ET01

Marvin W. Trimm, Westinghouse Savannah RiverCompany, Aiken, South Carolina (Parts 1 and 2)

Holger H. Streckert, General Atomics, San Diego,California (Part 3)

Andrew P. Washabaugh, Jentek Sensors, Waltham,Massachusetts (Part 2)

1C H A P T E R

Introduction toElectromagnetic Testing

PART 1. Nondestructive Testing

DefinitionNondestructive testing (NDT) has beendefined as comprising those methods usedto test a part or material or systemwithout impairing its future usefulness.1The term is generally applied tononmedical investigations of materialintegrity.

Strictly speaking, this definition ofnondestructive testing includesnoninvasive medical diagnostics.Ultrasound, X-rays and endoscopes areused by both medical and industrialnondestructive testing. Medicalnondestructive testing, however, has cometo be treated by a body of learning soseparate from industrial nondestructivetesting that today most physicians do notuse the word nondestructive.

Nondestructive testing is used toinvestigate specifically the materialintegrity or properties of the test object. Anumber of other technologies — forinstance, radio astronomy, voltage andamperage measurement and rheometry(flow measurement) — are nondestructivebut are not used specifically to evaluatematerial properties. Radar and sonar areclassified as nondestructive testing whenused to inspect dams, for instance, butnot when they are used to chart a riverbottom.

Nondestructive testing asks “Is theresomething wrong with this material?” Incontrast, performance and proof tests ask“Does this component work?” It is notconsidered nondestructive testing whenan inspector checks a circuit by runningelectric current through it. Hydrostaticpressure testing is another form of prooftesting, one that sometimes destroys thetest object.

Another gray area that invites variousinterpretations in defining nondestructivetesting is future usefulness. Some materialinvestigations involve taking a sample ofthe tested part for a test that is inherentlydestructive. A noncritical part of apressure vessel may be scraped or shavedto get a sample for electron microscopy,for example. Although future usefulnessof the vessel is not impaired by the loss ofmaterial, the procedure is inherentlydestructive and the shaving itself — inone sense the true test object — has beenremoved from service permanently.

MOVIE.Need fornondestructivetesting.

2 Electromagnetic Testing

The idea of future usefulness is relevantto the quality control practice ofsampling. Sampling (that is, less than100 percent testing to draw inferencesabout the unsampled lots) isnondestructive testing if the tested sampleis returned to service. If the steel is testedto verify the alloy in some bolts that canthen be returned to service, then the testis nondestructive. In contrast, even ifspectroscopy used in the chemical testingof many fluids is inherentlynondestructive, the testing is destructive ifthe samples are poured down the drainafter testing.

Nondestructive testing is not confinedto crack detection. Other discontinuitiesinclude porosity, wall thinning fromcorrosion and many sorts of disbonds.Nondestructive material characterizationis a growing field concerned with materialproperties including materialidentification and microstructuralcharacteristics — such as resin curing, casehardening and stress — that have a directinfluence on the service life of the testobject.

Methods and TechniquesNondestructive testing has also beendefined by listing or classifying thevarious techniques.1-3 This sense ofnondestructive testing is practical in that ittypically highlights methods in use byindustry.

In the Nondestructive Testing Handbook,the word method is used for a group of testtechniques that share a form of probingenergy. Ultrasonic test methods, forexample, use acoustic waves faster thansound. Infrared and thermal testing andradiographic testing both useelectromagnetic radiation, each in adefined wavelength range. A technique, incontrast, has features that adapt themethod to the application.Through-transmission immersion testingis a technique of the ultrasonic method,for example.

Purposes ofNondestructive TestingSince the 1920s, the art of testing withoutdestroying the test object has developed

FIGURE 2. Boilers operate with high internal steam pressure.Material discontinuities can lead to sudden, violent failurewith possible injury to people and property.

from a laboratory curiosity to anindispensable tool of fabrication,construction, manufacturing andmaintenance processes. No longer isvisual testing of materials, parts andcomplete products the principal means ofdetermining adequate quality.Nondestructive tests in great variety are inworldwide use to detect variations instructure, minute changes in surfacefinish, the presence of cracks or otherphysical discontinuities, to measure thethickness of materials and coatings and todetermine other characteristics ofindustrial products. Scientists andengineers of many countries havecontributed greatly to nondestructive testdevelopment and applications.

The various nondestructive testingmethods are covered in detail in theliterature but it is always wise to considerobjectives before details. How isnondestructive testing useful? Why dothousands of industrial concerns buy thetesting equipment, pay the subsequentoperating costs of the testing and evenreshape manufacturing processes to fit theneeds and findings of nondestructivetesting?

Modern nondestructive tests are usedby manufacturers (1) to ensure productintegrity and in turn reliability, (2) toavoid failures, prevent accidents and savehuman life (see Figs. 1 and 2), (3) to makea profit for the user, (4) to ensurecustomer satisfaction and maintain themanufacturer’s reputation, (5) to aid inbetter product design, (6) to controlmanufacturing processes, (7) to lowermanufacturing costs, (8) to maintainuniform quality level and (9) to ensureoperational readiness.

These reasons for widespread andprofitable nondestructive testing aresufficient in themselves but parallel

FIGURE 1. Fatigue cracks contributed to damage to aircraftfuselage in flight (April 1988).

developments have contributed to itsgrowth and acceptance.

Increased Demand on MachinesIn the interest of greater performance andreduced cost for materials, the designengineer is often under pressure to reduceweight. This can sometimes be done bysubstituting aluminum alloys, magnesiumalloys or composite materials for steel oriron but such light parts may not be thesame size or design as those they replace.The tendency is also to reduce the size.These pressures on the designer havesubjected parts of all sorts to increasedstress levels. Even such commonplaceobjects as sewing machines, sauce pansand luggage are also lighter and moreheavily loaded than ever before. The stressto be supported is seldom static. It oftenfluctuates and reverses at low or highfrequencies. Frequency of stress reversalsincreases with the speeds of modernmachines and thus parts tend to fatigueand fail more rapidly.

Another cause of increased stress onmodern products is a reduction in thesafety factor. An engineer designs withcertain known loads in mind. On thesupposition that materials andworkmanship are never perfect, a safetyfactor of 2, 3, 5 or 10 is applied. However,a lower factor is often used that dependson considerations such as cost or weight.

New demands on machinery have alsostimulated the development and use ofnew materials whose operatingcharacteristics and performance are notcompletely known. These new materials

3Introduction to Electromagnetic Testing

could create greater and potentiallydangerous problems. For example, anaircraft part was built from an alloy whosework hardening, notch resistance andfatigue life were not well known. Afterrelatively short periods of service, some ofthe aircraft using these parts suffereddisastrous failures. Sufficient and propernondestructive tests could have savedmany lives.

As technology improves and as servicerequirements increase, machines aresubjected to greater variations and towider extremes of all kinds of stress,creating an increasing demand forstronger or more damage tolerantmaterials.

Engineering Demands for SounderMaterialsAnother justification for nondestructivetests is the designer’s demand for soundermaterials. As size and weight decrease andthe factor of safety is lowered, moreemphasis is placed on better raw materialcontrol and higher quality of materials,manufacturing processes andworkmanship.

An interesting fact is that a producer ofraw material or of a finished productsometimes does not improve quality orperformance until that improvement isdemanded by the customer. The pressureof the customer is transferred toimplementation of improved design ormanufacturing. Nondestructive testing isfrequently called on to deliver this newquality level.

Public Demands for Greater SafetyThe demands and expectations of thepublic for greater safety are apparenteverywhere. Review the record of thecourts in granting high awards to injuredpersons. Consider the outcry for greaterautomobile safety, as evidenced by therequired automotive safety belts and thedemand for air bags, blowout proof tiresand antilock braking systems. Thepublicly supported activities of theNational Safety Council, UnderwritersLaboratories, the Occupational Safety andHealth Administration and the FederalAviation Administration in the UnitedStates, as well as the work of similaragencies abroad, are only a few of theways in which this demand for safety isexpressed. It has been expressed directlyby passengers who cancel reservationsfollowing a serious aircraft accident. Thisdemand for personal safety has beenanother strong force in the developmentof nondestructive tests.


Rising Costs of FailureAside from awards to the injured or toestates of the deceased and aside fromcosts to the public (because of evacuationoccasioned by chemical leaks) considerbriefly other factors in the rising costs ofmechanical failure. These costs areincreasing for many reasons. Someimportant ones are (1) greater costs ofmaterials and labor, (2) greater costs ofcomplex parts, (3) greater costs because ofthe complexity of assemblies, (4) greaterprobability that failure of one part willcause failure of others because ofoverloads, (5) trend to lower factors ofsafety, (6) probability that the failure ofone part will damage other parts of highvalue and (7) part failure in an integratedautomatic production machine, shuttingdown an entire high speed productionline. When production was carried out onmany separate machines, the broken onecould be bypassed until repaired. Todayone machine is tied into the productionof several others. Loss of such productionis one of the greatest losses resulting frompart failure.

Applications ofNondestructive TestingNondestructive testing is a branch of thematerials sciences that is concerned withall aspects of the uniformity, quality andserviceability of materials and structures.The science of nondestructive testingincorporates all the technology fordetection and measurement of significantproperties, including discontinuities, initems ranging from research specimens tofinished hardware and products in service.By definition nondestructive test methodsprovide a means for examining materialsand structures without disruption orimpairment of serviceability.Nondestructive testing makes it possiblefor internal properties or hiddendiscontinuities to be revealed or inferred.

Nondestructive testing is becomingincreasingly vital in the effective conductof research, development, design andmanufacturing programs. Only withappropriate nondestructive testingmethods can the benefits of advancedmaterials science be fully realized. Theinformation required for appreciating thebroad scope of nondestructive testing isavailable in many publications andreports.

Classification of MethodsThe National Materials Advisory Board(NMAB) Ad Hoc Committee onNondestructive Evaluation adopted a

system that classified techniques into sixmajor method categories: visual,penetrating radiation, magnetic-electrical,mechanical vibration, thermal andchemical/electrochemical.3 A modifiedversion is presented in Table 1.1

Each method can be completelycharacterized in terms of five principalfactors: (1) energy source or medium usedto probe the object (such as X-rays,ultrasonic waves or thermal radiation);(2) nature of the signals, image orsignature resulting from interaction withthe object (attenuation of X-rays orreflection of ultrasound, for example);(3) means of detecting or sensingresultant signals (photoemulsion,piezoelectric crystal or inductance coil);(4) means of indicating or recordingsignals (meter deflection, oscilloscopetrace or radiograph); and (5) basis forinterpreting the results (direct or indirectindication, qualitative or quantitative andpertinent dependencies).

The objective of each method is toprovide information about one or more ofthe following material parameters:(1) discontinuities and separations (cracks,voids, inclusions, delaminations andothers); (2) structure or malstructure(crystalline structure, grain size,segregation, misalignment and others);(3) dimensions and metrology (thickness,diameter, gap size, discontinuity size and

TABLE 1. Nondestructive testing method categories.

Categories

Basic Categories

Mechanical and optical color, cracks, dimensions, film thicfinish, surface flaws, through-cra

Penetrating radiation cracks; density and chemistry variamisalignment; missing parts; seg

Electromagnetic and electronic alloy content; anisotropy; cavities;corrosion; cracks; crack depth; chot tears; inclusions; ion concenpolarization; seams; segregation

Sonic and ultrasonic crack initiation and propagation; cdegree of sintering; delaminatiomechanical degradation; misaligsurface stress; tensile, shear and

Infrared and thermal anisotropy, bonding; composition;stress; thermal conductivity; thicmoisture; corrosion

Chemical and analytical alloy identification; composition; cmacrostructure; porosity; segreg

Auxiliary Categories

Image generation dimensional variations; dynamic pdistribution; anomaly propagatio

Signal image analysis data selection, processing and dispenhancement; separation of mu

others); (4) physical and mechanicalproperties (reflectivity, conductivity,elastic modulus, sonic velocity andothers); (5) composition and chemicalanalysis (alloy identification, impurities,elemental distributions and others);(6) stress and dynamic response (residualstress, crack growth, wear, vibration andothers); (7) signature analysis (imagecontent, frequency spectrum, fieldconfiguration and others); and(8) abnormal sources of heat.

Material characteristics in Table 1 arefurther defined in Table 2 with respect tospecific objectives and specific attributesto be measured, detected and defined.

The limitations of a method includeconditions to be met for methodapplication (access, physical contact,preparation and others) and requirementsto adapt the probe or probe medium tothe object examined. Other factors limitthe detection or characterization ofdiscontinuities, properties and otherattributes and limit interpretation ofsignals or images generated.

Classification Relative to TestObjectNondestructive testing techniques may beclassified according to how they detectindications relative to the surface of a testobject. Surface methods include liquid


Objectives

kness, gaging, reflectivity, strain distribution and magnitude, surfacecks

tions; elemental distribution; foreign objects; inclusions; microporosity;regation; service degradation; shrinkage; thickness; voids

cold work; local strain, hardness; composition; contamination;rystal structure; electrical conductivities; flakes; heat treatment;trations; laps; lattice strain; layer thickness; moisture content;; shrinkage; state of cure; tensile strength; thickness; disbonds; voids

racks, voids; damping factor; degree of cure; degree of impregnation;ns; density; dimensions; elastic moduli; grain size; inclusions;nment; porosity; radiation degradation; structure of composites;compressive strength; disbonds; wear

emissivity; heat contours; plating thickness; porosity; reflectivity;kness; voids; cracks; delaminations; heat treatment; state of cure;

racks; elemental analysis and distribution; grain size; inclusions;ation; surface anomalies

erformance; anomaly characterization and definition; anomalyn; magnetic field configurations

lay; anomaly mapping, correlation and identification; imageltiple variables; signature analysis

penetrant testing, visual testing, gridtesting and moiré testing.Surface/near-surface methods include tap,potential drop, holographic,shearographic, magnetic particle andelectromagnetic testing. When surface ornear-surface methods are applied duringintermediate manufacturing processes,they provide preliminary assurance that


TABLE 2. Objectives of nondestructive testing methods.

Objectives Attributes

Discontinuities and Separations

Surface anomalies roughness, scratches, gouges, crazingSurface connected anomalies cracks, porosity, pinholes, laps, seams,Internal anomalies cracks, separations, hot tears, cold shu

disbonds, poor bonds, inclusions, se

Structure

Microstructure molecular structure; crystalline structudeformation

Matrix structure grain structure, size, orientation and pdistribution; anisotropy; heterogene

Small structural anomalies leaks (lack of seal or through-holes), pGross structural anomalies assembly errors; misalignment; poor s

Dimensions and Metrology

Displacement; position linear measurement; separation; gap sDimensional variations unevenness; nonuniformity; eccentriciThickness; density film, coating, layer, plating, wall and s

Physical and Mechanical Properties

Electrical properties resistivity; conductivity; dielectric consMagnetic properties polarization; permeability; ferromagneThermal properties conductivity; thermal time constant anMechanical properties compressive, shear and tensile strengt

and embrittlementSurface properties color, reflectivity, refraction index, em

Chemical Composition and Analysis

Elemental analysis detection, identification, distribution aImpurity concentrations contamination, depletion, doping andMetallurgical content variation; alloy identification, verificatiPhysiochemical state moisture content; degree of cure; ion

Stress and Dynamic Response

Stress, strain, fatigue heat treatment, annealing and cold wMechanical damage wear, spalling, erosion, friction effectsChemical damage corrosion, stress corrosion, phase tranOther damage radiation damage and high frequencyDynamic performance crack initiation, crack propagation, pla

timing of events, any anomalous be

Signature Analysis

Electromagnetic field potential; intensity; field distribution aThermal field isotherms, heat contours, temperatureAcoustic signature noise, vibration characteristics, freque

emissions, ultrasonic emissionsRadioactive signature distribution and diffusion of isotopes aSignal or image analysis image enhancement and quantization

and correlation; discontinuity identidiscontinuity mapping and display

volumetric methods performed on thecompleted object or component willreveal few rejectable discontinuities.Volumetric methods include radiography,ultrasonic testing, acoustic emissiontesting and less widely used methods suchas acoustoultrasonic testing and magneticresonance imaging. Through-boundarytechniques include leak testing, some

Measured or Detected

, pitting, imbedded foreign material folds, inclusionsts, shrinkage, voids, lack of fusion, pores, cavities, delaminations,gregations

re and/or strain; lattice structure; strain; dislocation; vacancy;

hase; sinter and porosity; impregnation; filler and/or reinforcementity; segregationoor fit, poor contact, loose parts, loose particles, foreign objectspacing or ordering; deformation; malformation; missing parts

ize; discontinuity size, depth, location and orientationty; shape and contour; size and mass variationsheet thickness; density or thickness variations

tant and dissipation factortism; cohesive force, susceptibilityd thermoelectric potential; diffusivity; effusivity; specific heat

h (and moduli); Poisson’s ratio; sonic velocity; hardness; temper

issivity

nd/or profile diffusantson and sortingconcentrations and corrosion; reaction products

ork effects; stress and strain; fatigue damage and life (residual)

sformation voltage breakdownstic deformation, creep, excessive motion, vibration, damping,havior

nd patterns, heat flow, temperature distribution, heat leaks, hot spots, contrast

ncy amplitude, harmonic spectrum, harmonic analysis, sonic

nd tracers; pattern recognition; densitometry; signal classification, separation;fication, definition (size and shape) and distribution analysis;

FIGURE 3. Electromagnetic testing: (a) representative setupfor eddy current test; (b) in-service detection ofdiscontinuities.

Coil in eddy current probePrimary electromagnetic field

Direction ofprimary current

Eddy current strengthdecreases withincreasing depth

Direction of eddycurrents

Conducting specimen

Induced field

(a)

(b)

infrared thermographic techniques,airborne ultrasonic testing and certaintechniques of acoustic emission testing.Other less easily classified methods arematerial identification, vibration analysisand strain gaging.

No one nondestructive testing methodis all revealing. In some cases, onemethod or technique may be adequate fortesting a specific object or component.However, in most cases, it takes a series oftest methods to do a completenondestructive test of an object orcomponent. For example, if surface cracksmust be detected and eliminated and ifthe object or component is made offerromagnetic material, then magneticparticle testing would be the appropriatechoice. If the material is aluminum ortitanium, then the choice would be liquidpenetrant or electromagnetic testing.However, if internal discontinuities are tobe detected, then ultrasonic testing orradiography would be the selection. Theexact technique in each case woulddepend on the thickness and nature ofthe material and the types ofdiscontinuities that must be detected.

Value of NondestructiveTestingThe contribution of nondestructivetesting to profits has been acknowledgedin the medical field and computer andaerospace industries. However, inindustries such as heavy metals, althoughnondestructive testing may be reluctantlyaccepted, its contribution to profits maynot be obvious to management.Nondestructive testing is sometimesthought of only as a cost item and can becurtailed by industry downsizing. When acompany cuts costs, two vulnerable areasare quality and safety. When biddingcontract work, companies add profitmargin to all cost items, includingnondestructive testing, so a profit shouldbe made on the nondestructive testing.The attitude toward nondestructivetesting is positive when managementunderstands its value.

Nondestructive testing should be usedas a control mechanism to ensure thatmanufacturing processes are within designperformance requirements. When usedproperly, nondestructive testing savesmoney for the manufacturer. Rather thancosting the manufacturer money,nondestructive testing should add profitsto the manufacturing process.

Overview of OtherNondestructive TestingMethodsTo optimize the use of nondestructivetesting it is necessary first to understandthe principles and applications of all themethods. This volume featureselectromagnetic testing (Fig. 3) — onlyone of the nondestructive test methods.The following section briefly describesseveral other methods and theapplications associated with them.


FIGURE 5. Liquid penetrant indication ofcracking.

Visual TestingPrinciples. Visual testing (Fig. 4) is theobservation of a test object, either directlywith the eyes or indirectly using opticalinstruments, by an inspector to evaluatethe presence of surface anomalies and theobject’s conformance to specification.Visual testing should be the firstnondestructive testing method applied toan item. The test procedure is to clean thesurface, provide adequate illuminationand observe. A prerequisite necessary forcompetent visual testing of an object isknowledge of the manufacturing processesby which it was made, of its servicehistory and of its potential failure modes,as well as related industry experience.Applications. Visual testing provides ameans of detecting and examining avariety of surface discontinuities. It is themost widely used method for detectingand examining for surface discontinuitiesassociated with various structural failuremechanisms. Even when othernondestructive tests are performed, visualtests often provide a useful supplement.When the eddy current testing of processtubing is performed, for example, visualtesting is often performed to verify andmore closely examine the surfacecondition. The following discontinuitiesmay be detected by a simple visual test:surface discontinuities, cracks,misalignment, warping, corrosion, wearand physical damage.

Liquid Penetrant TestingPrinciples. Liquid penetrant testing (Fig. 5)reveals discontinuities open to thesurfaces of solid and nonporous materials.Indications of a wide variety ofdiscontinuity sizes can be found regardlessof the configuration of the work piece andregardless of discontinuity orientations.Liquid penetrants seep into various types


FIGURE 4. Visual test using borescope toview interior of cylinder.

of minute surface openings by capillaryaction. The cavities of interest can be verysmall, often invisible to the unaided eye.The ability of a given liquid to flow over asurface and enter surface cavities dependsprincipally on the following: cleanlinessof the surface, surface tension of theliquid, configuration of the cavity, contactangle of the liquid, ability of the liquid towet the surface, cleanliness of the cavityand size of surface opening of the cavity.Applications. The principal industrial usesof liquid penetrant testing includepostfabrication testing, receiving testing,in-process testing and quality control,maintenance and overhaul in thetransportation industries, in-plant andmachinery maintenance and in testing oflarge components. The following are someof the typically detected discontinuities:surface discontinuities, seams, cracks, laps,porosity and leak paths.

Magnetic Particle TestingPrinciples. Magnetic particle testing(Fig. 6) is a method of locating surfaceand slightly subsurface discontinuities inferromagnetic materials. It depends on thefact that when the material or part under

FIGURE 6. In magnetic particle testing, particles gather wherelines of magnetic force leak from discontinuity.

Magnetic particles

N SNS

Crack

LegendN = north poleS = south pole

test is magnetized, discontinuities that liein a direction generally transverse to thedirection of the magnetic field will cause aleakage field to be formed at and abovethe surface of the part. The presence ofthis leakage field and therefore thepresence of the discontinuity is detectedwith fine ferromagnetic particles appliedover the surface, with some of theparticles being gathered and held to forman outline of the discontinuity. Thisgenerally indicates its location, size, shapeand extent. Magnetic particles are appliedover a surface as dry particles or as wetparticles in a liquid carrier such as wateror oil.Applications. The principal industrial usesof magnetic particle testing include final,receiving and in-process testing; forquality control; for maintenance andoverhaul in the transportation industries;for plant and machinery maintenance;and for testing of large components. Someof the typically detected discontinuitiesare surface discontinuities, seams, cracksand laps.

Radiographic TestingPrinciples. Radiographic testing (Fig. 7) isbased on the differential absorption ofpenetrating radiation — eitherelectromagnetic radiation of very shortwavelength or particulate radiation(X-rays, gamma rays and neutron rays) —by the part or object being tested.Different portions of an object absorbdifferent amounts of penetrating radiationbecause of differences in density andvariations in thickness of the part ordifferences in absorption characteristics

FIGURE 7. Representative setup for radiographic testing.

Radiation source

Specimen

Void

Discontinuity imagesImage plane

caused by variation in composition. Thesevariations in the absorption of thepenetrating radiation can be monitoredby detecting the unabsorbed radiationthat passes through the object. Thismonitoring may be in different forms.The traditional form is through radiationsensitive film. Radioscopic sensors providedigital images. X-ray computedtomography is a radiographic technique.Applications. The principal industrial usesof radiographic testing involve testing ofcastings and weldments, particularlywhere there is a critical need to ensurefreedom from internal discontinuities. Forexample radiography is often specified forthick wall castings and weldments forsteam power equipment (boiler andturbine components and assemblies).Radiography can also be used on forgingsand mechanical assemblies, although withmechanical assemblies radiography isusually limited to testing for conditionsand proper placement of components.Typically detected discontinuities andconditions include inclusions, lack offusion, cracks, corrosion, porosity, leakpaths, missing or incomplete componentsand debris.

Ultrasonic TestingPrinciples. Ultrasonic testing (Fig. 8) is anondestructive method in which beams ofsound waves at a frequency too high tohear are introduced into materials for thedetection of surface and subsurfacediscontinuities in the material. Theseacoustic waves travel through the materialwith some attendant loss of energy(attenuation) and are reflected atinterfaces. The echoes are then analyzedto define the presence and locations ofdiscontinuities.Applications. Ultrasonic testing of metalsis widely used, principally for thedetection of discontinuities. This methodcan be used to detect internaldiscontinuities in most engineeringmetals and alloys. Bonds produced bywelding, brazing, soldering and adhesivescan also be ultrasonically examined.Inline techniques have been developed formonitoring and classifying materials asacceptable, salvageable or scrap and forprocess control. Other applicationsinclude testing of piping and pressurevessels, nuclear systems, motor vehicles,machinery, structures, railroad rollingstock and bridges and thicknessmeasurement.

Leak TestingPrinciples. Leak testing is concerned withthe flow of liquids or gases frompressurized or into evacuatedcomponents. The principles of leak testing


involve the physics of fluid (liquids orgases) flowing through a barrier where apressure differential or capillary actionexists. Leaking fluids (liquid or gas) canpropagate from inside a component orassembly to the outside, or vice versa, as aresult of a pressure differential betweenthe two regions or as a result ofpermeation through a barrier.

Leak testing encompasses proceduresthat fall into these basic functions: leaklocation, leakage measurement andleakage monitoring. There are several


FIGURE 8. Classic setups for ultrasonic testing:(a) longitudinal wave technique; (b) shear wave technique.

Transducer

Crack

Bolt

Time

Crack

Back surface

Crack

Entry surface

Crack

Skip distancewhere a = b

(a)

(b)

a b

subsidiary methods of leak testing,entailing tracer gas detection (Fig. 9),pressure change measurement,observation of bubble formation andother means.Applications. Like other forms ofnondestructive testing leak testing has animpact on the safety and performance ofa product. Reliable leak testing decreasescosts by reducing the number of reworkedproducts, warranty repairs and liabilityclaims. The most common reasons forperforming a leak test are to prevent theloss of costly materials or energy; toprevent contamination of theenvironment; to ensure component orsystem reliability; and to prevent thepotential for an explosion or fire.

Acoustic Emission TestingPrinciples. Acoustic emissions aremechanical waves produced by suddenmovement in stressed materials. Theclassic sources of acoustic emission arediscontinuity related deformationprocesses such as crack growth and plasticdeformation. Sudden movement at thesource produces a stress wave that radiatesout into the structure and excites asensitive piezoelectric sensor. As the stressin the material is raised, emissions aregenerated. The signals from one or moresensors are amplified and measured toproduce data for display andinterpretation.

FIGURE 9. Leakage measurement dynamic leak testing usingvacuum pumping: (a) pressurized system mode for leaktesting of smaller components; (b) pressurized envelopemode for leak testing of larger volume systems.

(a)

(b)

Leak detector

Envelope

Source of tracer gas

Source of tracer gas

Envelope

Leak detector

Systemundertest

Systemundertest

The source of acoustic emission energyis the elastic stress field in the material.Without stress there is no emission.Therefore, an acoustic emission test(Fig. 10) is usually carried out during acontrolled loading of the structure. Thiscan be a proof load before service; acontrolled variation of load while thestructure is in service; a fatigue, pressureor creep test; or a complex loadingprogram. Often a structure is going to beloaded hydrostatically anyway duringservice and acoustic emission testing isused because it gives valuable additionalinformation about the expectedperformance of the structure under load.Other times, acoustic emission testing isselected for reasons of economy or safetyand a special loading procedure isarranged to meet the needs of the acousticemission test.Applications. Acoustic emission is anatural phenomenon occurring in a widerange of materials, structures andprocesses. The largest scale eventsobserved with acoustic emission testingare seismic and the smallest are smalldislocations in stressed metals.

The equipment used is highly sensitiveto any kind of movement in its operatingfrequency (typically 20 to 1200 kHz). Theequipment can detect not only crackgrowth and material deformation but also

FIGURE 10. Acoustic emission testing setup in which eightsensors permit computer to calculate location of crackpropagation.

Computer

Preamplifier

Testobject

Acousticevent

Sensor

Preamplifier

such processes as solidification, friction,impact, flow and phase transformations.Therefore acoustic emission testing is alsoused for in-process weld monitoring;detecting tool contact and tool wearduring automatic machining; detectingwear and loss of lubrication in rotatingequipment; detecting loose parts andloose particles; detecting and monitoringleaks, cavitation and flow; preserviceproof testing; inservice weld monitoring;and leak testing.

Infrared and Thermal TestingPrinciples. Conduction and convectionare the primary mechanisms of heattransfer in an object or system. However,electromagnetic radiation is emitted froma heated body when electrons in thatbody change to a lower energy state.Thermal testing involves themeasurement or mapping of surfacetemperatures when heat flows from, to orthrough a test object. Temperaturedifferentials on a surface, or changes insurface temperature with time, are relatedto heat flow patterns and can be used todetect discontinuities or to determine theheat transfer characteristics of an object.For example, during the operation of anelectrical breaker, a hot spot detected atan electrical termination may be causedby a loose or corroded connection(Fig. 11). The resistance to electrical flow


FIGURE 11. Infrared thermography ofautomatic transfer switches of emergencydiesel generator. Hot spots appear bright inthermogram (inset).

through the connection produces anincrease in surface temperature of theconnection.Applications. There are two basiccategories of infrared and thermal testapplications: electrical and mechanical.The specific applications within these twocategories are numerous. Electricalapplications include transmission anddistribution lines, transformers,disconnects, switches, fuses, relays,breakers, motor windings, capacitorbanks, cable trays, bus taps and othercomponents and subsystems. Mechanicalapplications include insulation (in boilers,furnaces, kilns, piping, ducts, vessels,refrigerated trucks and systems, tank carsand elsewhere), friction in rotatingequipment (bearings, couplings, gears,gearboxes, conveyor belts, pumps,compressors and other components) andfluid flow (steam lines; heat exchangers;tank fluid levels; exothermic reactions;heating, ventilation and air conditioningsystems; leaks above and below ground;cooling and heating; tube blockages;environmental assessment of thermaldischarge; boiler or furnace air leakage;condenser or turbine system leakage;pumps; compressors; and other systemapplications).

Other MethodsThere are many other methods ofnondestructive testing, including opticalmethods such as holography,shearography and moiré imaging; materialidentification methods such as chemicalspot testing, spark testing andspectroscopy; strain gaging; and acousticmethods such as vibration analysis andtapping.


PART 2. Management of Electromagnetic Testing

Selection ofElectromagnetic Testing4

Electromagnetic tests are an importantand widely used method within the broadfield of nondestructive materials testing.The electromagnetic test method includesseveral subsidiary methods, sometimescalled submethods or techniques: eddycurrent testing, remote field testing, fluxleakage testing, alternating current fieldmeasurement testing and microwavetesting. Of these several submethods,conventional eddy current testing is themost widely used. Magnetic particletesting is an electromagnetic test thatindustry administers as a separatemethod.

Applications of eddy current tests inindustry are numerous and widespread.The total number of test measurementsmade annually by this nondestructive testmethod may exceed that of all othermethods combined. Eddy current testingis used for the following:

1. noncontacting measurement of thethickness of metallic foils, sheets,plates, tube walls and machined partsfrom one side only;

2. measurement of the thickness ofcoatings over base materials where thecoating and base material havesignificantly different electrical ormagnetic properties;

3. identifying or separating materials bycomposition or structure;

4. detecting material discontinuities thatlie in planes transverse to the eddycurrents, such as cracks, seams, laps,score marks or plug cuts, drilled andother holes and laminations at cutedges of sheet or plate;

5. identifying and controlling heattreatment conditions and evaluationof fire damage to metallic structures;

6. determining depths of case hardeningof steels and some ferrous alloys;

7. locating hidden metallic objects suchas underground pipes, buried bombsor ore bodies, or metallic objectsaccidentally packaged in foodstuffs;

8. timing or locating the motions ofhidden parts of mechanisms, countingmetallic objects on conveyor lines ordetecting metallic missiles in flight;and

9. precise dimensional measurement ofsymmetric, machined or ground andpolished metallic parts, such asbearings and bearing races, smallmechanism components and others.

Advantages of ElectromagneticTestingModern eddy current and electromagnetictest techniques offer low cost means forhigh speed, large scale testing of metallicmaterials such as those used in nuclear,aerospace, marine, high pressure, hightemperature and high speed engineeringsystems where premature failures couldrepresent economic disasters or theendangering of human life. The method’sspecial suitability for testing ofautomobiles, engines, machine parts andconsumer products has long beenrecognized.

Like other nondestructive methods,eddy current tests permit measurementsof material properties and dimensions anddetection of discontinuities. In general,electromagnetic tests provide nearlyinstantaneous measurements. The testspeed and modern signal analysis permitsuch analysis to be performed in realtime. Consequently, the method can beused in production lines to test swiftlymoving bars, tubes, sheets, plates, weldsand other symmetric parts. These partseither pass through test coils or arescanned by moving test probes. Theautomation of eddy current testing andtest data evaluation permits mass testingof similar parts at high rates, witheconomies not attainable by othercommonly used nondestructive tests. Theresults can be optimized for automationof test systems, for sorting of test parts,for control of manufacturing processesand for automatic documentation forprocess control and statistical qualitycontrol.

Small, portable forms of eddy currenttest instrumentation provide simple andrapid means for (1) manual quality testsby individual operators and(2) mechanized test systems to sort mixedlots of materials, to follow deterioration ofmaterials and equipment in service and toverify process quality.


Limitations of Eddy Current TestsLimitations of eddy current tests are adirect consequence of the specific natureof the test and of the response ofelectrically conductive test materials tothe externally applied, time varyingmagnetic fields used to excite eddycurrent flow. In general, eddy current testsare applicable only to test materials withsignificant electrical conductivity, such asmetals and alloys and composites withconducting layers or reinforcing fibers.They can be used, however, to measurethicknesses of nonconducting layers onthe surface of conducting metallicmaterials by the liftoff effect in which thecoating separates the test probe from theconducting material by the thickness ofthe nonconductive coating or sheetmaterial.

Eddy current tests provide maximumtest sensitivity for the surface and nearsurface layers of the test material adjacentto the source of excitation. In some casesit may be difficult or impossible topenetrate to the center of thick specimensbecause of skin effect and attenuation ofthe electromagnetic field at certain depthsbelow the surface. Eddy current tests tendto be insensitive to laminardiscontinuities, which lie parallel to theinduced eddy currents. They do tend torespond, however, to discontinuities thatlie transverse to the flow of eddy currentswithin test materials, where thesediscontinuities interrupt, lengthen ordistort the current flow paths.

Management ofElectromagnetic TestingProgramsManagement of an electromagnetictesting program will require considerationof many items before a program canproduce the desired results. Six basicquestions must be answered before a truedirection can be charted. They are asfollows.

1. Are regulatory requirements in placethat mandate program characteristics?

2. What is the magnitude of the programthat will provide desired results?

3. What provisions must be made forpersonnel safety and for compliancewith environmental regulations?

4. What is the performance date for aprogram to be fully implemented?

5. Is there a cost benefit ofelectromagnetic testing?

6. What are the available resources inpersonnel and money?

Once these questions are answered, then arecommendation can be made todetermine the best path forward. Three


primary paths are (1) service companies,(2) consultants and (3) in-house programs.

Although these are the primary paths,some programs may, routinely or asneeded, require support personnel from acombination of two or more of thesesources. Before a final decision is made,advantages and disadvantages of eachpath must be considered. Therefore thefollowing details must be considered.

Service Companies

1. Who will identify the componentswithin the facility to be examined?

2. Will the contract be for time andmaterials or have a specific scope ofwork?

3. If a time and materials contract isawarded, who will monitor the timeand materials charged?

4. If a scope of work is required, who istechnically qualified to develop andapprove it?

5. What products or documents (testreports, trending, recommendations,root cause analysis and others) will beprovided once the tests are completed?

6. Who will evaluate and accept theproduct (test reports, trending,recommendations, root cause analysisand others) within the servicecompany?

7. Do the service company workerspossess qualifications andcertifications required by contract andby applicable regulations?

8. Do the service company workersrequire site specific training (confinedspace entry, electrical safety, hazardousmaterials and others) or clearance toenter and work in the facility?

9. Does the service company retain anyliability for test results?

Consultants

1. Will the contract be for time andmaterials or have a specific scope ofwork?

2. If a scope of work is required, who istechnically qualified to develop andapprove it?

3. Who will identify the requiredqualifications of the consultant?

4. Is the purpose of the consultant todevelop or update a program or is it tooversee and evaluate the performanceof an existing program?

5 Will the consultant have oversightresponsibility for tests performed?

6. What products (trending,recommendations, root cause analysisand others) are provided once the testsare completed?

7. Who will evaluate the consultant’sperformance (test reports, trending,recommendations, root cause analysisand other functions) within thesponsoring company?

8. Does the consultant possessqualifications and certificationsrequired by contract and by applicableregulations?

9. Does the consultant require sitespecific training (confined space entry,electrical safety, hazardous materialsand others) or clearance to enter andwork in the facility?

10. Does the consultant retain anyliability for test results?

In-House Programs

1. Who will determine the scope of theprogram, such as which techniqueswill be used (eddy current, fluxleakage and others)?

2. What are the regulatory requirements(codes and standards) associated withprogram development andimplementation?

3. Who will develop a cost benefitanalysis for the program?

4. How much time and resources areavailable to establish the program?

5. What are the qualificationrequirements (education, training,experience and others) for personnel?

6. Do program personnel requireadditional training (safety, confinedspace entry or others) orqualifications?

7. Are subject matter experts required toprovide technical guidance duringpersonnel development?

8. Are procedures required to performwork in the facility?

9. If procedures are required, who willdevelop, review and approve them?

10. Who will determine the technicalspecifications for test equipment?

Test Procedures forElectromagnetic TestingThe conduct of facility operations(in-house or contracted) should beperformed in accordance with specificinstructions from an expert. This istypically accomplished using writteninstructions in the form of a technicalprocedure. In many cases, codes andspecifications will require a technicalprocedure to perform required tests.

The procedure process can take manyforms, including general instructions thataddress only major aspects of testtechniques. Or a procedure may bewritten as a step-by-step process requiring

a supervisor’s initial or signature aftereach step. The following is a typicalformat for an industrial procedure.

1. The purpose identifies the intent of theprocedure.

2. The scope establishes the latitude ofitems, tests and techniques coveredand not covered by the procedure.

3. References are specific documents fromwhich criteria are extracted ordocuments satisfied byimplementation of the procedure.

4. Definitions are needed for terms andabbreviations that are not commonknowledge to people who will read theprocedure.

5. Statements about personnel requirementsaddress specific requirements toperform tasks in accordance with theprocedure — issues such as personnelqualification, certification, accessclearance and others.

6. Equipment characteristics, calibrationrequirements and model numbers ofqualified equipment must be specified.

7. The test procedure provides a sequentialprocess to be used to conduct testactivities.

8. Acceptance criteria establish componentcharacteristics that will identify theitems suitable for service.

9. Reports (records) provide the means todocument specific test techniques,equipment used, personnel performingactivity, date performed and testresults.

10. Attachments may include (if required)items such as report forms, instrumentcalibration forms, qualified equipmentmatrix, schedules and others.

Once the procedure is completed,typically an expert in the subject matterevaluates it. If the procedure is judged tomeet identified requirements, the expertwill approve it for use. Some codes andstandards also require the procedure to bequalified — that is, demonstrated to thesatisfaction of a representative of aregulatory body or jurisdictionalauthority.

Test Specifications forElectromagnetic Testing4

An electromagnetic specification mustanticipate a number of issues that ariseduring testing.

Means of Induction and Detectionof Magnetic FieldsElectromagnetic nondestructive testmethods use either static or time varyingelectromagnetic fields as a probingmedium (1) to explore the properties oftest materials, (2) to locate discontinuities


or (3) to detect variations in geometry anddimensions of test materials. Themagnitudes, time lags, phase angles andflow patterns of the resulting fields aresensed by using probes such as sensingcoils or solid state magnetic field detectors(such as hall effect devices).

Eddy Current Test FrequenciesA single electromagnetic test system canbe used for many different measurementsthrough the selection of test frequencies.These frequencies are those of theexcitation current applied to the coils ofthe electromagnetic test probes.Frequency is measured in hertz (Hz),where 1 Hz = 1 cycle per second. Mostindustrial electromagnetic tests are madein the frequency range between 5 Hz and10 MHz.

Most types of electromagnetic testequipment provide either variablefrequency oscillators or several fixedfrequency steps. Thus, appropriate testfrequencies can be readily selected by theuser to meet special test requirements.Low excitation frequencies are used topenetrate deeper within a conducting testmaterial. High test frequencies can beused for selective examination of nearsurface regions, testing of thin materialsand for testing of materials that have lowelectrical conductivities.

InterpretationInterpretation may be complex, especiallybefore a procedure has been established.The interpreter must have a knowledge ofthe following: (1) the underlying physicalprocess, (2) techniques and equipmentused to obtain the data and displays,(3) details about the item being examined(configuration, material characteristics,fabrication process, potentialdiscontinuities and intended serviceconditions) and (4) acceptance criteria.

Ensuring Reliability of TestResultsWhen a test is performed, there are fourpossible outcomes: (1) a rejectablediscontinuity can be found when one ispresent; (2) a rejectable discontinuity canbe missed even when one is present; (3) arejectable discontinuity can be indicatedwhen none is present and (4) norejectable discontinuity is found whennone is present. A reliable testing processand a qualified inspector should find alldiscontinuities of concern with nodiscontinuities missed (no errors as incase 2 above) and no false calls (case 3above).


To achieve this goal, the probability offinding a rejectable discontinuity must behigh and the inspector must be bothproficient in the testing process andmotivated to perform with maximumefficiency. A reckless inspector may acceptparts that contain discontinuities, withthe result of possible inservice part failure.A conservative inspector may reject partsthat contain rejectable discontinuities butthe inspector also may reject parts that donot contain rejectable discontinuities,with the result of unnecessary scrap andrepair. Neither scenario is desirable.

Electromagnetic TestingStandardsTraditionally, the purpose of specificationsand standards has been to define therequirements that goods or services mustmeet. As such, they are intended to beincorporated into contracts so that boththe buyer and provider have a welldefined description of what one willreceive and the other will provide.

Standards have undergone a process ofpeer review in industry and can beinvoked with the force of law by contractor by government regulation. In contrast,a specification represents an employer’sinstructions to employees and is specificto a contract or work place. Specificationsmay form the basis of standards through areview process. Standards andspecifications exist in three basic areas:equipment, processes and personnel.

1. Standards for equipment includecriteria that address probes, artificialdiscontinuities and test results.Reference standards are work piecesthat contain artificial discontinuitiesfor instrument calibration and testprocedure verification.

2. ASTM International and otherorganizations publish standards fortest techniques. Some other standardsare for quality assurance proceduresand are not specific to a test methodor even to testing in general. Tables 3and 4 list some standards used inelectromagnetic testing. The UnitedStates Department of Defense hasreplaced most military specificationsand standards with industry consensusspecifications and standards. A sourcefor nondestructive testing standards isthe Annual Book of ASTM Standards.5

3. Qualification and certification of testpersonnel are discussed below withspecific reference to recommendationsof ASNT Recommended PracticeNo. SNT-TC-1A.6

TABLE 3.

MiscellanE 543, StE 1004, SE 1312, SE 1316, SE 1571, SE 1606, SE 1629, SF 673, St

Ed

Coating B 244, St

BaB 499, StB 659, StE 376, St

GeophysD 4748, D 6429, D 6432, D 6565, D 6639, D 6726,

MaterialE 566, StE 703, StE 1476, S

Tubular A 135, StE 215, StE 243, StE 309, StE 426, St

SimE 570, StE 571, StE 690, StE 1033, S

abE 2096, S

Personnel Qualificationand CertificationOne of the most critical aspects of the testprocess is the qualification of testpersonnel. Nondestructive testing issometimes referred to as a special process.The term simply means that it is verydifficult to determine the adequacy of atest by merely observing the process orthe documentation generated at itsconclusion. The quality of the test is

largely dependent on the skills andknowledge of the inspector.

The American Society forNondestructive Testing (ASNT) has been aworld leader in the qualification andcertification of nondestructive testingpersonnel for many years. By 1999, theAmerican Society for NondestructiveTesting had instituted three majorprograms for the qualification andcertification of nondestructive testingpersonnel.


Electromagnetic testing standards published by ASTM International.

eousandard Practice for Agencies Performing Nondestructive Testingtandard Practice for Determining Electrical Conductivity Using the Electromagnetic (Eddy-Current) Methodtandard Practice for Electromagnetic (Eddy-Current) Examination of Ferromagnetic Cylindrical Bar Product above the Curie Temperaturetandard Terminology for Nondestructive Examinations: Section C, Electromagnetic Testingtandard Practice for Electromagnetic Examination of Ferromagnetic Steel Wire Ropetandard Practice for Electromagnetic (Eddy-Current) Examination of Copper Redraw Rod for Electrical Purposestandard Practice for Determining the Impedance of Absolute Eddy Current Probesandard Test Methods for Measuring Resistivity of Semiconductor Slices or Sheet Resistance of Semiconductor Films with a Noncontact dy-Current Gage

Thicknessandard Test Method for Measurement of Thickness of Anodic Coatings on Aluminum and of Other Nonconductive Coatings on Nonmagneticsis Metals with Eddy-Current Instrumentsandard Test Method for Measurement of Coating Thicknesses by the Magnetic Method: Nonmagnetic Coatings on Magnetic Basis Metalsandard Guide for Measuring Thickness of Metallic and Inorganic Coatingsandard Practice for Measuring Coating Thickness by Magnetic-Field or Eddy-Current (Electromagnetic) Test Methods

ical MeasurementsStandard Test Method for Determining the Thickness of Bound Pavement Layers Using Short-Pulse RadarStandard Guide for Selecting Surface Geophysical MethodsStandard Guide for Using the Surface Ground Penetrating Radar Method for Subsurface InvestigationStandard Test Method for Determination of Water (Moisture) Content of Soil by the Time-Domain Reflectometry (TDR) Method Standard Guide for Using the Frequency Domain Electromagnetic Method for Subsurface InvestigationsStandard Guide for Conducting Borehole Geophysical Logging — Electromagnetic Induction

Identificationandard Practice for Electromagnetic (Eddy-Current) Sorting of Ferrous Metalsandard Practice for Electromagnetic (Eddy-Current) Sorting of Non-Ferrous Metalstandard Guide for Metals Identification, Grade Verification, and Sorting

Productsandard Specification for Electric-Resistance-Welded Steel Pipeandard Practice for Standardizing Equipment for Electromagnetic Examination of Seamless Aluminum-Alloy Tubeandard Practice for Electromagnetic (Eddy-Current) Examination of Copper-Alloy Tubesandard Practice for Eddy-Current Examination of Steel Tubular Products Using Magnetic Saturationandard Practice for Electromagnetic (Eddy-Current) Examination of Seamless and Welded Tubular Products, Austenitic Stainless Steel and

ilar Alloysandard Practice for Flux Leakage Examination of Ferromagnetic Steel Tubular Productsandard Practice for Electromagnetic (Eddy-Current) Examination of Nickel and Nickel-Alloy Tubular Productsandard Practice for In Situ Electromagnetic (Eddy-Current) Examination of Nonmagnetic Heat Exchanger Tubestandard Practice for Electromagnetic (Eddy-Current) Examination of Type F — Continuously Welded (CW) Ferromagnetic Pipe and Tubingove the Curie temperaturetandard Practice for In Situ Examination of Ferromagnetic Heat-Exchanger Tubes Using Remote Field Testing

18 El

TABLE 4. S

Iss

American

American

American

American

AmericanASTM InteCanadian

Chinese NDeutsche European

EuropeanInternatioJapanese Society of

United St

1. Recommended Practice No. SNT-TC-1Aprovides guidelines for personnelqualification and certification innondestructive testing. Thisrecommended practice identifies thespecific attributes that should beconsidered when qualifyingnondestructive testing personnel. Itrequires the employer to develop andimplement a written practice(procedure) that details the specificprocess and any limitation in thequalification and certification ofnondestructive testing personnel.6

2. ANSI/ASNT CP-189, Standard forQualification and Certification ofNondestructive Testing Personnelresembles SNT-TC-1A but alsoestablishes specific attributes for thequalification and certification ofnondestructive testing personnel.However, CP-189 is a consensusstandard as defined by the AmericanNational Standards Institute (ANSI). Itis recognized as the American standardfor nondestructive testing. It is notconsidered a recommended practice; it isa national standard.7

ectromagnetic Testing

ome standards for electromagnetic testing.

uing Organization Representative Standards and Related Documents

National Standards Institute ANSI B3.1, Rolling Element Bearings — Aircraft Engine, Engine Gearbox, and AccessoryApplications — Eddy Current Inspection

Petroleum Institute API 510, Pressure Vessel Inspection Code: Maintenance Inspection, Rating, Repair and AlterationAPI 570, Piping Inspection Code: Inspection, Repair, Alteration, and Rerating of In-Service Piping

SystemsAPI 650, Welded Steel Tanks for Oil StorageAPI 1104, Welding, Pipelines and Related Facilities

Society for Nondestructive Testing ASNT Recommended Practice No. SNT-TC-1AANSI/ASNT CP-189, ASNT Standard for Qualification and Certification of Nondestructive Testing

Personnel Society of Mechanical Engineers ANSI/ASME B31.1, Power Piping

ANSI/ASME B31.3, Process PipingASME Boiler and Pressure Vessel Code: Section V — Power Boilers: Article 8, Eddy Current

Examination of Tubular ProductsASME Boiler and Pressure Vessel Code: Section XI — Inservice Inspection of Nuclear Vessels.

N-553-1, Eddy Current Surface Examination Section XI, Division 1 ASME PTC 19-1, Performance Test Codes, Supplement on Instruction and Apparatus

Welding Society AWS D1.1, Structural Welding Code — Steelrnational See Table 3

General Standards Board CAN/CGSB-48.9712, Non-Destructive Testing — Qualification and Certification of Personnel48.14-M86-CAN/CGSB, Advanced Manual for: Eddy Current Test Method Amendment No. 1

May 1997 R(1997)ational Standards Z8005100, General Rules for Eddy Current TestingInstitut für Normung DIN 54141-3, Non-Destructive Testing; Eddy Current Testing of Pipes and Tubes; Procedure Association of Aerospace Industries AECMA PREN 2002-20, Aerospace Series Test Methods for Metallic Materials: Part 20: Eddy

Current Testing of Circular Cross-Section Tubes, Edition P 1 Committee for Standardization EN 12084, Non-Destructive Testing — Eddy Current Testing — General Principles and Guidelinesnal Organization for Standardization ISO 9712, Nondestructive Testing – Qualification and Certification of PersonnelStandards Association JIS Z 2314, Test Methods for Performance Characteristics of Eddy Current Testing Instruments Automotive Engineers SAE ARP 891A, Determination of Aluminum Alloy Tempers through Electrical Conductivity

Measurement (Eddy Current) (R 1988)SAE ARP 1926, Cure Monitor, Electrical MethodsSAE ARP 4402, Eddy Current Inspection of Open Fastener Holes in Aluminum Aircraft StructureSAE ARP 4462, Barkhausen Noise Inspection for Detecting Grinding BurnsSAE AS 4787, Eddy Current Inspection of Circular Holes in Nonferrous Metallic Aircraft Engine

HardwareSAE DFT K-89AW, Eddy Current Inspection of Circular Holes in Nonferrous MetallicSAE J 425, Electromagnetic Testing by Eddy Current Methods, Information Report; March 1991

ates Department of Defense MIL-P-85585, Probes, Eddy Current, Unshielded, Single Coil, AbsoluteMIL-STD-1537B, Electrical Conductivity Test for Verification of Heat Treatment of Aluminum

Alloys, Eddy Current MethodMIL-STD-2032, Eddy Current Inspection of Heat Exchanger Tubing on Ships of the United States

NavyMIL-STD-2195, Inspection Procedure for Detection and Measurement of Dealloying Corrosion on

Aluminum Bronze and Nickel-Aluminum Bronze Components

TABLE 5. Recommended training and experience forelectromagnetic testing personnel according toRecommended Practice No. SNT-TC-1A.6

Level I Level II

High school graduatea 40 h 40 hTwo years of collegeb 24 h 40 hWork experiencec 210 h 630 h

a. Or equivalent.b. Completion with a passing grade of at least two years of engineering or

science study in a university, college or technical school.c. Minimum work experience per level. Note: for Level II certification, the

experience shall consist of time as Level I or equivalent. If a person isbeing qualified directly to Level II with no time at Level I, the requiredexperience shall consist of the sum of the times required for Level I andLevel II and the required training shall consist of the sum of the hoursrequired for Level I and Level II.

3. The ASNT Central Certification Program(ACCP), unlike SNT-TC-1A and CP-189,is a third party certification processthat identifies qualification andcertification attributes for Level II andLevel III nondestructive testingpersonnel. The American Society forNondestructive Testing certifies thatthe individual has the skills andknowledge for many nondestructivetest method applications. It does notremove the responsibility for the finaldetermination of personnelqualifications from the employer. Theemployer evaluates an individual’sskills and knowledge for application ofcompany procedures using designatedtechniques and equipment identifiedfor specific tests.8

Selections from RecommendedPractice No. SNT-TC-1ATo give a general idea of the contents ofthese documents, the following items arespecified in the 2001 edition ofRecommended Practice No. SNT-TC-1A. (Thefollowing text has been excerpted andadapted. The original text is arranged inoutline format and includesrecommendations that are not specific toelectromagnetic testing.)

Scope. This recommended practice has beenprepared to establish guidelines for thequalification and certification ofnondestructive test personnel whose specificjobs require appropriate knowledge of thetechnical principles underlying thenondestructive tests they perform, witness,monitor or evaluate. This documentprovides guidelines for the establishment ofa qualification and certification program.

Written Practice. The employer shallestablish a written practice for the controland administration of nondestructivetesting personnel training, examination andcertification. The employer’s written practiceshould describe the responsibility of eachlevel of certification for determining theacceptability of materials or components inaccordance with the applicable codes,standards, specifications and procedures.

Education, Training, ExperienceRequirements for Initial Qualification.Candidates for certification innondestructive testing should havesufficient education, training andexperience to ensure qualification in thosenondestructive testing methods for whichthey are being considered for certification.Table 6.3.1A [Table 5 in this volume, forelectromagnetic testing] lists therecommended training and experiencefactors to be considered by the employer inestablishing written practices for initialqualification of Level I and II individuals.

Training Programs. Personnel beingconsidered for initial certification shouldcomplete sufficient organized training tobecome thoroughly familiar with theprinciples and practices of the specifiednondestructive test method related to thelevel of certification desired and applicableto the processes to be used and the productsto be tested.

Examinations. For Level I and II personnel, acomposite grade should be determined bysimple averaging of the results of thegeneral, specific and practical examinationsdescribed below. Examinations administeredfor qualification should result in a passingcomposite grade of at least 80 percent, withno individual examination having a passinggrade less than 70 percent. The examinationfor near vision acuity should ensure naturalor corrected near distance acuity in at leastone eye such that the applicant can read aminimum of jaeger size 2 or equivalent typeand size letter at a distance of not less than305 mm (12 in.) on a standard jaeger testchart. This test should be administeredannually.

Practical Examination for NDT Level I and II.The candidate should demonstrate ability tooperate the necessary nondestructive testequipment and to record and analyze theresultant information to the degreerequired. At least one selected specimenshould be tested and the results of thenondestructive test analyzed by thecandidate.

Certification. Certification of all levels ofnondestructive test personnel is theresponsibility of the employer. Certificationof nondestructive test personnel shall bebased on demonstration of satisfactoryqualification [in accordance with sectionson education, training, experience andexaminations] as described in theemployer’s written practice. Personnelcertification records shall be maintained onfile by the employer.


Recertification. All levels of nondestructivetesting personnel shall be recertifiedperiodically in accordance with thefollowing: evidence of continuingsatisfactory performance; and reexaminationin those portions of the examinationdeemed necessary by the employer’s NDTLevel III. Recommended maximumrecertification intervals are three years forLevel I and II and five years for Level III.

The minimum number of questionsthat should be administered in thewritten examination for eddy current testpersonnel is as follows: 40 questions inthe general examination and 20 questionsin the specific examination. The numberof questions is the same for Level I andLevel II personnel.

These recommendations fromRecommended Practice No. SNT-TC-1A arecited only to provide a general idea of thespecific items that must be considered inthe development of an in-housenondestructive testing program. Becausethe items are paraphrased, thosedeveloping a personnel qualificationprogram should consult the complete textof SNT-TC-1A and other applicableprocedures and practices. If an outsideagency is contracted for electromagnetictest services, then the contractor musthave a qualification and certificationprogram to satisfy most codes andstandards.

Central CertificationAnother standard that may be a source forcompliance is contained in therequirements of the InternationalOrganization for Standardization (ISO).The work of preparing internationalstandards is normally carried out throughtechnical committees of the InternationalOrganization for Standardization, aworldwide federation of nationalstandards bodies. Each ISO member bodyinterested in a subject for which atechnical committee has been establishedhas the right to be represented on thatcommittee. International organizations,governmental and nongovernmental, inliaison with the InternationalOrganization for Standardization, alsotake part in the work.

Technical Committee ISO/TC 135,Non-Destructive Testing SubcommitteeSC 7, Personnel Qualification, preparedinternational standard ISO 9712,Nondestructive Testing – Qualification andCertification of Personnel.9 In its statementof scope, ISO 9712 states that it“establishes a system for the qualificationand certification, by a certification body,of personnel to perform industrialnondestructive testing (NDT) using any ofthe following methods: (a) eddy currenttesting; (b) liquid penetrant testing;(c) magnetic particle testing;


(d) radiographic testing; (e) ultrasonictesting” and that the “system described inthis International Standard may alsoapply to visual testing (VT), leak testing(LT), neutron radiography (NR), acousticemission (AE) and other nondestructivetest methods where independentcertification programs exist.” Theapplicability of ISO 9712 toelectromagnetic testing therefore dependson activity of the national certifying body.

Safety in ElectromagneticTestingTo manage an electromagnetic testingprogram, as with any testing program, thefirst obligation is to ensure safe workingconditions. The following are componentsof a safety program that may be requiredor at least deserve serious consideration.

1. Before work is to begin, identify thesafety and operational rules and codesapplicable to the areas, equipment andsystems to be tested.

2. Provide proper safety equipment(protective barriers, hard hat, safetyharnesses, steel toed shoes, hearingprotection and others).

3. Before the test, perform a thoroughvisual survey to determine all thehazards and to identify necessarysafeguards to protect test personneland equipment.

4. Notify operative personnel to identifythe location and specific material,equipment or systems to be tested. Inaddition, it must be determinedwhether signs or locks restrict accessby personnel. Be aware of equipmentthat may be operated remotely or maybe started by time delay.

5. Be aware of any potentially explosiveatmosphere. Determine whether it issafe to take test equipment into thearea.

6. Do not enter any roped off or no entryareas without permission andapproval.

7. When working on or around movingor electrical equipment, the inspectorshould remove pens, watches, rings orobjects in pockets that may touch (orfall into) energized equipment.

8. Know interplant communication andevacuation systems.

9. Never let unqualified personneloperate equipment independentlyfrom qualified supervision.

10. Keep a safe distance between theinspector and any energizedequipment. In the United States, thesedistances can be found in documentsfrom the Occupational Safety andHealth Administration, the NationalFire Prevention Association (NationalElectric Code),10 the Institute ofElectrical and Electronics Engineers(National Electrical Safety Code)11 andother organizations.

11. Be aware of the personnelresponsibilities before entering aconfined space. All such areas must betested satisfactorily for gas and oxygenlevels before entry and periodicallythereafter. If odors are noticed or ifunusual sensations such as ear aches,dizziness or difficulty in breathing areexperienced, leave the areaimmediately.

Most facilities in the United States arerequired by law to follow therequirements in the applicable standard.Two Occupational Safety and HealthStandards in the United States that shouldbe reviewed are Occupational Safety andHealth Standards for general industry12 andthe Occupational Safety and HealthStandards for the Construction Industry.13

Personnel safety is always the firstconsideration for every job.


PART 3. Units of Measure for ElectromagneticTesting

TABLE 6. SI base units.

Quantity Unit Symbol

Length meter mMass kilogram kgTime second sElectric current ampere ATemperature kelvin KAmount of substance mole molLuminous intensity candela cd

TABLE 7. SI derived units with special names.a

Relationto Other

Quantity Units Symbol SI Unitsb

Capacitance farad F C·V–1

Catalytic activity katal kat s–1·molConductance siemens S A·V–1

Energy joule J N·mFrequency (periodic) hertz Hz 1·s–1

Force newton N kg·m·s–2

Inductance henry H Wb·A–1

Illuminance lux lx lm·m–2

Luminous flux lumen lm cd·srElectric charge coulomb C A·sElectric potentialc volt V W·A–1

Electric resistance ohm Ω V·A–1

Magnetic flux weber Wb V·sMagnetic flux density tesla T Wb·m–2

Plane angle radian rad 1Power watt W J·s–1

Pressure (stress) pascal Pa N·m–2

Radiation absorbed dose gray Gy J·kg–1

Radiation dose equivalent sievert Sv J·kg–1

Radioactivity becquerel Bq 1·s–1

Solid angle steradian sr 1Temperature, celsius degree celsius °C KTimea hour h 3600 sVolumea liter L dm3

a. Hour and liter are not SI units but are accepted for use with the SI.b. Number one (1) expresses dimensionless relationship.c. Electromotive force.

Origin and Use of SISystemIn 1960, the General Conference onWeights and Measures established theInternational System of Units. Le SystémeInternational d’Unités (SI) was designed sothat a single set of measurement unitscould be used by all branches of science,engineering and the general public.Without SI, this Nondestructive TestingHandbook volume could have contained aconfusing mix of obsoletecentimeter-gram-second (CGS) units,imperial units and the units preferred bycertain localities or scientific specialties.

SI is the modern version of the metricsystem and ends the division betweenmetric units used by scientists and metricunits used by engineers and the public.Scientists have given up their units basedon centimeter and gram and engineersmade a fundamental change inabandoning the kilogram-force in favor ofthe newton. Electrical engineers haveretained their ampere, volt and ohm butchanged all units related to magnetism.

Table 6 lists the seven SI base units.Table 7 lists derived units with specialnames. Table 8 gives examples ofconversions to SI units. In SI, the unit oftime is the second (s) but hour (h) isrecognized for use with SI.

For more information, the reader isreferred to the information availablethrough national standards organizationsand specialized information compiled bytechnical organizations.14-17

MultipliersIn science and engineering, very large orvery small numbers with units areexpressed by using the SI multipliers,prefixes of 103 intervals (Table 9). Themultiplier becomes a property of the SIunit. For example a millimeter (mm) is0.001 meter (m). The volume unit cubiccentimeter (cm3) is (0.01 m)3 or 10–6 m3.Unit submultiples such as the centimeter,decimeter, dekameter (or decameter) andhectometer are often avoided in scientificand technical uses of SI because of theirvariance from the 103 interval. However,dm3 and cm3 are commonly used. Notethat 1 cm3 is not equal to 0.01 m3.Nevertheless, in equations, submultiples


TABLE 8. Examples of conversions to SI units.

Quantity Measurement in Non-SI Unit Multiply by To Get Measurement in SI Unit

Angle minute (min) 2.908 882 × 10–4 radian (rad)degree (deg) 1.745 329 × 10–2 radian (rad)

Area square inch (in.2) 645 square millimeter (mm2)Distance angstrom (Å) 0.1 nanometer (nm)

inch (in.) 25.4 millimeter (mm)Energy British thermal unit (BTU) 1.055 kilojoule (kJ)

calorie (cal), thermochemical 4.184 joule (J)Power British thermal unit per hour (BTU·h–1) 0.293 watt (W)Specific heat British thermal unit per pound 4.19 kilojoule per kilogram per kelvin (kJ·kg–1·K–1)

degree fahrenheit (BTU·lbm–1·°F–1)

Force (torque, couple) foot-pound (ft-lbf) 1.36 joule (J)Pressure pound force per square inch (lbf·in.–2) 6.89 kilopascal (kPa)Frequency (cycle) cycle per minute 60–1 hertz (Hz)Illuminance footcandle (ftc) 10.76 lux (lx)

phot (ph) 10 000 lux (lx)Luminance candela per square foot (cd·ft–2) 10.76 candela per square meter (cd·m–2)

candela per square inch (cd·in.–2) 1 550 candela per square meter (cd·m–2)footlambert (ftl) 3.426 candela per square meter (cd·m–2)lambert 3 183 (= 10 000/π) candela per square meter (cd·m–2)nit (nt) 1 candela per square meter (cd·m–2)stilb (sb) 10 000 candela per square meter (cd·m–2)

Radioactivity curie (Ci) 37 gigabecquerel (GBq)Ionizing radiation exposure roentgen (R) 0.258 millicoulomb per kilogram (mC·kg–1)Mass pound (lbm) 0.454 kilogram (kg)Temperature (difference) degree fahrenheit (°F) 0.556 kelvin (K) or degree celsius (°C)Temperature (scale) degree fahrenheit (°F) (°F – 32)/1.8 degree celsius (°C)Temperature (scale) degree fahrenheit (°F) (°F – 32)/1.8) + 273.15 kelvin (K)

TABLE 9. SI prefixes and multipliers.

Prefix Symbol Multiplier

yotta Y 1024

zetta Z 1021

exa E 1018

peta P 1015

tera T 1012

giga G 109

mega M 106

kilo k 103

hectoa h 102

deka (or deca)a da 10decia d 10–1

centia c 10–2

milli m 10–3

micro µ 10–6

nano n 10–9

pico p 10–12

femto f 10–15

atto a 10–18

zepto z 10–21

yocto y 10–24

a. Avoid these prefixes (except in dm3 and cm3) forscience and engineering.

MOVIE.Radian of circle.

such as centimeter (cm) or decimeter (dm)are often avoided because they disturb theconvenient 103 or 10–3 intervals thatmake equations easy to manipulate.

In SI, the distinction between upperand lower case letters is meaningful andshould be observed. For example, themeanings of the prefix m (milli) and theprefix M (mega) differ by nine orders ofmagnitude.

SI Units forElectromagnetic Testing

CGS UnitsTable 10 gives examples ofcentimeter-gram-second (CGS) units.These units are not accepted for use withthe SI. Furthermore, no other units of thevarious CGS systems of units, whichincludes the CGS electrostatic, CGSelectromagnetic and CGS gaussiansystems, are accepted for use with SIexcept such units as the centimeter (cm),gram (g) and second (s) that are alsodefined in SI.

The oersted, gauss and maxwell arepart of the electromagnetic


TABLE 10. Units from the centimeter-gram-second (CGS) system of units and not accepted for use with SI. Factor toconvert each CGS unit to SI unit is given.

Physical Quantity CGS Unit Multiply by SI Unit SI Symbol

Basic CGS UnitsMagnetic field intensity oersted (Oe) 103·(4π)–1 ampere per meter A·m–1

Magnetic flux maxwell (Mx) 10–8 weber WbMagnetic flux density gauss (G) 10–4 tesla TMagnetic potential difference gilbert (Gb) 10·(4π)–1 ampere A

Electromagnetic CGS UnitsCapacitance abfarad 109 farad FCharge abcoulomb 10 coulomb CConductance abmho 109 siemens SCurrent abampere 10 ampere AInductance abhenry 10–9 henry HMagnetic field intensity abampere per centimeter 103 ampere per meter A·m–1

Potential abvolt 10–8 volt VResistance abohm 10–9 ohm Ω

Electrostatic CGS UnitsCapacitance statfarad 1.112 650 × 10–12 farad FCharge statcoulomb 3.3356 × 10–10 coulomb CConductance statmho 1.112 65 × 10–12 siemens SCurrent statampere 3.335 641 × 10–11 ampere AInductance stathenry 8.987 552 × 1011 henry HPotential statvolt 2.997 925 × 102 volt VResistance statohm 8.987 55 × 1011 ohm Ω

TABLE 11. Conversion of Units forConductivity σσ and Resistivity ρρ.

From Unit Operation To Get Unit

Conductivity Unit to Conductivity Unita

S·m–1 (S·m–1) × 10–6 MS·m–1

S·m–1 (S·m–1) × (1.724 × 10–6) %IACSMS·m–1 (MS·m–1) × 106 S·m–1

MS·m–1 (MS·m–1) × 1.724 %IACS%IACS %IACS × (5.800 × 105) S·m–1

%IACS %IACS × 0.580 MS·m–1

Conductivity Unita to Resistivity UnitS·m–1 1 ÷ (S·m–1) Ω·mS·m–1 (1 × 108) ÷ (S·m–1) µΩ·cmMS·m–1 (1 × 10–6) ÷ (MS·m–1) Ω·mMS·m–1 (1 × 102) ÷ (MS·m–1) µΩ·cm%IACS (1.724 × 10–6) ÷ %IACS Ω·m%IACS 172.4 ÷ %IACS µΩ·cm

Resistivity Unit to Conductivity Unita

Ω·m 1 ÷ (Ω·m) S·m–1

Ω·m (1 × 10–6) ÷ (Ω·m) MS·m–1

Ω·m (1.724 × 10–6) ÷ (Ω·m) %IACSµΩ·cm (1 × 108) ÷ (µΩ·cm) S·m–1

µΩ·cm (1 × 102) ÷ (µΩ·cm) MS·m–1

µΩ·cm 172.4 ÷ (µΩ·cm) %IACSa

Resistivity Unit to Resistivity UnitΩ·m (Ω·m) × 108 µΩ·cmµΩ·m (µΩ·m) × 10–8 Ω·m

a. %IACS: percentage of International Annealed CopperStandard.18

three-dimensional CGS system. Whenonly mechanical and electric quantitiesare considered, these three units cannotstrictly speaking be compared each to thecorresponding unit of SI, which has fourdimensions.Ampere per Meter Replaces Oersted. Oneampere per meter (A·m–1) equals aboutone eightieth of an oersted (Oe). Therelationship is 1 Oe = 1000·(4π)–1 A·m–1 =79.57747 A·m–1.Tesla Replaces Gauss. One tesla (T) equalsten thousand gauss (G). 1 G = 10–4 T =0.1 mT.Weber Replaces Maxwell. One weber (Wb)equals 108 maxwell (Mx). 1 Mx = 10–8 Wb= 0.01 µWb = 10 nWb.

Conductivity and ResistivityIn the twentieth century, the conductivityof a given metal was conventionallyexpressed as a percentage of pure copper’sconductivity with reference to theInternational Annealed Copper Standard(IACS).18 In SI, conductivity is expressedin siemens per meter (S·m–1). Theconductivity of pure copper (100 percentIACS) is 58 MS·m–1.

Resistivity is the inverse ofconductivity and is expressed in ohmmeter. Table 11 gives the formulas forconversion to and from units forconductivity and resistivity.



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