water inthe tube prevents the lance itself from melting. Similarly,plate steel is sometimes used to make cupolas for meltingcast iron. Again, the only thing that keeps the molten ironfrom damaging the steel shell is the external water spraycooling system. It is critical that the cooling system operate

TABLE 7. Crystal structures of common metals

Crystal Structure Metals

Body centered cubic Fe (room temperature and again near itsmelting point), Cr, Cb, W, V, Ti (at hightemperatures), Mo

Face centered cubic Fe (intermediate temperature), Cu, Au,Pb, Ni, Ag, Al

Hexagonal close packed Co, Mg, Sn, Ti (room temperature), Zn, ZrFROM THE AMERICAN WELDING SOCIETY. REPRINTED WITH PERMISSION.

properly to extract heat from the steel shell, which is other-wise unprotected from the molten metal within it.

Tests of Welded JointsAll types of welded structures are expected to possess cer-

tain service related capabilities. To ensure that theirintended function is fulfilled, welded structures are typicallytested with several nondestructive and some destructivetechniques. Ideally, components are observed in service. Inmany cases, in situ visual tests are difficult because of cost ortime factors.

Predicting the performance of structures from tests ofmanufactured reference standards is a complex procedurebecause test size, configuration, environment and loadingare rarely identical to those of service conditions.

MicroscopyStructure of Metals

Solid metals have a crystalline structure in which theatoms of each crystal are arranged in a specific geometricpattern. This arrangement of atoms is responsible for manyof the properties of the metal. The most common crystallinestructures found in metals are listed in Table 7 and theiratomic arrangements are shown in Fig. 27. In the liquidstate, the atoms in metals have no orderly arrangement.When the liquid approaches solidification temperature,nuclei begin to form at preferred sites (see Fig. 28a). Assolidification proceeds, the nuclei grow into larger solid par-ticles called grains (Fig. 28b). As grains grow and intersect,a so-called grain boundary (Fig. 28c) forms and is naturallyirregular. Figure 28 shows the grain boundaries of an ingotcast in a cylindrical mold.

The situation is much the same in a weld and, because ofthe similarity in grain shape, a fusion weld can be viewed insome respects as a small metal casting. Each grain in a puremetal, examined at a particular temperature, has the same

FIGURE 27. The three most common crystalstructures in metals and alloys: (al face centeredcubic, (bJ body centered cubic and (cJ hexagonalclose packed

(a1

( b J

lc 1

FROM THE AMERICAN WELDING SOCIETY. REPRINTED WITHPERMISSION.

FIGURE 28. Solidification of a metal: (a1 initialcrystal formation, (b) continued solidification and(cJ complete solidification

INITIAL CRYSTALS(al

SOLID GRAINS

( b J

LIQUID

SOLID GRANS WITH GRAINBOUNDARIES

1cl



crystalline structure and the same atomic spacing as all othergrains. However, because each grain grows independently ofthe others, orientation of the grain lattice differs from onegrain to another. Grain boundaries are regions where theperiodic and orderly arrangement of atoms is disrupted andfor this reason, there often are differences in the behavior ofthe metal at these locations.

Most common engineering metals are alloys, combina-tions of metals or of metals and nonmetals. Alloys provideengineering properties that, for specific applications, aresuperior to those of unalloyed metals. The atomic arrange-ment of an alloy, the purity of its alloying elements and thethermal and mechanical history all have an influence on theengineering properties of an alloy.

Alloying elements can be absorbed into the parent metalin different ways: (1) the atoms of the alloying elementreplace some of the parent metal atoms (direct substitutionalloys) as shown in Fig. 29a; (2) the atoms of the alloying ele-ment fit into the spaces between the atoms of the parentmetal (interstitial alloys) as shown in Fig. 29b; or (3) insteadof b.lng sulThstitntinn.11y ,,r int.rstiti.11y,atoms can form mixed atomic groupings with parent atoms.This new crystalline structure is called a phase and the alloyis called a multiphase alloy. The individual phases may bedistinguished by polishing and etching, followed by micro-scopic visual inspection at magnifications of 50 x to 2,000 xThese so-called metallographic tests are one way of studyingthe characteristics of metals and alloys.

Phase Diagrams

Events such as phase changes and solidification may beshown by a drawing called a phase diagram (sometimescalled an equilibrium diagram or a constitution diagram).

Phase diagrams of a given alloy system can show whatphases and what percentage of each phase are present for agiven alloy composition at a specified temperature. Thediagram can also help determine what phase changes tendto take place with a change in composition, temperatureor both. However, the phase diagram has a significant

FIGURE 29. Effects of the crystalline structureof alloys on the base metal Crystal lattice:lap substitutional solid solution where the basemetal lattice b distorted by presence of foreignatoms and 1bl solid solution hardening whereforeign atoms locate in Interstices of base metalcrystalline structure and cause distortion in lattice

1 8 1

FROM THE AMERICAN WELDINO sOCIETCREPINNTEDwITHPERFNSsion

lb) 0 0 00 0 0.

FIGURE 30. The nickel copper phase diagram:points along the top curve represent the liquidstate for each alloy composition; points along thebottom curve represent the solid state for eachalloy composition

3,400

3.000

2.600

2,200

1300

1.400

1,000

600

200!DO

L''7

LL

•

F-

NICKELpercenti

FROM THE AMERICAN WELDING sOCIETY, REPRINTED WITHPERiatiSION.

216 1 VISUAL AND OPTICAL TESTING

li mit at k,: it.eatise it describes the behavior of the allo y sys-tem under equilibrium. it is an apprarimation of how Alloys

behave. Equilibrium implies that a metal is stable indesired state fin- a Oro cnsinonttk nt. Obtaining this stabil-ity enquires slow heating and cooling conditions and longhold tin•s. Such eolahtions are rareh . eneountemd in prac-tice, espiviallv iti wilding with its fast heating and clxiling.

In addition, most phase diagrams describe alloy systemscontaining two elements, while ciiprieering alloys getieralli-contain many components. Phase diagrams for systems milliMore than No eolitiMerits are COTTIpleR and difklitt tointerpret.

A sirs' simple phase diagram for the copper nickel alloysystem is shown in Fig. 3(1. This is called 01 Alma/Thombittfirysysb941, a furl eirtrk111 SINtellt in which hiltll elenirllt sare completely !g ild de in each other in the lignid and thesolid states, in all proportions at all temperatures.

As shown in Fig. 30, phase diagrams an . dram' with tlwallov eotilent plotted on the horizontal axis. The extreme leftbaud edge of the diagra in represents A go percent nickel alai100 percent topper while the estreini• right land edge rep-resents MO 1:epeeist nickel and zero percent copper Tme-perature is pl. stted on the vertical axis. Figure 30 slows that,at temperatures above the top cony, the only Ouse is liquidmetal This is tow for all composim ins ecnered by this dia-gram. At temperatures inclose the bottom cunt, the onlypilaw' is solid metal. This too, is true for all compositionscoveird lir the diagram. All solid alloys funned ar singlephase only because topper and nickel are cornplet•ly solublein each tither in the solid state. "Ilus, An alloy with 30

percent (unser Mid 70 percent nickel is a hilmogi-neolis solidsolution. It remains solid up to a point below 1,200 T(2.200 'F), whew it begins to melt, Melting is complete at apoint higher than that temperature.

In the region lwtsseen the curs-es. solid and liquid .phasescoexist. This illustrates the fad that most allOV% solidify (goFrom n complete liquid to complete solid) Over a range of topermutes, As the diagrams shows, complete solidification inthe twtsper nickel system oentrs at a single 4-mi/endure onlyfor pore copper . 1nT pare Wad..

Figure 31 shows the pham., diagram For a Illdre cornpli-cated syNtern: the silver topper alloys (this diagr.ini is usedextensively for designing /ruing all oys), The ober capperalien" systern is use in which each element has only limitedsolubility hi the Other as can he seen in the phase diagram.The area marked as the alpha phase shows where a smallamount of copper is dissolved in the silver. As more copperis ZNIded bevond its solohilits beta phase (clipper with somesilver dissolved in it) begins to form.

This phase diagram also illustrates the feature knossuthe riderfir point, As a eutectic liquid (having the right pro-portion of (vaistiment inetals) cools and solidifies, both thealpha and beta phases are fimned at the sane temperature.Frequentl y, these phases occur as alternating platelets. giv-ing tilted ic colliposit loos their chstinctis• niicrostowt tire.

A final feature of the silver copper phase diagram is that it,lento solid solnInhh . in TVIabOn to teniperatilre, The hOlaal •ary line between the n + beta region and the alpha solid

FIGURE 31. Silver copper phase diagram:28 percent copper and 72 percent silver is theeutectic composition

ceLu °0_

LEGEND

1.300

1.200

1,100

1.000

900

800

600

500

400

300

200

2,000

1.500

1.000

500

LUCIL

LL )51-

2

LIQUID 1.083 't (1.981

%0 °C 11,760 °El

C/t 00'60 LIQUID"SOUD

°FI

SOLID 779 'c 11,439 °FI

EUTECTIC POINT

ot +

0 20 40 60 80

COPPER(percent)

100

= ALPHA OR COPPER= BETA OR SILVER



n

n

n

I

solution region shifts to lower alloy content as the tempera-ture decreases. This means that when all the possible copperis dissolved in silver at a certain temperature, the solubilitylimit is exceeded at any lower temperature and some betaphase forms when the alloy is cooled to that lower tempera-ture. This behavior is the principle behind precipitationhardening.

Effect of Deformation and Heat Treatment

Figure 32 shows some of the property changes and micro-structure changes that typically occur as a metal is workedand then heat treated. Working done below the metal's criti-cal temperature is known as cold work. Cold work graduallyincreases the hardness or strength of a metal and decreasesits ductility.

If the stainless steel in Fig. 32 is worked moderately orseverely, then heated to progressively higher temperatures,several things happen. At temperatures from 95 to 205 °C(200 to 400 °F), there is a steady decline in the residual stresslevel but virtually no change in microstructure or strength.At 205 to 230 °C (400 to 450 °F), a relatively low level ofstress remains and the microstructure is apparentlyunchanged. The strength of the metal remains relativelyunchanged compared to that of the original cold workedmaterial and the ductility is improved but still low. This

reduced stress level and increased ductility are attributed tothe metallurgical phenomenon known as recovery.

When cold worked metal is heated to a temperature above230 °C (450 °F), property changes occur, as do changes inmicrostructure. This process is called recrystallization, anecessary part of all annealing procedures. Annealing goesbeyond stress relief. If for example, stress relief is desirablein a structural member that had been cold worked toimprove strength, annealing is too severe and subsequentsoftening reduces the strength of the member below itsdesign strength.

Phase Transformations in iron and Steel

Pure iron solidifies at 1,535 °C (2,795 °F) and forms a bodycentered cubic structure called delta iron or delta ferrite.With slow cooling, the delta ferrite persists until at 1,390 °C(2,535 °F), it transforms into a face centered cubic structurecalled gamma iron or austenite. The austenite remainsdown to 910 °C (1,670 °F) where it transforms back into thebody centered cubic structure known as alpha iron or alphaferrite. Delta iron and alpha iron are similar in structure butthey are given different names to distinguish between thetemperatures at which they form. This transformation isreversed when iron is heated.

Steel is an iron alloy with less than two percent carbon.The presence of carbon alters the temperatures at whichfreezing and phase transformations occur. The addition ofother alloying elements also has some effect on these tem-peratures. As with other alloy systems, iron carbon alloysfreeze over a range of temperatures, with different liquidstate and frequently different solid state temperatures foreach composition. As the carbon content of iron carbonalloys increases up to 4.3 percent, the liquid state decreases(see Fig. 33).

Most of the alloying elements added to steel (nickel andchromium, for instance) alter the transformation tempera-tures, sometimes markedly; occasionally they suppress trans-formation completely. Phase changes attributed to thecarbon in steel are shown in the iron carbon diagram inFig. 33. Room temperature microstructures of iron carbonalloys at the equilibrium conditions covered by thisdiagram include one or more of the following constituents:(1) ferrite, a solid solution of carbon in alpha iron; (2) pear-lite, a platelet mixture of cementite and ferrite; (3) cemen-tite, an iron carbide Fe 3C present in pearlite or as massivecarbides in high carbon steels.

Pearlitic structure (see Fig. 34) imparts only moderatestrength to steel. When the cooling rate of an austenitizedsteel is increased sufficiently, there are two changes in trans-formation: (1) transformation occurs at lower temperaturesand (2) the resulting microstructure is changed and thehardness and tensile strength of the steel increase, with duc-tility undergoing a corresponding decrease.

.11

WORK - DEFORM RECRYSTALLIZE I GRAIN GROWTH

(c)ANNEAL

+ (Ws.--.N. STRESS -

RELIEFRECOVERY

(C )(a)

STRENGTH

uJ11.1

0

STRENGTH

10 10 30 40 50 00 95

AMOUNT OF COLD WORK Ipercent) 11001

1.11:41061-


DUCTILITY

315 425 540 650(600) (800) (1.0001 11,200)

TEMPERATURET 1°F)

20514001


FIGURE 32. Generalized effects of work hardening and recovery on chromium nickel stainless steel

A slight increase in cooling rate only depresses the trans-formation temperature a small amount and produces aslightly finer pearlitic structure. The lamellae are slightlyfiner, the hardness of the steel is somewhat greater than thatof steel with a coarse pearlitic structure and ductility is some-what lower.

At fast cooling rates, still lower transformation tempera-tures are encountered and bainite forms instead of pearlite.Bainite is a feathery arrangement of fine carbide needles ina ferrite matrix. It has significantly higher strength, higherhardness and lower ductility than the fine pearlitic structure.

With still faster cooling rates (severe quenching), martens-ite forms. Martensite is hard and relatively brittle. Figure 35illustrates its acicular (needlelike) appearance. As-quenchedmartensite usually is too brittle for practical use but its duc-tility can be restored by a process called tempering. This

transformation is important in any consideration of weldingbecause the weld metal and part of the heat affected zone(HAZ) of a welded joint are heated and cooled through thetransformation temperature range.

Isothermal Transformation

The iron carbon phase diagram is useful but does notprovide data about the transformation of austenite to anystructure other than equilibrium structures, nor does itfurnish details about the cooling rates required toproduce other structures or the temperatures at which trans-formations occur. A more practical diagram is the time tem-perature transformation (TTT) diagram. It graphicallydescribes the cooling rate required for the transformation ofaustenite to pearlite, bainite or martensite. It also gives the

14 16 18 20

CARBON[atomic percenl

2 4 6 8 10 12

Fe 2 3

CARBON[weight percerA

4 5

1.600

1,400

1,200

1,000

800

600

400

200

FIGURE 33. Iron carbon phase diagram

1111111111 LIOUID n•n111

-n

NE1111S,WME•

• SEEMEN

n•

E 1.130 .CI ow Fe3C

'AUSTEN TEJ pill •W nn

ME " Fe3C El Mmum A 1 1723 °C, M

11 IIIII

a[FERRITE)il 11111 11iiNM M n•

MEE In• II .MI • nME


2,800

2900

2,000

800

100

,600


temperature at which such transformations take place. Fig-

ure 36 shows a time temperature transformation diagram for0.8 percent plain carbon steel.

To produce this diagram, samples of the specific carbonsteel were heated to the austenitizing temperature of about845 °C (1,550 °F) and placed in environments in which theycould abruptly fall to a series of temperatures ranging from705 °C (1,300 °F) to room temperature. This was accom-plished by plunging the samples into various solutions ofhone, oil or water at the desired temperature and then hold-ing each test object at that temperature for a specified lengthof time. After each test object had reached its specified dwelltime, it was removed, cooled quickly and visually examinedunder a microscope.

The sample of plain carbon steel (Fig. 36) was held at705 °C (1,300 °F). It began to transform after about 500 S

and finished transforming after 4,000 s. The resulting struc-ture was coarse pearlite at R e 15.

FIGURE 34. Typical lamellar appearance of etched,magnified pearlite


FIGURE 35. Quenched martensite (etched andmagnified), showing acicular structure


Transformation was quicker for the test object held at565 °C (1,050 °F). It started in 1 s and was completed in 5 s.Transformation took place in the shortest length of time atthis temperature. For this reason, the nose of the curve islocated at 565 °C (1,050 °F) for this steel. The microstruc-ture of the steel transformed at this temperature is fine

tY

JO

40

41

11

SO

S5

57

06

66

0

OLL

2

LL

8

ll-4

FIGURE 36, The time temperature transformationdiagram for the transformation of austenite In aeutectold 10.8 percent carbonl plain carbon steel

900w

/0D

61;101.1J

540

400

300

u.

FAAKIENVE

I ) 4 A (s gr I I 4 4 ( 4A IsSECONDS MINUTES HOURS

TIME or TRANSFORMATION


STARTSALTSTENITE

A. TEMPERATURE ...........

TRANSFORMATION AT 705 °C 11,310 sf

co .w I'FARI 11 E

•1111E FORMINGM.ISTENITE pEAgLITE

FINE PEARtliE

-LA !HEFT BAINITE BAENITE

AUSTFNITEACICULAR 0AINITE

I EMPERATURt

- .1178 mATg9/1ri) msQ 1ROM AUSTEN if ON COOilhAa''

7.-A4, TEMPERATURE

•NOSE

ENDS

FIGURE 37. Maximum hardness of martensite as afunction of carbon content

1000

di

6 tie

500

L4.1

Li 400

5

rrsto

55

1.11 LH 05 00

CARBON COMPOSITIONIPCIO21111

FROM THE AMERICA'S/ WELDING SOCIETY, REPRINTED WITHPERMISSION,

45

40

Ii

30


pearlite with a hardness of 41. As temperature decreased,the time to start transformation increased, the mierostnie-tore lweattu• Unite and hardness increased still more.

The test objects that cooled to room temperature rapidlyenough to get past the nose of the cunr re had an entirely dif-ferent microstructure (inartensite)- Martensite forms 1w atransformation which occurs OIIIV on moling. It starts at theso-called M, temperature or the inrirteasite skirl tempera-ture. For the 0.8 percent carlxm steel in Fig, 36, the M,temperature is abotit 230 °C (450 OF), Martensitetransformation is complete when the temperature hasdropped below alxmt 120 °C 250 for this steel Thattemperatit re is known as the Mir trinivratrire OE the martens-* finish totiperelure, As the carbon content of steeldecreases, both the M and MI temperatures increase.

Hardenability

Although time temperature transformatitm diagramsportray certain characteristics of steels, the concept of hard.(liability presents another method of describing the transfor-mation of anstenite in various steels. Hardenahilitv shouldnot he confused with hardness. The maximum hardness of asteel is primarily a function of its carbon content (Fig. 37),

I lardenabilitv is a measure of how easily martensite firrins onquenching. Certain steels. s;ith very high alaide nahili ty, foolsmartensite even when they are being slowly cooled in air.'Mow steels with low hardenahilitY require cooling rates of540 °C.s .1 (LOW °F•s-') if they are to transform completely toinartensite. These hardenability characteristics determinethe extent to which a steel hardens during welding.

Tempering Martensite

Figure ati illustrates the mechanical properties obtainedby tempering a quenched low alloy steel. Quenching andtempering are frequently used to enhance the properties ofmachinery steels, pressure vessel steels and StrUillral steels

because that treatment develops high yield and high tensilestrengths, high yield to tensile ratios and improved notchtoughness enmpan .d to the same steel in the hot wiled,annealed or nonnali rted condition.

FIGURE 38. Tempering curve for AISI 4140chromium molybdenum steel

TEMPERING TEMPERATURE °C


E"

320

300280

260240

2.200

2.000

1,800

1.600220200 1,400

180 1.200

1̀-1 160 Q.

140 .000Cz 120 E

800100

60080

60 40040

20020

0 00 100 200 300 400 500 600 700

1210)1390) (570) (7501 I930) (1,100111,290)

=n nn-OEM nOUCIION —EINE

111=1111====Iri==

ELONGATION

70605040302010

7-

uJ

T I

T,u_r

T,

FIGURE 39. Hardening data for alloy X: fa) phasediagram of a precipitation hardening system andlb] changes during several stages in heattreatment

(al

LIQUID +

+ Is

100percerp0

(b)

Ot,6)21.G./4„

14

OLUTIONTREATED

AGING TIME



ITempering curves of the types shown in Fig. 38 are well

established for most plain carbon and low alloy steels.

Precipitation Transformations

Precipitation hardening or age hardening is anothermethod of strengthening steel. Figure 39 shows the phasediagram for an alloy in which the beta phase is increasinglysoluble in the alpha phase, as the temperature increases.

lk The Orecipitation hardenabIe alloy X shows there is a duplexstructure of a + 13 at room temperature.

By slowly heating this alloy, increased amounts of beta aregradually dissolved in the alpha. When the temperaturereaches T, (a temperature at which x percent of beta can bedissolved in alpha), the alpha solid solution has dissolved allof the available beta. This composition is stable as a singlephase solid solution at or above T 1 . If alloy X is cooled belowT„ the solid solution becomes supersaturated (the alpha canno longer retain the same amount of beta in solution) and thebeta, which cannot be retained at that temperature, must berejected from the solution. With slow cooling, this beta pre-cipitates at preferred locations such as the grain boundaries(see Fig. 40). If the same alloy is heated to T, and thencooled rapidly to room temperature, there is no time for dif-

fusion or rejection of the beta and the single phase supersat-urated structure is retained.

Table 8 lists some precipitation hardening alloys. Thereare several elements that produce age hardening in thesealloys. Titanium and aluminum are used for this purpose inboth the nickel and the iron alloys. Copper is commonlyused in aluminum as well as in iron alloys. In aluminum, the

0

0O

O FINEPRECIPITATE

a + 13 IN SOLUTION (d)

FIGURE 40. Microstructural changes in an agehardenable alloy during several stages In heattreatment: (a) annealed, (b) solutionized at T1,(c) reannealed, (d) solution treated, (e) agehardened and (1) overaged

C'Po, On ,

(c)

PROLONGED OREXCESSIVE HEATING

DURING AGING

(„/ + COARSER (f)( e )

TEMPERATURE T,

(a)


FIGURE 41. Coil breaks are creases or ridges whichappear as parallel lines, transverse to the directionof rolling, and which generally extend across thewidth of the sheet: (a) closeup; (b) appearanceafter flattening

(a )

(b)

ROLLING DIRECTION

FROM AMERICAN IRON AND STEEL INSTITUTE. REPRINTED WITHPERMISSION.


TABLE 8. Typical precipitation hardening alloys orseveral solvent elements or base metals

Iron Nickel Aluminum Magnesium Titanium

17-4 PH Inconel X® 2014 AZ 80A 6AI-4V17-7 PH Rene 41 2024 ZK 60A 4A1-3Mo-1VPH 15-7Mo Udimet 500 ® 6061 HM 21A 16V-2.5A1AM 350 Waspaloy® 7002 ZE 41 A 6AI-6V-6Sn19-9 DL Inconel 700® 7039 ZH 62 A 13V-11Cr-3A1A 286 M 252 7075 EZ 33 AFROM THE AMERICAN WELDING SOCIETY. REPRINTED WITH PERMISSION.

hardening agent is the chemical compound CuAl 2 . Similarcompounds can be produced by adding certain amounts ofcopper to steel, aluminum to magnesium or chromium totitanium.

Inspection for Discontinuities in SteelSurface Inspection

In the manufacturing processes, it is common practice toinspect the top surface of cut lengths as they are sheared to

length from coils. In the case of coils it is the outside surfaceof the coil that is inspected during processing and rewinding.

Sheet steel in coils or cut lengths may contain surfaceimperfections that can be removed with a reasonable amountof metal finishing by the purchaser. Because the top side ofa cut length or outside of a coil is ordinarily the inspectedside, the opposite side may contain more surface imperfec-tions. To minimize the amount of metal finishing, theinspected side can be used for the most critical surface of afabricated part.

FIGURE 42. A coil weld is a joint between twolengths of metal within a coil: coil welds are notalways visible in the cold reduced product

ROLLING DIRECTION


FIGURE 43. Edge breaks are short creases whichextend in varying distances from the side edge ofthe temper rolled sheet

ROLLING DIRECTION



When it is not possible to use the inspected side for themost critical surface of a fabricated part, the producer shouldbe notified. It is sometimes possible for a producer toinspect the bottom surface or reverse the inspected top sideto the bottom side of a lift of cut lengths or inside surface ofa coil.

Coils contain more surface imperfections than cut lengthsbecause the producer does not have the same opportunity tosort portions containing such imperfections as is possible inthe case of cut lengths.

Limitations of Inspection, Testing and Certification

Whenpurchaser's specifications stipulate that inspectionand tests (except product analysis) for acceptance of the steelbe made by his representative before shipment from themill, the producer affords the purchaser's representative allreasonable facilities to assure that the steel is being furnishedin accordance with the specification.

There are a number of intrinsic features of steel makingand finishing processes that affect the properties or condi-tions of the finished products, and those effects cannotalways be precisely known. Therefore, it is technicallyimpossible to give unconditional certification of complete

compliance with all prescribed requirements. That fact ismanifest to those having a technical knowledge of the subjectand those skilled in the manufacture of steel, and is recog-nized in applying a factor of safety. For example, the phe-nomenon of segregation causes variations in chemicalcomposition, mechanical test results and soundness. Varia-tions in manufacturing practice such as control of tempera-ture, which cannot always be regulated with exactness,sometimes cause variations in mechanical properties in dif-ferent parts of a heat or lot of steel.

Because of these and other conditions which are presentin steel mill operations, it is not possible at the present timeto identify any reasonable or practical methods of testing orinspection that will ensure the detection and rejection ofevery piece of steel that varies from the specified require-ments with regard to dimensional tolerances, chemical com-position, mechanical properties, surface or internalconditions.

Common Sheet Steel imperfections

The illustrations and definitions in Figs. 41 to 57 areincluded to promote better and more nearly uniform termi-nology On this subject.

FIGURE 44. Floppers are lines or ridges which arediagonally transverse to the direction of rollingand generally confined to the section midwaybetween the edges of a coil as rolled; they aresomewhat irregular and tend toward a flat arcshape

FIGURE 45. Fluting is a series of sharp parallelkinks or creases occurring in the arc when sheetsteel is formed cylindrically; photograph shown isfrom a test specimen


FIGURE 46. Friction digs are a series of relativelyshort scratches variable in form and severity


The imperfections listed are visible to the unaided eyebefore fabrication with the exception of orange peel, strain,fluting and ghost lines, which become evident only afterforming.

These imperfections are variable in appearance and sever-ity. To obtain suitable photographs, extreme conditions havebeen selected in some instances. Allowances must also bemade for image degradation due to reproduction forprinting.



ROLLING DIRECTION

ROLLING DIRECTION

FIGURE 48. Orange peel strain is a pebbly surfacecondition which develops during drawing

ROLLING DIRECTION


FIGURE 49. Pinchers are fern-like ripples or creasesusually diagonal to the rolling direction

-..*---)..-

ROLLING DIREC I ION



FIGURE 47. Orange peel is a coarse grain condition which becomes evident during drawing

fal Ibl

FROM AMERICAN IRON AND STEEL INSTITUTE. REPRINTED WITH PERMISSION.

FIGURE 50. Pipe lamination is a separation midwaybetween the surfaces containing oxide inclusions

ROLLING DIRECTION


FIGURE 51. Rolled-in dirt is extraneous matterrolled into the surface of the sheet

7 ROLLING DIRECTION



FIGURE 52. Rolled-in scale consists of scale partiallyrolled into the surface of the sheet

(al

imalir* .25'1"

(b)"vrRe...orpo,

ROLLING DIRECTION


FIGURE 53. Skin lamination is a subsurfaceseparation which usually results in a surface rupture

ROLLING DIRECTION


FIGURE 55. Sticker breaks are arc shaped types ofcoil breaks usually located near the middle ofthe sheet

4-alln-ROLI IND DIRECTION


FIGURE 54. Slivers are surface ruptures somewhatsimilar in appearance to skin laminations butusually more prominent

. 4010:111:11/-

.(---31nnROLLING DIRECTION


FIGURE 56. Stretcher strains are irregular surfacepatterns of ridges and valleys which developduring drawing


FIGURE 57. Ghost lines are lineal irregularities inthe surface which develop in drawing; they areparallel to the direction of rolling

la i

I b i

ROLLING DIRECTION




PART 2

VISUAL AND OPTICAL TESTING IN THESTEEL INDUSTRY

Visual tests are widely used for in-process and productinspections. The human eye is an excellent sensor and it isused in conjunction with a large, high-speed computer—thehuman brain. Human inspectors can instantly perceivemany material characteristics (shapes, colors, gloss, shades,speeds, perspective, etc.). In addition, humans can recog-nize patterns and reason from what they have experiencedand learned.

The customer sets parameters for discontinuity and gradeand specifies these limitations in the form of discontinuitysamples. These samples can standardize flaw characteriza-tion and accept/reject decisions from one inspection toanother. The acceptance criteria for microdiscontinuities aredetermined through discussion with each customer.

Organoleptic visual tests (i.e., those relying on the humaneye) have drawbacks. If momentarily distracted, an inspec-tor may overlook discontinuities. Also, the passage of yearsis increasing industry's need for automation: inspectors aregrowing older; younger workers are less willing to do visualinspections; production lines are getting faster; and users aredemanding better surface quality. Today's demands forhigher performance and faster throughput exceed the abili-ties of visual tests by humans. Consequently, visual testsmade by the human eye are being replaced by automatedvisual testing using optical equipment and unstaffed inspec-tion stations.

Humans still outperform automated test equipment in theability visually to classify and rank discontinuities and manyprocesses in the 1990s have not been fully automated.Human inspectors are usually deployed to complementsemiautomated testing situations; in the early 1990s, onesteel company in Japan had more than 300 inspectorsassigned to product inspection.

Surface Inspection TechnologiesTrends in Surface Inspection Technologies

This section discusses trends in surface discontinuityinspection. Equipment for sheet and strip steel in Japan isused as an example.

The discontinuity detection technologies available in199221'5 had almost all been developed by 1975. In the1980s, signal processing developed rapidly, removing the

discontinuity recognition limitations of hardware andincreasing inspection accuracy significantly. The improvedperformance of computers contributed to the technologyand led to the adoption of image processing. In parallel withthe development of signal and image processing, sensors forparticular imperfections or for special signal processing cir-cuits were introduced. Software and hardware thereby havecomplemented each other to improve performance andextend the range of applications.

Simultaneous efforts to increase performance anddecrease costs have pulled the technology in two directions.'The following developments could be anticipated in the1990s: developments of new sensors; new applications ofimage processing technologies; more enhancements of sig-nal processing; and the combination of new discontinuitydetection techniques with sensors based on other principles,such as magnetic testing.

Surface inspection entails not only optical considerationsbut also insight into the user industry and peripheral techno-logies, such as computer, system and human engineering;operational technology; quality control; and user demandtrends (see Fig. 58).

Development and Installation

The development and installation of surface discontinuityinspection equipment requires more manpower and timethan that of other sensors and must proceed efficientlybecause it is hard to make an absolute evaluation in terms ofcorrespondence with organoleptic examination. To ensureefficient inspection for surface discontinuities, the equip-ment must be erected and developed using the followingsteps.

1. Establish guidelines for installation; specify power andother requirements.

2. Evaluate items at the basic planning stage.a. Evaluate visual tests by humans.b. Determine basic specifications for equipment in

view of objectives (see Figs. 59 and 60).c. Check detection capabilities using test samples.d. Conduct an online test using a test machine.

3. Introduce equipment into production and continue toverify its accuracy online.

FIGURE 58. Various fields surrounding the surface inspection

PTIC5.

LASER TECHNOLOGYOPTICAL SENSORILLUMINATION TECHNOLOGYCOLORING ENGINEERING

BIOLOGYINFORMATION PROCESSING,

IN TRAIN AND VISION

SURFACEINSPECTION

ARTIFICIAL INTELLIGENCESAFEST SYSTEMEXPERTISELEARNING NEURAL NETWORK

DEMAND FOR DUALITYUNDERSTANDING THE NEEDS

AND MANUFACTURINGPROCESS OF CONSUMER

PHYSICSMETALL0GRAPHYSURFACE PROPERTIESDISLOCATION

CONTROL ENGINEERINGSIGNAL PROCESSING TECHNOLOGYINFORMATION PROCESSING TECHNOLOGYAUTOMATION TECHNOLOGYROBOTICS ERGONOMICS

ORGANOLEPTICSHUMAN RESOURCES

MANAGEMENTINVESTMENT EFFECTS

EXPERTISE 10 DEVELOP ANDCONSTRUCT AN ORGANIZATION

EXPERTISE TO DEVELOPHUMAN RELATIONS MATHEMATICS

FOURIER SERIESSTATISTICAL ANALYSISDISCRIMINANT FUNCTIONPRINCIPAL COMPONENTANAL YSISmu / TIVARIAIE ANALYSIS

ELECTRONICSSEMICONDUCTOR

TECHNOLOGYDIGITAL PROCESSINGTRANSPUTOR TECHNOLOGY

ELECTRICAL ENGINEERINGPOWERGROUNDNOISE

OPERATIONUNDERSTANDING THEMANUFACTURING PROCESS

QUALITY ASSURANCEQUALITY. CONTROLPROCESS CON TROi

COMPUTERTECHNOLOGY

SYSTEM ENGINEERING

MECHANICSDESIGNMANUFACTURING

NEEDS FOR INSPECTION - FACTORS TO DECIDE THE TARGET

FIGURE 59. Setting the target in order to introduce a surface inspection system

FINAL PRODUCTIONCOLD ROLLED SHEETSCOATED STEEL SHEETSMANY ITEMS ACCORDING

TO USERS [MALTYASSURANCE NEEDS

STRINGENT QUALITYCONTROL NEEDS

/E G. HOT STRIP)

WEAKNESSES OF ORGANOLEPTICVISUAL TESTING STRENGTHS OF ORGANOLEPTIC

VISUAL TESTING

NOT FOLLOWING HIGH SPEED MOVEMENTINDIVIDUAL DIFFERENCE['BANGING INSPECTORS CONDITIONTIRINGSCATTERING THE LEVEL OF INSPECTOR

THE EXCELLENT FUNCTIONOF HUMAN EYES

ICOLOR RECOGNITION PREPRO-CESSING FuNcTioN. ETC.)

PATTERN RECOGNITIONLEARNING AND INFERENCETOTAL JUDGMENT ABILITYIN T UITION

DECISION OF ABASIC INSPECTION

TO DECIDE DISPOSITION OFTEST OBJECT

TO CLEAR TEST STATION

4,

INCREASING THE IMPORTANCE FORAUTOMATIC INSPECTION

SPEEDING AND AUTOMATIONOF PRODUCTION

WEAKNESSES OF MACHINES

WITHOUT THE TOTAL JUDGMENT ABILITYWITHOUT INTUITIONWITHOUT PATTERN RECOGNITIONWITHOUT COLOR RECOGNITIONEXPENSIVE

STRENGTHS OF MACHINES

FOLLOWING HIGH SPEED MOVEMENTHOMOGENEOUS INSPECTIONINSPECTING BOTH SIDES SIMULTANEOUSLYWITHOUT INDIVIDUAL DIFFERENCESENSITIVE TO MINUTE DISCONTINUITIES


FIGURE 60. Flow chart for determination of basicspecification of automatic surface inspector

START

CONTINUOUS PROCESSING UNE SHEAR TINE

I OBSERVINGDISCONTINUITIES

THE FIRST SAMPLE TESTINSUFFICIENT AT STANDARD FUNCTIONS

HARDWARE ONLY

ORSPECIAL SEN OR EXCLUSIVEUSE CIRCUITS TO DETECTSPECIFIC DISCONTINUITIES

PATTERNREECGNiTIONF

SIMPLE OR NOT

HARDWARE WITHOUT SOFTWARE HARDWARE MTN SOFTWAREFOR SIMPLE PATTERN

FOR COMPLEX PATTERN

C END

UNIX WfIll-> NO FuNCTIONT

YESADDITION OF A FUNCTION TOCLASSIFY DISCONTINUITY TIES

FIGURE 61. Schematic diagram of an inspectionsystem charge coupled device (CCD} cameras

INSPECTION ROOMSIGNAL PROCESSING UNIT MICROCOMPUTER

CCD


In addition, data generated online may help generate orsimplify logic that classifies and grades discontinuities ortechniques. The online accuracy of the system must bechecked periodically.

Surface Discontinuity InspectionSystems for Hot Slabs27-28

Television Inspection

This system includes two techniques. The first reads selfradiation from a slab and infers the presence of defects froma difference of intensity between areas with and areas with-out discontinuities. The other approach, which offers ahigher signal-to-noise ratio, has been introduced into indus-try. The sensors used in the system include camera tubes andsolid state imaging devices such as charge coupled device(CCD) cameras. Charge coupled device cameras are mostcommonly used because of their long service life, enhancedaccuracy and low image distortion. Figure 61 shows aninspection system using a charge coupled device (CCD)camera. This noncontact technique allows fast discontinuitydetection but its detection capabilities are limited to discon-tinuities over 0.5 mm (0.02 in.) wide at the surface.

Laser Inspection

Laser beams are scattered by cracks and pinholes in away different from the way laser light is reflected from

discontinuity-free areas (Fig. 62). Applying this principlesystematically, a laser scattered light detection system canidentify discontinuities on a surface using an argon laser or ahelium neon laser as a light source and a photomultiplier ortelevision camera as a sensor. It can discern cracks of about0.3 mm (12 mil) wide at the surface, 10 mm (0.4 in.) long,and pinholes over 2.0 mm (0.08 in.) in diameter on slabs ofabout 900 °C (1,650°F). The difference between the surfaceof a slab and a discontinuity is not great enough for a sensinghead by itself to provide clear images of discontinuities, sothe equipment uses image processing to make anomalousareas conspicuous.

Photographic Inspection

In this system, the surface of the slab is photographed in aseries of images, the film is quickly developed and the imagesare projected to let an inspector discern imperfections. Theresolution of the film image is better than that of a televisionimage, thus providing sharper images of the surfaces. Thetechnique can detect discontinuities over 0.4 mm (16 mil) inopening widths (Fig. 63).

Inspection Using Thermometer

A slab surface is scanned by a microspot radiation meterto detect imperfections by changes in temperature. An

FIGURE 62. Schematic diagram of laser scanninginspection system

Ar + LASER

MIRRORPARABOLIC MIRROR

POLYGONAL MIRROR

PHOTO TUBE

SLAB

FIGURE 63. Schematic diagram of photographsystem

QUICK DEVELOPMENT

CAMERA

LAMP

0- -

SCREENSENSOR

A B

IT

FIGURE 64. Optical surface inspection usingscanning pyrometer as sensor

(a) PLANE

B ACROSS SECTION

DISTANCEPYROMETER OUTPUT

DISCONTINUITY PLANE

TEMPERATURE

= D

CROSS SECTION L=B

AAT 1-1 LBOr

DISTANCEPYROMETER OUTPUT

TEMPERATURE


infrared pyrometer is used as'a radiometer, taking into con-sideration the wavelength distribution of energy radiatedfrom the slab surface. Alternatively, anomalies can berevealed by induction heating the thin layer near a slab togenerate a difference in temperatures between the anoma-lous area on which current has concentrated and its sur-rounding area (Fig. 64). Radio frequencies of 100 to 150kHz are used for induction heating of the thin surface layer,and a cadmium-mercury-tellurium infrared pyrometer isused as a sensor. This method can detect slits penetrating 1.0to 2.0 mm (0.04 to 0.08 in.) deep.

Inspection for Surface Discontinuities inLarge Section Steel Products 29

Section steels tend to have irregular cross sections. Theresulting temperature patterns can interfere with identifica-tion of surface discontinuities generated on large sectionsteels by hot rolling. This problem is solved by correcting thetemperature pattern; the use of a charge coupled devicecamera with an electronic shutter, together with high speedimage processing, allows online detection of discontinuities.The inspection system looks for temperature deviation pro-duced by discontinuities in self radiation light during hotrolling. The charge coupled device used has a visible radia-tion range especially sensitive to temperature deviation pro-duced by discontinuities. The temperature pattern due tothe shape of the cross section is corrected by an optical tech-nique that uses slits to moderate the light from the flangecenter and both ends of the web. Image processing, the orig-inal image and smoothed one are differentiated and theresulting image is digitized. A system configuration is shownin Fig. 65.

VOTCRCONTROLER

VELCOTP METER

HOT METAL DETECTOR

ATTEMNEN DRAG MOTOR

VCR

MONITOR

HIGH SPED RATER

FIGURE 65. Diagram of surface discontinuitiesinspection system for large section steel products

_ _ u' t--

CAKE

ti

(MEC

AXE

ATTEMJATOR

RAW DEMCE cowl PtfiL ;- 1

CA*

CONTROLLER

tfIGH AID WI PR:CBS:11

146-1 SPEED IMAGE PRIES3011

IHGH VEED NAGE NO:ESS011

INCH WED MADE NOCESSORI

9 COMMR ROM F-- -

OPEKRYG

Nfl

InaCATHODE &IN

MTN COMPUTER

CRIOINC RCOV

!Loukt DEriciazi - - -LOPERAIM PAPE I--

FIGURE 66. Surface inspection system for hot strip

- - - - --- --

UPPER DETECTOR1-

iecurr LAM

1E3FRXESSO

STRIP

COMPUTERMAGE

PROCESSING

IMINCOMPUTERI

- ,

CHARGE COURTED

DEVICE NW

10 --

OSCOM1Nl

TYPE


Surface Inspection System for HotStrip30

A variety of hot strip surface inspection installations havebeen attempted but, because of the severity of their environ-ments, few have been practical. Nevertheless, demand forthem has been increasing as it has for cold strip. The systemshown in Fig. 66 has the following parts: a high voltage mer-cury lamp that illuminates the object at about 80,000 lux(7,400 ftc); a charge coupled device camera that photoelec-trically converts the image of the illuminated area; a signalprocessor that extracts discontinuity signals from thecamera's outputs; and a minicomputer that provides imageprocessing by converting discontinuity signals to two-dimen-sional images for discontinuity recognition. On a strip, it canlocate black spots of 2.0 mm (0.08 in.) in diameter at a speedof 1,500 m/min (5,000 ft/min) and at a temperature of about700 °C (1300 °F). Although its intrinsic sensitivity is low fordiscontinuities that cause the greatest trouble in hot rolling,that sensitivity can he improved by image processing thatanticipates the periodicity of flaws in rolled sheets and looksfor imperfections such as burrs and scaling.

In Europe, one particular testing installation uses a chargecoupled device camera with a capacity of 3,048 bits and aframe rate of 9,500 scans per second.

Surface Discontinuity Inspection ofCold Strip Steel

Requirements for surface quality of rolled steels aregetting more stringent, in particular for cold strip and sur-face treated steels. Thanks to advances in sensors and signalprocessing, recent inspection equipment can meet theserequirements." The most practical systems in use combineoptics suited to the surface conditions of the tested material"with a signal processor that turns sensor input into two-dimensional images to identify and grade discontinuities.

Optical System

The white light sources include halogen lamps, mercurylamps, fluorescent lamps and laser beams. Devices used asphotodetectors are silicon semiconductors, television cam-eras, solid state imaging devices and photomultipliers. Variousoptical systems combining photodetectors and these lightsources have been devised and applied. Fluorescent lightsare used with solid state imaging devices (e.g., charge cou-pled devices); laser lights are used with silicon semiconduc-tors or photomultipliers. Representative optical systems areshown in Fig. 67 and Table 9.

In addition to letting beams converge on a small spot, alaser brings to an optical system the advantages of coherentlight. When the laser light illuminates material under test,as shown in Fig. 68a, a diffraction pattern inherent in the tar-get surface is generated. The pattern shown in Fig. 68bcorresponds to one on a discontinuity-free area on a flat alu-minum sheet—rolling streaks appear brightly lit along the Xaxis. The pattern in Fig. 68c represents an area with dents;68d, one with scratches—clearly revealing pattern changesarising from surface irregularities along the Y axis. The sen-sors detect only reflected light and a spatial filter with high

FIGURE 67. Representative systems for surfacediscontinuity inspection of cold strip: (a) obliqueflying spot scanning system with halogen lamp,laser and silicon semiconductor sensor; (b) a simpleroblique flying spot scanning system with laser;(c) parallel flying spot scanning system withparabolic mirror; and (d) system using flying imagetechnique to detect scattering light (e.g.,television).

LASER IHe-NeJ ROTATING MIRROROR VIBRATING MIRROR

PHOTOMULTIPLIER

FILTER

OPTICAL FIBER

(b)

CHARGE COUPLEDDEVICE CAMERA

LIGHT SOURCE

(d)

COLLIMATORLASER

OSCILLATOR

(a) ROTARY MIRROR

PHOTOMULTIPLIERTUBESPECIAL MASK

ONDENSINGLENS

( C ) He-Ne LASER

PARABOLIC MIRROR

ROTATING MIRROSTRIP

PHOTOMULTIPLIEROPTICAL ROD

FIGURE 68. The laser diffraction pattern frombrighted aluminum changes at variousdiscontinuities: (a) laser diffraction patternreflected from a surface (optical Fourier transform);(b) standard without discontinuities; (c) patternshowing dents; and (d) pattern showing scratch

DIFFRACTION PATTERN

(a)

(b)

SCREEN

..4011010--(c)

(d)


signal-to-noise ratio lets discontinuities be detected in thearea of concern.

Image Processing and Pattern Recognition

Early surface inspection equipment was not equipped toclassify discontinuities but was concerned merely with thepresence of imperfections." If material under test con-tained many imperfections, some of them would be overde-tected, others would be missed, and the required detectionperformance would not be achieved. To solve this problem,image processing and parameter calculation features wereincorporated to classify discontinuities by combining param-eters. These additional features greatly improved inspectionperformance and were introduced into a variety of processlines. Figures 69 to 73 show image processing and patternrecognition methods for classifying discontinuities. Mean-while, developmental research is burgeoning to apply classi-fication of discontinuity types to neural networks.

FIGURE 69. Image processing and patternrecognition used in profile method of classifyingsurface discontinuities (classification parametersinclude discontinuity signal, length, width, numberof discontinuity spots, number of lineardiscontinuities and positions): (a) longitudinalprojections of discontinuity map and (b) profile ofdiscontinuity

(a)

(b)


TABLE 9. Optical systems for surface discontinuity inspection of cold strip

Light Optical SensorMethod Source System Characteristics

Flying spotOblique scanning I halogen lamp

laser beam (e g helium neon)

Flying image oblong light source(e.g., fluorescent lamp)

Silicon semiconductor

Spatial filter + photomultiplier

Two-dimensional imaging deviceCCD, ITV)

I. Detects minutediscontinuities

2. Overcomes limitation ofvelocity by using a polygonalmirror

3. Has complicated opticalsystem

1. Detect minute discontinuities2. Overcomes limitations of

velocity by using a polygonalmirror

3. Is highly sensitive by virtue ofuniformly intense reflection indirection of scanning

1. Uses electrical scanningwithout moving parts, forhigh speed operation

2. Has simple design3. Is limited to flaw detection

using scattering light

Oblique scanning II

laser beam (e.g., helium neon) Optical fiber + photomultiplierParallel scanning laser beam (e.g., helium neon) Optical rod + photomultiplier

Flatness Measurement of Hot RolledSteel33

Flatness measurement in hot rolling (of heavy plates andhot strips), unlike cold rolling, is characterized by manifestedflatness because of lack of tension or extremely low tension.

There are three main ways to measure malformation:(1) time series measurement of crosswise distribution ofsteel plate inclination; (2) time series measurement of cross-wise distribution of steel plate height; and (3) instantaneousmeasurement of height distribution of a given plane.

Time Series Measurement of Crosswise Distributionof Steel Plate Inclination

Using the steel plate as a minor, a television camera pho-tographs for signal processing a virtual image of an oblong(i.e., rod shaped) light source. The position of the virtualimage as it moves along the length of steel plate is adjusted tocompensate for the plate's inclination. Although the verticalmovements of the plate will not cause measurement errors,this method is susceptible to its surface properties because ituses a regular reflection technique. To overcome this diffi-culty devices have been incorporated, such as one thatdetects the brightest point of the image in signal processing.In this method, the steepness of steel plate is obtained beforethe ripple height of it is obtained as its integration (Fig. 74).

FIGURE 70. Image processing and patternrecognition used in the picture pyramid method ofclassifying surface discontinuities; templatematching using two-stage digital filter; classes ofdiscontinuities include scattered spots, crowdedgroups of spots, short lines, long lines andcontiguous nonlinear indications

SPREAD ED

LINEAR

SPOT

DIGITAL FILTER El

DIGITAL FLIER F2

FIORE ELEMENT 2 x 2 ram(0.08 x 0.08 in.J

FOURTH LAYER. CLASSIFICATION OF DISCONTINUITY TYPES BY TREE LOGIC I

GAGE

CRACK

FIGURE 71. Configuration and tree method ofclassifying discontinuity types

FIRST LAYER: SEPARATION OF CONFIGURATIONLENGTH (5 DIVISIONS) x WIDTH (5 DIVISIONSI = 25 TYPES

SECOND LAYER. SEPARATION BY DENSITY

THIRD LAYER: SEPARATION BY SHAPE

I INDEPENDENTLY LINEAR CROWDED COMPLEXED


Time Series Measurement of Crosswise Distributionof Steel Plate Height

Using a column of water as an electrical resistor (Fig. 75),the distance to a steel plate is obtained to calculate thecross-wise distribution of steel plate height; this distributionis then used to calculate differentially the steepness of theplate.34

A photodetector having a small field of view across thesteel plate (in the direction the beams move) is arranged nextto laser beams scanning parallel at a given speed v and at agiven angle 0 to the steel plate surface. As the height h ofthe surface changes according to the reference position andsurface irregularities, the position that the laser strikes willproduce a position displacement x in the horizontal directionby the principle of light section; accordingly the light receiv-ing timing of each photodetector changes by T. Displace-ment of steel plate can be calculated by the followingequations.

X = V • T (Eq. 1)

and

h = x • tan 0

(Eq. 2)

The laser light irradiating a projector is split into two beamsby a beam splitter, being formed into parallel scanning laserbeams by a polygonal mirror and a parabolic mirror, each ofwhich has 12 sides. The parallel scanning laser beamsilluminate the front and back surfaces. The light source is anionized argon laser and the detectors are photosensors, eachhaving 15 channels on its front and five channels on its back(Fig. 76).

FIGURE 72. Discontinuity classification by neuralnetwork

FIGURE 73. Calculating with weighting usingfeature parameter to select a representative kindof discontinuity (discontinuity group)

CALCULATION

4:).L = 24= r l • pfk,

j = I SCATTERED2 CROWDED3 LINEAR4 NONLINEAR.

CONTIGUOUS

dmax =max lljl

GRADING WTH WEIGHTING OF DISCONTINUITY TYPES

RECALCULATION OF di VALUES AT EVERY DISCONTINUITY TYPE

(1)' j = , r,•pu •kij

GRADE SELECTION USING FINAL= 1W)

LEGEND

= index for judgment = relement number of each flaw signal levelweighting value of each rsubscript describing flaw classification = psubscript for flaw signal level = kwetghfing value of a kind of flaw

d)

J

HCROWDED H

SPREADED

SPOT SCATTERED

r LINEAR

FIGURE 74. Flatness measurement system usingtelevision camera

TELEVISION CAMERA

LIGHTSOURCE

1RA THODEY TUBE

SIGNALPROCESSOR

HOT STRIP

FIGURE 75. Flatness measurement using water jetflow displacement detector

STRIP

TABLE ROLLER

SIGNALPROCESSOR

—6 OUTPUT VOLTAGE I


Instantaneous Measurement of Height Distribution

One technique instantaneously measures height distribu-tion in a given plane by using a moire fringe but this applica-tion is expensive and technically not easy.

Light Section Method Using LaserBeam33

The optical system in Fig. 77 comprises a 480 nm (1.9 x10 2 mil) ionized argon laser light source and detectors (infra-red region cutoff filter and high resolution television). Byusing the light section principle, the height h of a steel platesurface is obtained by horizontal displacement D from thereference position at which the light section plane intersectsthe steel plate surface. The equipment projects eight lightsection planes onto the steel plate, obtains three-dimen-sional coordinates at eight points along the length of plateand calculates the elongation by using polygons to approxi-mate it.

Surface MeasurementThe term surf-ace measurement covers an extensive range,

including gloss, reflectance, surface roughness and surfaceproperties related to roughness. Discussion here centers onthe measurement of surface roughness as well as on gloss andsurface properties.

Surface roughness of cold rolled steels is important inquality control because it determines glossiness, coatingproperties, plating properties and workability of steel plates.The optical technique described below has been proposed tomeasure 'surface roughness online (Fig. 78). 2 ' Equations 3to 5 hold with respect to the reflecting properties of irregularsurfaces, if a luminous flux of wavelength X. strikes a roughsurface at an incident angle

LASER BEAM PROJECTOR

SYNCHRONOUS %Not

FIGURE 76. Flatness measurement using light section method with laser beam: fa) principle and 113)system configurations

( b J

SENSOR UNIT

SIGNAL PROCESSOR

DISPLACEMENT].S.1` , ' I SIGNAL h,

•

DMSHAEMEM - STEEPNESSCOMMON COMPUrAllal

UNIT DISPLACEMENT um-T

SIGNAL h,

STEEPNESS

NIAT HEIGH TMMKRIAHON

UNIT

(al

PHOTOSE NS I NGPULSE

PHOTODETECTOR

COUNTER

CLOCK

SCANNING LASER [REAM.• •

.-• ...•-

S, rSTRIP SURFACEI

h SgEFERENCE PLANEJ

STRIP TRANSVERSEDIRECTION

I, TIM E

DISPLACEMENTSIGNAL

TO MONITOR

TO PROCESSCO/OVER

pEzcz11 11 11 11 11 11

FRONT UNIT

PARNIDUCMIRKY

I LASER I

BEAM SPUTTERPOLYGONALMIRROR

APPLICATIONS OF VISUAL AND OPTICAL TESTING IN THE METALS INDUSTRIES 1237

2ro-g (cosx2 01 + COS 0 2)2

and for g 1:

I, = e -g

and for g 1:

Is=f

Where:

g = intermediate variable;a- = variance of height distribution of surface;0, = incident angle;02 = reflective angle ( = 0, );k wavelength of luminous flux;T = self-correlation distance; and

= intensity of regular reflection.

Further, variance cr has the following relation with average(Eq. 3) roughness Ra :

I

cr = 1.25 Rr, (Eq. 6)

This surface roughness meter uses two light sources, eachhaving a unique incident angle, so as to calculate averageroughness R„ and self-correlation distance T by the abovebasic equation. The detector section, as shown in Fig. 78b,contains two helium-neon lasers provided as the lightsources: one is fixed to an incident angle of 75 degrees andthe other 10 degrees. Silicon photodiodes, provided as pho-tosensors, measure the distribution of reflection intensity foreach incident light by scanning a circular arc with its centerbeing the laser reflected point on the steel plate. The maxi-mum intensity and total reflection intensity are obtainedfrom the intensity distribution of each incident light, and avalue obtained from the division of the former by the latteris taken as I,. Average roughness R„ and self-correlationdistance T are calculated by the calibration curve based onthe empirical curve which has been obtained from the valueof I, for each incident angle. Experiments show that this sen-sor can measure surface roughness within -± 20 percent ofthe value of T.

(Eq. 4)

(Eq. 5)

FIGURE 77. Flatness measurement system withscanning laser displacement detector: (a) systemconfiguration; (b, c) measured results

2.500

0.4

0.3

02

O. i

00

500 1,000 1.500 2.000

WIDTH{rnfIllmeters)

0.4

0. 3

02

0.1

00 500 1.000 1,500 2,000 2,500

WIDTH(millimeters)

CENTER BUCKLES

(a)

A + LASERROTATING

MIRROR

TELEVISIONCAMERA

INTERFERENCEFILTER

{b)

lc)

zti C<zo

-w

F-

ADC

CRO-OMPUTE

INTERFACE

MOTOR DRIVER

INTERFACE

lorsPLAy lRECORDER

PROCESSCOMPUTER

STEP MOTOR

fa)


By selecting an appropriate service wavelength in a similaroptical system, the glossiness of steel surface can be mea-sured. The data processing, however, uses a liner discrimi-nant function because the glossiness is rated by organolepticexamination (Fig. 79).

In addition, a technique to measure metal surface proper-ties using a noncontact technique has been developed (seeFig. 80). In this technique, a 5 mW helium neon laser with1.2 mm (0.05 in.) spot irradiates a specimen at an incidentangle of 20 degrees; the scattered light projected on a screenis captured by a charge coupled device camera; and imageprocessing has been carried out on the area of scattered lightto obtain surface properties. 35

To improve sharpness and workability of steel plates pressmolded for vehicles, uneven micro patterns are lasermachined onto the rolls used for skin pass rolling." Duringthe processing of these rolls, still images of their surfaces aretaken by a charge coupled device, enlarged and imaged. Astroboscope and high speed rotating mirror are synchronizedwith the rolls being fed, after which the images are processedto calculate diameters of craters, pitches of craters andphase shift.

An offline, three-dimensional surface roughness analysissystem has been developed that uses a contact, three-dimen-sional surface roughness meter and a three-dimensionalcoordinate measuring unit (for swell analysis) as measuringequipment and a super minicomputer as host computer.

ColorimetersThe color of colored steel is a significant part of its com-

mercial value. In cold rolled and surface treated steels, too,the surface colors are an important consideration in qualitycontrol. This section describes one technique for online col-orimetiy used in color lines and in coating, drying andbaking.

FIGURE 78. Online gloss measuring system: (a) schematic diagram and (b) the optical system

300

I0 20

I 0

/arbitrary units)

FIGURE 79. Visual tests versus gloss measuringsystem with discrimination function

LEGEND0 GLOSSINESS 4V GLOSSINESS 30 GLOSSINESS 2• GLOSSINESS I

LINES OF DISCRIMINATION .G3 = IG2 = 1G1 = 0 9814. .

— 7)— ;— 7)

+ 0.98 II„o — 9.5)+ 2.45 — 0)+ 1.76 — 6.6)

FIGURE 80. Schematic diagram for surfacemeasuring system

LIGHT SOURCE IHe-Ne)

FILTER

SPECIMEN STAGE20 DEGREES

SCREEN


WORK-STATION

FIGURE 81. Optical system for measurement ofcolor: (a) optical system, (b} schematic diagram andRI results of measurement

SOO mm COLLIMATING LENS120 in)

GRATING_

4111111_ .

•E-m

fa)

COLLECTING SLITLENS MIRROR

AMPLIFIA

LIGHT SOURCE

32 CHANNELSPREAMPLIFIER

DETECTING UNIT

MAINNOLIFIER

ANALOG IT

alAKIER DiGITAL

4S ' qsDEGREE-L,DEGREES

SOO rryn ;20 in.I

MIWOMPUTER

fbiIMERFACE I

COLORED STEEL STRIP

i COLORED STEEL SIRE

IFECORDERI

AE( E3 )

LEGEND• SILVERA BROWNV BEIGE• SKY BLUE• WHITE

▪ GRAY• BRICK RED

= COLOR DIFFERENCE MEASURED BY THE INSTRUMENT BAE(NEW) = COLOR DIFFERENCE MEASURED BY THIS INSTRUMENT


The online color measuring system comprises the follow-ing: light sources that emit the light in a continuous spectrumtoward the object to be measured; a condenser lens systemthat converges the catoptric light on a 25 x 80 mm (1.0 x3.1 in.) field of view on the object to be measured;a reflecting plane grating that collimates the converged light;

detectors that simultaneously receive collimated light foreach monochromatic spectrum; a number of preamplifiersand main amplifiers that amplify the output from thedetectors on a monochromatic spectrum basis; and amicrocomputer that processes amplified signals and calcu-lates position coordinates X, Y, Z and color change 8E.

The system's configuration and components are shown inFig. 81.' 3 Two 650 W tungsten lamps serve as light sources,illuminating the test surface at an angle of 45 degrees in com-pliance with the geometry requirements stipulated in theapplicable standard for lighting and receptors. The catoptriclight converging along the normal from the center of the fieldof vision is collimated at regular wavelength intervals by

FILTER

FIGURE 79. Visual tests versus gloss measuringsystem with discrimination function

! PIOiarbitrary units)

LEGEND0 GLOSSINESS 4VGLOSSINESS 3

6 GLOSSINESS 2GLOSSINESS ILINES OF DISCRIMINATION:G3 = 134 111,, o – I + 0.98 (1„,, – 9_5;G2 = 1.75(1,., 0 – 11) + 245 11„, – E40;GI = 0 98p p ,, – 71 + 76 – 6.6)

C

dr7

FIGURE 80. Schematic diagram for surfacemeasuring system

LIGHT SOURCE [He-Nej

SPECIMEN STAGE20 DEGREES

SCREEN


MONITORIMAGE

PROCESSOR

WORK-STATION

FIGURE 81. Optical system for measurement ofcolor: la) optical system, f bischematic diagram and(c) results of measurement

0 1.0 150.5

LIGHT SOURCE

_ _4

RATING1/1116A

AMPLIFIER

500 mmITT in.1 COLLIMATING LENS

COLLECTING SLITLENS MIRROR

32 CHANNELSPREAMPUFIER

DETECTING UNIT

MAIN

AMPLIFIER

45 ! 55DEGREEk !,t3EGREES

910 mm I20 tn.;I COLORED STEEL STRIP

IRECOROCRI

I5

0.5

0

AE

i B )

COLORED STEEL STRIP- • -

(ai

lb] 1 IVIERFACE IANALOG TO h._

DOWCONVERTER

MIEROCOMFUTER

LEGENDO SILVERA BROWN3 BEIGE• SKY BLUE• WHITE▪ GRAY3 BRICK REDAE[13) = COLOR DIFFERENCE MEASURED BY THE INSTRUMENT B

COLOR DIFFERENCE MEASURED BY THIS INSTRUMENT


The online color measuring system comprises the follow-ing: light sources that emit the light in a continuous spectrumtoward the object to be measured; a condenser lens systemthat converges the catoptric light on a 25 X 80 mm (1.0 x3.1 in.) field of view on the object to be measured;a reflecting plane grating that collimates the converged light;

detectors that simultaneously receive collimated light foreach monochromatic spectnim; a number of preamplifiersand main amplifiers that amplify the output from thedetectors on a monochromatic spectrum basis; and amicrocomputer that processes amplified signals and calcu-lates position coordinates X, Y, Z and color change OE.

The system's configuration and components are shown inFig. 81.'3 Two 650 W tungsten lamps serve as light sources,illuminating the test surface at an angle of 45 degrees in com-pliance with the geometry requirements stipulated in theapplicable standard for lighting and receptors. The catoptriclight converging along the normal from the center of the fieldof vision is collimated at regular wavelength intervals by

FIGURE 83. Structure of surface puritymeasurement system

TENSIONREEL

FEEDINGREEL WHITE CLOTH BELT

DETECTING HEADNI

Nz.

STEEL STRIP

SENSOR

EWA "

EPA II, •

00• • III-

1

• mi• I

LIGHTSOURCE

•

• In •-ni

I\MOVABLEGUIDE

FIXED GUIDE

WIPING HEAD


lenses. A linear image sensor is used as a receptor. Thespectral wavelengths appropriate for practical use rangefrom 400 to 700 nm (1.5 x 10 2 to 2.8 x 102 mil). For mea-surement precision, the wavelength splitting width dividesthe spectrum into 31 sections of 10 nm (3.9 x 10 4 mil) each.Thirty-one spectral components after analog-to-digital con-version are transferred to the microcomputer, which calcu-lates and outputs the values of image components (L, a, b),(L*, a*, le), and M. The measurements are given inFig. 81c.

In addition, a technique has been developed in which aspectrophotometer is used to measure the spectralreflectance of the test surface.36 Before the color tone is digi-tized, tristimulus values are calculated to obtain the delta Efrom the reference color tone registered. In this technique,a pulsed xenon lamp emits light according to an externalinstruction for gatherings pectral data. Its wavelengthsrange from 360 to 740 nm (1.4 x 10 2 to 2.9 x 10' mil).Each measurement cycle comprises four emissions, twoemissions calibrating a white reference plate and the othertwo measuring the object.

Strip Surface Purity MeasuringEquipment3738

As the demand for steel products of high added valueincreases, the need for measurement of purity—that is, ofcleanliness—has also been rising in strip manufacturing.Strip surface purity measurement has so far been dependenton visual inspections or manual analysis. An offline quantita-tive evaluating technique and recent online measuringequipment have been introduced.

An offline measuring method' uses a light filtering sensorto calculate the diameters of particles and to count by voltagedrop the number of particles proportional to the area onwhich particles to be measured are projected (Fig. 82).Because particles to be measured include powdery iron,powdery rubber and dust, the sampler subjects the specimento pressurized air to ensure that particles of different specificgravities are measured. A shutoff valve or retractable parti-tion between the sampler and the sensor prevents heavy par-ticles, such as powdery iron, from passing the sensor beforebeing measured.

For online measurements, a purity meter that examinesstrip surfaces after washing has been developed (Fig. 83).The purity meter comprises (1) an automatic wiping unit thattransfers dirt on a strip surface to a white cloth by pushingthe cloth against the strip; (2) a sensor that optically mea-sures the dirt on the cloth; and (3) a signal processor.Table 10 indicates the basic specifications of the puritymeter. Features of this equipment are:

1. the wiper moving across the strip can measure thecrosswise purity profile;

2. before being measured, the purity profile within mea-suring range is averaged by analog area sensors that useone-chip silicon devices;

3. measurement is always made by comparison with thestandard plate, thus minimizing error from degradationof light source; and

4. an automatic inspection feature measures the whitecloth to determine zero point shift beforemeasurement.

FIGURE 82. Schematic diagram of particle counter

COMPRESSED AIR

r

SAMPLER

DRAIN

FIGURE 84. Optical inspection system

jiSPECIMEN

DEGREES' DEGREES

5 DEGREES

' 15 DEGREESLIGHT SOURCE

FIGURE 85. Stapleton measurement system fordistinctness of Image

FOCUSAPERTURE

LIGHTSOURCE

CONDENSERLENS

SENSOR SIGNALPROCESSOR

SPECIMEN

CONDENSER LENS

PLOTTER

FIGURE 86. Evaluation by waveform analysis ofstripe pattern images

PHOTO-DIODE ARRAY

LIGHT SOURCELIGHT INTENSITY

DISTRIBUTION

CONDENSER LENS

SLIT PATTERN

FOCUS LENS

it -k lbkb

SPECIMEN


TABLE 10. Specification of purity sensor

Items Specification

Detector type single silicon chip; area sensorLamp DC, 100 watt, white lightDetecting principle comparison with standard

sampleDetecting spot 413 x 30 mrn

Optional functions air purge; shutter in front ofcamera; auto zero check

Distinctness of Image of CoatedSurface'

Sharpness of image is closely related to gloss. The glossincludes specular gloss, contrast gloss, and distinctness ofimage. For purposes of this discussion, distinctness of imageis equivalent to sharpness.

Listed below are six groups of techniques for measuringthe distinctness of images: ,

1. techniques for reading images of characters of differentsizes, gloss box or gloss meter);

2. techniques using geometric patterns varying in size(Landholt ring, stripe patterns, and grid patterns);

3. evaluation by intensity of regular reflection vs. intensityof neighboring reflection (e.g., see Fig. 84);

4. evaluation by changes in light intensity at differentpoints in striated images created by stripe pattern ofparallel apertures (Fig. 85);

5. evaluation by contrast in stripe pattern images; and6. waveform analysis of stripe pattern images (Fig. 86).

ConclusionVisual tests are widely used for in-process and product

inspections throughout the steel industry. The human eye isunsurpassed as a sensor and human inspectors can instantlyrecognize patterns and characterize materials.

The drawbacks of human inspectors and especially theneed for faster throughput have together spurred on thedevelopment of unstaffed, computerized optical inspectionsystems. Advances in automated visual testing are expectedto continue into the indefinite future.


REFERENCES

1. "Properties and Selections of Metals." Metals Hand-book, eighth edition. Vol. 1. Taylor Lyman, ed. Materi-als Park, OH: ASM International (1961).

2. Handbook. J401. Warrendale, PA: Society of Automo-tive Engineers (1979): p 2.01-2.02.

3. "Properties and Selection: Iron and Steel." MetalsHandbook, ninth edition. Vol. 1. Hugh Baker, ed.Materials Park, OH: ASM International (1978).

4. Archer, R.S., J.Z. Briggs and C.M. Loeb, Jr. Molybde-num Steels, Irons, Alloys. New York, NY: HudsonPress (1948).

5. "Failure Analysis and Prevention." Metals Handbook,eighth edition. Vol. 10. Howard E. Boyer, ed. MaterialsPark, OH: ASM International (1975).

6. "Properties and Solution: Stainless Steel, Tool Materi-als and Special-Purpose Metals." Metals Handbook,ninth edition. Vol. 3. David Benjamin, ed. MaterialsPark, OH: ASM International (1980).

7. "Welding and Brazing." Metals Handbook, eighth edi-tion. Vol. 6. Taylor Lyman, ed. Materials Park, OH:ASM International (1971).

8. Rahinowicz, Ernest. "Wear." Scientific American.Vol. 206, No. 2. New York, NY: Scientific American(February 1962): p 127-135.

9. Baker, H.J., J. R. Lindsey and S.H. Weisbroth. TheLaboratory Rat: Research Applications. Vol. 2. NewYork, NY: Academic Press (1980): p 60.

10. Morin, C.R., K.F. Packer and J.E. Slater. Metallo-graphy in Failure Analysis. J.L. McCall and P.M.French, eds. Materials Park, OH: ASM International(1977): p 191-205.

H. Turasov, L.P. and C.O. Lundberg. "Nature and Detec-tion of Grinding Burn in Steel." Transactions. MaterialsPark, OH: ASM International (1949): p 893-939. Seealso: injury in Ground Surfaces: Detection, Causes,Prevention. Worcester, MA: Norton Company.

12. Sheehan, J.P., and M.A.H. Howes. "The Role of Sur-face Finish in Pitting Fatigue of Carburized Steel"(Paper 730580). Warrendale, PA: Society of Automo-tive Engineers (1973).

13. Sheehan, J.P. and M.A.H. Howes. "The Effect of CaseCarbon Content and Heat Treatment on the PittingFatigue of 8620 Steel" (Paper 720268). Warrendale,PA: Society of Automotive Engineers (1972).

14. Howes, M.A.H. and J.P. Sheehan. "The Effect of

Composition and Microstructure on the Pitting Fatigueof Carburized Steel Cases" (Paper 740222). Warren-dale, PA: Society of Automotive Engineers (1974).

15. Engine Bearing Service Manual, seventh edition.Detroit, MI: Federal Mogul Bearings (1956): p 93.

16. Pedersen, R. and S.L. Rice. "Case Crushing of Carbu-rized and Hardened Gears." SAE Transactions. Vol. 69.Warrendale, PA: Society of Automotive Engineers(1961): p 370-380.

17. Wulpi, Donald J. Understanding How ComponentsFail. Materials Park, OH: ASM International (1966).

18. Economic Effects of Metallic Corrosion in the UnitedStates. SP 511-1. Washington, DC: National Bureau ofStandards (May 1978).

19. Fontana, Mars. "Stress Corrosion." Corrosion. Lesson5. Materials Park, OH: ASM International (1968).

20. "Fractography and Atlas of Fractographs." MetalsHandbook, eighth edition. Vol. 9. Howard E. Boyer,ed. Materials Park, OH: ASM International (1974).

21. Asano, Yuichiro, et al. "Analysis of Light Reflectionfrom Cold-Rolled Steel Sheets and Its Application toOnline Measurement of Surface Roughness." ISIJInternational (in English) [formerly the Iron and SteelInstitute of Japan Transactions]. Vol. 70, No. 9. Tokyo,Japan: Iron and Steel Institute of Japan (1984):p 1,095.

22. Uchida, Hiroyuki, et al. "Development of the On-LineSurface Roughness Measuring Instrument." CAMP-/SIT Vol. 2 Tokyo, Japan: Iron and Steel Institute ofJapan (1989): p 508.

23. Azushima, Akira, et al. "On-Line Surface Measurementof Rolled Sheet by using Laser Beam with Image Proc-essor." CAMP-ISIJ. Vol. 4. Tokyo, Japan: Iron andSteel Institute of Japan (1991): p 1,376.

24. Uchida, Hiroyuki, et al. "Development of the PatternMeasuring Instrument in Laser Textured DullingOperation." CAMP-151j. Vol. 3. Tokyo, Japan: Iron andSteel Institute of Japan (1990): p 427.

25. Dejima, Katsuro, et al. "Surface Observation of Rotat-ing Roll in Finishing Mill." CAMP-154. Vol. 3. Tokyo,Japan: Iron and Steel Institute of Japan (1990):p 1,275.

26. Fukazawa, Chiald, et al. "Measuring Instruments forIron Steel Plants." Toshiba Review. Vol. 39, No. 5.Tokyo, Japan: Tokyo Shibaura Electric (1984): p 431.


27. Fukazawa, Chiaki, et al. "Knowledge-EngineeringApplication for Surface-Flaw Inspection System,TOSPECTRON." Toshiba Review. Vol. 44, No. 5.Tokyo, Japan: Tokyo Shibaura Electric (1989): p 413.

28. Iwai, Kunio, et al. "On-Line Inspection Techniques forSurface Defects of Hot Slabs." ISIJ International (inEnglish) [formerly the Iron and Steel Institute of JapanTransactions]. Vol. 70, No. 9. Tokyo, Japan: Iron andSteel Institute of Japan (1984): p 1,182.

29. Sugimoto, Takao, et al. "Development of SurfaceDefect Inspection System Using Detecting Self-Radia-tion Light for Large Section Steel Products." Transac-tions of the Society of Instrument and ControlEngineers. Vol. 27, No. 5. Tokyoto, Japan: Keisoku JidoSeigo Gakkai (1991): p 516.

30. Toyota, Toshio, et al. "Flatness Measurement of HotStrip by Laser Scanning Method." 'ski International(in English) [formerly the Iron and Steel Institute ofJapan Transactions]. Vol. 70, No. 9. Tokyo, Japan: Ironand Steel Institute of Japan (1984): p 1,071.

31. Nakano, Kimiaki, et al. "The Study of Neural Networkfor Surface Inspection of Cold Rolled Sheet." 7'. IEEEJapan. Vol. 111-D, No. 1, T. IEEE Japan (1991).

32. Onoe, Morio. "Non-Destructive Testing and ImageProcessing." ISIJ International (in English) [formerlythe Iron and Steel Institute of Japan Transactions].Vol. 70, No. 9. Tokyo, Japan: Iron and Steel Instituteof Japan (1984): p 1,000.

33. Kawaguchi, Kiyohiko, et al. "Development of PlateFlatness Meter Based on Light Section Method on HotRolling." ISIJ International (in English) [formerly theIron and Steel Institute of Japan Transactions]. Vol. 70,

No. 9. Tokyo, Japan: Iron and Steel Institute of Japan(1984): p 1,078.

34. Ebata, Sadao, et al. "Flatness Meter by using WaterCurrent Electric Resistance." Kawasaki Steel Giho.Vol. 10, No. 4. Tokyo, Japan: Kawasaki Steel Corpora-tion (1978): p 91.

35. Kubota, Naoji, et al. Three-Dimensional SurfaceRoughness Analysis System." CAMP-ISIJ. Vol. 1Tokyo, Japan: Iron and Steel Institute of Japan (1988)p583.

36. Torao, Akira, et al. "Apparatus for the On-Line Tele-scopic Color-Spectrophotometer." ISIJ International(in English) [formerly the Iron and Steel Institute ofJapan Transactions]. Vol. 70, No. 9. Tokyo, Japan: Ironand Steel Institute of Japan (1991): p 1,277.

37. Kawahara, Masahiro, Hitoahi Aizawa, et al. "Strip Sur-face Purity Measuring Equipment." CAMP-ISIJ. Vol. 3Tokyo, Japan: Iron and Steel Institute of Japan (1990)p 1,277.

38. Maegaki, Kenichi, et al. "Method of Surface CleannessEvaluation on Steel Strips." CAMP-ISIJ. Vol. 3 Tokyo,Japan: Iron and Steel Institute of Japan (1990) p 596.

39. Yokohari, Akira, et al. "Development of On-Line ColorDifferencemeter." CAMP-ISIJ. Vol. 2 Tokyo, Japan:Iron and Steel Institute of Japan (1989) p 1,454.

40. Morita, Misao. "Evaluation Method for Distinctness ofImage of Coated Surface." ISIJ International (inEnglish) [formerly the Iron and Steel Institute of JapanTransactions]. Vol. 77, No. 7. Tokyo, Japan: Iron andSteel Institute of Japan (1984): p 1,075.

SECTION 9

APPLICATIONS OF VISUAL ANDOPTICAL TESTS IN THE ELECTRICPOWER INDUSTRIESRichard Nademus, GPU Nuclear Corporation, Forked River, New Jersey (Part 2)James Hedtke, Nuclear Energy Services, Inc., Danbury, Connecticut (Part 2)

PART I AND PORTIONS OF PART 2 ADAPTED FROM EPRI LEARNING MODULES, © THE ELECTRIC POWER RESEARCH INSTITUTE. REPRINTED WITH PERMISSION.


INTRODUCTION

This section takes visual testing in nuclear power plants asthe focus of its discussion; however, the information may bemost valuable to individuals not involved in the nuclearindustry. The concepts arc applicable to coal, gas and oil firedplants, as well as to plants producing steam for the manufac-turing and transportation industries.

Related discussion may be found in the section on proce-dures and the section on standards.

FIGURE 1. The five basic joint types are: (a) butt,(Il) corner, (c) edge, (d) tee and (e) lap

(a)

(b)

(C)

(d)

(e)

FROM THE ELECTRIC POWER RESEARCH INSTITUTE. REPRINTED WITHPERMISSION.

FIGURE 2. Forces may be introduced at variouspoints and transmitted to different areasthroughout the various weld joints: (a) square,(b) single J, (c) single bevel, (d) single V cap,(e) double bevel, and (f) single U

(al 1

(b)

•••nn

(c) I I

(d) 1 Na(e1

=1 /


APPLICATIONS OF VISUAL AND OPTICAL TESTS IN THE ELECTRIC POWER INDUSTRIES / 247

PART 1

JOINING PROCESSES

To perform accurate visual tests of metallurgical joints, thefollowing general information should be known: (1) jointconfigurations, (2) welding processes, (3) weld joint termi-nology and (4) the fabrication process.

Even if the visual inspector may only be examining thefinal welds, a more thorough test can be done by understand-ing how a weld is made. This understanding is especially val-uable if the visual inspector is involved with weld repair orreplacement.

Metallurgical Joint ConfigurationsThe geometry of metal joints is determined by the

requirements of their host Structure. The five basic jointtypes are the butt, corner, edge, tee and lap joints (seeFig. 1).

The purpose of a weld joint is to transfer forces from onemember to the other through the joint. The forces may beintroduced at various points and transmitted to differentareas throughout the weld joint (see Fig. 2). The amount ofstress transferred across the joint depends on the type ofloading and the service of the weldment. These factors affectjoint preparation designs, which are typically classified as:(1) complete joint penetration and {2) partial joint penetra-tion.

Complete Penetration

Complete joint penetration (see Fig. 3) may be defined asjoint penetration in which the weld metal completely fills thegroove and is fused to the base metal throughout itsthickness.

Assuming that the weld metal strength is equal to orgreater than that of the base metal, which is nearly always the

FIGURE 3. In full penetration joints, the weld metalcompletely fills the groove and is fused to the basemetal throughout its thickness: (a) double V and(b) single V

(a) 1111111g


FIGURE 4. Partial joint penetration in (a) singleV butt joint and (b) double V butt joint


(a)

(b)


case, the weld joint in Fig. 3 could be considered 100 percentefficient. That is, for purposes of loading, the member couldbe considered a uniform solid structure. When weld jointsare subject to static loads as Well as dynamic, reversing orimpact loads at varying temperatures, complete joint pene-tration is usually required.

Partial Penetration

Partial joint penetration (see Fig. 4) is designed to have anunfused area and the weld does not completely penetrate thejoint. The load rating or joint efficiency is based on thepercentage of the weld metal depth to the total joint depth.Partial penetration joints are reliable for specific loads andparticular service environments.

Basic Welding ProcessesAccording to the American Welding Society, welding is a

materials joining process which produces coalescence ofmaterials by heating them to suitable temperatures, with orwithout the application of pressure or by the application ofpressure alone and with or without the use of filler metal. Theprocesses shown in Fig. 5 are grouped according to themeans of energy transfer. A secondary consideration is theinfluence of capillary attraction in distribution of filler metalin the joint.

Metallurgical joints are typically formed by: (1) soldering,(2) brazing, (3) oxyfuel gas welding, (4) resistance weldingand (5) arc welding. Some other processes, including diffu-sion bonding and electron beam welding, are widely used incertain industries. There are also metallurgical joints that aremechanical in character (bolted or riveted connections, forinstance).

Soldering

Soldering joins materials by heating them to a suitabletemperature and by using filler metal with a liquid statebelow 450 °C (840 °F) and below the solid state threshold,or solidus, of the base metals. The filler metal is distributedbetween the closely fitted faying surfaces of the joint by cap-illary action. Solder is normally a nonferrous alloy used toaccelerate wetting and to remove oxides. Many metals can besoldered, including aluminum, copper base alloys, nickelbase alloys, steel and stainless steel. The mechanism for sol-dering involves three closely related phenomena: (1) wet-ting, (2) alloying and (3) capillary action.

Wetting is the bonding or spreading of a liquid filler metalor flux on a solid base metal. When molten solder leaves acontinuous permanent film on the surface of a base metal, itis said to wet the surface. Wetting occurs when there is astronger attraction between certain atoms of the solder andthe base metal than between the atoms of the solder. Wettingis essentially a chemical process.

The ability of a solder to alloy with the base metal isrelated to its ability to wet the surface. Alloying also is depen-dent on the cleanliness of the base metal—there must beintimate contact between the solder and the base metal foralloying to occur at the interface.

The fluidity of the molten solder must be such that it canflow into narrow spaces by capillary action. Fluidity is theproperty that influences the spreading of the solder over thebase metal surface.

The strength of a soldered joint depends on the jointdesign and its clearance. Heating of the joint and filler metalcan he accomplished by a number of methods including dip,furnace, induction, infrared, iron, resistance, torch and wavesoldering.

AHWBMAWCAWCAW-GCAW-SCAW-TEGWFCAW

ATOMIC HYDROGEN WELDINGBARE METAL ARC WELDINGCARBON ARC WELDING

GASSHIELDEDTWIN

ELECTRoGas WELDINGFLUX CORED ARC WELDING

FLASH WELDINGHIGH FREQUENCY RESISTANCE WELDINGPERCUSSION WELDINGPROJECTION WELDINGRESISTANCE SEAM WELDINGRESISTANCE SPOT WELDINGUPSET WELDING

FIRHFRWPEWRPWREEWRSWUW

ABSRDEBDBFLBFBIBIRORBTBTCAB

ARC BRAZINGBLOCK BRAZINGDIFFUSION BRAZINGDIP BRAZINGFLOW BRAZINGFURNACE BRAZINGINDUCTION BRAZINGINFRARED BRAZINGRESISTANCE BRAZINGTORCH BRAZINGTWIN CARBON ARC BRAZING

EBWEBW-HvEETW-MVEBW-NVESWFLOWLE

TW

ELECTRON BEAM WELDINGHIGH VACUUMMEDIUM VACUUMNONVACUUM

ELECTROSLAG WELDINGFLOW WELDINGINDUCTION WELDINGLASER BEAM WELDINGTHERMIT WELDING

MWCAW-CHWPGW

AIR ACETYLENE WELDINGOXYACETYLENE WELDINGOxYHYD,OGEN WELDINGPRESSURE GAS WET DING

FOCPOCOFCDEC -AOFC•iiOFC-NOFC•PAOCLOC

CHEMICAL FLUX CUTTINGMETAL POWDER CUTTINGOXYFUEL GAS CUTTING

OXYACETYLENE CUTTINGOXYHYDROGEN CUTTINGOXYNATURAL GAS CUTTINGOXYPROPANE CUTTING

OXYGEN ARC CUTTINGOXYGEN LANCE CUTTING

MCCACGMACGTACMACPACSMAC

AIR CARBON MC CUTTINGCARBON ARC CUTTINGGAS METAL ARC CUTTINGGAS TUNGSTEN ARC CUTTINGMETAL ARC CUTTINGPLASMA ARC CUTTINGSHIELDED METAL ARC CUTTING

FIGURE 5. Welding, allied processes and their abbreviations

COEXTRUSIONWEWING CEWCOLD WELDING CU/DIFFUSION WELDING DEWEXPLOSION WELDINGFORGE WELDING FOWFRICTION WELDING FEWHOT PRESSURE WELDING HEWROLL WELDING ROWULTRASONIC WELDING USW

DIP SOLDERING DSFURNACE SOLDERING FSINDUCTION SOLDERING ISINFRARED SOLDERING IRSIRON SOLDERING INSRESISTANCE SOLDERING RSTORCH SOLDERING TSWAVE SOLDERING WS

ELECTRIC ARC SPRAYINGFLAME SPRAYINGPLASMA SPRAYING

GAS METAL ARC WELDINGPULSED ARCSHORT CIRCUITING ARC

GAS TUNGSTEN ARC WELDINGPULSED ARC

PLASMA ARC WELDINGSHIELDED METAL ARC WELDINGSTUD ARC WELDINGSUBMERGED ARC WELDING

SERIES

GMAWGMAW-PGMAW-SGTAWGTAW-PPAWSMAWSWSAWSAW-S

I

ELECTRON BEAM CUTTINGLASER BEAM CUTTING

EBC ISOMETIMES A WELDING PROCESS


FROM THE ELECTRIC POWER RESEARCH INSTITUTE. REPRINTED WITH PERMISSION.

Brazing

Brazing joins materials by heating them to a suitable tem-perature and by using a filler metal with a liquidus above450 °C (840 °F) and below the solidus of the base metal. Thefiller metal is distributed between the closely fitted faying

surfaces of the joint by capillary action. This is considered ametallurgical joint because the members are held togetherby the adhesion of the filler metal to the joint surfaces. Mostmetals can be joined by brazing. Filler metals commonlyused for low carbon and low alloy steels are silver alloys andcopper zinc alloys.

FIGURE 6. Diagram of a shielded metal arc weldingsetup, where the electrode and the work piece arepart of the electrical circuit

ELECTRODE HOLDERKM/ER SOURCE

(')

0

ELECTRODE LEAD

BASE METALWORK LEAD



The selection of the filler metal, the nominal alloy compo-sition and the melting and brazing temperature dependmainly on the joint design and the method of assembly. Heat-ing of the joint and filler metal can be accomplished by a gasfurnace, induction heating, resistance heating, infrared heat-ing or immersion into molten salt. Fluxes and inert atmo-spheres must be used to prevent surface oxidation and toensure wetting action. Surface preparation and precleaningare also important in this process.

Oxyfuel Gas Welding

Oxyfuel gas welding joins materials by heating with an oxy-fuel gas flame with or without the application of pressure andwith or without the use of filler metal. The process involvesthe melting of the base metal and a filler metal, if used, bymeans of a welding torch. This process normally uses acety-lene as the fuel gas and uses pure oxygen instead of air. Mol-ten metal from the base material edges and filler metalintermix in a common pool and coalesce to form a continuousmaterial.

An advantage of this welding process is that the rate ofheat input can be controlled, as is the temperature of theweld zone. Because the filler metal is added independentlyof the heat source, weld bead size and shape are also con-trolled. Oxyfuel gas welding is ideally suited for thin sheet,tubes and small diameter pipes, as well as for repair welding.

Resistance Welding

Resistance welding joins metals with the heat obtainedfrom resistance of the metal to electric current and by theapplication of pressure.

In both spot and seam welds, a nugget (the weld metal) isproduced at the electrode site. Seam welding is a variation ofspot welding in which a series of overlapping nuggets is pro-duced to obtain a continuous seam. Projection welding is asimilar procedure, except that the nugget location is deter-mined by a projection on one closely fit (faying) surface or bythe intersection of parts. These resistance processes can beused to make corner, tee, edge and the common lap joints.

Flash, upset and percussion welding are also resistancewelding techniques. These techniques can be used to pro-duce a butt joint between components with similar cross sec-tions by making a weld simultaneously across the entire jointwithout the addition of filler metal. A force is applied before,during or after the heat energy is applied to bring the partsinto intimate contact. These three processes are distin-guished by the method of heating and the time of the forceapplication.

Arc Welding

Arc welding joins metals by heating them with an electricarc, with or without pressure and with or without filler metal.For all forms of arc welding, a means of shielding the arc isneeded to block out harmful elements found in theatmosphere.

The work piece serves as one electrode of a circuit. Theother electrode can be a consumable or a nonconsumablematerial. The electric arc is generated between these twoelectrodes. Nonconsumable electrodes do not melt in the arcand filler metal is not transferred across the arc. Weldingprocesses that use a nonconsumable electrode are carbon arcwelding (CAW), plasma arc welding (PAW) and gas tungstenarc welding (GTAW). Consumable electrodes melt in the arcand are transferred across the arc to become deposited fillermetal. Welding processes that use consumable electrodesare shielded metal arc welding (SMAW), gas metal arc weld-ing (GMAW), flux cored arc welding (FCAW) and sub-merged arc welding (SAW).

Shielded Metal Arc Welding

Shielded metal arc welding joins metals by heating themwith an arc between the work piece and a covered metal elec-trode. Shielding is obtained from decomposition of the elec-trode covering. Pressure is not used and filler metal isobtained from the electrode.

This process, sometimes called stick welding, is a manualprocess, which accounts for its high versatility. Shieldedmetal arc welding uses the heat of the arc to melt the basemetal and the tip of a consumable flux covered electrode.The electrode and the work piece are part of the electricalcircuit (see Fig. 6).

This circuit includes a power supply with controls, weldingcables, an electrode holder and an arc welding electrode.One of the cables from the power supply is attached to the

FIGURE 8. Diagram of a gas tungsten arc weldingsetup

TORCHFILLER META!

1'tpnn

y/n/".

BASE METAL

FOOT RHEOSTAT

INERT POWERGAS SUPPLY SOURCE

fr 0 0 0

000•• n:Pci"

11114111I

0!— WATERINLET

1WATERDRAIN GAS

WORK LEAD ELECTRODE I SAD



work piece and the other is connected to the electrodeholder.

When an arc is struck between the electrode tip and thework piece, the intense heat of the are melts the electrode tipas well as the work piece beneath the arc. Tiny globules ofmolten metal rapidly form on the tip of the electrode and arethen transferred across the arc into the molten weld pool. Inthis manner, filler metal is deposited and the electrode isconsumed. The arc is one of the hottest commercial heatsources, with temperatures above 5,000 °C (9,000 °F) at itscenter. Melting takes place almost instantaneously when thearc contacts the metal. If welds are made in the horizontalposition, the metal transfer is induced by gravity, gas expan-sion, electric and electromagnetic forces and surface tension.For welds in other positions, gravity works against the otherforces.

Shielded metal arc welding (see Fig. 7) is one of the mostwidely used welding processes for the following reasons:

1. the equipment is relatively simple, inexpensive andportable;

2. the shielding characteristics of the electrodes make theprocess less sensitive to drafts than some otherprocesses;

3. the position of welding is limited only by the size andtype of electrode;

4. the process is suitable for most of the commonly usedmetals and alloys; and

5. it is the most tolerant arc welding process in regard toundesirable fit up conditions, such as wide rootopenings.

One disadvantage is that slag removal is required beforeperforming shielded metal arc welding.

Gas Tungsten Arc Welding

Gas tungsten arc welding joins metals by heating themwith an arc between a tungsten, nonconsumable electrodeand the work piece. Shielding is obtained from gas, pressuremay or may not be used and filler metal may or may not beused.

This process, sometimes called tungsten inert gas (TIG)welding, can be manual, semiautomatic machine or auto-matic (see Fig. 8).

The equipment needed for gas tungsten arc welding arethe welding machine, the welding electrode holder, the tung-sten electrode, the shielding gas supply and controls. Severaloptional accessories are available, including a foot rheostatwhich permits the welder to initiate or extinguish the arc andcontrol the current while welding. Water circulating systemsto cool the electrode holder, arc pulsers and wire feed sys-tems are a few of the other accessories available.

Pure tungsten electrodes (99.5 percent) are less expensiveand are generally used on less critical operations than tung-sten alloy electrodes containing thorium or zirconium. Apure tungsten electrode has a relatively low current carryingcapacity with alternating current power and a low resistanceto contamination. Tungsten electrodes with 1 or 2 percentthorium have some advantages over pure tungsten elec-trodes, including higher electron emissivity, better currentcarrying capacity, longer life, greater resistance to contami-nation, easier arc initiation and a more stable arc.

The electric arc is produced by the passage of currentthrough the ionized inert shielding gas. To prevent oxidiza-tion, the heated weld zone, the molten metal and the tung-sten electrode are shielded from the atmosphere by a blanketof inert gas fed through the electrode holder. The inertshielding gas is usually helium, argon or a mixture of the two.Figure 9 shows the positions of the gas tungsten arc welding

FIGURE 7. Components of the shielded metal arcwelding process

PROTECTIVE GAS

FROM ELECTRODE COATING

ELECTRODECOATING

SLAG MOLTEN -% -- /-"` ELECTRODE WIRE

WELD METAL ARC ,

imgpnw.446v=4.44,4".3L....4k ,) METAL DROPLETSSOLIDIFIED METAL

BASE METAL


FIGURE 10. Diagram of setup for gas metal arcwelding (GMAW), also called metal inert gas(MIG) welding

GAS OUT

HANDHELD \GUN

CONTROL1GUN

FEED CONTROL

I tifliji

I 7

WIRE SPOOLSHIELDING GAS SOURCE

VOLTAGECONTROL

0 04'B

Z.- POWERSOURCE


FIGURE 11. Components of a gas metal arc weld

NOZZLE

MOLTENWELD METAL SHIELDING GASii

SOLIDIFIED I"----'s BASE METAL

WELD METAL METAL , , ELECTRODE.a111116 DROPLETS iii\A

I /.



FIGURE 9. Components of the gas tungsten arcwelding IGTAW) process, also called tungsten inertgas (TIG) welding

DIRECTION OF TRAVEL

WELDING TORCH

SHIELDING GAS TUNGSTEN ELECTRODEMOLTEN WELD METAL., ARC

SOLIDIFIED

FILLER RODWELD METAL -

BASE METAL


torch, the arc, the tungsten electrode, the shielding gas andthe filler metal.

Because of its excellent control of heat input, gas tungstenarc welding is very good for joining thin base materials, smalldiameter pipe, tubing and root passes of piping for criticalapplications. Because the electrode is nonconsumable, theprocess can be used to weld by fusion alone or with the addi-tion of filler metal in the form of inserts or wire. It can beused on almost all metals and is especially useful for joiningaluminum and magnesium which form refractory oxides.The process lends itself to high quality welding but removalof surface contaminants before welding is very important.Some limitations of gas tungsten arc welding include:

1, speed slower than consumable electrode arc weldingprocesses;

2. possible transfer of tungsten from the electrode to theweld causing contamination (the resulting tungsteninclusion is hard and brittle);

3. exposure of the hot filler rod to air causing weld metalcontamination;

4. additional costs of inert shielding gases and tungstenelectrodes; and

5. higher equipment costs.

Gas Metal Arc WeldingGas metal arc welding joins metals by heating them with

an arc between a consumable electrode and the work piece.Shielding is achieved using an externally supplied gas. Gasmetal arc welding can be semiautomatic or automatic. Theprocess is used particularly where high production quantitiesare needed. Important metals such as carbon steel, alumi-num and copper can be welded with this process in all posi-tions by proper choice of shielding gas, electrode andwelding parameters.

The filler metal can be transferred from the electrode tothe work piece in two ways: (1) short circuiting transfer, inwhich the electrode contacts the molten weld pool andestablishes a short circuit; or (2) drop transfer, in which dis-crete drops are moved across the arc gap under the influenceof gravity or electromagnetic forces. Drop transfer can beglobular or a spray. The type of transfer is determined by thetype and magnitude of the welding current, the current den-sity, the electrode composition and the shielding gas.

Equipment used for gas metal arc welding includes awelding gun, a power supply, a shielding gas supply and awire drive system that pulls the wire electrode from a spooland pushes it through a welding gun. After passing throughthe gun, the wire becomes energized by contact with a cop-per contact tube, which transfers current from a powersource to the arc (see Figs. 10 and 11).

High quality welds are produced by this process whenproper welding procedures are used. The absence of flux orelectrode covering eliminates slag inclusions in the weld.Some dross formation may occur when highly deoxidizedsteel electrodes are used and it should be removed before the

FIGURE 12. Terms used for describing the featuresof fillet welds

ACTUAL THROAT THEORETICAL THROAT

CROWN OR FACE

TOE

ROOTL to

TOE


FIGURE 13. Components of a concave fillet weld

LEGACTUAL THROAT AND

EFFECTIVE THROAT

CONCAVITYLEG

SIZE

THEORETICAL THROAT


FIGURE 14. Components of a convex fillet weldCONVEXITY

LEG AND SIZEACTUAL THROAT

EFFECTIVE THROAT

THEORETICALTHROAT

LEG AND SIZE



next weld bead or pass is made. The inert gas shielding pro-vides excellent protection of the weld area from oxygen andnitrogen contamination. Hydrogen is virtually eliminated asa concern in the weld and heat affected zones of low alloysteels. On the other hand, the process permits low cost weld-ing of carbon steels with the use of inexpensive CO, gasshielding.

One of the chief advantages of gas metal arc welding isthat, in general, it does not require the degree of operatorskill essential to shielded metal arc welding or gas tungstenarc welding. Other advantages include: (1) high depositionrates, (2) good use of filler metal, (3) no slag and flux removal,(4) reduction of smoke and fumes and (5) versatility.

Fabrication ProcessFor a visual inspector to fully understand a metallurgical

joint and to effectively communicate test results, welding ter-minology and symbols must be used. Figures 12 to 16 showthe terms commonly used for describing the features ofwelds.

In making a welded joint, the various configurations mustfirst be fabricated and suitably prepared. In most instances,it is also necessary to fit up or hold one or more joint mem-bers in place with special external fixturing to prevent move-ment during welding. The melting and refusing of base metaland filler metal is usually done so that refused metal developsas a bead along the long dimension of the weld (see thestringer bead in Fig. 17).

Laying down a longitudinal bead along the entire weldlength is called a pass. The weld depicted in Fig. 17 is calleda multipass weld. In those welding techniques using flux, slag

removal is required after each pass or after each stop, if weld-ing is interrupted during a pass.

Because there is usually a depression (crater) at the end ofa weld bead, the welder must take special measures to ensurefilling this crater when restarting the head after a stop. Thewelder must also avoid melting a groove in the base metalnear the toe of the weld. This undesirable groove is calledundercut (see discussion below of weld joint discontinuities).

Visual Tests of Metal JointsThe text below is p rovided for educational purposes and in

no way represents actual visual testing requirements for anyapplication. The information has been adapted to the style ofthe NDT Handbook, regardless of the language of the sourcedocuments. For example, as presented here, SI units in deci-mal forrn always precede other units of measure, despite theoriginal measurement formats. The information below is pre-sented with enough detail to he helpful but such detail maychange with code revisions. All visual testing must be done

FIGURE 17. A stringer bead weld pattern



FIGURE 15. Components of typical welds

LEGENDI ROOT OPENING: THE SEPARATION BETWEEN THE MEMBERS TO BE JOINED

AT THE ROOT OF THE JOINT (SOMETIMES CALLED THE GAP)2. ROOT FACE: GROOVE FACE ADJACENT TO THE ROOT OF THE JOINT (ALSO

CALLED THE LAND)3. GROOVE FACE: THE SURFACE OF A MEMBER INCLUDED IN THE GROOVE

(ALSO CALLED THE BEVEL FACE)4. BEVEL ANGLE: THE ANGLE FORMED BETWEEN THE PREPARED EDGE OF A

MEMBER AND A PLANE PERPENDICULAR TO THE SURFACE OF THE MEMBER5. GROOVE ANGLE: THE TOTAL INCLUDEDANGLE OF THE GROOVE BETWEEN

PARTS TO BE JOINED BY A GROOVE WELD (ALSO CALLED THE INCLUDEDANGLE)

6 SIZE OF WELD: THE JOINT PENETRATION (DEPTH OF CHAMFERING PLUSROOT PENETRATION WHEN SPECIFIED)

7 PLATE THICKNESS: THICKNESS OF WELDED PLATE8 COUNTERBORE: BORING OF THE PIPE INSIDE DIAMETER TO CORRECT FOR

OUT 9F-ROUNDNESS CAUSED DURING MANUFACTURE


FIGURE 16. Common weld terminology

CROWNREINFORCEMENT

CROWN OR FACE

ROOTREINFORCEMENT


according to the codes or standards that are in force, usingspecified revisions of or addenda to the documents.

Visual tests of welds are normally performed before thestart of fabrication, in process, after completion of fabrica-tion, periodically inservice and after repair of discontinuitiesfound during the course of inservice tests. Exact frequenciesof inservice tests are governed by the applicable code.Although inservice inspection may not specifically call forvisual examination, a preliminary visual test under suitableconditions before required surface or volumetric tests candetect serious material discontinuities.

The text below discusses typical requirements for visualtests, the inspector's role in the examination process andsome methods for determining the quality of welded joints.

Before performing an inservice visual test, the inspectorneeds to verify that the following requirements have beenmet: (1) personnel qualifications, (2) testing procedures,(3) drawings, (4) equipment, (5) surface preparation and(6) test conditions.

Personnel Qualification

Typically, the applicable code is consulted to determinerequirements for personnel qualification and vision acuityFor welding inspections done under the American Society ofMechanical Engineers' guidelines (ASME Section XI),requirements are partially given by the International Weld-ing Association in IWA-2300:

1. training and qualification to comparable levels of com-petency as those given by the American Society forNondestructive Testing in Recommended PracticeSNT-TC- IA;

2. personnel qualification comparable to that given bythe American National Standards Institute in ANSIN45.2.6;

3. an owner established qualification program based oneducation, experience, training, testing and evaluation;

4. vision requirements such that natural or corrected neardistance vision acuity is equivalent to the ability to readJaeger Number 1 text at a specified distance such as350 mm (14 in.) or 400 mm (16 in.); natural or cor-rected far distance vision acuity equivalent to a Snellenexamination at 4.6 m (15 ft); and a color vision examina-tion to determine the ability to distinguish colors anddifferentiate contrasts; and

5. recertification every three years based on retrainingand retesting.

FIGURE 18. Typical welding reference standard

DUPLICATE FILLETWELD ON THIS SIDE

POLISH AND ETCHTHIS SURFACE

MACRO SAMPLE

mm II In.)MINIMUM

MACRO SAMPLE UPRIGHTTO PLATE WITH ETCHEDSURFACE OUT

50 mm12 MINIMUM

200 mm (8 in.} MINIMUM


FIGURE 19. Typical weld gages

(al1/13

5f8

(bJ

111

JdJ

[e l


APPLICATIONS OF VISUAL AND OPTICAL TESTS IN THE ELECTRIC POWER INDUSTRIES 1 255

Procedures

Procedures used in visual testing of welds are often gov-erned by certain codes. The ASME Boiler and Pressure Ves-sel Code and the American Welding Society's AWS D1.1Code are commonly used. A written procedure based on theapplicable code should be prepared by the organizationresponsible for performing the visual test. The procedureshould include:

1. visual test performance guidelines;2. type of surface condition;3. method of surface preparation and tools used;4. type of viewing (direct or remote);5. special illumination, instruments or equipment;6. sequence of the test;7. data to be tabulated; and8. report forms or statements needed.

A copy of the visual test procedure should be referred toat test site by the inspector. A reference standard can be avaluable tool for judging preweld fit up, welding techniqueand completed weld quality (see Fig. 18).

Drawings

A thorough understanding of all drawing requirements isnecessary before a visual test begins. The inspector is notonly responsible for detecting surface discontinuities in thecompleted weld but also must determine if all weld size andcontour requirements are met. Included is the responsibilityfor ensuring that the full extent of welding specified in thedrawing has been performed. These tasks require a thoroughknowledge of welding terminology and symbols.

Equipment

Basic tools for direct visual testing include: (1) artificiallight source, (2) mirrors, (3) magnifiers, (4) straight edges orrules and (5) weld gages (see Fig. 19).

Surface Preparation

The test surface should be free of slag, dirt, grease, weldspatter or other contaminants that might make it obscure tothe unaided eye. Surface preparation may also include thosesteps needed for valid interpretation of subsequent nonde-structive tests.

The test area normally consists of 100 percent of thereadily accessible exposed surfaces of the test object, includ-ing the entire weld crown surface at a specified distance suchas 25 mm (1 in.) of the adjacent base metal.

Test Conditions

Direct visual tests may be used when there is access to thearea of interest without personal injury and the unaided eye


may be placed within 600 mm (24 in.) of the test surface atan angle no less than 30 degrees. Mirrors can be used toimprove the angle of vision.

Natural or artificial lighting of sufficient intensity andplacement is needed to illuminate the test areas and to allowproper reading of weld gages and other equipment. TheBoiler and Pressure Vessel Code Section XI (IWA-2211)states that visual test resolution is considered adequatewhen the inspector, by combination of access, lighting andangles of vision (either unaided or corrected), can resolve a0.75 mm (0.03 in.) wide black line on an 18 percent neutralgray card placed on the test surface.

Remote visual test systems may be needed if access isimpaired or if personal injury could result from direct visualtesting.

Testing Procedure

The following are some of the issues an inspector shouldaddress during a visual test before welding.

I. Verify welding procedure qualification status.2. Verify welder or welding operator qualification status

and limits of qualification, including limits of base andwelding material type, limits of base material thicknessand diameter limits, limits of welding position and cur-rency of qualification date.

3. Verify that lighting on the test surface is sufficient.4. Verify identification of base material and visually exam-

ine joint preparation. This should include: base mate-rial (including hacking ring, if used) type compatiblewith the detailed weld procedure; weld being made inaccordance with the drawing; weld preparation freefrom base material discontinuities such as laminations,laps, nonmetallic inclusions or pinhole porosity.

5. Verify that the weld preparation geometry is to thedimensions required on the joint design drawings. Thisshould include: alignment of parts to be welded; size ofroot face (land) and root gap; groove angle; identifica-tion mismatch of butt joints; clearance of backing rings,strips or consumable inserts.

6. Verify that the conditions of the general welding proce-dure and the conditions of the detailed welding proce-dure are followed.

7. Verify that tack welds are completely removed or pre-pared for incorporation into the final weld.

8. Visually examine prepared tack welds for discon-tinuities.

9. Visually examine inside of piping (when applicable) forcleanliness.

During welding, the visual inspector should do the followingthings.

1. Verify preheat temperature as required by the detailedwelding procedure.

2. Visually inspect for cleanliness, weld spatter, slag andoxide removal between passes.

3. Visually inspect for discontinuities in surfaces of weldbeads and side walls of preparation.

4. Verify interpass temperatures as required by thedetailed weld procedure.

5. Verify that amperage and voltage specifications whereappropriate are being met.

After welding, the inspector should do the following things.

1. Verify postheat temperature as required by the detailedwelding procedure.

2. Visually inspect the finished weld for adequacy of thedimensions required in the test procedure, including:leg size, throat and profile of fillet welds; root concavityand convexity where possible; weld reinforcement;transition of weld metal for thick to thin sections;acceptable weld slope for weld joint offsets.

3. Visually inspect the weld surface for discontinuities andworkmanship.

4. Verify the removal of fitup lugs and any other tempo-rary attachments and the proper preparation of theaffected base metal.

Visual Testing for Weld DiscontinuitiesThe purpose of visual weld testing is to identify critical

surface discontinuities at a point in the fabrication processwhen repair is still possible. The principal tools needed bythe visual inspector are training, good vision acuity and theability to distinguish relevant discontinuities.

Weld Joint Configuration Discontinuities

Configuration discontinuities occur when the weld is notwithin the specified size or the shape required by the govern-ing documents. This condition includes butt joint reinforce-ment (Fig. 20), where weld metal extends above the basemetal.

Most codes closely control butt joint reinforcementheight. Many weld gages can be used for measurement ofreinforcement height. One leg of the gage is placed on thebase metal and the other is moved to touch the tip of thereinforcement. The reinforcement height is then read off asliding scale. To comply with most acceptance standards, thegage should be read to 7.5 mm (0.3 in.). Most inspectors areinterested in adequacy, not actual dimensions. In practice, itis convenient to set the gage to the allowable height and then

FIGURE 20. Butt joint reinforcement

FACE REINFORCEMEN f

l a i

ROOT REINFORCEMENT

FACE REINFORCEMENT

(b)

ROOT REINFORCEMEN I


FIGURE 21. Deter"` nation of equal leg fillet weldsize, where size = a: (a) convex fillet weld and(b) concave fillet weld


a)

(b)


use it as a go/no-go gage. If the gage clears the reinforce-ment, fine. If it hits, then the reinforcement is excessive.

For an equal leg fillet weld, size is the leg length of thelargest isosceles right triangle that can be inscribed withinthe weld cross section (see Fig. 21). The size of an unequalleg fillet weld is the leg lengths of the largest right trianglethat can he inscribed within the fillet weld cross section (seeFig. 22). In each case, the weld size is based on the length ofthe fillet weld leg.

Fillet weld throat dimension requirements are commonlygiven for three different throats, two of which are veryimportant to the visual inspector.

FIGURE 22. Determination of unequal leg filletweld size, where size = a + b

a

b


1. Theoretical throat is 0.707 times the fillet weld size foran equal leg weld.

2. The actual throat is the shortest distance from the rootof the weld to the face of the weld. It must be equal toor greater than 0.707 times the fillet weld size. Becausethe visual inspector does not have access to the rootarea of the fillet weld, base metal penetration cannot beconsidered.

3. The effective throat includes the amount of weld pene-tration but ignores excess metal between the theoreti-cal face and the actual face (the visual inspector doesnot consider this throat).

The actual fillet weld throat of a T-joint can be measuredby placing the perpendicular surfaces of a weld gage in con-tact with the base metal of the joint. The effective throat isthen read off the sliding pointer of the gage.

Fillet weld length and spacing are critical design parame-ters when intermittent fillet welding is specified in a con-struction drawing. The welding symbols contain all of theinformation necessary to determine the extent of weldingrequired. Fillet weld length and spacing is easily measuredwith a ruler.

A concave fillet or butt joint groove weld surface is one thatcurves inward (see Fig. 13). Concavity is a smooth transitionin thickness with complete fusion at both sides of the joint.This should not be confused with inadequate fusion or over-lap, which is an abrupt change in thickness where weld metalis not fused to the base metal.

Concavity in a buttjoint is unacceptable when the weldthickness is less than the thinnest member being joined. Asshown in Fig. 23, this situation is equivalent to underfill.

The acceptability of concavity in a fillet weld is based onthe actual throat being equal to or greater than the theoreti-cal throat calculated from the specified weld size. One way toverify this is to use a standard fillet weld gage. The appro-priate gage (see Fig. 24) for the specified weld size may beused to square the edges of the gage with the welded parts.If all three points of the gage's double arc make contact withthe weld and home metal, the weld concavity is acceptable(see Fig. 25). If the center point does not make contact withthe weld metal, the concavity is excessive and the weld isunacceptable.

FIGURE 23. Concavity in a butt joint: (a) unaccept-able concavity with underfill present and(b) thinnest section of base metal with acceptableconcavity and no underfill

RULER

(a)

(b(


FIGURE 24. Gage to determine fillet weld concavity

SIZE OF WELD


(13)

1-SIZE--*C

LEGS

FIGURE 25. Desirable fillet weld profiles:(a) diagram of weld components and (b) weldcross section

SIZE

ACTUAL THROATCONVEXITY

THEORETICAL THROAT

(a)

45DEGREES SIZE

LEGS

(b)

SIZE-SIZE-'-1> C


FIGURE 26. Acceptable fillet weld profiles:(a) diagram of weld components and (b) weldcross section

ACTUAL THROATCONVEXITY

Apt,THEORETICAL THROAT


(a)


A convex weld surface is one that bulges or curves outward(see Fig. 14). Excessive convexity in a butt joint is the sameas excessive reinforcement. The few codes that address con-vexity specify acceptability if convexity is not greater than0.1 times the measured fillet weld leg length (or the longerleg in the case of an unequal leg fillet weld) plus 0.75 or1.5 mm (0.03 or 0.06 in.), depending on the standard beingused. The actual weld throat minus the allowable convexitymust be less than or equal to the theoretical throat (seeFig. 26).

In axially aligned members of different materials, thick-nesses are required so that the slope in the transition zonedoes not exceed some specified amount. The transition isaccomplished by chamfering the thicker part, tapering thewider part, sloping the weld metal or a combination of these.Figures 27 to 29 show examples of transition weld require-ments adapted from the American Welding Society's

Structural Welding Code, which requires that the transitionzone not exceed 25 mm (1 in.) in 64 mm (2.5 in.).

A simple gage (see Fig. 30) may be used for determiningacceptable transitions. The gage is positioned with the flatedge on the thicker base metal and the sloped edge over thetransition zone. To be acceptable, the slope of the transitionzone must follow the slope of the gage.

FIGURE 27. Transition formed by a sloping weldsurface

fa) 25 mm in.J64 mm

64 mm [2 5in125 mm in .l

25 mm I I in.J64 mm1251a1


(a) 25 mm (1 in.)

64 mm 125 in .1

FIGURE 28. Transition formed by a sloping weldsurface and chamfering

64 mm 12.5 in]25 mm 11

(14 25mmI1in1

64 mm1.2 5 01REMOVE AFTER WELDING

REMOVE AFTER WELDING

REMOVE AFTER WELDING

PROM THE ELECTRIC POWER RESEARCH INSTITUTE. REPRINTED WITHPERMISSION.

FIGURE 29. Transition by chamfering thicker part:(a) center line alignment (particularly applicable toweb plates) and (b) offset alignment (particularlyapplicable to flange plates)

25 mm {1 in 1 I63 mm 12.5 In.)

63mm(25 pn)25 mm in.)

( a ) CHAMFER BEFORE WELDING

CHAMFER BEFORE WELDING

lb) 25 mm (1 in.)63 mm {2.5 inT

CHAMFER EFFORE WELDING


FIGURE 30. Example of an acceptable slope oftaper where a 4:1 slope is specified

4:1 TEMPLATE—TEMPLATE SHOWS MORE THAN 4'1 TAPER

APPLICATIONS OF VISUAL AND OPTICAL TESTS IN THE ELECTRIC POWER INDUSTRIES 1259

Weld Joint DiscontinuitiesPorosity

Porosity is a group of gas pockets or voids (see Fig. 31),inside or on the surface of weld metal. Porosity is commonlyobserved as spherically shaped voids °that are uniformlyscattered, clustered or linear. Uniformly scattered porosityconsists of individual pores which can vary in diameter frommicroscopic to easily visible. Cluster porosity is a grouping ofsmall pores. Linear porosity typically occurs in the root pass.When porosity has tails some standards, such as Section VIII

of the ASME Boiler and Pressure Vessel Code, base the sizeof the bore to include the tail.

Surface porosity is usually visible with the naked eye if thetest area is properly illuminated. Surface porosity is generallyundesirable, but the fabrication document should be con-sulted for acceptance standards. In cases where standardsare given for porosity (ASME Sections III and VIII, AWSD1.1, ANSI B31.1, USAS B31.7), acceptance limits arebased on either maximum single pore diameter, number ofpores below a given size per unit area, or aggregate porediameter per unit area. A rule with increments of 0.8 mm(0.03 in.) or less is needed to make this determination. A lowpower magnifying lens may also be a useful tool for measur-ing the size of pores.

FIGURE 31. Porosity beneath a weld surface:(a) end view, (b) section view

(a)

(b)


FIGURE 32. If overlap is present, the angle betweenthe tangent to the weld metal and the base metalshould be less than 90 degrees; acceptable profileswithout overlapping in (a) a V joint and (13) a buttjoint

(a)90 DEGREES

FIGURE 33. Presence of unacceptable overlap In(a) a V joint and (t)) a butt joint

(a)

(b)



OverlapOverlap or cold lap is the protrusion of weld metal beyond

the toe or root of the weld (see Figs. 32 and 33). Overlap isgenerally not allowed in a completed weld because the notchformed between the crown and base metal concentratesstresses.

The presence of overlap is determined by visually examin-ing the weld metal-to-base metal transition at the toe of theweld. The transition should be smooth, without rollover ofweld metal.

Consider a tangent to the weld metal at the intersectionbetween the weld metal and base metal. If no overlap is

present, the angle between the tangent to the weld metal andthe base metal is greater than or equal to 90 degrees. If over-lap is present, the angle between the tangent to the weldmetal and the base metal is less than 90 degrees.

Undercut

Undercut is a groove formed at the toe or root of a weldwhen base metal is melted away and left unfilled by weldmetal (see Fig. 34). Undercut can be difficult to measureaccurately. When a gage is used to measure the depth ofundercut in butt joints, the body of the gage is positioned to

FIGURE 34. Presence of undercut in (a) a fillet weldjoint and (b) a double butt weld joint


(b)

FIGURE 35. Limits for undercut from the AWS D1.1structural welding code

2.0

1.8

1 6

(0.08)

(0.07)-

(0 06)-

C 1.3 10.05) -

D 1.0 (0.04) -CC CUE 0.8o.

(0.03) )1

ZE 0.5

0.3

(0.02)

1001)

7

;5- 55'CD -

o 0(1)25

(2) (31 (4)50 75 100

15)125

16)150

,Daz.- MEMBER THICKNESS

millimeters iinchesj

LEGEND NO CALCULATED STRESS

PRIMARY TENSILE STRESS PARALLEL TO UNDERCUT; SHEAR.COMPRESSION IN ANY DIRECTION

PRIMARY TENSILE STRESS TRANSVERSE TO UNDERCUT

FIGURE 36. Inadequate joint penetration


FIGURE 37. Typical weld cracks

TRANSVERSE CRACK IN WELDAND HEAT AFFECTED ZONE

THROAT CRACK

TOE CRACK

ROOT CRACK


CRATERCRACK


straddle the weld crown and a pointed tip on a semicirculardial is placed in the undercut.

Undercut results in a reduction of base metal thicknessand may materially reduce the strength of a joint, particularlythe fatigue strength. For this reason, excessive undercut isundesirable in a completed weld and most codes specifyacceptance criteria for undercut. For example, Section III ofthe ASME Boiler and Pressure Vessel Code (1992, Div. 1,Subsections NB, NC, NE, NF and NG Paragraph 4424)requires that undercut be less than 0.8 mm (0.03 in.) deep.Figure 35 shows acceptable limits for undercut according toAWS D1.1.

Inadequate Joint Penetration

Inadequate penetration is a failure of base material to fusewith filler metal in the root of the weld joint (see Fig. 36).Where full penetration is specified by the construction draw-ing, the inspector must visually determine that the root hasbeen completely filled with weld metal.

Cracks

Surface cracks detectable by visual testing may occur lon-gitudinally or transversely along the weld root, face, toe andheat affected zone (see Fig. 37) or in arc strikes outside theweld. A low power (5 x) magnifying lens is the best tool for

visually detecting surface cracks. Care must be exercisedwhen using a magnifier—minor surface irregularities can beaccentuated, affecting the accuracy of interpretation.

With few exceptions, no visible surface cracks are allowedin completed welds.

Arc Strikes

Arc strikes are caused by unintentional rapid heating ofthe base metal or weld metal and subsequent rapid coolingof the molten material (Fig. 38). Melting of the base metaland deposition of filler metal are often associated with strikescaused by a welding arc.

Arc strikes may also be caused by a poorly connectedwelding ground clamp or by instruments used for magneticparticle testing. The extremely high heat input that occursduring an arc strike can cause localized hardness andcracking.

Visual Testing of Brazed and SolderedJointsBrazed Joints

In power plants, brazed joints are commonly used oninstrumentation lines in low criticality systems, called Class


FIGURE 38. Arc strikes on the workpieces andportions of a multipass weld

ARC STRIKES

ARC STRIKES



III systems in the ASME Boiler and Pressure Vessel Code.Lap joints are the most common configurations for theseapplications. The soundness of these ,joints is largely deter-mined before the braze is made by control of cleanliness,joint overlap and the joint gap.

The objective in visual testing of brazed lap joints is todetermine that a continuous flow of braze filler metal hasbeen achieved. If filler metal is applied from the same sideof the joint as is visually examined, there is no assurance ofpenetration within the joint, even if the appearance of thefillet is good. Ultrasonic testing may he used to determinefiller metal penetration in such cases.

To simplify verification of a sound braze, good joint designrequires that filler metal is placed inside the joint beforebrazing. Visual testing is then conducted on the outside filletto determine if it is continuous. If the outside fillet is contin-uous, there is reasonable assurance that a sound joint hasbeen made.

Soldered JointsWetting is the most common problem encountered with

soldering, resulting in incomplete coverage of the surfacewith solder. Nonwetting is apparent during visual testing bythe appearance of the original surface finish. Dewetting isthe flow and retraction of solder, caused by contaminated

surfaces, dissolved surface coatings or overheating beforesoldering.

Dewetting may look like nonwetting but it is identified bya colored residual film with beads of solder where the solderreceded. Excessive movement of the soldered joint duringsolidification may materially weaken the joint and is visuallydetermined by a frosty appearance.

Visual testing is the most widely used method for nonde-structive inspection of soldered joints. Reference standardsare used to aid the inspector in checking for general designconformance, wetting, finished cleanliness of the productand the quality and quantity of solder.

Acceptance StandardsEvery feature of the joint configurations and dimensions

subjected to visual testing has an associated quantitative tol-erance limit or qualitative criteria of acceptability. These areestablished with the drawing or incorporated into therequirements of the governing document. Discontinuitiesalso have quantitative and qualitative criteria.

The inspector should record all test results. Acceptabilitymay be determined at the site or later, depending on therequirements of the application.

Recording and Reporting Visual TestResults

Proper recording of visual test results is typically doneusing forms developed for specific applications. If no form isavailable, the inspector should develop one that provides thefollowing data: (1) joint identification and description,(2) joint inspector's identification, (3) testing dates,(4) instrument and equipment identification (may refer toprocedure), (5) procedure identification (and checklist) and(6) record of the test results.

In the record of results, identify all measurements to berecorded, provide space for all recorded measurements withidentification, and provide space for recording indications.

Through the use of standard recording formats and pre-pared forms, the inspector and supervisor can determine ifall required observations have been made properly.


PART 2

SPECIFIC VISUAL INSPECTION APPLICATIONS

Visual Testing of Reactor PressureVessels

Visual testing of a reactor vessel and its internal compo-nents is one of the most critical operations in any inspectionprogram. Not only is the test closely audited by the NuclearRegulatory Commission (NRC) and the American NuclearInstitute (ANI), it is usually done on a critical outage sched-ule, so there is no opportunity to substitute operations ifthere are problems with the inspection.

A Level III inspector is typically involved in all phases ofthe test, beginning with the selection of equipment, thetraining of test personnel, supervision of the test itself andthe final interpretation and review of the results. The textbelow presents situations typical to such a visual inspectionand suggests techniques that might help resolve anticipatedproblems.

Use of the Applicable Code

Visual tests of reactor vessel internals are performedaccording to the rules of the plant's inservice test program,which is in turn based on visual test requirements set forth inappropriate standards. It is the responsibility of the inspectorto perform the tests so that the requirements of the standardand Nuclear Regulatory Commission bulletins are satisfied.

The primary requirement is that the visual test be doneaccording to a written procedure and that a checklist be usedto plan and perform the test. The specific items referencedby an ASME nuclear code are shown in Table 1.

Visual tests of the vessel are performed to determine thecondition of critical inner surfaces. Such critical surfacesinclude high stress points at the junctions of nozzles and thevessel or nozzles and the cladding. Cladding is criticalbecause it protects the vessel from corrosion.

The visual inspection of a reactor vessel may involve manytechniques. Vessel tests may be made with the vesselempty or filled with water with the internal components inplace—by remote viewing or direct viewing (in any suchtest, it is impossible for the inspector to place his unpro-tected eye close to the test site).

The purpose of the test is to determine how the pressurevessel system hat-been operating. Many small componentfailures have been detected by such tests, preventing seriousfailures. It is almost impossible to make a complete visualtest of anything as complex as pressure vessel internals. The

TABLE 1. Typical visual Inspection checklist for apressurized water nuclear reactor's vessel Internalsleach component listed is given a complete visualinspection)

1. Core barrel alignment key2. Core barrel thermal pad mounting3. Core barrel irradiation specimen basket4. Core barrel lower internals head alignment pin5. Core barrel outlet nozzle/vessel interface surface6. Core barrel midplane girth weld7. Instrumentation mounted in core barrel8. Upper internals alignment key in core barrel9. Bathes in core barrel

I0. Guide tube attachment to upper core plate11. Upper core plate!Z. Upper core plate alignment keyway13.Alignment of keyway upper support plate14. Core barrel15. Upper support plate instrument conduits and supports16. Lower core plate showing fuel assembly alignment pins,

flow holes, in core instrument guides, supports andattachments

17.Lower core support columns and instrument guides18. Support column instrument guide tube attachments to

lower core support plate19. Upper internal keyway insert attachments20. Fuel pin and support column attachment upper internals21. Upper internals guide types and support columns22. Guide tube note: split pin attachments23. Baffle bolting24. Upper tube assemblies on upper support plate with

instrument port guide post25. Secondary core support assembly and instrument guide

tubes26. Reactor vessei internal surfaces27. Interior attachments and core support structures28. Core support structures

visual inspector has limited time available because time inthe vessel may he critical during a refueling outage. Somelimitations are set by equipment, because each test methodhas its drawbacks.

ASME Code, Sections V and XI

Nuclear plants are examined on a routine basis accordingto the requirements of their license, the laws of the state inwhich they are operated and special requirements of theNuclear Regulatory Commission (NRC). The majority of


these tests must meet the requirements of the ASME Boilerand Pressure Vessel Code which forms a part of the inservicetest program. In certain instances, visual tests are requestedby the NRC when component failures are suspected.

Before the 1977 edition of the Boiler and Pressure VesselCode, visual tests were conducted to determine the generalcondition of a part, component or surface. These tests weremade according to requirements of Article 9, Section V, withan additional requirement for lighting sufficient to resolve a0.75 mm (0.03 in.) wide black line on an 18 percent neutralgray card.

In 1992, Section XI of the document divides such visualtests into three categories:

1. VT-1: to determine the condition of a part, componentor surface.

2. VT-2: to detect leakage from pressure retaining com-ponents or abnormal leakage during system pressure orfunctional inspection.

3. VT-3: to determine the general mechanical and struc-tural condition of components and supporting struc-tures and to determine conditions relating tocomponent or device operability.

The VT-1 test (for condition of part, component or sur-face) is required on the following components, according toASME Section XI:

1. The reactor vessel bushings and closure washers. Thesecomponents are available for close, direct visual obser-vation when the head is removed from the reactor. Thebushings, which are part of the head, may require thatthe inspector wear protective clothing to guard againstinhaling loose contaminant. The washers should pre-sent no contamination problem because they are usu-ally kept separate from the head in special bins. TheASME Cork also requires volumetric examination ofthe vessel bushing area and ultrasonic testing of vesselbushing areas.

2. All bolts, studs and nuts. The reactor vessel studs andnuts are kept clear of the head and are relatively easy toinspect. The root of the threads in the studs may befouled with a threading compound which is nearlyimpossible to remove, making a close visual testimpractical. The eddy current method for surface test-ing of these components has yielded some favorableresults. Volumetric and surface examinations of thestuds and surface examination of the nuts are required,in addition,to ultrasonic testing of reactor vessel headbolts.

3. The reactor vessel internal attachments within the belt-line region. These attachments are visually inspectedusing closed circuit television.

The visual inspector is expected to be (1) trained and quali-fied in a manner comparable to that described in AS NT'sRecommended Practice No. SNT-TC-1A for other nonde-structive evaluation methods; (2) able to read Jl text (at a dis-tance of 30 cm 112 in.] from a Jaeger chart, describedelsewhere) or equivalent in a standard acuity test and(3) tested for his ability to distinguish colors applicable to thetest.

The VT-2 test (for evidence of leaks) is normally per-formed for the record during a pressure test of the systembut it can also be performed in practice every time an inspec-tor works around the reactor vessel, particularly in the areaof penetrations or underneath the vessel (in boiling waterreactors).

The VT-2 inspector looks for stains, dislodged insulationand water on the floor or dripping from overhead insulation.A leak in a vessel nozzle or safe end can result in water travel-ing along the insulation until it has a path for escape, so thevessel inspector should be alert to conditions in pipingaround the vessel.

Vertical insulated components may be examined at thelowest point where leakage may be detected. Horizontalinsulated components are examined at each insulation joint.If accessible, the VT-2 test is performed directly on the ves-sel. If this is not possible, the test may be made on the sur-rounding areas and floor, looking for evidence of leakage.

Much of the VT-2 leak test is performed by the visualinspectors when the plant is cold. These individuals haveclose access to the nozzles and safe ends when they areuncovered. The inspector should form the habit of making avery close inspection for leaks during other visual tests.

There is a tendency for some inspectors to limit them-selves to the specialized test underway, neglecting othervisual evidence. Inspectors should remember that the reac-tor is in theory a water tight and gas tight system. Any waterwhere none is expected, no matter how small the amount, iscause for concern.

The VT-3 test (for mechanical and structural conditions)is used to determine the general condition of the vessel out-side of the beltline region and the core supports. This test isgenerally performed using closed circuit television and videotape recording. The object of the test is to disclose corrosionor broken, worn or missing components and may requirephysical measurements. Test personnel are qualified by thesame program as VT-1 and VT-2.

Proper performance of a remote visual test depends onthe use of appropriate fixturing and lighting. A televisioncamera on a long pole is like a pendulum—unless care istaken to stabilize it, the inspector can spend as much timewaiting for the camera to settle down as for the actual perfor-mance of the test.

FIGURE 39. A calibration standard for remotevisual examination

POLE CONNECTIONTO HANDLING TOOL

ALUMINUM FRAME

WIRE 1"


FIGURE 40. Pressurized water reactor vessel

LEGENDI. CONTROL ROD DRIVE SHAFT2. LIFTING LUG3. UPPER SUPPORT PLATE4. INTERNALS SUPPORT LEDGE5 CORE BARREL6. OUTLET NOZZLE7. UPPER CORE PLATE8. REACTOR VESSEL9. LOWER INSTRUMENTATION GUIDE TUBE

10 BOTTOM SUPPORT FORGINGII. RADIAL SUPPORT12 TIE PLATES13.CONTROL ROD DRIVE MECHANISM14.THERMAL SLEEVE15.CLOSURE HEAD ASSEMBLY16.HOLD-DOWN SHARING17. INLET NOZZLE1B. FUEL ASSEMBLIES19. BAFFLE211 FORMER21. LOWER CORE PLATE22. IRRADIATION SPECIMEN GUIDE23. NEUTRON SHIELD PAD24. CORE SUPPORT COLUMNS

FROM WESTINGHOUSE ELECTRIC CORPORATION. REPRINTED WITHPERMISSION.

13

14

15

16

17

18

19

2a

21

22

23

24

2

7

8

9

IC

12

APPLICATIONS OF VISUAL AND OPTICAL TESTS IN THE ELECTRIC POWER INDUSTRIES I 265

Test Procedures

The ASME code requires that a direct VT-1 test be per-formed with the eyes no more than 60 cm (24 in.) away fromthe test object, at an angle no greater than 30 degrees tothe surface. The illumination on the object is such that a0.75 mm (0.03 in.) wide line inscribed on an 18 percent neu-tral gray card can be seen by the inspector. Remote viewingaids such as closed circuit television are allowed if conditionsequivalent to direct inspection can be met.

It is very unlikely that a gray card test could be used withan underwater closed circuit television system. In somecases, the closed circuit television must resolve a stretchedwire of a known diameter placed against the test material(Fig. 39).

Pressurized Water Reactor Internals

All of the internals of the pressurized water reactor(Fig. 40) may be removed from the vessel for testing. To dothis, the fuel must also be removed. Consequently, the inser-vice inspection program recommended for this type of reac-tor calls for the visual testing of internals at the end of everyten year interval, as defined in the ASME code.

During a normal refueling operation on a pressurizedwater reactor, the head and control rod drive assembly areremoved and stored. Access to the lower portions of the ves-sel must, in many cases, be through openings left by fuel that

FIGURE 41. Boning water reactor vessel

LEGENDI. VENT AND HEAD SPRAY2. STEAM OUTLET3. CORE SPRAY INLET4. LOW PRESSURE COOLANT INJECTION INLET5. CORE SPRAY SPARGER6. JET PUMP ASSEMBLY7. FUEL ASSEMBLIES8. JET PUMP/RECIRCULATION WATER INLET9. VESSEL SUPPORT SKIRT

10 CONTROL ROD DRIVES11.IN-CORE FLUX MONITOR12.STEAM DRYER LIFTING LUG13. STEAM DRYER ASSEMBLY14 STEAM SEPARATOR ASSEMBLYIS FEEDWATER INLET16.FEEDWATER SPARGER17.CORE SPRAY LINE18. TOP GUIDE19 CORE SHROUD20. CONTROL BLADE21. CORE PLATE22. RECIRCULATION WATER OUTLET23. SHIELD WALL24. CONTROL ROD DRIVE HYDRAULIC LINES

FROM GENERAL ELECTRIC NUCLEAR ENERGY, REPRINTED WITHPERMISSION.

12

13

14

1516

4

5

B

9

10

11

17

18

19

20

2122

23

24


has been removed. In some cases, access openings for a tele-vision camera may be provided so that a small video unit maybe inserted between the thermal shield wall and the vessel.

Before beginning the test, the inspector should have awritten procedure and a checklist. The checklist is used toensure that nothing is overlooked (see Table 2).

At the end of the ten year interval, the pressurized waterreactor internals are completely removed from the vessel andstored separately. At this time, they are examined in detail.Because this test is repeated over such long time intervals, itis important that complete records he maintained. Becausean index and inspection logs may become separated from avideo tape, it is a good idea to make the tape self identifying.If the recorder has a voice channel, it can be used but a titleblock, made by writing the essential data on a card before thetaping, should be a requirement.

Boiling Water Reactor Internals

When the boiling water reactor is opened for refueling,the head, the steam separator and the steam dryer areremoved and placed on stands. The head is stored out of thewater on the refueling building floor and the other compo-nents are on stands in a holding pool which may be gated offfrom the reactor vessel. Fuel may or may not he completelyremoved. A cross section of a typical boiling water reactor isshown in Fig. 41. It is over 24 m (80 ft) from the refuelingbridge to the bottom of the vessel and it is 6 m (20 ft) fromthe vessel flange to the refueling bridge.

TABLE 2. Summary of reactor vessel and internals visualtesting requirements (1992 ASME Section XII

Test ExtentTest Object Method

Every 10 Years

Partial penetration weld VT-2Pressurizer heater

penetration VT-2Vessel nozzles VT-2CRD nozzles VT-2

Instrumentation nozzles VT-2Range surfaces VT-1

Nuts, bushings andwashers VT-1

Bolts, studs, nuts VT-1Vessel interior VT-3Interior attachment within

beltline VT-1Interior attachment beyondbeltline VT-3

Core support structurePressure retaining

boundaryPressure retaining

boundary

25 percent nozzles

all nozzles25 percent nozzles25 percent nozzles25 percent nozzlesall when disassembled

allallaccessible areas

accessible welds

accessible weldsVT-3 accessible surfaces

VT-2 coolant system leak test

VT!,' coolant system hydrotest

FIGURE 42. Video camera positionerLIGHTCONTROL CONSOLE

QUICK DETACHABLESUPPORT CLAMPS

VERTICAL DRIVE WINCH

ROTATOR/LIGHT CABLECAMERA CABLE

MOTION CONTROL CONSOLE

QUICK CLAMPTURNING HANDLE

REFUELING BRIDGETROLLEY

SUPPORT FRAME

SWIVEL PIN CONNECTION

360 DEGREE PAN ROTATION

WINCH CABLE

QUICK CONNECTEXTENSION TUBE

CAMERA SUPPORT YOKE

j150 DEGREE TILT


APPLICATIONS OF VISUAL AND OPTICAL TESTS IN THE ELECTRIC POWER INDUSTRIES 1 267

With the components removed from the vessel, a televi-sion camera can view the upper nozzles and interior piping.Visual inspection of these items is very important and mustbe carefully performed. A positioning fixture is required forclose examination of the piping.

The remaining fuel in the vessel can be a cable trap. Thetops of the fuel bundles have many closely grouped springs,nuts, antirotation devices and other hardware. It is very easyto snag a small cable if it is allowed to loop into the fuel. Thiscan be prevented by attaching floats to the cable.

The inspector working over the open reactor vessel musttake extreme care that equipment does not become dis-lodged and fall into the vessel. Operating personnel makesure that the inspector has all pockets taped, eyeglassestaped on and all small tools moved away from the vessel. Itmay be worthwhile to situate the inspector and the datarecorder away from the vessel and to have an operator posi-tion the camera on the bridge, using a small monitor andintercom.

Two types of closed circuit camera positioning equip-ment—a camera positioner mounted to the trolley of therefueling bridge and a submersible remotely operatedvehicle—may be used.

The closed circuit television camera positioner, shown inFig. 42, is designed to support and position an underwatercamera and a lighting system. It uses a quick-connect mount-ing for the refueling bridge and can be positioned in X, Y andZ coordinates. The unit's design and simplicity of operationallow stable positioning of the camera for quality pictureswith a minimum of experience.

The positioner consists of a support frame with verticaldrive winch, extension tubes, camera support yoke (alsohouses the lighting) and rail mounted control console. Thesupport frame incorporates vertical guides through whichthe extension tube extends. The vertical drive winch cableconnects directly to the bottom-most extension tube andprovides the vertical support and drive for the camera andextension tube system. The winch is a self-locking worm gearunit with 115/230 V, variable speed, alternating current drivemotor and electric brake. A safety brake is included to pre-clude the assembled extension tubes from falling in theunlikely event that the winch cable broke.

The camera support yoke incorporates an electricallydriven underwater rotator tilt system which supports thecamera in a protective tube and could rotate it 150 degreesupward from the vertical down position. Camera rotationabout the vertical axis (360 degrees pan) is achieved by man-ually rotating the extension tubes.

The extension tube sections are assembled to the requiredoperating length by releasing the turning handle clamp andinserting a tube section. The camera is lowered, another sec-tion added and he turning handle is then clamped to thetopmost tube section. The extension tube sections are quick-connect assemblies with air compartments for buoyancy. The

bottom-most section is allowed to fill with water to provideoperator protection from direct radiation streaming throughthe tubes.

The positioner is furnished with 30 m (100 ft) of connect-ing cable. The console incorporates the controls necessary tocontrol the positioner remotely. A 115/230 V alternating cur-rent power supply operates the system.

The positioner is fabricated from aluminum and stainlesssteel with commercial components selected for compatibilityin a high radiation underwater environment. All parts andhardware are retained in a manner to prevent their loss in thepool or reactor.

The horizontal inspection boom (see Fig. 43) attaches tothe bottom-most extension tube and is designed for scanningthe reactor pressure vessel circumference with the posi-tioner located at the center of the vessel. This eliminates theneed to move the refueling bridge or trolley when per-forming circumferential examinations. The boom has amotorized extension with a variable stroke to allow pan-oramic as well as closeup viewing of the circumferentialcomponents (feedwater and core spray spargers andattachments) without moving the bridge or trolley. The

1•1•111111111•1111111113

P

FIGURE 43. Horizontal inspection boom for videocamera

• SILV/

9 C..4:7101 HE--- I 2

10 .11= sue elm

1415

LEGENDI. BOOM CONTROL CONSOLE2. BOOM IOWA MOTOR3. REACTOR PRESSURE VESSEL AND VERTICAL EXTENSION TUBES4. BOTTOM EXTENSION TUBE5. BOOM ADAPTER6. HORIZONTAL BOOM7. TOP \ARV OF FEEDWATER SPARGER OR CORE SPRAY HEADER8. BUOYANCY CHAMBER9, CAMERA SUPPORT YOKE0, CORE SPRAY SPARGERSI. CAMERA2. TRAVEL3. TOP GUIDE4. CORE SHROUD5, PRESSURE VESSEL WALL


V


extension boom (with mounted support yoke and camera) isbuoyant.

Another visual testing tool that saves a great deal of time isthe remotely operated vehicle. It is controlled manually, withthe controller connected to the vehicle by a coaxial cable. Apower supply is connected to the vehicle by an umbilicalcable. The vehicle has a pan and tilt color camera, twin lights,two horizontal thrusters and one vertical thruster formaneuverability.

lnservice inspection components that are inspectedinclude core spray sparger and core spray piping, feedwatersparger, top guide, core plate, steam dryer, moisture separa-tor, shroud head bolts, fuel support pieces, stub tubes andclad patches. Components requiring visual testing becauseof license agreements or regulatory requirements vary fromplant to plant. Typical augmented inspection components arethe intermediate range monitors, source range monitors,local power range monitors, control rod drive return nozzleshroud and shroud separator mating surface.

The expense of remote camera handling equipment is jus-tified when inexperienced personnel are visual testing theinternals of a reactor vessel. Often the remote handlingequipment can provide steadier pictures and can speed upthe visual tests. Manufacturers are making remotely oper-ated vehicles (ROVs) small and gearing some strictly for thenuclear industry. These vehicles are small enough to get toareas that are extremely difficult to access with hand heldcameras and produce pictures superior in clarity andsteadiness.

The environment of the visual test mandates cameraselection. Charge coupled device (CCD) cameras start toloose the image as the radiation field increases and would notbe a good choice for visual testing of reactor vessel internals.Metal oxide semiconductor (MOS) chip cameras providecolor images and are usually used in remotely operated vehi-cles. For high radiation fields, however, MOS chip camerasmust be replaced with tube cameras. The image tube iseither an orthicon tube, which is sensitive to low levels oflight and requires only slight external illumination, or a vid-icon tube. The orthicon tube would he the choice for lowerradiation areas of a vessel whereas the vidicon tube would beused for higher radiation areas despite its illuminationrequirements.

Hand held cameras will probably always be needed. Thetechnicians performing the visual testing of reactor vesselinternals should get hands-on training with the cameraequipment as well as training on what to look for during avisual test. It is very difficult and laborious to position a cam-era and record a steady picture in a timely manner.

Familiarization with Visual Recording Methods

Visual test data are recorded on videotape recorders. Theinspector should remember that the velocity of light in wateris less than it is in air. This means that the focal settings onthe camera must be modified to be meaningful underwater.It also means that a test cannot be qualified on the bench—it must be qualified underwater.

Where possible, the use of a zoom lens on the camera sim-plifies the test, as it is extremely difficult to increment a cam-era on a long pole. The camera fixture should allow thecamera to be easily tilted. This is easy to accomplishmanually.

Performance of the closed circuit television test shouldalways be planned and performed systematically. If the testis recorded, the tape counter readings can be marked on anappropriate drawing to index each test.

The video tape has the advantage of recording motion.This is important when attempting to analyze certain visualdiscontinuity indications. For example, a crack may cause asurface disruption which produces shadows, while a stain orsmall scratch does not.

FIGURE 44. Drawing with video and photographicrecord locations in a horizontal cross section of thereactor vessel

JET PUMPAND RISERBRACE PAIR

RISER (TYPICAL)

JET PUMP fTYPICAO

7 BC DEGREES

FROM THE ELECTRIC POWER RESEARCH INSTITUTE. REPRINTED WITHPERM/5510N.

APPLICATIONS OF VISUAL AND OPT/CAL TESTS IN THE ELECTRIC POWER INDUSTRIES 1269

It must be remembered, however, that video taperecording loses detail. Commonly used video recorders havea resolution of about 200 lines, about one half that of the tele-vision camera.

For other types of records, it is possible to photograph atelevision monitor. The problem is that the monitor bright-ness usually does not match the surroundings and a properexposure of the entire scene is not favorable for the monitor.If the camera exposure suits the monitor screen and thespeed of the camera is kept slower than half the frame fre-quency, it is possible to get usable pictures from the monitor.Even if the picture is not as detailed as desired, it still canbe used as a guide when the video tape is replayed at a latertime.

Visual Test Reporting

The test report should be a record of all pertinent dataresulting from the test, as well as the procedures, personneland equipment used. The report may be a specific form foreach test object or a general form made specific by theentries of the inspector. The test'report may be microfilmedand the hard copy removed to archive storage while the pho-tographs or the video tapes are stored in logbooks or thequality analysis vault. If possible, the filed copy of the reportshould indicate the disposition of the remaining records ofthe test for future reference.

Drawings included as part of the test report should indi-cate the location of photos taken or recorded scans of thetelevision camera (see Fig. 44) Video counter readings canbe entered on the drawing to facilitate review of the test.

Visual Testing of PumpsTesting Condensate and Boiler Feed Pumps

Always refer to the dismantling procedure outlined in themanufacturer's instruction manual. During disassembly,check the impellers for signs of erosion and cavitation dam-age. Typically, the areas most affected are near the inlet ofthe impeller vane. The vane inlet should be smooth and radi-used. Vanes exhibiting abnormal wear may be blunt edged,jagged and may appear porous if cavitation was present. Eachimpeller is also inspected using liquid penetrant methodsover 100 percent of its surface. All cracks are repaired or theimpeller is replaced.

Diffuser elements for pumps are visually inspected forerosion and crfks in the same way as the impeller. Disconti-nuity indications are referred to the pump manufacturer fordisposition if the component is to he reused.

The dimensions of sleeves and rings should be verifiedagainst the manufacturer's tolerances and replaced if worn

beyond the tolerance limit. Excessive clearances reduce thepump's efficiency and can cause hydraulic imbalance.

A total indicator reading should be taken on the pumpshaft to ensure its straightness. Even slightly warped or bentshafting may precipitate excessive vibration during use. Shaftbearing journals should he checked for proper finishes. Totalindicator reading or runout is the deviation from a perfectform which is normally detected by full rotation of the com-ponent. A runout tolerance applied to a surface means thatthe considered surface must lie within a tolerance zone nor-mal to the true profile of the component. The zone limits areseparated by a distance equal to the specified total tolerance.

Because runout is applied as a composite form of relatedfeatures having a common axis, measurements should betaken under a single setup. Runout is a geometric form con-trol which can include such qualities as roundness,straightness, flatness and parallelism of individual surfaces.When reviewing the condition of a shaft or element, nevertry to mount the shaft in the original machining centers. It islikely that the centers could be damaged through erosion orcorrosion and true readings are difficult to obtain.

Always try to set journal surfaces of the shaft on a set ofV-blocks. The readings should be taken on all bearing andring surfaces with a dial indicator. In any case, the pumpmanufacturer should be consulted for tolerance limitsacceptable for runout readings.


Babbitted surfaces must be inspected for smoothness andwear. Babbit material is very soft and can be scored by eventhe smallest particles encountered during operation. Wheninspecting habbitted components, liquid penetrant tests arealso used to check the babbit for lamination or separationfrom its backing.

The pump casing should be visually inspected for erosionor washout. Casing contours should be smooth and continu-ous. Pits or ridges can reduce efficiency and accelerate cas-ing wear. Casing joints should be checked for erosion andeffective flange sealing area.

All sleeve bearings should be visually inspected for pitting,finish, scoring and size. The design clearance should bechecked with feeler gages. Frictionless ball or roller bearingsmust be inspected for surface finishes on the rotatingassemblies.

Testing of Circulating Water Pumps

Always refer to the dismantling procedure outlined in themanufacturer's instruction manual. If the pump has rubberbearings, use only use fresh water for cleaning (solvents maydamage the rubber). After all components have beencleaned, check them and the pump casing for corrosion.Deposits or scaling must be removed before reassembly.

The components are checked for cracks and signs of ero-sion or cavitation damage. Particular attention is given to theinlet and discharge areas of each impeller vane. The vane tipsand edges are inspected on vertical column pumps with animpeller-impeller cone design.

The diffuser vanes are inspected in the same manner asthe impeller vanes. Areas of wear or erosion should berepaired and ground smooth.

Check the shaft sleeves for scoring. A scored sleeve accel-erates hearing wear and should be replaced. Polishing isacceptable if the scoring is minor.

Sleeve, rubber or habbitted bearings should be checkedfor wear or damage. Worn or damaged bearings are replaced.Visual inspection is also used to check rubber and babbit foradherence to the shell. The preferred method of checkingbabbit adherence is the liquid penetrant method.

Remove the packing and check the shaft sleeve for scoring.Make sure that the stuffing box and the seal water pipinglines are flushed.

Check all running clearances with a feeler gage. Consultmanufacturer's instructions for allowable wear limits.

Inspect the war pump for erosion or loss of protectivecoatings (if used). Pits and ridges can reduce efficiency andcan accelerate wear. Check flanges and welded joints finsigns of erosion. Shafts should be checked for runout. Visu-ally inspect shafts for corrosion or signs of washout.

Visual Testing of ValvesA valve is a mechanical device that controls flow into,

inside of or out of enclosed conduits such as piping and tub-ing. When fully open, the perfect valve offers no more flowresistance than an equal length of pipe. Closed, the perfectvalve permits no fluid to pass. In addition, it completelyresists distortion from internal fluid gas pressure. Also neces-sary is resistance to fluid dynamic effects, temperature, pres-sure drop, vibration, corrosion, wear, erosion by smallparticles and damage from large objects in the fluid stream.This performance has to be constant over the life of the valve.

Because a valve rarely approaches perfect performance,and because valve failure can he costly from an operationaland safety standpoint, periodic tests are performed duringthe life of the valve in order to minimize failures.

Before visual testing of any valve, the inspector must firstknow the type of valve, its function, temperatures and pres-sures, how long it has been in service and its maintenancehistory. With this information, the inspector can more accu-rately evaluate questionable conditions.

Dismantling valves is always done according to a detailedprocedure or in accordance with the manufacturer's techni-cal instruction manual. Documentation of the as found con-dition is a very important step for determining continuanceof service. The as found condition is also compared to thevalve's maintenance record.

Inservice Visual Tests of Valves

Inservice testing allows the performance of a completevisual test of the valve, plus any other nondestructive testthat may be needed. It is best to remove the valve from theline. This permits a more detailed inspection by exposingmore of the internal surfaces. More reliable seat or shell leaktests are also possible with the valve offline.

The first step in performing an inservice test is to reviewthe preservice, maintenance and past inspection records.

Tests of Gate Valves

Sealing surfaces of the wedge and body are inspected forevidence of physical damage (cracks, scratches, galling, wiredrawing, pits, indentations). If facilities are available, liquidpenetrant testing is also performed. Guide surfaces (stuffingbox and yoke area) are inspected for evidence of wear or gall-ing. The wear pattern on guide surfaces often reveals mis-alignment of working components. Excessive clearance inguide surfaces can lead to excessive rubbing between seatfaces when the valve is closing, causing premature leakagefailure (Fig. 45).

In gate valves, the upstream seat of the wedge and down-stream body seat are the most likely places for erosion and

FIGURE 45. Bolted bonnet gate valve

STEM

YOKE SLEEVE

FIXED SEAT

SEAT FACE

STUFFING BOX

BONNET

BODY

WEDGE

STEM NUT


FIGURE 46. Bolted bonnet globe nonreturn valve

STEM NUT

STEM

YOKE SLEEVE BONNET

DISK OR PLUGBODY



wear. The sealing surface behind removable seat rings aregood candidates for leakage from erosion (wire drawing orsteam cutting). The stuffing box is the second most vulnera-ble spot in a valve. Remove the packing and check for corro-sion in the walls of the box. Check the stem in the areapassing through the bonnet guide surface and the packinggland for evidence of rubbing or corrosion. The stem in thisarea should have a surface finish no greater than 0.8 p.m(32 µM.) to ensure reasonable packing life. Another area ofconcern is the stem-to-wedge connection.

In an outside stem and yoke gate valve, the stem-to-wedgeconnection (in addition to opening the valve) must preventthe stem from turning in order to allow the stem nut to oper-ate the valve. Check this connection for wear or erosion.Excessive wear, allowing the stem to turn or to becomedisengaged, renders the valve inoperative. Check the stem-to-stem nut threads for excessive wear. Excessive wear canonly be corrected by replacing the worn components.

Because stem threads can be observed during an inservicetest, a judgment can be made regarding the rate of wear andthe need for replacement. Stem nuts can usually be replacedwith the valve inservice but stem replacement requiresremoval of the valve from service. Wear and refitting thewedge of a gate valve make the wedge fit lower in the body.Thus, when inspecting a valve, it is important to be sure thatthe reconditioned wedge fits properly. There must be ade-quate contact between the wedge seats and body seats toprevent leakage.

Tests of Globe VOves and Stop Check Valves

Sealing surfaces of the disk and body are inspected for evi-dence of physical damage (cracks, scratches, galling, wire

drawing, pits, indentations). Liquid penetrants are used, ifpossible. Inspect the guide surfaces of the disk and seat ringor body for evidence of galling and wear. In a globe valve,excessive clearance in the guides increases the possibility ofthe bottom edge of the disk catching on the top edge of theseat in the body and damaging one or both.

This is especially significant if the stem is not verticalinservice. Check the body seat for leakage behind the ringwhen performing an offline pressure test. If it is necessary toresurface seat faces, verify that the clearance between thehandwheel and yoke permits full closure of the recondi-tioned valve. Remove the packing and check the walls of thestuffing box for corrosion. Check the stem in the stuffing boxfor evidence of rubbing or corrosion. A 0.8 11,m (32 On.) fin-ish is the maximum permitted in the area through the pack-ing. In a globe valve, the stem-to-disk connection isespecially important for proper operation. The fit should betight but not rigid. Excessive clearance at this point leads toerratic operation, excessive noise and accelerated wear andthis connection should be carefully inspected for excessiveclearance. The stem-to-nut threads are an important testsite. Excessive wear is corrected by replacement of the com-ponent (Fig. 46).

Tests of Lift Check Valves and Swing Check Valves

Lift check valves are inspected the same way as globevalves, without the stem or stuffing box (Fig. 47).

In a swing check valve, inspect sealing surfaces of the diskand body for evidence of physical damage and perform a liq-uid penetrant test if possible. Check for leakage behind

FIGURE 47. Bolted bonnet lift check valvePLUG

BODY


FIGURE 49. Floating ball valve

CLOSURE DEVICE


FIGURE 48. Bolted bonnet swing check valve

HINGE

LAP

MIMI!! 3

.4011-

FROM THE ELECTR D: POWER RESEARCH INSTITUTE. REPRINTED WITHPERMISSION.

RINGDISK

BODY HINGE PIN


the seat ring and verify that the disk is not installed back-ward. Inspect the hinge pin, hinges and disk for evidence ofwear. Excessive clearance at these points can lead to mis-alignment of the seat surfaces and leakage (see Fig. 48).

Tests of Ball Valves

Sealing surfaces are visually inspected for evidence ofphysical damage, wear or corrosion that causes leakage. Thehorizontal plane at the flow centerline is the first location toshow wear because it always seals against the full differentialpressure. Check the stuffing box and stem thrust bearing sur-faces for evidence of corrosion or wear (Fig. 49).

Tests of Rug and Butterfly Valves

Plug valves are tested the same way as ball valves. In atapered plug valve, wear and refitting of the plug in the bodycauses the plug to fit lower. When inspecting a valve of this

type, it is important to be sure that the reconditioned com-ponent fits properly. Proper alignment between thecomponents in the plug and body prevent excessive turbu-lence and pressure drop that may cause accelerated erosion.

In butterfly valves, inspect the seating surface of the diskfor evidence of physical damage, wear or corrosion. Again,the horizontal plane at the centerline is the most vulnerablepoint. Inspect the plastic seal or body liner for evidence ofdamage or cold flow. Alignment of the seal and disk whenclosed is vital to the satisfactory operation of a butterfly valve.Inspect the bearing surfaces and position stops for excessivewear (Fig. 50).

Tests of Diaphragm Valves

Inspect the sealing surface of the body partition for evi-dence of corrosion, erosion or damage. Most importantly,visually inspect the diaphragm for evidence of aging andcracking, especially where it is retained by the body and bon-net or on the surface in contact with the body partition. Thestuffing box is inspected the same way as it is in a globe valve.

Visual Tests of BoltingVisual testing of bolts (see Fig. 51) is usually done in order

to detect conditions such as cracks, wear, corrosion, erosionor physical damage on the surfaces of the components.

The test setup and environment are specified in detail. Forexample, tests of pressure vessel bolts may be done withdirect methods when access is sufficient to place the eyewithin 60 cm (24 in.) of the test surface at an angle not lessthan 30 degrees to the surface. Mirrors may be used toimprove the viewing angle. Lighting, natural or artificial,must be sufficient to resolve a 0.75 mm (0.03 in.) black lineon an 18 percent neutral gray card.

FIGURE 50. Butterfly valve


DISK

FIGURE 51. Diagram of nondestructive test forbolting

INSPECT VOLUMETHREADS IN

FLANGE 1E F G H)

INSPECT VOLUMETHREADS INFLANGE (A B C DI

CENTER DRILL HOLE..7— (WHERE USEDJ

I KI

IN PLACE ULTRASONICTESTING (J K L

EDGE OF NUT INBOLTED POSITION

25 mmll in)

!- FACE OF FLANGEOF COMPONENTB

25 mmii in.)

Ds

EF

STUD

H



Remote visual tests may be substituted for direct visualtesting, using devices such as telescopes, borescopes, cam-eras or other suitable instruments. Such devices must haveresolution at least equivalent to that attainable by directvisual testing.

Equipment and Preparation

Steel rules, micrometers, vernier calipers, depth microm-eters, thread gages and magnifying glasses are necessary fordirect visual testing of bolts.

Visual tests often require clean surfaces or decontamina-tion for valid interpretation of results. Special precautionsshould be taken before any cleaning process is used.

Discontinuity Classification

Discontinuities on bolts, studs, washers and nuts can beclassified in four groups. Inherent discontinuities originatefrom solidification of metal in the ingot. Pipe and nonmetal-lic inclusions are the most common and can lead to othertypes of discontinuities in service. Primary processing dis-continuities are produced from the hot or cold working of theingot into rod and bar. Secondary processing discontinuitiesare produced during manufacture of studs, washers, boltsand nuts in machining, grinding, heat treating, plating or

other finishing operations. Finally, service induced disconti-nuities may be caused by vibration, over tensioning and cor-rosion. The presence of inherent, primary processing andsecondary processing discontinuities is sometimes revealedin service.

The most common locations for fastener failures are in thehead-to-shank fillet, through the first thread inside the nuton threaded fasteners or at the transition from the thread tothe shank. Sources of failure are discontinuities in the metalcaused either by segregation in the form of inclusions in theingot or by folds, laps or seams that have formed because offaulty working in the semifinishing or finishing mills (seeFig. 52).

THREADED BUSHING(WHERE USED)

Discontinuities Visible in Bolting

Bursts in bolting materials may be external or internal.External bursts often occur where forming is severe or wheresections are thin (see Fig. 53). An internal burst is a subsur-face discontinuity found in bars and forgings. Internal burstsare caused by rupturing of metal extruded or forged at tem-peratures that are too low or too high.

Seams are generally inherent in the raw material fromwhich a fastener is made. Seams are usually straight or

SHEAR BREAK EXTERNAL BURST

(a)

THREADS HEAD-TO-SHANK MIDGRfPFILLET

DISHED HEAD

MIDGRIP THREADSHEAD

HEAD

id)

FIGURE 52. Common failures in threaded fasteners:(a) tension failures, lb] shear failures and(c) cracked nut

EMI


FIGURE 53. Visible bolting discontinuities


274 / VISUAL. AND OPTICAL TESTING

smoothly curved discontinuities running parallel to the lon-gitudinal axis.

Laps are surface discontinuities caused by folding of themetal. If laps occur in threads, they generally show a pattern

of consistency—they are located in the same place on allthreads of the nut or bolt.

Surface tears occur along the length of a bar or threadedfastener and are caused by faulty extrusion dies or inade-quate lubrication during extrusion. Surface tears can resem-ble seams.

Folds may occur during forging, at or near the intersectionof diameter changes.

Tool marks are longitudinal or circumferential shallowgrooves produced by the movement of manufacturing toolsover the surface of the fastener. A'nick or gouge is an indenta-tion on the surface of a fastener produced by forceful abra-sion or by impact of the fastener against other components.

Shear breaks or shear cracks are open breaks in the metallocated at the periphery of a bolt or nut, at about a 45 degreeangle to the long axis. Shear breaks occur most often withflanged products. They can be caused by overstressing themetal during forging, insufficient ductility and high strainrates.

Necking down is a localized reduction in area of a compo-nent in overload conditions.

Erosion is destruction of metals or other materials by theabrasive action of moving fluids, usually accelerated by thepresence of solid particles in suspension.

Crevice corrosion cracks are a type of concentration cellcorrosion corrosion of a metal caused by the concentrationof dissolved salts, metal ions, oxygen or other gases. It occursin crevices or pockets remote from the principal fluid stream,with a resulting build up of differential cells that ultimatelycause deep pitting.

Recording

Any area where a visual test reveals surface discontinuities(physical damage, wear, cracks, gouges, corrosion, erosion,misalignment, nicks. oxidation, scratches) on studs, washers,nuts or bolts is recorded on a data sheet regardless of discon-tinuity size. Unspecified movement and the looseness ofbolts is also recorded.

If there are areas or indications that cannot be easilyrecorded on a data form, a sketch or photograph is includedwith the report, to clarify the results.

Visual Tests for Forging DiscontinuitiesForgings are often large but simple in shape. They can

usually be visually inspected without complex viewing equip-ment. The forging process usually occurs at an elevated tem-perature, so scaling or oxidation can sometimes be foundinside the discontinuity.

The processing discontinuities that a visual inspectorcould detect on a forging are bursts, laps and cracks.

FIGURE 54. Typical crack in a forged billet


FIGURE 55. Seam in a billet: (a) typical location and(b) magnified view of a rough, oxidized surfaceseam

(al

(b1



These primary processing discontinuities are a result ofthe forging process. Inherent discontinuities caused by pipe,porosity or inclusions could also be detected if the forgingprocess moves them to an exposed surface. In visual testing,a discontinuity must appear on an accessible surface to hedetectable. It is important to plan and perform the visual testduring the manufacturing cycle to provide opportunities forviewing all finished surfaces.

Bursts are internal forging discontinuities. They appear asscaly, ragged cavities inside the forging. Large forgings gen-erally receive secondary processing during the manufactur-ing cycle. Trimming, descaling and machining are allprocesses that could expose a hidden discontinuity such as aburst. A trim cut on the end of a fbrged shaft is a good exam-ple. By scheduling visual testing during the manufacturingcycle after each individual operation, discontinuities may bedetected that would otherwise remain hidden.

Laps are folds of metal forced into the surface of the com-ponent during forging. A lap can he shallow or very deep andits appearance is that of an oddly shaped crack on the sur-face. A lap indication can vary from a tight, straight, lineardiscontinuity to a wide U-shaped indication. When a lap isviewed through a low magnification lens, the inside surfacesoften contain oxidized scale a gray, porous material.

Cracks are different from laps in that cracks follow thestress distribution within the forging while laps do not (seeFig. 54). Both laps and cracks can appear on the surface of aforging as thin, jagged, linear indications.

It is often helpful for the inspector to use a 5 x to 10 xmagnifier. Lenses higher than 10 x are typically large anddifficult to keep steady.

Visual Tests for Rolled StockRolled products are probably the most commonly encoun-

tered visual test objects. A visual inspector should becomefamiliar with the rolling processes in order to identify discon-tinuities by their location on the component.

Processing discontinuities encountered in rolled productinclude tears and cracks. Such discontinuities exhibit charac-teristics similar to some forging discontinuities, including anoxidized, scaly interior.

Inherent discontinuities such as pipe, inclusions and gasholes are affected by the rolling process. They usually formlaminar discontinuities with primary dimensions parallel tothe rolling direction. When any of these discontinuities aremoved to the surface of a rolled product, a seam (Fig. 55),stringer (Fig. 56) or crack can form.

The location of the discontinuity on the component helpsto classify it. Laminations detected by visual inspectionappear on the edges of plate or the ends of pipe (Fig. 57).They are linear and parallel with the top and bottom surfacesof the plate. On rolled shapes, laminations are parallel to therolling direction and appear on the edges of the shape.

Pipe causes laminations that are oxidized inside. Gas holescause laminations that may he oxidized if oxygen wasoriginally present in the hole. Inclusions cause laminationsthat contain a layer of the included material.

Seams, cracks and stringers can appear anywhere on arolled product. Seams and stringers follow the direction ofrolling. Cracks caused by gas holes reaching the surface ofthe component during processing also follow the direction ofrolling. Visual tests of rolled products are complicated by the

FIGURE 56. Rolled bar containing stringers orinclusions elongated during the rolling process


FIGURE 57. Typical I-beam lamination and seamlocations



fact that many complex structures are fabricated from plateand rolled shapes. Visual testing must be performed beforeany fabrication operation that may hide portions of thematerial.

In addition to magnifiers (5 x to 10 x ), a mirror is usefulfor visual testing of rolled shapes with obstructed areas.

Drawing, Extruding and Piercing Discontinuities

The discontinuities associated with drawing, extrudingand piercing are all on the surfaces of the component andtherefore detectable during a visual test.

Drawn products usually exhibit gross failure if any discon-tinuity is present. Because most drawn products have thinwalls, failure usually appears as a through-wall break.

Extrusions can have surface discontinuities that appear asscrapes and tears.

Pierced pipe can contain slugs of metal that are easilyidentified. Severe score marks usually lead to the slug.

Visual Tests for Casting DiscontinuitiesBecause casting is a primary process, the discontinuities

associated with casting are considered to be inherent. These

discontinuities include (1) hot tears, (2) inclusions, (3) poros-ity (4) unfused chills, (5) unfused chaplets and (6) cold shuts.

Hot tears appear as ragged cracks or, in severe cases, as agroup of cracks. Tears are always open to the tsurface and canbe detected visually.

Inclusions in a casting may be detectable during a visualtest—a visual inspector must draw conclusions based on asmall portion of the discontinuity being visible at the surface.Inclusions are usually sand or refractory material and appearas irregularly shaped cavities containing a nonmetallicmaterial.

Porosity appears as a series of hemispherical depressionsin the surface of a casting. This pockmarked appearance iseasily recognized during visual examination.

Unfiised chills and chaplets appear as irregularly shapedcavities on the surface of the casting. The cavity varies fromthe entire shape of the chill or chaplet, such as a square, to aportion of the shape, depending on the amount of fusion.Chills and chaplets differ in shape, so their cavities also dif-fer. Each foundry uses chills and chaplets of preferred shape.Therefore, no definite description of the shape of these dis-continuities is possible.

Cold shuts appear as folds or smooth cracklike discontinu-ities, depending on the location and the severity They areusually visible on the surface of a casting.

SECTION 10

APPLICATIONS OF VISUAL ANDOPTICAL TESTS IN THETRANSPORTATION INDUSTRIESDavid Aiman, DuPont Color Operations Group, Troy, Michigan (Part 1)Roman Baldur, Walsh Automation, Montreal, Quebec, Canada (Part 1)Bruce Bates, Douglas Aircraft Company, Long Beach, California (Part 2)Donald Christina, Douglas Aircraft Company, Long Beach, California (Part 2)Farshid Farrokhnia, Innovision, Madison, Wisconsin (Part 1)Donald Hagernaier, Douglas Aircraft Company, Long Beach, California (Part 2)Richard Horth, Walsh Automation, Montreal, Quebec, Canada (Part 1)Anil K. Jain, Michigan State University, East Lansing, Michigan (Part 1)


INTRODUCTION

Visual testing has traditionally been carried out byhumans. For the transportation industries, tedious jobs arenow increasingly performed by machines, especially wherelarge quantities or repetitive operations are involved, as inassembly line fabrication. The technology of machine visionis also discussed in an earlier section.

Inspection as a part of aircraft maintenance entails manycritical and individual interrogations specific to the compo-nent under inspection. Human inspectors and hand oper-ated instruments such as borescopes are indispensable.

APPLICATIONS OF VISUAL AND OPTICAL TESTS IN THE TRANSPORTATION INDUSTRIES / 279

PART

OPTICAL TESTS IN THE AUTOMOBILEINDUSTRIES

The automotive industry was among the first to embracemachine vision. Applications cover many techniques; one isin the verification of color in an automobile's instrumentcluster. Machine vision can be used to help design the light-ing system behind the cluster to provide even illumination. Itcan also be used to assist in printing overlays to verify that thesilk-screening process is performing satisfactorily. Coloranalysis indicates that certain colors are not being added cor-rectly as sheets are printed. The vision system informs theoperator of clogging screens, preventing scrap from beingproduced.

Machine vision is being used successfully to calibratespeedometers and other gages: A given voltage on the gageshould correspond to certain positions of the needle. An ana-log output from the vision system controls the voltage, andthe system measures the angle between the needle and a ref-erence line. If the measurement is out of specification, thevision system advises the operator to make the neededadjustments.

Texture Analysis of AutomotiveFinishes

Value of Texture Analysis

The increased automation of production lines has turnedmany inspection stations into bottlenecks.' To achieve higherspeed and increased reliability, machine vision systems arebeing used with increasing frequency to perform variousinspection tasks. For example, the electronics industry usesvisual inspection to monitor mass produced printed circuitboards, integrated circuit chips and photomasks."

Because the quality of a surface is often best characterizedby its texture, texture analysis plays an important role in auto-matic visual testing of surfaces. The texture of a paper, forexample, controls its printability because the random fiberdistribution on the surface affects the contact area betweenpaper and the printing medium. In another industry, textureanalysis techniques were made part of an automated lumberprocessing system used to detect and classify common sur-face discontinuities in wood.' Visual inspection of productappearance, as assessed by the customer, is anotherimportant area where texture analysis techniques have

proved useful. For example, textural features have been suc-cessfully used to determine the degree of carpet wear. 5

The appearance of metallic finishes used in the automo-tive industry is affected by their color and their visual tex-ture. One of the factors that determines the acceptability ofthe finish is the visual texture's degree of uniformity. The goalof the inspection program is to find quantitative measuresthat capture the characteristics of the metallic finish texture.These texture features are then used to grade the uniformityof finish samples.

The texture analysis technique used in this application ismotivated by a multichannel filtering model of visual infor-mation processing in the early stages of the human visual sys-tem. It has been proposed that the visual system decomposesthe retinal image into a number of filtered images, each ofwhich contains intensity variations over a narrow range offrequency (size) and orientation . 6 A multichannel filteringapproach to texture analysis is intuitively appealing becausefiner textures are rich in higher spatial frequencies andcoarser textures are rich in lower spatial frequencies. Simi-larly, differences in directionality or orientation are also use-ful for discriminating many natural and manufacturedtextures.

Characteristics of Metallic Automotive Finish

The sparkle and color directionality of metallic automotivefinishes are caused by metal particles such as aluminumflakes added to the paint. The nonuniform distributions ofposition and tilt angle of these flakes within the paint filmgive rise to a visual texture consisting of light and dark colorregions. The distributions of the flakes (and therefore theperceived finish texture) is influenced by parameters of thepaint itself and by various paint application parameters suchas pot pressure, air pressure, gun distance and rheology (sci-ence of flow) treatment. Ideally, the finish texture shouldlook uniform. Judging the degree of uniformity of a finishtexture, however, is a subjective process—even paint inspec-tors tend to have different opinions of uniformity.

Over time, automotive paint inspectors have adopted vari-ous terms for describing the appearance of metallic finishes.Two of these terms, mottle and blotch, represent two poten-tial components of uniformity. Mottle refers to an apparentlyrandom positioning of metallic flakes that creates an acciden-tal pattern. The size of such patterns is usually about 1 mm


(0.04 in). Blotch refers to the nonuniformity characterized byirregularly spaced areas of color change. The size of theseirregularities is usually about 25 mm (1 in.).

In one experiment, the metallic finishes consisted of metalpanels painted under various application parameters, givingrise to finish textures with different degrees of texture uni-formity. Specifically, in two sets of finish samples in lightblue and medium blue, each included thirteen panels sizedat 10 x 15 cm (4 x 6 in.). Panels in each set varied in paintapplication parameters (flash time, gun distance and air pres-sure) as well as in the grade (size) of the aluminum flakes.

A group of visual paint inspectors were asked to judge theuniformity of finish samples in each set. First, ten inspectorswere asked to rank the panels from most to least uniform.The rank order average was then used as initial ranking in apaired comparison experiment. Each panel was compared toeight other panels nearest to its rank order. For example, thepanel with rank order 7 was compared to panels with rankorders 3, 4, 5, 6, 8, 9, 10 and 11. The pairs were presented inrandom order to four observers. Each observer was asked toselect the more uniform panel from the pair shown and eachobserver performed the comparisons ten times. Using theresults of the paired comparisons, a preference frequencymatrix was constructed for each set and the number of timesa panel in row i was preferred over a panel in column j wastabulated. Ordinal scale values for the panels were thenobtained using a scaling technique.' The resulting visualscales values (given in the uniformity column in Tables 1 and2) may be used for evaluation.

In another visual scaling experiment, (en paint inspectorswere asked to grade the finish samples in each set along withother visual components that might be related to the

TABLE 1. In the light blue set, visual scale valuesfor texture uniformity may be related to visiblecharacteristics such as mottling, flake size andblotching-sample 10, for example, had both thehighest score for uniformity and the lowest subjectivelevel of blotching

Panel Uniformity Mottle Flake Size Blotch

I 3.02493 5.72 4.8 5.852 0.21339 8.61 4.65 9.463 3.62269 3.79 11.10 3.024 2.14104 5.91 6.10 7.345 2.42717 5.63 7.75 7.276 1.32080 8.01 8.10 8.797 1.04008 8.38 5.25 9.728 3.36581 3.17 2.25 4.009 0.24747 9.20 6.60 9.69

10 4.80362 3.34 9.85 3.32I 1 0.00000 9.90 11.30 11.8912 1.39585 6.65 3.45 7.3113 4.28204 2.51 9.80 3.35

TABLE 2. In the medium blue set, visual scale valuesfor texture uniformity may be related to values formottling, flake size and blotching

Panel Uniformity Mottle Flake Size Blotch1 1.10309 6.35 5_40 4.702 0.60587 10.10 4.25 8.853 2.32165 5.70 12.40 6.054 2.03682 3.85 5.00 3.805 0.00000 11.20 8.05 11.256 1.35577 9.30 8.05 7.957 1.11753 5.40 4.55 8.058 1.77176 3.75 4.10 5.909 0.12917 9.05 5.35 9.10

10 3.42172 5.20 11.75 4.4011 2.28994 5.35 11.85 4.8012 0.43111 7.65 3.95 6.9513 0.85181 8.10 6.30 9.20

perceived uniformity of the finish. These components arelisted as mottle, flake size and blotch. Each technician wasasked to place the panels on a scale of Ito 10 for each of theabove components, with 10 indicating severe mottle effect,extremely coarse flake size or severe blotchiness. Becausethe observers had no reference samples to define their base-lines, significant individual biases in the resulting values arepossible. The rank order of the scale values, on the otherhand, are less likely to suffer from these biases. The rank val-ues from one to thirteen for each of the visual componentswere averaged and are given in the last three columns ofTables 1 and 2. The goal is to develop, through regressionmodeling as discussed below, quantitative measures ofmetallic finish texture that explain these subjective eval-uations.

Image Acquisition and Preprocessing

Although there are guidelines for lighting and imaging set-ups, every machine vision application has peculiarities. Acrucial issue in any imaging problem is the selection of lightsources that highlight features of interest. The lightinggeometry (positions of light source, camera and the testobject) is equally important.

Based on experiment, directed lighting (not diffused light-ing) is more appropriate for highlighting the texture of anautomotive finish. The specular (mirror-like) nature of themetallic finish poses a challenge to achieving uniform illumi-nation. In general, to image objects with specular finishes-including most smooth, metal objects-is more difficult thanto image lambertian (nonshiny, matte or diffusing) objects.One technique for dealing with the problem of specularreflections is to use a pair of polarizing filters. The first filterpolarizes the light source. The object is then viewed throughthe second, cross polarized, filter (the analyzer). This

FIGURE 1. Perspective plots of (a) a frequencyselective filter tuned to radial frequency of16 cycles (a black line and an adjacent whiteline together constituting one cycle) per imagewidth and (b) an orientation selective filter tunedto 0 degrees; the image array Is 64 x 64 andthe origin fu,v) = (0,0) is at (r,c) = (32,32)

(a)

100000,

0 00000

I c

63 0

lb)

9863 U

I 00000

0.00000


technique did not work this time because almost all thereflected light from the panel surface was blocked by theanalyzer, resulting in a significant loss of detail and contrastin the acquired images. A similar problem occurred whendulling spray was used to reduce the specularity of the finish.

To alleviate the problems arising from the specular natureof the metallic finish, it was necessary to reduce the size ofthe image area on the panel surface. The relatively highimage resolution required for analyzing the finish texturealso dictated the use of high magnification and a small areaon the panel surface (see Fig. I). A single light projector illu-minates the finish sample (panel). To minimize illuminationvariations, the angle between the axis of the camera and theaxis of the light projector is kept at angles between 15 and20 degrees. Too small an angle produces large specularreflection in the camera.

The maximum resolution of the human eye is estimated tobe about 60 cycles per degree of visual angle (a black line andan adjacent white line together constituting one cycle). 8Assuming a standoff of 0.5 m (1.5 ft), the resolution trans-lates into 0.073 mm (2.9 mil) per individual receptor. Theresolution of images obtained using this imaging setup is0.08 mm (3 mil) per pixel, which is close to the above value.Several images from each panel were acquired by shiftingthe panel with respect to a fixed position of the camera andthe light projector. The image data base used here containseight 256 x 256 images from each panel in each of the two

sets. Each image corresponds to about 20 x 15 mm (about0.8 x 0.6 in.).

Note that the physical area is not square. This occursbecause the camera used for acquiring the images has a4:3 aspect ratio. The resolution indicated here is along thehorizontal direction. The resolution in the vertical directionis slightly higher (by a factor of 4/3).

Preprocessing

A number of preprocessing operations are used to com-pensate for nonuniform illumination of the panels. Theseoperations include image subtraction, where the smoothedimage of the background is subtracted from the originalimage. When the intensity variations inherent in the lightsource or its position are known, it is possible to compensatefor the resulting nonuniformities in illumination by sub-tracting these variations from the acquired images. In prac-tice, these variations can be approximated by an intensityimage acquired from the background. The backgroundimage is obtained from an unpainted metal panel and is thensmoothed to suppress the fine texture of the unpaintedmetal.

Further preprocessing is needed to compensate for differ-ences in the first order statistics (the gray level histograms)of acquired images. Such differences are usually caused byvariations in lighting. In this test, differences in factors suchas color and size of the aluminum flakes are also responsiblefor variations. Because histograms of different finish sampleshad similar shape (like a Gaussian distribution), it wasdecided that linear scaling of the gray values should be suffi-cient for suppressing differences in the first order statistics.

The effective width or spread of a histogram can be mea-sured in several ways. Here, the average absolute deviationfrom the mean value was used. The average absolute devia-tion measure is given by:

Nr N,

I s (a ,b ) gl (Eq. 1)

Where:

= number of rows;= number of columns;

a and b = coordinates;s(a,b) = the acquired image; andg = the mean gray level in the image.

Image normalization is achieved by dividing the gray levelsin each acquired image by its average absolute deviation.


Characterization of Finish Texture

The diversity of natural and artificial textures makes itimpossible to give a universal definition of texture and manytechniques for analyzing image texture have been pro-posed. 9." One technique for texture analysis, known as themultichannel filtering approach, employs psychophysicaland neurophysiological findings about biological visual sys-tems. This approach is motivated by a multichannel filteringmodel of visual information processing in the early stages ofthe human visual system. It has been proposed that the visualsystem decomposes the retinal image into a number of fil-tered images, each of which contains intensity variations overa narrow range of frequency (size) and orientation. 6 The psy-chophysical experiments that suggested such a decomposi-tion used various grating patterns as stimuli and were basedon adaptation techniques.

Subsequent psychophysiological and neurophysiologicalexperiments provided additional evidence supporting themultichannel filtering theory. In one case, scientists decodedthe response of simple cells in the visual cortex of theMacaque monkey to sinusoidal gratings with different fre-quencies and orientations." It was observed that each cellresponds to a narrow range of frequency and orientationonly. Therefore, it appears that there are mechanisms in thevisual cortex of mammals that are tuned to combinations offrequency and orientation in a narrow range. These mecha-nisms are often referred to as channels and are appropriatelyinterpreted as bandpass filters.

The main issues involved in the multichannel filteringapproach to texture analysis are: (1) functional characteriza-tion of the channels and the number of channels, (2) extrac-tion of appropriate texture features from the filtered images,(3) the relationship between channels (dependent versusindependent) and (4) integration of texture features fromdifferent channels to produce an interpretation. Multichan-nel filtering techniques proposed in the literature differ intheir approach to these issues.

In the optical analysis of metallic automotive finish tex-ture, a multichannel filtering technique with a proven utilityfor texture classification was used."' The text belowdescribes the functional form of the channels, the choice ofthe filter parameters and the definition of texture features.An important advantage of the multichannel filteringapproach is that it accommodates simple statistics of gray val-ues in the filtered images as texture features. This simplicityis the direct result of decomposing the original image intoseveral filtered images with limited spectral information. Incontrast, texture features that are based on the statistics ofthe gray level distribution in the original image (gray levelco-occurrence features, for example) are usually very com-plicated and lack physical interpretation.

Filter Functions and Parameters

The functional characterization of the set of filters used inthis application are detailed in the literature.' 2 Each filter inthe set has either frequency selective or orientation selectiveproperty only. The filters are specified in the spatial fre-quency domain by their modulation transfer function. Themodulation transfer functions H of the frequency selectivefilters are given by:

1 • [ln(u2 + — In p.1 2 }

H(u,v) = exp { 2(Eq. 2)

where u is the horizontal frequency component, v is the ver-tical frequency components, is the center radial frequencyand a, determines the bandwidth of the filter. For Eq. 2 and3, (u,v) (0,0). Note that these filters are defined on a loga-rithmic scale. The modulation transfer functions of the ori-entation selective filters are given by:

H(u,v) = (Eq. 3)

{minlitan-1 — — (a + 7r)I]}2

exp1 • 1

2

ffy

Where:

0 tan (•) IT; anda = the center orientation of the filter (in radians).

The character (•) is a place holder for a variable.The value of a, determines the orientation of the filter

bandwidth. In this application, the value of modulationtransfer functions at (u,v) = (0,0) is set to zero for both typesof filters so that the mean gray values of the filtered imagesare zero (the direct current component is blocked). Modula-tion transfer functions of a frequency selective and an orien-tation selective filter are plotted in Fig. 1.

Metallic finish textures do not possess significant orienta-tion tendencies (they are practically isotropic). In quantifi-cation of metallic finish textures, therefore, only the isotropicfrequency selective filters defined by Eq. 2 were used. Avalue of u, = 0.275 is used for all filters. This results in a ban-dwidth of about one octave (the frequency bandwidth, inoctaves, between frequency f, to frequency f„ is given bylog,(f,/f,). This bandwidth is close to the estimated band-width of simple cells in mammalian visual cortex?'

The problem is to determine the appropriate values forthe center frequencies of the fitters by searching among alarge but finite number of filters whose center frequencies

(a)

(e)

(b) (c) (d)

(g ) fh)


are a half octave apart. The number of filters considereddepends on the size of the input image. For a 256 x 256image, a total of fourteen frequency selective filters tuned to1, 1(2) 12, 2, 2(2)", 4(2) 1 ", 8, 8(2) 1` 16(2)", 32, 32(2) 112 ,64, 64(2)" cycles per image width are used. This choice ofcenter frequencies for the filters provides a nearly uniformcoverage of the spatial frequency domain. Any significantrange of spatial frequencies in the input image should fall inthe passband of one of these filters.

A fast Fourier transform is used to perform the filteringoperations in the spatial frequency domain. Figure 2 showsan image of a finish sample along with some of the filteredimages. The ability of the filters to exploit differences in spa-tial frequency (size) is evident in these filtered images.

Texture Features

The texture features are defined as the average absolutedeviation in the filtered images. The texture feature f, for thejth (zero mean) filtered image ri ( • ,• ) is computed as follows:

N, N,

Ir.,(a,b)1 (Eq. 4)=1 13=-1

where N, and N, are the number of rows and columns in theimage. Each filtered image is therefore summarized by onefeature and there are as many features as filtered images.With a total of ten filters, for example, there are ten texturefeatures, resulting in a ten-dimensional feature vector foreach image. These feature vectors are then used in gradingthe texture uniformity of panels as described later.

Image normalization can be achieved by dividing each tex-ture feature by the average absolute deviation measure in theinput image defined by Eq. 1. Formally, the normalized fea-turef,' corresponding to texture featuref is given by:

f; = f1-- for j = 1, , 14

where fo is the average absolute deviation measure of theinput image. In the text below, normalized texture features

FIGURE 2. Images of a metallic finish sample (Figures 2b through 2h illustrate responses of frequency selectivefilters with center frequencies listed): (a) a 256 x 256 nonfiltered image, (b) 4 cycles per image width,(c1 8 cycles per image width, (d) 16 cycles per image width, (e) 1612) 1 /2 cycles per image width, (f) 32 cyclesper image width, (g) 32(4 12 cycles per image width and (hi 64 cycles per image width

r


are referred as f„ f, and so on. the first two features, f, andf,, are not used in the grading experiments. These featurescorrespond to filters that respond to very slow intensity varia-tions. Such variations, however, are more likely to result fromvariations in lighting than from variations in finish texture.

Reference Based Grading of Finish TextureUniformity

A few painted panels with extreme appearances are usedas reference panels. Because finish samples with highly uni-form or highly nonuniform texture are much easier to iden-tify, these reference panels can be selected with highconfidence. In this application, two panels with lowest visualscale values and two panels with highest visual scale values ineach set are used. Using the feature vectors correspondingto images from these four panels, a least uniform and mostuniform reference cluster is constructed and a texture uni-formity grade is assigned to each panel based on the dis-tances of its mean feature vector to these reference clusters.

Formally, let f denote the feature vector for the in imagefrom a panel. Each panel is represented by its mean featurevector:

i= 1

where m is the number of images taken from a panel (here,m = 8). Let d„ and d, he the distances between the meanfeature vector f and the least uniform and most uniformclusters, respectively (see Fig. 3).

The distance of a point from a cluster of points can bedefined in several different ways. Here, the Euclidean dis-tance between the point and the centroid (mean) of the clus-ter was used. The texture uniformity grade y for the panel isdefined by the following ratio.

y= d d,

FIGURE 3. Reference based grading in a two-dimensional feature space

u LEAST UNIFORM CLUSTERa

o a

MOST UNIFORM CLUSTERd,

0

0 0

Note that -y lies between 0 and 1. A value of -y close to 1indicates that the corresponding panel can be classified asuniform.

Feature Selection

There is a one-to-one correspondence between the tex-ture features and the filters. Selecting a subset of featurestherefore is equivalent to selecting a subset of filters.Described below are feature selection experiments based onmaximizing the rank correlation between the visual scale andthe texture uniformity grade given by Eq. 5. This rank corre-lation is given by:

– —

(Eq. 6)

n 1 )2

n(n2 — I)12

Where:= the rank of texture uniformity grade among all -ys;

S, = the rank of visual scale value u, among all vs, andthe total number of panels.

Note that unlike correlation, which measures linear associa-tion, the rank correlation measures the monotone associationbetween two sets of data.

The feature selection procedure can be summarized asfollows. A texture grade is assigned to each panel in a givenset. Next, the rank correlation between texture grade andvisual scale value is computed. This procedure is repeatedfor every subset of texture features. The feature subset ischosen to give the highest rank correlation. Starting with alltwelve texture features (corresponding to filters with 2through 64(2) 1" cycles per image width center frequencies),exhaustive feature selection is performed. The best featuresubsets for the light blue and the medium blue sets of panels,along with the corresponding rank correlations, are given inTables 3 and 4.

As expected, there were occasionally ties between differ-ent feature subsets. These ties were resolved based on thedirect correlation between the texture grade and the visual

(Eq. 5)

f- f1=I

sse =

sstot = Ey

L


TABLE 3. Reference based grading of finish textureuniformity for the light blue set; shows the bestfeature subsets of size i to 7 and corresponding rankcorrelations (the higher number being better)between texture grade and visual scale

Size Best Subset Rank Correlation

1 {15} 0.912 115, f141 0.983 {f5, 112, f14} 0.984 {f5, f6, 112,114} 0.975 {f5, f6, f7, f12, f14} 0.976 {14, 16, 17, 112, f13, 114) 0967 {14,15,16, 17, 11 2, f13, f14} 0.96

TABLE 4. Reference based grading of finish textureuniformity for the medium blue set; shows the bestfeature subsets of size I to 7 and corresponding rankcorrelations (the higher number bring better)between texture grade and visual scale

Size Best Subset Rank Correlation

1 {11O} 0.812 {f3, 0.893 {15, f10,113} 0,894 {f3,17, f10,111} 0 915 {13, f7, fI0, 11 1, 113} 0.916 17, f9 f10, (131 0.917 {f3, f6, f7, HO, f 1 1, f13, f14} 0.88

scale value (the feature subset having the highest correlationwas chosen).

The highest rank correlation for the light blue set is 0.98and is achieved by feature subsets ff,,f NI and If f, f14. • Thehighest rank correlation for the medium blue set is 0.91 andis achieved by feature subset f7 , fi ,„f„}. Even though thelight blue and medium blue sets of panels have different col-ors, the best feature subsets for both sets of panels wouldlikely be the same. By comparing the feature subsets of size4 for both sets of panels, a common subset was sought thatcould be used for grading both sets of panels. The featuresubset f7, f9, f„}. results in a rank correlation of 0.95 forthe light blue set and 0.89 for the medium blue set. Althoughthe best feature subsets for the two sets of panels are not thesame, there does exist a feature subset that results in accept-able performance for both sets.

Regression Based Grading

Below is proposed a different approach to relate the visualscale values for finish texture uniformity to the texture

features. This alternative grading scheme is based on theclassical linear regression model. Unlike the previous texturegrading scheme, which uses only the panels with extremeappearance qualities as training samples, in the followingregression based grading scheme all the panels in a given setare used for estimating the parameters of the regressionmodels.

In the regression setting, the visual scale for texture uni-formity is viewed as the dependent variable and the texturefeatures computed from the filtered images are viewed asindependent (or predictor) variables. Specifically, let v be thevisual scale for a panel and f be the texture features associ-ated with the panel. Note that texture features for a panel areobtained by averaging the texture features for all eightimages corresponding to the panel. The regression coeffi-cients are estimated using the least square mettod,15 Let theleast square estimates of these coefficients be 13 0, Pi, ,Then the predicted visual scale values are given by:

= Rn + + R2 f2 + •-• + lc, ( Eq . 7)

The quality of the fit can be measured by the coefficient ofdetermination, given by:

sseR2 =- 1

sstot (Eq. 8)

where

and where n is the number of panels (observations). Thisquantity is an indicator of the proportion of the total varia-tion in the t), values explained by predictor variables. In theprocess of deciding which subset of features explains thevisual scale values better, regression models are comparedwith a different number of predictors. In order to comparethese models with one another, the adjusted coefficient ofdetermination is used:

= 1sstot/(n - I)

ssel(n - p)

(Eq. 9)

where p is the total number of parameters (including (30) inthe fitted model.

and


Model Selection

In the following experiments, the texture features com-puted from images normalized by their average absolutedeviation measure are used. Although examining the regres-sion models corresponding to all possible subsets of featuresis computationally demanding, the required computations inthis case were not prohibitive. The best regression modeland the best feature subset is determined as follows. First,the best regression model for a given size of the feature sub-set is determined. The criterion for best model is to max-imize the coefficient of determination (R2 ). Among thesefeature subsets, the subset with the highest adjusted coeffi-cient of determination (11„,,, 2 ) is singled out as the best model.This model can then be used for grading (predicting) thedegree of uniformity of finish texture for future samples.

Tables 5 and 6 give the feature subsets of size 1 through 7from the set {A, ,f, 4} that give the best regression modelsfor the light blue and medium blue sets. As seen in Table 5,for example, the adjusted coefficient of determination firsttends to increase as more variables are included but it beginsto flatten when more and more variables are used. Note thatthe number of samples used for estimating the regressioncoefficients is small (12 or 13). The estimated -Coefficients forregression models with a larger number of independent vari-ables are therefore not very reliable. Also, the referencebased grading scheme indicated that a feature subset of size4 results in acceptable performance, so that the model selec-tion is restricted only to those with no more than 4 indepen-dent variables.

Based on the above constraints, the best regression modelfor the light blue set is:

= 13.859 - 1.5315f, + 3.1389f4 (Eq. 10)

- 2.1269f, - 0.4014f8

The best regression model for the medium blue set is:

= 82.9490 - 3.3293f, + 3.3342f, (Eq. 11)

- 1.4469fm - 0.9514f 4

Note that the best regression models for the light blue andthe medium blue sets are not the same. A common regres-sion model that gives acceptable performance for both setsof finish samples proved to be the subset {f3,f4,f6>fii}• Thecorresponding regression model has a coefficient of determi-nation of 96.93 percent for the light blue set and 88.6 percentfor the medium blue set. Therefore, the same subset of fea-tures may be used for grading both sets of panels.

TABLE 5. Regression based grading of finish textureuniformity for the light blue set; shows selectedvariables (texture features) for regression modelswith 1 to 7 variables and corresponding coefficientof determination values

Size Best SubsetR 2

(percent) (percent)

{15} 84.79 83.272 {13,16) 91.75 89.913 {13, f4, 16} 96.92 95.774 {13, f4, f5, 18) 98.37 97.445 113, fl, 15. fl 0. fill 98.99 98.166 113, f4,15, f 10, fll, 1121 99.30 98.477 }13, 14,15, 17, 18, 19, 113} 99.35 98.22

TABLE 6. Regression based grading of finish textureuniformity for the medium blue set; shows selectedvariables (texture features) for regression models with1 to 7 variables and corresponding coefficient ofdetermination values

Size Best SubsetR2

{percent)R.d1 2

(percent)1 {15} 56.17 52.182 (13, f4} 70.38 64.463 }f3, f1i,f14} 83.10 77.474 {f7, f8,110,114} 93.32 89.995 {f7, V0,112,113, f14} 96.91 94.716 {f 3. f7. f10. f12. f13,114} 97.87 95.747 {f4, 15, f6, f11, 112, 113, f14} 98.70 96.88

Grading Examples

With more panels in each set, part of the set is used to esti-mate the parameters of the regression model and the balanceis used to test the model. Because there are only a few panelsin each set in this application, all were used for parameterestimation. An alternative strategy is to use the regressionmodel obtained by the samples in one of the sets to predictthe visual scale values for the other set. It must be remem-bered that, since each set of panels was evaluated separately,the visual scale values for the two sets are not on the samescale. In fact, the range of visual scale values for the light blueset is wider than that of the medium blue set-see the sec-ond columns (for uniformity) in Tables 1 and 2. Therefore,the rank correlation between predicted and actual visualscale values is perhaps a more suitable figure of merit thanthe direct correlation between them.

The feature subset if,,f4, is used in the following twograding examples. First, the visual scale values for textureuniformity for panels in the light blue set were used to esti-mate the parameters of the regression model. Next, the


predicted visual scales for panels in the medium blue setwere obtained using this model. The correlation and rankcorrelation between predicted and actual scales are 0.87and 0.88, respectively. Similarly, the visual scale values forpanels in the medium blue set were used to estimate theparameters of the regression model. The predicted visualscale values for panels in the light blue set were thenobtained. The correlation and the rank correlation betweenpredicted and actual scales are 0.76 and 0.74, respectively.These results indicate that the regression based gradingscheme is fairly robust.

Mottle, Flake Size and Blotch Components

Now consider additional visual scale values that rank theappearance of the finish samples along with other compo-nents. In the text below, the linear regression setting is usedto obtain best regression models explaining mottle, flake sizeand blotchy appearance of metallic finish samples. The crite-rion for the best regression model is also the same—R A2 .Based on these experiments, the best regression modelexplaining the mottle appearance of panels in the light blueset is:

t3 = — 12.61 + 2.0233f, — 2.9597f, (Eq. 12)

+ 1.6562f, + 0.5511f,

The best regression model for the medium blue set is:

= —149.079 + 4.713f, — 13.042f, (Eq. 13)

+ 9.861f, + 2.309f13

The corresponding coefficient of determination values forthese models are 97.5 and 85.49 percent, respectively.According to the model selection, feature subsets {1:3,f,,f,,f,}and {f,, f5, f,,} form the best feature subsets for the lightblue and medium blue sets of panels, respectively. In the textabove, it was shown that the feature subset tf,,f,,,f„„f,31 givesacceptable performance for both sets of panels, when grad-ing texture uniformity. Note that the above feature subsetoverlaps considerably with this feature subset. This observa-tion suggests that the mottle appearance of the metallic fin-ish is a dominant component of the perceived uniformity ofthe finish texture.

The best regression models explaining the flake sizeappearance of finish texture for panels in the light blue andmedium blue sets respectively are:

= 178.909 — 1.528f, + 2.029f, (Eq. 14)

— 1.863f11 — 2.092f,

and

13 — 20.988 — 2.611f, + 2.037f, (Eq. 15)

+ 2.710f,

The corresponding coefficient of determination values forthese models are 87.4 and 93.34 percent, respectively.

Similarly, the best regression models explaining theblotchy appearance of panels in the light blue and mediumblue sets are:

= 85.1814 + 1.6055f, — 0.7237f, (Eq. 16)

+ 1.6886f„ — 2.9040f12

and

1.3 = — 216.432 + 3.476f7 + 2.989f2(Eq. 17)

—2.201f13 + 3.012A9

The corresponding coefficient of determination values forthese models are 97.88 and 75.25 percent.

The strong association between the texture features andthe visual scale for the panels is reflected in the high coeffi-cient of determination values for the above regression mod-els. The mottle, flake size and blotchy appearance of themetallic finish all contribute to the perceived uniformity ofthe finish texture. As pointed out earlier, the contribution ofthe mottle appearance seems to be more significant thanothers.

Conclusions

This text addresses techniques for automatic opticalinspection of the textural appearance of metallic automotivefinishes. A multichannel filtering technique is used to obtaina number of texture features, each of which captures thecharacteristics of the finish texture in a narrow range of fre-quencies. Two methods are proposed to use these texturefeatures to grade the degree of uniformity of the finish tex-ture. Feature (filter) selection experiments are detailed todetermine the best subset of features (filters) for the gradingtask.

Nonuniform illumination of the panels resulting fromtheir specular nature and resolution requirements causedthe need to acquire multiple images from small areas on thepanel surface. It is more desirable to have a single imageacquired from the entire panel surface because humanobservers are more likely to base their judgment on simulta-neous examination of the entire panel. Currently, the resolu-tion of most sensors is limited to 1,024 x 1,024 pixels. Forimaging larger areas, the panels must be scanned with a sen-sor array under a linear light source.

In this texture analysis technique, the filtering and featurecomputation operations account for most of the required

FIGURE 4. Brake shoe in the testing system fixture

LIGHT

BRAKE SHOEO

FIXTURE

INDEXI NGTABLE

INDEXING PUSHERS

LOADING STATION

WIDTHMEASUREMENT

(a) LOADING STATION

FIGURE 5. Brake shoe sorting system; la) top viewand (b) side view

VIDEO STATION

eACKLIGHTINE,

TESTING PHASE DISTRIBUTION PHASE

CONTINUOUSLY MOVING PUSHERS

INDEXING PUSHERS

STATIONARY TABLE

TRANSFER CONVEYOR FIXED PLATES

GATES


computations. However, these operations can be performedin parallel, regardless of the number of filters. Moreover,grading experiments showed that a small number of filters issufficient. The results of feature selection indicate that usinga smaller number of features is not only possible but alsoleads to improved performance. These results indicate thata common feature subset can give acceptable performanceacross different sets of metallic finish samples.

The texture features used in the grading experiments werea measure of energy in each filtered image. Each finish sam-ple was then represented by the mean feature vector for alleight images from the panel. This representation assumesthat the variation of texture features across images is negligi-ble. However, when grading texture uniformity the variationof texture features across the panel could he a good indicatorof finish texture's degree of uniformity. That is, larger varia-tions indicate that the texture is less uniform.

Automotive Brake Shoe Sorting SystemManufacturers that refurbish brake shoes usually carry out

the following operations. Boxes containing a random mix ofused brake shoes are visually sorted and stored according totheir model numbers. Specially trained operators memorizepertinent characteristics of each model out of several hun-dred currently in use. The main difficulties in the sortingprocess stem from two things: (1) certain models areidentical in all respects except for specific, often very small,details; and (2) because of wide manufacturing tolerances,brake shoe models can vary significantly. Errors in assigningwrong model numbers can be very, costly in subsequent pro-,,sses, •including cleaning and recovering the shoes -with anew lining.

In a machine vision sorting system, brake shoes are loadedonto a conveyor that transports them through a width mea-suring device and an optical contour detection station. Thebacklighted parts are analyzed by special purpose softwarethat extracts only the significant characteristics from theimage. These data are combined with the width measure-ments to determine the model number.

There are several hundred brake shoe models in use andthese can differ from each other in minute details, whileidentical models can vary significantly, depending on themanufacturer. Sorting is further confused when brake shoesfrom older cars are modified to fit current vehicles.

The optical recognition system acknowledges these diffi-culties, is virtually error-free and performs the inspections ata rate of about 4,000 units per hour.

Sorting Equipment

Figure 4 shows a typical brake shoe. Figure 5 illustratesequipment for automatic optical sorting of brake shoes. Thesystem consists of two main elements: the inspection tableand the distribution conveyor.

Used brake shoes are first loaded onto the inspectiontable. A series of arms, driven intermittently by an indexingmechanism, carries the brake shoes around the table past a

FIGURE 6. Detail of the sorting system width sensor

DIRECTION OF BRAKE SHOE MOTION

ANGULAR ENCODER

LEVERS

SKI

INDEXING TABLE7 SURFACE

FIGURE 7. Brake shoe image and measurementdetails

WEB HEIGHTMEASUREMENTS(UP TO 51

FIXTURETHICKNESSMEASUREMENT

SHOE LINING

SHOE WEBBING

RECTANGLE FEATURE WINDOWFOR HOLE AREA MEASUREMENT

HOLE FIXTURE HEIGHTMEASUREMENT(RIGHT SIDE)

COARSE SPAN MEASUREMENT

FIXTURE HEIGHTMEASUREMENT(LEFT SIDE(

FIXTURE


width measurement station and a video camera. Dataobtained from these two subsystems are fed into a computerthat determines the specific model number of each compo-nent using a look-up table. Subsequently, the brake shoes aretransferred to a conveyor equipped with a series of gates.The computer software maintains a record of the position ofeach shoe within the system. Each conveyor gate is associ-ated with a different model number and is opened to receivea brake shoe when that specific model arrives in position.

6Visual Testing Facility

The inspection table includes two manual loading stations.Because brake shoes can be sorted by the system at a rate ofone per second, a single operator is not able to keep up withthe equipment. A series of arms, attached to a centralindexing mechanism, pushes the shoes around a stationaryannular table. The station following the loading position isused to determine the width of the brake shoes.

This is performed by means of a gage suspended above thetable as shown in Fig. 6. A ski at the bottom of the gagedeflects the arms of the gage as it slides over each shoe. Anoptical encoder, used to measure the gage rotation, sends thedata to the central comp_uter for subsequent use.

The fourth station in the system is the machine visionhardware. A camera located above the table acquires theimage of the brake shoe during the short dwell time of theindexing mechanism. At the vision station, the steel table iscut away and replaced by a frosted glass pane]. The cameraconsequently takes backlighted images that are converted tobinary data for later calculations. A typical image is shown inFig. 7. In principle, the system determines the span of thebrake shoe, the contour of the central web plate and certainfeatures that could be holes or other characteristic details.

The Distribution Conveyor

The brake shoes discharged from the inspection table fallonto the distribution conveyor. The forward motion of theshoes is obtained by means of pusher arms that constrain themotion of the parts as they slide over a steel base plate (seeFig. 8).

The pusher arms of the distribution conveyor move at aconstant speed but they are, synchronized with the indexingmechanism of the inspection table. The position of each shoetransferred to the conveyor is predictable and the systemsoftware monitors the position of each brake shoe. This posi-tioning information is updated with each cycle of theindexing mechanism.

The distribution conveyor has openings cut out of the bot-tom steel plate. They are normally covered by pivoted gatesand permit the brake shoes to slide freely over them. Eachgate is activated by a separate solenoid that opens the gate so

that each brake shoe can drop into a bin assigned to its modelnumber. The distribution chart maintained in the computeris used to determine when an opening signal should be sentto a specific gate. After discharge, each gate is closedmechanically when the advancing bar comes into contactwith the gate's restoring lever.

FIGURE 8. Detail of the distribution conveyorCHAIN DRIVE FOR PUSHERS

RESTORINGLEVER

FIXED PLATE


Sorting Requirements

The algorithm used to recognize specific brake shoes mustsatisfy the following conditions.

1. Used brake shoes often contain features that are notpresent on the original design. This occurs becauseadditional machining is added to obsolete shoes so thatthey fit more recent vehicles. These modified shoes areacceptable for restoring.

2. Brake shoes made by various manufacturers are oftendifferent in some minor details. They are usually madeto different dimensions so that the total span may varyby as much as 3 mm (0.125 inf. The width may alsovary this much. The web plate thickness cannot be usedas a recognition criterion.

3. There exists a significant number of pairs of brake shoemodels that are identical in all respects except for aminor feature. This could he a slightly different loca-tion of a specific opening or the size of a specific hole.Those differences are often smaller than the tolerancescreated by differences in manufacturing origins.

4. In some instances, complex chains of similarities exist.Several brake shoe models may share identical featuresand they must be distinguished by a combination of upto three distinct features.

5. Several brake shoes have been assigned two modelnumbers, depending on their application, althoughthey are physically identical in all respects.

6. Brake shoes are sorted before cleaning so that holesmay he partially or completely covered. Some shoeshave reasonably good linings, while other have no lin-ings. Occasionally, brake shot's are visibly damagedbefore sorting.

7. After sorting, brake shoes undergo various automaticoperations. The first of these, cleaning, is expensive anddamaged shoes should be eliminated. Subsequentoperations are designed for specific models. A wrong

shoe can damage machinery if allowed to enter thewrong machine. Consequently, errors related to wrongmodel numbers cannot be tolerated. Rejection in caseof doubt is preferable but the efficiency of the opera-tion is clearly affected.

Creation of the Data Base

The recognition process relies on the data base kept in thecomputer memory. The following procedure is used to createthe data base.

All brake shoe models are loaded singly onto the inspec-tion table and the indexing mechanism is advanced one step.After the width measurement is stored in the computermemory, a video image of the shoe is displayed on the moni-tor. All pertinent measurements are taken automatically andstored. The operator then flips the shoe. Because brakeshoes are not symmetrical, and may subsequently need to berecognized in either orientation, allowances are made for thelack of symmetry. A second set of measurements is takenwith the brake shoe in its new position.

The operator may then request a list of models that haveidentical (or very similar) contour characteristics. Referenceto the catalog permits the definition of distinguishing fea-tures. This is done by tracing windows around an area of thevisible image (existing software helps trace the windows andseveral windows may be defined for each shoe). The softwareis also designed to account for lack of symmetry so that thewindows in the memory model are always correctly locatedrelative to the rest of the contour.

The Sorting Algorithm

The working version of the sorting algorithm is designedto satisfy all the requirements listed above and the followingprincipal steps have been taken.

A catalog of all current brake shoe models has been pre-pared, including all features needed to recognize eachspecific model. Two important parameters are width andspan. Using those values, it is possible to assign each modelto a class of items that have similar dimensions. Because ofthe variable manufacturing tolerances, each class spans a sig-nificant range of values, from about 3 to 6 mm (0.125 to0.25 in.). The classes are chosen so that there is an overlapthat includes all dimensional deviations.

When brake shoes are sorted, the first measurements arethe width and span. These determine the brake shoe's classand immediately narrows the selection to about 10 percentof all possible models. The next step in recognition is mea-surement of the central web plate contour. Because the toler-ance for the span is about 3 mm (0.125 in.), it is not possibleto locate a reliable point of reference for coordinates of theweb contour. Measurements are therefore taken for a num-ber of groups of adjoining points called pillars, in order to


enclose a possible shift of the origin of the previously storedcontour in the computer memory.

The span is determined by finding the two extremes of thecontour of the shoe at rest in an arbitrary orientation on theindexing arm of the inspection table (see Fig. 7). The leftside is taken as the origin and the X coordinate of the mid-point is found. Several pillars are located symmetricallyabout that midpoint. Each pillar contains an odd number ofcolumns spaced one pixel apart. As many pillars as possibleare included within the complete span.

Within each pillar, the height from the baseline to the con-tour of the web is determined for every column. Theminimum and maximum heights within the pillar areretained in memory and the original arbitrary orientation ofthe shoe is designated as the normal position.

The brake shoe is flipped and the same measurements arerepeated, taking into account that pillars formerly left of themidpoint in the normal position are right of the midpoint inthe flipped position. The minimum and maximum for thecolumn heights are again determined and compared withprevious readings. Final values are kept in the memory withlimits of the minima and maxima.

In summary, the brake shoe span and width are measured,the midpoint is determined. Next, symmetrically and equallyspaced midpoints of the pillars are located. The columnheights of those midpoints_are then found. Those values arecompared with the data corresponding to all model numbersselected on the basis of the width and span for each model(the normal and flipped positions are analyzed). A match is

achieved if all the midpoints lie within the extremesrecorded in the data base.

If more than one match is found, further elimination isperformed by analyzing special features. These have beendefined during the system's learning phase by describingwindows around significant details and recording the num-ber of pixels. The combination of the web comparison aug-mented by the feature elimination leads to the recognition ofa unique model.

This method allows for dimensional variations within thesame models but was found very accurate in identifyingmodel numbers.

ConclusionsA brake shoe sorting system has been designed and built,

using optical, machine vision technology. The system canrecognize a specific model number out of several hundredpossibilities in less than 1 s. In addition, the optical systemallows for dimensional variations caused by manufacturingpractices and yet can distinguish brake shoe models that dif-fer only in minor details. The distribution conveyor attachedto the inspection system loads each shoe into a bin assignedto its model.

Certain brake shoes can damage processing equipmentdesigned for other models. Accurate sorting with the opticalsystem significantly reduces this possibility and markedlyreduces the amount of subsequent processing done ondefective brake shoes.


PART 2

OPTICALLY AIDED VISUAL TESTING OFAIRCRAFT STRUCTURE

Visual testing is the primary method used in aircraft main-tenance and such tests can reveal a variety of discontinuities.Generally, these tests cover a broad area of the aircraft struc-ture. More detailed (small area) tests are conducted usingoptically aided visual methods. Such tests include the use ofmagnifiers and borescopes. The text below details opticallyaided tests of structures where access is poor for other non-destructive tests. A short description of optically aided visualtest devices is given, along with seven examples where suchdevices are used to inspect aircraft structures and operatingmechanisms. The detection of corrosion pitting, stress corro-sion cracks and fatigue cracks is discussed and illustrated.

Many discontinuities are revealed during visual tests ofaircraft, including rust stains (that reveal corroded steel),paint blisters around fasteners in wing skins (that revealunderlying intergranular corrosion of the skin), yellowishstains on the fuselage (that reveal through-thickness cracksin the skin), and fuel leaks in the lower wing skin (that canindicate a cracked spar cap or skin). Even the pilot's walk-around visual inspection can reveal some serious faults. Forexample, an improper attitude of the aircraft can indicate afailed main landing gear attachment fitting and a floatingspoiler can reveal a failed torsion bar.

Testing with Visual and Optical AidsVisual testing is the oldest and most economical form of

nondestructive testing. The use of optical instruments invisual testing is beneficial for two purposes: (1) to magnifydiscontinuities that cannot be detected by the unaided eyeand (2) to permit visual checks of areas not accessible to theunaided eye.

It is important to know the type of discontinuities that maydevelop and to recognize the areas where such problems mayoccur. Magnifying devices and lighting aids are used and thegeneral area is checked for cleanliness, presence of foreignobjects, security of the component, corrosion and cracks orother damage. In many cases, the area to be inspected iscleaned before visual inspection.

Magnifying Lenses

An optical microscope is a combination of lenses used tomagnify an image. The object is placed close to the lens in

order to obtain as great a magnification as desired. The dis-tance from lens to object is adjusted until the object is in thelens's depth of field and is in focus.

The simplest form of a microscope is a single converginglens, often referred to as a simple magnifier Magnification Mof a single lens is determined in millimeters by the equation:

250M = —

f(Eq. 18)

or in inches:

10M=

In this equation,f is the focal length of the lens in millimeters(or inches) and 250 (or 10) is a constant that represents theaverage minimum distance at which objects can be distinctlyseen by the unaided eye. Using the equation, a lens with afocal length of 125 mm (5 in.) has a magnification of twowidths or is said to be a two power (2 x ) lens.

For a simple magnifier, the focal length and working dis-tance are about the same. For example, suppose that a com-ponent must be inspected without moving it and that amagnifier cannot be placed nearer than 75 mm (3 in.). A lenswith a working distance (focal length) of at least 75 mm(3 in.) is required. From Eq. 18, that is shown to be a 3 x ora three power lens.

The field of view is the area seen through the magnifier.With a simple magnifier, the diameter of the field of view isless than its focal length. Selection of a magnifier with theproper field of view is important. For example, if the testobject is large, it takes too much time to use a 20 x magnifier,with a field of view slightly greater than 10 mm (0.37 in.).The proper procedure is to use a low power magnifier first,marking questionable areas and then to inspect questionedareas with a higher powered magnifier.

Depth of field denotes the distance a magnifier can bemoved toward or away from a subject with the subjectremaining in good focus (sharply defined). At other dis-tances, the subject is out of focus and not sharply defined.Depth of field varies with the power of the lens and is

FIGURE 9. types of borescope viewing angles:la) straight fotward. {b) laterai, {-c) forwardoblique and id) retrograde

C ap

(b I

•

i0 .0VinDEr gIlS

icl

0 DEGRE ES- -

APPLICATIONS OF OVAL AND OPTICAL TESTS IN TI-IF TRANSPORTATION moil gritiEc 1 243

comparative's, greater in lower power magi sificrs decreasingas the power of the Imes increases.

BareSCOpeS

A horescopt is a ilPt •eise optical instniment with built-inillumination, It can be used to visually check internal arrasand deep and [mires.

horescopen are available in rigid and flexible models from2.5 min (0,1110 in diampter to 19 MnI (0.75 in.) in ski ISIrterand meters (several lect) in length. 'Ilicsc devices an• wiser-ally provided with fixed diameters and fixed working lengths,Willi qitical systems designed to provide direct. right angle.retrosp••iii re and oblique vision (see Fig. 9).

Cone nil's', the (6211 t CT of tlic bOrestOpe is (141tingentthe diameter of the licie or 11451 -e laci ng inspected. The lengthof tlic horoscope l`; governed by the distance behaTen theavailable at.t.'eliN and the distnnue to tilt' tr.14 area, The chiriceof viewing angle is determined lay discontinuity type nailtuCttion.

Microborescopes

The iniensly iiise.cspe is I t} se of optical device, rine .c .gev-eral designs known In the medical professions as endoseopes.

It is midi like a borescripti with superior opt ical yvstrms ;sadhigh intensity cold light piped to the ivorking tip thottighlibel opt it • /ruirklies.

Othemamilble features inefude eonstant firms from aliout4 ram (0.15 in.) to infinity. Actually, when the tip is about4 tom (0.15 in) from the test surfice, a magnification factorof alxint 10 >c is achievc9.1. M icroLorescopes arx alltilith.le indiameters down to 1.7 Hun (0.07 in.) and in lengths from 100to 150 mm 14 to 6 in.).

Flexible Fiber Optic Borescopes

Flexible fiborrinic lx irescopes pennit 111:1-111pulation of tl wrant inctrnment annind •arners and through passages withseveral directional changes. Woven main3ess stet! sheathinglii otec-tai•agt- rela y !t•.:rand Hexing in Idmaneuve ring. Thetie devicts Lac &signed ill prwide !;harp,clear images of eoniixinents and interior stafaccs that arenormally impossible to inspect. Remote end tip deflectionalloy s the viewer to thread the filler optic borescopc through

u•rplex s.erivi of -bunk fide end tip is deflected with arotating contral mounted on the handle,

Most ,,r the devices have a wide single objective lens. thatprinrides a 100 degree field of %rim and tip deflection -of190 clegrees. They all loom a fiber optic image bundle midare equip ix-d with a filclei Control to bring dw subject into8harp Ic Kms over a wide Flinge of viowing distances, Theworking lengths are normally Friuli 0.fi to 3.1 in (2 to 12with diameters from 3 to t3 nim (0.12 to 0.5 in.).

Microelectronic Video Borescopes

Au electronic sensor el:dm-tided in the movable [I li of theprobe transmits Sigiiiihi to a v kir() nrINCZeatf, vdIrre thedodge is sent to a In011itor. Tire vids-0 I aresc a bright,high renollition with no distortMns or spcits. The(11. • .1(e dries not have an P►eriVil' like ether isorescopes. It

a freeze fraine Ccat tire that allows closer viewing of theimage. 'the image can lie .rIcctionically transferred for per-iniiiiciit tiOvItItientation .

C;eids or rneaautv inent references may he entered into theinaTgin of the onagii anti can beeinne part of the permanentrevoni, The linage inay be magnified fur precise vicsiring.The field of view is up to 9() (Iegrces and the prol-ir tipfi..111r-way articulation. p resenti)', the weediest prohediameteris 1(1 mm (0.37 in.) with working lengths up to 30 m 100 Ft).

Diffracted Light

A te,,linique llSiRg; diffracted fight has been developed forializing surface distortions, depression. or protrusions as

as urn (0_39 mil). A real-time leellfli(1111_7 partiell:11.411y

FIGURE 10. Optical setup for the diffracted lighttechnique

SCREEN

90 DEGREES TO SCREEN

SURFACE

LENSLAMP

DISCONTINUITY I.2 m )4 ft)

No DEGREEN .

I .5 m ft)

1.2 m(4 ft)

FIGURE 11. Borescope inspection of spoiler torsionbars

BORE INSPECTION FOR PITS

BORESCOPE

EYEPIECE


applicable to rapid inspection of large surfaces, the dif-fracted light technique is currently used to inspect automo-bile body panels and metal-working dies. There are severalcommercial versions in production. These range from a man-ual handheld system, in which the operator directly views theinspected part, to systems with video cameras and computer-based image processors. Komorowski of the National Aero-nautical Establishment of the National Research Council ofCanada suggested using this technique to inspect compositestructures for barely visible impact damage.' 618 Computer-based image processing has been applied to diffracted lighttechnique images. An image from a previous inspection canbe directly compared to current results for quick identifica-tion of areas where surface features have changed.

The optical setup for the diffracted light technique con-sists of a light source, a retroreflective screen, and the objectbeing inspected (Fig. 10). The surface being inspected mustbe reflective. Both flat and moderately curved surfaces canbe inspected using this method.

The diffracted light effect can be explained using geomet-ric optical principles. If a flat surface with an indentation isinspected, the light striking the indentation is deflected. Itthen strikes the retroreflective screen at a point removedfrom the light rays reflected from the area surrounding theindentation. The retroreflective screen attempts to return allthese rays to the points on the inspected surface from whichthey were first reflected. However, the screen, consisting ofnumerous glass beads, returns a cone of light to the surface.This imperfection of the retroreflective screen creates thediffracted light effect. By backlighting the defect, the tech-nique increases the light intensity on one side of the indenta-tion and reduces it on the opposite side.

Typical ApplicationsFollowing are examples of optically aided visual tests used

by airline maintenance personnel to ensure the structuralintegrity of transport aircraft.

Torsion Bar Core Corrosion Pitting

Stress corrosion cracks can cause failure in high strengthsteel spoiler torsion bars. In one investigation, corrosion pit-ting on the inside surface of the torsion bar cavity (bore) wasfound to have led to stress corrosion cracks and subsequentfailure (Fig. 11). It was determined that torsion bar failurecan be greatly minimized by removing corrosion from thebore within repairable limits and resealing the cavity with apolysulfide sealant (to prevent moisture ingress). For thisparticular case, a service bulletin was issued requesting oper-ators to remove the torsion bars, clean the exterior surfaceand perform a magnetic particle test for cracks. If the testshows the bar is cracked, it is scrapped.

If no cracks are detected, sealant (if present) and primerare removed from the bore and it is visually inspected forcorrosion pits using la 70 or 90 degree borescope. If corrosionpits are detected, the bore is reamed to a maximum oversizeand reinspected. If the second borescope test reveals pits,the component is scrapped. If no pits are detected, the boreis cleaned with solvent, given two coats of corrosion inhib-iting primer and then filled with polysulfide sealant. The tor-sion bar is then radiographed to verify sealant integrity.

Slat Drive Mechanism Bell Crank Cracking

Instances of slat bell crank failures were reported by oper-ators. Investigation revealed that failures were caused byfatigue cracks that initiated at the bell crank-to-collar attach-ment holes (see Fig. 12).

FIGURE 12. Flexible borescope inspection of theslat drive belt crank

SLAT DRIVE MECHANISM ASSEMBLY

BELL CRANKTYPICAL CRACK

COLLAR

TEST STTEBORESCOPE

EYEPIECE

TYPICAL CRACK CABLE DRUM

FIGURE 13. Borescopic inspection of spoileractuating mechanism

la) TYPICAL CRACKS

LINE ASSEMBLY

"'YMCA/ CRACK

70 DEGREE FORWARD OBUOUE BORESCOPE

LIGHT SOURCE

TYPICAL CRACK

7• IjIFITTING ASSEMBLY

TYPKAL CRACK

BORESCOPE

UGHT SOURCE


With the bell crank installed on the aircraft, access to thecrack location is extremely poor, preventing the use of ultra-sonic, eddy current or radiographic nondestructive testing.This leaves the fiber optic borescope as the only option forinspecting this component.

A right angle flexible borescope with deflecting tip, 5 mm(0.2 in.) in diameter and about 1 m (40 in.) in length, is rec-ommended for this test. Before the inspection, the bore ofthe bell crank is cleaned with solvent to remove grease. Thetip of the flexible borescope is placed in the forward end ofthe bell crank and fed aft until it is aligned with the twoattachment holes. To control the position of the flexibleborescope, it can he placed within a stiff but flexible plasticor thin metal tube. When the operator has the tip alignedwith the attachment bolts, it can be rotated to inspect forcracks originating at the attachment holes. If cracks aredetected, the hell crank is removed from the aircraft. If nocracks are detected, repetitive tests can be conducted untilthe bell crank is replaced with a better component.

Spoiler Actuating Mechanism Lube Hole Cracks

Failures of the link or fitting assemblies of the slat drivemechanism may be reported by operators. In one investiga-tion, the faitures were found to be caused by fatigue cracksgenerated at the inner surface of the lubrication holes in thelink and fitting assemblies (see Fig. 13). The initial servicebulletin called for an ultrasonic shear wave test to detect sub-surface cracks adjacent to the lubrication holes.

This test proved to be unreliable (cracks were missed) andwas replaced by an eddy current test of the bores of threeholes. This test required removal of the lubrication fittings

and grease before inserting the 3 mm (0.125 in.) eddy cur-rent probe into the holes. Borescope tests were consideredwhen the microborescopes discussed above became avail-able commercially.

If the visual test is performed, the lubrication fittings andgrease are removed and the bores are cleaned with solvent.The visual test is performed using a 70 degree forwardoblique borescope, 2.7 mm (0.11 in.) in diameter by 180 mm(7 in.) long. Crack indications may appear at the inboard andoutboard sides of the lubrication bores. Cracks, if detected,are not acceptable and the component must be replaced.

Wing Rear Spar Doubler and Web Cracks UnderTrapezoidal Fitting

Fatigue cracks may occur in the wing rear spar cap weband doubler under the trapezoidal panel attachment fitting.

FIGURE 14. Wing rear spar doubler and web cracklocation

FORWARD

INBOARDTEE CAP

REAR SPAR WEB

FITTINGDOUBLER OR WEB CRACK

DOUBLER

UP

CRACK POSITION IEDGE OF DOUBLER AND WEB

FIGURE 15. Borescope inspection of wing rear spardoubler and web for cracks under fitting: (a) viewlooking inboard and (131 view looking forward atlower portion of rear spar left side

TRAPEZOIDALPANELREAR

SPAR

bj(a)

U r --<0 FORWARD

WING TRACE

CRACKSBORE SCOPE

CRACK C.BORESCOPE POSITION 3 UP

CRACK POSITION 2 INBOARD


These fatigue cracks may occur in both the web and doubleror in each member separately. The cracks originate at thelower edge of both members (see Fig 14). Direct access tothe cracked areas of the web and doubler require removal ofthe trapezoidal fitting.

If both members are cracked, fuel may leak from underthe fitting, indicating the existence of through-thicknesscracks. Other methods of nondestructive testing cannot beused because of the poor access caused by the fitting. Thedoubler and web are made from 7075-T6 clad aluminumsheet about 4 mm (0.16 in.) thick. Access to the area is aft ofthe rear spar plane, inboard and outboard of the trapezoidalpanel and fitting.

The visual test can be optically aided using a 0 degreeborescope along with a 70 to 90 degree borescope, 300 to480 mm (12 to 19 in.) in length and 4 to 5 mm (0.16 to0.20 in.) in diameter.

The test area is cleaned using a cotton swab wetted withsolvent. The area is first viewed using the 0 degree borescopeto check for cracks in the radius areas of the doubler and web(position 1, Fig. 14). If a crack exists and it does not rununder the fitting, its length in the doubler may be deter-mined by a liquid penetrant or high frequency eddy currenttest using a shielded pencil point surface probe. To measurethe length of a crack in the web (foremost member) under anuntracked doubler requires removal of the fitting and appli-cation of a low frequency eddy current test.

Detecting a crack in the doubler or web hidden by the fit-ting (Fig. 15, positions 2 and 3) requires the use of the 70 or90 degree borescope. The area is cleaned with a cotton swabwetted with solvent and the borescope is inserted throughthe small opening between the forward side of the fitting andthe aft side of the doubler (Fig. 15). To measure the lengthof doubler cracks under the fitting at position 2 or 3 requiresultrasonic shear wave (angle beam) techniques.

Rudder Rib Flange Cracks

Radiographic tests can reveal cracks developing in the ribflanges of the rudder. In one case, analysis determined thatcracking of the rib flanges resulted from acoustically inducedvibration. It was also determined that installation of stiffen-ers on the rudder ribs strengthens the rudder and minimizesthe possibility of further crack development.

A service bulletin was issued giving criteria for flyablecrack lengths based on the number of cracked ribs and thelength of the cracks. Unrepaired rib flange cracks may causecracks to occur in the rudder skins, thereby requiring moreextensive repairs.

A radiographic test is first conducted and if cracks aredetected in the rib flanges at or adjacent to the skin attach-ment fastener holes, their lengths must be determined. Thecracks may run upward into the flange radius and progressinto the rib area, where their lengths may be difficult to

determine from radiographs alone (see Fig. 16). If cracksoccur within or progress into the flange upper radius, theirlengths must be determined by use of a 3.2 mm (0.125 in.)maximum diameter rigid borescope or flexible borescope(see Fig. 16).

FIGURE 16. Determining crack length in rudder ribsby optically aided inspection

RIGIDBORESCOPE

FLANGE TYPICALUPPER RIB WEBRADIUS c'D

RIGIDBORESCOPE •UP

(a)

(b) FIBER OPTICLIGHT SOURCE

RUDDERSKIN

FBER OPTICLIGHT SOURC

FLEXIBLE FLANGEBORESCOPE UPPER

CRACKED FLANGE RADIUS

FLEXIBLEBORESCOPE

TYPICAL CRACKS

FIGURE 17. Case history of corrosion leading tocomponent failure: (a) main landing gear truckbeam; (b) fractured surface of failed component,due to failure to apply lubricant (the lubricationhole is visible opening on the inside surface)

(a) L OBE HOLES

(b)


To perform this test, the fastener common to the rudderskin and the rib flange (opposite the cracked flange) must beremoved. A 0 degree borescope is inserted through the openhole and the opposite flange radius and web are inspected todetermine crack position and length. An alternative methodrequires removal of a fastener common to the rudder skinand the cracked flange that is judged to be about 50 mm

(2 in.) beyond the crack image on the radiograph.A flexible borescope or rigid retrograde borescope {Fig. 9)

is inserted through the open hole and articulated to allowviewing of the rib flange, radius and web to determine crackposition and length. If cracks exceed flyable length, the ribsare repaired. If cracks are of tolerable length, an easilyreplaced fastener is installed in the open holes to allow forrepetitive evaluation.

Main Landing Gear Truck Beam Pitting Corrosion

Three instances of main landing gear truck beam assemblyfailures were reported as resulting in major secondary dam-age to the aircraft. Investigation revealed that failure was aresult of stress corrosion fracture that 'initiated at or immedi-ately adjacent to the intersection of the lubrication hole andthe pivot bore (see Fig. 17). The stress corrosion fracture isa result of severe pitting in the lubrication hole caused byinadequate lubrication.

If this condition is not corrected, the truck beam assem-blies are vulnerable to failure. Removing corrosion or pittingfrom the surface of the four pivot bore lubrication holes andincreasing the frequency of lubrication minimizes the possi-bility of failure and extends the service life of the beamassemblies.

Inservice testing for corrosion pitting requires removal ofthe lubrication fitting and grease from each of four holes inthe subject beam. The internal surface of each bore ischecked using a 0 degree (forward looking) 2.8 mm (0.11 in.)diameter borescope. If corrosion or pitting is revealed, the

hole is checked a second time with a 70 or 90 degree (lateral)borescope.

When heavy pitting is detected, the beam is removed fromthe aircraft and the pits are removed by reaming the affectedholes to a maximum oversize. Beams showing slight corro-sion may continue in service for a limited time provided thatperiodic borescope and ultrasonic shear wave tests are madeto detect possible stress corrosion cracks that may originateat a pit.

Figure 18a shows an acceptable condition at the innerbore chamfer. Figure 18b shows a typical heat treat pit—an acceptable condition away from the inner bore chamfer.Figures 17b and 18c show unacceptable corrosion pittingat the inner pivot bore intersection. Figure 18d showsacceptable machining marks (rifling) in the bore outer edge,close to the lubrication fitting threaded area.

FIGURE 19. Borescope inspection of wing lowerforward spar cap tang for fatigue cracks

FOOTSTOOL/FITTING

FOOTSTOOL FITTING

FASTENER

FORWARDSPAR CAP

EDGE SPAR I firyBc•FORWARD.,-- •

TANGACCESS HOLE -----

OF FOOTSTOOL FITTINGINBOARD SIDE J TUBE

GUIDEPYLON FITTING LIGHT SOURCEAND FOOTSTOOLFITTING LEFT SIDE .

FORWARDS

4 INBOARD

TYPICAL CRACK

(.10:

PYLONICING

AFT',FLEXIBLEBORESCOPEI


FIGURE 18. Surface conditions possible in a typical main landing gear truck beam: fa) acceptable borechamfer, (b1 typical heat treat pit, icl severe bore chamfer corrosion and fd) machining marks (rifling)

(a)

(b)

(c)

(d)

Wing Front Spar Lower Cap Cracks

The purpose of this test is to check the lower spar cap for-ward tang for fatigue cracks at the fastener locations. Thearea of interest is located at the four wing pylons that supportthe jet engines. The total inspection for fatigue cracks at thepylons includes X-ray, ultrasonic shear wave and borescopetests. In order to accomplish the ultrasonic and borescopetests inboard of the pylons, an access hole is made in thewing leading edge, as shown in Fig. 19. In addition, a smalleraccess hole is made in the footstool fitting, so that the fasten-ers located under the fitting can be visually inspected.

Inspection of the six fasteners at the inboard side of thefitting is accomplished using a flexible borescope with a mini-mum length of 0.6 m (2 ft). The inspector places his handthrough the access hole and positions the tip of the 90 degreeflexible borescope forward and aft of each fastener to detectfatigue cracks in the spar cap forward tang.

The area under the footstool fitting is inspected using twotechniques: (1) the seven fastener locations are inspectedusing a 0.6 m (2 ft) long, 90 degree, rigid borescope or a flex-ible borescope supported by semirigid plastic tubing and(2) the forward edge of the spar cap is inspected using a flex-ible borescope placed in a semirigid tube bent into a J shape(Fig. 19). These tests have been successful in detecting smallcracks at the forward and aft side of the fasteners and largercracks that propagate to the leading edge (forward) or verti-cal leg (aft) of the cap.

Impact Damage and Exfoliation in Composite Panels

Impact damage and exfoliation are conditions that causeanomalies in the surface contours of composite panels likethose used in aviation. The diffracted light techniquedescribed above has been investigated for possible use inevaluating the condition of such panels.

FIGURE 20. Typical diffracted light techniqueimages and C-scans of impacted carbon-epoxycomposite panels

ULTRASONIC C-SCAN DIFFRACTED LIGHT TECHNIQUE

SPECIMEN A SPECIMEN A

33

SPECIMEN C

56

SPECIMEN 8

11

SPECIMEN 8

SPECIMEN C

18.6

24.3

IMPACT ENERGY{JOULES)

SPECIMEN A 1.4SPECIMEN B 4.1SPECIMEN C 41

FROM THE NATIONAL AERONAUTICAL ESTABLISHMENT OF THE NRC OFCANADA. REPRINTED WITH PERMISSION.

FIGURE 21. Images and C-scan results forexfoliation corrosion around steel Fasteners in7075-T6 aluminum plate: fa) C-scan, (b) ambientlight and (c) diffracted light

lc)

APPLICATIONS OF VISUAL. AND OPTICAL TESTS IN THE TRANSPORTATION INDUSTRIES / 299

Graphite-epoxy panels of three thicknesses (8, 24, and 48plies) using a [0/45/90/ — 45]s layup were numbered and eachwas cut into three specimens (A, B and C). The specimenswere then painted and ultrasonically C-scanned. No discon-tinuities were found. The specimens were inspected withdiffracted light technique before being subjected to impactenergy ranging from 0.7 to 21.7 joules (0.5 to 16.0 ft-lb). Theresulting indentations ranged from nonvisible to barely visi-ble. The depth of the indentations were measured and thespecimens were ultrasonically C-scanned to detect and mea-sure delaminations.

The diffracted light images of the impacted specimens areshown in Fig. 20 along with the C-scans. 19 The test resultsclearly show that the diffracted light technique can detectbarely visible impact damage in composite laminates and canalso detect cold worked holes in 7075-T6 panels and fatiguecracks extending from holes in 7075-T6 pane/s5 19 Some7075-T6 panels containing steel fasteners, some of whichhad exfoliation corrosion around them, were then evaluated.

Figure 21 shows the result of exfoliation corrosion detec-tion under ambient lighting, ultrasonic C-scan, and dif-fracted light technique imaging. The diffracted light


technique shows corrosion around six of the twelve fastenerholes. C-scan definitely shows corrosion in four of the six.The remaining two fastener holes probably would haveindicated corrosion if a more sensitive ultrasonic method hadbeen used. In any case, the results are encouraging.

ConclusionAs indicated above, optically aided visual tests include the

use of magnifiers, and both rigid and flexible horescopes.These devices are typically used during on-aircraft testing todetect corrosion and cracks where access is limited.

The examples given are typical of other such applicationsfor maintenance testing of aircraft structures and operatingmechanisms. Optically aided visual testing is a viable andeconomical method that can be used to monitor the struc-tural integrity of inservice aircraft.


References

1. Cielo, P Optical Techniques for Industrial Inspection.New York, NY: Academic Press (1988).

2. Chin, R.T. "Algorithms and Techniques for AutomatedVisual Inspection." Handbook of Pattern Recognitionand Image Processing. T.Y. Young and K.S. Fu, eds. NewYork, NY: Academic Press (1986): p 587-612.

3. Hara, Y., H. Doi, K. Karasaki and T. Iida. "A System forPCB Automated Inspection Using Fluorescent Light."Transactions on Pattern Analysis and Machine In-telligence. Vol. PAMI-10, No. 1. New York, NY: Insti-tute of Electrical and Electronics Engineers (1988):p 69-78.

4. Conners, R.W,, C.W. McM ilin, K. Lin and R.E.Vasquez-Espinosa. "Identifying and Locating SurfaceDefects in Wood: Part of an Automated Lumber Pro-cessing System." Transactions on Pattern Analysis andMachine Intelligence. Vol. PAMI-5, No. 6. New York,NY: Institute of Electrical and Electronics Engineers(1983): p 573-583.

5. Siew, L.H., R.M. Hodgson and E.J. Wood. "TextureMeasures for Carpet Wear Assessment." Transac-tions on Pattern Analysis and Machine Intelligence.Vol. PAMI-10, No. 1. New York, NY: Institute of Electri-cal and Electronics Engineers (1988): p 92-105.

6. Campbell, F.W. and J.G. Robson. "Application of Fou-rier Analysis to the Visibility of Gratings." Journal ofPhysiology. Vol. 197. Cambridge, England: CambridgeUniversity Press 968): p 551-566.

7. Torgerson, W.S. Theory and Methods of Scaling. NewYork, NY: Wiley and Sons Publishing (1958).

8. De Valois, R.L. and K.K. De Valois. Spatial Vision. Lon-don, England: Oxford University Press (1988).

9. Haralick, R.M. "Statistical and Structural Approaches toTexture." Proceedings of IEEE. Vol. 67, No. 5. New York,NY: Institute of Electrical and Electronics Engineers(1979): p 786-804.

0. Van Goo', L., P. Dewaele and A. Oosterlinck. "TextureAnalysis 1983." Computer Vision, Graphics and Image

Processing. Vol. 29, No. 3. New York, NY: AcademicPress (1985): p 336-357.

11. De Valois, R.L., D.C. Albrecht and L.G. Thorell. "Spa-tial-Frequency Selectivity of Cells in Macaque VisualCortex." Vision Research. Vol. 22, No. 5. New York, NY:Pergamon Press (1982): p 545-559.

12. Coggins, J.M. A Framework for Texture Analysis Basedon Spatial Filtering. PhD dissertation. East Lansing,MI: Michigan State University (1982).

13. Coggins, J.M. and A.K. Jain. "A Spatial FilteringApproach to Texture Analysis." Pattern Recognition Let-ters. Vol. 3, No. 3. Amsterdam, Netherlands: ElsevierScience Publishers (1985): p 195-203.

14. Jain, A.K. "Experiments in Texture Analysis Using Spa-tial Filtering." Proceedings of the Workshop on Lan-guages for Automation. New York, NY: Institute ofElectrical and Electronics Engineers (1985): p 66-70.

15. Draper, N.R. and H. Smith. Applied Regression Analy-sis, second edition. New York, NY: John Wiley Publish-ing (1981).

16. Komorowski, J.P., D.L. Simpson and R.W. Gould."Enhanced Visual Technique for Rapid Inspection ofAircraft Structures." Materials Evaluation. Vol. 49,No. 12. Columbus, OH: The American Society for Non-destructive Testing (1991): p 1,486-1,490.

17. Komorowski, J. et a!. "A Technique for Rapid ImpactDamage Detection with Implications for CompositeAircraft Structures." Composites. Vol. 21, No. 2.Guildford, Surrey, England: Butterworth-HeinemannLtd. (March 1990): p 169-173.

18. Hageniers, 0. "Diffracto-Sight A New Form of Sur-face Analysis." Photomechanics and Speckle Metrology.Vol. 814, Bellingham, WA: Society of Photo InterpretiveEngineers (1987): p 193-198.

19. Komorowski, J. and R. Gould. "A Technique for RapidInspection of Composite Aircraft Structures for ImpactDamage." AGARD Conference Proceedings. No. 462.Neuilly sur Seine, France: NATO Advisory Group forAerospace Research and Development (1990).


PART 1 INTERFACE OF VISUAL TESTING WITHOTHER NONDESTRUCTIVE TESTINGMETHODS

Data from all types of nondestructive tests are producedso that they may be recorded and interpreted visually. Forthis reason, almost any nondestructive test could be consid-ered a visual test, particularly at the detection or interpreta-tion stages. With certain of the nondestructive methods, thelink with purely visual testing is even more direct.

Visibility criteria are specified for magnetic particle tests,liquid penetrant tests and some leak tests. The vision acuityof radiographers is medically verified before they are allowedto interpret radiographic images. The same is true in someindustries for inspectors using magnetic particle and ultra-sonic testing. Light levels, indication sizes, viewing angles,color sensitivity and many other phenomena pertaining tohuman vision are strictly controlled in order to achieve reli-able accuracy in visual tests as well as other nondestructivetechniques.

The link between visual testing and these other techniquesis enhanced by their shared NDT hardware. Borescopes arebasic to many visual testing procedures and they are oftenused to view obstructed magnetic particle or liquid pene-trant test indications. Magnifiers are used to visually testmaterial surfaces, as well as to study the details in radio-graph's and to measure indication in magnetic particle andliquid penetrant testing. Optic systems are used in cineradi-ography, in the photographic recording of various testresults, in machine vision, in remote television pickups andin virtually all the automated NDT methods. Imageenhancement and other software approaches to data manip-ulation open a further link between visual and, for example,the radiographic, ultrasonic or microwave methods.

This points to a final and more substantive tie betweenvisual testing and the other nondestructive techniques.Visual methods are often used to inspect or verify the data ofthe other tests. It might be said, for example, that radiogra-phy is ultimately a visual test of the radiograph—to deter-mine if the radiographic images are properly exposed, thatthe areas of interest are free of artifacts and that the imagescan show the characteristics of interest. As another example,it might also be said that accurate wet magnetic particle testscannot occur until the bath has passed a preliminary visualtest and it can be proven that the inspector can see indica-tions. (In some industries where it is not possible to usevisual testing—for example, the inspection of casing in an oil

well—then a second form of nondestructive testing is oftenused to add credence to the first.)

There are a lot of details in the conduct of a nondestruc-tive test that are as important as the ability to see. In order tointerpret indications in the various methods of nondestruc-tive testing, the inspector must have a great deal of knowl-edge of and experience with the test method and theapplication in question. In other words, the visual aspect isimportant but not sufficient. The truth of this is self evident;otherwise, vision acuity testing would have made training inother methods unnecessary. A test is affected as much bywhat an inspector knows as by lighting or optical aids. Thefollowing brief discussions of other test methods should beread with the awareness that much more is involved than thevisual aspects being emphasized.

Human vision is basic to nondestructive testing and visualtesting is directly or indirectly linked with all the NDT meth-ods. The text below illustrates some of this importantinterplay.

Visual Aspects of Leak Testing'

Leak testing is done by detecting a tracer medium, a gasor liquid that has escaped from confinement. The tracer canbe an added fluid or in some cases it can be the fluid that thevessel is designed to hold. Testing is done visually, aurally orelectronically. Occasionally, tracers are designed to interactwith materials applied or naturally present outside the vessel,to produce highly visible evidence of leakage. The visual por-tion of a typical leak test is that which determines the pres-ence and location of leakage. The rate of leakage and itseffect on fluid flow may be determined by visual observationof meters and gages.

The ASME Boiler and Pressure Vessel Code requires thata visual test be conducted to locate evidence of leakage frompressure retaining components. In nuclear power plants,visual tests are required for locating abnormal leakage fromcomponents with or without leakage collection systems. Forcertain components, visual testing is performed using thereactor coolant (water) as the tracer medium. The visual test-ing of noninsulated pressure retaining components is per-formed by inspecting external, exposed surfaces for visible

4

4

OTHER APPLICATIONS OF VISUAL AND OPTICAL TESTS / 305

evidence of leakage, Components whose external surfacesare inaccessible for direct viewing are examined by visuallychecking the surrounding area including drip pans or sur-faces located beneath the components of interest.

Color detection and ("odor differentiation can he essentialsteps in eel tain leak testing procedures. For example, bro-mo•resol purple dye is used in chemical reaction leak totingwith ammonia gas tracers. The dye is sprayed or brushedonto the outside surfaces of pressure vessel welds andallowed to dry. After drying, the dye turns into a vellum pow-der The vessel is then pressurized with ammonia gas. If aleak exists, it is indicated fn change in the color of the pow-dered dye—from a light yellow to a vivid purple.

Minims visible color and fluorescent dyes are also used inleak testing. These materials are sensitive to the concentta-titm of active ions, either acid or alkaline. Such ions deter-mine the potential of hydrogen (phi) ) of a solution mid the pHvalue can be shifted by addition of an acid or an alkali. Anumber of lea .fid indicator dyes are sensitive to smallchanges 111 phi and the most direct way to use these effects isto use the dyes with a truer gas that produces a change inpH. This Nit. of gas-phase leak indicator typically employsa liquid applied onto areas suspected of leakage, One dyesuitable for producing visible color leak indications isphenolphthalein,

Color differentiation is also needed in high voltage dis-charge leak testing In this technique, a spark coil is used toexcite a visible glow discharge in SySterm where du , pressureis behsren 1 and 1,000 Pa (10 = to 10 torr). The tracer eaube a gas such as ear lion dioxide or a volatile liquid such Rsbenzene. acetone or methyl alcohol. When the tracer gasenters the system through a leak, the color of the dischargechangt1 from purple (the color of air) to the color character.Wit . of the tracer (see Table I ). Visual detection of this colorchange is required fin accurate completion of the leak test.

Visual Aspects of Liquid PenetrantTestingz

In liquid iwnetrant tests, visual techniques are used torompare pities of penetrant systems, to detect discon-ti nuity in, i ins and to verify the cleanliness of the testingmaterials When a hydrophilic emulsifier bath is used, visualmonitoring of the bath can pnnide clues to its condition. Afresh solution of emulsifier in water exhibits a typical colorin visible light and in ultra\ kilo light. Traces of fluoresecntpenetrant contamination in the bath will darken its color andcause the fluorescence to shift. thigh levels o:penetrant con-tamination cause a hydrophilic emulsifier solution to becomecloudy and at higher levels, free penetrant can ix, seen tofloat on the bath sinful . .

TABLE 1, Discharge colors In gases and vapors at lowpressures

Gas Negative Glow Positive Column

Air blue redNitrogen blue yellow (red-gold)Oxygen yellow-white lemonHydrogen blue-pink (bright blue) pink (rose)Helium pale green violet-redArgon blue deep red (violet)Neon red-orange red -orange (blood red)Krypton greenXenon blue -whiteCarbon monoxide green-white whiteCarbon dioxide blue whiteMethane red-violetChlorine green light greenBrame yellow-green rediodine orange-yellow peachLithium bright redSodium yellow-green (white( yellowPotassium green greenMercury greed (gold-whited green-blue (green(

Penetrant developers may also sider degradation that isvisually detectable. The hest test of a devcioper is a visualcomparison with new material tinder visible and ultravioletlight. Penetrant contamination in dry developer causesbright color spots or bright fluorescent spots,

During liquid penetrant tests, color detection and colordifferentiation are often critical to eompletion of the test.Certain visual data produced by penetrant testing proce-dures are knon.rn to cuirrelate with specific aspects of n tat rrialdiscontinuities; depth of surface discontinuities may be cor-related with penetrant color intensity; unbroken linear indi-cations often represent cracks, random dots um indicatefine porosiht In many codes, it is Npedfied that penetrantindications mar he visually detected in natural or artificiallight.

Because lienetrant tests normally rely on an inspector'svisual detection, the lighting for this procmhire is veryimportant, It not only affects the sensitivity of the testmethod but it is also an important factor in inspector fatigue.Visible light sources for penetrant tests are identical to thosespecified kw other visual test applications. Spectral thane-teristivx are usually not critical for visible light col 'ryes hilt itmay be better to use a light source deficient in the lightreflected by the penetrant and rich in the other componentsof the visible speculum When such a light is used on a testobject having good white developer badsgrotual, the pene-trant indication appears darker and maximal! contrast isobtained. Floodlights are advantageous for large and riga-tivdy flat test surfaces. the intricate or small test objects,manually directed spotlights may the most effective visi-ble light maim,.

10 20

TIME IN THE DARK)minutes)

30

FIGURE 1. park adaptation curves measured witha 25 mm (1 in.) test stimulus

FROM HATTWICK-ACADEMIC PRESS. REPRINTED WITH PERMISSION.

_ 2

SUBJECT PREADAPTEDTO 6,650 TROLANDS FOR

FIVE MINUTES

FOVEA

SUBJECT PREADAPTEDTO 389 TROLANDS FOR

PARAFOVEA FIVE MINUTES

•


The proper intensity of visible light is determined by therequirements of the penetrant test. For gross discontinuities,a brightness level of 300 to 550 lx (30 to 50 ftc) at the surfaceis typically sufficient. Intensity levels of 1,000 lx (100 ftc) arenecessary for small but critical discontinuities.

Because liquid penetrant tests use the human eye as adetection device, the condition of the inspector's eyes is animportant test specification. For fluorescent penetrant tests,the inspector must be dark adapted (see Fig. 1) and theintensity of the ultraviolet source must reach specified min-ima. Photosensitive eye glasses should not be worn. The eyeitself will fluoresce under certain conditions, causing tempo-rarily clouded vision.

Optical technologies are used in penetrant tests in a vari-ety of ways. For example, a flying spot laser can be used fordetection of penetrant test indications. When the laser beamstrikes fluorescent penetrant materials, a pulse of differentwavelength light is generated. A photodetector converts thispulse into an electrical signal that is analyzed, using patternrecognition techniques, to determine the discontinuity'sshape and size.

Visual Aspects of Radiography 3Vision acuity is vital to the radiographic interpretation pro-

cess. Individual visual acuity can and does vary from test totest depending on physiological and psychological factors.

Annual vision acuity examinations cannot detect daily fluc-tuation or its influence on interpretation and the frequencyof a vision acuity examination may he specified.

In radiographic testing, the physical measure of interestfor vision acuity is the discontinuity as displayed on the film,regardless of how much it may differ from the actual discon-tinuity in the test object. Daily vision acuity tests can bebased on microdensitometric scans of discontinuities takendirectly from actual radiographs. Specifications for visionacuity exams typically include factors such as the figure-to-ground relationship, background luminance, contrast, linewidth, line length, viewing distance, blur, line orientationand characteristics of the light source.

During radiographic tests, viewing conditions are veryimportant. A finished radiograph should be inspected underconditions that afford maximum visibility of detail togetherwith maximum comfort and minimum fatigue for the inter-preter. Subdued lighting in the viewing area is preferable tototal darkness. The room lighting must be arranged so thatthere are no reflections from the surface of the film.

If slight density variations in radiographs are not seen,rejectable conditions may go unnoticed. This emphasizes theimportance of the interpreter's vision acuity as well as theimportance of the viewing equipment. Illuminators are use-ful for varying radiographic densities. Density requirementsthrough an area of interest typically range between 2.0 and4.0 H and D ( I to 0.01 percent light transmission). Heat,diffusion and intensity controls are needed for accurateviewing.

Magnifiers and densitometers are among the other opticalequipment used to view industrial radiographs. Scanningmicrodensitometry equipment can be very useful for certainindustrial applications, including focal spot measurementsand determination of total radiographic unsharpness. Theresulting graph (see Fig. 2) shows the relationship of densitydifferences to material thickness differences—best studiedwhen there is an item of known thickness (a shim, for exam-ple) on the radiograph.

Digital image processing may also be used during radio-graphic tests to extract data from film radiographs. Muchresearch has been aimed at development of computerizedX-ray scanners. The most innovative of these techniques iscomputer assisted tomography for radiographic and radioiso-tope imaging. In addition, scanned projection radiographysystems have been designed to develop data in point scan,line scan or area scan modes. Most of these image processingtechniques have the digitizing step built into the detector.

For an image to be digitally enhanced, it must be in digitalform. Digital radioscopy and computed tomography areinherently digital. Radioscopic images from luminescentscreens (fluoroscopy) or cathode ray tubes may be scannedand presented in a two-dimensional digital array of pictureelements (pixels). Conventional film radiographs also may bedigitized. Once the radiographic image is digitized, a variety

AVERAGELOWEST POINT

AVERAGE ••AVERAGE

AD,0 37 H AND D

ODD0.42 H AND D


FIGURE 2. Scanning microdensitometry graph(H and D = Hurter and Driffield log relative exposure)

SHIM SCAN WELD SCAN

FILM SCAN PATH (2:1 SCALE(

of enhancement methods can be used, including brightnesstransfer functions, gradient removal, digital filtering, fieldflattening, smoothing and a number of other transforms.

The coding of several scale intervals with a different colorresults in an enhancement technique known as pseud-molar.Though these color images are striking to the viewer, thetechnique has the disadvantage of presenting color changesthat do not necessarily ccFespond to abrupt changes in opti-cal density on the image.

Another optical technique that is used with radiography ishigh speed videography. Such systems use data in its digitalform and provide two advantages: live camera setup(increases the chance of success on the first recording) andthe immediate playback feature common to all video sys-tems. When used with an X-ray image intensifier, dynamicradiographic images can be recorded up to 12,000 partialframes per second.

Real-time imaging systems may• be used for radiographictesting. Fluorescent screens may be used to convert X-rays tolight or image intensifier systems may present the radio-graphic image to the television system.

Visual Aspects of ElectromagneticTesting 4

In electromagnetic methods such as the eddy current andmicrowave techniques, test data can be presented on a stripchart or oscilloscope. Adjustments to an oscilloscope's scales

are sometimes made to increase the visibility of the testre-sults and, except for the interpreter's ability to see such a dis-play, there is tittle overlap with the unaided visual testmethods.

Imaging techniques and data enhancement proceduresmay be used to advantage during some electromagnetic test-ing procedures. In microwave tests, for instance, imaging canbe used to produce two-dimensional tomograms of the testobjects. Such images can have high lateral resolution and,although they may be in any plane through the test object(see Fig. 3), they are commonly equivalent to a cross section(like an ultrasonic B-scan) or a planar view (like a C-scan).These imaging processes can improve lateral resolution byartificially creating a large aperture from a small one. Thissynthetic aperture can be created from a side-looking algo-rithm or from a phased array.

Visual Aspects of Magnetic ParticleTesting5

In magnetic particle tests, the vision acuity of the inspec-tor and the visibility of the test results are as important asthey are for liquid penetrant tests.

Dry magnetic particles are commercially available in visi-ble colors, fluorescent colors and daylight fluorescent colors.Visible particles are typically available in gray, red, black,

PLASTIC BLOCK0HOLE

(b)

FIGURE 3. Frequency modulated microwaveB-scan image of a drill hole in a plastic block:(a) diagram of the plastic block and (b) B-scanimage of hole in plastic block

(aJ

MICROWAVE HORN

FROM FOLSOM RESEARCH INC. REPRINTED WITH PERMISSION.

SCAN


yellow, blue and metallic pigments. Colors of visible particlesare chosen based on the highest contrast with the test objectsurface.

Daylight fluorescent colors were developed to increasevisibility in those applications where somewhat lessened sen-sitivity is acceptable. Daylight fluorescent colors haveenhanced visibility in light from the sun, blue mercury vaporlamps or white fluorescent tubes. The yellow light fromsodium vapor sources does not excite fluorescence in thesemagnetic particles. The dyes absorb photons of one energyand emit photons at a lower energy.

Much of the specified technique for magnetic particle andother tests is established to enhance visibility of the disconti-nuity indications. Choice of color, magnetization levels andapplication of the particles themselves are all done in waysthat maximize visibility. In certain magnetic particle applica-tions, a thin white lacquer is applied to the test object surfaceto make dark colored particles more visible (smaller particlesare more responsive to magnetism and the addition of a layerof pigment to the particles decreases their sensitivity).

Wet method particles are available in fluorescent and non-fluorescent forms. Particle color is again chosen to increasecontrast with the test object surface.

Not only are particle characteristics chosen to maximizevisibility but light levels are also carefully monitored. Mili-tary standard MIL-STD-1949A calls for a minimum lightintensity of 1,000 lx (100 ftc) for viewing nonfluorescentmagnetic particle indications. Other standards call for otherintensities. The optical light level, however, is sometimes acompromise between operator fatigue and visibility. Onbright or reflective surfaces, high light intensities can causeglare that interferes with vision and subsequent interpreta-tion. On darker surfaces or those covered with thin scale, the1,000 lx (100 ftc) level may be barely adequate for visibility.

In wet method fluorescent tests, the US military stan-dard requires a minimum ultraviolet light intensity of1,000 ji.W•mm - 2, with a maximum allowable visible lightintensity at the surface of 20 lx (2 ftc). Even small amounts ofvisible light can lower the contrast of a fluorescent indication.

Appropriate light levels are so critical to the completion ofmagnetic particle tests that visible light intensity measure-ments are routinely performed and logged. Typically, suchentries are made from a light meter that is itself calibratedevery six months or at intervals prescribed by the relevantspecification.

Visual capabilities are critical to magnetic particle indica-tion detection but they are also used to verify the quality ofthe testing materials. In an agitation system, where the parti-cle bath constantly passes through a centrifugal pump, parti-cles are subject to constant high speed impact and shearingfrom the pump's impeller. As breakdown of the particlesincreases, test indications become dimmer and backgroundfluorescence is seen to increase (indication-to-backgroundcontrast diminishes).

Optical equipment used for magnetic particle tests is thesame as for liquid penetrant tests: illuminated rigidborescopes, fiber optic borescopes and video borescopes.These devices are used in the same way, with the sameadvantages and precautions, as in a purely visual test. Thedifference is that the object of interest is a magnetic particleindication of a material discontinuity. Light intensities forborescopes and remote viewing instruments must be set toinclude losses between the eyepiece and the distal tip of thedevice. Quartz fiber borescopes typically need less lightintensity than conventional borescopes used for viewing visi-ble particle indications. In a rigid borescope, light transmis-sion is governed by an optical system. For video borescopes,the sensitivity of the camera chip is the critical factor. Refer-ence standards and comparative tests with known disconti-nuities are used to verify remote viewing of magnetic particleindications. Figure 4 is an example of magnetic particle indi-cations imaged through a video horoscope.

FIGURE 4. Fluorescent magnetic particleindications on a forging, viewed with a videoborescope system and a rigid borescope adapter

FROM SANDRA T. BRUNK AND ASSOCIATES. PRINTED WITH PERMISSION.

FIGURE 5. Laser ultrasonic test of a curved graphiteepoxy composite with an artificial delaminationintroduced at mid-thickness: (a) test configuration(generation and detection performed at the samelocation) and (b) display of test results

(b)

I J 3 4 5 6 / 8 9

TIME(microseconds)

(a)

wO

0 DE LAMINATION


Visual Aspects of Ultrasonic Testing 6

Optical Generation of Ultrasound

One advanced technique of ultrasonic testing involves theoptical generation of ultrasound. Its primary advantage isthat no mechanical contact is needed with the test object sur-face. Transformation of light energy into acoustic energy isperformed by the test material lnd no intervening couplantis necessary. Likewise, material surface vibrations aredirectly encoded onto a light beam, also without couplant.These techniques make ultrasonic tests possible in condi-tions that are difficult for other techniques.

When light radiating from a laser source is absorbed by atest object, thermal expansion results, producing elasticultrasonic waves. Optical methodsfor ultrasonic wave detec-tion can be grouped into two categories. The first includesthose methods that permit real-time detection of ultrasonicdisturbances at a single point or over a single zone on a testobject surface. The second category includes full field meth-ods that provide maps of the acoustic energy distributionsover an entire field of view at one instant in time.

The laser technique produces an ultrasonic source at thesurface of a test object and allows detection of the object sur-face, independently of shape and orientation. Curved andcomplex geometries such as pipes, rotor blades and edges ofaircraft wings can be tested. Detection of delaminations inflat or curved graphite epoxy laminates is also possible (seeFig. 5).

Precision Acoustic Imaging

Ultrasonic imaging can he performed by several tech-niques. The most common is that used by commerciallyavailable ultrasonic immersion C-scanning systems. An ultra-sonic wave of known amplitude A, is transmitted through atest material (see Fig. 6). The final amplitude Al or C-scanimage is a representation of relative attenuation or energylost by the ultrasonic wave during its trip from the transmit-ter to the receiver.

C-scan images represent the energy lost by the ultrasonicwave but, unfortunately, not solely because of the internalstructure of the test object. The ultrasonic reflection coeffi-cients at the water-to-object interfaces are generallyassumed to be constant but actually can vary widely. Thesereflection coefficients must be included in the analysis todetermine the actual energy loss or attenuation caused bythe inner structure of the test object. They must also be usedin determining the accuracy of the acoustic image as well asthe accuracy of the imaging technique.

A precision acoustic scanning system is shown in Fig. 7. 7 'Here, an ultrasonic transducer is scanned over the test objectsurface while a load cell gages coupling pressure. After

FIGURE 6. Standard immersion through-transmission ultrasonic C-scanning arrangement(Ao is intercepted by delamination and receiversignal goes to zero)

- TEST OBJECT

DELAMINATION

A,

::MITTER

Hp

RECEIVER

H

XYZ POSITIONINGTABLE

WAVEFORM DIGITIZERTIME BASE

VOLTAGC BASE

PRESSURE SENSORVOLTMETERTIME DELAY

FIGURE 7. Precision acoustic scanning system:(a) system diagram and (b) schematic diagram

ULTRASONICPULSER RECEIVER

(b) PRESSUREGAGE SUPPORT -,.,

,--- PRESSURE GAGE

1111111.1111001111/TRANSDUCER

TEST OBJECTHOLDER

--- CERAMIC

0111"-111",---- n••motoluin

TABLE WITH SURFACE ADJUSTABLEON THREE AXES0 RING

SUPPORT

la)

TO CENTRAL GPIB

PROCESSING UNIT


collection and subsequent Fourier analysis of the appro-priate waveforms, an accurate determination of velocity andattenuation can be made. This is done at different positionson the test object, in an organized array. The resulting data

yield velocity and attenuation maps that are displayed on avideo system.

A typical data set, from which about 25 images can beobtained, contains about 2,000 waveforms, 2,000 Fourierspectra and 40,000 attenuation and velocity values. The orig-inal acquired data use about 20 megabytes of computermemory. The reduced data set containing the attenuation,velocity and reflection coefficient' images occupies about5 megabytes.

Evaluating an image often requires tests of both the rawand reduced data for a particular point in the image. A dataretrieval program provides the necessary interface to allowimmediate retrieval of data displayed in a video image over-lay. For example, the data for one point (determined by thecursor location) in an ultrasonic image of velocity (Fig. 8) aredisplayed in an overlay as shown in Fig. 9. This display con-tains waveforms and their Fourier spectra, reflection andattenuation coefficients (may be used to determine accu-racy), phase velocity as a function of frequency and groupvelocity (determined by cross correlation).

All these data are inspected for irregularities when evalu-ating a specific point on the ultrasonic image. For example,if a quantitative measure of the density difference betweentwo points is needed, it can be determined from the group

FIGURE 8. Ultrasonic image of SiC (cursor indicatesthe physical point on the test object that needsto be evaluated); area shown is 20 x 20 mm(0.8 x 0.8 in.)

VELOCITYmrn• LS

- 115

_ 11.3

FIGURE 9. Screen revealing data obtained forcursor position shown in Fig. 8

A 113 pm

DEnTHIVP.M. (C711 .393100.7 : V41, 7;1AZ :

,17 CO.V YEL. uM21

14329nwSE WIA7C17,1114 21. 4

FIGURE 10. Acoustic scanning system image ofSIC disk of 40 mm (1.6 in.) diameter

ATTENUATIONNc •rnm - 1

- 0 25

1)

FIGURE 11. Acoustic scanning ultrasonic velocityimage of SIC showing a pear shaped pattern inporosity structure (area shown is 20 x 20 mm)

VELOCITYmn •o - 1

- 114

- 10v

- 10.4

- 0.99


velocity at these points on the appropriate overlays. Also, thefrequency dependence of attenuation has been related to thesubsurface pore or discontinuity sizes, 9 the state of therecrystallization in metals' and the mean grain size." Easyaccess to this type of information is made possible by the useof a video system.

Role in Data Interpretation

Video and computer systems also play a crucial role in theinterpretation of ultrasonic d13.ta. In the past, only a few mea-surements were taken at a small number of positions to eval-uate a material ultrasonically. The data from these measure-ments were averaged and the standard deviation deter-mined. These two values (the average and the standarddeviation) were used to classify the test object but thetwo quantities may not contain sufficient information forproviding a full understanding of the internal structure of thetest material.

For example, from nine measurements, the average ultra-sonic attenuation and its standard deviation at 100 MHz in aceramic test object can be calculated as 0.11 and 0.03 nepersper millimeter. But this information does not describe thecloudlike attenuation structure revealed in the video imageof the test object (Fig. 10).

Another example of video imaging's impact on the inter-pretation of ultrasonic data is shown in Figs. 11 and 12. Pre-viously, ultrasonic attenuation in ceramics was believed to becaused by diffractive scattering from individual grain andpore boundaries. An increase in the number or size of pores

FIGURE 12. Acoustic scanning ultrasonicattenuation image of sample region inFigure 11, showing high attenuation at sharphigh-to-low density boundaries (area shown is20 x 20 mm)

ATTENUATIONNp•mm - I

01

-03

- 0 1

-o

- 0.2


produced an increase in the attenuation. Figures 11 and 12show that high ultrasonic attenuation is caused by diffractivescattering at regions exhibiting large velocity (density) gradi-ents, not increased porosity.

ConclusionThe importance of optical technologies to the broad field

of nondestructive testing is typified in the interplay of visualand optical techniques with ultrasonic technology, asdetailed above. Visual testing has a long and vital history.Despite their origins and their value in the past, the visualand optical techniques are also directly linked to the mostcomplex nondestructive testing technologies of the present.

FIGURE 14. Archival photograph of exfoliationcorrosion damage on an aircraft wing spar


PART 2APPLICATIONS OF PHOTOGRAPHY INVISUAL TESTING

Photographs as a Permanent Record forVisual Testing

The eyes have no match when it comes to scanning andevaluating small objects or large structures. There is no sub-stitute for a direct view of a test object and this is the reasonfor the continued value of visual testing—the oldest of thenondestructive testing methods.

Trying to recall details such as the exact size, location andorientation long after a visual test is at best difficult, at worstreckless. A permanent visual record is often needed for engi-neering purposes, personnel training, failure analysis andcrack growth monitoring (Fig. 13): Photography is often themost practical and least expensive way to preserve visual testresults permanently.

Photographs are historical records with many scientificand technical applications. Using 35 or 70 mm slides or

FIGURE 13. Cracked engine truss mount showingcritical areas and crack orientation

prints for a permanent record of a test makes it possible tosubsequently review, scrutinize and evaluate a discontinuity.Photographs can be used as reference documents (Fig. 14),just as other nondestructive testing methods use radio-graphic film, computer printouts or tape from strip chartrecorders. The primary use of photography is as a backuprecord of visual tests.

Photographic Equipment

Typical equipment for producing a photographic record ofvisual tests includes a camera (a single lens reflex cameraallows viewing through its lens), a sturdy tripod, a macrolens for closeup capabilities (Fig. 15), a battery poweredautomatically regulated flash and a cable shutter release. The35 and 70 mm film formats are widely accepted and allowmaximum flexibility. The 70 mm format is essential to only afew specific applications and is substantially more expensiveto start.

The macro lens with a 28 to 80 mm zoom lens can be madeto focus on objects some 30 cm (a foot) from the lens or tomagnify objects that are difficult or dangerous to approach.The tripod and cable release are used to stabilize the camerafor long exposures in dim lighting.


FIGURE 15. A macro lens view revealing cracksand corrosion pitting on a casting

Slides (color positives) have several advantages overprints, including the fact that high quality prints can be pro-duced from slides. Prints made from negatives are developedand processed in two steps. Transparancies are cheaper tomake because they are developed in one process and this onestep image reconstruction process allows slide films to cap-ture sharp, colorful and greatly detailed images very accu-rately. A single 24 x 36 mm transparancy is visually equal to10 billion pixels. In addition, slides are easy to store andretrieve.

Usually, buying a 28 to 80 mm zoom lens means that thecamera body and lens are purchased separately. A camera,no matter how costly, is no better than its lens. An inexpen-sive lens can drastically reduce an expensive camera's imagereproduction quality.

Archiving Test Documentation

Once film has been exposed and professed, images shouldbe edited by choosing the best exposure and composition forthe archival record. Both slides and prints are most conve-niently filed in vinyl pockets sized for the image type anddesigned for storage in binders. It is helpful if the vinyl pock-ets are of recent manufacture and are designed specificallyto hold slides or prints.

A file management system should be developed by label-ing the vinyl pages for ease of retrieval. When the need arisesto review a document, finding it should not be a problem.The most recent government legislation should be consultedto determine how long such records must be kept. Qualityassurance personnel should seek the advice of a corporatelibrarian, archivist or consultant as to the safest way to storeinspection documents.

Photogrammetry for Documenting theCondition of Petrochemical Furnaces

Photogrammetry is the science of obtaining quantitativemeasurements of physical objects through processes of

recording, measuring and interpreting photographic images,Photogrammetry is used to make topographic maps and sur-veys based on measurements and information obtained fromaerial photographs.'

Considerable use has been made of photogrammetry fornontopographic purposes such as architecture, civil andmechanical engineering and structural analysis. Close range,terrestrial applications are used for solving problems inremote measurement and permanent documentation ofdeformation, deflection or damage to a wide variety of largeand small objects with surprisingly good accuracy."'

Close range photogrammetry also has the potential forremote, noncontact, onst ream monitoring of the condition offurnace tubes through measurement of bulging, bowingand creep as well as the ability to identify the extent of de-terioration of other components such as cracks in hangers,tubesheets or spilling of refractory and brickwork. Photo-grammetry could prove useful as a quantitative tool appliedperiodically for charting the condition of furnace internals.

Basic Principles of Photogrammetry

Close range photogrammetry generally involves camera-to-object distances less than 30 m (100 ft). Specially designedcalibrated cameras are available for this work.' ' 8 However,many readily available amateur and professional types canalso be successfully used.' Most of the close range applica-tions use an analog approach that involves taking a pair ofphotographs with the camera oriented normal to the objectat a known distance. By viewing this pair of stereo photo-graphs through a stereoscope, a three-dimensional image ofthe object is reconstructed (see Fig. 16).

Instead of looking directly at the object, it can be photo-graphed from two points analogous to the location of the twoeyes. If these pictures are then viewed, left picture with lefteye and right picture with right eye, the object appears againin three dimensions. Figure 17 shows how stereo photo-graphs are viewed through a mirror stereoscope.

For various points on an object, relative differences in dis-tance from the camera can be found by determining theirparallax differences. With the aid of a parallax bar or stereoplotter, parallaxes can he measured with a precision of 10

(0.4 mil). If the distance between camera stations (base),camera focal distance and object-to-camera distance areknown, absolute measurements on the object can be made.

Stereo Photogrammetry Applied to Measurements ofFurnace Components

The purpose of measuring parallax difference betweentwo object points is to develop the contour or shape of a sur-face so that information on deformation, relief (contour) ormovement of the component can he determined. In Fig. 18,


FIGURE 16. Visual depth perception viewing a realobject: a rectilinear object 1-2-3 is perceived as/ '-2'-3' by the right eye and as 1"-2"-3" by the lefteye, so that !for example) the left eye perceives agreater distance between points 2 and 3 than theright eye does

RIGHT EYE

LEFT EYE

FIGURE 17. Visual depth perception usingstereoscope viewer

RIGHT EYE

LEFT EYE

a point on a furnace tube, possibly a bulge or high creep area,is imaged in a stereo view. Similar triangles (shaded areas)demonstrate the relation.

h = f x —P

X' — X" = p

hb

7_- 1)

h 2b = x

112 x dpdh =

f x b

(Eq. 1)

(Eq. 2)

(Eq. 3)

(Eq. 4)

(Eq. 5)

Where:

h the object-to-lens distance;f = the focal length of the camera lens;b = the distance between exposure stations or camera

bases; andp the parallax or the difference in the position in the

two photographs (X'—X").

FIGURE 18. Parallax measurements from stereophotographs

FIGURE 19. Parallax bar—the millimeter readingon the spindle scale plus the micrometer readingon the rotating drum together equal the totalparallax

2 3 4LEGENDI. REMOVABLE GLASS PLATES WITH DOT, CIRCLE AND CROSS MARKS2. THUMB SCREW OF SPINDLE EXTENSION FOR COARSE ADJUSTMENTS3. MILLIMETER SCALE OF SPINDLE4. MICROMETER DRUM READING TO 0.01 MILLIMETER FOR FINE

ADJUSTMENTS

FIGURE 20. Mirror stereoscope used with aparallax bar


Parallax difference between corresponding points on stereophotographs can be measured by means of a parallax bar(Fig. 19). The parallax bar is essentially a micrometer withone fixed and one movable measuring mark on a glass reticle.Turning the micrometer screw changes the distance betweenthe measuring marks. The measuring range is 44 mm(1.72 in.), the smallest division being 10 gm (0.4 mil).

After appropriate positioning under the stereoscope pic-tured in Fig. 20, the parallax bar is placed on the photo-graphs, aligning the two reference plates on the parallax barto exactly coincide with one point on the two photographs.As viewed through the stereoscope, when this condition isachieved, both marks on the parallax bar appear to fusetogether into one point apparently floating into space abovethe model of the object. If two points of different and knownelevation can be identified in a stereo model, their difference

is measured by:

AhC = f xb

= Ap

(Eq. 6)

With this equation applied to two known points, it becomespossible to measure the difference in distance of otherunknown points on the model. This technique can be used infilm radiography.

Factors Affecting the Accuracy of PhotogrammetricMeasurements

The accuracy of distance measurements from photogram-metry is related to camera internal orientation, lens distor-tion, focal length, object-to-lens distance, photographic baselength and precision of the plotting method.

Using conventional and readily available photogrammetriccameras and plotting equipment, sufficient accuracy is

attainable to measure distortions and deformations associ-ated with creep and bulging of components such as furnacetubes. The following relationship is used to determine theminimum difference in distance that can be measured byphotogrammetry:

h2h — x

f dh(Eq. 7)

Where:

b= camera base;object-to-lens distance;

f

lens focal length;dp = accuracy to which parallax can he measured; anddh accuracy to which measurements can be made on

the test object.

Example: compute the accuracy attained using a first orderstereo plotting instrument for taking stereo photographs ofa deformed 150 mm (6 in.) diameter tube, where h is

FIGURE 21. Photogrammetric set up for measuringdeformation in 150 mm (6 In.) outside diameterfurnace tube: (a) geometry of test setup,(b) position 1 photograph and (c) position 2photograph

I 27ml9f0

(C)

(a)

(b)

MIN

For SI units:

(75)(2,740)— ±1.8 mm

(5,490)2(1.25 x 10- 2 )

or in inches:

= ±0.072 in.(3)(108)

(216)2(5 x 10 -4 )

OTHER APPLICATIONS OF VISUAL AND OPTICAL TESTS / 3/7

2,740 mm (108 in.), h is 5,490 mm (216 in.), f is 75 mm(3 in.) and dp is 1.25 x 10- 2 min (5 x 10' in.). Rearrang-ing Eq. 9 gives:

(Eq. 8)

This example represents the test setup shown in Fig. 21. Typ-ical results from these tests are shown in Fig. 22, whichgives a comparison of photogrammetric and mechanicalmeasurements at one cross section of a deformed 150 mm(6 in.) diameter section of furnace tube. Copies of the stereophotographs of this pipe are shown in Fig. 21.

In this example, the possible error of ± 1.8 mm ( ± 0.072in.) is unacceptable. This error can be reduced by reducingh and dp or by increasing for b. Because this test representsconditions on an actual furnace, the distances b (distancebetween viewing ports) and h (distance across furnace) arefixed. The value of dp is the accuracy available for first orderstereo plotters and cannot be practically reduced. The cam-era lens focal lengthf can be most readily increased and thefollowing example shows how an improvement in accuracyof 0.6 mm ( ± 0.024 in.) is achieved by increasing f from75 mm (3 in.) to 225 mm (9 in.). For millimeters:

(5 490) 2(1 25 x 10- 2 1dh = ' = ± 0.6 mm

(225)(2,740)

or in inches

(216)2(5 x 10- 4 )dh = — ±0.024 in.

(9)(108)

For tube condition monitoring, the acceptability of accu-racies of this order depends on the magnitude of the totalcreep deformation to failure and particularly on the size ofthe growth increments after the onset of tertiary creep. Forthin walled furnace tubes constructed of wrought low alloyand austenitic stainless steels, expected creep deformation tofailure are in the range of 10 to 20 percent. Assuming a tubeoutside diameter of 150 mm (6 in.), the range of outsidediameters at failure is between 165 mm (6.6 in.) and 180 mm(7.2 in.) or a growth in outside diameter of 15 mm (0.6 in.)and 30 mm (1.2 in.), respectively. Assuming that a measuring

accuracy of 0.6 mm ( ± 0.024 in.) can be achieved, theresulting accuracies for this example are ±0.5 percent and± 1.0 percent, respectively.

Greater measuring accuracies are probably required fordetermining creep growth of HK — 40 tubes and those fabri-cated from similar cast high alloy materials where the totalcreep growth to failure is on the order of 5 percent.

For evaluating photogrammetry as a means of determin-ing the, condition of furnaces, tube sheets, hangers orrefractories, an assessment should be made of the mini-mum distortion, deformation, cracking or other significant

h2 x dpdh f x h

FIGURE 22. Comparison of measurements taken byphotogrammetry and mechanical measurementon deformed 150 mm (6 in.) outside diameterfurnace tube

LARGEST ERROR =4 mm10 16 rn

LEGENDPROFILE OF PIPE MEASURED WITH MICROMETER

— — — CIRCULAR PROFILE OF UNDEFORMED TUBEPROFILE OF PIPE MEASURED BY STEREO PHOTOGRAMMETRICPLOTTING

FIGURE 23. A refinery pipestill furnace: (a) stereophotography setup and (b) stereo views

(a)N.\ \ \ \NV...NAN \ \ \ \ \ NAN \ \

I 138 cm 154.5

1

II

541 cm 213 In.}


precursor to failure. This size estimate can in turn be com-pared to the calculated accuracy limits of a particular photo-grammetric setup used to decide whether results areacceptable.

Effects of High Temperature Environments onPhotogrammetry

The use of cameras for documenting conditions in hightemperatures is subject to several constraints and limitations.The most serious problemn furnaces, for example, is theprotection of the camera from high intensity radiant energyemitted through the viewing ports. This high temperatureenvironment usually limits the placement period and expo-sure times to a few minutes. The relatively small size of theviewing ports cuts down the angle of view and limits the areathat can be pictured within the furnace. Lighting conditionssignificantly affect the production of shadows and modelingof the surface of subjects. In this respect, a single lightingsource is desirable. However, in a furnace the many burners,as lighting sources, tend to produce diffuse lighting effectsthat reduce overall shadow contrast and surface modeling.Flame luminosity, particularly from oil fired burners, isundesirable because the unburned incandescent carbon par-ticles tend to hide details of the subject if it is located behindthe flame. Uneven heat distribution also creates problems

resulting from local variations in flue gas density causingso-called heat waves that distort images of objects picturedin the firebox.

Because furnace firing conditions can be varied, somecontrol can be exerted on the visual environment. Morefavorable periods, such as when gas rather than oil is beingburned, offer lighting conditions more suitable for takingphotographs. The stereo photographs shown in Fig. 23 weretaken in a refinery pipestill atmospheric furnace. Eventhough this furnace was oil fired at the time, good definitionand image quality are apparent.

Other considerations for furnace photogrammetry con-cern selection of the camera base and lens focal length. Thecamera base is limited to the location of viewing ports in thesides of the furnace. In general, it is desirable to select aslarge a base as can be accommodated by the lens that is used.This results in the best accuracy for measurements on thetest objects. For the same reason, a lens with a long focal


length is desirable. The effects of camera base and lens focallength can be quantitatively evaluated by use of Equation 7.

ConclusionsClose range photogrammetry appears to have applica-

tions, under favorable conditions, for inservice, remote, non-contact monitoring of the condition of furnace tubesand other internal furnace components. For such onlineapplications, the following conditions are required for favor-able results:

1. viewing port location and spacing appropriate for line ofsight positioning of camera stations;

2. adequate lighting intensity from furnace burners;3. proper quality and source position of light to give good

surface contrast and modeling;4. minimum variations in flue gas density to prevent dis-

tortion of viewed images;5. high quality 35 mm cameras; and6. by proper selection of parameters, photogrammetry can

offer the means for the direct measurement of deforma-tion, bulging, creep of furnace tubes and damage toother furnace components.

FIGURE 24. Visually detected open crack on anaPrnToace engine ceramic turbine blade


PART 3 VISUAL TESTS OF CERAMICS

Ceramic materials are being developed for many uses andas applications of ceramics expand so does the need for reli-able nondestructive testing techniques. Concurrent with thedevelopment of new materials is the study of NDT tech-niques and their ability to detect discontinuities approachingcritical size in engineering ceramic materials. Visual meth-ods are widely used for this purpose, along with liquid pene-trant, radiographic and ultrasonic techniques.

For example, with sintered ceramics, optical holographyand optically enhanced visual techniques are used to detectsurface and near surface discontinuities. Potential applica-tions for these inspection methods include materials devel-opment and statistical sampling of manufactured productionlots.

The text below briefly describes several ways visual andoptical tests may be used with specific advanced ceramiccomponents.

Visual Tests of Injection MoldedTurbine Blades

In an aerospace engine, the material integrity of turbineblades must be high to sustain the service li=e. The blade isengineered to transfer combustion energy to the enginecompressor (providing mostly air as driving force) and, in theprocess, experiences severe thermal and mechanical loading.

For a ceramic turbine blade of silicon nitride and 6 per-cent glass, penetrant tests are used to detect and locatecracking and porosity." Radiography is used to detect severaltypes of inclusions as well as subsurface cracking. Visualtechniques are also commonly used for inspection of theseinjection molded blades.

In one application, optical magnifications from 5 x to40 x are used for crack detection. 2° Cracking previouslylocated with penetrants is typically not detected visually butopen surface cracks along a turbine blade's split lines are visi-ble (see Fig. 24). It is possible that such cracks occur duringremoval from the mold and are subsequently sinteredsmooth so that they do not hold liquid penetrant.

Visual techniques may also be used in this application tolocate pores in the turbine blade's thick sections.

Automatic Testing of Thin CeramicComponents21

Prefired thin ceramics are commonly used in radiofrequency

filters and as insulation in capacitors. Minute but critical dis-continuities in the ceramic can escape typical visual detec-tion but could be discovered using a neon light source in areflecting chamber. The light iscintensified within the cham-ber and directed through a slot onto the ceramic wafer. Pho-toelectric cells on the opposite side of the ceramic wafersense light transmitted through discontinuities in the testobject (see Fig. 25).

Photocell output is amplified and applied to a logic circuitthat in turn activates an electromechanical marking system.

Laser Based and MicroscopicInspections of Ceramics22High Speed Laser System

A variety of optical techniques can be used for processcontrol of ceramics. One device for detecting surface discon-tinuities at high speeds uses laser light scattering to detect

`I PHOTOELECTRIC CELLS

LOGICAND

DRIVINGCIRCUITRY

FIGURE 25. Diagram for an automatic opticalsystem used to detect discontinuities inceramic wafers

CONCENTRATING REFLECTOR

DRIVERS AND PENS

CERAMIC TEST OBJECT

LIGHT SOURCE


and count discontinuities. The system has scanning speedsup to 100 crii2•s 1 , requires little computer memory and,because of its statistical base, uses no image processing.

A scanning laser beam and focusing optics are used to pro-duce a 60 p.m diameter spot on the test object. As the scan-ning beam encounters surface discontinuities, laser light isscattered and optics coupled to a photomultiplier tube col-lect the scattered light. Pulses from the photomultiplier tubeare counted and stored as a function of their position, pro-ducing a discontinuity map.

Video System for Detecting Stress in Glasses

Stress in glass has been conventionally measured using

manual visual techniques based on stress birefringence orphotoelasticity. These techniques are labor intensive andpotentially inaccurate. With the introduction of fiber optics,computer systems and cameras using charge coupleddevices, it is now possible to acquire data rapidly on discretepoints in a stress field. Such a digital image analysis systemhas the advantages of acquiring information quickly andobtaining accurate measurements that are independent ofstress directions.

One such system is based on the concept known as half-fringe photoelasticity. First, a video camera acquires animage on a 512 x 512 pixel frame. The light intensity at eachpixel is measured and digitized with a precision of -± 0.5 per-cent. The speed of data acquisition is a function of the cam-era framing rate (30 to 1,000 frames per second). A full fieldstress image is then produced in 2 to 15 s. The software canpresent the data in several formats.

Such a digital image analysis system may be used for a vari-ety of quality control applications. For instance, it can mea-sure the retardation within a prescribed area—in opticalmicroscopy, retardation is the difference in the optical pathof light passing through the test object and light bypassingthe test object. Such a digital test system can also displaymaximum measured retardation. For analysis of edge stressin tempered glass, for quality control of optical glass and con-trol of cooling performance in molded or cast ceramic prod-ucts, a graph of stress distribution also can be generated. Incomponents where the birefringence is very weak and onlygray shades are detected visually (annealed glass), it is possi-ble to display a full field pattern of partial orders at selectedretardation intervals. Digital image analysis systems may beused at temperatures up to 1,100 °C (2,000 °F) and have arange of up to 8,000 nm.

FIGURE 26. Casing and tubing round thread—

nominal tolerances

DESIGNED CLEARANCE0.08 mm (0.003 in.)30 DEGREES 30 DEGREES

DESIGNED CLEARANCE0.08 mm (0.003 In.)

FIGURE 27. Buttress casing thread—nominaltolerances

CREST TO ROOT CONTACTG r

r—u<

Z

PIPECREST TO ROOT CONTACT

COUPLINGDESIGNED CLEARANCE

.001 (NOMINAL)


PART 4VISUAL TESTING OF THREADS IN OILCOUNTRY TUBULAR GOODS

Before the assembly of the downhole structure of an oilwell, casing and tubing threads must undergo visual testing.These types of tubular products in the oil field are referredto as oil country tubular goods (OCTG). The term casingapplies to the many strings of pipe that are used to line thehole during and after drilling. This pipe protects the holefrom formation collapse, keeps the formation fluids out ofthe hole and—perhaps most importantly keeps the oilwell fluids out of the water tables. The casing strings are apermanent part of the well and many are cemented into theformation.

The tubing string is referred to as the production string.This is the pipe through which the oil or gas is brought to thesurface. To do this, it is important that the connection notallow the fluids to leak out.

RequirementsThreads on both tubing and casing are required to per-

form two functions: seal the connection and provide thestrength to support the weight of the string as it is loweredinto the well.

Extensive service data for the older connections andextensive design testing for new connections together helppredict the lifetime of a properly manufactured and undam-aged threaded connections. Inspectors visually testingthreads are looking for manufacturing errors or damagecaused by handling or for corrosion that would affect theability of the connection to seal." The second function of avisual test is to detect any discontinuity that would interferewith the ability of the connection to be properly "made up,"that is, screwed together.

Types of SealThere are three types of seals used on oil field tubing and

casing: interference sealing threads, gasket seals and metalto metal seals. The interference sealing threads, or interfer-ence fitted threads, use a tapered connection made up undergreat pressure, forcing the mating surfaces together moretightly than is possible by hand alone. Figure 26 shows theflank engagement for American Petroleum Institute (API)

round threads and Figure 27 shows the root and crestengagement for API buttress threads.

Interference Seal

Because two mass produced machined parts cannot bemade to fit perfectly, there are designed clearances in themating pieces. If these clearances are not plugged theywould provide a helical leak path through the connection.Thread lubricant is used to close the gap. If properly placedin the gaps the thread lubricant, a heavy grease, will com-plete the seal by plugging up these gaps.' The smallness ofthe gap and the long helical_ distances make an effective seal.

Gasket Seal

The gasket seal uses a ring of resilient material somewherein the connection (see Fig. 28). The ring is ductile enough to

FIGURE 28. Gasket seal using a resilient ring

FIGURE 29. Metal to metal seals are machined toprovide a pressured interference fit 360 degreesaround the connection

PIPE AXIS

FIGURE 30. API round threads: (a) field end andit)) mill end (L, Indicates full crested threads)

(a)

PIPE

I II 1 D END

(1,J

COUPLING

16 awl

PERFECTTHREAD LENGTH 5 i

COI TERBOREA ND FACE

It film 10.5 on J

MILL ENDAREA

F./Kt. ICHAAM ER

I, NON L k


limn itself to the shape of the mating piece. This type of sealis always used with at least one other seal.

Metal to Metal Seal

Metal to metal seals are considered the premium seals inthe oil field. The mating surfaces on the external connection(the pin) and internal connection (the box) are machined toprovide a pressured interference fit 360 degrees around theconnection (see Fig. 29).

These three types of seals are used either alone or withothers in the various connections used in oil country tubulargoods.

SpecificationsAmerican Petroleum Institute (API) threads arc public

property governed by API Standard 513, where the inspec-tion guidelines are very well defined. Also, over a hundredthread designs used on oil country tiiblilargo(ids are iliiprie-taiy, that is, the design is owned by someone and in manycases patented. For these non-API threads, the inspectioncriteria are confidential, usually closely guarded by theinanufa•tmer.

The third party inspector of these connections can onlyexamine the threads and set aside any imperfect thread forthe manufacturer's evaluation. To recognize any deviationfrom normal in proprietary connections, the third partyinspector must be familiar with the published literature onthe connection.

Visual Testing ProceduresThreads of oil country tubular goods are visually tested as

a separate service or in conjunction with other services suchas mechanical gaging of the threads for API threads or mag-netic particle or liquid penetrant testing of the ends,depending on whether the host material is ferromagnetic.

Before inspection, the threads must be cleaned with sol-vents and brushes. The waste materials must be captured forproper disposal. During cleaning, the inspector should beginhis inspection of the threads. Any obvious imperfectionsshould be marked as soon as they are found.

The Pin ThreadsThe pin thread of API round and buttress threads have

four distinct areas with different criteria for each (seeFigs. 30 and 31). The threads toward the end of the pipe arethe sealing threads, referred to as the L,. threads because APIStandard 5B uses the term L. to designate the minimumlength of hill crested threads.

Pin Sealing Area Criteria

The L, length is a measured distance that is closely testedvisually fiir anything that might cause a leak path through the

FIGURE 31. API buttress threads: (a) field end and(b) mill end

(a)

FIRST PERFECT THREADCHAMFER \

FACE

LOCATION OF TRIANGLESTAMP ON PIPE

PIPELc THREADS NON Lc

HELD END

(b)

FACE

17.8 mm (07FIRST PERFECT THREAD

COUPLING

— 12 7 mm (0 5 In.)MILL END

PIPE

J AREA


connection. The 4 thread must be free of visible tears, cuts,grinds, shoulders or any other anomaly that breaks the conti-nuity of the threads. 25 All threads in the 4 must have fullcrests on round threads—threads with less than full crestsare called black crested threads, so called because threadinghas not removed the dark, carburized original mill surface(Fig. 32). On buttress threads, there may be two blackcrested threads in the L, area as long as neither is longer than25 percent of the pipe circumference. The tables in APIStandard 5B give the distance from the end of the pipe to theend of the 4 area.

While there may be superficial discoloration in this area,that is the limit of discontinuities allowed in the L area. Themost critical consideration throughout the threaded area,including the L, area, is that there be no protrusions on thethread surfaces that could score the mating surface. Because

the thread flanks slide past each other during makeup, thesurfaces must be smooth. If the surface has a protrusion, thepressure, instead of being distributed across the flank, will beconcentrated in the high spot causing friction and galling.Minor repair of high spots with a hand file may be permittedwith the pipe owner's permission.

Pin Nonsealing Threads Criteria

The threads between the end of the L, area and the vanishpoint of the threads are not considered as sealing threads sothey are allowed to have imperfections that would be consid-ered to be leak paths. In fact, the manufacturer may repairthreads in this area by grinding as long as the grind does notgo either below the root cone of the thread or 12.5 percentof the specified pipe wall thickness measured from the pro-jected pipe surface, whichever is greater. The most criticalfactor in this area of the threads is that there be no protru-sions on the thread flanks that will remove the protectivecoating or score the mating surfaces.

Chamfer Criteria

The chamfer area on the end of the pipe is beveled to pro-vide a place for the thread to start. This 65 to 70 degree bevelmust be present for 360 degrees around the pipe face and thestarting thread must run out on the chamfer. 25 This require-ment is to prevent a feather edge, which could be folded overduring stabbing at the rig floor. If a ridge were present andfolded over, the coupling thread would not have a place to gosince the fold would occupy the thread groove designed forthe coupling. The required length of the chamfer, to be suf-ficiently long to allow the thread to run out on the chamfer,must be tempered with the industry standard that the cham-fer not come to a sharp edge."

If the starting thread is not continuous, that is, if a portionof the groove is missing, this condition in itself is acceptablebut may be a sign that the pipe was misaligned during thefinishing of the thread. There are tolerances for angular mis-alignment and this condition must be evaluated. A false start-ing thread is not acceptable if it runs into the true startingthread." The chamfer smoothness is not critical for it doesnot contribute to the thread after it provides a place to startmakeup.

Pin Face Criteria

The fourth and final area where threads are critical is thepipe end. The ends must be free of burrs on the inside andoutside. 27 Freedom from burrs is actually important to theentire threaded area because the burrs might be dislodgedduring makeup. If they become dislodged they could inter-fere with makeup and promote galling (see Fig. 33).


The Coupling Threads'Mere are three areas on the coupling threads (see

Figs. :30 and 31), The area on the coupling referred to as theperfect thread length is longer than the L, area on the pin. Itstarts with the first threads in the coupling and continues tothe plane located near the made up p9sition of the first fullthread (lithe pin threads. The length of this area provides fora good thread throughout the travel area during makeup ofthe pin thread. This area has the same criteria as the L, onthe pin threads. These threads must be nearly perfect.

On smaller diameter pipe, a mirror is required to view theload flanks, that is, the flanks facing the center of the coup-ling. The repair of minor anomalies in the coupling threadsis normally not as practical as repairs on the pin threads. lbimprove conosion resistance, antigalling and sealing ability,the internal threads are coated with zinc, tin or metalliclhosphate.'

The threads in the center of the coupling are required onlyo be present. Sometimes these are not full threads but areonsidered acceptable if the thread root is present. Seams,aps or cracks in the coupling threads are always consideredelectable but are not normally fbund by visual testing alone.e cause of the coating applied to the internal threads. Dou-le cut threads in this area are ameptable. While cutting the'cond side, the thread cutter may dip the crests of thetreads from the first side. This condition is acceptablenless the cutting extends into the perfect thread area."The counter bore and face are the other areas of the coup-lg. The diameter of the recess shall be sufficient to preventMingghost thread mots on the surface of the recess.' Also,ere should be no burrs (Fig. 34) or protrusions in thelinter bore area that could damage the pipe threads duringtithing at the rig site.

Presence of Makeup TriangleThe visual thread inspector checks fin• the presence of the

makeup triangle (a manufacturer's stamp indicating wheremakeup should stop) on buttress threads and round threads

larger than 400 mm (16 in.). The lack of a triangle is not nor-mally cause for rejection, but the customer should be noti-fied since the triangle is used to aid proper makeup on therig floor. These criteria are summarized in Table 2 for roundthreads and Table 3 for buttress threads.

Makeup Connections'Me "power tight" (that is, completely tightened) makeup

is also checked visually during a visual thread test. The pinthread face should be made up to 13 mm (0.5 in.) from thecenter of the coupling (see Fig. 3.5) for most pipe. The centerof the coupling can usually he visually located. By countingthreads between the center of the coupling and the face ofthe pin on the opposite side, the distance can be quite accu-rately estimated. Further evaluation by measurement maybe required for classification if visual evaluation shows a sig-nificant e rror.

Use of a Profile GageAs an aid in detecting thread form problems, at profile gage

(see Fig. 36) should be used on each thread tested. There areseveral thread form problems that cannot be detected with-out a profile gage. Shaved and other threads that exhibit anexcessive narrowness of thread width" will not seal properlybecause the flanks will not mate. This condition would cause


TABLE 2. American Petroleum Institute (API) criteriafor visual testing of round threads

In accordance with these guidelines, visually test thesethreads for the following imperfections. Imperfectionsdescribed below are considered defects.

1. On the pin face examine for:a. a knife edgeb. any burrs

2. On the chamfer examine for:a. a feather edgeb. any burrsc. chamfer is not present for the full 360 degree around the

Piped. starting thread running out to the end of the pipe

3. On the L, area (pin), examine for:a. any imperfection or distortion of the thread form which

will produce a longitudinal or helical leak pathb. black crested threadsc. any imperfection that visibly bulges the flanks or results in

a protrusion of metal from one or more threads4. On the non Lc area (pin), examine for:

a. any imperfection traveling through the root of the threador 12.5 percent of the specified wall thickness measuredfrom the projected pipe surface, whichever is greater

b. any metal protrusion that may prevent proper makeup5. On the Jarea (coupling):

a. threads not extending to the center of the couplingb. any metal protrusion that may prevent proper makeup

6. This area of the threads must be inspected the same asnumber 3 above

7. On the coupling face and counterbore, examine for:a. any metal protrusion that would prevent proper makeup

the thread to fail in the well. Thick threads, improper threadheight and steps on the thread flank or roots are all condi-tions that a profile gage can help detect.

Profile gages may also help determine whether an anomalyis causing a protrusion or whether all protruding metal hasbeen removed.

Arc BurnsArc burns are localized points of surface melting caused by

arcing between electrode and ground and the pipe surface."(The condition can be caused during magnetic particle test-ing by passing current into the pipe through steel thread pro-tectors.) Because arc burns cause localized changes in themetallurgical structure and frequently cause protrusions,they always cause the thread to be rejected.

ShouldersAPI round threads are designed to run out at the pipe

TABLE 3. American Petroleum Institute (API) criteriafor visual testing of buttress threads

In accordance with these guidelines, visually test thesethreads for the following imperfections. Imperfectionsdescribed below except triangle stamps are considereddefects. Triangle stamp errors need to be reported.

1. on the pin face examine for:a. a knife edgeb. any burrs

2. on the chamfer examine for:a. a feather edgeb. any burrsc. chamfer is not present for the full 360 degrees around the

piped. starting thread running out to the end of the pipe

3. on the L, area, examine for (pin):a. any imperfection or distortion of the thread form that will

produce a longitudinal or helical leak pathb. black crested threads, no more than two with each no

longer than 25 percent of circumferencec. any imperfection that visibly bulges three flanks, crests, or

results in a protrusion of metal from one or more threadsd. superficial corrosion covering more than half the

circumference of the L, area4. on the non L, area (pin), examine for (thread area stops at

the apex of makeup triangle):a. any imperfection traveling through the root of the thread

or 12.5 percent of the specified wall thickness measuredfrom the projected pipe surface, whichever is greater

b. any metal protrusion that may prevent proper makeupc. examine for makeup triangle present on pin end

5. on the J area (coupling):a. threads not extending to the center of the coupling

6. this area of the threads must be inspected the same asnumber 3 above with the exception that no black crestedthreads are allowed

7. on the coupling face, examine for:a. any metal protrusion that would prevent proper makeupb. any feather edges

8. ensure proper makeup to triangle on the coupled end

surface. Excessive metal, machined for threading but in factnot threaded, where the thread stops on the outside surfaceof the pipe is referred to as a shoulder." If the shoulder goesall the way around the connection, it indicates that either thepipe is too big or the thread is too small.

Further investigation is required to determine which ofthese conditions exists. A small thread size is a serious condi-tion because makeup and sealing depend on both threadmembers' being the proper size. Because of the threaddesign, shoulders do not occur on buttress threads butthreads that are too small may be recognized by the lack ofblack crested threads (described above) near the L, area. Ifthere are not black crested threads in this area, either thepipe is too big or the threads are too small.


An alignment problem may he indicated by shoulders ononly one side for round threads or by lack of black crestedthreads on one side for buttress. The threads may be angu-larly or axially misaligned. There are API limits for both

conditions because the angular misalignment (hookedthreads) would cause makeup problems in the field and theaxial misalignment would cause joint strength problems.

FIGURE 35. Power tight makeup is visuallyinspected by counting threads between the centerof the coupling and the face of the pin on theopposite side (not to scale)

FIGURE 36. Handheld pipe thread gage

AIL


PART 5 VISUAL TESTING OF COMPOSITE MATERIALS

Visual and optical testing are important methods to thecomposite materials industry as primary evaluation methods,quick-look cursory evaluations and as backups to other formsof nondestructive evaluation (NDE). The need for visualtesting often results from several associated requirements,such as the desire to visualize an anomalous situation, thedesire to perform a low cost evaluation and the desire to visu-alize or document the findings of another nondestructiveevaluation method.

Visual testing is often used with other forms of nonde-structive evaluation—looking at a composite structure's sur-face for pits or voids, for example, before performingradiography. In spite of its "low tech," low cost nature, avisual evaluation may be all that is required by engineeringpersonnel in assessing quality or service life conditions. Sev-eral topics will be covered in the following text: problems inthe applications of visual testing, what visual testing candetect in composite materials, anomalous situations callingfor visual testing, specific applications and recent image pro-cessing techniques for visual testing.

Problem AreasVisual testing of composite materials is more difficult than

that of most other engineering materials, due to the hetero-geneous nature of composites. A perfectly good structuremay have variations in resin, causing color variations on theexterior surface, surface roughness or waviness due to wovenmaterials used to apply pressure during the layup, or benignsurface features such as small voids or wrinkles.

Preparation of the part to be viewed is important in visualtesting of composites. Often, structures cannot be viewedimmediately after manufacture and edges and bag side sur-faces must be trimmed extensively to remove excess resin.After these trimming operations there may be dust and resi-due on the surfaces. If the edge of the structure must beviewed, sanding or polishing of the edge may be necessary.After the cleaning, good lighting is essential. Straight onlighting may be needed to see the features at the bottom of avoid; oblique lighting, to distinguish between a protruding orconcave surface feature.

Because of inhomogeneity, it is important to choose anappropriate scale factor when viewing composite materialswith magnification. Many visual tests are performed with 5 xto 10 x power. This relatively low power evaluation is a bal-ance of close viewing of the small surface imperfections and

retention of the ability to view a significant surrounding areafor perspective and comparison. Figure 37 shows a question-able area on a cross section of a Kevlar ®/polyester com-posite. Without proper lighting and scale factor, it appearsthat the area in the center of the figure has two matrix cracks.Figure 38 shows the same area, with higher magnificationand with better lighting. The two lines are not matrix cracks.Figure 39 shows actual matrix cracks (notice the prominenceof the crevice).

Because of the visualization -potential of a visible lightimage, documentation of an evaluation is important. Oftenthe visual test will be requested and documentation requiredbecause of an anomaly detected with another form of nonde-structive evaluation. Both film based and video photographyare used extensively. Low power magnification film photog-raphy is useful to document surface discontinuities ( seeFig. 40); microscope photography is useful in imaging dis-continuities that have been cross sectioned and polished (seeFig. 41). Video photography is useful in documenting inac-cessible areas, particularly when moving the imaging devicefrom the external structure into the inaccessible area whilethe video recorder is turned on.

What to Look for in CompositeMaterials

Visual testing is the first evaluation a composite structure

FIGURE 37. Possible matrix cracks

FIGURE 38. Coloration change only, no cracks FIGURE 40. Voids exposed on composite surface(scale shows sixteenths of an inch; 0.0625 in. =1.5875 mm)


FIGURE 39. Actual matrix cracks

FIGURE 41. Voids in cross section of composite

receives after manufacture. This may be simply a quick eval-uation of the condition of the structure after removal fromthe autoclave. After removal of the tooling and bagging mate-rials, the surfaces of the component may be viewed for exces-sive wrinkles, holes, gaps between adjacent plies, surfacevoids, cracks, buckling, etc. This is normally done withoutthe aid of optical magnification. Just as the actual part itselfis viewed, the tooling and bond forms may be viewed for dis-continuities and alignment problems. These evaluations arenormally performed by manufacturing personnel.

A more thorough evaluation may be required by engi-neering personnel. This may entail looking for resin rich orresin starved areas, blisters or delaminations in the exteriorplies, surface voids and edge separations. If the matrix

material is semitransparent, this evaluation may also includelooking for internal voids by shining a high intensity lightthrough the structure and viewing in a darkened area.

Visual evaluations often are required as an in-service timerequirement. Structures that experience stresses, such asweather phenomena, temperature fluctuations, or impact orhandling damage, are routinely checked for damage initia-tion or damage growth. Delaminations, cracks and bucklingcan occur. This visual evaluation may reveal suspicions aboutan area and may necessitate the use of followup nondestruc-tive evaluation methods. Often the visual indication is the

FIGURE 42. Dye enhancement of surface cracks(scale shows sixteenths of an inch; 0.0625 in. =1.5875 mm)


first sign of something wrong, and then the followup nonde-structive evaluation reveals the details.

ApplicationsVisual testing may be all that is required to evaluate a

structure in low stress applications. For example, thin sheetsof composite material bonded to a honeycomb core mayshow visibly the large voids and delaminations that need tobe detected. Even though the structure may be used in anaerospace application, it may not be necessary to spend largedollar amounts performing sophisticated nondestructiveevaluation procedures. An example of this type of structuremay be the fold down door on the overhead baggage com-partment in an airliner. As an added test, a tap test may beperformed (also a low cost evaluation).

Visual testing may be done on large, costly aerospacestructures as a first look to see if extensive discontinuitiesexist, making it uneconomical to continue with more nonde-structive testing. If no surface voids, excessive wrinkles orbuckles are noted, ultrasonic testing may be used to look fordelaminations and radiographic techniques may be used tolook for voids/porosity. The cost of the ultrasound and radiog-raphy may be avoided if the first visual evaluation discoversrejectable discontinuities.

Borescope assisted visual testing has been used to aug-ment other methods of nondestructive evaluation, whereprobes or film placements are in areas difficult to access.Some ultrasound composite testing applications havemounted probes on borescope devices that are inserted intoclosed structures. Viewing the borescope image allows place-ment of the probe in an otherwise inaccessible area.

Visual testing is often the aircraft commander's first line ofdefense from runway debris, hail impact damage, or light-ning strikes. Looking at the structure for dents, buckledareas or discoloration is part of every flight. Some aircraftradomes have had excessive static discharges that haveresulted in burns to the composite. These burns are visuallydetected; then the composite structure is measured by ultra-sound and radiographic methods and the results provided toengineering personnel.

Magnifiers, light sources, wetting agents such as alcohol,and inking dyes are all necessary tools for performing visualtesting on composite materials. As noted above, 5 x to 10 xseems to be an appropriate viewing magnification. A com-mon light source is a small, bright flashlight. It is portableand its beam is easily directed to the viewing area. The alco-hol is used as a visible penetrant to enhance the contrast onedge discontinuities and voids. After wiping the alcoholonto the composite surface, the surface will dry very quickly,whereas the alcohol trapped in cracks or voids will takelonger to dry. The dark marks go away as the alcohol evapo-rates. The inking dyes are useful in imaging small surfacebreaking cracks or voids, particularly when trying to docu-ment the cracks with photography. A small amount of dye ispoured onto a cloth and wiped over the area. After the resid-ual surface dye is wiped away, some dye stays trapped in

the cracks, contrasting them against the background color.Figure 42 shows surface cracks in polyester resin after animpact damage event.

Image ProcessingRecent advances and lowering costs of image processing

tools have made their use in visual testing more popular.Some commercially available video borescopes now can beattached to computer systems with the ability to capture,store and manipulate the images. This ability to manipulatethe images makes visual testing very powerful in visualizingfaint images, in tracking features over time by storing andcomparing images and by using measurement capabilities.

Captured images of a composite structure can be filtered,de-noised and enhanced. Faint matrix cracks can be scaledup, contrast enhanced, pseudocolored, have an edgedetecting filter applied to them—all to make them more visi-ble. The capturec image can be measured to determine thelength of a void or crack.

SummaryVisual testing as applied to composite structures has been

discussed, showing the importance of the method as a pri-mary nondestructive evaluation method and as a supplement

--,, to one or more other nondestructive evaluation methods.Composite structures have some unique problem areas thatrequire attention during visual testing. Several applicationswere briefly presented, as was the importance of visual test-ing to the verification of discontinuities noted by other meth-ods. Looking to the future, it can be said that imageprocessing techniques will play an increasing role in thechoice of visual testing as applied to composite materials.

FIGURE 43. A 50/50 lead-tin hemispherical heatexchanger fluid joint failed because of inadequatesolder coverage

LEGENDI NO SOLDER PRESENT2. SECTION OF HEAT EXCHANGER POLAR CAP REMOVED FOR FAILURE

ANALYSIS3. FRACTURED SOLDER AREA IS LIGHT GRAY, SHOWING WHERE FITTING

LEAKED AFTER SHEAR FAILURE OF JOINT


PART 6VISUAL TESTING OF MICROELECTRONICCOMPONENTS

Visual Testing of Solder JointsSoldering is defined by the Metals Handbook" as a group

of welding processes that produces coalescence of materialsby heating them to a suitable temperature and by using a fil-ler metal having a liquidus, or liquid state threshold, notexceeding 450 °C (840 °F) and below the solidus, or solidstate threshold, of the base metals. The filler metal is distrib-uted between the faying (or closely fitted) surfaces of thejoint by capillary action. Various methods of nondestructivetesting such as thermography and radiography are used forinspection of solder joints but visual testing under magnifi-cation continues to play the major role in their evaluation.Visual testing is well suited for the qualitative evaluation ofimportant solder joint parameters such as wetting, proper-ties, filleting, coverage, contact angle and degree of flux—allof which directly relate to the degree of solder joint qualityachieved. Solder quality visual testing can be divided intostructural and electrical soldering on the basis of the physicalrequirements for the bonding.

General criteria for the visual acceptance of solder jointscan be found in soldering texts." However, many industrialapplications of soldering result in the formation of a work-manship standards manual. A workmanship standardattempts to provide quantitative acceptance and rejectionlimits for each type of joint condition. Frequently there willbe a desired condition illustrated or photographed alongwith a minimum acceptable and unacceptable condition foreach potential anomaly.

Structural Soldering

Nonelectrical soldering has a wide variety of usages fromhome pipe joining to aerospace heat exchanger fabrication.Depending on the materials to be joined, solder materialsrange from familiar lead-tin alloys to more exotic alloys con-taining cadmium, antimony and pure indium. Physicalrequirements for the joint include strength, leak tightnessand—in applications such as heat exchangers—thermalconduction. Examples of acceptable and unacceptable struc-tural solder joint conditions can be seen in Figs. 43-45.

Electrical Soldering

Electrical soldering is used to ensure that good and reli-able electrical point to point connections of the joint are

achieved. Applications include printed wiring boards(Fig. 46), hybrid microcircuits (Fig. 47), leadless chip carri-ers, flex circuits and an endless variety of interconnectsbetween electrical devices. The acceptance criteria for elec-trical solder joints depend on good wetting, filleting and cov-erage and on low electrical resistance of the joint. Thischaracteristic, while not evaluated directly, is related to sol-der deposition size and joint cleanliness.

Visual Testing of ElectronicsThe electronics industry relies heavily on visual testing

and electrical (performance) testing of discrete electroniccomponents and complex electrical assemblies to determineelectrical system reliabilities. Electrical device designs,achieving smaller and more dense configurations, areincreasingly sensitive to small contaminants that can severely

FIGURE 46. Visual presentation of partial flex-rigidprinted circuit motherboard; visual test evaluatesintegrity of flexible and rigid portions, strip andJ connector solder joints; fracture flex portioncan be seen below large J connector


FIGURE 44. Magnified view of recessed solder jointin structural indium solder joint, joining stainlesssteel tubing to aluminum structure

3

LEGEND1. CORROSION RESISTANT STAINLESS STEEL TUBING2. 6061 ALUMINUM PLATE3. RECESSED INDIUM SOLDER FILLET

FIGURE 45. Fillet cracking in indium structuralsolder joint joining stainless steel tubing toaluminum structure

LEGENDI CORROSION RESISTANT STAINLESS STEEL TUBING2 INDIUM SOLDER FILLET CRACKING3. 6061 ALUMINUM PLATE

degrade electrical performance. Visual testing of printedwiring boards is performed at various stages of manufactur-ing, from the bare board through the completed assembly, toidentify mechanical and electrical overstresses, conductive

and nonconductive mask requirements and anomalies suchas delaminations.

Visual testing of semiconductor chips requires the identi-fication of bridging or near bridging conductive particles thatcan short the device (Fig. 48), as well as requiring the identi-fication of damage to the tiny electrical traces, damage thatcould cause a circuit to open (Fig. 49).

Evaluation of components which have been proof testedor used includes the identification of electrical and/ormechanical overstresses. Examples of electrical componentand printed wiring board visual anomalies are shown inFigs. 50 and 5L

Workmanship standards are frequently applied to thevisual and electrical testing of electronic devices, printedwiring boards and assemblies. The standards, in addition toproviding subjective appearance criteria, often directly spec-ify parameters such as dielectric spacing, wire bond dimen-sions and component spacing. 31

Visual Testing of Hybrid Microelectronics, Substratesand Discrete Components

Visual testing of ceramic substrates for microelectronicsbegins with handling of the structure. Substrates should be

FIGURE 47. The Kovar — metal used in this hybridmicrocircuit can incorporate leads through glassmetal seals to achieve electrical continuity fromceramic substrate to printed wiring boards wherethe hybrid package and 40 leads are solderattached; visual testing evaluates the integrity ofceramic substrate, integrated circuits, discretedevices (capacitors, resistors), wire bonding andsolder attachment of components

ir


FIGURE 48. Integrated circuit micrographdepicting smeared aluminum metallization thatresults in electrical short, showing smearedmetallization between traces

FIGURE 49. Magnified visual test image of hybridmicrocircuit revealing power transistor whosegold indium solder attach has scavanged goldmetallization, resulting in a failed solder attachand loss of component

LEGENDI POWER TRANSISTOR WITH NORMAL SOLDER ATTACHMENT2. EMPTY COMPONENT PAD WHERE POWER TRANSISTOR FELL OFF DUE TO

SCAVENGING OF GOLD INDIUM SOLDER ATTACHMENT

FIGURE 50. Close up visual test image revealsanomalies such as foreign material on die surfaceand evidence of electrical overstress shown below

FIGURE 51. Visual evidence of integrated circuitelect rirml ovPrstress


handled with stainless steel or plastic coated tweezers exceptwhen photo-resist strippers are used or when working tern-peratures are excessive. Vacuum tweezers-will be used for thehandling of microelectronic component dice (wafer thin,semiconductive substrates on which individual componentsare diffused by a photoresistive process). Metallized sub-strates without components may be handled with rubbergloves. At all times precautions should be taken to avoiddamage of the thick film on a substrate by handling the edgesonly. Inspections should be handled in a clean room environ-ment as specified by a detailed specification.

Low magnification evaluations are frequently performedat magnifications of 7 to 60 x with a binocular microscope atan angle of approximately 30 degrees from perpendicularto the substrate surface with the device under suitableillumination.

Evaluated characteristics of microelectronic substratesinclude active metallization separations from the edge of thedevice and from other metallization paths/components.Bonding pads for components will have their locations speci-fied relative to other components and relative to the sub-strate boundaries. These spacings, often referred to asminimum dielectric spacings, are to prevent hard and softshorts. Metallization patterns must be properly registered onthe substrate. Where multilayer substrates are employed,aluminum bonding pads must completely cover via regionsfrom subsurface layers so as not to mix gold and aluminummetallization areas. Substrates will also be evaluated forphysical damage such as chipouts, cracks, blistering of metal-lization, peaks, pits or holes.

Evaluated characteristics of substrate components varywith the component type. Components must be precisely

aligned on the bonding area. Bonding material such as poly-mers or gold eutectics must be visible during visual testingaround a specified portion of the component. If componentsare removed and replaced, visual testing must verify thatadequate metallization remains and no conductive materialis loose. The substrate is itself attached by solder or adhesiveto a can, a hermetic housing that provides dielectric andmagnetic isolation as well as leads to complete the circuits.The visual evaluation of this attachment must also be per-formed to verify alignment, the proper amount of bondingmaterial around the periphery and absence of flux or bridg-ing conductive material between the package and lead bond-ing areas.

High magnification visual testing of microelectronic sub-strates and components ranges between 50 and 200 x . Inthis environment, the microscopic wire bonding and diceevaluation takes place.

There are various types of wire bonding such as gold ballbonds of the thermosonic and thermocompression types,wedge bonds and tailless bonds. Bond sizes are frequentlyspecified as a function of the wire diameter (25-125[1-5 mil]). Also the position of the wire as it exits the bondarea must be evaluated. Bonds must be centered on thebonding pad and surrounding glassivation for die and sub-strate bonds. If rebonding is performed the new bonds mustbe adequately separated from the initial bond attempt.

Semiconductor devices such as transistors, gates, logicunits, field effect transistors (FETs) and discrete compo-nents such as thick and thin film resistors and capacitors arevisually tested at high magnification for a variety of construc-tion and process anomalies. In addition to the aforemen-tioned bond evaluations carried out on substrates, thedice/components are visually evaluated for cracks, chipouts,precision of laser trim areas (resistors), embedded foreignmaterials, smeared or separated metallizations and voids inindividual integrated circuit components.

Visual Testing of Printed Wiring Boards andAssemblies

Printed wiring board evaluation includes a variety of boardtypes, including rigid glass epoxy or polyamid, flexibleprinted circuits or a hybrid incorporating both rigid and flex-ible portions. The circuit patterns may be on one side, twosides, or contain many inner layers (20 is not uncommon).Visual testing occurs at many stages of fabrication of theprinted wiring board (PWB) and subsequent assemblies.Printed circuits are fabricated through a variety of platingprocesses. The quality of the circuit forming process andboard lamination is verified through visual testing. Charac-teristics such as trace width, registration, dielectric spacingand joining to plated through-holes are visually tested.High magnification visual testing of prepared printed wiring


board cross sections is performed to assist in the evalua-tion of the construction requirements. Specificationssuch as MIL-P-55110 and MIL-P-50884 provide detailedrequirements for conductor sizes, spacing, registration, con-dition of laminate, coverlay, masks, conductive material, con-dition of the interface between circuit traces and the platedthrough-holes as well as general workmanship. Most of theevaluations made in receiving visual testing are performedon board coupons strategically placed on the board artworkto represent the manufacturing variance envelope.

Once acceptable boards are identified through visual and

electrical tests, components are mounted onto the board.Here visual testing is concerned with evaluating the integrityof the component attachment process, usually soldering. Fil-let and hole fill must be verified, as well as inspection for sol-der related damage and contamination such as pad lift, boardcracking, excessive residual flux and solder bridging of con-ductive traces.

Advances in automation and computer applications," inthis as in other areas of visual testing, promise to make prac-tical the broader application of this technology in quality andprocess control.


REFERENCES

1. Nondestructive Testing Handbook, second edition. LeakTesting. Vol. 1. Columbus, OH: The American Societyfor Nondestructive Testing (1982): p 23, 362-364, 772.

2. Nondestructive Testing Handbook, second edition. Liq-uid Penetrant Tests. Vol. 2. Columbus, OH: The Ameri-can Society for Nondestructive Testing (1982): p 84, 95,192-197, 214-219, 362-364.

3. Nondestructive Testing Handbook, second edition.Radiography and Radiation Testing. Vol. 3. Columbus,OH: The American Society for Nondestructive Testing(1985): p 382-393, 642-672, 760-762.

4. Nondestructive Testing Handbook, second edition. Elec-tromagnetic Testing: Eddy Current, Flux Leakage andMicrowave Nondestructive Testing. Vol. 4. Columbus,OH: The American Society for Nondestructive Testing(1986).

5. Nondestructive Testing Handbook, second edition.Magnetic Particle Testing. Vol. 6. Columbus, OH: TheAmerican Society for Nondestructive-Testing (1989):p 202-206, 213, 369, 388, 390-392.

6. Nondestructive Testing Handbook, second edition.Ultrasonic Testing. Vol. 7. Columbus, OH: The Ameri-can Society for Nondestructive Testing (1991): p 313-319, 393-397.

7. Generazio, E.R. "The Role of the Reflection Coefficientin Precision Measurement of Ultrasonic Attenuation."Materials Evaluation. Vol. 43, No. 8. Columbus, OH:The American Society for Nondestructive Testing( July 1985): p 995-1,004.

8. Generazio, E.R., D.B. Stang and D.J. Roth. AnnualReview of Progress in QNDE. Vol. 9. D.O. Thompsonand D.E. Chimenti, eds. New York, NY: Plenum Press(1990).

9. Truell, R., C. Elbaum and B.B. Chick. Ultrasonic Meth-ods in Solid State Physics. New York, NY: AcademicPress (1969).

10. Generazio, E.R. "Ultrasonic Attenuation MeasurementsDetermine Onset, Degree and Completion of Recrys-tallization." Materials Evaluation. Vol. 46, No. 9.Columbus, OH: The American Society for Nondestruc-tive Testing (August 1988): p 1,198-1,203.

11. Generazio, E.R. "Scaling Attenuation Data Character-izes Changes in Material Microstructure." MaterialsEvaluation. Vol. 44, No. 2. Columbus, OH: The Ameri-can Society for Nondestructive Testing (February1986): p 198-202.

12. Manual of Photogrammetry. Falls Church, VA: Ameri-can Society of Photogrammetry and Remote Sensing(formerly American Society of Photogrammetry)(1980).

13. Moser, C. and W.R. Schriever. "Photogrammetric Mea-surements of Deformations of Structures." Proceedingsof the Rilem Symposium on the Observation of Struc-tures. Paper 30. Lisbon, Portugal: Laboratorio Nacionalde Engenharia Civil (October 1955).

14. Hardegen, L. "The Application of Photogrammetry tothe Conservation of Monuments." Schweizerische Tech-nische Zeitscrift. Vol. 66, No. 35. arich, Switzerland:Weinbergstrasse (1969): p 721-731.

15. Kenefick, J.F. "Photogrammetry in Shipbuilding."PB-262-130/AS. Springfield, VA: National TechnicalInformation Service ( July 1976).

16. Turpin, R.D. "Photogrammetry-A Versatile MeasuringTechnique." Photogrammetric Engineering. Vol. 21,No. 1. Bethesda, MD: American Society for Photogram-metry and Remote Sensing (1963): p 64-67.

17. Konecny, G. "Structural Engineering Application of theStereometric Camera." Photogrammetric Engineering.Vol. 31, No. 1. Bethesda, MD: American Society forPhotogrammetry and Remote Sensing (1965): p 96-103.

18. Schlienger, R. "Cameras for Terrestrial and Close RangePhotogrammetry." Proceedings of the UN Seminar onPhotogrammetric Techniques Zurich, Switzerland 1971.Zurich, Switzerland: Institut fur Geodasie and Photog-rammetrie (1971).

19. Karara, H.M. "Simple Cameras for Close-Range Appli-cations." Photogrammetric Engineering. Vol. 38, No. 5.Bethesda, MD: American Society for Photogrammetryand Remote Sensing (May 1972): p 447-451.

20. Dunhill, Tony. "Some Experiences of Applying NDETechniques to Ceramic Materials and Components."Nondestructive Testing of High-Performance Ceramics.Westerville, OH: American Ceramic Society (1987):p 268.

21. Honaker, James. "Automatically Inspecting ThinCeramics for Pinholes." NASA Tech Briefs. Vol. 12,No. 1. New York, NY: Associated Business Publications( January 1988): p 49.

22. Sheppard, Laurel. "Evolution of NDE Continues forCeramics." Ceramic Bulletin. Vol. 70, No. 8. West-erville, OH: American Ceramic Society (1991): p 1,265.

23. Raulins, Max. "How Loading Affects Tubular ThreadShoulder Seals." Petroleum Engineering International.

SECTION 12

GLOSSARY OF VISUAL ANDOPTICAL TESTING TERMS


Most of the definitions in this glossary have been modifiedto satisfy peer review and editorial style. For this reason, ref-erences given in this glossary are not attributions but ratheracknowledgments and suggestions for further reading.

Most definitions are adapted from the preceding text.There, published sources are acknowledged individually onsection title pages.

This glossary is provided for instructional purposes. Noother use is intended.

A

AOQ: Average outgoing quality.AOQL: Average outgoing quality limit.AQL: See acceptable quality level.ASNT: American Society for Nondestructive Testing.ASNT Recommended Practice No. SNT-TC-1A: A set of

guidelines for employers to establish arid conduct anondestructive testing personnel qualification and cer-tification program. SNT-TGlA was first issued in 1968by the Society for Nondestructive Testing (SNT, nowASNT) and has been revised every few years since.

acceptable quality level (AQL): The maximum percentdefective (or the maximum number of units with dis-continuities per hundred units) that, for the purposes ofsampling tests, can be considered satisfactory as a pro-cess average.

accommodation: Of the eye, adjustment of focus made bychanging the thickness and curvature (the focusingpower) of the lens and done by the action of tiny mus-cles attached to the lens.

acuity: See neural acuity, vision acuity.adaptive thresholding: Threshold value varying with incon-

stant background gray level.adhesive wear: See wear, adhesive.alpha ferrite: The form of pure iron that has a body centered

cubic structure stable below 910 °C (1,670 °F). Alsocalled alpha iron.

alpha iron: See alpha ferrite.ambient light: Light in the environment as opposed to illu-

mination provided by a visual testing system.angle: See field angle.angstrom unit (A): Unit of length, equal to 0.1 nanometer.annealing: Process of heating and cooling a material, usually

to reduce residual stresses or to make it softer.anode: Negatively charged terminal, which may corrode

electrochemically during production of an electric cur-rent. Compare cathode.

arc welding: See electric arc welding.

austenite: A solid solution with iron as the solvent in a facecentered cubic structure formed by slow cooling ofdelta ferrite. Austenite 's characteristic lattice struc-ture is stable between 906 °C (1,663 °F) and 1,390 °C(2,535 °F). Also called gamma iron.

automated system: Acting mechanism that performsrequired tasks at a determined time and in a fixedsequence in response to certain conditions.

background cylinder and difference cylinder: Two devicesused to calculate illuminance by using the equivalentsphere illumination technique.'

binary system: In metallurgy, a two element alloy system.See also isomorphous binary system.

birefringence: The splitting of a light beam into two partsthrough a translucent material.

black body: See blackbody.black light: Term sometimes used for ultraviolet radiation,

particularly in the near ultraviolet range of about 320 to400 nm.

blackbody: A theoretical object that radiates more totalpower and more power at any wavelength than anyother source operating at the same temperature.'

blacldight: See black light.blind spot: Portion of the retina wher,e the optic nerve

enters, without rods and cones and hence insensitive tolight.

blotch: (1) An irregularly spaced area of color change on asurface. (2) The nonuniform condition of a surfacecharacterized by such blotches.

blue hazard: Exposure to high frequency visible light atintensities and durations that may damage the retina,particularly in conjunction with overheating.

borescope: An industrial endoscope; a periscope or tele-scope using mirrors, prisms, lenses, optic fibers or tele-vision wiring to transmit images from inaccessibleinteriors for visual testing. Borescopes are so calledbecause they were originally used in machined aper-tures and holes such as gun bores. There are bothflexible and rigid, fiber optic and geometric lightborescopes.

borescope, angulated: Borescope bent for viewing at for-ward oblique, right angle or retrospective angles forvisual testing of surfaces not accessible with conven-tional borescopes.

borescope, calibrated: Borescope with gage on externaltube to indicate the depth of insertion during a test.Borescopes with calibrated reticles are used to deter-mine angles or sizes of objects in the field when held ata predetermined working distance.

GLOSSARY OF VISUAL AND OPTICAL TESTING TERMS / 341

borescope, cave: Multiangulated, periscopic borescope usedfor remote observation of otherwise inaccessible areas.

borescope, fiber optic: Borescope that uses fiber opticmaterials (such as glass or quartz) in the optical path andfor transmission of light to and from the test surface.

borescope, indexing: Borescope that can be bent90 degrees by rotation of a knob after the instrumenthas been inserted through an aperture. A knob at theeyepiece can rotate the objective head through360 degrees for scanning a circumferential weld seam.

borescope, micro-: Borescope with an outside diametergenerally from 1 to 5 mm (0.04 to 0.2 in.), typically usingquartz filaments. Compare miniature borescope.

borescope, miniature: Borescope with an outside diametergenerally less than 13 mm (0.5 in.). Sometimes calledminiborescope. See also microborescope.

borescope, panoramic: Borescope with a scanning mirrormounted in front of the objective lens system. Rotationof the mirror is adjusted at the ocular end of* instru-ment to scan in forward oblique, right angle and retro-spective directions.

borescope, retrospective: Borescope that looks backwards,180 degrees or nearly so from the distal line of interro-gation normal to the plane of a conventional objectivelens.

borescope, rigid: Borescope that does not bend, typically inorder to keep the geometrical optics in alignmentthrough a light train system.

borescope, ultraviolet: Borescope equipped with ultravioletlamps, filters and special transformers to transmit radia-tion of ultraviolet wavelengths.

borescope, waterproof/vaporproof: Borescope completelysealed and impervious to water or other types of fluid,used for internal tests of liquid, gas or vapor envi-ronments.

borescope, wide field: Borescope with rotating objectiveprism to provide fields of view up to 120 degrees.

brinelling: Stripe indentations made by a spherical object.False brinelling refers to a type of surface wear.

burr: A raised or turned over edge occurring on a machinedpart and resulting from cutting, punching or grinding.'

burst: In metal, external or internal rupture caused byimproper forming.

butt weld or butt joint: Weld joining two metal pieces in thesame plane.

C

CCD: See charge coupled device.candela: Base unit of measure in the SI system for luminous

intensity. The luminous intensity in a given direction of

a source that emits monochromatic radiation of fre-quency 540 x 1012 Hz and that has a radiant intensityin that direction of 1.4641 milliwatt per steradian. Sym-bolized cd. Formerly known as candle.

candle: Former name for candela.case crushing: A mechanism producing fracture of the case,

like subcase fatigue but attributable to static overload-ing rather than to fatigue alone. In many instances themovement of the subcase causes the case to crack orspall.

casing: The many strings of pipe that are used to line thehole during and after drilling of a gas or oil well. Thispipe protects the hole from formation collapse, keepsthe formation fluids out of the hole and keeps the oilwell fluids out of the water tables.

casing string: Tubular structure on the outer perimeter of agas or oil well hole. The casing string is a permanentpart of the well and many are cemented into theformation.

cathode: Positively charged terminal in an arrangement thatproduces current by chemical reactions. Compareanode.

cavitation fatigue: A form of pitting caused by erosion fromvibration and movement in liquid environments.

cementite: Iron carbide (Fe3C), present in steels.certification: The process of providing written testimony

that an individual is qualified. See also certified.certified: Having written testimony of qualification. See

also certification.chafing: See wear, fretting.channels: In biology, mechanisms functioning as bandpass

filters in the visual cortex of mammals, causing sensitiv-ity to visual stimuli in particular frequencies and ranges.

charge coupled device (CCD): Solid state image sensor.CCDs are widely used in inspection systems because oftheir accuracy, high speed scanning and long servicelife.

closing: In image processing, dilation followed by erosion. Asingle pixel closing connects a broken feature separatedby one pixel. See also opening.

closure: Process by which a person cognitively completespatterns or shapes that are incompletely perceived.

cocoa: Debris (usually oxides of the contacting metals) offretting wear, retained at or near the site of its forma-tion—a condition especially helpful during visual tests.With ferrous metals, the debris is brown, red or black,depending on the type of iron oxide formed. For thisreason, ferrous debris is called cocoa or, when mixedwith oil or grease, red mud.

code: A standard enacted or enforced as a law.coefficients of the filter: Values in a mask that serves as a fil-

ter in image processing.


cold light: Obsolete word for fluorescence.color: Sensation by means of which humans distinguish light

of different intensities (brightness) and wavelengths(hue).

color blindness: Deficiency in the ability to perceive or dis-tinguish hues.

color discrimination: The perception of differences be-tween two or more hues.

color temperature: Rating of a light source for color vision.complete testing: Testing of an entire production lot in a

prescribed manner. Sometimes, complete testingentails the inspection of only the critical regions of apart. One hundred percent testing requires the inspec-tion of the entire part by prescribed methods. Comparesampling, partial.

compound microscope: See microscope, compound.cone: In biology, a retinal receptor that dominates the retinal

response when the luminance level is high and providesthe basis for the perception of color. Compare rod.'

constitution diagram: See phase diagram.contrast: The difference between the amount of light

reflected or transmitted by an object and by the visualtask or the observer's field of view.

control: See in control, process control and quality control.corrosion: Loss or degradation of metal because of chemi-

cal reaction.corrosion-erosion: Simultaneous occurrence of erosion and

corrosion.corrosion, crevice: A kind of galvanic corrosion caused by

differences in metal ion concentrations in neighboringportions of the corrodent.

corrosion, fretting: Corrosion facilitated by fretting, par-ticularly where a protective surface has been chafed ina corrosive environment.

corrosion, poultice: Corrosion occurring under a layer offoreign material (e.g., under mud in automobile rockerpanels).

creep: Gradual and permanent change of shape in a metalunder constant load, usually at elevated temperature.Occurs in three stages: primary creep, secondary creepand tertiary creep. See also deformation.

crevice corrosion: See corrosion, crevice.

D

dark adaptation: The process by which the retina becomesadapted to a luminance that is less than approximately0.034 candela per square meter.'

dark adapted vision: See scotopic vision.

defect: A condition or discontinuity having a size, shape, ori-entation, nature, frequency or location that impairs theuseful service of the part or that is rejectable accordingto a specification or standard. Compare discontinuity,indication.'

deformation: Change of shape under load. See also creepand elastic deformation.

delta ferrite: Solid solution with body centered cubic struc-ture and iron as solvent. Also called delta iron.

delta iron: See delta ferrite.depth of field: In photography, the range of distance over

which an imaging system gives satisfactory definitionwhen its lens is in the best focus for a specific distance.

dewetting: In soldering, the flow and retraction of solder,caused by contaminated surfaces, dissolved surfacecoatings or overheating before soldering.

difference cylinder: See background cylinder.dilation: In image processing, the condition of a binary

image where the pixel in the output image is a 1 if anyof its eight closest neighbors is a 1 in the input image.See also closing, erosion and opening.

direct photometry: Simultaneous comparison of a standardlamp and an unknown light source.'

direct substitution alloy: Alloy in which the atoms of thealloying element can occupy the crystal lattice spacesnormally occupied by the atoms of the parent metal.

direct viewing: Viewing of a test object in the viewer'simmediate presence. The term direct viewing is used inthe fields of robotics and surveillance to distinguish con-ventional from remote viewing.

direct vision instrument: Device offering a view directly for-ward. A typical scene is about 20 mm (0.75 in.) wide at25 mm (1 in.) from the objective lens.

directional lighting: Lighting provided on the work plane orobject predominantly from a preferred direction.'

discontinuity: An intentional or unintentional interruptionin the physical structure or configuration of a part.Compare defect, dislocation, indication. 3

discontinuity, inherent: Material anomaly originating fromsolidification of cast metal. Pipe and nonmetallic inclu-sions are the most common and can lead to other typesof discontinuities in fabrication.'

discontinuity, primary processing: Material anomaly pro-duced from the hot or cold working of an ingot intoforgings, rod and bar.'

discontinuity, secondary processing: Material anomaly pro-duced during machining, grinding, heat treating, plat-ing or other finishing operations.'

discontinuity, service induced: Material anomaly caused bythe intended use of the part.

dislocation: Void or discontinuity in the lattice of a metalcrystalline structure.


distal: In a manipulative or interrogating system, of or per-taining to the end opposite from the eyepiece and far-thest from the person using the system. Objective; tip.

effective throat: In welding,

E

the weld throat including theamount of weld penetration but ignoring excess metalbetween the theoretical face and the actual face.

elastic deformation: Temporary change in shape under aload. The material returns to its original size and shapeafter the load is removed. Elastic deformation is thestate in which most metal components are used inservice.

elasticity: The ability of a material to resume its formershape after deformation.

electric arc welding: Joining of metals by heating with elec-tric arc. Also called arc welding.

endoscope: Device for viewing the interior of objects. Fromthe Greek words for inside view, the term endoscope isused mainly for medical instruments. Nearly everymedical endoscope has an' integral light source; manyincorporate surgical tweezers or other devices. Com-pare borescope.

equilibrium diagram: A phase diagram showing the phasespresent at equilibrium in a material system.

equivalent sphere illumination: Level of perfectly diffuse(spherical) illuminance that makes the visual task asphotometrically visible within a comparison test sphereas it is in the real lighting environment.

equivalent 20/20 near vision acuity: Vision acuity withremote viewing or other nondirect viewing means thatapproximates 20/20 direct viewing closely enough to beconsidered the same for visual testing purposes.

erosion-corrosion: Simultaneous occurrence of erosion andcorrosion.

erosion: (1) Loss of material or degradation of surface qual-ity through friction or abrasion from moving fluids,made worse by solid particles in those fluids or by cavi-tation in the moving fluid. See wear. (2) In image pro-cessing, condition of a binary image where the pixel inthe output image is a 1 if each of its eight neighbors is a1 in the input image. See also closing, dilation andopening.

etch crack: Shallow crack in hardened steel containing highresidual surface stresses, produced in an embrittlingacid environment.'

eutectic liquid: A liquid having a proportion of metals suchthat two or more solid phases form at the same tempera-ture during cooling.

eutectic point: Temperature and proportion of metals atwhich two or more phases of a eutectic liquid form.Compare eutectoid.

eutectoid: Similar to eutectic but in a solid system duringcooling.

evaluation: A review, following interpretation of the indica-tions noted, to determine whether they meet specifiedacceptance criteria.'

eye sensitivity curve: Graphic expression of vision sensitivitycharacteristics of the human eye. In the case of a physi-cal photometer, the curve should be equivalent to thestandard observer. The required match is typicallyachieved by adding filters between the sensitive ele-ments of the meter and the light source.

false brinelling: Fretting wear indentations. Comparebrinelling.

false indication: An indication that is not produced by a dis-continuity. Compare defect.

far vision: Vision of objects at a distance, generally beyondarm's length. Compare near vision.

farsightedness: Vision acuity functionally adequate for view-ing objects at a distance, generally beyond arm's length.Compare nearsightedness.

feature extraction: From an enhanced image, derivation ofsome feature values, usually parameters for distinguish-ing objects in the image.

ferrite: Solid solution of one or more other elements inalpha iron.

fiber optic borescope: See borescope, fiber optic.fiber optics: The technology of light transmission through

crystalline fibers such as glass or quartz.fiberscope: Jargon for fiber optic borescope.field: In video technology, one of two video picture compo-

nents that together make a frame. Each picture isdivided into two parts called fields because a frame atthe rate of thirty frames per second in a standard videooutput would otherwise produce a flicker discernible tothe eye. Each field contains one half of the total pictureelements. Two fields, then, are required to produce onecomplete picture or frame so the field frequency is sixtyfields per second and the frame frequency is thirtyframes per second.

field angle: The included angle between those points onopposite sides of the beam axis at which the luminousintensity from a theatrical luminaire is 10 percent of themaximum value. This angle may be determined from anilluminance curve or may be approximated by use of anincident light meter.'

field of view: The range or area where things can be seenthrough an imaging system, lens or aperture. Comparedepth of field.


field of vision: The range or area where things can be per-ceived organoleptically at a point in time, assuming theeye to be immobile.

fillet weld: Weld on the inside corner of two metal pieces ata right angle.

filter: A processing component or function that excludes aselected kind of signal or part of a signal.

filtering: See low pass filtering.fit up: To secure one or more joint members with special

external fixturing in order to prevent movement duringwelding.'

flakes: Short discontinuous internal fissures in ferrous metalsattributed to stresses produced by localized transforma-tion and/or decreased solubility of hydrogen duringcooling usually after hot working. On a fractured sur-face, flakes appear as bright silvery areas; on an etchedsurface they appear as short, discontinuous cracks.'

fluorescence: The emission of visible light from a material inresponse to ultraviolet or X-radiation. Formerly calledcold light.

flux method: See lumen method.focus: Position of a viewed object and a lens system relative

to one another to offer a distinct image of the object asseen through the lens system. See accommodation anddepth of field.

focus, principal plane of: The single plane actually in focusin a photographic scene.

focusing, automatic: (1) Feature of camera, usually incorpo-rating a range finder, whereby the lens system adjusts tofocus on an object in part of the field of view. (2) Meta-phorical attribute of a borescopic instrument's depth offield (the range of distance in focus). The depth of fieldis so great in the case of video borescopes that focusingis unnecessary for most applications. Despite the name,no mechanism is actively adjusted. The expanded depthof field is due both to the small diameter of the lensaperture and to the proximity of the lens to the chargecoupled device.

focusing, primary: Focusing by the lens of the image onto afiber optic bundle at the tip of a probe.

focusing, secondary: Focusing at the eyepiece of aborescope or other optical instrument, specifically themanual refocusing needed when the viewing distancechanges.

footcandle: Former unit of measure for illumination, equiv-alent to one lumen evenly distributed over a squarefoot, or to a surface illumination at a distance of one footfrom a point of one candela. Abbreviatedftc or fc. Seealso lux.

footlambert: Former unit of luminance. Measured in theSI system by candela per square meter.

forging crack: Discontinuity formed during mechanicalshaping of metal.

fovea centralis: Region of sharpest vision in the retina,where the layer of blood vessels, nerve fibers and cellsabove the rods and cones is far thinner than in periph-eral regions.

foveal vision: See photopic vision.frame: A complete raster scan projected on a video screen.

There are thirty frames per second in a standard videooutput. A frame may be comprised of two fields, eachdisplaying part of the total frame.

fretting corrosion: See corrosion, fretting.fretting wear: See wear, fretting.friction oxidation: See wear, fretting.

galling: A type of adhesive wear more gross than fretting.galvanic series: List of metals, alloys and graphite (a non-

metal) in sequence with the most anodic (easily cor-roded) in liquids at one end and the most cathodic (leasteasily corroded) at the other end. For practical reasons,this sequence is compiled using seawater as the electro-lyte-3 to 5 percent sodium chloride and other saltsdissolved in water.

gamma iron: Iron with face centered cubic structure formedby slow cooling of delta ferrite. This characteristic lat-tice structure is stable between 906 °C (1,663 °F) and1,390 °C (2,535 °F). Also called austenite.

gas tungsten arc welding (GTAW): Inert gas shielded arcwelding using a tungsten electrode. Also called tungsteninert gas (TIG) welding.

gasket seal: Resilient ring, usually virgin polytetrafluoro-ethylene (PTFE), in a piping or tubing connection.Compare interference sealing thread and metal tometal seal.

general examination: A test or examination of a person'sknowledge, typically (in the case of nondestructive test-ing personnel qualification) a written test on the basicprinciples of a nondestructive testing method and gen-eral knowledge of basic equipment used in the method.(According to ASNT's guidelines, the general examina-tion should not address knowledge of specific equip-ment, codes, standards and procedures pertaining to aparticular application.) Compare practical examinationand .specific examination.

geometrical optics: The mathematical study of how lightrays are reflected and refracted and practical tech-niques based on such understanding, including thetransmission of images by lenses and mirrors. Alsocalled lens optics.

glare: Excessive brightness (or brightness varying by morethan 10:1 within the field of view) which interferes withclear vision, critical observation and judgment.


glare, blinding: Glare so intense that for an appreciablelength of time after it has been removed, no object canbe seen.'

glare, direct: Glare resulting from high luminances or insuf-ficiently shielded light sources in the field of view.Direct glare is usually associated with bright areas, suchas luminaires, ceilings and windows which are outsidethe visual task or region being viewed.' —

glare, reflected: Glare resulting from specular reflections ofhigh luminances in polished or glossy surfaces in thefield of view. It usually is associated with reflectionsfrom within a visual task or nearby areas.'

glossmeter: Reflectometer used to measure specularreflectance.'

gouge: Surface indentation caused by forceful abrasion orimpact or flame cutting. Also called nick. Compare toolmark.

grain: Solid particle or crystal of metal. As molten metalsolidifies grains grow and lattices intersect, formingirregular grain boundaries.

grain boundary: Interface that forms between grains ofsolidifying metal as the random oriented crystal latticesmeet. See grain.

gray level: Integer number representing the brightness ordarkness of a pixel or, as a composite value, of an imagecomprised of pixels.

graybody: Radiator whose spectral emissivity is uniform forall wavelengths.

green rot: Form of attack due to simultaneous carburizationand oxidation of stainless heating elements common tonickel chromium and nickel chromium iron alloys,especially in furnace environments.

grinding crack: Shallow crack formed on the surface of rela-tively hard materials due to excessive grinding heat, toallotropic transformation of the surface material or tothe high sensitivity of the material surface containinghigh tensile residual stress. Grinding cracks typically are90 degrees to the direction of grinding.'

Hadfield's steel: An austenitic manganese specialty steelthat is easily work hardened.

halitation: Rings of light visible around a spot on a videoscreen where an electron scanning beam is held.

heading: Upsetting wire, rod or bar stock in dies to formparts having some of the cross-sectional area larger thanthe original. Examples are bolts, rivets and screws.'

heat checking: Surface cracking caused when metal rapidlyheated (or cooled and heated repeatedly) is preventedfrom expanding freely by colder metal below the sur-face. Friction may produce the heat. Heat checking issometimes called thermal fatigue.

heat wave: Thermally produced variation in flue gas densitythat distorts images of objects in a firebox.

hot tear: Fracture formed in a cast metal during solidifica-tion and due to extensive tensile stress associated withvolumetric shrinkage. Hot tears often occur whereareas of different thickness adjoin.

hue: Characteristic of light at a particular bandwidth thatgives a color its name.

hundred percent testing: See one hundred percent testing.hyperthermia: Heating so excessive that it can damage or

kill plant or animal cells.

illuminance: The density of luminous flux on a surface. Mea-sured in the SI system by lux.

illuminate: Shed light on.illumination: The act of illuminating or state of being illumi-

nated. See also illuminate. Compare illuminance.'image: Visual representation of a test object or scene.image enhancement: Any of a variety of image processing

steps, used singly or in combination to improve thedetectability of objects in an image.

image guide: Fiber bundle that carries the picture formedby the objective lens at the distal end of a fiber opticborescope back to the eyepiece.

image orthicon: Television tube that uses the photoemissionmethod. Compare vidicon.

image processing: Actions applied singly or in combinationto an image, in particular the measurement and alter-ation of image features by computer. Also called pic-ture processing.

image segmentation: Process in which the image is parti-tioned into regions, each homogeneous.

in control: Within prescribed limits of process control.incandescence: The emission of visible radiation due to

thermal excitation.incandescent: Emitting visible radiation as a result of

heating.indication: A nondestructive testing response that requires

interpretation to determine its relevance. Comparedefect, discontinuity, false indication.

indication, nonrelevant: An indication that has no relationto a discontinuity that might constitute a defect.'

indication, relevant: An indication from a discontinuity (asopposed to a false indication) requiring evaluation by aqualified inspector, typically with reference to an accep-tance standard, by virtue of the discontinuity's size orlocation.'

inert gas shielded arc welding: Joining of metals by heatingthem with an electric arc between electrode(s) and thework piece, using an inert gas to shield the electrode( s).See also gas tungsten arc welding.


infrared radiation: Electromagnetic radiant energy of wave-lengths longer than 770 nm. 2

interference fitted thread: Interference sealing thread.interference objective: Small, metallized glass mounted in

contact with the test object and adjustable for tilt tocontrol fringe spacing.

interference sealing thread: Piping seal using a taperedconnection made up under great pressure, forcing themating surfaces together more tightly than is possibleby hand alone. Compare gasket seal and metal to metalseal.

interlaced scanning: A process whereby the pictureappearing on a video screen is divided into two parts.Interlaced scanning reduces flicker by increasing theelectron beam's downward rate of travel so that everyother line is sent. When the bottom is reached, thebeam is returned to the top and the alternate lines aresent. The odd and even line scans are each transmittedat 1/60 s, totaling 1/30 s per frame and retaining thestandard rate of 30 frames per second. The eye's persis-tence of vision allows the odd and even lines to appearas a single image without flicker.

interpretation: The determination uf whether indicationsare relevant or nonrelevant or false.'

interstitial alloy: Alloy in which the atoms of the alloying ele-ment fit into the spaces between the atoms of the par-ent metal.

iris: Ring of variable area around the pupil and in front of thelens of the eye. The surface area of the iris adjusts spon-taneously to change the amount of light entering theeye.

irradiance: Power of electromagnetic radiant energy inci-dent on the surface of a given unit area. Compareradiance.

Ishihara" plates: Trade name for a kind of pseudoisochro-matic plates.

isomorphous binary system: A two element alloy system inwhich both elements are completely soluble in eachother in the liquid and the solid states, in all proportionsat all temperatures.

Jaeger eye chart: An eye chart used for near vision acuityexaminations.

kinetic vision acuity: Vision acuity with a movinvarget.Studies indicate that 10 to 20 percent of visual efficiencycan be lost by target movement.

LTPD: Lot tolerance percent defective.laboratory microscope: Conventional compound micro-

scope. See microscope.

lambertian: Having a surface that diffuses light uniformlyrather than reflecting it. Matte. Most objects have alambertian surface. Compare specular.

lap: A forging discontinuity caused by a folding over ofmetal. Laps are found in rolled bar stock and at or neardiameter changes.'

laser: An acronym (light amplification by stimulated emis-sion of radiation). The laser produces a highly mono-chromatic and coherent (spatial and temporal) beam ofradiation. A steady oscillation of nearly a single electro-magnetic mode is maintained in a volume of an activematerial bounded by highly reflecting surfaces, called aresonator. The frequency of oscillation varies accordingto the material used and by the methods of initiallyexciting or pumping the material. 2

lens: Translucent object that refracts light passing through itin order to focus the light on a target.

lens optics: See geometrical optics.light: Radiant energy that can excite the retina and produce

a visual sensation. The visible portion of the electromag-netic spectrum extends from about 380 to 770 nm. 2

light adapted vision: See photopic vision.light guide bundle: Bundle of filaments, usually glass, that

carries noncoherent light from a high intensity sourcethrough a fiber optic borescope to illuminate the object.

lighting, back: Placement of light source and image sensoron opposite sides of the test object, used when the sil-houette of a feature is important.

lighting, flash: See lighting, strobe.lighting, front: Placement of light source and image sensor

on the same side of the test object.lighting, strobe: Lighting that flashes intermittently at a rate

that may be adjusted and is often perceived as a flicker,used to image moving objects or still objects with poten-tial movement.

lighting, structured: Combining a light source with opticalelements to form a line or sheet of light.

limited certification: Individuals who are certified only forspecific operations are usually called limited Level (I,or or are designated as having limited certificationbecause they are not qualified to perform the full rangeof activities expected of personnel at that level ofqualification.

line pair: Pair of adjacent, parallel lines used to evaluate theresolution of a specific imaging system. See also mini-mum line pair.

lot tolerance percent defective: In quality control, the per-cent defective at which there is a 10 percent probabilityof acceptance in a production run.

low pass filtering: A linear combination of pixel values tosmoothen abrupt transitions in a digital image. Alsocalled smoothing.


lumen: Unit of measure in the SI system for luminous flux,equivalent to candela times steradian (cd•sr). Abbrevi-ated lm.

lumen method: A lighting design procedure used for prede-termining the relation between the number and typesof lamps or luminaires, the room characteristics and theaverage illuminance on the work plane. It takes intoaccount both direct and reflected flux. Also called fluxmethod,'

luminance: The ratio of a surface's luminous intensity in agiven direction to a unit of projected area. Measured incandela per square meter.

luminosity: The luminous efficiency of radiant energy.luminous efficacy: The ratio of the total luminous flux of a

light source to the total radiant flux or to the powerinput. Sometimes called luminous efficiency.

luminous efficiency: See luminous efficacy.luminous flux: Radiant energy's time rate of flow. Measured

in lumens.luminous intensity: Luminous flux on a surface normal to

the direction from its light source, divided by the solidangle the surface subtends at the source. Measured incandela. Also known as candlepower.

lux: Unit of measure for illuminance in the SI system. Equiv-alent to lumens per square meter and symbolized lx.Formerly known as meter-candle.

machine vision: Automated system function of acquiring,processing and analyzing images to evaluate a testobject or to provide information or interpretation forhuman interpretation. A typical machine vision systemconsists of a light source, a video camera, a video digi-tizer, a computer and an image display.

macular lutae: Irregular, diffuse ring of yellow pigmentwhich partly overlaps the fovea and surrounds it out toaround 10 degrees and which absorbs blue light, thuschanging the color of the light reaching receptorsbeneath.

martensite: (1) Acicular (needlelike) microstructure pro-duced by fast cooling or quenching of metals and alloyssuch as steel. (2) The hard steel with such microstruc-ture produced by fast cooling of austenite.

martensite finish temperature: Temperature at which mar-tensite formation is completed as steel cools.

martensite start temperature: Temperature at which mar-tensite starts to form as steel cools.

mask: (1) A spatial filter in the sensing unit of a surfaceinspection system. (2) An n x n square matrix with dif-ferent values that serves as a filter in image processing.

match bend effect: Optical illusion whereby an area of uni-form brightness appears to be nonuniform because ofcontrast with the brightness of an adjacent area.

mathematical morphology: Image processing technique ofexpanding and shrinking. The basic operators in mathe-matical morphology are dilation (expanding), erosion(shrinking), opening and closing.

matte: Tending to diffuse light rather than reflect it; notshiny. Also called lambertian. The term matte is gener-ally applied to smooth surfaces or coatings. Comparespecular.

mesopic vision: Vision adapted to a level of light betweenphotopic at 3.4 x 10 - 2 cd•m (3.2 x 10 - 3 cd-ft - 2 ) andscotopic at 3 x 10- 5 edmi - 2 (2.7 x 10- s cd•ft- 2 ).

metal to metal seal: Piping seal in which the mating surfaceson the external connection (the pin) and internal con-nection (the box) are machined to provide a pressuredinterference fit 360 degrees around the connection.Compare gasket seal and interference sealing thread.

metallograph: Short term for metallographic microscope.metallographic microscope: See microscope, metallo-

graphic.metallography: The science and practice of microscopic

testing, inspection and analysis of a metal's structure,typically at magnifications in the range of 50 x to2,500 x

metallurgical microscope: See microscope, metallurgical.microborescope: See borescope, micro-.microscope: An instrument that provides enlarged images of

very small objects.microscope, compound: Conventional microscope, using

geometrical optics for magnification. Also called labora-tory microscope.

microscope, interference: Magnifier using the wavelengthof light as a unit of measure for surface contour andother characteristics.

microscope, metallographic: Metallurgical microscopeincorporating a camera. Also called a metallograph.Most metallographic microscopes share these features:(a) stand with concealed shock absorbers, (b) intenselight source, (c) inverted stand so that the test object isface down, (d) viewing screens for prolonged tasks suchas dirt count or grain size measurements, (e) bright,dark and polarized illumination options.

microscope, metallurgical: Microscope designed with fea-tures suited for metallography.

microscope, phase contrast: Laboratory microscope withtwo additional optical elements to transmit both dif-fracted and undiffracted light, revealing refractive indexdiscontinuities in a completely transparent test object.


microscope, polarizing: Microscope with polarizing ele-ments to restrict light vibration to a single plane forstudying material with directional optical properties. Asfibers, crystals, sheet plastic and materials under strainare rotated between crossed polarizers on the micro-scope stage, they change color and intensity in a waythat is related to their directional properties.

miniature borescope: See borescope, miniature.miniborescope: Jargon for miniature borescope.minimum line pair: The closest distance that a specific

imaging system can resolVe between a pair of adjacent,parallel lines (line pair) that are used to evaluate sys-tem resolution.

modulus of elasticity: The ratio between stress and strain ina material deformed within its elastic range.

monochromatic: Light from a very small portion of the spec-trum is called monochromatic.

monochromator: Device that uses prisms or gratings to sep-arate or disperse the wavelengths of the spectrum intononcontinuous lines or bands.

morphology: See mathematical morphology.mottle: An apparently random positioning of metallic flakes

that creates an accidental pattern.multipass weld: A weld made by more than one pass.multiphase alloy: Alloy in which several phases are present.

nick: Surface indentation caused by forceful abrasion orimpact. Also called gouge. Compare tool mark.

nit: A former unit for measuring luminance, equivalent toone candela per square meter. Abbreviated nt.

noble metals: Cathodic metals (such as gold, platinum andsilver), which strongly resist corrosion.

nondestructive evaluation (NDE): Another term for non-destructive testing. In research and academic commu-nities, the word evaluation is often preferred because itemphasizes interpretation by knowledgeable personnel.

nondestructive examination (NDE): Another term for non-destructive testing. In the utilities and nuclear industry,examination is sometimes preferred because testing canimply performance trials of pressure containment orpower generation systems.

nondestructive inspection (NDI): Another term for nonde-structive testing. In some industries (utilities, aviation),the word inspection often implies maintenance for acomponent that has been in service.

nondestructive testing (NDT): The determination of thephysical condition of an object without affecting thatobject's ability to fulfill its intended function. See alsonondestructive evaluation, nondestructive examinationand nondestructive inspection.

nonrelevant indication: See indication, nonrelevant.

NDE: (1) Nondestructive evaluation. (2) Nondestructiveexamination.

NDI: Nondestructive inspection.NDT: Nondestructive testing.near ultraviolet radiation: Ultraviolet radiation with wave-

lengths ranging from about 320 to about 400 nm. For-merly called black light.

near vision: Vision of objects nearby, generally within arm'slength. Compare far vision.

nearsightedness: Vision acuity functionally adequate forviewing objects nearby, generally within arm's length.Compare farsightedness.

necking down: Localized reduction in area of a specimen orstructural member during welding under overload.'

negative sliding: The rolling and sliding of meshing gears orrollers when the rolling and sliding are in oppositedirections.

neural acuity: The ability of the eye and brain together todiscriminate patterns from background. Discriminationis influenced by knowledge of the target pattern, by thescanning technique and by the figure/ground relation-ship of a discontinuity. The figure/ground relationshipcan be referred to as having a level of visual back-ground noise.

0

OCTG: Oil country tubular goods.objective: In discussion of a lens system (camera, borescope,

microscope, telescope), of or pertaining to the end orlens closest to the object of examination—at the endopposite from the eyepiece. Distal; tip.

oil country tubular goods (OCTG): Hollow cylindrical com-ponents used to convey petroleum and relatedproducts.

one hundred percent testing: Testing of all parts of anentire production lot in a prescribed manner. Some-times, complete testing entails the testing of only thecritical portions of the part. Compare sampling, partial.

opening: Image processing operation of erosion followed bydilation. A single opening eliminates isolated single pix-els. See also closing.

opsin: See visual purple.optic disk: Area in the retina through which the fibers from

the various receptors cross the inner (vitreous humor)side of the retina and pass through it together in theoptic nerve bundle. This transitional area is completelyblind.

organoleptic: Relying on or using sense organs, such as thehuman eye.

orthicon: See image orthicon.


p

parafoveal vision: See scotopic vision.parallax: The apparent difference in position of an imaged

point according to two differently positioned sensors.pass: In welding, a single bead along the entire weld length

or the process of laying down that bead.pearlite: Platelet mixture of cementite and ferrite in steels or

in alpha and beta phases in nonferrous alloys.peripheral vision: The seeing of objects displaced from the

primary line o)sight and outside the central visual field.'phase: In metallurgy, a physically homogeneous portion of a

material system, specifically the portion of an alloy char-acterized by its microstructure at a particular tempera-ture during melting or solidification.

phase contrast microscope: See microscope, phase contrast.phase diagram: Graph showing the temperature, pressure

and composition limits of phase fields in a material sys-tem. Also called a constitution diagram. Compare equi-librium diagram.

photoconduction: Method by which a vidicon televisioncamera tube produces an electrical image, in which theconductivity of the photosensitive surface changes inrelation to the intensity of the light reflected from thescene focused onto the surface. Compare photo-emission.

photoelasticity: The effect of a material's elastic propertieson the way that it refracts or reflects light.

photoelectric effect: Emission of electrons from a surfacebombarded by sufficiently energetic photons. Suchemissions may be used in an illuminance meter and canbe calibrated in lux. 2

photoemission: Method by which an image orthicon televi-sion camera tube produces an electrical image, in whicha photosensitive surface emits electrons when lightreflected from a viewed object is focused on the surface.Compare photoconduction.

photometer: The basic measuring instrument of photome-try. Early photometers requiring visual appraisal by theoperator have been replaced by more accurate metersthat measure radiant energy incident on a receiver, pro-ducing measurable electrical quantities.

photometric brightness: The luminance of a light source.photometry: The science and practice of the measurement

of light or photon emitting electromagnetic radiation.See also relative photometry.

photons: Particle of light, hypothesized to explain thosebehaviors of light in which its behavior is corpuscularrather than wavelike.

photopic vision: Vision adapted to daylight and mediatedmainly by the cones. Vision is wholly photopic when theluminance of the test surface is above 0.034 cd.m(0.0032 cd•ft - 2 ). Also known as foveal vision and lightadapted vision. Compare mesopic vision and scotopicvision.'

photoreceptor: Light sensor.picture element: See pixel.picture processing: See image processing.pipe: A longitudinal centerline discontinuity inherent in

ingots, imparted to some rolled metal and consisting ofa concavity or voids.

pitting: Discontinuity consisting of surface cavities. See alsocavitation fatigue and pitting fatigue.

pitting fatigue: Discontinuity consisting of surface cavitiestypically due to fatigue and abrasion of contacting sur-faces undergoing compressive loading. See also cavita-tion fatigue and pitting.

pixel: A lighted point on the screen of a digital image. Theimage from a conventional computer is an array of over256,000 pixels, each of which has a numerical value.The higher the number for a pixel, the brighter it is.Formerly called picture element.

plane of focus: See focus, principal plane ofplatelet: Flat crystallites in certain phases of steel.polarizing microscope: See microscope, polarizing.porosity: A discontinuity in metal resulting from the creation

or coalescence of gas. Very small pores are calledpinholes.'

positive sliding: The rolling and sliding of meshing gears orrollers when the directions of rolling and sliding arethe same.

poultice corrosion: See corrosion, poultice.practical examination: In certification of nondestructive

testing personnel, a hands-on examination using testequipment and sample test objects. Compare generalexamination and specific examination.

primary creep: First stage of creep, marked by elastic strainplus plastic strain.

principal plane of focus: See focus, principal plane ofprocess: Repeatable sequence of actions to bring about a

desired result.process control: Application of quality control principles to

the management of a repeated process.production string: See tubing string.pseudocolor: Image enhancement technique wherein colors

are assigned to an image at several gray scale intervals.pseudoisochromatic plates: Color plates used for color

vision examinations. Each plate bears an image whichmay be difficult for the examinee to see if his or hercolor vision is impaired.

psychophysics: Interaction between vision performance andphysical or psychological factors. One example is the so-called vigilance decrement, the degradation of reliabil-ity based on performing visual activities over a periodof time.

pupil: Black aperture in the center of the eye's lens, throughwhich light enters the lens to impinge on the retina.

purple: See visual purple.


qualification: Process of demonstrating that an individualhas the required amount and the required type of train-ing, experience, knowledge and capabilities. See alsoqualified.

qualified: Having demonstrated the required amount andthe required type of training, experience, knowledgeand abilities. See also qualification.

quality: The ability of a process or-product to meet specifi-cations or to meet the expectations of its users in termsof efficiency, appearance, longevity and ergonomics.

quality assurance: Administrative actions that specify,enforce and verify a quality program.

quality control: Physical and administrative actions requiredto ensure compliance with the quality assurance pro-gram. Quality control may include nondestructive test-ing in the manufacturing cycle.

quality of lighting: Level of distribution of luminance in avisual task or environment. 0

I?

radiance: Radiant flux per unit solid angle and per unit pro-jected area of the source. Measured in watts per squaremeter steradian. Compare irradiance.

radiant energy: Energy transmitted through a medium byelectromagnetic waves. Also known as radiation.

radiant flux: Radiant energy's rate of flow, measured inwatts.

radiant intensity: Electromagnetic energy emitted per unittime per unit solid angle. Measured in watts persteradian.

radiant power: Total radiant energy emitted per unit time.radiometer: Instrument for measuring radiant power of

specified frequencies. Different radiometers exist fordifferent frequencies.

radiometric photometer: Radiometer for measuring radiantpower over a variety of wavelengths.

raster: A repetitive pattern whereby a directed element(a robotic arm or a flying dot on a video screen) followsthe path of a series of adjacent parallel lines, takingthem successively in turn, always in the same direction(from top to bottom or from left to right), stopping atthe end of one line and beginning again at the start ofthe next line. Following a raster pattern makes it possi-ble for electron beams to form video pictures or framesand for a sensor bearing armature to cover a predeter-mined part of the surface of a test object.

rat's tooth principle: (1) The tendency for hard material ona tooth's front surface to wear more slowly than softmaterial on the back surface, keeping the edge sharp.(2) Mechanism of wear whereby adjacent hard and soft

surfaces wear at different rates, producing a self sharp-ening edge.

recommended practice: A set of guidelines or recom-mendations.

Recommended Practice SNT-TC-1A: See ASNT Recom-mended Practice No. SNT-TC-1A.

recovery: Reduced stress level and increased ductility ofmetal after work hardening. See creep.

recrystallization: Changes in microstructure and propertiesupon heating of cold worked metal.

red mud: Debris (usually oxides of the contacting metals) offretting wear, mixed with oil or grease and retained ator near the site of its formation. See also cocoa.

reference standard: Work piece or energy source preparedaccording to precise instructions by an approved agencyfor tests and calibrations requiring precise and consis-tent measurements. (In the case of photometry, thestandard is a light source).

reflectance: The ratio of reflected wave energy to incidentwave energy. Also known as reflectivity.

reflection: A general term for the process by which the inci-dent flux leaves a surface or medium from the incidentside, without change in frequency. Reflection is usuallya combination of regular and diffuse reflection. 2

reflectometer: Photometer used to measure diffuse, specu-lar and total reflectance.

reflector: A device used to redirect the luminous flux from asource by the process of reflection. 2

refraction: The bending of radiation's path by the mediumthrough which it passes.

relative photometry: (1) Evaluation of a desired photomet-ric characteristic based on an assumed lumen output ofa test lamp. (2) Measurement of an uncalibrated lightsource relative to another uncalibrated light source.

relevant indication: See indication, relevant.remote viewing: Viewing of a test object not in the viewer's

immediate presence. The word remote previouslyimplied either closed circuit television or fiber optic sys-tems remote enough so that, for example, the eyepieceand the objective lens could be in different rooms. Highresolution video and digital signals can now be transmit-ted around the world with little loss of image quality.Compare direct viewing.

replica: Piece of malleable material, such as polyvinyl orpolystyrene plastic film, molded to a test surface for therecording or analysis of the surface microstructure.

replication: A method for copying the topography of a sur-face by making its impression in a plastic or malleablematerial.

reserve vision acuity: The ability of an individual to maintainvision acuity under poor viewing conditions. A visualsystem with 20/20 near vision acuity under degradedviewing conditions has considerable reserve vision


acuity compared to that of an individual with 20/70 nearvision acuity.

resolution: An aspect of image quality pertaining to a sys-tem's ability to reproduce objects, often measured byresolving a pair of adjacent objects or parallel lines. See

r also minimum line pair, resolving power.resolution test: Procedure wherein a line is detected to ver-

ify a system's sensitivity.resolution threshold: Minimum distance between a pair of

points or parallel lines when they can be distinguishedas two, not one, expressed in minutes of arc. Vision acu-ity, in such a case, is the reciprocal of one-half of theperiod expressed in minutes.'

resolving power: The ability of vision or other detection sys-tem to separate two points. Resolving power dependson the angle of vision and the distance of the sensorfrom the test surface. Resolving power is often mea-sured using parallel lines. Compare resolution.

retina: In the eye, the tissue that senses light.retinene: See visual purple.rhodopsin: See visual purple.robotic system: Automated system .programmed to perform

purposeful movements in variable sequences.rod: Retinal receptor that responds at low levels of lumi-

nance even down below the threshold for cones. Atthese levels there is no basis for perceiving differencesin hue and saturation. No rods are found in the foveacentralis. 2

SI: The International System of units of measurement.Includes most of the base units formerly called metric.

SNT-TC-1A: See ASNT Recommended Practice No. SNT-TC-1A.

sampling, partial: Testing of less than one hundred percentof a production lot. See one hundred percent testing.

sampling, random partial: Partial sampling that is fullyrandom.

sampling, specified partial: Partial sampling in which a par-ticular frequency or sequence of sample selection isprescribed. An example of specified partial sampling isthe testing of every fifth unit.

saturation: Relative or comparative color characteristicresulting from a hue's dilution with white light.

scaling: (1) Forming a layer of oxidation product on metals,usually at high temperatures. (2) Deposition of insolu-ble constituents on a metal surface, as in cooling tubesand water boilers.'

scoring: (1) Marring or scratching of any formed part bymetal pickup on a punch, die or guide. (2) Reducing thethickness of a part along a line to weaken it purposely ata specific location.'

scotopic vision: Dark adapted vision, using only the rods inthe retina, where differences in brightness can bedetected but differences in hue cannot. Vision is whollyscotopic when the luminance of the test surface is below3 x 10- 6 cd•m (2.7 x 10- 6 cd•ft - 2). Also knownas parafoveal vision. Compare mesopic vision andphotopic vision.

scuffing: A type of adhesive wear.seam: Linear discontinuity formed by a lack of metal from

folds produced by an underfilled pass during metal roll-ing. Squeezed tight on subsequent passes, the underfillruns parallel to the longitudinal axis of the bar.

second stage replica: A positive replica made from the firstcast to produce a duplicate of the original surface iscalled a second stage replica.

secondary creep: Second stage of creep, where deformationproceeds at a constant rate and less rapidly than as inprimary creep. Essentially an equilibrium conditionbetween the mechanisms of work hardening andrecovery.

sensitization: Precipitation of chromium carbides in thegrain boundaries of a corrosion resistant alloy, resultingin intergranular corrosion that would otherwise beresisted.

shadow casting: Nondestructive technique of vapor depos-iting a thin metal film onto a replica at an oblique anglein order to obtain a micrograph of a test surface of anopaque specimen.

shear break: Open break in metal at the periphery of a bolt,nut, rod or member at approximately a 45 degree angleto the applied stress. Shear breaks occur most oftenwith flanged products. Also called shear crack.'

shear crack: See shear break.shoulder: Cylindrical metal component surface, machined

to receive threading indentations but in fact notthreaded, where the thread stops on the outside surfaceof the pipe.

signal electrode: Transparent conducting film on the innersurface of a vidicon's faceplate and a thin photoconduc-tive layer deposited on the film.

signal processing: Acquisition, storage, analysis, alterationand output of digital data through a computer.

simple magnifier: A microscope having a single converginglens.

smoothing: In image processing, use of positive coefficientsin a linear combination of pixel values to smoothenabrupt transitions in a digital image. Also called lowpass filtering.

spalling fatigue: See subcase fatigue.specific examination: In certification of nondestructive test-

ing personnel, a written examination that addresses thespecifications and products pertinent to the application.Compare general examination and practical exami-nation.


specification: A set of instructions or standards invoked by aspecific customer to govern the results or performanceof a specific set of tasks or products.

spectral powier distribution: The radiant power per unitwavelength as a function of wavelength. Also known asspectral energy distribution, spectral density and spec-tral distribution.

spectral reflectance: The radiant flux reflected from a mate-rial divided by the incident radiant flux.

spectral transmittance: The radiant flux passing through amedium divided by the incident radiant flux.

spectrophotometer: Instrument used for spectropho-tometry.

spectrophotometry: Measurement of electromagnetic radi-ant energy as a function of wavelength, particularly inthe ultraviolet, visible and infrared wavelengths.

spectroradiometer: Instrument used for spectroradiometry.spectroradiometry: Measurement of electromagnetic radi-

ant power and spectral emittance, used particularly toexamine colors and to measure the spectral emittanceof light sources.

spectroscope: Instrument used for spectroscopy.spectroscopy: Spectrophotometry or spectroradiometry in

which the spectrum, rather than being analyzed only bya processing unit, is presented in a visible form to theoperator for organoleptic examination.

spectrum: Representation of radiant energy in adjacentbands of hues in sequence according to the energy'swavelengths or frequencies. A rainbow is a well knownexample of a visible spectrum.

specular reflection: When reflected waves and incidentwaves form equal angles at the reflecting surface.

specular: Pertaining to a mirror-like reflective finish, as of ametal. Compare lambertian.

speed of light: The speed of all radiant energy, includinglight, is 2.997925 x 10 8 meters per second in vacuum(approximately 186,000 miles per second). In all materi-als the speed is less and varies with the material's indexof refraction, which itself varies with wavelength.'

speed of vision: The reciprocal of the duration of the expo-sure time required for something to be seen.'

standard observer response curve: See eye sensitivitycurve.

standard: Document to control and govern practices in anindustry or application, applied on a national or interna-tional basis and usually produced by consensus. See alsoworking standard and reference standard.

steel: An iron alloy, usually with less than two percentcarbon.

stereo photography: Close range photogrammetric tech-nique involving the capture and viewing of two imagesof the same object in order to reconstruct a threedimensional image of the object.

strain: The alteration of the shape of a material by externalforces.

stress: (1) In physics, the force in a material that resists exter-nal forces such as tension and compression. (2) Loadper unit area.

stress raiser: Contour or property change that causes localconcentration of stress.

stress riser: See stress raiser.subcase fatigue: Fatigue originating below the case depth.

Compare case crushing. Also spalling fatigue.subcase origin fatigue: See subcase fatigue.subsurface fatigue: Fatigue cracking that originates below

the surface. Usually associated with hard surfaced orshot peened parts but may occur anytime subsurfacestresses exceed surface stresses.

TIG welding: Tungsten inert gas welding.TTT: Time temperature transformation.Tarasov etching technique: Way of visually inspecting for

the presence of deleterious effects in hardened steels byusing specific etching solutions and methods ofinspection.

temperature diagram: See time temperature transformation(TTT) diagram.

tempering: Process of heating a material, particularly hard-ened steel to below the austenite transformation tem-perature, to improve ductility.

tertiary creep: Third stage of creep, marked by steadyincrease in strain to the point of fracture under con-stant load.

threshold: See adaptive threshold, resolution threshold,threshold.

thresholding: Digital data processing technique thatreduces a gray level image into a binary image.

throat, actual: Shortest distance from the root of a fillet weldto its face, as opposed to theoretical throat or weld size.

throat, theoretical: The distance from the beginning of theroot of the weld perpendicular to the hypotenuse of thelargest right triangle that can be inscribed within thecross section of the fillet weld. Compare weld size.

throat, weld: Distance from the root of a fillet weld to itsface. Compare weld size and throat, actual.

time temperature transformation (TM') diagram: A graphshowing time required at any temperature to transformaustenite to pearlite, bainite or martensite.

tip: In casual usage, the distal or objective end of aborescope.

tool mark: Shallow indentation or groove made by the move-ment of manufacturing tools over a surface. Comparegouge or nick.


trace: Line formed by electron beam scanning from left toright on a video screen to generate a picture.

tracer: In leak testing, a gas that is sensed as it escapes fromconfinement.

transformation diagram: See time temperature transforma-tion (TTT) diagram.

troland: A unit of retinal illuminance equal to that producedby a surface whose luminance is 1 nit when the pupilmeasures 1 square millimeter. (1 nit = 1 candela persquare meter.)

tubing string: Pipe with which oil or gas has contact as it isbrought to the earth's surface. Also called productionstring.

tungsten inert gas (TIG) welding: See gas tungsten arcwelding.

ultraviolet borescope: See borescope, ultraviolet.ultraviolet radiation: Electromagnetic radiation with wave-

lengths ranging from about 4 to about 400 nm, betweenvisible light and X-rays. Compare near ultravioletradiation.

ultraviolet radiometer: A meter, usually calibrated at365 nm, used in fluorescent liquid penetrant and mag-netic particle testing to detect output of ultravioletlamp.

undercut: Undesirable groove left unfilled by weld metal,created during welding and located in base plate at thetoe of a weld.

V

VT: Visual testing.vaporproof borescope: See waterproof borescope.video: Pertaining to the transmission and display of images

in an electronic format that can be displayed on a cath-ode ray screen.

videoscope: Jargon for video borescope. See borescope,video.

vidicon tubes: Television tube that uses the photoconduc-tion method. Compare image orthicon.

vigilance decrement: Degradation of reliability during per-formance of visual activities over a period of time. Seealso psychophysics.

visibility: The quality or state of being perceivable by theeye. In many outdoor applications, visibility is definedin terms of the distance at which an object can be justperceived by the eye. In indoor applications it usually isdefined in terms of the contrast or size of a standard testobject, observed under standardized view conditions,having the same threshold as the given object.'

vision: Perception by eyesight. See far vision, machinevision, mesopic vision, near vision, peripheral vision,photopic vision, scotopic vision, speed of vision.

vision acuity: The ability to distinguish fine details visually.Quantitatively, it is the reciprocal of the minimumangular separation in minutes of two lines of width sub-tending one minute of arc when the lines are just resolv-able as separate.'

visual acuity: See vision acuity.visual angle: The angle subtended by an object or detail at

the point of observation. It usually is measured inminutes of arc.'

visual background noise: Formations on or signals from atest object that constitutes the background to a disconti-nuity. The higher the level of visual background noise,the more difficult it is to distinguish a discontinuity.

visual efficiency: Reliability of a visual system. The termvisual efficiency uses 20/20 near vision acuity as a base-line for 100 percent visual efficiency.

visual field: The locus of objects or points in space that canbe perceived when the head and eyes are kept fixed.The field may be monocular or binocular.'

visual perception: The interpretation of impressions trans-mitted from the retina to the brain in terms of informa-tion about a physical world displayed before the eye.Visual perception involves any one or more of the fol-lowing: recognition of the presence of something(object, aperture or medium); identifying it; locating itin space; noting its relation to other things; identifyingits movement, color, brightness or form.'

visual performance: The quantitative assessment of the per-formance of a visual task, taking into considerationspeed and accuracy.'

visual purple: Chromoprotein called rhodopsin, the photo-sensitive pigment of rod vision. The mechanism of con-verting light energy into nerve impulses is aphotochemical process in the retina. Chromoprotein istransformed by the action of radiant energy into a suc-cession of products, finally yielding the protein calledopsin plus the carotenoid known as retinene.

visual task: The appearance and immediate background ofthose details and objects that must be seen for the per-formance of a given activity. The term visual task is amisnomer because it refers to the visual display itselfand not the task of extracting information from it.'

visual testing: Method of nondestructive testing using elec-tromagnetic radiation at visible frequencies.

voids: Hollow spots, depressions or cavities. See also discon-tinuity and dislocation.


wear: See erosion; rat's tooth principle; wear, adhesive; andwear, fretting.

wear, adhesive: Degradation of a surface because of micro-welding and consequent fracture due to the sliding ofone surface against another. Types include fretting,galling and scuffing.

wear, fretting: Surface degradation caused by microweldingand microfractures on surfaces rubbing each other. Alsocalled chafing, friction oxidation and wear oxidation.See also cocoa and false brinelling.

wear oxidation: See fretting wear.weld size: Thickness of weld metal—in a fillet weld the dis-

tance from the root to the toe of the largest isosceles

right triangle that can be inscribed in a cross section ofthe weld.

weld throat: See throat.welder's flash: Clinical condition, specifically keratocon-

junctivitis, commonly caused by overexposure to ultra-violet radiation of welding arc.

white light: Light combining all frequencies in the visiblespectrum.

work hardening: Increase in hardness accompanying plasticdeformation of a metal. Usually caused in a metal byrepeated bending or flexing. Compare creep andrecovery.

working standard: Work piece or energy source calibratedand used in place of expensive reference standards. Inthe calibrating of photometers, the standard would be alight source.


REFERENCES

1. EPRI Learning Modules. Charlotte, NC: The ElectricPower Research Institute.

2. IES Lighting Handbook: Reference Volume. New York,NY: The Illuminating Engineering Society of NorthAmerica (1984).

3. 1992 Annual Book of ASTM Standards, Section 3, MetalsTest Methdds and Analytical Procedures, Volume 03.03,Nondestructive Testing. Philadephia, PA: The AmericanSociety for Testing and Materials (1992).

INDEX / 357

INDEX

Page numbers in italics refer to Section 12, "Glossary ofVisual and Optical Testing Terms."

AAFNOR (French Association for Standardization)AIA (Aerospace Industries Association)ANSI. See American National Standards InstituteAPI. See Anierican Petroleum InstituteASME. See A lmerican Society of Mechanical EngineersASNT. See American Society for Nondestructive TestingASNT Recommended Practice No. SNT-TC-1A. See American Society for

Nondestructive TestingASTM. See American Society for Testing and MaterialsAWS. See American Welding SocietyAbrasive wear 198Acceptable quality level (AQL) 160-161, 340Acceptance criteria, weld testing. See Weld testing, acceptance criteriaAcetone, for surface preparation 118Acoustic imaging 309-311Actual required resolution 130Acuity. See Vision acuityAdaptive threshholdingAdhesive wearAerospace Industries Association (AIA)Aircraft inspection

borescope applicationscomposite materials discontinuities main landing gear truck beam pitting corrosionrudder rib flange cracks slat drive mechanism bell crank cracking spoiler actuating mechanism lube hole cracks torsion bar core corrosion pitting turbine blade inspection wing front spar lower cap cracks wing rear spar doubler and web cracks

Alloysphase diagrams

Alpha ferrite Alpha iron Ambient light American Bureau of Shipping (ABS) American Conference of Governmental Industrial HygienistsAmerican National Standards Institute (ANSI)

ANSIIASNT CP-189 ANSI B13.1

American Nuclear Institute (ANI) American Petroleum Institute (API)

API RP 7G API RP 8B API Standard 1104 API Specification 5CT API Standard 5B tubular goods inspection

American Society for Nondestructive Testing (ASNT)ANSVASNT CP-189 ASNT Level III program ASNT Recommended Practice No. SNT-TC-1A visual committees visual testing qualification/certification

American Society for Testing and Materials (ASTM)

E380, Standard Metric Practice Guide.. American Society of Mechanical Engineers (ASME) .

Boiler and Pressure Vessel Codeand SNT-TC-1Aand nuclear industry visual testing (VT 1,-2,-3) in

American Welding Society (AWS)AWS D1.1, Structural Welding Code-Steel (D1.1) ....

AWS Certified Welding Inspector (CWI) See also Weld inspection; Welding

Angle of visionfor boreseopes

AngstromAngulated borescopes Annealing Arc burns Arc strikes Arc welding Archiving photographs Aspirin safety seal inspection Attenuation. See Optical filters and filteringAttribute testing .AusteniteAutomated visual testing Automatic focusing, video borescopes Automation. See Automated visual testingAutomotive inspection

borescope applicationsbrake shoe sortingfinish texture analysisglassshock absorbers

Average outgoing quality (AO(?) Average outgoing quality limit (AOQL) Aviation applications. See Aircraft inspection

BBSI (British Standards Institute) Babbitted surface inspectionBack lighting Ball valve inspectionBandpass filtersBidirectional stressesBilletsBimodal histogram threshholding Binary systems Birefringence Black light. See Ultraviolet radiationBlackbody radiation Blind spot Blobs, in image segmentation Blotch Blue hazard function

269-270 138

266-268

178 178

100 200-201

178

6-8, 86, 138-139, 293, 294-300298-300,330 297-298

296-297294-295

295294320298

295-296 215

215-217217, 340217, 340

340178 26

178179, 180, 187

179263

178,180, 322188,190

191180,186186, 188

323188-191, 322-327 8,178

179,180, 187187

8, 180-187, 254vi-ix, 8

69, 187

178-180

74,178 179-180, 255, 264

185-186263-266

179-180, 255, 264-266 8, 69, 178180, 186, 258, 261

63, 69

13-14, 55-6087-90

5585

340326

261, 262250,340

314105

160, 161 217, 340

92 107, 128-130, 228-243, 279-291137

86 288-291

279-288 321

133161161

178 270

93 272 94

196-197 275

100216,340

340

31-33,34011, 58, 61

100-101279, 280, 287, 340

26, 340Boiler and Pressure Vessel Code. See American Society of Mechanical

EngineersBoiler feed pump inspectionBoiler tube inspectionBoiling water reactor vessel inspection

358 / INDEX

272-274, 298 82-91, 134-139, 293, 340-341

6-8, 86, 138-139, 293, 294-30188-91

4-684, 85, 88-89

7582-83, 137, 293 75, 88

83-84, 137magnetic particle testing, use in 308

84, 85, 89, 29387-88

91 83-85

85-87, 89-9174-75

Certification and qualification of personnel. See American Society for Non-destructive Testing; American Welding Society, Certified WeldingInspector; Qualification

Certified Welding Inspector program. See American Welding SocietyChafing fatigue 201Charge coupled devices (CCDs)

in nuclear reactor inspection size in video borescopesCharge injected devices (CID) 96

Check valve inspection 271-272Chemical etching. See EtchingChemical industry. See Petroleum and chemical industry applications

118-119331-335

10154

99, 341341

178, 263, 341

Bolting inspection Borescopes

aircraft inspection applicationsconstruction development of direction of viewentry port size fiber optic field of viewfocusing..

miniature optical components photography with rigidspecial purpose "test object effects video instruments. See Video borescopesSee also specific applications

Bottle inspectionBozzini, PhilippBrake shoe sorting (automotive brakes)BrazingBrightness

spot metersand visionSee also Contrast; Intensity; Light

Brinell microscopeBrinellingBritish Standards Institute (BSI)BurrBursts Butt welds or butt joints Butterfly valve inspection

CCCD. See Charge coupled devicesCEN (European Committee for Standardization) 178CRT. See Cathode ray tubeCWI. See American Welding Society, Certified Welding InspectorC-scanning 299-300,309-310Cable reefs 150Calibrated borescopes 85Cameras. See Charge coupled devices; Photography; Video componentsCandela x, xi, 341Candy inspection 104-107Caps, for color vision examinations 16, 17-20Car inspection. See Automotive inspectionCarbon arc weldingCarburizationCase crushingCasingCastings

discontinuities 276See also Steel

Cataracts . Cathode, in galvanic corrosion Cathode ray tubes Cavitation fatigue Cellulose acetate replication Cementite Centralizers, for video camera Ceramics

inspectionmicrostructure size distribution

Chlorinated hydrocarbons, for surface preparation Circuit board inspection

Classification, in image segmentation Cleanliness, importance of Closing of images Cocoa Codes

See also StandardsCoefficients, of spatial filtersCoil breaks, in steelCoil welds, in steelCold drawingCold lightCold strip (steel) inspectionCold working Color cameras Color chips Color temperature Color vision

aging and classification color blindness/deficiencyexaminations

caps/diskspseudoisochromatic plates/charts

spectrumin weld inspection

Colorimeters, for steel inspectionColors

discoloration indicating discontinuitiesspectrum .. See also Color vision

Comparators, surfaceComplete inspection/testing Composite materials inspection

aircraft panelsmicrostructure size distribution determination

Compound microscope Compressive strength.. Condensate pump inspection ConesContact stress fatigue Contrast

See also Brightness; Light; ResolutionControl chartsCooling methods, to prevent temperature discontinuities Corn cob inspection Cornea Corpuscular theory Correlated color temperatureCorrosion

crackingcreviceelevated temperature

105-107 4

288-291 249-250,261-262

41 62

79 341

178 341

275, 341165, 168, 170, 171, 247, 248, 341 272, 273

250 213 205 322

23, 59207-208140-141

205-206, 341108-111217, 341150-151

320-321145

95-96, 230, 341268137

134-135

96, 341 222 223 196 16

232-234217149

135, 13716, 32-33

9, 14-20, 61-6215

15-16, 61-62 14-20

16-2016,349

61-62, 9463

238-240 61-62, 342

62, 32961 62, 94

76-77160,342328-330

298-300, 330 14579, 80, 342

197269-270

36, 57-58, 61, 342201-20562, 342

161,162214106

11, 5730

33 197, 206-210, 342 209-210 208-209

213

INDEX / 359

fatigue fracture 210fretting. See Wear, frettinggalvanic rust uniform

Cracksin aircraft parts composite matrix cracking in forgings replication analysis stress-corrosion cracking in welds

Crampton, George Sumner Crawlers

See also Positioning and transport systemsCreep

replication analysis ..Creep fracture Crevice corrosion Crystalline structure Cylinders for photometry Cystoscopes

DDIN (German Institute for Standardization)Dark adapted vision

See also VisionDe Broglie DefectDeformation

effect on metal structure Delamination

See also Exfoliation; LaminationDelta ferriteDelta ironDensitometer Department of Defense (DOD) Depth of field

in photography: effect of aperturevideo borescopes

Depth of focus, borescopes Design criteria Desmutting Detergents, for surface preparationDewetting Diabetic retinopathy Diaphragm valve inspection Difference measure Diffracted light technique Dilation, of images Dimensional inspection

image processingof pipesof pumps weld profiles

Direct photometry Direct substitution alloys Direct viewing Direct vision borescopes Direct vision instrument Directional lighting Dirt, rolled in steel Discoloration, indicating discontinuitiesDiscontinuities

in bolting in castings

in forgings 274-276size of, and detection 74-75in steel 222-241See also Weld inspection, discontinuities; specific discontinuities by name

Disks, colored. See Caps, coloredDislocationDistribution photometers

EEddy current testing, visual aspects Edge breaks, in steelEdge findingEffective throat Elastic deformationElasticityElectric arc welding Electromagnetic energy

spectrum wave theory

Electromagnetic testing, visual aspects ofElectron microscopes . . .

Electronic component inspection Electrons

Elevated temperature discontinuities Elevated temperature fatigue Endoscopes

See also BorescopesEnergy. See Electromagnetic energy; Radiant energyEngine inspection. See Aircraft inspection; Automotive inspectionEquilibrium diagram . Equivalent 20/20 near vision acuity Equivalent sphere illumination photometersErosion, of images Erosion (wear)Etch crackEtching

chemical etchants heat generation during macroetching process safety precautions for welds

Ethyl alcohol, for surface preparation European Committee for Standardization (CEN)Eutectic liquid and eutectic point Eutectoid Evaluation Exfoliation

See also DelaminationEye

anatomy ofprotective filters sensitivity curve spectral response See also Safety; Vision

Eyeglasses 11-12Eyesight. See Vision

F116

201, 343 343 11, 343

272-274

207-20862,119

208 344

294-298328, 329

275111, 112209-21067, 261

4-7128-129, 149-150

Remote visual testing 211-212,342

111, 112212

208-209214-215

424

178 36,342

30 342

194-195, 342217,218

329

217, 342 217

37178

142-143, 342 142-143

137-13887-8852-53

121 118-119

262, 34259

272103

293-294, , 299-30099, 342

101 188-191, 322-327

270156-159, 163-176, 253-262

38, 342215, 342

34289

342342226

62, 329342

273-274 276

34244-46

307 223 100-101

343194-195,343

194, 343343

30-3416, 31, 61-62, 94

30-31 307-308

133-134331-335

33210-214

2124, 343

34369

41-4299

198-199, 213, 343 343 118-124

119,122-124120118

121-123120124118 178

216, 343343343296

11, 36, 57-6126

34333-34

Fabric seam temperature indication False brinellingFalse indicationFarsightednessFastener inspection

360 / INDEX

Fatiguecavitation contact stresscorrosion fatigue fracture elevated temperature fatigue strengththermal cycling

Feature extraction in automotive finish texture analysisin high speed optical tests

FerriteFiber optics

See also Borescopes, many of which use fiberFiberscope. See Borescopes, fiber opticField of view, borescopes Fields Fillet weldsFilm speed Filters, image processing Filters, optical. See Optical filters and filteringFinish texture analysis (automotive finishes)Fixed multiple cell photometersFlakesFlange weld Flash lighting. See Stroboscopic lightingFlatness measurement, hot rolled steel ........................... 234-236, 237, 238Flexible borescopes. See Borescopes, fiber optic;,Video borescopes

2243, 16, 21, 305-309, 344

132306224

250129344

83-84, 87-88of eye principal plane of

Food product inspection systemsFootcandleFootlambertForgings

discontinuities 274-276, 344See also Steel

Forward oblique borescopesFourier transformsFovea centralisFoveal vision

See also VisionFracture surface replicationFrameFrames (video) French Association for Standardization (AFNOR)Fretting wear. See Wear, frettingFriction digs, in steel 224Friction oxidation. See Wear, frettingFront lightingFrozen dinner inspectionFull wrapFurnace photogrammetry

GGages, for tubular goods inspection 190Galling 200-201,344Galvanic corrosion 207-208,344Gamma iron

217,344

Gas-cooled borescopes Gas metal arc weldingGas tungsten arc weldingGasket sealsGastroscopes Gate valve inspection Gears, contact fatigue Geometrical optics German Institute for Standardization (DIN)Ghost lines, in steel GlareGlass stress detection Glaucoma . Global threshholding Globe valve inspection Gloss measurement, in steel Goniometers Gouging wear Gradients, for image enhancement Grain boundaries Grains Gray level Graybody radiation Green rot Grinding Grinding wear Grinding crack . Groove welds. See Weld inspection; Welding

HHadfield's steel Half-fringe photoelasticityHalitationHard facing Hardenability Heat affected zone

replication analysisHeat checking Heat treatment Heisenberg, Werner Helicopter blade inspection, borescopes in High pass spatial filters High speed optical testing Histogram thresholding, bimodalHoisting equipment, oil field Hooke's law Hot rolled steel flatness measurement Hot slab surface discontinuity inspectionHot strip inspection Hot tearHueHuman vision. See VisionHuygens, ChristianHybrid microelectronic circuit inspectionHyperthermia

IGSCC (Intergranular Stress Corrosion Cracking). See Stress corrosioncracking

ISO. See International Organization for StandardizationIconoscopes 132Illuminance 345Illuminance photometers 39Illuminated magnifiers 77

205-206 201-205

210212197

21292, 100, 343 283-285

101-107 217, 343

82-83, 137, 293, 295, 343opti-c-technolog

75, 85, 88139, 343

164, 167, 344143-144

96-97

279-288 45

34417

Floppers, in steel Fluorescence Fluoroscopes Fluoroscopy Fluting, in steel Flux cored arc weldingFlying rigs Focus

of borescopes60

142, 143, 344102-107

xi, 344xi, 344

89,90 101,102 103,105 106 57,61,344

36

108-109,112-113 344

139178

92105-106

116314-319

86 252-253

251-252, 344 322-323, 344

4270-271204, 2052-3, 344 178

227 62, 344-345

32159

100271

42-43, 238, 239, 24144-45

200, 34597-99

214-215,345214-215,345

92, 34532, 33, 345

213, 345119

199-200345

200 321

140-141, 345199-200

220218 111212,345217,218

307

96-9792-107, 228-243, 279-291

100191194

234-236230-231

232 345 345

30332-334

23, 25, 345

Jaeger eye chart 12, 55, 190, 346See Vision acuity

Japanese Industrial Standards Committee (JISC) 178Joint configurations 247

Keratoconjunctivitis Kinetic vision acuity 64, 346

K

Machine shops, borescope applications Machine vision systems

for automotive brake shoe sortingfor automotive surface inspectionimage sensors

23 lightingobject recognition/classification

M86

92-107, 347288-291279-288

94-96 92-94

92, 101, 284-291

INDEX / 361

Illuminating Engineering Society Illumination. See LightingImage amplifiers, solid stateImage guides, fiber opticImage orthiconsImage processing

automotive finish -)

coatings, surface gloss of composite materials electromagnetic testing feature extraction image convolution image enhancement image segmentation mathematical morphologyin radiographysize determinationsmoothingstrip steelsee also Video components

Image tubes 94-95, 132-133, 140Imaging devices, photoelectric 94-95, 132-133, 140Imaging devices, solid state

charge coupled devices. See Charge coupled devices (CCD)charge injected devices (CID)

Inadequate joint penetration Incandescence Indexing borescopesIndications Industrial practicesInert gas shielded arc weldingInfrared radiationIntegrated circuit inspection

Integrating sphere photometers Intensity

requirements for light sources See also Brightness; Light

Interference microscopes Interference objective Interference seals Intergranular Stress Corrosion Cracking. See Stress corrosionInterlaced scanning . Internal stresses International Organization for Standardization (ISO)

ISO DIS-9712 Interstitial alloys Interpretation, of indication Inverse square lawIrisIron, phase transformationsIrradiance Ishihara plates/charts Isomorphous binary systemsIsothermal transformation

Laboratory microscopesLambert cosine lawLambertian . .Lamination (steel defect)

See also DelaminationLang, John 7Lap joints. See Weld inspection; WeldingLapsLaplacian operator Lasers

for ceramic inspection hazards.. for hot slab surface discontinuity inspection for strip steel inspection for surface measurement

Leak testing, visual aspects of Lens, of the human eye Lens systems of borescopes Levels of (visual testing) qualification . Light Lighting

detection and measurement of. See Photometers; Photometryelectromagnetic spectrum and 16, 31, 94measurement units x-xi, 37speed of 31theories of 30-31See also Brightness; Lighting; Radiant energy

Light adapted vision 36, 346See also Vision

Light guides ..Lighting

for automotive finish inspection color temperature illumination for visual testing intensity attenuation light guides luminous efficiency for machine vision systems.. measureable quantities and units for photography placement of for weld inspection

Line pair Liquid crystal devices Liquid metal contact Liquid penetrant testing, visual aspects ofLong pass filters Lot tolerance percent defective (LTPD) Low pass spatial filters LumenLuminance Luminance photometersLuminous efficiency Luminous flux Luminous intensity Lux

55

132 82-83,345 132-133, 140, 268, 345

96-101,345280-285

241330307

92, 100, 101-10797

92, 96-99, 144-147, 34592, 100-101, 345 99-100

306-307101

97 233-234, 235

96261345

89-90345

178345

23, 34, 346333,334

46

55

81346

322,346cracking

139,346196178179

215,346346

37,38 11,57,346

217-222346

16,346216,346218-220

79, 80, 346 37, 38

346226, 275

275,34699

346320-321

22-23230

232-233,236238, 239304-305

11, 57, 34683-91

183346346

83, 91, 346 2, 67

28016, 32-33

54-5638-3983, 9133-3492-94

37143

54, 92-93, 23767-68

346102

213-21458, 85, 93, 305-306

94, 95161

97, 346 x, xi, 347

347 4133-34, 347

x, xi, 347x, xi, 347

x, xi, 347

362 / INDEX

programming of test rirocedures 103-107for steel 228-243See also Image processing

MacroetchingMacroscopesMacular lutae Magnetic particle testing \.Magnification

borescopes 88vs. resolution and aperture 80television systems video borescopes

Magnifierssurface comparatorstypical working characteristics

Manhattan Project borescope Martensite Mask in image processing Match bend effect Mathematical morphology Matrix cracking, in composites Maxwell, James Clark Measurement, units of Measuring magnifiers Mechanical properties

See also Discontinuities; Steel; Weld inspectionMedian spatial filters Melting points. See Temperature indicating materialsMesopic vision

See also VisionMetal crystalline structureMetal inert gas (MIG) weldingMetal to metal sealsMetallographic microscopes MetallographyMetallurgical instabilities Metallurgical microscopes Metric units of measurement Microborescopes Microelectronic component inspection

microscopeshigh powerlow powermedium power for metallographic tests for replicate analysis See also specific types of microscopes

Microstructureelectron microscopyreplicationgrain structureof steel

Microwelding.. Military specifications

MIL-P-50884MIL-P-55110MIL-STD-271, SNT-TC-1A recommendationsMIL-STD-410, SNT-TC-1A recommendationsMIL-STD-1949A MIL-STD-105D, sampling plans in

Miniature borescopes Minimum enclosing rectangle method Minimum line pair.. Mirror stereoscopes Modulus of elasticity Molten metal contact Monochromatic light

Monochromator MottleMoving cell photometers Moving mirror photometers Mfiller-Lyer illusion Multichannel filtering approachMultiphase alloys Multiple cell photometers

NNDE. See Nondestructive testingNDI. See Nondestructive testingNDT. See Nondestructive testingNanometer 55Naphthas, for surface preparation . 118NAV SHIPS 250-100-1 . . 8Naval Submarine Research Laboratory (color vision classification) ... 15-16Near ultraviolet radiationNearsightednessNecking down Negative slidingNeural acuityNeural networks, discontinuity classification by

Neutral filters Newton, Sir Isaac Nick Nit Nitze, MaxNondestructive testing

quality tolerancesvisual testing asSee also Visual testing, visual aspects of other test methods

Nonlinear mechanical property behaviorNonwettingNuclear power plants

borescope applications reactor pressure vessel inspection

Nuclear Regulatory Commission

0Objective distance 74Oil field pipe and tube inspection. See Tubular goods inspectionOpening, of images Operating characteristic (OC) curvesOptic diskOptic nerve Optical bench photometers Optical filters and filtering

in machine vision systemsprotective filters

Optical testing high-speed safetysteel, surface inspection of cold strip systems/components. See specific entry-e.g., Borescopes

vision; MicroscopesOptics, geometricalOrange peel, on steel Organoleptic visual testingOrthiconsOven temperature verificationOverlapOxidation Oxyfuel gas welding

118 78

58,3472,63,85,93,307-309

142 136-137 76-81, 292-293

76-7776

6, 7218, 219, 220-221, 347

96-97, 347144, 347

99-100328, 329

30x-xi, 37, 55

77, 78

97

36,347

214-215252-253

323, 34780

215, 347212-213

79-80,347x-xi, 37, 55

293,347 331-33576-81, 347-348

79-81 77-78

78-79215

108-109

108, 110, 133-134 108-110

145-147 217-222

200-201

335 335 186 186

308160-162

84, 85, 89, 348101348316

194-195,348213-214

2, 61, 348

2, 348 279-280, 287, 348

4545-46

59279, 282-283

215, 34845

34811, 348

348 203, 204, 348 69, 348

23538-39,94

30348

xi, 348 4

181, 348 53 vii, 2

195-196 262

245-27686, 138-139 263-269

263

99 160-161

58,3481144

38-39 94,95

2652

101-107 22-26

232-233, 234Mac"-

2-3225

228,348 132-133, 140, 268

115 260

213250

INDEX / 363

PPanoramic borescopes 85, 90, 91Parafoveal vision 36

See also Vision314-316,349

160 233 234, 235 217, 219, 349 58, 85, 93, 305-306

57, 65, 69

65-66

13,349

Petroleum and chemical industry applicationsborescope applications furnace photogrammetry remote pipeline inspection standards, specifications, etc. threads in tubular goods See also Pipe inspection

Phase contrast microscopesPhases

Phase diagrams Phase transformations

Photocathode materials Photoconductive components

light detection and measurementphotoconductive lagphotoconductive tubesphotoconductor cells photodiodesvideo technology See also Charge coupled devices; Solid state devices

Photoelasticity, half-fringePhotoelectric effect

devices tubes

Photoemissive devicesPhotogrammetiyPhotography

with borescopes film choice of microstructure for surface inspection of steel

Photometersdistribution photometers equivalent sphere illumination photometersilluminance photometers luminance photometers photomultiplier tube photometers photovoltaic cells radiometers reflectometers spectometers types ofzeroingSee also Photometry

Photometrycosine cubed lawdirect inverse square law Lambert cosine law

relativesource intensity attenuation standard observer response curvessubstitution See also Photometers

Photomultiplier tube photometersPhotonsPhotopic vision

See also VisionPhototransistors 36Photovoltaic cell photometers 39-41Photovoltaic cells 35, 132Picture pyramid method, of discontinuity classification 235

pigs 150See also Positioning and transport systems; Remote visual testing

Pill pack inspection 104-107Pin stretch Pinchers, in steelPipe (the defect)Pipe inspection

gradespipeline coating temperature indicatorsremote visual testing standards and specifications thread inspection 190, 322-327See also Petroleum and chemical industry applications; Tubular goods

inspection, Weld inspectionPittingPixelsPlanck's constant Planck's equation Planck's radiation law Plasma arc welding Plastic deformation Platelet Plug valve inspection Polar log coordinate transform Polarizing microscopes Polishing .. Porosity .Positioning and transport systems

automated systemsfor nuclear reactor inspection for remote visual testing of pipes

Positive slidingPostforming heat control, temperature indicators forPosturePoultice corrosionPower plants

boiler weld inspection 164-166borescope applicatiOns 86, 91See also Nuclear power plants; Weld inspection, acceptance criteria

Practice 178See Recommended Practice; Standards

Preattentive processing . Precipitation hardening.PresbyopiaPressure vessels

nuclear reactorsweld inspection acceptance criteria 166-167, 172See also American Society of Mechanical Engineers, Boiler and Pressure

Vessel Code; Weld inspectionPressurized water reactor vessel inspectionPrewitt gradientPrewitt, J.M.SPrimary focusing, borescopesPrincipal plane of focus .

Parallax measurement (in photogrammetry) Partial sampling Pattern recognition, in strip steel inspection

Pearlite Penetrant testing, visual aspects ofPerception

See also VisionPerformance standards

See also Reference standardsPeripheral visionPeriscopes Personnel qualification and certification. See Certification; Qualification

6686-89

314-319148-153

180, 186, 188 191, 322-327322-327

80,81 215, 349

215-217, 349217-222

131

35-36,43 133

132,14035,131

36,131131-133

321,349 35, 131

94 95, 131-133, 14035-36

131, 132, 140, 349 314-319 142-144,313-314

91143-144146-147

230, 231 35-50,349

44-46 41-42

394141

35, 39-4124, 43-44

42-4344, 240

44-46 40-41

35-50, 34937, 38

3837, 3837, 38

measurement units and characteristics of light 37reference standards 37-38

3838-39 36

38

41 16,21,349

36,349

190 225 226,275,349

188 117148-153188-191

202-205, 349 100, 144, 349

30, 333332

250194-195

349272

102-10380-81

119259-260, 349

128-130267-268148-150

203, 349116-117

13208

9 217, 221-222 59

245-276263-269

265-266 98-99 98

137142, 143

364 / INDEX

Printed wiring board inspectionProceduresSee also Standards; specific applications

Process controltemperature indicators for ..See also Quality

Profile gages 325-326Profile method, of discontinuity classification .

Project MilkbottlePseudocolorPseudoisochromatic platesPsychophysics Pump inspectionPupilPyrometers

Qualification 8, 68, 181, 254See also Certification and qualification of personnel

Qualityquality assurance quality control sampling plans andSee also Process control; specific applications, e.g., Automotive inspection;

SteelQuantum theory 30, 31Quenching 220-221

RROVs (remotely operated vehicles). See Remote visual testingRadiance Radiant energy

blackbody radiation

graybody radiation luminous efficiencyradiometersselective radiatorsspectrumtheories ofSee also Brightness; Electromagnetic energy; Light

Radiant fluxRadiant intensity Radiation. See Radiant energyRadiators, selective 32, 33

See also Blackbody; GraybodyRadiography

of aircraft visual aspects ofof welds, vs. visual testing

RadiometersRailway bearing inspection Random partial sampling Rasters Rat's tooth principle Reactor pressure vessel inspectionReading borescopes 90ReceptorsRecommended Practice

API RP-7G or API RP-8B. See American Petroleum InstituteASNT Recommended Practice No. SNT-TC-1A. See American Society for

Nondestructive Testingvs. standards. See Standards

RecoveryRecrystallization Reference standards

for photometry for surface finishfor weld inspection

Red mud Reflectance

as illumination reflectivityreflectometersSee also Gloss measurement; Surface finish

ReflectionReflectometerReflectorRefraction . Regression, in automotive finish texture analysisRegulations

See also CodesRelative photometryRemote visual testing

pipes and vesselspositioning and transport remotely operated vehicles (ROVs)video technology See also Automated visual testing; Machine vision; Positioning and trans-

port systems.Remotely operated vehicles (ROVs). See Remote visual testingReplication 108-113, 350

cellulose acetatesilicone rubber

Reserve vision acuityResistance welding Resolution

acuity examinationactual required of eyefocusing of borescopesvs. magnification and aperture 80television/video 129-130, 141-142, 265See also Vision; Vision acuity

RetinaRetrospective borescopesReturn traces Rhodopsin Right angle borescopes Rigid borescopes

miniature borescopes See also Borescopes

Roberts gradientRoberts, L.G.Robotics. See Automated visual testing; Positioning and transport systemsRods 36, 57-58, 61, 351Roll forming machines 116-117Rolled-in dirt and scale .................. ....................................... .................. 226Rolled stock inspection 275-276Rolling 203-205Rust. See Corrosion

SI units for measurement x-xi, 37, 55, 351SNT-TC-1A. See American Society for Nondestructive Testing, Recom-

mended Practice No. SNT-TC-1ASaccadic movementSafety

blue hazard etchant useinfrared hazardslaser hazards

332-335 155-176

349 115-116

............ ............... ............ 234 7 307, 349

16, 3499-10, 65, 349

269-270 11, 57, 349

231

35053,350

53,228,279,350 160-162,351

350 30-34,350

31-3332,3333-34

24, 43-4432-33

16, 31, 94 30-33

350350

296-297 306-307

156 24, 43-44, 350

115160

139, 350199, 350263-269

36, 57-58, 61 178

211,217,218,350217,218,350 350

37-38 76-77, 240, 284-286

65-67, 69, 157-158, 164350

35054

74-75 42-43

350350

350350

285-287178,263

38,35068-69, 267-268, 350

148-153128-13.0

128-130, 149-150, 267-268137-140

108-111 111-113

65, 350250

35163-65,265

130 55,63-65

83-84

11, 22, 24-26, 57-61, 351 89-91

13958, 6188-9183-85

84, 85, 89

98 98

5822-26

26 120

2322-23

INDEX / 365

retina damageultraviolet hazards

Sampling plansScaleScanning

automated quality control of steelborescopes electron microscopy video screen formationweld inspection

Schindler, Rudolph Scoring (wear) Scotopic vision

See also VisionScuffingSeals, on tubular goodsSeamsSeason cracking Secondary emission in photomultiplier tubesSecondary focusing, borescopes Section steel inspection Sectioned borescopes Seizing Semiconductor components, inspection of

Semiconductors Sensitization Sequential lighting.. Shadow casting Sheet steel inspection Shielded metal arc welding Ship Structure Committee (SSC) Shop microscopes Short pass filters Shoulders Silicone rubber replication Simple magnifiers Sintered metals Sistering Skin lamination Slab surface discontinuity inspection Slides, vs. prints Sliding Slip marks Slivers, in steel Smoothing, in image processing Smutting Sobel, I Sobel gradient Society of Automotive Engineers (SAE) Socket weld Soda bottle inspection Solder joint inspection Solid state devices

image amplifiersimaging devicesphotodiodesSee also Photoconductive devices

SolidificationSolvent cleaning Sorting system, for automotive brake shoesSpalling fatigue Spatial filters Spatial light modulators Specifications. See StandardsSpecified partial samplingSpectral emissivitySpectrophotometry

SpectroradiometersSpectroscopySpectrum

colorelectromagnetic

Speedometer calibration Standard observer response curvesStandards

vs. codes, specifications, practicesperformance standards See also Military specifications; Reference standards; specific standards

under names of issuing organizationsStapelton measurement systemSteel

discontinuities mechanical properties phase transformations P flatness atness measurement surface purity measurement of strip surface inspection of cold strip torsion bars See also Pipe inspection; Weld inspection

Stefan-Boltzmann lawStepping pipe crawlersStereo

microscopes photogrammetry See also Vision, binocular

Sticker breaks, in steelStiffnessStorage vessels

borescopes 86, 148acceptance criteria 166-167, 168, 172, 175See also American Society of Mechanical Engineers, Boiler and Pressure

Vessel CodeStrainStrain replication Stress Stress concentration Stress corrosion cracking

replication analysis Stress fatigue, contact Stress rupture Stress-strain curves

bidirectional stresses and nonlinear

Stretcher strains, in steel String shot, in steel pipe Stringers Strip steel inspection

Strip surface purity measurement Stroboscopic lighting

for machine vision systems for video borescopes

Structural Welding Code-Steel (DI.1)See also American Welding Society

Structured lightingSubcase origin fatigueSubsurface fatigueSubmerged arc welding Substitution photometry Subsurface origin fatigue Subtraction, in image segmentationSurface, inspection of

comparatorsequipment forsurface finish

22,24-2623-24

160-162, 351 119, 226, 351

230-23889

108-110 139

674

200-201,276,35136,35/

200-201, 351322-323

275, 351209

41137

231, 23289, 90

200-201332-334

131214, 351

136133, 134, 138, 351

222-227250178

7994

326-327,351111-113292, 35/

195130227

230-231314

203-205191227

97,35112198

98-99178174105

248, 261-262, 331, 332

132 95-96 36, 131

214,215118-119288-291205, 351

96-97101-102

160 32

44, 240, 352

37, 43-44, 352 352 352 61-62, 94

16, 31, 94279

36,352352 178

65-66

241 193-243, 352

222-241194-197217-222234-236240-241

232-233, 234294

32 128-129

35278-79, 316

314-319

227 194-195

352110-111, 112

352197

209-210,214111, 112201-205

212194-195196-197195-196

227191

275-276232-234240-241

293,94

135-136 180, 186

93205, 352

35225038

202 100222-227

76-77 228-24176-77, 240, 284-286

366 / INDEX

surface roughness of steels 236-238Surface origin fatigue 203-205Surface preparation 118-119, 255Syrup bottle inspection 105-107

TTIG. See Tungsten inert gasTalbot's law 38, 40Tanks. See Storage vesselsTarasov etching techniqueTarget angleTelevision camera tubes Television systems

for glass stress detection for hot slab surface discontinuity inspectionfor nuclear reactor inspection remote closed circuit scanning (producing an image) video resolution See also Image processing; Remote visual testing

Temperature effect, on mechanical properties elevated temperature discontinuities

Temperature indicating materials applications

Tempering Tensile strength Tensile stress and stress corrosion cracking Textile mills

microscope use intemperature indicator use in

Texture, importance of surface Texture analysis (automotive finishes)Thermal fatigueThin ceramic inspection Thread inspection, in tubular goodsThreshholdingTime temperature transformation diagramsToilet paper inspection Tone pulse encoding Tool and die shops, borescope applicationsTool markTracesTransport systems. See Positioning and transport systemsTrolandTubing string Tubular goods inspection

borescopesgradeshoisting equipmentnew pipe thread inspection used drill pipeSee also Petroleum and chemical industry applications; Pipe inspection

Tungsten inert gas (TIC) welding 251-252, 353Tungsten lamps.. 33Turbine blade inspection 320

UUltrasonic testing

visual aspects of Ultraviolet radiation

borescopes hazardsfor machine vision systemsradiometers

Undercut 253, 260-261, 353Underwater visual testing 63, 66-67, 68, 142, 268Unified theory 30-31Uniform corrosion 208Units of measure x-xi, 37, 55

VVT-1,-2,-3 tests 179-180, 255, 264, 266

See also American Society of Mechanical Engineers, Boiler and PressureVessel Code

Valve inspection 270-272, 273Vaporproof borescopes 85,353Vessels. See Pressure and storage vesselsVideo borescopes

black and white vs. color imaginglightingmagnification factor for remote visual testing of pipessize limitations video taping See also Video components

Video componentscablescameras digitizers . nuclear reactor inspectionrecorders.. resolution systems tube vs. chip type See also Charge coupled devices; Image orthicons; Image processing;

Machine vision systems; Positioning and transport systems; Remotevisual testing; Resolution; Solid state devices; Television systems; Videoborescopes; Vidicon tubes

Vidicon tubesVigilance decrementVision

acuity. See Vision acuityaging and.anatomy of angle of viewing attitude effects (of observer)background noise binocularblind spot brightness contrast color vision. See Color visiondifferentiation 9distancefatigue effects health effects light and dark adapted mesopic state observer differences perception peripheral physiology of spectral response visual efficiency Weber's law and See also Machine vision; Resolution; Vision acuity

Vision acuityexaminations

color vision for underwater inspection

far vision

/- 201, 352 13-14, 64-65

139-140132-133

321230

267-269139, 141

139141-142

Video components195,196210-214114-115115-117

218, 220-221196,197209-210

7911654

279-288 212

320, 321190, 322-327

100, 352218-219, 352

105, 106145-147 86

352 139, 352

xi, 353322,353

148-153, 188, 190-191 188

191189-190

190,322-327 190-191

186, 191309-312

16, 21, 34, 35385

23-24 9324, 353

91, 134-139, 293135

135-136136-137148-153

137138

353 150

68-69, 94-95, 129-130, 150-151, 26892

267-269267-269

129-130,26568,92

129

94-95,132,133,14065,353

353

57, 5911, 36, 57-58, 61

13-14, 55-60, 87-90, 353 60

69,353 60-61, 314-316

11, 58, 6162

56, 6159

59-603636

59-60, 62, 65, 6958-60, 353

13-14, 589-11, 60-61

33-3465, 353

10

10-12, 35312-13, 55, 254

14-2063, 66-67, 68

10, 12

INDEX / 367

farsightedness 11kineticnear vision nearsightedness neural acuity weld inspection and See also Color vision; Resolution; Vision

Visual task photometersVisual testing

angle of viewing design considerationsenvironmental factorsremoteand nondestructive testingsafetytest object effects visual aspects of other test methodsSee also numerous specific applications and aspects of visual testing-e.g.,

Aircraft inspection; Lighting; Quality; Safety; VisioiMVision acuity;Weld inspection

Voids 353in composites 328, 329See also Porosity

Wafer ceramic inspection Wappler, Reinhold Water circulation pump inspectionWater-cooled borescopesWaterproof borescopesWave theoryWear

replication analysis abrasive adhesive erosive fretting gouging grinding oxidation in pumps

Weber's law Weld inspection

acceptance criteria (weld profile/size)butt welds fillet welds flange welds pipe welds pipelines power boilers pressure and storage vessels socket welds weld discontinuities

after welding before welding

chemical etchants for 124codes and standards 178-179

See also Certification and qualification of personnel; names oforganizations that issue standards

discontinuitiesduring welding gaging of weld profile/sizeinspection plan, typical marking repair welds personnel qualification reference standards remotesampling plans underwater vision acuity visual performance verificationvisual vs. radiographic 156See also American Welding Society; Remote visual testing; Welding

Welder's flash Welding

arc welding basic joint configurations brazingcomplete penetration effect on stress corrosion cracking fabrication processes gas metal arc welding gas tungsten arc welding metal structure in.. oxyfuel gas welding partial pentetration resistance welding shielded metal arc welding soldering ............... ............. ........ temperature indicator applications types/processes typical weld componentsSee also American Welding Society;

White lightWide field instruments Wien radiation law Wiring board inspection

Wolf, Georg

Work hardening 211, 217, 218, 354Working standard 354

YYield point

194

Yield strength 194,196elevated temperature and

210-211

Zoom lenses 313-314

6410, 12, 69

1169

63-65, 254

42vii, 2-3, 52, 353

13-14, 55-60, 87-90 53 54-56

68-69, 148-1532, iv, vii

22-2674-75

304-312

320, 3214

270 86 85 30-31 197-201, 213, 354

112198

200, 354198

201, 354200199

201, 354270

10156, 253-254

163-176, 253-262165-171, 247, 248

164, 167174

167-176172-175164-166

166-167, 172174

163-164,259-261157-158,256

156, 256

163-164, 172, 256-262156-157,256

255, 2.56-259, 354179158

68,25465-67, 69, 157

152-153160-162

63, 66-67, 68 63-68

23, 354

250247

249-250, 261-261247-248

210253

252-2532.51-252214-215

250248250

250-251................................. 248, 262, 331

, 249 254

117248

Weld inspection 354

77-79,8532

332-3354

nondestructive testing handbook vol.8 (second edition)

Documents

optical testing

scope of visual testing

acoustic emission testing

visual committee

handbook development

materials science

technical information

technical societies