ndt structural steel

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Non Destructive Testing Of Structures

Sri Harsha Gamidi09304012

M.techStructural Engineering

Guide: Dr. K.M.BajoriaDepartment of civil Engineering

Indian Institute of Technology,Bombay

November 2009


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Abstract

A large range of Non-Destructive Testing (NDT) techniques are employed for a wide variety ofapplications within Civil Engineering. The majority of these applications may be regarded

as structural but they also include site surveying and highways problems. Testing maybe used during planning and construction phases, but the majorities of applications areconcerned with troubleshooting, maintenance and repair. The normal maintenance methodis to inspect the structure periodically, and when the degrees of deterioration exceed certainlimit, repair or strengthening of the structures is performed. To inspect existing structures,visual inspection is the easiest and the most fundamental method. But this method may notbe applicable for inspecting defects which does not appear on the surface. For such defects,Non-destructive inspection is the only method which can be applied. Nondestructive testingplays an important role for quality evaluation. India has been producing concrete structuressince long times. And now it is becoming important problem to maintain these existingconcrete and steel structures. This presentation aims on the explanation of NDT techniques

and their application to structures.


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Contents

List of Figures iv

1 Introduction 1

2 Destructive testing 2

2.1 Definitions of Hardness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.2 Testing of large structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.3 Destructive software testing . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3 Non Destructive Testing 5

4 Ultrasonic testing 74.1 How it works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

5 Liquid Penetration Test 105.1 Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

5.2 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105.3 Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

5.3.1 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125.3.2 Precautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

6 Magnetic Particle inspection 146.1 Magnetic Flux Leckage(MFL) . . . . . . . . . . . . . . . . . . . . . . . . . . 15

6.1.1 Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156.1.2 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156.1.3 Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166.1.4 Crack detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

7 ElectroMagnetic testing 177.0.5 Eddy current testing . . . . . . . . . . . . . . . . . . . . . . . . . . . 177.0.6 Remote field testing (RFT) . . . . . . . . . . . . . . . . . . . . . . . 187.0.7 Differences Between RFT and ECT . . . . . . . . . . . . . . . . . . . 197.0.8 Alternating Current Field Measurement (ACFM) . . . . . . . . . . . 197.0.9 Pulsed eddy current method . . . . . . . . . . . . . . . . . . . . . . . 19

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8 Radiographic Testing 208.1 X-ray Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

8.1.1 Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208.1.2 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

8.2 Gamma Ray method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208.2.1 Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228.2.2 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

9 Miscellaneous methods 239.1 Visual inspection method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239.2 LASER Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239.3 Infrared and Thermal Testing . . . . . . . . . . . . . . . . . . . . . . . . . . 239.4 Schmidt hammer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249.5 RADAR Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

10 Case Study: Bridge 25

11 Conclusions 29


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List of Figures

2.1 Destructive testing of a 6-story non-ductile concrete building using a shaketable[2] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

5.1 Dye penetration test Steps[3] . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

8.1 X-Ray Equipment[8] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

8.2 X-Ray image of reinforced concrete column[9] . . . . . . . . . . . . . . . . . 21

9.1 Carrying of RADAR Test on pavement[5] . . . . . . . . . . . . . . . . . . . . 24

10.1 Schmidt Hammer[7],GPR Test being conducted on a bridge[5] . . . . . . . . 2810.2 Figure showing the RADAR data[5] . . . . . . . . . . . . . . . . . . . . . . . 28

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Chapter 1

Introduction

Nondestructive testing (NDT) is a wide group of analysis techniques used in science andindustry to evaluate the properties of a material, component or system without causing

damage. Because NDT does not permanently alter the article being inspected, it is a highly-valuable technique that can save both money and time in product evaluation, troubleshoot-ing, and research. Common NDT methods include ultrasonic, magnetic-particle, liquid pen-etrant, radiographic, and eddy-current testing NDT is a commonly-used tool in forensicengineering, mechanical engineering, electrical engineering, civil engineering, systems engi-neering, medicine, and art. Specialist high risk areas such as nuclear and offshore structures,and gas and oil pipelines, make extensive use of Non-Destructive Testing of metallic compo-nents during manufacture and construction as part of quality assurance procedures as well asduring routine maintenance inspections to detect cracking and corrosion. Radiography andultrasonics are most widely used for checking of welds, although eddy current and magnetic

methods are also available. Alternating current field measurement techniques permit non-contacting crack detection and sizing in welded joints both in air and underwater.The focusof this presentations concentrates on mainstream engineering activities, where the extent ofN.D.T. usage varies considerably [1].

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Chapter 2

Destructive testing

In destructive testing, tests are carried out to the specimens failure, in order to understand aspecimens structural performance or material behavior under different loads. These tests are

generallymuch easier to carry out, yield more information, and are easier to interpret thannondestructive testing. Destructive testing is most suitable, and economic, for objects whichwill be mass produced, as the cost of destroying a small number of specimens is negligible.It is usually not economic to do destructive testing where only one or very few items are tobe produced (for example, in the case of a building). Some types of destructive testing.

Stress testing: It is a form of testing that is used to determine the stability of agiven system or entity. It involves testing beyond normal operational capacity, oftento a breaking point, in order to observe the results. Stress testing may have a morespecific meaning in certain industries, such as fatigue testing for materials.

Crash testing: It is a form of destructive testing usually performed in order to ensuresafe design standards in crashworthiness and crash compatibility for automobiles orrelated components. Some of the examples are Frontal-Impact Tests, Offset Tests, andSide-Impact Tests, Roll over Tests, Roadside hardware crash tests e.t.c. The tests arenot discussed here as it is beyond the scope of this presentation

Hardness testing: Hardness refers to various properties of matter in the solid phasethat gives it high resistance to various kinds of shape change when force is applied.Macroscopic hardness is generally characterized by strong intermolecular bonds. How-ever, the behavior of solid materials under force is complex, resulting in several differentscientific definitions of what might be called hardness in everyday usage.

2.1 Definitions of Hardness

In materials science, there are three principal operational definitions of hardness.

Scratch hardness: Resistance to fracture or plastic (permanent) deformation due tofriction from a sharp object.

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Figure 2.1: Destructive testing of a 6-story non-ductile concrete building using a shaketable[2]

Indentation hardness: Resistance to plastic (permanent) deformation due to a con-stant load from a sharp object.

Rebound hardness: Height of the bounce of an object dropped on the material,related to elasticity.

The mathematical definition of hardness is the pressure applied over the projected contactarea between the indenter and the material being tested. As a result hardness values aretypically reported in units of pressure, although this is only a true pressure if the indenterand surface interface is perfectly flat[2].

2.2 Testing of large structuresBuilding structures or large nonbuilding structures (such as dams and bridges) are rarelysubjected to destructive testing due to the prohibitive cost of constructing a building, ora scale model of a building, just to destroy it. Earthquake engineering requires a goodunderstanding of how structures will perform at earthquakes. Destructive tests are morefrequently carried out for structures which are to be constructed in earthquake zones. Suchtests are sometimes referred to as crash tests, and they are carried out to verify the designedseismic performance of a new building, or the actual performance of an existing building.The tests are, mostly, carried out on a platform called a shake-table which is designed toshake in the same manner as an earthquake(Fig. 2.1).

2.3 Destructive software testing

Destructive software testing is a type of software testing which attempts to cause a pieceof software to fail in an uncontrolled manner, in order to test its robustness. As structural


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performance at earthquakes is better understood, testing of structures in earthquakes isincreasingly done by modeling the structure using specialist finite element software[2].


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Chapter 3

Non Destructive Testing

Nondestructive testing (NDT) are noninvasive techniques to determine the integrity of amaterial, component or structure or quantitatively measure some characteristic of an object.

In contrast to destructive testing, NDT is an assessment without doing harm, stress or de-stroying the test object. The destruction of the test object usually makes destructive testingmore costly and it is also inappropriate in many circumstances. NDT plays a crucial rolein ensuring cost effective operation, safety and reliability of plant, with resultant benefit tothe community. NDT is used in a wide range of industrial areas and is used at almost anystage in the production or life cycle of many components. The mainstream applications arein aerospace and civil structures, power generation, automotive, railway, petrochemical andpipeline markets. NDT of welds is one of the most used applications. It is very difficult toweld or mold a solid object that has no risk of breaking in service, so testing at manufactureand during use is often essential.

While originally NDT was applied only for safety reasons it is today widely ac-cepted as cost saving technique in the quality assurance process. Unfortunately NDT is stillnot used in many areas where human life or ecology is in danger. Some may prefer to pay thelower costs of claims after an accident than applying of NDT. That is a form of unacceptablerisk management.

For implementation of NDT it is important to describe what shall be found andwhat to reject. A completely flawless production is almost never possible. For this reasontesting specifications are indispensable. Nowadays there exists a great number of standardsand acceptance regulations. They describe the limit between good and bad conditions, but

also often which specific NDT method has to be used. The reliability of an NDT Methodis an essential issue. But a comparison of methods is only significant if it is referring tothe same task. Each NDT method has its own set of advantages and disadvantages and,therefore, some are better suited than others for a particular application. By use of artificialflaws, the threshold of the sensitivity of a testing system has to be determined. If the thesensitivity is to low defective test objects are not always recognized. If the sensitivity is toohigh parts with smaller flaws are rejected which would have been of no consequence to the

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serviceability of the component. With statistical methods it is possible to look closer into thefield of uncertainly. Methods such as Probability of Detection (POD) or the ROC-methodRelative Operating Characteristics are examples of the statistical analysis methods. Alsothe aspect of human errors has to be taken into account when determining the overall reli-

ability. Personnel Qualification is an important aspect of non-destructive evaluation. NDTtechniques rely heavily on human skill and knowledge for the correct assessment and inter-pretation of test results. Proper and adequate training and certification of NDT personnel istherefore a must to ensure that the capabilities of the techniques are fully exploited. Thereare a number of published international and regional standards covering the certification ofcompetence of personnel.

The most common NDT Methods are discussed in this presentation. In order ofmost used, they are: Ultrasonic Testing (UT), Radiographic Testing (RT), Liquid penetrantTesting, Magnetic particle Testing, Electromagnetic Testing (ET) in which Eddy CurrentTesting (ECT) is well know and Acoustic Emission (AE or AET). Besides the main NDTmethods a lot of other NDT techniques are available, such as Shearography Holography,Microwave and many more and new methods are being constantly researched and developed.In the next sections the methods are explained and their applications to structures arediscussed[3].


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Chapter 4

Ultrasonic testing

In ultrasonic testing, very short ultrasonic pulse-waves with center frequencies ranging from0.1-15 MHz and occasionally up to 50 MHz are launched into materials to detect internal

flaws or to characterize materials. The technique is also commonly used to determine thethickness of the test object, for example, to monitor pipework corrosion. Ultrasonic testing isoften performed on steel and other metals and alloys, though it can also be used on concrete,wood and composites, albeit with less resolution. It is a form of non destructive testing usedin many industries including aerospace, automotive and other transportation sectors[3].

4.1 How it works

In ultrasonic testing, an ultrasound transducer connected to a diagnostic machine is passedover the object being inspected. The transducer is typically separated from the test object

by a couplant (such as oil) or by water, as in immersion testing.

There are two methods of receiving the ultrasound waveform, reflection and at-tenuation. In reflection (or pulse-echo) mode, the transducer performs both the sending andthe receiving of the pulsed waves as the sound is reflected back to the device. Reflected ul-trasound comes from an interface, such as the back wall of the object or from an imperfectionwithin the object. The diagnostic machine displays these results in the form of a signal withan amplitude representing the intensity of the reflection and the distance, representing thearrival time of the reflection. In attenuation (or through-transmission) mode, a transmittersends ultrasound through one surface, and a separate receiver detects the amount that has

reached it on another surface after traveling through the medium. Imperfections or otherconditions in the space between the transmitter and receiver reduce the amount of soundtransmitted, thus revealing their presence

Advantages:

High penetrating power, which allows the detection of flaws deep in the part.

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High sensitivity, permitting the detection of extremely small flaws.

Only one surface need be accessible.

Greater accuracy than other nondestructive methods in determining the depth of in-ternal flaws and the thickness of parts with parallel surfaces.

Some capability of estimating the size, orientation, shape and nature of defects.

Nonhazardous to operations or to nearby personnel and has no effect on equipmentand materials in the vicinity.

Capable of portable or highly automated operation.

metals, nonmetals and composites

surface and slightly subsurface flaws can be detected

can be applied to welds, tubing, joints, castings, billets, forgings, shafts, structuralcomponents, concrete, pressure vessels, aircraft and engine components

used to determine thickness and mechanical properties

monitoring service wear and deterioration

Disadvantages:

Manual operation requires careful attention by experienced technicians

Extensive technical knowledge is required for the development of inspection procedures

Parts that are rough, irregular in shape, very small or thin, or not homogeneous aredifficult to inspect.

Surface must be prepared by cleaning and removing loose scale, paint, etc, althoughpaint that is properly bonded to a surface usually need not be removed.

Couplants are needed to provide effective transfer of ultrasonic wave energy betweentransducers and parts being inspected unless a non-contact technique is used. Non-contact techniques include Laser and Electro Magnetic Acoustic Transducers (EMAT).

Inspected items must be water resistant, when using water based couplants that donot contain rust inhibitors.

usually contacting, either direct or with intervening medium required (e.g. immersiontesting)

special probes are required for applications


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sensitivity limited by frequency used and some materials cause significant scattering

scattering by test material structure can cause false indications

not easily applied to very thin materials


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Chapter 5

Liquid Penetration Test

Dye penetrant inspection (DPI), also called liquid penetrant inspection (LPI), is a widely ap-plied and low-cost inspection method used to locate surface-breaking defects in all non-porous

materials (Can be applied to welds, tubing, castings, forgings, aluminum parts, turbineblades and disks, gears, metals, plastics). The penetrant may be applied to all non-ferrousmaterials, but for inspection of ferrous components magnetic-particle inspection is preferredfor its subsurface detection capability. LPI is used to detect casting and forging defects,cracks, and leaks in new products, and fatigue cracks on in-service components.The meritsof this technique are,Limited training is required for the operator - (although experience isquite valuable), Low testing costs.

5.1 Principle

DPI is based upon capillary action, where low surface tension fluid penetrates into cleanand dry surface-breaking discontinuities. Penetrant may be applied to the test componentby dipping, spraying, or brushing. After adequate penetration time has been allowed, theexcess penetrant is removed, a developer is applied. The developer helps to draw penetrantout of the flaw where a visible indication becomes visible to the inspector. Inspection isperformed under ultraviolet or white light, depending upon the type of dye used - fluorescentor nonfluorescent (visible).

5.2 Materials

Penetrants are classified into sensitivity levels. Visible penetrants are typically red in color,and represent the lowest sensitivity. Fluorescent penetrants contain two or more dyes thatfluoresce when excited by ultraviolet (UV-A) radiation (also known as black light). SinceFluorescent penetrant inspection is performed in a darkened environment, and the exciteddyes emit brilliant yellow-green light that contrasts strongly against the dark background,this material is more sensitive to small defects. When selecting a sensitivity level one mustconsider many factors, including the environment under which the test will be performed,

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the surface finish of the specimen, and the size of defects sought. One must also assure thatthe test chemicals are compatible with the sample so that the examination will not causepermanent staining, or degradation. This technique can be quite portable, because in itssimplest form the inspection requires only 3 aerosol spray cans, some paper towels, and ad-

equate visible light. Stationary systems with dedicated application, wash, and developmentstations, are more costly and complicated, but result in better sensitivity and higher samplethrough-put.

5.3 Procedure

Below are the main steps of Liquid Penetrant Inspection: 1. Pre-cleaning: The test sur-face is cleaned to remove any dirt, paint, oil, grease or any loose scale that could either keeppenetrant out of a defect, or cause irrelevant or false indications. Cleaning methods mayinclude solvents, alkaline cleaning steps, vapor degreasing. The end goal of this step is a

clean surface where any defects present are open to the surface, dry, and free of contamina-tion. 2. Application of Penetrant: The penetrant is then applied to the surface of theitem being tested. The penetrant is allowed time to soak into any flaws (generally 5 to 30minutes). The dwell time mainly depends upon the penetrant being used, material beingtesting and the size of flaws sought. As expected, smaller flaws require a longer penetrationtime 3. Excess Penetrant Removal: The excess penetrant is then removed from thesurface. The removal method is controlled by the type of penetrant used. Water-washable,solvent-removable, lipophilic post-emulsifiable, or hydrophilic post-emulsifiable are the com-mon choices. Emulsifiers represent the highest sensitivity level, and chemically interact withthe oily penetrant to make it removable with a water spray. When using solvent remover andlint-free cloth it is important to not spray the solvent on the test surface directly, becausethis can the remove the penetrant from the flaws. This process must be performed undercontrolled conditions so that all penetrant on the surface is removed (background noise), butpenetrants trapped in real defects remains in place.

4. Application of Developer: After excess penetrant has been removed a white devel-oper is applied to the sample. Several developer types are available, including: non-aqueouswet developer, dry powder, water suspendable, and water soluble. Choice of developer isgoverned by penetrant compatibility (one cant use water-soluble or suspendable developerwith water-washable penetrant), and by inspection conditions. When using non-aqueouswet developer (NAWD) or dry powder, the sample must be dried prior to application, whilesoluble and suspendable developers are applied with the part still wet from the previous

step. NAWD is commercially available in aerosol spray cans, and may employ acetone, iso-propyl alcohol, or a propellant that is a combination of the two. Developer should form asemi-transparent, even coating on the surface. The developer draws penetrant from defectsout onto the surface to form a visible indication, a process similar to the action of blottingpaper. Any colored stains indicate the positions and types of defects on the surface underinspection. 5. Inspection: The inspector will use visible light with adequate intensity (100foot-candles or 1100 lux is typical) for visible dye penetrant. Ultraviolet (UV-A) radiation


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of adequate intensity (1,000 micro-watts per centimeter squared is common), along withlow ambient light levels (less than 2 foot-candles) for fluorescent penetrant examinations.Inspection of the test surface should take place after a 10 minute development time. Thistime delay allows the blotting action to occur. The inspector may observe the sample for

indication formation when using visible dye. Also of concern, if one waits too long afterdevelopment, the indications may bleed out such that interpretation is hindered. 6. PostCleaning: The test surface is often cleaned after inspection and recording of defects, espe-cially if post-inspection coating processes are scheduled. The flaws are more visible, becauseThe defect indication has a high visual contrast (e.g. red dye against a white developerbackground, or a bright fluorescent indication against a dark background). The developerdraws the penetrant out of the flaw over a wider area than the real flaw, so it looks wider[2].

5.3.1 Limitations

need access to test surface

efects must be surface breaking

decontamination and precleaning of test surface may be needed

vapour hazard.

very tight and shallow defects difficult to find

depth of flaw not indicated

5.3.2 PrecautionsProper cleaning is necessary to assure that surface contaminants have been removed and anydefects present are clean and dry. Some cleaning methods have been shown to be detrimentalto test sensitivity, so acid etching to remove metal smearing and re-open the defect may benecessary. Penetrant dyes stain cloth, skin and other porous surfaces brought into contact.One should verify compatibility on the test material, especially when considering the testingof plastic components.

Steps,Fig 5.1

1.Section of material with a surface-breaking crack that is not visible to the naked eye.

2.Penetrant is applied to the surface.

3.Excess penetrant is removed.

4.Developer is applied, rendering the crack visible.


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Figure 5.1: Dye penetration test Steps[3]


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Chapter 6

Magnetic Particle inspection

Magnetic particle inspection processes are non-destructive methods for the detection of sur-face and sub-surface defects in ferrous materials. They make use of an externally applied

magnetic field through the material, and the principle that the magnetic flux will leave thepart at the area of the flaw. The presence of a surface or near surface flaw (void) in thematerial causes distortion in the magnetic flux through it, which in turn causes leakage ofthe magnetic fields at the flaw. This deformation of the magnetic field is not limited to theimmediate locality of the defect but extends for a considerable distance; even through thesurface and into the air if the magnetism is intense enough. Thus the size of the distortion ismuch larger than that of the defect and is made visible at the surface of the part by meansof the tiny particles that are attracted to the leakage fields. The most common method ofmagnetic particle inspection uses finely divided iron or magnetic iron oxide particles, heldin suspension in a suitable liquid (often kerosene). This fluid is referred to as carrier. The

particles are often colored and usually coated with fluorescent dyes that are made visiblewith a hand-held ultraviolet (UV) light (sometimes referred to as black light). The suspen-sion is sprayed or painted over the magnetized specimen during magnetization with a directcurrent or with an electromagnet, to localize areas where the magnetic field has protrudedfrom the surface. The magnetic particles are attracted by the surface field in the area ofthe defect and hold on to the edges of the defect to reveal it as a build up of particles.This inspection can be applied to raw material in a steel mill (billets or slabs), in the earlystages of manufacturing (forgings, castings), or most commonly to machined parts beforethey are put into service. It is also very commonly used for inspecting structural parts (e.g.,landing gear) that have been in-service for some time to find fatigue cracks. Usually testedpieces needs to be demagnetized before use. Parts are demagnetized by applying AC current

through the part and reducing the current which scrambles the magnetic domains causing itto demagnetize. It is a quite economic non-destructive test because it is easy to do and muchfaster than ultrasonic testing and radiographic testing. Because of the left hand rule, thereare two different ways of magnetizing a part, Longitudinal and Circular magnetization. Lon-gitudinal Magnetization passes current through a coil and the magnetic flux lines go throughthe part. Circular magnetization passes current through the part and establishes a magneticfield around the part. The two different methods are used because cracks can only be seen

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45 to 90 degrees to the magnetic flux lines. Magnetic Particle Inspection cannot be used fornon-ferrous materials and non-magnetic ferrous materials such as austenitic stainless steels.In such cases, other methods such as dye penetrant inspection are used[4].

6.1 Magnetic Flux Leckage(MFL)

It is a magnetic method of nondestructive testing that is used to detect corrosion and pittingin steel structures, most commonly pipelines and storage tanks. The basic principle is that apowerful magnet is used to magnetize the steel. At areas where there is corrosion or missingmetal, the magnetic field leaks from the steel. In an MFL tool, a magnetic detector isplaced between the poles of the magnet to detect the leakage field. Analysts interpret thechart recording of the leakage field to identify damaged areas and hopefully to estimate thedepth of metal loss. This method is best suitable for pipeline examination and tank floors.

6.1.1 Procedure

Typically, an MFL tool consists of two or more bodies. One body is the magnetizer withthe magnets and sensors and the other bodies contain the electronics and batteries. Themagnetizer body houses the sensors that are located between powerful rare-earth magnets.The magnets are mounted between the brushes and tool body to create a magnetic circuitalong with the pipe wall. As the tool travels along the pipe, the sensors detect interruptionsin the magnetic circuit. Interruptions are typically caused by metal loss and which in mostcases is corrosion. Mechanical damage such as shovel gouges can also be detected. The metalloss in a magnetic circuit is analogous to a rock in a stream. Magnetism needs metal to flowand in the absence of it, the flow of magnetism will go around, over or under to maintainits relative path from one magnet to another, similar to the flow of water around a rockin a stream. The sensors detect the changes in the magnetic field in the three directions(axial ,radial, or circumferential) to characterize the anomaly. An MFL tool can take sensorreadings based on either the distance the tool travels or on increments of time. The secondbody is called an Electronics Can. This section can be split into a number of bodies dependingon the size of the tool. This can, as the name suggests, contains the electronics or brainsof the instrument. The Electronics Can also contains the batteries and is some cases an IMU(Inertial Measurement Unit) to tie location information to GPS coordinates.

6.1.2 Features

Although primarily used to detect corrosion, MFL tools can also be used to detect featuresthat they were not originally designed to identify. When an MFL tool encounters a geometricdeformity such as a dent, wrinkle or buckle, a very distinct signal is created due to the plasticdeformation of the pipe wall.


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6.1.3 Corrosion

High-resolution MFL tools collect data approximately every 2 mm along the axis of a pipeand this superior resolution allows for a comprehensive analysis of collected signals. PipelineIntegrity Management programs have specific intervals for inspecting pipeline segments.Byemploying high-resolution MFL tools an exceptional corrosion growth analysis can be con-ducted. This type of analysis proves extremely useful in forecasting the inspection intervals.

6.1.4 Crack detection

There are cases where large non-axial oriented cracks have been found in a pipeline that wasinspected by a magnetic flux leakage tool. To an experienced MFL data analyst, a dent iseasily recognizable by trademark horseshoe signal in the radial component of the vectorfield. What is not easily identifiable to an MFL tool is the signature that a crack leaves[4].


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Chapter 7

ElectroMagnetic testing

Electromagnetic Testing (ET), as a form of nondestructive testing, in which the process ofinducing electric currents or magnetic fields or both inside a test object and observing the

electromagnetic response. If the test is set up properly, a defect inside the test object creates ameasurable response. The term Electromagnetic Testing is often intended to mean simplyEddy-Current Testing (ECT). However with an expanding number of electromagnetic andmagnetic test methods, Electromagnetic Testing is more often used to mean the wholeclass of electromagnetic test methods, of which Eddy-Current Testing is just one.CommonMethods of Electromagnetic Testing are:

7.0.5 Eddy current testing

Eddy-curren testing uses electromagnetic induction to detect flaw in conductive materials. Itis used to detect near-surface cracks and corrosion in metallic objects such as tubes and air-craft fuselage and structures. ECT is more commonly applied to nonferromagnetic materials,since in ferromagnetic materials the depth of penetration is relatively small[3].

Procedure

In a standard eddy current testing a circular coil carrying current is placed proximity to thetest specimen (electrically conductive).The alternating current in the coil generates changingmagnetic field which interacts with test specimen and generates eddy current. Variations inthe phase and magnitude of these eddy currents can be monitored using a second search coil,or by measuring changes to the current flowing in the primary excitation coil. Variations

in the electrical conductivity or magnetic permeability of the test object, or the presence ofany flaws, will cause a change in eddy current flow and a corresponding change in the phaseand amplitude of the measured current. This is the basis of standard (flat coil) eddy currentinspection, the most widely used eddy current technique. However, eddy-current testing candetect very small cracks in or near the surface of the material, the surfaces need minimalpreparation, and physically complex geometries can be investigated. The testing devices areportable, provide immediate feedback, and do not need to contact the item.

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Limitations

Only conductive materials can be tested, the surface of the material must be accessible, thefinish of the material may cause bad readings, the depth of penetration into the material islimited, and flaws that lie parallel to the probe may be undetectable.

7.0.6 Remote field testing (RFT)

It is an electromagnetic method of nondestructive testing whose main application is findingdefects in steel pipes and tubes. RFT may also referred to as RFEC (remote field eddycurrent) or RFET (remote field electromagnetic technique). An RFT probe is moved downthe inside of a pipe and is able to detect inside and outside defects with approximatelyequal sensitivity (although it can not discriminate between the two). Although RFT worksin nonferromagnetic materials such as copper and brass, its sister technology eddy-currenttesting is more effective in these materials.

Procedure

The basic RFT probe consists of an exciter coil (also known as a transmit or send coil)which sends a signal to the detector (or receive coil). The exciter coil is pumped with an ACcurrent and emits a magnetic field. The field travels outwards from the exciter coil, throughthe pipe wall, and along the pipe. The detector is placed inside the pipe two to three pipediameters away from the exciter and detects the magnetic field that has traveled back infrom the outside of the pipe wall (for a total of two through-wall transits). In areas of metalloss, the field arrives at the detector with a faster travel time (greater phase) and greatersignal strength (amplitude) due to the reduced path through the steel. Hence the dominant

mechanism of RFT is through-transmission.

Benefits

commonly applied to examination of boilers, heat exchangers, cast iron pipes, and pipelines.No need for direct contact with the pipe wall. Probe travel speed around 30 cm/s (1 footper second), usually slower in pipes greater than 3 inch diameter.

Limitations

Less sensitive to probe wobble than conventional eddy current testing (its sister technology

for nonferromagnetic materials). Because the field travels on the outside of the pipe, RFTshows reduced accuracy and sensitivity at conductive and magnetic objects on or near theoutside of the pipe, such as attachments or tube support plates. Two coils generally createtwo signals from one small defect - a headache for the analyst.


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7.0.7 Differences Between RFT and ECT

The main differences between RFT and conventional eddy-current testing (ECT) is inthe coil-to-coil spacing. The RFT probe has widely spaced coils to pick up the through-transmission field. The typical ECT probe has coils or coil sets that create a field andmeasure the response within a small area, close to the object being tested.

7.0.8 Alternating Current Field Measurement (ACFM)

It is similar to eddy current applied to steel. Its most common application is to detectand size cracks in welds. The ACFM probe induces a uniform alternating current in thearea under test and detects the resulting current flow near to the surface. This current isundisturbed if the area is defect free. A crack redirects the current around the ends andfaces of the crack. The ACFM instrument measures these disturbances in the field and usesmathematical modeling to estimate crack size[4].

Benefits

Detects and sizes cracks both length and depth. Can inspect any electrically conductivematerial. Data recorded electronically for off-line evaluation if necessary. Permanent recordof indications. Non-Invasive, inspection without removing any protective coating. Workswith surface temperatures up to 500 degrees Celsius.

Limitations

Not recommended for short sections or small items. Locations of weld repairs and localized

grinding can cause spurious indications. Crack length needs to be longer than 5 mm. Multipledefects reduce the ability to estimate defect depth. Equipment more bulky than for MT andindications may be more difficult to interpret.

7.0.9 Pulsed eddy current method

A method for the detection and the characterization of corrosion in multi-layer metallicstructures. For this technique, a coil (or coils) is used both as field source (driven by asquare wave voltage-controlled excitation), and/or as field sensor (measuring a transientresponse). The field sensor allows the capture of information about the condition of the areaof the structure under inspection. The ability of this technique to detect corrosion hinges

on the use of a transient response feature (i.e., Lift-off Point of Intersection) to infer thepresence of material loss.


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Chapter 8

Radiographic Testing

Radiographic Testing (RT), or industrial radiography, is a nondestructive testing (NDT)method of inspecting materials for hidden flaws by using the ability of short wavelength

electromagnetic radiation (high energy photons) to penetrate various materials. Since theamount of radiation emerging from the opposite side of the material can be detected andmeasured, variations in this amount (or intensity) of radiation are used to determine thicknessor composition of material. Penetrating radiations are those restricted to that part of theelectromagnetic spectrum of wavelength less than about 10 nanometers. The extra featureof these methods is films of the objects being tested can be produced.

8.1 X-ray Method

In this method x rays of high frequency are used for inspection.The instrument is as shown

in the Fig. 8.1.

8.1.1 Benefits

Metals, nonmetals, composites and mixed materials can be tested. Used on all shapes andforms; castings, welds, electronic assemblies, aerospace, marine and automotive components.

8.1.2 Limitations

Access to both sides of test piece needed. Voltage, focal spot size and exposure time crit-ical radiation hazards. cracks must be oriented parallel to beam for detection. sensitivitydecreases with increasing thickness[8].

8.2 Gamma Ray method

In this method Gamma rays emitted from radio active source are used for inspection.

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Figure 8.1: X-Ray Equipment[8]

Figure 8.2: X-Ray image of reinforced concrete column[9]


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8.2.1 Benefits

Usually used on dense or thick material. Used on all shapes and forms; castings, welds,electronic assemblies, aerospace, marine and automotive components. Used where thicknessor access limits X-ray generators.

8.2.2 Limitations

There some radiation chances of hazards. Cracks must be oriented parallel to beam fordetection. Sensitivity decreases with increasing thickness. Access to both sides of test pieceneeded. Not as sensitive as X-rays[8].


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Chapter 9

Miscellaneous methods

9.1 Visual inspection method

This system consists of stretch pole, which can turn and horizontal arm that can stretch from1.5m to 3m, and this horizontal arm has high sensitive camera equipment for the inspectionon the arm tip.The cameras record the data while traveling.Blind spots(supports) can beeasily checked by this method.Best suitable for pipelines and railway tracks[8].

9.2 LASER Testing

Laser, stands for Light Amplification by Stimulated Emission of Radiation. It is a devicethat emits light (electromagnetic radiation) through a process called stimulated emission.Lasers have applications like

The application of laser are (laser induced breakdown spectroscopy (LIBS)) to deter-mine the composition of building materials.

we can use lasers for cutting metals and measuring distances.

9.3 Infrared and Thermal Testing

Infrared Thermography is the science of measuring and mapping surface temperatures. Aninfrared thermographic scanning system can measure and view temperature patterns basedupon temperature differences as small as a few hundredths of a degree Celsius. Infrared

thermographic testing may be performed during day or night, depending on environmentalconditions and the desired results.Infrared thermography, a nondestructive, remote sensingtechnique, has proved to be an effective, convenient, and economical method of testingconcrete. It can detect internal voids, delaminations, and cracks in concrete structures suchas bridge decks, highway pavements, garage floors, parking lot pavements, and building walls.As a testing technique, some of its most important qualities are that (1) it is accurate; (2) itis repeatable; (3) it doesnt cause any inconvenience to the public; and (4) it is economical.

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Figure 9.1: Carrying of RADAR Test on pavement[5]

9.4 Schmidt hammer

In this the test hammer hits the concrete with a spring-driven pin at a defined energy, and

then measures the rebound (in rebound units). Its rebound is dependent on the hardnessof the concrete and is measured by test equipment. When conducting the test the hammershould be held perpendicular to the surface which in turn should be flat and smooth. Notethat the Schmidt hammer does not work well for small samples and will make marks. Byreference to the conversion tables, the rebound value can be used to determine the compres-sive strength. Schmidt hammers are available from their original manfacturers in severaldifferent energy ranges[9].

9.5 RADAR Technology

The RADAR technology based on Electromagnetic waves has been used not only in ar-chaeology but also in civil engineering. Suitable frequency for carrying experiment must bechosen based on the site conditions, materials involved e.t.c. Example: EM wave tomog-raphy of underground, locating the life lines buried at underground as gas or water pipes,finding air pockets under roads, etc. Moreover, radar system installed on the shield machinewhich excavates a tunnel at urban area and the system is utilized to find the life lines asgas or water pipes and the metal or woody old piles existing ahead of the machine.Radaris not much influenced by weather, and it is used for void inspection for tunnels and roadpavement. Therefore, radar is considered as the most suitable method for inspecting voidsunder pavement at airports. The Fig.9.1 shows the application of RADAR technology topavments.


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Chapter 10

Case Study: Bridge

Nondestructive testing can be an effective tool in the inspection and condition assessment ofbridge structures. It can provide knowledge that may not be possible to deduce from visual

observation alone. The integration of both visual and nondestructive inspection methods iskey to complete bridge condition assessment and management. Some simple nondestructivetechniques, such as hammer sounding, rebound hammer testing, dye penetration, and mag-netic particle testing, can be easily integrated into visual inspections. The results of theseintegrated inspections will improve bridge data files, and will yield more technically basedrecommendations for further inspection and maintenance, and more accurate estimations ofremaining service life. Once a full representation of the overall bridge condition is determined,appropriate and economical decisions regarding the possible rehabilitation or replacement ofbridge members or the entire structure can be made[9]. In the case of concrete bridge thedecks consist of a concrete slab covered by an asphalt coating. The concrete slabs generally

have a thickness of approximately 25 cm and contain two mats of steel rebar reinforcements.The most serious form of deterioration in concrete bridge decks is the corrosion of steel re-bars caused by the excessive use of chloride deicing salts during winter for the maintenanceof the structures. As the reinforcement steel corrodes, it expands and creates a cracks orsurface fracture plane in the concrete at, or just above, the level of the reinforcement. Thefracture plane, or delamination, may be localized or may extend over a substantial area.Recent advances in NDE techniques have improved the functional characteristics of manyNDE methods and have led to systems that are more reliable. Increased use of NDE meth-ods will depend on several factors including the ability of the systems to accurately detectdeteriorated conditions, the ease of use and field portability of the systems, and the totalcost of completing the NDE based inspections. Since bridges are build in almost hundreds

of different kind of types and using so many different materials of supporting components,so that it is not possible to use just one NDT method for all tasks. For instants, microwaveor ground penetrating radar (GPR) may be used for reinforced concrete decks but is notsuitable for weld testing of steel members. Also there are many tasks which need furtherresearch to make NDT methods suitable. A lot of application reports of NDT methods forbridge testing are there. Several methods are available, some are in research and some areused for further inspection after regularly inspections indicates their needs[7].

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They include:

Impact-Echo (IE) for checking integrity of concrete[9].

Impact-Echo (IE) is suitable to determine the thickness of the concrete[9].

Magnetic flux leakage (MFL) to detect corrosion in strands and bars in post-tensionedconcrete structures[].

The nuclear magnetic resonance (NMR) method can determine the presence and loca-tion of water. This enables the determination of pore size and pore distribution as theconcrete cures.

Infrared imaging technologies to find defects in the concrete parts of bridges..

Strain Transducers to record the induced strains.

Potential mapping is the most simple electrochemical technique used for obtainingcorrosion information on site. The technique informs qualitatively on the corrosionrisk of the reinforcement in the reinforced concrete structures.

Acoustic emission monitoring can play a very effective role in enhancing safety, ensuringavailability and reducing the repair/refurbishment costs of bridges.

A well-established and accepted application of GPR is the accurate condition assess-ment of bridge decks as well as other reinforced concrete structures. The ability ofGPR to be used without requiring the removal of an existing asphalt.

Ultrasonic testing allows also the visualization of perpendicularly arranged reinforce-ment bars, tendon ducts. UT can compensate deficits of the radar method[4].

The combined application of radar, impact-echo and ultrasonic echo for the assessmentof post-tensioned concrete structures[5].

Liquid-penetration testing - Visual check of dye that penetrates hairline fractures inpavement.

Ultrasonic testing of welds, bolts and rivets steel members.

Divers inspect underwater concrete piers, which may have been damaged by erosion.

Magneto-inductive is used for evaluation of cables and wires.

Laser Measurement Technologies for numerous applications using laser-based distancemeasurements for highway infrastructure. Applications for this technology includemeasuring bridge deflections under calibrated load to evaluate structural behavior,calculating out-of-plane distortions in girder webs and flanges, and evaluating the as-built construction of large structures such as abutments.


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Bridge Monitoring Systems using acoustic emission sensors or eddy-current sensor.Generally, these instruments are dedicated, remote data-acquisition systems that col-lect information on the behavior of a structure over time. The acoustic emissionsdetection system can evaluate sounds emanating from any materials, including steel

cables, trusses and concrete in a bridge. Cracks can be recognized many months beforethey are visible on the surface.

Thermographic methods for evaluating composite bridges and composite bridge repairs.

Both ultrasonic and radiographic testing are used to inspect steel bridges during fab-rication to ensure weld quality.

Automated Ultrasonic Testing (AUT) can be an effective inspection tool that could beused in place of radiography under certain conditions.

Ultrasonic velocity measurements can be used as a quality-control tool during con-

struction and how ultrasonic testing may be used for in-service inspection of bridgesconstructed of reactive powder concrete (RPC).

Electromagnetic acoustic transducers could detect broken wires within a strand.

Electrochemical Fatigue Sensor (EFS) can be used to determine if actively growingfatigue cracks are present. An EFS sensor is first applied to the fatigue sensitivelocation on the bridge or metal structure, and then is injected with an electrolyte atwhich point a small voltage is applied. An algorithm automatically indicates the levelof fatigue crack activity at the inspection location.

Brillouin fiber optic sensor technology is a promising technology for structural healthmonitoring (SHM) thanks to its unique feature of distributed strain and temperaturemeasurement by means of low-cost optical fibers.

X-ray computed tomography for determination of crack propagation in concrete us-ing[8].

X-ray computed tomography for determination of void percentage and distribution inconcrete.

Prompt Gamma Neutron Activation Analysis for determination of chloride concentra-tion and depth profiles in concrete.

Neutron scattering technique measuring cement hydration using.

Ultrasonic method for direct measurement of preload force of bolted connections. Thisis an innovative technique for direct measurement of actual bolt stresses


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Figure 10.1: Schmidt Hammer[7],GPR Test being conducted on a bridge[5]

Figure 10.2: Figure showing the RADAR data[5]

Robotic Tacheometry System (RTS) (Total Station) offers the capability to measure thespatial coordinates of discrete points on a bridge without having to touch the structure.There is incredible technology out there to help monitor and diagnose problems, andresearch continues to develop new technologies to keep up with the infrastructure ofbridges. Older bridges need more than just a regular inspection. As bridge get older,it needs more tests[7].


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Chapter 11

Conclusions

The present report aimed at explaining the methods of NDT and their techniques. A casestudy of bridges also studied. Engineering is not always complete, and further research works

are needed. To set up a good system for maintenance of existing concrete structures, thereare still many things to be done. Different methods can be applied to the same problem,but the best method is choosen based on the features of the problem.

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Bibliography

[1] J H BUNGEY, Non destructive testing in U K.,Seiken Symposium,2000.

[2] www.wikipedia.com, Non destructive testing., 14 November 2009.

[3] www.ndt.org, Non destructive testing 14 November 2009.

[4] Y.H.Cha anb J H shu. Differential approach to Ultrasonic Testing of Strength andHomogenety of concrete Seiken symposium.,2000.

[5] K R Maser and I sande , Application of Ground Penetration RADAR technique forevaluation of Air field pavment, Seiken Symposium 2000 .

[6] N.Tamura and K.Takada, High Evaluation Inspection vehicle, Seiken Symposium,2000.

[7] K Brandes,J Herther and R.Helmerich. Non - destructive testing being essential partof the safety assesment of steel bridges Seiken Symposium 2000.

[8] J F Hinslay, text book of Non destructive Testing.

[9] A P Ferrerira and P F Castro. NDT for assessing concrete strength Seiken sympo-sium.,2000.

ndt structural steel

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