nondestructive testing handbook, third edition: volume 5

Paul M. Gammell, Gammell Applied Technologies,Exmore, Virginia

19C H A P T E R

Electromagnetic TestingGlossary

From Nondestructive Testing Handbook, Third Edition: Volume 5, Electromagnetic Testing © 2004. Reprinted with permission of The American Society for Nondestructive Testing Inc.

IntroductionMost of the definitions in this glossary areadapted from the second edition of theNondestructive Testing Handbook.1-10 Thedefinitions in this glossary have beenmodified to satisfy peer review andeditorial style. For these reasons,references in this glossary should beconsidered not attributions but ratheracknowledgments and suggestions forfurther reading.

The definitions in this NondestructiveTesting Handbook volume should not bereferenced for tests performed accordingto standards or specifications or infulfillment of contracts. Standards writingbodies take great pains to ensure thattheir standards are definitive in wordingand technical accuracy. People working towritten contracts or procedures shouldconsult definitions referenced instandards when appropriate.

This glossary is provided forinstructional purposes. No other use isintended.

Terms

A

absolute coil: Coil that responds to theelectromagnetic properties of thatregion of the test part within themagnetic field of the coil, withoutcomparison to the response of asecond coil at a different location onthe same or similar material.4

absolute measurement: (1) Measurementmade with an absolute coil.4(2) Measurement of a propertywithout reference to anothermeasurement of that property.

acceptance criterion: Benchmark againstwhich test results are to be comparedfor purposes of establishing thefunctional acceptability of a part orsystem being examined.

acceptance level: Measured value orvalues above or below which testspecimens are acceptable in contrast torejection level.4,11

acceptance limit: Test signal value usedin electromagnetic testing,establishing the group to which amaterial under test belongs.4,11

acceptance standard: Specimen, similarto the product to be tested, containingnatural or artificial discontinuities thatare well defined and similar in size orextent to the maximum acceptable inthe product.4 Acceptance standards areavailable also for material propertiessuch as conductivity and hardness.

algorithm: Set of well defined rules orprocesses that prescribe the solution ofa problem in a finite number ofsteps.4,12

alternating current: Electrical currentthat reverses its direction at regularintervals.6,10

alternating current field: Varyingmagnetic field produced around aconductor by alternating currentflowing in the conductor.6,10

alternating current magnetization:Magnetization by a magnetic fieldgenerated when alternating current isflowing.6,10,13

ampere (A): SI unit of electric current.6,10

ampere per meter (A·m–1): SI compoundunit for magnetic field intensity. Themeasurement 1 A·m–1, for example,describes a current of 1 A flowingthrough a coil of 1 m diameter.6,10,13

amplitude response: Property of a testsystem whereby the amplitude of thedetected signal is measured withoutregard to phase. See also phaseanalysis.4,11

analog-to-digital converter: Circuitwhose input is information in analogform and whose output is essentiallythe same information in digitalform.4,12

annular coil: See encircling coil.annular coil clearance: Mean radial

distance between the inner diameterof an encircling coil assembly and testobject surface in electromagnetictesting. See fill factor.4,10,11

anomaly: Variation from normal materialor product quality.4

argand diagram: Graphicalrepresentation of a vector quantity onthe complex plane.

artifact: In nondestructive testing, anindication that may be interpretederroneously as a discontinuity.10,15

artificial discontinuity: See discontinuity,artificial.

artificial discontinuity standard: Seeacceptance standard.

ASNT: American Society forNondestructive Testing.

ASNT Recommended PracticeNo. SNT-TC-1A: See RecommendedPractice No. SNT-TC-1A.

attenuation: Decrease in signal amplitudeover distance, often called loss; can beexpressed in decibels or as a scalarratio of the input magnitude to theoutput magnitude.4,12

automated system: Acting mechanismthat performs required tasks at adetermined time and in a fixedsequence in response to certainconditions.10

498 Electromagnetic Testing


B

band pass filter: Frequency filter that hasa single transmission band betweentwo cutoff frequencies, neither of thecutoff frequencies being zero orinfinity.4,10,12

bandwidth: Difference between the cutofffrequencies of a bandpass filter.14

blister: Discontinuity in metal, on or nearthe surface, resulting from theexpansion of gas in a subsurface zone.Very small blisters are called pinheadsor pepper blisters.10

blowhole: Hole in a casting or a weldcaused by gas entrapped duringsolidification.10

bobbin coil: Cylindrically wound coil.brittleness: Characteristic of a material

that leads to crack propagationwithout appreciable plasticdeformation.10

bucking coil: See differential coils.

C

calibration, instrument: Adjustment ofinstrument readings to knownreference standard.

casing: Many strings of pipe that are usedto line the hole during and afterdrilling of a water, gas or oil well.10

casing string: Tubular structure on theouter perimeter of a water, gas or oilwell hole. The casing string is apermanent part of the well and manycasing strings are cemented into theformation.10

central conductor: Electric conductorpassed through the opening in a partwith an aperture, or through a hole ina test object, for the purpose ofcreating a circular magnetic field inthe object.10

certification: With respect tonondestructive test personnel, processof providing written testimony that anindividual is qualified. See also certifiedand qualified.

certified: With respect to nondestructivetest personnel, having writtentestimony of qualification. See alsocertification and qualification.

circular magnetization: Magnetization inan object resulting from currentpassed longitudinally through theobject itself or through an insertedcentral conductor.10,15

circumferential coil: See encircling coil.coil: One or more loops of a conducting

material; a single coil may be anexciter and induce currents in thematerial or it may be a detector orboth simultaneously.4

coil clearance: See liftoff.

coil spacing: In electromagnetic testing,the axial distance between twoencircling or inside coils of adifferential or remote field testsystem.4,11

comparative measurement: Inelectromagnetic testing, ameasurement based on the imbalancein a system and using comparator coilsin contrast to differential and absolutemeasurements. See also comparatorcoils.4,11

comparator coils: In electromagnetictesting, two or more coils electricallyconnected in series opposition andarranged so that there is no mutualinduction (coupling) between them.Any electromagnetic condition that isnot common to the test specimen andthe standard will produce animbalance in the system and therebyyield an indication. See also differentialcoils.4,11

complex plane: Plane defined by twoperpendicular reference axes, used forplotting a complex variable (such asimpedance) or functions of thisvariable (such as a transferfunction).4,12

complex plane diagram: Graphicalpresentation of complex quantitieswhere the real and imaginarycomponents are represented along thehorizontal and vertical axes,respectively.4 Types of complex planediagram include impedance planediagram, voltage plane diagram andphase amplitude diagram.

conductance (G): Transmission of electriccurrent through material. Measured insiemens (S). Inversely related toresistance R (ohm).

conductivity (σσ): Ability of material totransmit electric current. Measured insiemens per meter. Inversely related toresistivity ρ.

contact head: Electrode assembly used toclamp and support an object tofacilitate passage of electric currentthrough the object for circularmagnetization.10,15

coupled: (1) Of two electric circuits,having an impedance in common sothat a current in one causes a voltagein the other.10,11 (2) Of two coils,sharing parts of their magnetic fluxpaths.

σρ

= 1

GR

= 1

499Electromagnetic Testing Glossary


coupling: Percentage of magnetic fluxfrom a primary circuit that links asecondary circuit; effectiveness of acoil in inducing eddy currents in thetest object.4

coupling coefficient: Fraction ofmagnetic flux from one circuit (testcoil) that threads a second circuit (testobject); the ratio of impedance of thecoupling to the square root of theproduct of the total impedances ofsimilar elements in the two meshes.4,12

crack: (1) Break, fissure or rupture,sometimes V shaped and relativelynarrow and deep. Discontinuity thathas a relatively large cross section inone direction and a small or negligiblecross section when viewed in adirection perpendicular to the first.(2) Propagating discontinuity causedby stresses such as heat treating orgrinding. Difficult to detect unaidedbecause of fineness of line and pattern(may have a radial or latticedappearance).10

crack, cold: Crack that occurs in a castingafter solidification, because ofexcessive stress generally resultingfrom nonuniform cooling.10

crack, cooling: Crack in bars of alloy ortool steels resulting from unevencooling after heating or hot rolling.Cooling cracks are usually deep and liein a longitudinal direction but areusually not straight.10

crack, fatigue: Progressive crack thatdevelops on the surface and is causedby the repeated loading and unloadingof the object.10

crack, forging: Crack developed in theforging operation because of forging attoo low a temperature, resulting inrupturing of the material.10

crack, hot: Crack that develops before thecasting has completely cooled, ascontrasted with cold cracks, thatdevelop after solidification.10

crack, longitudinal: Crack parallel to thelength of the test object.10

crack, quenching: During quenching ofhot metal, rupture produced by morerapid cooling and contraction of oneportion of a test object than occurs inadjacent portions.10

crack, transverse: Crack at right angle tothe length of the test object.10

current flow technique: Magnetizing bypassing current through an objectusing prods or contact heads. Thecurrent may be alternating current orrectified alternating current.10,15

current induction technique:Magnetization in which a circulatingcurrent is induced in a ringcomponent by a fluctuating magneticfield.10,15

cycle: Single period of a waveform orother variable. See period.

D

defect: Discontinuity whose size, shape,orientation or location make itdetrimental to the useful service of itshost object or which exceeds theaccept/reject criteria of an applicablespecification.10,16 Note that somediscontinuities may not exceedspecifications and are therefore notdefects. Compare discontinuity andindication.10

demodulation: Process wherein a carrierfrequency modulated with a signal oflower frequency than the carrierfrequency is converted to a closerepresentation of the originalmodulating signal.14 See modulation.

depth of penetration: See skin effect andstandard depth of penetration.

detector coil: See sensing coil.differential amplifier: Amplifier whose

output signal is proportional to thealgebraic difference between two inputsignals.4,12

differential coils: Two or more physicallyadjacent and mutually coupled coilsconnected in series opposition suchthat an imbalance between them,causing a signal, will be produced onlywhen the electromagnetic conditionsare different in the regions beneathtwo of the coils. In contrast,comparator coils are not adjacent ormutually coupled.

differential measurement: Inelectromagnetic testing, themeasurement of system imbalance byusing differential coils, in contrast toabsolute and comparativemeasurements.4,11

differentiated signal: In electromagnetictesting, an output signal proportionalto the input signal’s rate of change.4,11

direct current: Electric current flowingcontinually in one direction withoutvariation in amplitude through aconductor.10,16 See also full-waverectified direct current and half-wavedirect current.

direct current field: Active magnetic fieldproduced by direct current flowing ina conductor or coil.10,16

discontinuity: Interruption in thephysical structure or configuration of atest object.10,17 After nondestructivetesting, unintentional discontinuitiesinterpreted as detrimental to theserviceability of the host object maybe called flaws or defects.10 Comparedefect and indication.

discontinuity, artificial: Referencediscontinuity such as hole,indentation, crack, groove or notchintroduced into a reference standardto provide accurately reproducibleindications for determining sensitivitylevels.10



discontinuity inversion: Technique formeasuring some dimension(s) of adiscontinuity by the application of amathematical algorithm to themeasured test data.4

discontinuity resolution: Property of atest system that enables the separationof indications due to discontinuitiesnear each other in a test specimen.4,11

E

eddy current: Electrical current inducedin a conductor by a time varyingmagnetic field.4

eddy current testing: Nondestructive testtechnique in which eddy current flowis induced in the test object. Changesin the flow caused by variations in thespecimen are reflected into a nearbycoil, coils, hall effect device or othermagnetic flux sensor for subsequentanalysis by suitable instrumentationand techniques.4,11

edge effect: In electromagnetic testing,the disturbance of the magnetic fieldand eddy currents because of theproximity of an abrupt change ingeometry, such as an edge of the testobject. Sometimes called end effect.The effect generally results in themasking of discontinuities within theaffected region.4,11

effective depth of penetration: Inelectromagnetic testing, the minimumdepth beyond which a test system canno longer practically detect a furtherincrease in specimen thickness.

electric field: Vector field of either theelectric field intensity (volt per meter)or of the electric flux density(coulomb per meter squared).

electrical center: Center established bythe electromagnetic field distributionwithin a test coil. A constant intensitysignal, irrespective of thecircumferential position of adiscontinuity, is indicative of electricalcentering. The electrical center may bedifferent from the physical center ofthe test coil.4,11

electrode: Conductor by which a currentpasses into or out of a test object.10,13

electromagnet: Ferromagnetic coresurrounded by a coil of wire thattemporarily becomes a magnet whenan electric current flows through thewire.10,15

electromagnetic acoustic transducer(EMAT): Electromagnetic device usinglorentz forces and magnetostriction inconductive and ferromagneticmaterials to generate and receiveacoustic signals for ultrasonicnondestructive tests.10

electromagnetic testing (ET):Nondestructive test method formaterials, including magneticmaterials, that uses electromagneticenergy, either alternating or directcurrent, to yield information regardingthe quality and characteristics of thetested material.10,11

EMAT: Electromagnetic acoustic transducer.encircling coil: In electromagnetic

testing, a coil or coil assembly thatsurrounds the test object. Such a coilis also called an annular coil,circumferential coil or feed-throughcoil.10,11

end effect: In bar and tube testing, edgeeffect.

ET: Electromagnetic testing.evaluation: Review following

interpretation of indications, todetermine whether they meet specifiedacceptance criteria.

excitation coil: Coil that carries theexcitation current. Also called primarycoil or winding. See sensing coil.10

external discontinuities: Discontinuitieson the outside or exposed surface of atest object.10

F

false indication: Test indication thatcould be interpreted as originatingfrom a discontinuity but whichactually originates where nodiscontinuity exists in the test object.Distinct from nonrelevant indication.Compare defect.10

feed-through coil: See encircling coil.ferrite: Any of several magnetic

substances that consist essentially ofan iron oxide combined with one ormore metals (such as manganese,nickel or zinc) having high magneticpermeability and high electricalresistivity.6

ferromagnetic material: Material such asiron, nickel or cobalt whose relativepermeability is considerably greaterthan unity and depends on themagnetizing force and often exhibitshysteresis.10,12 Materials that are moststrongly affected by magnetism arecalled ferromagnetic.10

fill factor: For encircling coilelectromagnetic testing, the ratio ofthe cross sectional area of the testobject to the effective cross sectionalcore area of the primary encircling coil(outside diameter of coil form, notinside diameter that is adjacent to theobject).10,11,13 For internal probeelectromagnetic testing, the ratio ofthe effective cross sectional area of theprimary internal probe coil to thecross sectional area of the tubeinterior.10,11



fill factor effect: Effect of fill factor oncoupling between coil and test object.See coupling coefficient.4

filter: Network that leaves a signalunaffected over a prescribed range offrequencies and attenuates signalcomponents at all otherfrequencies.4,11

finite element analysis: Numericaltechnique for the analysis of acontinuous system whereby thatsystem is decomposed into acollection of finite sized elements.4

flaw: Rejectable or unintentionalanomaly. See also defect anddiscontinuity.10

flaw inversion: See discontinuity inversion.flux density: See magnetic flux density.flux leakage: See magnetic flux leakage

field; magnetic flux leakage technique;magnetic flux meter.

flux meter: See magnetic flux meter.full-wave rectified direct current:

Single-phase or three-phase alternatingcurrent rectified to produceunidirectional current. The rectifiedcurrent contains ripple.

G

gauss (G): Obsolete unit of magnetic fluxdensity, replaced in SI by tesla (T).1 G = 0.1 mT.10

gauss meter: Gage that measuresmagnetic flux density in gauss (ortesla).10

general examination: In personnelqualification, test or examination of aperson’s knowledge, typically (in thecase of nondestructive testingpersonnel qualification) a written teston the basic principles of anondestructive testing method andgeneral knowledge of basic equipmentused in the method. (According toASNT’s guidelines, the generalexamination should not addressknowledge of specific equipment,codes, standards and procedurespertaining to a particularapplication.)10

grinding crack: Shallow crack formed inthe surface of relatively hard materialsbecause of excessive grinding heat orthe high sensitivity of the material.Grinding cracks typically are90 degrees to the direction ofgrinding.10

H

half-wave direct current: Single-phasealternating current half-wave rectifiedto produce a pulsating unidirectionalcurrent. Also called half-wavecurrent.10,15

hall detector: Semiconductor elementthat produces an output electromotiveforce proportional to the product ofthe magnetic field intensity and abiasing current.10

hall effect: Potential difference developedacross a conductor at right angles tothe direction of both the magneticfield and the electric current. Producedwhen a current flows along arectangular conductor subjected to atransverse magnetic field.10,13

hardness: Resistance of metal to plasticdeformation, usually by indentation.However, the term may also refer tostiffness or temper or to resistance toscratching, abrasion or cutting.10

heat affected zone (HAZ): Base metalthat was not melted during brazing,cutting or welding but whosemicrostructure and physical propertieswere altered by the heat.10

hertz: Measurement unit of frequency,equivalent to one cycle persecond.10,12

horseshoe coil: Probe coil in which theferrite core of the coil is horseshoeshaped. Also called a U shaped coil.4

hysteresis: Apparent lagging of themagnetic effect when the magnetizingforce acting on a ferromagnetic bodyis changed; phenomenon exhibited bya magnetic system wherein its state isinfluenced by its previous history.10

hysteresis loop: Curve showing fluxdensity B plotted as a function ofmagnetizing force H as magnetizingforce is increased to the saturationpoint in both negative and positivedirections sequentially. The curveforms a characteristic shaped loop.

I

IACS: International Annealed CopperStandard.

impedance: Opposition that a circuitpresents to the flow of an alternatingcurrent, specifically the complexquotient of voltage divided bycurrent.10,11

impedance analysis: In electromagnetictesting, an analytical technique thatconsists of correlating changes in theamplitude, phase, quadraturecomponents or all of these of acomplex test signal voltage to thecondition of the test specimen.10,11

impedance plane diagram: Graphicalrepresentation of the locus of pointsindicating the variations in theimpedance of a test coil as a functionof a parameter, such as conductivity orliftoff.



incremental permeability: Ratio of thechange in magnetic induction to thecorresponding change in magnetizingforce.

indication: Nondestructive testequipment response to a discontinuitythat requires interpretation todetermine its relevance. Comparedefect, discontinuity and falseindication.10

indication, discontinuity: Visibleevidence of a material discontinuity.Subsequent interpretation is requiredto determine the significance of anindication.10

indication, false: See false indication.indication, nonrelevant: Indication due

to misapplied or improper testing.May also be an indication caused byan actual discontinuity that does notaffect the usability of the test object (achange of section, for instance).10

indication, relevant: Indication from adiscontinuity (as opposed to anonrelevant indication) requiringevaluation by a qualified inspector,typically with reference to anacceptance standard, by virtue of thediscontinuity’s size, shape, orientationor location.10,17

induced current technique: See currentinduction technique.

inductor: Device consisting of one ormore associated windings, with orwithout a magnetic core, whichimpedes the flow of current.

initial permeability: Slope of theinduction curve at zero magnetizingforce as the test specimen begins to bemagnetized from a demagnetizedcondition (slope at the origin of theB,H curve before hysteresis isobserved).

inserted coil: See inside diameter coil.inside coil: See inside diameter coil.inside diameter coil: Coil or coil

assembly used for electromagnetictesting by insertion into the test piece,as with an inside probe for tubing.Also called inserted coil.4,11

intergranular stress corrosion cracking(IGSCC): Anomaly caused byintergranular corrosion as a result ofsensitized material, stress andcorrosive environment (typical in theheat affected zone of stainless steelwelds).

International Annealed CopperStandard (IACS): Conductivitymeasurement system in which theconductivity of annealed, unalloyedcopper is arbitrarily rated at100 percent and in which theconductivities of other materials areexpressed as percentages of thisstandard. See also conductivity andpercent International Annealed CopperStandard.

interpretation: Determination of thesignificance of test indications fromthe standpoint of their relevance orirrelevance, that is, from thestandpoint of whether they aredetrimental or inconsequential.10

inversion, discontinuity: See discontinuityinversion.

L

leakage flux: (1) Magnetic flux of the coilthat does not link with the test object.(2) Magnetic flux that leaves asaturated or nearly saturated specimenat a discontinuity.4

level, acceptance: See acceptance level.level, rejection: See rejection level.liftoff: Distance between the probe coil

and the test object.4liftoff effect: In an electromagnetic test

system output, the effect observed dueto a change in coupling between a testobject and a probe whenever thedistance between them is varied.10,11

longitudinal magnetic field: Magneticfield wherein the flux lines traversethe component in a directionessentially parallel with itslongitudinal axis.10,15

M

magnetic circuit: Closed path that allowsmagnetic flux to flow.

magnetic field: Distribution of a vectorquantity that is a measure of anexerted magnetic force.

magnetic field indicator: Device used tolocate or determine relative intensityof a flux leakage field.10,15

magnetic field intensity: Strength of amagnetic field at a specific point.Measured in ampere per meter.10

magnetic flux density: Normal magneticflux per unit area, measured intesla (T).10,13

magnetic flux leakage field: Magneticfield that leaves or enters the surfaceof an object.10,15

magnetic flux leakage technique:Electromagnetic test technique for thedetection and analysis of a surfacediscontinuity or near surfacediscontinuity using the flux thatleaves a magnetically saturated, ornearly saturated, test object at adiscontinuity.10

magnetic flux meter: Electronic devicefor measuring magnetic flux leakage.10

magnetic flux leakage: Excursion ofmagnetic lines of force from thesurface of a test specimen.4,11



magnetic particle testing (MT):Nondestructive test method usingmagnetic leakage fields and indicationmaterials to disclose surface and nearsurface discontinuities.10,15

magnetic saturation: That degree ofmagnetization where a further increasein magnetizing force produces nosignificant increase in magnetic fluxdensity in an object.10,11

magnetometer: Device for measuring thestrength of magnets or the intensity ofmagnetic fields.10,16

magnitude: Absolute value of a complexquantity (number) without referenceto the phase of the quantity.4

Maxwell’s equations: Fundamentalequations of electromagnetic fieldtheory:

where B = magnetic flux density,D = electric flux density, E = electricfield intensity, H = magnetic fieldstrength, J = current density andρ = volume charge density.4

microwave testing: Nondestructivetesting method that uses, for itsprobing energy, electromagneticradiation at radio frequencies — from0.3 to 300 GHz, with wavelengthsfrom 1 mm to 1 m.10

model, analytical: Mathematicalrepresentation of a process orphenomenon.

modulation: Process of impartinginformation to a carrier signal by theintroduction of amplitude or phaseperturbation.14

MT: Magnetic particle testing.multifrequency: Two or more frequencies

applied sequentially or simultaneouslyto the test coil.

multifrequency technique: Use of theresponse of a test specimen to morethan one frequency, usually toseparate effects that would beindistinguishable at a singlefrequency.4

multiparameter: Of or pertaining to atest system having many parametersthat affect the response. Theseparameters can often be distinguishedwith a multifrequency technique.4

multivariable: See multiparameter.4

mutual inductance: Property of twoelectrical circuits whereby a voltage isinduced in one circuit by a change ofcurrent in the other circuit.4,12

N

NDC: Nondestructive characterization.NDE: (1) Nondestructive evaluation.

(2) Nondestructive examination.NDI: Nondestructive inspection.NDT: Nondestructive testing.noise: In electromagnetic testing, any

nonrelevant signal that tends tointerfere with the normal reception orprocessing of a desired discontinuitysignal. Such noise signals may be dueto an extraneous source or generatedby heterogeneities in the test part thatare not detrimental to the use of thepart.4,11

nondestructive characterization (NDC):Branch of nondestructive testingconcerned with the description andprediction of material properties andbehaviors of components and systems.

nondestructive evaluation (NDE):Another term for nondestructive testing.In research and academiccommunities, the word evaluation issometimes preferred because it impliesinterpretation by knowledgeablepersonnel or systems.10

nondestructive examination (NDE):Another term for nondestructive testing.In the utilities and nuclear industry,examination is sometimes preferredbecause testing can imply performancetrials of pressure containment orpower generation systems.10

nondestructive inspection (NDI):Another term for nondestructive testing.In some industries (utilities, aviation),the word inspection often impliesmaintenance for a component thathas been in service.10

nondestructive testing (NDT):Determination of the physicalcondition of an object withoutaffecting that object’s ability to fulfillits intended function. Nondestructivetest methods typically use anappropriate form of energy todetermine material properties or toindicate the presence of materialdiscontinuities (surface, internal orconcealed).10

nonferromagnetic material: Material notmagnetizable and essentially notaffected by magnetic fields.4,11

=∇ • D ρ

= 0∇ • B

∇ × ∂∂

+HD

J=t

∇ × − ∂∂

EB

=t



normalized impedance diagram:Diagram in which the impedance ofthe probe in air is a reference value towhich impedance values in otherconditions are compared. Usually theplotted data are (1) the measuredreactance divided by the reactance ofthe coil in air versus (2) the measuredresistance less the resistance in airdivided by the coil reactance in air.

null: To adjust a bridge circuit so that thetest sample and reference armsproduce equal and opposite currentsthrough the detector.4

null signal: Fixed component of the testcoil signal that is subtracted from theoutput signal leaving only that part ofthe signal that varies with test objectconditions; it reduces dynamic rangerequirements.4

numerical analysis: Technique togenerate numbers as the solution to amathematical model of a physicalsystem; used in place of a closed formanalytic expression; usually requiresdigital computation.4

O

OCTG: Oil country tubular goods.10

oersted (Oe): Obsolete measurement unitof magnetic field intensity, replaced inSI by ampere per meter (A·m–1).1 Oe = 79.57747 A·m–1.

ohm (Ω): Measurement unit of electricalresistance.

oil country tubular goods: Hollowcylindrical components used toconvey petroleum and relatedproducts.10

optimum frequency: In electromagnetictesting, that frequency that providesthe largest signal-to-noise ratioobtainable for the detection of anindividual material property.

P

pancake coil: Probe coil whose axis isnormal to the surface of the testmaterial and whose length is notlarger than the radius.4

paramagnetic material: Inelectromagnetic testing, a materialthat has a relative permeability slightlygreater than unity and is practicallyindependent of the magnetizingforce.10,11

percent International Annealed CopperStandard (%IACS): Measurement ofconductivity as a percentage of theconductivity of pure copper, arbitrarilyrated at 100 percent. See alsoInternational Annealed Copper Standard.

period: Absolute value of the minimuminterval after which the samecharacteristics of a periodic waveformor a periodic feature repeat.4

permeability: Ratio of magneticinduction to magnetizing force. Thisrelationship is either (1) absolutepermeability, in general the quotientof magnetic induction divided by themagnetizing force, or (2) relativepermeability (or specific permeability),a pure number that is the same in allunit systems. The value anddimension of absolute permeabilitydepend on the system of units used. Inanisotropic media, permeability is amatrix.4,12

phase analysis: Analytical technique thatdiscriminates between variables in apart undergoing electromagnetictesting by the different phase angleand amplitude changes that theseconditions produce in the test signal.See also phase detection.4,11

phase angle: Angular equivalent of thetime displacement betweencorresponding points on two sinewaves of the same frequency.4,11

phase detection: Derivation of a signalwhose amplitude is a function of thephase angle between two alternatingcurrents, one of which is used as areference.4,11

phase sensitive system: System whoseoutput signal depends on the phaserelationship between the voltagereturned from a pickup or sensing coiland a reference voltage.4,11

phase shift: Change in the phaserelationship between two alternatingquantities of the same frequency.4,11

phasor: Complex number that representsthe amplitude and phase of a quantitythat varies sinusoidally with time. Aphasor is not a vector, because theorientation of a vector representsdirection.

physical properties: Nonmechanicalproperties such as density, electricalconductivity, heat conductivity andthermal expansion.10

probe coil clearance: Perpendiculardistance between adjacent surfaces ofthe probe and test part. See liftoff.4,11

pulse technique: Multifrequencytechnique in which a broadbandexcitation such as an impulse is used.Either the frequency components areextracted and analyzed or theinterpretation is based directly oncharacteristics of the time domainwaveform.4



Q

Q of a coil: Quality factor of a coil; relatedto the ratio of maximum energy storedto the total energy lost per period.

quadrature: Relation between twoperiodic functions when the phasedifference between them is one fourthof a period.4,12

qualification: Process of demonstratingthat an individual has the requiredamount and the required type oftraining, experience, knowledge andcapabilities.10

qualified: Having demonstrated therequired amount and the requiredtype of training, experience,knowledge and abilities.10

quality: Ability of a process or product tomeet specifications or to meet theexpectations of its users in terms ofefficiency, appearance, longevity andergonomics.10

quality assurance: Administrative actionsthat specify, enforce and verify aquality program.10

quality control: Physical andadministrative actions required toensure compliance with the qualityassurance program. Quality controlmay include nondestructive testing inthe manufacturing cycle.10

quality factor: Of a coil, the ratio ofreactance to resistance defined at theoperating frequency.

R

recommended practice: Set of guidelinesor recommendations.10

Recommended Practice No. SNT-TC-1A:Set of guidelines published by theAmerican Society for NondestructiveTesting, for employers to establish andconduct a nondestructive testingpersonnel qualification andcertification program.10

recovery time: Time required for a testsystem to return to its original stateafter overload or signal reception.

reference coil: In electromagnetic testing,the section of the coil assembly thatexcites or detects the electromagneticfield in the reference standard of acomparative system.4,11

reference number: Number associatedwith the impedance of a coil adjacentto a test sample.

reference standard: Reference used as abasis for comparison or calibration. Intube testing, a tube with artificialdiscontinuities used for establishingthe test sensitivity setting and forperiodically checking and adjustingthe sensitivity setting as required. Seealso acceptance standard.4,11

reflection probe: Coil system that usesboth an excitation and a detection orsensing coil on the same side of thesample.4,11

rejection level: Value established for atest signal above or below which testspecimens are rejectable or otherwisedistinguished from the remainingspecimens. This level is different fromthe rejection level as defined forultrasonic and other test systems.4,11,20

relative permeability: Ratio of thepermeability of the material to thepermeability of vacuum. See alsopermeability.4

resistance, electrical (R): Opposition totransmission of electric currentthrough material; ratio of voltage tocurrent. Measured in ohm (Ω).Inversely related to conductance:

where A is the conductor’s crosssectional area (square meter), G isconductance (siemens), L is the lengthof the conductor (meter) and ρ isresistivity (ohm meter).

resistivity (ρρ): Ability of material to resistelectric current. Measured in ohmmeter (Ω·m), which is the resistance ofa cube made of the material whosedimensions are 1 m on each side.Inversely related to conductivity σ(siemens per meter):

response function: Ratio of response toexcitation, both expressed as functionsof the complex impedance.4,12

S

scalar: Quantity completely specified by asingle number and unit.4,12

search coil: Detection coil, usuallysmaller than the excitation coil.4

secondary magnetic flux: Magnetic fluxdue to induced flow of eddy currents.4

selectivity: Characteristic of a test system,a measure of the extent to which aninstrument can differentiate betweenthe desired signal and disturbances ofother frequencies or phases.4,11

self-inductance: Property of an electriccircuit whereby an electromotive forceis induced in that circuit by a changeof current in the circuit.4,12

sensing coil: Coil that detects changes inthe flow of eddy currents induced byan excitation coil; sensing andexcitation coils can be one and thesame.4 Also called detector coil.

ρσ

= 1

RG

LA

= =1 ρ



shielding: Conducting or magneticmaterial (or a combination of both)placed so as to decrease susceptibilityto interference.4

SI (International System of Units):Universal, coherent system ofmeasurement in which the followingseven units are considered basic:meter, mole, kilogram, second,ampere, kelvin and candela.4,12

siemens per meter (S·m–1): SI unit ofconductivity.

signal: Physical quantity, such aselectrical voltage, that containsrelevant information.4,11

signal-to-noise ratio: Ratio of signalvalues (responses that contain relevantinformation) to baseline noise values(responses that contain nonrelevantinformation). See noise.10,11

skin depth: SStandard depth of penetration.See also skin effect.

skin effect: Phenomenon wherein thedepth of penetration of electricalcurrents into a conductor decreases asthe frequency of the current isincreased. At very high frequencies,the current flow is restricted to anextremely thin outer layer of theconductor. See standard depth ofpenetration.10,11

SNT-TC-1A: See Recommended PracticeNo. SNT-TC-1A.

specification: Set of instructions orstandards invoked by a specificcustomer to govern the results orperformance of a specific set of tasksor products.10

spectrum: Signal aspect showing thedistribution of the various frequencycomponents of the signal.14 Alsocalled fourier spectrum.

SQUID: Superconducting quantuminterference device, a sensitivedetector of magnetic fields usingquantum effect.4

standard: (1) Physical object with knownmaterial characteristics used as a basisfor comparison or calibration; referencestandard. (2) Concept established byauthority, custom or agreement toserve as a model or rule in themeasurement of quantity or theestablishment of a practice orprocedure. (3) Document to controland govern practices in an industry orapplication, applied on a national orinternational basis and usuallyproduced by consensus. See alsoacceptance standard and referencestandard.10,11,18

standard depth of penetration: Inelectromagnetic testing, the depth atwhich the magnetic field intensity orintensity of induced eddy currents hasdecreased to 37 percent of its surfacevalue. The square of the depth ofpenetration is inversely proportionalto the frequency of the signal, theconductivity of the material and thepermeability of the material. See alsoskin effect.10,11

standardization, instrument: Adjustmentof instrument readout before use to anarbitrary reference value.

stationary: Of a signal, having statisticalproperties such as mean and variancethat do not vary with time.14

T

tangential magnetic field: Magnetic fieldat an object’s surface parallel to thesurface. The tangential field iscontinuous (equal on either side) withthe interface of material to air.Measurement can be influenced byexternal fields.10

tape head probe: Head of a tape recorderused as an eddy current coil; a type ofhorseshoe coil.4

tesla (T): SI unit of measure for magneticflux density. 1 T = 1 Wb·m–2 =10 000 G.10

tesla meter: Gage that measures magneticflux density in tesla.10

test coil: Section of a coil assembly thatexcites or detects the magnetic field inthe material under electromagnetictest.4,11

test frequency: In electromagnetictesting, the number of complete cyclesper unit time of the alternatingcurrent applied to the primary testcoil.4,11

test quality level: See rejection level.text information: Information stored on

recording medium to support recordededdy current data.

three-way sort: Electromagnetic sortbased on a test object signal responseabove or below two levels establishedby three or more calibrationstandards.4,11

threshold level: Setting of an instrumentthat causes it to register only thosechanges in response greater or lessthan a specified magnitude.4,11

through-transmission: Of or pertainingto electromagnetic techniques wherethe excitation field penetrates the testobject so that the detected signal isresponsive to features external to oron the opposite surface.

toroidal field: Induced magnetic fieldoccurring in a ring test object whencurrent is induced. See current inductiontechnique.10



trace: Line formed by an electron beamscanning from left to right on a videoor computer screen to generate animage.10

transducer: Device by means of whichenergy can flow from one or moretransmission systems or media to oneor more other transmission systems ormedia; sensor or probe.

tubing string: Pipe with which oil or gashas contact as it is brought to theearth’s surface.10

two-way sort: Electromagnetic sort basedon a test object signal response aboveor below a level established by two ormore calibration standards.4,11

U

U shaped coil: See horseshoe coil.Unified Numbering System:

Alphanumeric system for identifyingalloys according to a registrymaintained by ASTM Internationaland SAE International.21

unit of data storage: Discrete physicalrecording medium on which textinformation is stored.

V

vector quantity: Any physical quantitywhose specification involves bothmagnitude and direction and thatobeys the parallelogram law ofaddition.4,12

volt (V): Measurement unit of electricpotential.

Wwobble: In electromagnetic testing, an

effect that produces variations in anoutput signal of a test system andarises from coil spacing (operationalliftoff) variations due to lateral motionof the test specimen in passingthrough an encircling coil or of abobbin coil passing through acylindrical test object.4,11

Y

yoke: Magnet that induces a magneticfield in the area of a part that liesbetween its poles. Yokes may bepermanent magnets or eitheralternating current or direct currentelectromagnets.4,11



1. Nondestructive Testing Handbook,second edition: Vol. 1, Leak Testing.Columbus, OH: American Society forNondestructive Testing (1982).

2. Nondestructive Testing Handbook,second edition: Vol. 2, Liquid PenetrantTests. Columbus, OH: AmericanSociety for Nondestructive Testing(1982).

3. Nondestructive Testing Handbook,second edition: Vol. 3, Radiography andRadiation Testing. Columbus, OH:American Society for NondestructiveTesting (1985).

4. Nondestructive Testing Handbook,second edition: Vol. 4, ElectromagneticTesting. Columbus, OH: AmericanSociety for Nondestructive Testing(1986).

5. Nondestructive Testing Handbook,second edition: Vol. 5, AcousticEmission Testing. Columbus, OH:American Society for NondestructiveTesting (1987).

6. Nondestructive Testing Handbook,second edition: Vol. 6, Magnetic ParticleTesting. Columbus, OH: AmericanSociety for Nondestructive Testing(1989).

7. Nondestructive Testing Handbook,second edition: Vol. 7, UltrasonicTesting. Columbus, OH: AmericanSociety for Nondestructive Testing(1991).

8. Nondestructive Testing Handbook,second edition: Vol. 8, Visual andOptical Testing. Columbus, OH:American Society for NondestructiveTesting (1993).

9. Nondestructive Testing Handbook,second edition: Vol. 9, SpecialNondestructive Testing Methods.Columbus, OH: American Society forNondestructive Testing (1995).

10. Nondestructive Testing Handbook,second edition: Vol. 10, NondestructiveTesting Overview. Columbus, OH:American Society for NondestructiveTesting (1996).

11. E 268-81, Definitions Approved for Useby Agencies of the Department of Defenseas Part of Federal Test Method StandardNo. 151b and for Listing in the DoDIndex of Specifications and Standards.West Conshohocken, PA: ASTMInternational (1981).

12. IEEE Standard Dictionary of Electricaland Electronic Terms. New York, NY:Wiley-Interscience, for the Institute ofElectrical and Electronics Engineers(1984).

13. Glossary of Terms Used in NondestructiveTesting, Part 2. London, UnitedKingdom: British Standards Institute(November 1984).

14. Couch, L.W. Digital and AnalogCommunication Systems. Upper SaddleRiver, NJ: Prentice-Hall (1997).

15. E 269-89, Standard Definitions of TermsRelating to Magnetic ParticleExamination. West Conshohocken, PA:ASTM International (1989).

16. API RP5A5, Recommended Practice forField Inspection of New Casing, Tubingand Plain End Drill Pipe, third edition.Washington, DC: American PetroleumInstitute (1987).

17. Annual Book of ASTM Standards:Section 3, Metals Test Methods andAnalytical Procedures. Vol. 03.03,Nondestructive Testing. WestConshohocken, PA: ASTMInternational (2001).

18. Nondestructive Testing Methods.TO33B-1-1 (NAVAIR 01-1A-16)TM43-0103. Washington, DC:Department of Defense, United StatesAir Force (June 1984): p 1.25.

19. Libby, H. Introduction to ElectromagneticNondestructive Test Methods. New York,NY: Wiley-Interscience (1971).

20. Webster’s Ninth New CollegiateDictionary. Springfield, MA:Merriam-Webster Incorporated (1990).

21. ASTM DS-56H [SAE HS-1086], Metalsand Alloys in the Unified NumberingSystem, ninth edition. Warrendale, PA:SAE International (2001).


References


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