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NondestructiveTesting

TableofContents

ChapterNo:

NameoftheChapter PageNo

1 Coursedailyschedule 1

2 CourseContents 2

3 IntroductionNDTprocesses&theirUses 311

4 IdentificationofweldDiscontinuities 1220

5 PenetrantTesting 2130

6 MagneticParticleTesting 3148

7 UltrasonicTesting 4960

8 RadiographicTesting 6177

9 EddyCurrentTesting 7880

10 ComparisonandSelectionofNDT Methods 81

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ChapterI

INTRODUCTION

NondestructiveTesting

ThefieldofNondestructiveTesting(NDT) isaverybroad, thatplaysacritical role inassuringthatstructuralcomponentsandsystemsperformtheirfunctioninareliableandcosteffectivefashion.NDTtechniciansandengineersdefineandimplementteststhatlocate and characterize material conditions and flaws that might otherwise causeseriousaccidentssuchas,planestocrash,reactorstofail,trainstoderail,pipelinestoburst,andavarietyoftroublingevents.

Thesetestsareperformedinamannerthatdoesnotaffectthefutureusefulnessoftheobjectormaterial.Inotherwords,NDTallowspartsandmaterialstobeinspectedandevaluatedwithoutdamagingthem.Becauseitallowsinspectionwithoutinterferingwithaproduct's finaluse,NDTprovidesanexcellentbalancebetweenqualitycontrolandcosteffectiveness.

NondestructiveEvaluation

NondestructiveEvaluation(NDE)isatermthatisoftenusedinterchangeablywithNDT.However,technically,NDEisusedtodescribemeasurementsthataremorequantitativeinnature.Forexample,aNDEmethodwouldnotonlylocateadefect,butitwouldalsobe used to measure something about that defect such as its size, shape, andorientation. NDE may be used to determine material properties such as fracturetoughness,ductility,conductivityandotherphysicalcharacteristics.

UsesofNDE

FlawDetectionandEvaluation LeakDetection,LocationDetermination DimensionalMeasurements StructureandMicrostructureCharacterization EstimationofMechanicalandPhysicalProperties Stress(Strain)andDynamicResponseMeasurements MaterialSortingandChemicalCompositionDetermination

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BackgroundonNondestructiveTesting(NDT)

Nondestructive testing has been practiced for many decades. One of the earliestapplicationswasthedetectionofsurfacecracksinrailcarwheelsandaxles.Thepartsweredippedinoil,thencleanedanddustedwithapowder.Whenacrackwaspresent,theoilwouldseepfromthedefectandwettheoilprovidingvisualindicationindicatingthat the component was flawed. This eventually led to oils that were specificallyformulatedforperformingtheseandotherinspectionsandtheseinspectiontechniquesarenowcalledpenetranttesting.

Xrayswerediscoveredin1895byWilhelmConradRoentgen(18451923)whowasaProfessor at Wuerzburg University in Germany. Soon after his discovery, Roentgenproduced the first industrial radiographwhenhe imageda set ofweights in a box toshow his colleagues. Other electronic inspection techniques such as ultrasonic andeddy current testing started with the initial rapid developments in instrumentationspurredbytechnologicaladvancesandsubsequentdefenseandspaceeffortsfollowingWorldWar II. In the early days, the primary purpose was the detection of defects.Criticalpartswereproducedwitha "safe life"design,andwere intended tobedefectfree during their useful life. The detection of defects was automatically a cause forremovalofthecomponentfromservice.

Thecontinued improvementof inspection technology, inparticular theability todetectsmallerandsmaller flaws, led tomoreandmorepartsbeingrejected.At this time thedisciplineoffracturemechanicsemerged,whichenabledonetopredictwhetheracrackof a given size would fail under a particular load if a particular material property orfracture toughness, were known. Other laws were developed to predict the rate ofgrowthofcracksundercyclicloading(fatigue).Withtheadventofthesetools,itbecamepossible to accept structures containing defects if the sizes of those defects wereknown. This formed the basis for a new design philosophy called "damage tolerantdesigns."Components having knowndefects could continue to beused as longas itcouldbeestablishedthatthosedefectswouldnotgrowtoacriticalsizethatwouldresultin catastrophic failure. A new challenge was thus presented to the nondestructivetestingcommunity.

Mere detection of flaws was not enough. One needed to also obtain quantitativeinformationabout flawsize toserveasan input to fracturemechanicscalculations topredicttheremaininglifeofacomponent.Theseneeds,ledtothecreationofanumberofresearchprogramsaroundtheworldandtheemergenceofnondestructiveevaluation(NDE)asanewdiscipline.

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NDT/NDEMethods

The list of NDT methods that can be used to inspect components and makemeasurementsislargeandcontinuestogrow.Researcherscontinuetofindnewwaysof applying physics and other scientific disciplines to develop better NDT methods.However, there are six NDTmethods that are usedmost often. Thesemethods areVisual Inspection, Penetrant Testing, Magnetic Particle Testing, Electromagnetic orEddyCurrentTesting,Radiography,andUltrasonicTesting.

VisualandOpticalTesting(VT)

Visual inspectioninvolvesusingan inspector'seyes to look fordefects.The inspectormayalsousespecialtoolssuchasmagnifyingglasses,mirrors,orborescopestogainaccessandmorecloselyinspectthesubjectarea.Visualexaminersfollowproceduresthatrangefmsimpletoverycomplex.

PenetrantTesting(PT)

Test objects are coated with visible or fluorescent dye solution. Excess dye is thenremoved from thesurface,andadeveloper isapplied.Thedeveloperactsasblotter,drawing trappedpenetrantoutof imperfectionsopento thesurface.Withvisibledyes,vividcolorcontrastsbetweenthepenetrantanddevelopermake"bleedout"easytosee.With fluorescentdyes,ultraviolet light isused tomakethebleedout fluorescebrightly,thusallowingimperfectionstobereadilyseen.

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MagneticParticleTesting(MT)

Thismethod isaccomplishedby inducingamagnetic field ina ferromagneticmaterialand then dusting the surface with iron particles (either dry or suspended in liquid).Surfaceandnearsurfaceimperfectionsdistort themagnetic fieldandconcentrateironparticlesnearimperfections,previewingavisualindicationoftheflaw.

ElectromagneticTesting(ET)orEddyCurrentTesting

Electrical currents are generated in a conductive material by an induced alternatingmagnetic field This electrical currents is called eddy currents because they flow incirclesat and just below the surfaceof thematerial. Interruptions in the flowof eddycurrents, causedby imperfections, dimensional changes, or changes in thematerial'sconductiveandpermeabilityproperties,aredetected.

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Radiography(RT)

Radiography involves the use of penetrating gamma or Xradiation to examine partsandproductsforimperfections.AnXraygeneratororradioactiveisotopeisusedasasourceofradiation.Radiationisdirectedthroughapartandontofilmorotherimagingmedia. The resulting radiograph shows thedimensional featuresof the part. Possibleimperfections are indicated as density changes on the film in the samemanner as amedicalXrayshowsbrokenbones.

UltrasonicTesting(UT)

Ultrasonics use transmissionofhighfrequency soundwaves into amaterial to detectimperfections or to locate changes in material properties. The most commonly usedultrasonictestingtechniqueispulseecho,whereinsoundisintroducedintoatestobjectandreflections(echoes)arereturned toa receiver from internal imperfectionsor fromthepart'sgeometricalsurfaces

.

crack

0 2 4 6 8 10

Initialpulse

Crackecho

Backsurfaceecho

Soundwaves

Xrayfilm

Source

Rays

ObjectwithdefectFilm

DefectImage Filmwithimage

Probe

Couplant

PlateScreen

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AcousticEmissionTesting(AE)

Whenasolidmaterialisstressed,imperfectionswithinthematerialemitshortburstsofacousticenergycalled"emissions."Asinultrasonictesting,acousticemissionscanbedetectedbyspecialreceivers.Emissionsourcescanbeevaluatedthroughthestudyoftheirintensity,rate,andlocation.

LeakTesting(LT)

Severaltechniquesareusedtodetectandlocateleaksinpressurecontainmentparts,pressure vessels, and structures. Leaks canbedetectedbyusing electronic listeningdevices,pressuregaugemeasurements,liquidandgaspenetranttechniques,and/orasimplesoapbubbletest.

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TestMethod

UT Xray EddyCurrent

MPI LPT

Capitalcost Mediumtohigh

High Lowtomedium

Medium Low

Consumablecost

Verylow High Low Medium Medium

Timeofresults

Immediate Delayed Immediate Shortdelay

Shortdelay

Effectofgeometry

Important Important Important NottooImportant

NottooImportant

Accessproblems

Important Important Important Important Important

Typeofdefect

Internal Most External ExternalNearSurface

Surfacebreaking

Relativesensitivity

High Medium High Low Low

Operatorskill

High High Medium Low Low

Operatortraining

Important Important Important Important NotImportant

Trainingneeds

High High Medium Low Low

Portabilityofequipment

High Low Hightomedium

Hightomedium

High

Capabilities Thicknessgauging,compositiontesting

Thicknessgauging

Thicknessgauging,gradesorting

Defectsonly

Defectsonly

The Relative Uses and Merits of Various NDT Methods

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Table1ReferenceGuidetoMajorMethodsfortheNondestructiveExaminationofWeldsInspectionMethod

EquipmentRequired

EnablesDetectiortof

Advantages Limitations Remarks

Visual MagnifyingglassWeldsizinggaugePocketruleStraightedgeWorkmanshipstandards

Surfaceflawscracks,porosity,unfilledcraters,slaginclusionsWarpage,underwelding,overwelding,poorlyformedbeads,misalignments,improperfitup

Lowcost.Canbeappliedwhileworkisinprocess,permittingcorrectionoffaults.Givesindicationofincorrectprocedures.

Applicabletosurfacedefectsonly.Providesnopermanentrecord.

Shouldalwaysbetheprimarymethodofinspection,nomatterwhatothertechniquesarerequired.Istheonly"productive"typeofinspection.Isthenecessaryfunctionofeveryonewhoinanywaycontributestothemakingoftheweld.

Radiographic CommercialXrayorgammaunitsmadeespeciallyforinspectingwelds,castingsandforgings.Filmandprocessingfacilities.Fluoroscopicviewingequipment.

Interiormacroscopicflawscracks,porosity,blowholes,nonmetallicinclusions,incompleterootpenetration,undercutting,icicles,andburnthrough.

Whentheindicationsarerecordedonfilm,givesapermanentrecord.Whenviewedonafluoroscopicscreen,alowcostmethodofinternalinspection

Requiresskillinchoosinganglesofexposure,operatingequipment,andinterpretingindications.Requiressafetyprecautions.Notgenerallysuitableforfilletweldinspection.

Xrayinspectionisrequiredbymanycodesandspecifications.Usefulinqualificationofweldersandweldingprocesses.Becauseofcost,itsuseshouldbelimitedtothoseareaswhereothermethodswillnotprovidetheassurancerequired.

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MagneticParticle

Specialcommercialequipment.Magneticpowders dryorwetformmaybefluorescentforviewingunderultravioletlight.

Excellentfordetectingsurfacediscontinuitiesespeciallysurfacecracks.

Simplertousethanradiographicinspection.Permitscontrolledsensitivity.Relativelylowcostmethod.

Applicabletoferromagneticmaterialsonly.Requiresskillininterpretationofindicationsandrecognitionofirrelevantpatterns.Difficulttouseonroughsurfaces.

Elongateddefectsparalleltothemagneticfieldmaynotgivepatternforthisreasonthefieldshouldbeappliedfromtwodirectionsatornearrightanglestoeachother.

LiquidPenetrant

Commercialkitscontainingfluorescentordyepenetrantsanddevelopers.Applicationequipmentforthedeveloper.Asourceofultravioletlight iffluorescentmethodisused.

Surfacecracksnotreadilyvisibletotheunaidedeye.Excellentforlocatingleaksinweldments.

Applicabletomagneticandnonmagneticmaterials.Easytouse.Lowcost.

Onlysurfacedefectsaredetectable.Cannotbeusedeffectivelyonhotassemblies.

Inthinwalledvesselswillrevealleaksnotordinarilylocatedbyusualairtests.irrelevantsurfaceconditions(smoke,slag)maygivemisleadingindications.

Ultrasonic Specialcommercialequipment,eitherofthepulseechoortransmissiontype.StandardreferencepatternsforinterpretationofRForvideopatterns.

Surfaceandsubsurfaceflawsincludingthosetoosmalltobedetectedbyothermethods.Especiallyfordetectingsubsurfacelaminationlikedefects.

Verysensitive.Permitsprobingofjointsinaccessibletoradiography.

Requireshighdegreeofskillininterpretingpulseechopatterns.Permanentrecordisnotreadilyobtained.

Pulseechoequipmentishighlydevelopedforweldinspectionpurposes.Thetransmissiontypeequipmentsimplifiespatterninterpretationwhereitisapplicable.

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ChapterII

IDENTIFICATIONOFWELDDISCONTINUITIES

Discontinuities are interruptions in the typical structure of a material. These interruptions may occur in the base metal, weld material or "heat affected" zones. Discontinuities, which do not meet the requirements of the codes or specification used to invoke and control an inspection, are referred to as defects.

General Welding Discontinuities The following discontinuities are typical of all types of welding. Cracks: Crack is tight linear separations of metal that can be very short to very long indications. Cracks are grouped as hot or cold cracks. Hot cracks usually occur as the metal solidifies at elevated temperatures. Cold cracks occur after the metal has cooled to ambient temperatures ( delayed cracks). Cracks can be detected in a radiograph only when they are propagating in a direction that produces a change in thickness that is parallel to the x-ray beam. Cracks will appear as jagged and often very faint irregular lines. Cracks can sometimes appear as "tails" on inclusions or porosity.

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Lack of Fusion: Lack of fusion (Cold Lap) is a condition where the weld filler metal does not properly fuse with the base metal or the previous weld pass material (inter pass cold lap). The arc does not melt the base metal sufficiently and causes the slightly molten puddle to flow into base material without bonding.

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Porosity: Porosity is the result of gas entrapment in the solidifying metal. Porosity can take many shapes on a radiograph but often appears as dark round or irregular spots or specks appearing singularly, in clusters or rows. Sometimes porosity is elongated and may have the appearance of having a tail This is the result of gas attempting to escape while the metal is still in a liquid state and is called wormhole porosity. All porosity is a void in the material it will have a radiographic density more than the surrounding area.

Cluster porosity: Cluster porosity is caused when flux coated electrodes are contaminated with moisture. The moisture turns into gases when heated and becomes trapped in the weld during the welding process. Cluster porosity appear just like regular porosity in the radiograph but the indications will be grouped close together.

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Slag inclusions: Slag inclusions are nonmetallic solid material entrapped in weld metal or between weld and base metal. In a radiograph, dark, jagged asymmetrical shapes within the weld or along the weld joint areas are indicative of slag inclusions.

Incomplete penetration (IP): Incomplete penetration (IP) or lack of penetration (LOP) occurs when the weld metal fails to penetrate the joint. It is one of the most objectionable weld discontinuities. Lack of penetration allows a natural stress riser from which a crack may propagate. The appearance on a radiograph is a dark area with well-defined, straight edges that follows the land or root face down the center of the weldment.

Root concavity:

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Root or Internal concavity or suck back is condition where the weld metal has contracted as it cools and has been drawn up into the root of the weld. On a radiograph it looks similar to lack of penetration but the line has irregular edges and it is often quite wide in the center of the weld image.

Internal or root undercut: Internal or root undercut is an erosion of the base metal next to the root of the weld. In the radiographic image it appears as a dark irregular line offset from the centerline of the weldment. Undercutting is not as straight edged as LOP because it does not follow a ground edge.

External or crown undercut:

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External or crown undercut is an erosion of the base metal next to the crown of the weld. In the radiograph, it appears as a dark irregular line along the outside edge of the weld area.

Offset or mismatch: Offset or mismatch are terms associated with a condition where two pieces being welded together are not properly aligned. The radiographic image is a noticeable difference in density between the two pieces. The difference in density is caused by the difference in material thickness. The dark, straight line is caused by failure of the weld metal to fuse with the land area.

Inadequate weld reinforcement:

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Inadequate weld reinforcement is an area of a weld where the thickness of weld metal deposited is less than the thickness of the base material. It is very easy to determine by radiograph if the weld has inadequate reinforcement, because the image density in the area of suspected inadequacy will be more (darker) than the image density of the surrounding base material.

Excess weld reinforcement : Excess weld reinforcement is an area of a weld that has weld metal added in excess of that specified by engineering drawings and codes. The appearance on a radiograph is a localized, lighter area in the weld. A visual inspection will easily determine if the weld reinforcement is in excess of that specified by the engineering requirements.

Discontinuities in TIG welds

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The following discontinuities are peculiar to the TIG welding process. These discontinuities occur in most metals welded by the process including aluminum and stainless steels. The TIG method of welding produces a clean homogeneous weld which when radiographed is easily interpreted.

Tungsten inclusions. Tungsten is a brittle and inherently dense material used in the electrode in tungsten inert gas ( TIG ) welding. If improper welding procedures are used, tungsten may be entrapped in the weld. Radiographically, tungsten is denser than aluminum or steel; therefore, it shows as a lighter area with a distinct outline on the radiograph.

Oxide inclusions: Oxide inclusions are usually visible on the surface of material being welded (especially aluminum). Oxide inclusions are less dense than the surrounding materials and, therefore, appear as dark irregularly shaped discontinuities in the radiograph.

Discontinuities in Gas Metal Arc Welds (GMAW) The following discontinuities are most commonly found in GMAW welds.

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Whiskers: Whiskers are short lengths of weld electrode wire, visible on the top or bottom surface of the weld or contained within the weld. On a radiograph they appear as light, "wire like" indications.

Burn-Through: Burn-Through results when too much heat causes excessive weld metal to penetrate the weld zone. Often lumps of metal sag through the weld creating a thick globular condition on the back of the weld. These globs of metal are referred to as icicles. On a radiograph, burn through appears as dark spots, which are often surrounded by light globular areas (icicles).

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ChapterIII

PENETRANTINSPECTION

Introduction

Liquidpenetrationinspectionisamethodthat isusedtorevealsurfacebreakingflawsbybleedoutofacoloredorfluorescentdyefromtheflaw.Thetechniqueisbasedontheabilityofaliquidtobedrawnintoa"clean"surfacebreakingflawbycapillaryaction.After a period of time called the "dwell," excess surface penetrant is removed and adeveloper is applied. This acts as a "blotter." It draws thepenetrant from the flaw torevealitspresence.

Colored(contrast)penetrantsrequiregoodwhitelightwhilefluorescentpenetrantsneedtobeviwedindarkenedconditionswithanultraviolet"blacklight".

A very early surface inspection technique involved the rubbing of carbon black onglazedpottery,wherebythecarbonblackwouldsettleinsurfacecracksrenderingthemvisible. Later it became the practice in railway workshops to examine iron and steelcomponents by the "oil and whiting" method. In this method, heavy oil commonlyavailable in railway workshops was diluted with kerosene in large tanks so thatlocomotive parts such as wheels could be submerged. After removal and carefulcleaning,thesurfacewasthencoatedwithafinesuspensionofchalkinalcoholsothatawhitesurfacelayerwasformedoncethealcoholhadevaporated.Theobjectwasthenvibratedandstrokedwithahammer,causing the residualoil inanysurfacecracks toseepoutandstainthewhitecoating.Thismethodwasinusefromthelatterpartofthe19thcenturythroughtoapproximately1940, when the magnetic particle method was introduced and found to be moresensitive for the ferromagnetic iron and steels. Penetrant Inspection Improves theDetectabilityofFlaws

The advantage that a liquid penetrant inspection (LPI) offers over an unaided visualinspection is that itmakesdefectseasier tosee for the inspector.Therearebasicallytwowaysthatapenetrantinspectionprocessmakesflawsmoreeasilyseen.First,LPIproducesaflawindicationthatismuchlargerandeasierfortheeyetodetectthantheflawitself.Manyflawsaresosmallornarrowthattheyareundetectablebytheunaidedeye.

ThesecondwaythatLPIimprovesthedetectabilityofaflawisthat itproducesaflawindicationwithahighlevelofcontrastbetweentheindicationandthebackgroundwhichalsohelpstomaketheindicationmoreeasilyseen.Whena

visible dye penetrant inspection is performed, the penetrantmaterials are formulatedusingabrightreddyethatprovidesforahighlevelofcontrast

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betweenthewhitedeveloperthatservesasabackgroundaswellastopullthetrappedpenetrant from the flaw. When a fluorescent penetrant inspection is performed, thepenetrantmaterialsareformulatedtoglowbrightlyandtogiveofflightatawavelengththattheeyeismostsensitivetounderdimlightingconditions.

BasicProcessingStepsofaLiquidPenetrantInspection

1. Surface Preparation: One of the most critical steps of a liquid penetrantinspection is the surface preparation. The surfacemust be free of oil, grease,water,orothercontaminantsthatmaypreventpenetrantfromenteringflaws.Thesample may also require etching ifmechanical operations such asmachining,sanding, or grit blasting have been performed. These and other mechanicaloperationscansmearthesurfaceofthesample,thusclosingthedefects.

2. PenetrantApplication:Oncethesurfacehasbeenthoroughlycleanedanddried,the penetrant material is applied on the surface by spraying, brushing, orimmersingthepartsinapenetrantbath.

3. PenetrantDwell:Thepenetrantisleftonthesurfaceforasufficienttimetoallowas much penetrant as possible to be drawn from or to seep into a defect.Penetrantdwelltimeisthetotaltimethatthepenetrantisincontactwiththepartsurface.Dwelltimesareusuallyrecommendedbythe

penetrant producers or required by the specification being followed. The timesvarydependingon theapplication,penetrantmaterialsused, thematerialbeinginspected,andthetypeofdefectbeinginspected.Minimumdwelltimestypicallyrange from 5 to 60 minutes. Generally, there is no harm in using a longer

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penetrant dwell time as long as the penetrant is not allowed to dry. The idealdwelltimeisoftendeterminedbyexperimentationandisoftenveryspecifictoaparticularapplication.

4 Excess Penetrant Removal: This is a most delicate part of the inspectionprocedurebecause theexcesspenetrantmustberemoved from thesurfaceofthe sample while removing as little penetrant as possible from defects.Dependingonthepenetrantsystemused,thisstepmayinvolvecleaningwithasolvent, direct rinsing with water, or first treated with an emulsifier and thenrinsingwithwater.

5 DeveloperApplication:Athinlayerofdeveloperisthenappliedtothesampleto drawpenetrant trapped in flawsback to the surfacewhere itwill be visible.Developers come in a variety of forms that may be applied by dusting (drypowdered),dipping,orspraying(wetdevelopers).

6 IndicationDevelopment:Thedeveloperisallowedtostandonthepartsurfaceforaperiodoftimesufficienttopermittheextractionofthetrappedpenetrantoutofanysurfaceflaws.Thisdevelopmenttimeisusuallyaminimumof10minutesandsignificantlylongertimesmaybenecessaryfortightcracks.

7 Inspection: Inspection is then performed under appropriate lighting to detectindicationsfromanyflawsthatmaybepresent.

8 Clean Surface: The final step in the process is to thoroughly clean the partsurfacetoremovethedeveloperfromthepartsthatwerefoundtobeacceptable.

PenetrantTestingMaterials

The penetrantmaterials used today aremuchmore sophisticated than the keroseneandwhitingfirstusedbyrailroadinspectorsneartheturnofthe20thcentury.Today'spenetrants are carefully formulated to produce the level of sensitivity desired by theinspector.

1 Penetrant: Penetrant materials are classified in the various industry andgovernmentspecificationsbytheirphysicalcharacteristicsandtheirperformance Penetrant materials come in two basic types. These types are listed below:

Type 1 - Fluorescent Penetrants Type2VisiblePenetrants

Fluorescentpenetrantscontainadyeorseveraldyes that fluorescewhenexposed toultraviolet radiation. Visible penetrants contain a red dye that provides high contrastagainst the white developer background. Fluorescent penetrant systems are moresensitive than visible penetrant systemsbecause the eye is drawn to the glowof thefluorescingindication.However,visiblepenetrantsdonotrequireadarkenedareaandan ultraviolet light in order to make an inspection. Visible penetrants are also lessvulnerable to contamination from things such as cleaning fluid that can significantlyreducethestrengthofafluorescentindication.

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Penetrantsarethenclassifiedbythemethodusedtoremovetheexcesspenetrantfromthepart.Thefourmethodsarelistedbelow:

MethodAWaterWashable MethodBPostEmulsifiable,Lipophilic MethodCSolventRemovable MethodDPostEmulsifiable,Hydrophilic

Waterwashable(MethodA)penetrantscanberemovedfromthepartbyrinsingwithwateralone.Thesepenetrantscontainsomeemulsifyingagent(detergent)thatmakesitpossibletowashthepenetrantfromthepartsurfacewithwateralone.Waterwashablepenetrantsaresometimesreferredtoasselfemulsifyingsystems.Post emulsifiable penetrants come in two varieties, lipophilic and hydrophilic. In postemulsifiers,lipophilicsystems(MethodB),thepenetrantisoilsolubleandinteractswiththe oilbased emulsifier to make removal possible. Post emulsifiable, hydrophilicsystems(MethodD),useanemulsifierthatisawatersolubledetergentwhichliftstheexcess penetrant from the surface of the part with awater wash. Solvent removablepenetrantsrequiretheuseofasolventtoremovethepenetrantfromthepart.

PropertiesofgoodPenetrantToperformwell,apenetrantmustpossessfollowingimportantcharacteristics.

spread easily over the surface of the material being inspected to providecompleteandevencoverage.

bedrawnintosurfacebreakingdefectsbycapillaryaction. remaininthedefectbutremoveeasilyfromthesurfaceofthepart. remainfluidsoitcanbedrawnbacktothesurfaceofthepartthroughthedrying

anddevelopingsteps. behighlyvisibleorfluorescebrightlytoproduceeasytoseeindications. mustnotbeharmfultothematerialbeingtestedortheinspector.

2Emulsifiers: Whenremovalofthepenetrantfromthedefectduetooverwashingof the part is a concern, a post emulsifiable penetrant system can be used. Postemulsifiablepenetrantsrequireaseparateemulsifier tobreak thepenetrantdownandmake it water washable. Most penetrant inspection specifications classify penetrantsystemsintofourmethodsofexcesspenetrantremoval.Thesearelistedbelow:

1. MethodA:WaterWashable2. MethodB:PostEmulsifiable,Lipophilic

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3. MethodC:SolventRemovable4. MethodD:PostEmulsifiable,Hydrophilic

Method C relies on a solvent cleaner to remove the penetrant from the part beinginspected.MethodAhasemulsifiersbuiltintothepenetrantliquidthatmakesitpossibletoremovetheexcesspenetrantwithasimplewaterwash.MethodBandDpenetrantsrequireanadditionalprocessingstepwhereaseparateemulsificationagentisappliedto make the excess penetrant more removable with a water wash. Lipophilicemulsification systemsare oilbasedmaterials that are supplied in readytouse form.Hydrophilic systems are waterbased and supplied as a concentrate that must bedilutedwithwaterpriortouse .Lipophilicemulsifiers(MethodB)wereintroducedinthelate1950'sandworkwithbothachemicalandmechanicalaction.After theemulsifierhascoated thesurfaceof theobject,mechanicalactionstarts to removesomeof theexcesspenetrantasthemixturedrainsfromthepart.Duringtheemulsificationtime,theemulsifier diffuses into the remaining penetrant and the resulting mixture is easilyremovedwithawaterspray.Hydrophilicemulsifiers(MethodD)alsoremovetheexcesspenetrantwithmechanicaland chemical action but the action is different because no diffusion takes place.Hydrophilic emulsifiers are basically detergents that contain solvents and surfactants.The hydrophilic emulsifier breaksup the penetrant into small quantities and preventsthesepiecesfromrecombiningorreattachingtothesurfaceofthepart.Themechanicalaction of the rinse water removes the displaced penetrant from the part and causesfreshremovertocontactandliftnewlyexposedpenetrantfromthesurface.

Thehydrophilicpostemulsifiablemethod(MethodD)wasintroducedinthemid1970'sandsinceit ismoresensitive than thelipophilicpostemulsifiablemethod ithasmadethe latermethod virtually obsolete. Themajor advantage of hydrophilic emulsifiers isthat they are less sensitive to variation in the contact and removal time. Whileemulsificationtimeshouldbecontrolledascloselyaspossible,avariationofoneminuteormoreinthecontacttimewillhavelittleeffectonflawdetectabilitywhenahydrophilicemulsifier is used. However, a variation of as little as 15 to 30 seconds can have asignificanteffectwhenalipophilicsystemisused.

3DevelopersTheroleofthedeveloperistopullthetrappedpenetrantmaterialoutofdefectsandtospreadthedeveloperoutonthesurfaceofthepartsoitcanbeseenbyaninspector.Thefinedeveloperparticlesbothreflectandrefracttheincidentultravioletlight,allowingmore of it to interact with the penetrant, causing more efficient fluorescence. Thedeveloper also allowsmore light to be emitted through the samemechanism.This iswhy indicationsarebrighter than the penetrant itself underUV light.Another functionthat somedevelopers performs is to createawhite background so there is a greaterdegreeofcontrastbetweentheindicationandthesurroundingbackground.

DeveloperForms

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The AMS 2644 and MilI25135 classify developers into six standard forms. Theseformsarelistedbelow:

1. FormaDryPowder2. FormbWaterSoluble3. FormcWaterSuspendible4. FormdNonaqueousType1Fluorescent(SolventBased)5. FormeNonaqueousType2VisibleDye(SolventBased)

Thedeveloper classifications are based on themethod that the developer is applied.Thedevelopercanbeappliedasadrypowder,ordissolvedorsuspended ina liquidcarrier.Eachofthedeveloperformshasadvantagesanddisadvantages.

A)DryPowderDry powder developer is generally considered to be the least sensitive but it isinexpensivetouseandeasytoapply.Drydevelopersarewhite,fluffypowdersthatcanbe applied to a thoroughly dry surface in a number of ways. The developer can beappliedbydippingparts inacontainerofdeveloper,orbyusingapuffer todustpartswith thedeveloper.Partscanalsobeplaced inadustcabinetwhere thedeveloperisblown around and allowed to settle on the part. Electrostatic powder spray guns arealso available to apply the developer. The goal is to allow the developer to come incontactwiththewholeinspectionarea.

Unlessthepartiselectrostaticallycharged,thepowderwillonlyadheretoareaswheretrapped penetrant has wet the surface of the part. The penetrant will try to wet thesurfaceof thepenetrantparticleand fill thevoidsbetween theparticles,whichbringsmore penetrant to the surface of the part where it can be seen. Since dry powderdevelopersonlysticktothepartwherepenetrantispresent,thedrydeveloperdoesnotprovide a uniform white background as the other forms of developers do. Having auniform light background is very important for a visible inspection to beeffective andsincedrydevelopersdonotprovideone,theyareseldomusedforvisibleinspections.When a dry developer is used, indications tend to stay bright and sharp since thepenetranthasalimitedamountofroomtospread.

B) - Water SolubleAsthenameimplies,watersolubledevelopersconsistofagroupofchemicalsthataredissolvedinwaterandformadeveloperlayerwhenthewaterisevaporatedaway.Thebestmethodforapplyingwatersolubledevelopersisbysprayingitonthepart.Thepartcan be wet or dry. Dipping, pouring, or brushing the solution on to the surface issometimes used but these methods are less desirable. Aqueous developers containwettingagentsthatcausethesolutiontofunctionmuchlikedilutehydrophilicemulsifierand can lead to additional removal of entrapped penetrant. Drying is achieved byplacingthewetbutwelldrained

partinarecalculatingwarmairdryerwiththetemperatureheldbetween70and75F.Ifthepartsarenotdriedquickly,theindicationswillwillbeblurredandindistinct.Properlydevelopedpartswillhaveaneven,palewhitecoatingovertheentiresurface.

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C)WaterSuspendibleWater suspendible developers consist of insoluble developer particles suspended inwater.Water suspendible developers require frequent stirring or agitation to keep theparticles fromsettlingoutofsuspension.Watersuspendibledevelopersareapplied toparts in the same manner as water soluble developers. Parts coated with a watersuspendibledevelopermustbe forceddried justaspartscoatedwithawatersolubledeveloper are forced dried. The surface of a part coated with a water suspendibledeveloperwillhaveaslightlytranslucentwhitecoating.

B)NonaqueousNonaqueousdevelopers suspend thedeveloper in a volatile solvent andare typicallyappliedwithaspraygun.Nonaqueousdevelopersarecommonlydistributedinaerosolspray cans for portability. The solvent tends to pull penetrant from the indications bysolvent action. Since the solvent is highly volatile, forced drying is not required. Anonaqueousdeveloper should beapplied to a thoroughly dried part to forma slightlytranslucentwhitecoating.

PreparationofPart

Oneofthemostcriticalstepsinthepenetrant inspectionprocessispreparingthepartfor inspection.Allcoatings,suchaspaints,varnishes,plating,andheavyoxidesmustbe removed toensure thatdefectsareopen thesurfaceof thepart. If thepartshavebeenmachined,sanded,orblastedpriortothepenetrantinspection,itispossiblethatathin layerofmetalmayhavesmearedacross thesurfaceandclosedoffdefects. It isevenpossibleformetalsmearingtooccurasaresultofcleaningoperationssuchasgritorvaporblasting.Thislayerofmetalsmearingmustberemovedbeforeinspection.

Contaminants

Coatings, such as paint, aremuchmore elastic thanmetal andwill not fracture eventhough a large defect may be present just below the coating. The part must bethoroughlycleanedassurfacecontaminatescanpreventthepenetrantfromenteringadefect.Surfacecontaminantscanalsoleadtoahigherlevelofbackgroundnoisesincetheexcesspenetrantmaybemoredifficulttoremove.

Common coatings and contaminates that must be removed include: paint, dirt, flux,scale,varnish,oil,etchant, smut,plating,grease,oxide,wax,decals,machining fluid,rust,andresiduefrompreviouspenetrantinspections.

Someof thesecontaminantswouldobviouslypreventpenetrant fromenteringdefectsand it is, therefore, clear that they must be removed. However, the impact of othercontaminantssuchastheresiduefrompreviouspenetrantinspectionsislessclear,buttheycanhaveadisastrousaffectontheinspection.Takethelinkbelowtoreviewsome

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of the research that has been done to evaluate the effects of contaminants on LPIsensitivity.

A good cleaningprocedurewill removeall contamination from the part andnot leaveanyresiduethatmayinterferewiththeinspectionprocess.Ithasbeenfoundthatsomealkaline cleaners canbedetrimental to thepenetrant inspection process if they havesilicatesinconcentrationsabove0.5percent.Sodiummetasilicate,sodiumsilicate,andrelatedcompoundscanadheretothesurfaceofpartsandformacoatingthatpreventspenetrantentryintocracks.ResearchersinRussiahavealsofoundthatsomedomesticsoaps and commercial detergents can clog flaw cavities and reduce thewettability ofthemetal surface, thus, reducing the sensitivity of the penetrant.ConradandCaudillfound thatmedia fromplasticmedia blastingwas partially responsible for loss of LPIindication strength. Microphotographs of cracks after plastic media blasting showedmediaentrapmentinadditiontometalsmearing.

It isveryimportant that thematerialbeinginspectedhasnotbeensmearedacrossitsown surface during machining or cleaning operations. It is well recognized thatmachining, honing, lapping, hand sanding, hand scraping, shot peening, grit blasting,tumbledeburring,andpeeningoperationscancauseasmallamountofthematerialtosmear on the surface of some materials. It is perhaps less recognized that somecleaning operations, such as steam cleaning, can also causemetal smearing in thesoftermaterials.TakethelinkbelowtolearnmoreaboutmetalsmearinganditsaffectsonLPI.

CommonUsesofLiquidPenetrantInspection

Liquid penetrant inspection (LPI) is one of the most widely used nondestructiveevaluation (NDE)methods. Itspopularitycanbeattributed to twomain factors,whichare its relative ease of use and its flexibility. LPI can be used to inspect almost anymaterialprovided that itssurface isnotextremely roughorporous.Materials thatarecommonlyinspectedusingLPIincludethefollowing:

Metals(aluminum,copper,steel,titanium,etc.) Glass Manyceramicmaterials Rubber Plastics

LPIoffersflexibilityinperforminginspectionsbecauseitcanbeappliedinalargevarietyof applications ranging from automotive spark plugs to critical aircraft components.

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Penetrantmaterialcanbeappliedwithaspraycanoracottonswabtoinspectforflawsknowntooccurinaspecificareaoritcanbeappliedbydippingorsprayingtoquicklyinspect large areas. At right, visible dye penetrant being locally applied to a highlyloadedconnectingpointtocheckforfatiguecracking.

Penetrant inspection systems have been developed to inspect some very largecomponents. In this picture, DC10 banjo fittings are being moved into a penetrantinspection system at what used to be the Douglas Aircraft Company's Long Beach,California facility. These large machined aluminum forgings are used to support thenumber3engineinthetailofaDC10aircraft.

Liquid penetrant inspection is used to inspect of flaws that break the surface of thesample.Someoftheseflawsarelistedbelow:

Fatiguecracks Quenchcracks Grindingcracks Overloadandimpactfractures Porosity Laps Seams Pinholesinwelds Lackoffusionorbraisingalongtheedgeofthebondline

Asmentionedabove,oneofthemajorlimitationsofapenetrantinspectionisthatflawsmustbeopentothesurface.

AdvantagesandDisadvantagesofPenetrantTestingLikeallnondestructiveinspectionmethods,liquidpenetrantinspectionhasbothadvantagesanddisadvantages.TheprimaryadvantagesanddisadvantageswhencomparedtootherNDEmethodsaresummarizedbelow.PrimaryAdvantages

Themethodhashighsensitivetosmallsurfacediscontinuities. Themethodhas fewmaterial limitations, i.e.metallicandnonmetallic,magnetic

and nonmagnetic, and conductive and nonconductive materials may beinspected.

Largeareasandlargevolumesofparts/materialscanbeinspectedrapidlyandatlowcost.

Partswithcomplexgeometricshapesareroutinelyinspected. Indicationsareproduceddirectlyonthesurfaceofthepartandconstituteavisual

representationoftheflaw. Penetrantmaterialsandassociatedequipmentarerelativelyinexpensive.

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PrimaryDisadvantages

Onlysurfacebreakingdefectscanbedetected. Onlymaterialswitharelativenonporoussurfacecanbeinspected. Precleaningiscriticalascontaminantscanmaskdefects. Metal smearing from machining, grinding, and grit or vapor blasting must be

removedpriortoLPI. Theinspectormusthavedirectaccesstothesurfacebeinginspected. Surfacefinishandroughnesscanaffectinspectionsensitivity. Multipleprocessoperationsmustbeperformedandcontrolled. Postcleaningofacceptablepartsormaterialsisrequired. Chemicalhandlingandproperdisposalisrequire

ChapterIV

MagneticParticleInspection

Introduction:

Magnetic particle inspection is a nondestructive testingmethod used for surface andnear surface defect detection.MPI is a fast and relatively easy to apply and surfacepreparationisnotascriticalasitisforsomeotherNDTmethods.ThesecharacteristicsmakeMPIoneofthemostwidelyutilizednondestructivetestingmethods.

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MPI usesmagnetic fields and smallmagnetic particles, suchas iron filings to detectflawsincomponents.Theonlyrequirementisthatthecomponentbeinginspectedmustbemadeofaferromagneticmaterialsuchiron,nickel,cobalt,orsomeoftheiralloys.

Ferromagneticmaterialsarematerialsthatcanbemagnetizedtoalevelthatwillallowtheinspectiontobeeffective.

Themethodisusedtoinspectavarietyofproductformssuchascastings,forgings,andweldments.Manydifferentindustriesusemagneticparticleinspectionfordeterminingacomponent's fitnessforuse. Some examples of industries that usemagnetic particleinspection are the structural steel, automotive, petrochemical, power generation, andaerospace industries. Underwater inspection is another areawheremagnetic particleinspection may be used to test items such as offshore structures and underwaterpipelines.

BasicPrinciples

Intheory,magneticparticleinspection(MPI) isarelativelysimpleconcept.Considerabar magnet. It has a magnetic field in and around the magnet. Any place that amagnetic line of force exits or enters the magnet is called a pole. A pole where amagneticlineofforceexitsthemagnetiscalledanorthpoleandapolewherealineofforceentersthemagnetiscalledasouthpole.

Whenabarmagnetisbrokeninthecenterofitslength,twocompletebarmagnetswithmagneticpolesoneachendofeachpiecewillresult. Ifthemagnetisjustcrackedbutnotbrokencompletelyintwo,anorthandsouthpolewillformateachedgeofthecrack.Themagneticfieldexitsthenorthpoleandreenterstheatthesouthpole.Themagneticfieldspreadsoutwhenitencounterthesmallairgapcreatedbythecrackbecausetheaircannotsupportasmuchmagneticfieldperunitvolumeasthemagnetcan.Whenthefield spreads out, it appears to leak out of thematerial and, thus, it is called a fluxleakagefield.

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Ifironparticlesaresprinkledonacrackedmagnet,theparticleswillbeattractedtoandclusternotonlyatthepolesattheendsofthemagnetbutalsoatthepolesattheedgesofthecrack.Thisclusterofparticlesismucheasiertoseethantheactualcrackandthisisthebasisformagneticparticleinspection.

Thefirststepinamagneticparticleinspectionistomagnetizethecomponentthatistobeinspected.Ifanydefectsonornearthesurfacearepresent,thedefectswillcreatealeakagefield.Afterthecomponenthasbeenmagnetized,ironparticles,eitherinadryorwetsuspendedform,areappliedtothesurfaceofthemagnetizedpart.Theparticleswillbeattractedandclusterat the flux leakagefields, thus formingavisibleindicationthattheinspectorcandetect.

HistoryofMagneticParticleInspection

Magnetismistheabilityofmattertoattractothermatter.TheancientGreekswerethefirst to discover this phenomenon in a mineral they named magnetite. Later onBergmann, Becquerel, and Faraday discovered that all matter including liquids andgasseswereaffectedbymagnetism,butonlyafewrespondedtoanoticeableextent.

Theearliestknownmagneticinspectionanobjecttookplaceasearlyas1868.Cannonbarrels were checked for defects by magnetizing the barrel then sliding a magneticcompassalongthebarrel'slength.Theseearlyinspectorswereabletolocateflawsinthebarrelsbymonitoringtheneedleofthecompass.

In the early 1920s,WilliamHoke realized thatmagnetic particles couldbeusedwithmagnetism as a means of locating defects. Hoke discovered that a surface orsubsurface flaw in a magnetized material caused the magnetic field to distort andextend beyond the part. This discovery was brought to his attention in the machineshop. He noticed that themetallic grindings from hard steel parts, whichwere beingheldbyamagneticchuckwhilebeingground,formedpatternsonthefaceoftheparts

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whichcorrespondedtothecracksinthesurface.Applyingafineferromagneticpowdertothepartscausedabuildupofpowderoverflawsandformedavisibleindication.

Today, the MPI inspection method is used extensively to check for flaws in a largevarietyofmanufacturedmaterialsandcomponents.MPIisusedtocheckmaterialssuchassteelbarstockforseamsandotherflawspriortoinvestingmachiningtimeduringthemanufacturingofacomponent.Criticalautomotivecomponentsareinspectedforflawsafterfabricationtoensurethatdefectivepartsarenotplacedintoservice.MPIisusedtoinspectsomehighlyloadedcomponentsthathavebeeninserviceforaperiodoftime.Forexample,manycomponentsofhighperformanceracecarsareinspectedwhenevertheengine,drivetrainandothersystemsareoverhauled.MPIisalsousedtoevaluatethe integrity of structural welds on bridges, storage tanks, pipelines and other criticalstructures.

Magnetism

Magnets are very common items in the workplace and household. Uses of magnetsrangefromholdingpicturesontherefrigeratortocausingtorqueinelectricmotors.Theterm"magneticfield"simplydescribesavolumeofspacewherethereisachangeinenergywithinthatvolume.Thischangeinenergycanbedetectedandmeasured.Thelocationwhereamagneticfieldcanbedetectedexitingorenteringamaterialiscalledamagneticpole.Magneticpoleshaveneverbeendetectedinisolationbutalwaysoccurinpairsand,thus,thenamedipole.

AbarmagnetcanbeconsideredadipolewithanorthpoleatoneendandSouthPoleattheother.AmagneticfieldcanbemeasuredleavingthedipoleattheNorthPoleandreturning the magnet at the South Pole. If a magnet is cut in two, two magnets ordipolesarecreatedoutofone.Thissectioningandcreationofdipolescancontinuetotheatomiclevel.Therefore,thesourceofmagnetismliesinthebasicbuildingblockofallmatter...theatom.

TheSourceofMagnetism

Allmatter is composedof atoms, and atomsare composedof protons,neutronsandelectrons.Theprotonsandneutronsarelocatedintheatom'snucleusandtheelectronsareinconstantmotionaroundthenucleus.Electronscarryanegativeelectricalchargeandproduceamagneticfieldastheymovethroughspace.Amagneticfieldisproducedwhenever an electrical charge is in motion. The strength of this field is called themagneticmoment.

consider electric current flowing through a conductor. When the electrons (electriccurrent)areflowingthroughtheconductor,amagneticfieldformsaroundtheconductor.

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Themagnetic fieldcanbedetectedusingacompass.Themagnetic fieldwillplaceaforceonthecompassneedle.

Since all matter is comprised of atoms, allmaterials are affected in some way by amagneticfield.However,notallmaterialsreactthesameway.

Diamagnetic,Paramagnetic,andFerromagneticMaterialsInmostatoms,electronsoccur inpairs.Eachelectron inapairspins in theoppositedirection. So when electrons are paired together, there opposite spins cause theirmagnetic fields to cancel each other. Therefore, no net magnetic field exists.Alternately,materialswithsomeunpairedelectronswillhaveanetmagnetic fieldandwill reactmore toanexternal field.Mostmaterialscanbeclassifiedas ferromagnetic,diamagneticorparamagnetic.

Diamagneticmetals havea veryweakandnegative susceptibility tomagnetic fields.Diamagneticmaterialsareslightlyrepelledbyamagneticfieldandthematerialdoesnotretainthemagneticpropertieswhentheexternalfieldisremoved.Mostelementsintheperiodictable,includingcopper,silver,andgold,arediamagnetic.

Paramagneticmetalshaveasmallandpositivesusceptibilitytomagneticfields.Thesematerialsareslightlyattractedbyamagneticfieldandthematerialdoesnotretainthemagneticpropertieswhentheexternalfieldisremoved.Paramagneticmaterialsincludemagnesium,molybdenum,lithium,andtantalum.

Ferromagnetic materials have a large and positive susceptibility to an externalmagneticfield.Theyexhibitastrongattractiontomagneticfieldsandareabletoretaintheir magnetic properties after the external field has been removed. They get theirstrongmagneticpropertiesduetothepresenceofmagneticdomains.Inthesedomains,largenumbersofatomsmoments(1012 to1015)arealignedparallelsothatthemagneticforcewithinthedomainisstrong.Whenaferromagneticmaterialisintheunmagnitizedstate,thedomainsarenearlyrandomlyorganizedandthenetmagneticfieldforthepartasawholeiszero.Whenamagnetizingforceisapplied,thedomainsbecomealignedtoproduceastrongmagneticfieldwithinthepart.Iron,nickel,andcobaltareexamplesofferromagneticmaterials.

MagneticDomains

Ferromagneticmaterialsgettheirmagneticpropertiesbecausethematerialismadeupofsmallregionsknownasmagneticdomains.Ineachdomain,alloftheatomicdipolesarecoupledtogetherinapreferentialdirection.Thisalignmentdevelopsasthematerialdevelopsitscrystallinestructureduringsolidificationfromthemoltenstate.

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During solidification a trillion or more atom moments are aligned parallel so that themagnetic forcewithin thedomain isstronginonedirection.Eventhough thedomainsare magnetically saturated, the bulk material may not show any signs of magnetismbecausethedomainsdevelopthemselvesarerandomlyorientedrelativetoeachother.

Ferromagnetic materials become magnetized when the magnetic domains within thematerial are aligned. This can be done by placing the material in a strong externalmagnetic fieldorbypassingelectricalcurrent through thematerial.Someorallof thedomains can become aligned. The more domains are aligned, the stronger themagnetic field in the material. When all of the domains are aligned, the material ismagnetically saturated and additional amount of externalmagnetization forcewill notcauseanyincreaseinitsinternallevelofmagnetization.

UnmagnetizedMaterial MagnetizedMaterial

MagneticFieldCharacteristics

Magneticlinesofforcehaveanumberofimportantproperties,whichinclude:

Theyseekthepathofleastresistancebetweenoppositemagneticpoles.Inasinglebarmagnetasshowntotheright,theyattempttoformclosedloopfrompoletopole.

Theynevercrossoneanother. Theyallhavethesamestrength. Theirdensitydecreases(theyspreadout)whentheymovefromanareaof

higherpermeabilitytoanareaoflowerpermeability. Theirdensitydecreaseswithincreasingdistancefromthepoles. Theyareconsideredtohavedirectionasifflowing,thoughnoactualmovement

occurs.Theyflowfromthesouthpoletothenorthpolewithinthematerialandnorthpoletosouthpoleinair.

ElectromagneticFields

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Inmostconductors,themagneticfieldexistsonlyaslongasthecurrentisflowing

thedirectionofthemagneticfieldisdependentonthedirectionoftheelectricalcurrentin thewire.A threedimensional representation of themagnetic field is shownabove.There is a simple rule for remembering the direction of themagnetic field around aconductor.Itiscalledtherighthandrule.Ifapersongraspsaconductorinonesrighthandwith the thumbpointing in the direction of the current, the fingerswill circle theconductorinthedirectionofthemagneticfield.

MagneticFieldProducedbyaCoil

Whenacurrentcarryingconductorisformedintoalooporseveralloopstoformacoil,a magnetic field develops that flows through the center of the loop or coil alonglongitudinalaxisandcirclesbackaround theoutsideof the looporcoil.Themagneticfieldcirclingeachloopofwirecombineswiththefieldsfromtheotherloopstoproduceaconcentratedfielddownthecenterofthecoil.Alooselywoundcoilisillustratedbelowtoshow the interactionof themagnetic field.Themagnetic field isessentiallyuniformdownthelengthofthecoilwhenitiswoundtighter.

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Thestrengthofacoil'smagneticfieldincreasesnotonlywithincreasingcurrentbutalsowitheachloopthatisaddedtothecoil.Alongstraightcoilofwireiscalledasolenoidand can beused to generate a nearly uniformmagnetic field similar to that of a barmagnet. The concentrated magnetic field inside a coil is very useful in magnetizingferromagnetic materials for inspection using the magnetic particle testing method.Pleasebeawarethatthefieldoutsidethecoilisweakandisnotsuitableformagnetizeferromagneticmaterials.

TheHysteresisLoopandMagneticProperties

Agreatdealofinformationcanbelearnedaboutthemagneticpropertiesofamaterialby studying its hysteresis loop.A hysteresis loop shows the relationship between theinducedmagneticfluxdensityBandthemagnetizingforceH.ItisoftenreferredtoastheBHloop.Anexamplehysteresisloopisshownbelow.

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Plotting the change in magnetic flux B induced a ferromagnetic material while themagnetizingforceHischangedgeneratesthehysteresisloop.Aferromagneticmaterialthathasneverbeenpreviouslymagnetizedorhasbeenthoroughlydemagnetizedwillfollow the dashed line as H is increased. As the line demonstrates, the greater theamountofcurrentapplied(H+),thestrongerthemagneticfieldinthecomponent(B+).Atpoint"a"almostallofthemagneticdomainsarealignedandanadditionalincreaseinthemagnetizingforcewillproduceverylittleincreaseinmagneticflux.Thematerialhasreachedthepointofmagneticsaturation.

WhenHisreducedbackdowntozero,thecurvewillmovefrompoint"a"topoint"b."Atthispoint,itcanbeseenthatsomemagneticfluxremainsinthematerialeventhoughthemagnetizingforceiszero,thisisreferredtoasthepointofretentivityonthegraphand indicates theremanenceor levelof residualmagnetism in thematerial. (Someofthe magnetic domains remain aligned but some have lost there alignment.) As themagnetizingforceisreversed,thecurve

movestopoint"c",wherethefluxhasbeenreduced to zero.Thisiscalledthepointofcoercivity on the curve. (The reversed magnetizing force has flipped enough of thedomainsso that thenet fluxwithin thematerial iszero.)Theforcerequired toremovetheresidualmagnetismfromthematerial,iscalledthecoerciveforceorcoercivityofthematerial.

As themagnetizing forceis increasedin thenegativedirection, thematerialwillagainbecomemagneticallysaturatedbut intheoppositedirection(point"d").ReducingHto

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zerobringsthecurvetopoint"e."Itwillhavealevelofresidualmagnetismequaltothatachievedintheotherdirection.IncreasingHbackinthepositivedirectionwillreturnBto zero.Notice that the curvedidnot return to the origin of thegraph because someforceisrequiredtoremovetheresidualmagnetism.Thecurvewilltakeadifferentpathfrompoint"f"backthesaturationpointwhereitwithcompletetheloop.

From thehysteresisloop,anumberofprimarymagneticpropertiesofamaterialcanbedetermined.

Retentivity A measure of the residual flux density corresponding to the saturationinduction of a magnetic material. In other words, it is a material's ability to retain acertainamountofresidualmagneticfieldwhenthemagnetizingforceisremovedafterachievingsaturation.(ThevalueofBatpointBonthehysteresiscurve.)

ResidualMagnetismorResidualFlux themagnetic fluxdensity that remains inamaterial when the magnetizing force is zero. Note that residual magnetism andretentivityarethesamewhenthematerialhasbeenmagnetizedtothesaturationpoint.However,thelevelofresidualmagnetismmaybelowerthantheretentivityvaluewhenthemagnetizingforcedidnotreachthesaturationlevel.

Coercive Force The amount of reversemagnetic field whichmust be applied to amagneticmaterialtomakethemagneticfluxreturntozero.(ThevalueofHatpointConthehysteresiscurve.)

Permeability Apropertyofamaterialthatdescribestheeasewithwhichamagneticfluxisestablishedinthecomponent.

ReluctanceIstheoppositionthataferromagneticmaterialshowstotheestablishmentofamagneticfield.Reluctanceisanalogoustotheresistanceinanelectricalcircuit.

Theshapeofthehysteresislooptellsagreatdealaboutthematerialbeingmagnetized.Thehysteresiscurvesoftwodifferentmaterialsareshowninthegraph.

MagneticFieldOrientationandFlawDetectability

To properly inspect a component for cracks or other defects, it is important tounderstand that orientation between themagnetic lines of force and the flaw is veryimportant.Therearetwogeneraltypesofmagneticfieldsthatcanbeestablishedwithinacomponent.

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A longitudinal magnetic field hasmagnetic lines of forcethat run parallel to the long axis of the part. Longitudinalmagnetizationofacomponentcanbeaccomplishedusingthelongitudinalfieldsetupbyacoilorsolenoid.Itcanalsobeaccomplishedusingpermanentorelectromagnets.

A circular magnetic field has magnetic lines of force thatrun circumferentially around the perimeter of a part. Acircular magnetic field is induced in an article by eitherpassing current through the component or by passingcurrentthroughaconductorsurroundedbythecomponent.

To magnetize the part in two directions is important because the best detection ofdefectsoccurswhen the linesofmagnetic forceareestablishedat rightangles to thelongestdimensionof thedefect, if themagnetic fieldisparallel to thedefect, the fieldwillseelittledisruptionandnofluxleakagefieldwillbeproduced.

Anorientationof45to90degreesbetweenthemagneticfieldandthedefectisnecessarytoformanindication.Sincedefectsmayoccurinvariousdirections,eachpartisnormallymagnetizedintwodirectionsatrightanglestoeachother.Todeterminemostofthedefects.

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Demagnetization

Afterconductingamagneticparticleinspection,itisusuallynecessarytodemagnetizethecomponent

Remanentmagneticfieldscan:

Affectmachiningbycausingcuttingstoclingtoacomponent. Interferewithelectronicequipmentsuchasacompass. Createaconditionknownas"arkblow"intheweldingprocess.Arcblowmay

causetheweldarctowonderorfillermetaltoberepelledfromtheweld. Causeabrasiveparticletoclingtobearingorfayingsurfacesandincreasewear.

MagnetizingEquipmentforMagneticParticleInspection

Toproperlyinspectapartforcracksorotherdefects,itisimportanttobecomefamiliarwiththedifferenttypesofmagneticfieldsandtheequipmentusedtogeneratethem.Asdiscussed previously, one of the primary requirements for detection of a defect in aferromagneticmaterial is that themagnetic field inducedin thepartmust interceptthedefectata45to90degreesangle.Flawsthatarenormal(90degrees)tothemagneticfield will produce the strongest indications because they disrupt more of themagnetflux.

AvarietyofequipmentexisttoestablishthemagneticfieldforMPI.Someequipmentisdesigned to be portable so that inspections can be made in the field and some isdesigned to be stationary for ease of inspection in the laboratory or manufacturingfacility.

Permanentmagnets

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Permanentmagnetsaresometimesusedformagneticparticleinspectionasthesourceof magnetism. The two primary types of permanent magnets are bar magnets andhorseshoe(yoke)magnets.Theseindustrialmagnetsareusuallyverystrongandmayrequire significant strength to remove them from a piece of metal. Some permanentmagnetsrequireover50poundsofforcetoremovethemfromthesurface.Becauseitisdifficult to remove themagnets from the component being inspected, and sometimesdifficult and dangerous to place the magnets, their use is not particularly popular.However,adiverforinspectioninanunderwaterenvironmentorotherareassometimesusespermanentmagnets,suchasinanexplosiveenvironment,whereelectromagnetscannot beused.Permanentmagnets canalso bemade small enough to fit into tightareaswhereelectromagnetsmightnotfit

ElectromagnetsToday,mostoftheequipmentusedtocreatethemagneticfieldusedinMPIisbasedonelectromagnetism.Thatis,usinganelectricalcurrenttoproducethemagneticfield.Anelectromagneticyokeisaverycommonpieceofequipmentthatisusedtoestablishamagneticfield.Itisbasicallymadebywrappinganelectricalcoilaroundapieceofsoftferromagneticsteel.Aswitchisincludedintheelectricalcircuitsothatthecurrentand,therefore, also themagnetic field can be turn onand off. They canbepoweredwithalternatingcurrentfromawallsocketorbydirectcurrentfromabatterypack.Thistypeofmagnetgeneratesa

verystrongmagneticfieldinalocalareawherethepolesofmagnettouchtheparttobeinspected.Someyokescanliftweightsinexcessof40pounds.

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Portableyokewithbatterypack Portablemagneticparticlekit

ProdsProdsarehandheldelectrodes thatarepressedagainst thesurfaceof thecomponentbeing inspected tomakecontact forpassingelectrical current through themetal.Thecurrentpassingbetween theprodscreatesacircularmagnetic fieldaround theprodsthat is can be used in magnetic particle inspection. Prods are typically made fromcopperandhaveaninsulatedhandletohelpprotecttheoperator.Oneoftheprodshasa trigger switch so that the current can be quickly and easily turned on and off.Sometimesthethetwoprodsareconnectedbyanyinsulatorasshownintheimagetofacilitateonehandoperation.Thisisreferredtoasadualprodandiscommonlyusedforweldinspections.

If proper contact is not maintained between the prods and the component surface,electricalarcingcanoccurandcausedamage to thecomponent.For this reason, theuseofprodsarenotallowedwheninspectingaerospaceandothercriticalcomponents.Tohelptopreventarcing,theprodtipsshouldbe

inspected frequently toensure that theyarenotoxidized,coveredwithscaleorothercontaminant,ordamaged.

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Portable Coils and Conductive CablesCoilsandconductivecablesareusedtoestablishalongitudinalmagneticfieldwithinacomponent.Whenapreformedcoilisused,thecomponentisplacedagainsttheinsidesurfaceonthecoil.Coilstypicallyhavethreeorfiveturnsofacoppercablewithinthemoldedframe.Afootswitchisoftenusedtoenergizethecoil.Conductivecablesarewrappedaroundthecomponent.Thecableusedistypically00extraflexibleor0000extraflexible.Thenumberofwrapsisdeterminedbythemagnetizingforceneededand,ofcourse,thelengthofthecable.Normallythewrapsarekeptasclosetogetheraspossible.Whenusingacoilorcablewrappedintoacoil,amperageisusuallyexpressedinampereturns.Ampereturnsistheamperageshownontheampmetertimesthenumberofturnsinthecoil.

Portablecoil ConductiveCable

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central conductor.

Thistypeofasetupisusedtoinspectpartsthatarehollowsuchasgears,tubes,andother ringshapedobjects.Acentral conductor isanelectricallyconductivebar that isusually made of copper or aluminum. The bar is inserted through the center of thehollow part and the bar is then clamped between the contact pads.When current ispassedthroughthecentralconductor,acircularmagneticfieldflowsaroundthebarandentersintothepartorpartsbeinginspected.

LightsforMagneticParticleInspection

Magnetic particle inspection can be performed using particles that are highly visibleunder white lighting conditions or particles that are highly visible under ultravioletlighting conditions. When an inspection is being performed using the visible colorcontrastparticles,nospeciallightingisrequiredaslongastheareaofinspectioniswelllit.Alightintensityofatleast1000lux(100fc)isrecommendedwhenavisibleparticlesareused,butavarietyoflightsourcescanbeused.

When fluorescent particles are used, special ultraviolet light must be used.Fluorescenceisdefinedas thepropertyofemittingradiationasa resultofandduringexposure to radiation. Particles used in fluorescent magnetic particle inspections arecoatedwithamaterialthatproduceslightinthevisiblespectrumwhenexposedtothenearultraviolet light. This "particle glow" provides high contrast indications on thecomponent anywhere particles collect. Particles that fluoresce yellowgreen aremostcommonbecausethiscolormatchesthepeaksensitivityofthehumaneyeunderdarkconditions. However, particles that fluoresce red, blue, yellow, and green colors areavailable.

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UltravioletLight

Ultravioletlightor"blacklight"islightinthe1,000to4,000Angstroms(100to400nm)wavelength range in theelectromagneticspectrum. It isaveryenergetic formof lightthat is invisible to the human eye.Wavelengths above 4,000 Angstroms fall into thevisible light spectrum and are seen as the color violet. UV is separatedaccording towavelength into three classes: A, B, and C. The shorter the wavelength, the moreenergythatiscarriedinthelightandthemoredangerousitistothehumancells.

ClassUVAUVBUVC

WavelengthRange3,2004,000Angstroms2,8003,200Angstroms2,8001,000Angstroms

Thedesiredwavelengthrange foruse innondestructive testing isbetween3,500and3,800Angstromswithapeakwavelengthatabout3,650A.Thiswavelength range isusedbecauseitisintheUVArange,whichisthesafesttoworkwith.UVBwilldoaneffective job of causing substances to fluoresce, however, it should not be usedbecause harmful effects such as skin burns, and eye damage can occur. Thiswavelengthof radiation is found in thearc createdduring theweldingprocess.UVC(1,000 to2,800) isevenmore dangerous to living cells and is used to killbacteria inindustrialandmedicalsettings.

ThedesiredwavelengthrangeforuseinNDTisobtainedbyfilteringtheultravioletlightgenerated by the light bulb. The output of a UV bulb spans a wide range ofwavelengths.Theshortwavelengthsof3,120Ato3,340Aareproducedinlowlevels.Apeakwavelengthof3650Aisproducedataveryhighintensity.Wavelengthsinthevisible violet range (4050 A to 4350 A), greenyellow (5460 A), yellow (6220 A) andorange(6770A)arealsousuallyproduced.Thefilterallowsonlyradiationintherangeof3200to4000angstromsandalittlevisibledarkpurpletopass.

MagneticParticles

Asmentionedpreviously,theparticlesthatareusedformagneticparticleinspectionareakeyingredientastheyformtheindicationsthatalerttheinspectortodefects.Particlesstart out as tinymilled (amachiningprocess) pieces of iron or iron oxide.A pigment(somewhatlikepaint)isbondedtotheirsurfacestogivetheparticlescolor.Themetalusedfortheparticleshashighmagneticpermeabilityandlowretentivity.Highmagneticpermeabilityisimportantbecauseitmakestheparticlesattracteasilytosmallmagneticleakagefields fromdiscontinuities,suchas flaws.Lowretentivityisimportantbecause theparticlesthemselvesneverbecomestronglymagnetizedsotheydonotsticktoeachotherorthesurfaceofthepart.Particlesareavailableinadrymixorawetsolution.

Dry Magnetic Particles

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Drymagneticparticlescantypicallybepurchasedinred,black,gray,yellowandseveralother colors so thata high level of contrastbetween the particles and the part beinginspectedcanbeachieved.Thesizeof themagneticparticles isalsovery important.Drymagneticparticleproductsareproduced to includearangeofparticlesizes.Thefine particles are around 50 m (0.002 inch) in size are about three times smaller indiameter andmore than20 times lighter than the coarse particles (150 mor 0.006inch), which make them more sensitive to the leakage fields from very smalldiscontinuities. However, dry testing particles cannot bemade exclusively of the fineparticles.Coarserparticlesareneededtobridgelargediscontinuitiesandtoreducethepowder's dusty nature. Additionally, small particles easily adhere to surfacecontamination,suchasremanentdirtormoisture,andgettrappedinsurfaceroughnessfeaturesproducingahigh levelofbackground. It shouldalsobe recognized that finerparticleswillbemoreeasilyblownawayby thewindand, therefore,windyconditionscan reduce the sensitivity of an inspection. Also, reclaiming the dry particles is notrecommendedbecausethesmallparticlearelesslikelytoberecapturedandthe"onceused"mixwillresultinlesssensitiveinspections.

The particle shape is also important. Long, slender particles tend align themselvesalong the lines of magnetic force. However, research has shown that if dry powderconsists only of long, slender particles, the application process would be less thandesirable.Elongatedparticlescomefromthedispenserinclumpsandlacktheabilitytoflow freely and form the desired "cloud" of particles floating on the component.Therefore, globular particles are added that are shorter. The mix of globular andelongatedparticlesresultinadrypowderthatflowswellandmaintaingoodsensitivity.MostdryparticlemixeshaveparticlewithL/Dratiosbetweenoneandtwo.

WetMagneticParticlesMagneticparticlesarealsosuppliedinawetsuspensionsuchaswateroroil.Thewetmagneticparticle testingmethod isgenerallymoresensitive than thedrybecause thesuspensionprovidestheparticleswithmoremobilityandmakesitpossibleforsmallerparticles tobeusedsincedustandadherence tosurfacecontaminationis reducedor

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eliminated. The wet method also makes it easy to apply the particles uniformly to arelativelylargearea.

Wetmethodmagneticparticlesproductsdifferfromdrypowderproductsinanumberofways. One way is that both visible and fluorescent particle are available. Mostnonfluorescentparticlesareferromagneticironoxides,whichareeitherblackorbrownincolor.Fluorescentparticlesarecoatedwithpigmentsthatfluorescewhenexposedtoultraviolet light. Particles that fluoresce greenyellow are most common to takeadvantageofthepeakcolorsensitivityoftheeyebutotherfluorescentcolorsarealsoavailable. (Formore informationon the color sensitivity of the eye, see the penetrantinspectionmaterial.)

Theparticlesusedwiththewetmethodaresmallerinsizethanthoseusedinthedrymethodforthereasonsmentionedabove.Theparticlesaretypically10 m(0.0004inch)andsmallerandthesyntheticironoxideshaveparticlediametersaround0.1 m(0.000004inch).Thisverysmallsizeisaresultoftheprocessusedtoformtheparticlesandisnotparticularlydesirable,astheparticlesarealmosttoofinetosettleoutofsuspension.However,duetotheirslightresidualmagnetism,theoxideparticlesarepresentmostlyinclustersthatsettleoutofsuspensionmuchfasterthantheindividualparticles.Thismakesitpossibletoseeandmeasuretheconcentrationoftheparticlesforprocesscontrolpurposes.Wetparticlesarealsoamixoflongslenderandglobularparticles.Thecarriersolutionscanbewater oroilbased.Waterbasedcarriersformquickerindications,aregenerallylessexpensive,presentlittleornofirehazard,giveoffnopetrochemicalfumes,andareeasiertocleanfromthepart.Waterbasedsolutionsareusuallyformulatedwithacorrosioninhibitortooffersomecorrosionprotection.However,oilbasedcarriersolutionsoffersuperiorcorrosionandhydrogenembrittlementprotectiontothosematerialsthatarepronetoattackbythesemechanisms.

ChapterIV

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UltrasonicTesting

BasicPrinciplesofUltrasonicTesting

UltrasonicTesting(UT)useshighfrequencysoundenergytoconductexaminationsandmakemeasurements.Ultrasonic inspectioncanbeused for flawdetection/evaluation,dimensionalmeasurements,materialcharacterization,andmore.

The soundenergy is introduced and propagates through thematerials in the formofwaves.When there is adiscontinuity (suchasacrack) in thewavepath,partof theenergy will be reflected back from the flaw surface. The reflected wave signal istransformedintoelectricalsignalbythetransducerandisdisplayedonascreen.

SCREEN

Ultrasonic Inspection is a very useful and versatile NDT method for detecting bothsurfaceandsubsurfacevolumetricdefectsand iswidelyused inpipeline,oilandgasandprocessingindustry.

Plate Crack

0 2 4 6 8 10

Initialpulse

Crackecho

Backsurfaceecho

Oscilloscope,orflawdetectorscreen

Probe

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AdvantagesofUltrasonicInspection

Someoftheadvantagesofultrasonicinspectionthatareoftencitedinclude:

Itissensitivetobothsurfaceandsubsurfacediscontinuities. Thedepthofpenetrationforflawdetectionormeasurementissuperiorto

otherNDTmethods. Only singlesided access is needed when the pulseecho technique is

used. Itishighaccuracyindeterminingreflectorpositionandestimatingsizeand

shape. Minimalpartpreparationrequired. Electronicequipmentprovidesinstantaneousresults. Detailedimagescanbeproducedwithautomatedsystems. It has other uses such as thicknessmeasurements, in addition to flaw

detection.

DisadvantagesofUltrasonicInspection

As with all NDTmethods, ultrasonic inspection also has its limitations, whichinclude:

16Hz20kHz 200kHz 15MHz

256Hz 70kHz

Audiblerange

Ultrasonictestingrange

15MHz

Usualsteeltestingrange

SoundSpectrum

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Surfacemustbeaccessibletotransmitultrasound. Skillandtrainingismoreextensivethanwithsomeothermethods. It normally requires a coupling medium to promote transfer of sound

energyintotestspecimen. Materialsthatarerough,irregularinshape,verysmall,exceptionallythin

ornothomogeneousaredifficulttoinspect. Castironandothercoarsegrainedmaterialsaredifficulttoinspectdueto

lowsoundtransmissionandhighsignalnoise. Lineardefectsorientedparalleltothesoundbeammaygoundetected. Reference standards are required for both equipment calibration, and

characterizationofflaws.

Propertiesofsoundwave

WavePropagationUltrasonic testing is based on timevarying deformations or vibrations inmaterials,whichisgenerallyreferredtoasacoustics.Allmaterialsubstancesarecomprised of atoms, which may be forced into vibrational motion about theirequilibrium positions.

Insolids,soundwavescanpropagateinfourprinciplemodesthatarebasedonthewaytheparticlesoscillate.Soundcanpropagateaslongitudinalwaves,shearwaves, surfacewaves, and in thinmaterials as platewaves. Longitudinal andshearwavesare the twomodesofpropagationmostwidelyused inultrasonictesting. The particlemovement responsible for the propagation of longitudinalandshearwavesisillustratedbelow.

Longitudinalwaves:In longitudinal waves the oscillations occur in the longitudinal direction of thedirection of wave propagation. Since compressional forces are active in thesewaves, theyarealsocalledcompressionalwaves.Compressionwavescanbe

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generated in liquids, aswell as solids because theenergy travels through theatomic structure by a series of comparison and expansion (rarefaction)movements.

Transverseorshearwave:In the transverse or shear wave, the particles oscillate at a right angle ortransverse to thedirectionofpropagation.Shearwavesrequireanacousticallysolid material for effective propagation and, therefore, are not effectivelypropagated inmaterials suchas liquids or gasses.Shearwavesare relativelyweakwhencomparedtolongitudinalwaves

SurfaceorRayleighwaves:SurfaceorRayleighwavestravelonthesurfaceofarelativethicksolidmaterialpenetratingtoadepthofonewavelength.Theparticlemovementhasanellipticalorbit. Raleigh waves are useful because they are very sensitive to surfacedefects and since theywill follow the surfacearoundcurves, thereforecanbeusedtoinspectareasthatotherwavesmighthavedifficultyinreaching.

Platewaves:Platewaves canbepropagated only in very thinmetals. Lambwavesare themostcommonlyusedplatewavesinNDT.Lambwavesareacomplexvibrationalwavethattravelsthroughtheentirethicknessofamaterial.PropagationofLambwavesdependsondensity,elastic,andmaterialpropertiesofacomponent,andtheyareinfluencedbyagreatdealbyselectedfrequencyandmaterialthickness.

Velocity:Howquicklyasoundwavewilltravel

Frequency:Howmanyvibrationspersecond

Wavelength:

HowfarasoundwavewilladvanceincompletingonecycleThewavelengthisdirectlyproportionaltothevelocityofthewaveandinverselyproportionaltothefrequencyofthewave.Thisrelationshipisshownbythefollowingequation.

1SecondA2H

1SecondB=5H

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Achangeinfrequencywillresultinachangeinwavelength.Inultrasonictesting,the shorterwavelength resulting froman increase in frequencywillhelp in thedetectionofsmallerdiscontinuities.

Sensitivity:Sensitivity is the ability to locate small discontinuities. Sensitivity generallyincreaseswithhigherfrequency(shorterwavelengths).

Resolution:Resolution is the ability of the system to locate discontinuities that are closetogether within the material or located near the part surface. Resolution alsogenerallyincreasesasthefrequencyincreases.

Velocityofsoundtravelingthroughmaterials:Velocityofsoundvarieswiththematerialinwhichitistraveling. Material Compression

Velocity m\sec Shear Velocity m\sec

Steel 5960 3245

Water 1490 NA

Air 344 NA

Copper 4700 2330

AttenuationofSoundWavesWhen sound travels through a medium, its intensity diminisheswith distance.This weakening results from two basic causes, which are scattering andabsorption. The combined effect of scattering and absorption is calledattenuation.

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RefractionandSnell'sLaw

Whenanultrasoundwavepassesthroughaninterfacebetweentwomaterials,itproduces both reflected and refracted waves. Refraction takes place at aninterface due to the different velocities of the acoustic waves within the twomaterials.Thevelocityofsound ineachmaterial isdeterminedby thematerialproperties(elasticmodulesanddensity)forthatmaterial.

Snell'sLawdescribes therelationshipbetweentheanglesandthevelocitiesofthewaves.Snell's lawequates theratioofmaterialvelocitiesv1andv2 to theratio of the sine's of incident ( ) and refraction ( )angles,asshown in thefollowingequation.

Where:

VL1 is the longitudinal wave velocity in material 1.

VL2 is the longitudinal wave velocity in material 2.

UltrasonicProbes

Theconversionofelectricalpulsestomechanicalvibrationsandtheconversionof returned mechanical vibrations back into electrical energy is the basis for

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ultrasonic testing. The active element is the Probe. It converts the electricalenergytoacousticenergy,andviceversa.

CharacteristicsofProbes

Theprobeisaveryimportantpartoftheultrasonicinstrumentationsystem.Theprobeconvertselectricalsignalsintomechanicalvibrations(transmitmode)andmechanical vibrations into electrical signals (receive mode). Many factors,including material, mechanical and electrical construction, and the externalmechanical and electrical load conditions, influence the behavior a transducer.Mechanical construction includes parameters such as radiation surface area,mechanicaldamping,housing,connectortype

.

TypesofProbes

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Ultrasonictransducersaremanufacturedforavarietyofapplicationandcanbecustom fabricatedwhennecessary.Carefulattentionmustbepaid toselectingthe proper transducer for the application It is important to choose transducersthathavethedesiredfrequency,bandwidth,andfocusingtooptimizeinspectioncapability.Mostoften the transducer ischoseneither toenhancesensitivityorresolutionofthesystem.

Transducersareclassifiedintogroupsaccordingtotheapplication.

Contact transducers are used for direct contact inspections, and aregenerally handmanipulated. Theyhave elementsprotected ina ruggedcasing to withstand sliding contact with a variety of materials. Thesetransducersaredesignedsothattheyareeasytogripandmovealongasurface. Theyalso often have replaceablewearplates to lengthen theiruseful life. Coupling materials of water, grease, oils, or commercialmaterialsareusedtoremovetheairgapbetweenthetransducerandthecomponentinspected.Contactprobesareclassifiedas.

Singlecrystalprobe Twincrystalprobe Normalbeamorzerodegreeprobe Anglebeamprobe

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Contacttransducersareavailableinavarietyofconfigurationsto improvetheirusefulnessforavarietyofapplications.

Singlecrystalprobenormalprobe:

The flat contact transducer shown above is used normal beam inspections ofrelatively flat surfaces, andwhere near surface resolution is not critical. If thesurfaceiscurved,ashoethatmatchesthecurvatureofthepartmayneedtobeaddedtothefaceofthetransducer.

Twincrystalnormalprobe:

contain two independently operating elements in a single housing.One of theelements transmits and the other receives.Activeelementscanbechosen fortheir sending and receiving capabilities providing a transducer with a cleanersignal, and transducers for special applications, such as inspection of coursegrainmaterial.Dual element transducers are especiallywell suited formakingmeasurements in applications where reflectors are very near the transducersincethisdesigneliminatestheringdowneffectthatsingleelementtransducersexperience. (When singleelement transducers are operating in pulse echomode,theelementcannotstartreceivingreflectedsignalsuntiltheelementhasstopped ringing from it transmit function.) Dual element transducers are veryuseful when making thickness measurements of thin materials and wheninspectingfornearsurfacedefects.Thetwoelementsareangledtowardseachothertocreateacrossedbeamsoundpathinthetestmaterial.

Anglebeamtransducers:

Anglebeamsaretypicallyusedtointroducearefractedshearwaveintothetestmaterial.Inthefixedangleversions,theangleofrefractionthatismarkedonthetransducer isonlyaccurateforaparticularmaterial,whichisusuallysteel.Theangledsoundpathallowsthesoundbeamtobereflectedfromthebackwalltoimprovedetectabilityofflawsinandaroundweldedareas.Theyarealsousedtogenerate surface waves for use in detecting defects on the surface of acomponent.

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Couplant

A couplant is a material (usually liquid) that facilitates the transmission ofultrasonic energy from the transducer into the test specimen. Couplant isgenerallynecessarybecausetheacousticimpedancemismatchbetweenairandsolids

CalibrationBlocks

Standard blocks are used tocalibrate the instrumentand tocalculatedifferentfeaturesofprobeandtheinstrument.Theseblocksconsistsaccuratelycutandfinepolishedsurfaces,holes,anglesetc.

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InspectionofWeldedJoints

The most commonly occurring defects in welded joints are porosity, slaginclusions, lack of sidewall fusion, lack of interrun fusion, lack of rootpenetration,undercutting,andlongitudinalortransversecracks.

Ultrasonicweldinspectionsaretypicallyperformedusingastraightbeamprobein conjunction with an angle beam probe A straight beam probe, producing alongitudinalwaveatnormal incidence into the testpiece, is firstused to locateanylaminationsinorneartheheataffectedzone.Thisisimportantbecauseananglebeamtransducermaynotbeabletoprovideareturnsignalfromalaminarflaw.

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ChapterVI

RADIORGAPHICTESTING

60

Introduction:

In thismethod ofNondestructive testing the penetration property ofXray andGamma rays todetect thediscontinuities.Theobject tobe inspected isplacedbetween the radiation source and a piece of film. Xrays or gamma rays passthroughtheobject.Theobjectwillstopsomeoftheradiation.Thickeranddenserareawillstopmoreof theradiationandshowonthe film lighter than thinnerorless dense area. Most weld defects will show on the film darker than thesurroundingarea.

NatureofPenetratingRadiation

Xrays and gamma rays are part of the electromagnetic spectrum. They arewaveformsasarelightrays,microwaves,andradiowave,butxraysandgammarayscannotbeenseen,felt,orheard.Theypossessnochargeandnomassand,therefore, are not influenced by electrical andmagnetic fields andwill alwaystravelinstraightlines.Theycanbecharacterizedbyfrequency,wavelength,andvelocity

TheElectromagneticSpectrum

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TheInternationalSystem(SI)unitforactivityistheBecquerel(Bq),w

WavelengthsofElectroMagneticSpectrum

ElectroMagneticRadiationType Wavelengthinnm

VisibleLight 700-400

Ultraviolet light 400-100

X-Rays

Gamma -Rays

1nm=109Meters

AdvantageofRadiography1.Givesapermanentrecord2.DetectsinternalFlaws3.Detectsvolumetricflawsreadily4.Canbeusedonmostmaterials5.Cancheckforcorrectassembly

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6.GivesdirectImages7.RealtimeImageispossible

DisadvantagesofRadiography

1RadiationHealth2Canbesensitivetodefectorientationandcouldmissplanarflaws3Haslimitedabilitytodetectfinecracks4Accessisrequiredtobothsidesoftheobject5Limitedthicknessofthematerialcanbepenetrated6Skilledradiographicinterpretationisrequired7Requirehighcapitalcost8Relativelyslowprocess9Requirehighcapitalcost10Requirehighrunningcost

PropertiesofXraysandgammarays

1.Theyhavenoeffectonthehumansenses2.Theyhaveadverseeffectonthebodytissuesandblood3.Theypenetratematter4.Theymoveinstraightline5.Theyarepartofelectromagneticspectrum6.Theytravelatthespeedoflight7.Theyobeytheinversesquarelaw8.Theyionizegases9.Theymaybescattered10.Theymakecertainmaterialsfluoresce11.Theymayberefracted,diffractedandpolarized

XrayTube

HighElectricalPotentialElectrons

+XrayGeneratororRadioactiveSource

CreatesRadiation

ExposureRecordingDevice

RadiationPenetratetheSample

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PropertiesofXrays

1.Potentialdifferenceofaround300kvisused2.Approximatelyaround9799%heat&13%ofxraysaregenerated

3.Anodeismadeupofcuppertocarryouttheheat.Additionalcoolingusingoil,airorwaterisalsoused

4Targetismadeupoftungsten5.Areaofthetargetstruckbytheelectronsiscalledasfocalspot6. Focalspotsizeshouldbebigtoabsorbmoreheatbuttoproduce

goodqualityradiographthissizeshouldbethesmallest7.Importantcontrolpointsofthexraymachinearetimer,Amperage

controlandVoltagecontrol8.Moretimemoreradiationmoreexposure9.AmperagecontrolstheintensityorqualityofXray,612Ampareusuallyused10.Morevoltagegeneratestheshorterwavelengthorqualityofx

raysmorepenetratingpower11.Increaseinvoltageincreasesthespeedoftheelectrons,therefore

highkineticenergyandhighpenetration

Gammarays:

Gammarays are electromagnetic radiation emitted by the disintegration of aradioactiveisotopeandhaveenergyfromabout100keVtowellover1MeV.Themost useful gammaemitting radioactive isotopes for radiological purposes arefoundtobecobalt(Co60),iridium(Ir192),cesium(Cs137),ytterbium(Yb169),andthulium(Tm170).

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PropertiesofGammarays

1.Gammaraysareemittedfromartificialradioactiveisotope2.Radioactiveisotopeisanunstablestateofelementwhichhasdifferentnumberofneutronstothenormalstateofthesameelement

3.ThemassnumberofRadioactiveIsotopewillbedifferentfromsameelement4.Theradioactiveisotopedisintegratecontinuouslyreleasingelectromagneticenergy(gammarays)

5Gammaraysourcesareusuallydisc,cylindricalorsphericalshape6Thediscs:3.0mmdiameterand1mmthick,stackedtogether7Cylindrical:Typicallyupto4mminlength8Spherical:0.63.0mmdiameter9Sourcesareencapsulatedinthecapsulesof316\S12gradeStainlesssteel

IsotopeDecayRate(DecayoftheGammaSource)

LossofactivityofaradioactivenucleaseduetoDisintegration

HalfLifeofGammasource:

TimetakenforaradioactiveIsotopetoreduceitsoutputbyhalf

Source Halflife Penetrationrangesteel

60Cobalt 26Years 75150mm

192Iridium 74days 2045mm

Ytterbium169 31days 115mm

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AdvantagesofGammaraysoverXrays

1.Noelectricalorwatersupplyareneeded

2.Gammaequipmentisusuallysmallerandlighterandthereforemoreportable

3.Theequipmentismoresimple

4.Placesinaccessibletoxrayequipmentareaccessibletogammaequipment

5.Becauseofhighenergythereislessscatter

6.Gammaequipmentislessexpensivethanxrayequipment

7.Greaterpenetratingpowerthanxrays

Disadvantagesofgammaraysoverxrays1.Duetothehigherenergy,poorercontrastanddefinition

2.Exposuretimesarelonger

3.Sourcesneedreplacingatregularintervals

4.Theradiationcannotbeswitchedoff

5.SFDisshorter,resultinginpoorergeometricunsharpness

6.Remotehandlingisnecessary

RadiographicTechniques

1) SWSI:(FilmInsideSourceOutside)2) SWSI:(FilmOutsideSourceInside)3) DWSI:(FilmOutsideSourceOutside)4) DWDI:(FilmOutsideSourceOutside

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RadiographicContrast

Radiographic contrast describes the differences in photographic density in aradiograph.Thecontrastbetweendifferentpartsoftheimageiswhatformstheimage and the greater the contrast, the more visible features become.Radiographiccontrasthastwomaincontributors:subjectcontrastanddetectororfilmcontrast.

Subjectcontrastisdeterminedbythefollowingvariables:AbsorptiondifferencesinthespecimenWavelengthoftheprimaryradiationScatterorsecondaryradiation

Filmcontrastisdeterminedbythefollowing:GrainsizeortypeoffilmChemistryoffilmprocessingchemicalsConcentrationsoffilmprocessingchemicalsTimeofdevelopmentTemperatureofdevelopmentDegreeofmechanicalagitation(physicalmotion)

Exposingthefilmtoproducehigherfilmdensitieswillgenerallyincreasecontrast.In other words, darker areas will increase in density faster than lighter areasbecauseinanygivenperiodoftimemorexraysarereachingthedarkerareas.

ReasonsforlowcontrastRadiationwavelengthtooshortOverexposureProlongeddevelopmentToocolddeveloper

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InsufficientfixingFogonthefilm

ReasonsforHighcontrast

RadiationwavelengthtoolongIncorrectdeveloperUnderexposure

Definition

Radiographicdefinitionistheabruptnessofchangeingoingfromonedensitytoanother.ThereareanumberofgeometricfactorsoftheXrayequipmentandtheradiographic setup that have an effect on definition. These geometric factorsinclude:Focalspotsize,whichistheareaoforigin