Nondestructive Testing Glossary

Richard D. Lopez, Iowa State University,Ames, Iowa

16C H A P T E R

Nondestructive TestingGlossary

From Nondestructive Testing Handbook, Third Edition: Volume 10, Overview © 2012. Reprinted with permission of The American Society for Nondestructive Testing Inc.

Introduction

PurposeStandards writing bodies take great painsto ensure that their standards aredefinitive in wording and technicalaccuracy. People working to writtencontracts or procedures should consultdefinitions referenced in standards whenappropriate. For example, persons whowork in accordance with standardspublished by ASTM International areencouraged to refer to definitions in theASTM standards.1

The definitions in this NondestructiveTesting Handbook volume should not bereferenced for tests performed accordingto standards or specifications or infulfillment of contracts. This glossary isprovided for instructional purposes. Noother use is intended.

On ReferencesDefinitions from other volumes of theNondestructive Testing Handbook are notreferenced. This volume’s antecedent inthe second edition was the NondestructiveTesting Handbook: Volume 10,Nondestructive Testing Overview (1996).2However, most of the definitions in thisglossary are from the various, supersedingmethod volumes.3-11

Measurement units and their symbolsare covered in the introduction to thisvolume.

For physical quantities and propertiesin materials science, the reader is servedby reference books such as the CRCHandbook of Chemistry and Physics12 andLeonard Mordfin’s Handbook of ReferenceData for Nondestructive Testing.13

Aabsolute measurement: (1) Measurement

made with an absolute coil. (2)Measurement of a property withoutreference to another measurement ofthat property. Compare comparativemeasurement; relative measurement.

absolute pressure: Pressure aboveabsolute zero value or pressure abovethat of space empty of all molecules.Equal to sum of local atmosphericpressure and gage pressure.

absolute temperature: Thermodynamictemperature measured from absolutezero temperature, expressed inkelvin (K).

absorbed dose: In radiographic testing,amount of energy imparted to matterby an ionizing event per unit mass ofirradiated material at the place ofinterest. Absorbed dose is expressed ingray (Gy) or rad. See also dose rate;dosimeter.14

absorptance; absorptivity: Proportion (asa fraction of 1) of the radiant energyimpinging on a material’s surface thatis absorbed into the material. For ablackbody, this is unity (1.0).Technically, absorptivity is the internalabsorptance per unit path length. Inthermography, the two terms havesometimes been used interchangeably.

absorption: In nondestructive testing,reduction of the intensity of any formof radiated energy as a result of energyconversion (absorption) in a medium,such as the conversion of soundenergy into heat. Compare attenuation.

absorption coefficient, linear (µL):Fractional decrease in transmittedintensity per unit of absorberthickness. Expressed in units of cm–1.15

acceptable quality level (AQL):Maximum percent defective (or themaximum percentage of units withrejectable discontinuities) that, for thepurposes of sampling tests, can beconsidered satisfactory as a processaverage. Compare lot tolerance percentdefective.

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

acceptance level; acceptance limit:(1) Test signal value used to establishthe group to which a material underevaluation belongs (2) Measured valueor values above or below which testobjects are acceptable. Comparerejection level.

516 Nondestructive Testing Overview

Terms


acceptance standard: (1) Specimen,similar to the product to be tested,containing natural or artificialdiscontinuities that are well definedand similar in size or extent to themaximum acceptable in the product.(2) Document defining acceptablediscontinuity size limits. See alsoreference standard; standard.

accommodation: Of the eye, adjustmentof the lens’ focusing power bychanging the thickness and curvatureof the lens through its movement bytiny muscles.

accumulation test technique: In leaktesting, detecting the total amount ofleakage by enclosing the componentunder test within a hood, bag, box,shroud or container. For pressuretesting, any gas leaking from thecomponent accumulates in the space(volume) between the component andthe enclosure. For vacuum testing, anygas leaking into the componentaccumulates in the leak detectorsampling the evacuated component.Accumulation of tracer gas in ameasured time period provides ameasure of the leakage rate.

accuracy: Degree of conformity ofmeasurement to a standard or truevalue.

ACGIH: American Conference ofGovernmental Industrial Hygienists.

acoustic emission: (1) Transient elasticwaves resulting from local internalmicrodisplacements in a material.

acoustic emission activity: Number ofbursts (or events, if the appropriateconditions are fulfilled) detectedduring a test or part of a test.

acoustic emission count: Number oftimes the signal amplitude exceeds thepreset reference threshold. Sometimescalled ringdown counts.

acoustic emission event: Microstructuraldisplacement that produces elasticwaves in a material under load orstress.

acoustic emission hit: Acoustic emissionsignal received on one channel.

acoustic emission rate: Number of timesthe acoustic emission signal amplitudehas exceeded the threshold in aspecified unit of time.

acoustic emission signal: Electrical signalobtained through the detection ofacoustic emission.

acoustic emission testing (AE): Passivenondestructive testing method thatmonitors a component or assembly fortransient elastic waves and convertsthese ultrasonic waves into electricalsignals. Acoustic waves may beproduced by the formation ormovement of microstructuraldislocations during crack propagation,

melting, phase transformations orthermal stresses.

acoustic impedance (z): Frequencydependent property of a mediumthrough which acoustic wavespropagate in units of kg·s–1·m–2. In itssimplified form, acoustic impedance isthe product of longitudinal ultrasonicwave velocity (m·s–1) and materialdensity (kgm·m–3). The relativetransmission and reflection at aninterface are governed in part by theacoustic impedances of the materialson each side of the interface.

acoustic impedance, characteristic: Inultrasonic testing, acoustic impedancetypical or characteristic of a particularmaterial.

acoustic impedance, specific: Inultrasonic testing, acoustic impedancein a particular test object or a definedvolume of a specified material.

acoustic microscopy: In ultrasonictesting, general term referring to theuse of high resolution, high frequencyultrasonic techniques to produceimages of features beneath the surfaceof a test object.

activation: In radiographic testing,process by which neutrons bombardstable atoms and make themradioactive.

activity: In radiographic testing, degree ofradioactivity of a particular isotope.Activity is expressed as the number ofatoms disintegrating per unit of time.Measured in becquerels.

AE: Acoustic emission testing.acuity: See neural acuity, vision acuity.adhesive wear: See wear, adhesive.agency: Organization selected by an

authority to perform nondestructivetesting, as required by a specificationor purchase order.

agglomeration: Clustering where smallerparticles collide and adhere as groups.

aging: (1) The effect of long termenvironmental exposure on materialsor components. (2) Heat treatmentmethod that alters material propertiesand microstructure because of theduration of time at ambient (naturalaging) or elevated (artificial aging)temperature. Aging is commonlyapplied to alloys after hot working,quenching from an elevatedtemperature or cold working. See alsoprecipitation hardening.

air flow: In leak testing, flow of air fromthe probe inlet to the sensitive elementof the halogen leak detector thatcarries the tracer gas from the leak tothe sensing diode.

algorithm: Prescribed set of well definedrules or processes for the solution of amathematical problem in a finitenumber of steps.16

517Nondestructive Testing Glossary


alkali ion diode: Sensor type for halogengases. In this device, positive ions(cations) of an alkali metal areproduced on the heated surfaces(usually platinum) of the diode. Oneelectrode is at a negative potential andattracts cations that are released whena halogen gas passes between thesensor electrodes. Provides an outputcurrent to operate the indicator on thehalogen leak detector.

alpha iron: See ferrite.alpha particle: Positively charged helium

ion emitted by certain radioactivematerials. It is made up of twoneutrons and two protons; hence, it isidentical with the nucleus of a heliumatom.14

alpha ray: Ionizing radiation in the formof a fast stream of alpha particles.Compare beta ray; gamma ray; X-ray.

alternating current (AC): Electric currentwhose waveform changes cyclically inmagnitude and direction.17

alternating current field: Inelectromagnetic and magnetic testing,varying magnetic field producedaround a conductor by alternatingcurrent flowing in the conductor.

alternating current magnetization: Inmagnetic particle testing, techniquefor inducing an active magnetic stateby a cyclically reversing waveform, astate generally characterized by itsform following ability and by shallowpenetration.

ambient light: Light in the environmentas opposed to illumination providedby a testing system.

ambient temperature; atmospherictemperature: Temperature ofsurrounding atmosphere. Also calleddry bulb temperature. Compare standardatmospheric conditions.

ampere (A): SI unit of electric current.ampere per meter (A·m–1): SI derived

unit of magnetic field intensity. Themeasurement 1 A·m–1, for example,describes a current of 1 A flowingthrough a coil that is 1 m in diameter.Compare oersted.

ampere turn (At): In magnetic particletesting, unit for expressing themagnetomotive force required formagnetization using a coil in terms ofthe product of the number of coilturns and the current in amperesflowing through the coil.

amplitude, echo: In ultrasonic testing,the vertical height of a received signalon an A-scan, measured from base topeak for a video presentation or frompeak to peak for a radio frequencypresentation.

amplitude response: That property of atest system whereby the amplitude ofthe detected signal is measuredwithout regard to phase.1

analog-to-digital converter: Circuitwhose input is information in analogform and whose output is essentiallythe same information in digitalform.16

angle beam: In ultrasonic testing,ultrasound beam traveling at an acuteangle into a medium. The angle ofincidence or angle of refraction ismeasured from the normal to theentry surface.15

angle beam test technique: In ultrasonictesting, inspection technique in whichtransmission of ultrasound is at anacute angle to the entry surface.18

angle of field: (1) In visual testing,included angle between those pointson opposite sides of a beam axis atwhich the luminous intensity is10 percent of the maximum value.This angle may be determined from anilluminance curve or may beapproximated by use of an incidentlight meter. Also known as field ofview. Compare depth of field.19 (2) Ininfrared and thermal testing, angularsubtense (expressed in angular degreesor radians per side if rectangular andangular degrees or radians if round)over which an instrument willintegrate all incoming radiant energy;the projection of the detector at thetarget plane. In a radiationthermometer, this angle defines thetarget spot size; in a line scanner orimager, it represents one resolutionelement in a scan line or athermogram and is a, index of spatialresolution.

angle of incidence: In ultrasonic testing,the angle included between the beamaxis of the incident wave and thenormal to the surface at the point ofincidence.18

angle of reflection: In ultrasonic testing,included angle between the beam axisof the reflected wave and the normalto the reflecting surface at the point ofreflection.18

angle of refraction: In ultrasonic testing,the angle between the beam axis of arefracted wave and the normal to therefracting interface.18

angle of view: In visual testing, the anglein degrees between the field of viewaxis and the axis of the fiberscope’sarticulating section. Also calleddirection of view

angstrom (Å): Disused unit of length.1 Å = 0.1 nm.

angular subtense: (1) Angular diameter ofan optical system or subsystem,expressed in angular degrees ormilliradians. (2) In thermography, theangle over which a sensing instrumentcollects radiant energy.



anisotropy: Material characteristic inwhich different values of a property(acoustic velocity, for example) arenoted in different directions. Compareisotropy.

annealing: Process of heating a materialto, and holding at, a desiredtemperature followed by cooling at adesired rate, usually to reduce residualstresses or bring about some otherdesired change.

annular coil: See coil, encircling.anode: (1) In radiography, the positive

electrode of a cathode ray tube thatgenerates ionizing radiation.(2) Positively charged terminal, whichmay corrode electrochemically duringproduction of an electric current.Compare cathode.

anomaly: Variation from normal materialor product quality. (1) Innondestructive testing, a nonrelevantindication. (2) In nondestructivetesting, an unintentional or undesiredmaterial condition that may qualify asa defect. Compare defect; discontinuity.

antinode: Point in a standing wave wherecertain characteristics of the wave fieldhave maximum amplitude. Comparenodal point.18

apposing field: See bucking field.arc: Current flow across a gap, producing

intense heat and light.arc strike: Localized thermal damage to

object from an electric arc caused bybreaking an energized circuit. Alsocalled arc burn.

arc welding: See welding, arc.area linearity: See linearity, area.argand diagram: In electromagnetic

testing, graphical representation of avector quantity on the complex plane.

articulate: Ability of a device, such as aflexible borescope, to be remotelydeflected in a plane with respect to theaxis of the undeflected workingsection.

articulated pole piece: In magneticparticle testing, independentlyadjustable legs of a contour probe thatenable satisfactory contact on irregulartest object profiles.

artifact: See indication, false.A-scan: One-dimensional display of

ultrasonic echo amplitude as functionof time or depth in test object. Seealso marker. Compare C-scan.

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

ASNT: American Society forNondestructive Testing.

atmospheric pressure: Ambient pressurecaused by the weight of the earth’satmosphere. Because the weight of theearth’s overlying atmosphere variesinversely with altitude, atmosphericpressure decreases with elevation. Also

called barometric pressure. At sea level,standard barometric pressure is taken as101.325 kPa (14.696 lbf·in.–2). It is alsoequal to the pressure exerted by amercury column 760 mm (29.92 in.)high — that is, equal to 760 mm Hg(29.92 in. Hg) or 760 torr.

attenuation: (1) Decrease in energy orsignal magnitude in transmission fromone point to another. Can beexpressed in decibels or as a scalarratio of the input magnitude to theoutput magnitude.16 (2) Change insignal strength caused by an electronicdevice such as an attenuator.(3) Decrease in intensity caused byabsorption, leakage, reflection,scattering or other materialcharacteristics. See also neper.

attenuation coefficient: Factordetermined by the degree ofdiminution in sound wave energy perunit distance traveled. It is composedof two parts, one (absorption)proportional to frequency, the other(scattering) dependent on the ratio ofgrain size or particle size towavelength.20 See also ultrasonicabsorption.

atmospheric windows (infrared): Ininfrared and thermal testing, spectralintervals within the infrared spectrumin which the atmosphere transmitsradiant energy well (atmosphericabsorption is a minimum). These areroughly defined as 2 to 5 µm and 8 to14 µm.

austenite: Face centered cubic phase ofiron, which phase is stable between906 °C (1663 °F) and 1390 °C (2535 °F)and often acts as a solvent for carbon.Also called gamma iron.

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

axial: See longitudinal.

Bbackground: Formations on or signals

from a test object that constitutes thebackground to a discontinuity. Thehigher the level of background noise,the more difficult it is to distinguish adiscontinuity. Background signals mayarise from visual, acoustic, chemical,electrical or radiation sources that thesensor responds to. See also neuralacuity; sensitivity; signal-to-noise ratio.

back reflection: In ultrasonic testing,signal received from the far boundaryor back surface of a test object.



backscatter: (1) In radiographic testing,interaction of radiation with mattersuch that the direction of travel afterscattering is over 90 degrees and oftenclose to 180 degrees to the originaldirection of travel. (2) In transmissionradiography, interaction of radiationwith matter behind the image planesuch that scattered radiation returns tothe image plane, often adding fog andnoise that interfere with production ofan image of the specimen. (3) Ofscatter imaging, interaction ofincident radiation with a specimenthat scatters the radiation throughlarge angles frequently greater than90 degrees to the original direction oftravel. Such radiation is used to forman image or to measure a parameter ofthe specimen, usually through digitaltechniques.

backscatter imaging: In radiographictesting, a family of radioscopictechniques that use backscatter.

backstreaming: Movement of pumpingfluids from a pump back to thevacuum chamber.21

baffle: System component, typically aplate, that condenses pump fluidsbefore they reach the vacuumchamber and returns fluid to thepump.21

barium clay: Molding clay containingbarium, used to eliminate or reducethe amount of scattered or secondaryradiation reaching an X-ray sensor.

barometric pressure: Ambient pressurecaused by the weight of the Earth’satmosphere. See atmospheric pressure.

baseline: Standard, average, priormeasurements or other criteria forcomparison and evaluation.

bath: In magnetic particle testing,combination of well agitated waterbased or oil based carrier fluid with acontrolled concentration of suspendedmagnetic particles.

beam: In radiographic testing, definedstream of radiation particles in whichstream all particles are traveling inparallel paths.

beam quality: In radiographic testing,penetrating energy of a radiationbeam.

beam spread: (1) In radiographic testing,divergence from a beam of radiationin which all particles are traveling inparallel paths. (2) In ultrasonic testing,divergence of a sound beam as it travelsthrough a medium.18 Specifically, thesolid angle that contains the mainlobe of an ultrasonic beam in the farfield.

bearding: See furring.becquerel (Bq): SI unit for measurement

of radioactivity, equivalent to onedisintegration per second. Replacescurie (Ci), where 1 Ci = 3.7 × 1010 Bq.

beta particle: Electron or positronemitted from a nucleus duringradioactive decay.14

beta ray: Radiation beam consisting ofbeta particles. Compare alpha ray,gamma ray, X-ray.

betatron: Circular electron acceleratorthat is a source of either high energyelectrons or X-rays. The electrons areinjected by periodic bursts into aregion of an alternating magneticfield.14 Sometimes the electrons areused directly as the radiation.

berthold penetrameter: Shared fluxindicator of magnetic field orientation,for use during continuousmagnetization. Similar to a pie gagebut containing a cover plate withheight adjustable to vary the magneticflux density required to form anindication. See also shared fluxindicator. Compare pie gage.

binary system: In metallurgy, a two-element alloy system. See also phasediagram.

birefringence: Splitting of light into twobeams, through double refraction, as itpasses through specific types oftranslucent materials.

blackbody: In physics, a theoreticalobject whose incandescent radiationemission distribution (intensity versuswavelength) depends only on theabsolute temperature of the blackbodyand not on its internal nature orstructure. A blackbody absorbs allenergy falling on it. As the blackbodytemperature increases, peak emissionwavelength decreases. See alsoabsorptivity; Planck’s law; stefan-boltzmann law; Wien’s displacement law.

black light: See UV-A.bleed back technique: In liquid

penetrant testing, procedure forverifying fluorescent penetrantindications by fully removing theindication and then reinspecting thearea of interest. The technique beginsby removing the indication smoothlyusing a soft paintbrush or cottontipped applicator lightly moistenedwith a volatile solvent (acetone). If theindication was linear, apply a lightcoating of solvent based nonaqueouswet developer to the area of interest. Ifthe indication was nonlinear nodeveloper is applied. The indication isconfirmed if the fluorescent indicationreappears within 600 s. Also calleddoubt removal technique, rebleedtechnique or wipeoff technique.

bleedout: In liquid penetrant testing,action by which liquid penetrant isdrawn from a discontinuity into thedeveloper layer, thus forming anindication on the surface of thespecimen.



blind spot: Portion of the retina wherethe optic nerve enters, without rods orcones, and where the retina isinsensitive to light.19

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.

blotch: (1) An irregularly spaced area ofcolor change on a surface. (2) Thenonuniform condition of a surfacecharacterized by such blotches.

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

blue haze: Temporary blurred visioncaused by UV-A photons entering theeye and exciting fluorescence of thevitreous humor. See also vision acuity.

blue light hazard: Danger of long termretinal damage posed to the eye byexposure to visible light with awavelength between 400 and 520 nmat elevated intensities and/or extendeddurations. See also American Conferencefor Industrial Hygienists.

bolometer, infrared: Thermal infrareddetector in which electricalconductivity changes withtemperature.

borescope: Remote viewing deviceconsisting of fiber bundles and/or aseries of lenses with an objective lensat one end and an eyepiece at theother, for viewing objects notaccessible to direct viewing.Borescopes are so called because theywere originally used in machinedapertures and holes such as gun bores.Borescopes, which may have diametersas small as 0.5 mm (0.02 in.), fall intotwo categories: flexible and rigid.

borescope, blending: Borescopecomprised of a flexible shaft and arotary tool to smooth out (blend)damage. There are generally customdesigned kits for use with specificapplications.

borescope, calibrated: Borescope with agage on external tube to indicate thedepth of insertion during a test.Borescopes with calibrated reticles areused to determine angles or sizes ofobjects in the field when held at apredetermined working distance.

borescope, fiber optic: Flexible industrialendoscope that uses glass or quartzfibers to transmit light and the opticalpath to and from the test object.Generally used in areas where tortuousbends or curves necessitate a flexibledevice, a fiber optic borescope consistsof a coherent fiber optic bundle, lightguide fiber and a flexible protectivesheath enclosing wires for probedeflection.

borescope, panoramic: Borescope with arevolving prism mounted in front ofthe objective lens system. Prism angleis adjusted at the ocular end of theinstrument to scan in forward oblique,right angle and retrospective(backward) directions.

borescope, rigid: Borescope that does notbend, typically to keep the geometricaloptics in alignment through a lighttrain system.

borescope, ultraviolet: Borescope withthe ability to transmit ultravioletradiation to the distal end whiletransmitting visible light to theeyepiece.

borescope, video: Borescope that uses avideo camera instead of an eyepieceand transmits the image electronically.Compare borescope.

borescopy: Viewing or inspection with aborescope.

boundary echo: In ultrasonic testing,reflection of an ultrasonic wave froman interface.15

brazing: Joining of metals and alloys byfusion of nonferrous alloys that havemelting points above 430 °C (806 °F),but below melting points of materialsbeing joined.

brehmsstrahlung: Electromagneticradiation produced when electrons’path and kinetic energy brings themclose to the positive fields of atomicnuclei — as when, for example,electrons strike a target provided forthis purpose. The electrons slow down,giving up kinetic energy asX-radiation.

brinell hardness testing: Evaluationmethod for determining the hardnessof a material by forcing a hard steel orcarbide ball of specified diameter(often 10 mm) into it under aspecified load. The diameter of theindent is measured, and the result isreported as the material’s brinellhardness number. Compare rockwellhardness testing.

brinelling: Permanent surfacedeformation caused by contact stressabove the material’s limit. Comparefalse brinelling.

brittle crack propagation: Very suddenpropagation of crack with absorptionof no energy except that storedelastically in body. Microscopicexamination may reveal somedeformation invisible to the unaidedeye. Compare ductile crack propagation;fatigue.

brittleness: Quality of material that leadsto crack propagation withoutappreciable plastic deformation.Compare ductility.



broad band: In ultrasonic testing, havinga relatively wide frequency bandwidth.Used to describe pulses that display awide frequency spectrum and receiverscapable of amplifying them. Comparenarrow band.

B-scan: In ultrasonic testing, datapresentation technique typicallyapplied to ultrasonic pulse echotechniques. It produces a two-dimensional view of a cross sectionalplane through the test object. Thehorizontal sweep is proportional tothe distance along the test object andthe vertical sweep is proportional todepth, showing the front and backsurfaces and discontinuities between.15

bubbler: See water column.bucking field technique; apposing field

technique: In magnetic particletesting, field flow magnetizationtechnique where magnetic poles oflike polarity are induced on the endsof a test object to force magnetizationinto extremities that are normally fieldfree. Bucking fields are generallyimparted with a pair of iron coreinduction coil pole extenders on a wethorizontal machine.

buckle: (1) Indentation on a flat face of acasting that may be caused byexpansion of molding sand or by thedip coat of an investment castingpeeling away from the pattern.(2) Local waviness in rolled metalsheet or bar stock, usually transverseto the rolling direction. (3) Failuremode of a compressed componentthat is characterized by unstablelateral deflection.

burr: Raised or turned over edge occurringon a machined part and resulting fromcutting, punching or grinding.22

burn through: In welding, coalescence ofmetal protruding beyond the root ofthe weld. Sometimes called icicles.

burnt-in sand: In manufacturing,discontinuity consisting of mixture ofsand and metal cohering to surface ofcasting.

burst: (1) In metal forming operations,external or internal rupture caused bypoor process control or inherentmaterial discontinuities. (2) In acousticemission, signal whose oscillationshave a rapid increase in amplitudefrom an initial reference level(generally that of the backgroundnoise), followed by a gradual decreaseto the initial level. Compare pulse.

Ccaked: In penetrant testing, condition of

dry developer powder having asemisolid condition caused bymoisture or other liquidcontaminants.

calibration: (1) Ratio of the output from adevice to a reference input. Knowledgeof this ratio helps to infer a device’sinput from its output. (2) Statement ofthe scale of a device. Compareverification. (3) Adjustment ofinstrument readings to known referencestandard.

calibration reflector: In ultrasonictesting, reflector with a knowndimensioned surface in a specifiedmaterial, established to provide anaccurately reproducible reference levelin ultrasonic testing,. See also flatbottom hole; reference standard; workingstandard.

candela (cd): Base SI unit of luminousintensity, in a given direction, of amonochromatic radiation source thathas a frequency of 5.4 × 1014 Hz andthat has a radiant intensity in thatdirection of 1.464 mW·sr–1.

capacitance, thermal: Amount of heatthat an object can store. The termthermal capacitance describes heatcapacity in an electrical analogy,where loss of heat is analogous to lossof charge on a capacitor. Structureswith high thermal capacitance changetemperature more slowly than thosewith low thermal capacitance.Compare capacity, heat.

capacitor discharge technique: Inmagnetic particle testing,magnetization technique generallycharacterized by a short duration, highintensity electrical pulse, oftenperformed on oil country tubular goods.

capacity, heat: Ability of a material orstructure to store heat. The product ofthe specific heat and the density ofthe material. This means that densermaterials generally will have higherheat capacities than porous materials.Heat capacity is the amount of energy(J·m–3·K–1) required to elevate by onedegree a given volume of material.Among common materials, water hasone of the highest heat capacities; air,one of the lowest. Comparecapacitance, thermal; conductivity,thermal.

capillary action: Tendency of liquids topenetrate or migrate into smallopenings, such as cracks, pits orfissures. The positive force that causesmovement of certain liquids alongnarrow or tight passages.



carrier fluid: (1) Liquid that acts as atransport mechanism for the activematerials. In magnetic particle testing,for example, the fluid may be oil orwater based. See also centrifuge tube;conditioning agent. (2) In liquidpenetrant testing and leak testing,fluid in which fluorescent and visibledyes or particles are dissolved orsuspended.

case crushing: Longitudinal gouges andfracture of a case hardened surface,such as the tooth of a gear.

casing: In the drilling industry, manysections of pipe that line the holeduring and after drilling of a water, gasor oil well. See also oil country tubulargoods.

casing string: In the drilling industry,tubular structure on the outerperimeter of a water, gas or oil wellhole. The casing string is a permanentpart of the well, and many casingstrings are cemented into theformation. See also oil country tubulargoods; tubing string.

casette, film: Often spelled cassette. Inradiographic testing, lightproofcontainer for holding radiographicfilm in position during theradiographic exposure. The casettemay be rigid or flexible and maycontain intensifying screens, filterscreens, both or neither.14

casting: In manufacturing, an objectproduced through the solidification ofa material within a mold.

casting, die: (1) Casting made in areusable metallic cavity. (2) Castingprocess where molten metal is forcedunder high pressure into the cavity ofa metal mold. See also parting line.

casting, investment: (1) Casting metalinto a mold produced by surrounding(investing) an expendable pattern witha refractory slurry that sets at roomtemperature after which the wax,plastic or frozen mercury pattern isremoved. Also called precision castingor lost wax process. (2) A casting madeby the process.

cathode: (1) Negatively charged terminalin an arrangement that producescurrent by chemical reactions.Compare anode. (2) In radiography,the negative electrode of an X-raytube, the electrode from whichelectrons are emitted.

cathode ray: Stream of electrons emittedby a heated filament and projected ina more or less confined beam underthe influence of a magnetic or electricfield.15

cavitation errosion: Loss of material dueto the repeated formation and collapseof bubbles at the surface of an objectin contact with a rapidly flowingliquid.

CCD: See charge coupled device.celsius (centigrade): Temperature scale

based on 273 K (0 °C = +32 °F) as thefreezing point of water and 373 K(100 °C = 212 °F) as the boiling pointof water at standard atmosphericpressure. A relative scale related to thekelvin scale (0 °C = 273.12 K;1 °C = 1 K).

cementite: A hard brittle compound ofiron and carbon known chemically asiron carbide (Fe3C) and found in steelsand cast irons.

central conductor: See internal conductor.centrifuge tube: In magnetic particle

testing, vial that holds liquids and hasgraduations to indicate theconcentration of solids that settle outof a known suspension volume.

certification: With respect tonondestructive test personnel, theprocess of providing written testimonythat an individual has met thequalification requirements of a specificpractice or standard. See alsoqualification.

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

cesium-137: Radioactive isotope ofelement cesium, having a half life of30 years and photon energy of about660 keV.

CGS system: Obsolete system ofmeasurement units based on thecentimeter, gram and second.Compare SI.

channel: In biology, mechanismfunctioning as a band pass filter in thevisual cortex of mammals, causingsensitivity to visual stimuli inparticular frequencies and range. Seealso vision.

chaplet: In manufacturing, metal supportused to hold a core in place on a mold.

charge coupled device (CCD): Solid stateimage sensor. Charge coupled devicesare widely used in inspection systemsbecause of their accuracy, high speedscanning and long service life.

chatter: (1) In machining or grinding,vibration of tool, wheel or workpieceproducing a wavy surface on the work.Chatter marks on the surface finish areproduced by a vibrating machiningtool

check cracking: Clustered small surfacecracks often caused by overheating orthermal cycling. See also grinding crack.

chill: (1) Metal insert embedded in thesurface of a sand mold or core orplaced in a mold cavity to increase thecooling rate at that point. (2) Hardshell of an iron casting formed byrapid cooling and/or careful control ofalloy chemistry. Chill depth may beevaluated using a wedge test.



choked flow: In leak testing,phenomenon where, while pressuredownstream is gradually lowered,velocity through an orifice increasesuntil it reaches the speed of sound inthe fluid. Also known as sonic flow.Compare transition flow.

circular magnetic field: In magneticparticle testing, active or residualmagnetization oriented along thecircumference.

circular magnetization: Result of currentflow technique or internal conductortechnique where a circular magnetic fieldis imparted. See also field flowtechnique; right hand rule. Comparelongitudinal magnetization.

circumferential: Direction around theperimeter of a cylindrical surface.Compare longitudinal; radial; transverse.

circumferential coil: See coil, encircling.clean: Free from interfering solid or liquidcontaminants on the test surface andwithin voids or discontinuities. Seealso water break free.

cleaning, chemical: Use of detergents,solvents or vapors at carefullycontrolled temperatures,concentrations, pH and contact timesto remove contaminants from thesurface and within discontinuities of acomponent.

cleaning, mechanical: Method ofremoving contaminants or materialfrom a surface, through an acceleratedstream of media. Media include glassbeads, plastic particles, metallic shotor wire, natural products and dry ice.Pressurized air, liquid or a rotatingwheel may propel the media stream.Acid etching is required if liquidpenetrant testing will be performedafter abrasive blasting. Comparepeening.

cleanup time; cleanup: In leak testing,time (time constant) required after atracer gas has ceased to enter a leak testsystem, for the system to reduce itssignal output to 37 percent of thesignal indicated before the tracer gashad ceased to enter the leak testingsystem.

closing: In image processing, dilationfollowed by erosion. A single pixel byclosing connects a broken featureseparated by one pixel. See alsoopening.

closure: Process by which a personcognitively completes patterns orshapes that are incompletelyperceived.

cobalt-60: Radioactive isotope of elementcobalt, having half life of 5.3 yearsand photon energies of 1.17 and1.33 MeV.

cocoa: Debris (usually oxides of thecontacting metals) of fretting wear,retained at or near the site of itsformation — a condition easilyidentified during visual tests. Withferrous metals, the debris is brown, redor black, depending on the type ofiron oxide formed. For this reason,ferrous debris is called cocoa or, whenmixed with oil or grease, red mud.

code: Standard enacted or enforced as alaw. Compare recommended practice;standard.

coefficients of the filter: Values in a maskthat serves as a filter in imageprocessing.

coefficient of thermal expansion (cte):Rate of expansion or contraction perunit length, volume or area per degreeof temperature change betweenspecified lower and upper temperaturelimits.

coercive force: Reverse external magneticfield intensity required to reduce thetest object’s bulk magnetism to zero.See also hysteresis loop.

coil: One or more loops of a conductingmaterial. A single coil may be anexciter and induce currents in thematerial, or a detector, or bothsimultaneously.

coil, absolute: In electromagnetic testing,a coil that responds to theelectromagnetic properties of thatregion of the test object within themagnetic field of the coil, withoutcomparison to the response of asecond coil at a different location onthe same or similar material. Comparecoil, comparator; coil, differential.

coil, bobbin: In electromagnetic testing,cylindrically wound absolute ordifferential probe useful for inspectingthe inside diameter of tubularproducts.

coil clearance, annular: Inelectromagnetic testing, mean radialdistance between the inner diameterof an encircling coil assembly and testobject surface. See also fill factor.

coil clearance, probe: In electromagnetictesting, perpendicular distancebetween adjacent surfaces of the probeand test part. See liftoff.23

coil, comparator: 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 coil,differential.23



coil, demagnetizing: In magnetic particletesting, solenoid or coil carrying thecurrent for demagnetization. Currentwaveform may be alternating forpass-through solenoids or a rectifiedcurrent for a multiple-step downcycledemagnetization. Some residualmagnetization may remain in largeparts magnetized with direct currentor rectified current, but subsequentlydemagnetized with alternatingcurrent. See also direct current downcycledemagnetization.

coil, differential: In electromagnetictesting, 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.

coil, encircling: In electromagnetictesting, a solenoid or coil assemblythat surrounds the test object. Such acoil is also called an annular coil,circumferential coil or feed-through coil.23

See also coil technique.coil, excitation: In electromagnetic

testing, coil that carries the excitationcurrent. Also called primary coil orwinding. Compare coil, sensing.

coil, horseshoe: In electromagnetictesting, probe coil in which the ferritecore of the coil is horseshoe shaped.Also called a U shaped coil.

coil, inside diameter: In electromagnetictesting, coil or coil assembly used forelectromagnetic testing by insertioninto the test piece, as with an insideprobe for tubing. See also coil,bobbin.23

coil, pancake: In electromagnetic testing,probe coil whose axis is normal to thesurface of the test material and whoselength is not larger than the radius.

coil, reference: In electromagnetictesting, the section of the coilassembly that excites or detects theelectromagnetic field in the referencestandard of a comparative system.23

coil, search: In electromagnetic testing,detection coil, usually smaller than theexcitation coil.

coil, sensing: In electromagnetic testing,coil that detects changes in the flow ofeddy currents induced by anexcitation coil; sensing and excitationcoils can be one and the same. Alsocalled detector coil. Compare coil,excitation.

coil shot: In magnetic particle testing,one instance of the coil technique, orone pulse of current in the coiltechnique. See also shot.

coil spacing: In electromagnetic testing,the axial distance between twoencircling or inside coils of adifferential or remote field testsystem.23

coil technique: In magnetic particletesting, field flow magnetizationtechnique using an encircling currentcarrying solenoid that imparts alongitudinal magnetic field inferromagnetic components with alength-to-diameter ratio greater than3. See also end effect; L·D–1 ratio;self-demagnetizing factor.

coil, test: In electromagnetic testing,section of a coil assembly that excitesor detects the magnetic field in thematerial under electromagnetic test.23

cold shut: (1) Casting discontinuitycaused by two streams of semimoltenmetal coming together within a moldbut failing to fuse. (2) A cracklikediscontinuity caused by forging, wheretwo surfaces of metal fold against eachother without joining. See lap.(3) Freezing of the top surface of aningot before the mold is full.

cold trap: Device that condenses vaporsand prevents oil or water moleculesfrom entering a vacuum chamber.

cold working: Permanent deformationproduced by an external force in ametal below its recrystallizationtemperature. Compare hot working.

collimator: In radiographic testing, devicefor restricting the size, shape anddirection of the irradiating beam,thereby limiting beam spread and itsconsequences.

cold light: Disused word for fluorescence.color: Visual sensation by means of which

humans distinguish light of differinghue (predominant wavelengths),saturation (degree to which thoseradiations predominate over others)and lightness. See also vision.

color blindness: Deficiency in ability toperceive or distinguish hues.

color discrimination: Perception ofdifferences between two or more hues.

color temperature: Rating of a lightsource, in degrees kelvin, for colorvision.

comparative measurement: Inelectromagnetic testing, ameasurement based on the imbalancein a system and using comparator coilsin contrast to differential and absolutemeasurements. See also coil,comparator. Compare absolutemeasurement; relative measurement.23



comparator, penetrant: (1) Transparentplastic device containing circles and/orlines of known lengths used toevaluate indication dimensions.(2) Aluminum test block with artificialcracks or special surface conditions,typically having two separate butadjacent areas for application ofdifferent liquid penetrants orprocessing materials or operations sothat direct visual comparison can bemade between different liquidpenetrant processes or materials. Thisblock is 50 mm wide by 76 mm longby 10 mm thick and is divided intohalves by a machined groove. Alsocalled aluminum test block and ASMEpenetrant cracked test piece.

compensator: Electrical matchingnetwork to compensate for electricalimpedance differences.15

complex plane: Plane defined by twoperpendicular reference axes, used forplotting a complex variable (such asimpedance) or functions of thisvariable (such as a transfer function).16

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

compton scatter: Reduction of energy ofincident photon by its interactionwith an electron. Part of the photonenergy is transferred to the electron,giving it kinetic energy, and theremaining photon is redirected withreduced energy. Compare diffraction.

computed tomography technique (CT):In radiographic testing, inspectionprocedure by which computer analysisof the radiation passing through arotating object is used to constructvirtual two-dimensional slices orthree-dimensional representations of acomponent. Three-dimensional datauses volumetric picture elements(voxel) rather than theirtwo-dimensional counterpart (pixel).

conditioning agent: In magnetic particletesting, additive to water based carrierfluid aiding with defoaming, surfacewetting, particle dispersion, corrosivitypH or or antifungal properties.

conductance: (1) In leak testing, the flowcharacteristics of a tube, manifold orleak path expressed in m3·s–1.(2) Transmission of electric currentthrough material. Conductance (G) ismeasured in siemens (S). Inverselyrelated to electrical resistance (R):

conduction: Heat transfer occurring whenmore energetic particles collide with— and thus impart some of their heatenergy to — adjacent less energetic(slower moving) particles. This actionis passed on from one atom (or freeelectron) to the next in the directionof cooler regions. Thus, heat alwaysflows from a warmer to a coolerregion. Compare convection; radiation.

conductivity, electrical (σ): Ability ofmaterial to transmit electric current,measured in siemens per meter.Reciprocal or inverse of resistivity ρ:

conductivity, thermal (k): Materialproperty defining the relative abilityto carry heat by conduction in a statictemperature gradient. Conductivityvaries slightly with temperature insolids and liquids and withtemperature and pressure in gases. It ishigh for metals (copper has a k of380 W·m–1·K–1) and low for gases andporous materials (concrete has a k of1.0 W·m–1·K–1). Compare capacity,thermal.

cone: Part of the eye; color sensitivephotoreceptor at the fovea centralis.Cones assist with mesopic vision andare responsible for photopic vision.Compare rod.

confidence level: Level of assurance fordetecting a specified discontinuity sizewith a specified probability. See alsoprobability of detection.

contact pad, contact head: In magneticparticle testing, replaceable metal pad,usually made of braided copper and/orlead, that prevents electrical arcingbetween the headstock and test objectin the current flow technique.

contact technique: (1) In ultrasonictesting, technique in which anultrasonic transducer face makes directcontact with the test object through athin film of couplant. Compareimmersion technique.15 (2) In magneticparticle inspection, see current flowtechnique.

contact time: See dwell time.contaminant: Any foreign substance on

the surface of a part, in a discontinuityor in the inspection materials, thatadversely affects an inspection.

continuous casting: Manufacturingmethod in which an ingot, billet, tubeor other shape is continuouslysolidified while being poured so thatits length is not determined by molddimensions.

GR

=1

σρ

=1



continuous technique: In magneticparticle testing, a test sequence wherethe particles flow over the test objectonly during active magnetization.Compare residual technique.

contour probe: In magnetic particletesting, electromagnetic yoke witharticulated pole pieces for evaluatingspecimens with an irregular shape. Seealso lifting power; field flowmagnetization.

contracted sweep: In ultrasonic testing,misnomer that refers to extending theduration of the ultrasonic sweep topermit viewing discontinuities or backreflections from deeper in the testobject. The sweep appears to becompressed.

contrast: (1) Difference in color orbrightness between a test indicationand background. (2) Differencebetween the amount of light reflectedor transmitted by an object and by thebackground in the field of view.

control: See in control; process control;quality control.

control cable: In radiographic testing,cable connected to isotopicradiographic source and used to movethe source in and out of the exposuredevice. See also guide tube; pigtail; pill.

convection: Type of heat transfer thattakes place in a moving medium andis almost always associated withtransfer between a solid (surface) and amoving fluid (such as air), wherebyenergy is transferred from highertemperature sites to lower temperaturesites. Compare conduction; radiation.

core: (1) In manufacturing, speciallyformed material inserted in a mold toshape the interior of another part of acasting that cannot be shaped as easilyby the pattern. (2) In a heat treatedferrous alloy, the inner portion that issofter than the outer portion or case.

corner effect: In ultrasonic testing, strongreflection obtained when an ultrasonicbeam is directed toward theintersection of two or threeintersecting surfaces.15

corrosion: Deterioration of a metal bychemical or electrochemical reactionwith its environment. Removal ofmaterial by chemical attack, such asthe rusting of automobilecomponents.

corrosion, crevice: Localized corrosionfound in regions where part orassembly geometry limits full exposureto the environment.

corrosion embrittlement: Severe loss ofductility of a metal, resulting fromcorrosive attack, usually intergranularand often not visually apparent.

corrosion erosion: Accelerated loss ofmaterial because of the simultaneousactions of erosion and corrosion whenexposed to a moving corrosive liquid.

corrosion, exfoliation: Corrosion thatprogresses parallel to the outer surfaceof the metal along grain boundariesoriented in the rolling directioncausing layers of the metal to beelevated by the formation of corrosionproduct. Usually associated withaluminum products.

corrosion fatigue: Fatigue crackingcaused by repeated load applicationson metal in a corrosive environment.

corrosion, fretting: Wear caused byrepeated small relative movementsbetween mating surfaces. Transferredmaterial and freshly exposed surfacesquickly corrode, often forming aproduct harder than the parentmaterial, thus increasing the wear rate.See also false brinelling.

corrosion, poultice: Corrosion occurringunder a layer of foreign material (forexample, under mud in automobilerocker panels).

cosine law: In nondestructive testing,physical law stating that theillumination of a surface varies as thecosine of the incidence angle.Maximum illumination is obtainedwhere the cosine equals one and whenthe source is perpendicular to thesurface.

coulomb (C): SI unit for electric charge,replaces faraday and ampere hour,where 1 A·h = 3600 C. X-ray orgamma ray intensity is measured incoulomb per kilogram (C·kg–1).

couplant: In ultrasonic testing and leaktesting, substance used between atransducer and the contacting surfaceto permit or improve transmission ofultrasonic energy into or from the testobject.15

coupled: (1) Of two electric circuits,having an impedance in common sothat a current in one causes a voltagein the other.23 (2) Of two coils,sharing parts of their magnetic fluxpaths. See also coupling.

coupling: In electromagnetic testing,percentage of magnetic flux from aprimary circuit that links a secondarycircuit; effectiveness of a coil ininducing eddy currents in the testobject.

coupling coefficient: In electromagnetictesting, fraction of magnetic flux fromone circuit (test coil) that threads asecond circuit (test object); the ratio ofimpedance of the coupling to thesquare root of the product of the totalimpedances of similar elements in thetwo meshes. See also fill factor effect.16



coupon: Piece of metal from which a testobject is prepared, often an extrapiece, as on a casting or forging.

crack: (1) Stress induced break, fissure orrupture, sometimes V shaped in crosssection and relatively narrow. Byconvention, a crack is called linear if itis at least three times longer than it iswide. (2) Propagating discontinuitycaused by fatigue, corrosion or stressessuch as heat treating or grinding. Maybe difficult to detect unaided becauseof fineness of line and pattern (mayhave a radial or latticed appearance).Compare fracture.

crack, cold: (1) Discontinuity that formsnear room temperature while a castingcools due to stresses caused duringnonuniform cooling. (2) Discontinuitythat may form in a weld either as itcools or later, if stress, hydrogencontamination and microstructuralconditions allow.

crack, crater: Multisegment crack in aweld crater. Segments radiate from acommon point, often called starcracks.

crack, fatigue: See fatigue.crack, forging: Stress induced

discontinuity formed duringmechanical shaping of metal; seecrack; discontinuity, primary processing.

crack, grinding: Shallow discontinuityformed in the surface of relativelyhard materials because of excessivegrinding heat or the brittleness of thematerial. Grinding cracks typically areoriented perpendicular (90 degreerotation) to the direction of thegrinding wheel. See also grinding burn.

crack, hot: (1) Discontinuity formed in aweldment caused by the segregation atgrain boundaries of low meltingconstituents in the weld puddle.(2) Postsolidification castingdiscontinuity caused by internalstresses.

crater: (1) In machining, depression incutting tool face eroded by chipcontact. (2) In arc or gas fusionwelding, cavity in the weld beadsurface, typically occurring when heatsource is removed and insufficientfiller metal is available to fill thecavity. See also crack, crater.

creep: Gradual and permanent change indimensions of a solid body subjectedto constant load at a stress level belowthe material’s yield strength. Creepoften occurs at elevated temperature.See also deformation.

crevice corrosion: See corrosion, crevice.

critical angle: (1) In ultrasonic testing,incident angle of the ultrasound beamwhere the refracted beam is parallel tothe surface and above which a specificmode of refracted energy no longerexists. See also mode conversion.18 (2) Invisual testing, incident angle abovewhich total internal reflection occurs.Critical angle is key to the operationof fiber optics.

cross talk: Unwanted signal leakage(acoustic or electrical) across anintended barrier, such as leakagebetween the transmitting andreceiving elements of a dualtransducer.15 Also called cross noise andcross coupling.

crush: Casting discontinuity caused by apartial destruction of the mold beforethe metal was poured.

crystal: See transducer element.crystal, X cut: In ultrasonic testing, cut

with face perpendicular to the Xdirection of the piezoelectric crystal.In a quartz slice so cut, a thicknessmode of vibration occurs when theslice is electrically stimulated in the Xdirection. See also transducer element.15

crystal, Y cut: In ultrasonic testing,piezoelectric crystal whose cut face isperpendicular to the Y direction. Inquartz, a transverse mode of vibrationis obtained when the slice iselectrically stimulated in the Ydirection. See also transducer element.15

crystal mosaic: In ultrasonic testing,multiple crystals mounted in the samesurface on one holder and connectedso as to cause all to vibrate as oneunit. See also transducer element.15

C-scan: In ultrasonic testing, presentationtechnique applied to acoustic data anddisplaying an image of two-dimensional test object with scaledgrays or colors representing theultrasonic signals. The amplituderepresented in each pixel may be apulse echo, through-transmission or pitchcatch value calculated from eachA-scan datum.

cumulative characteristic distribution:In acoustic emission signal processing,a display of the number of times acharacteristic meets a preselectedcriterion.

curie (Ci): Disused unit for measurementof the quantity of radioactivity,corresponding originally to radiationfrom atomic disintegrations from 1 gof radium; replaced by Becquerel (Bq)in SI, where 1 Ci = 3.7 × 1010 Bq.

curie point (Tc): Temperature at which aphase transformation causesferromagnetic materials to lose theirmagnetic properties.



current flow magnetization: In magneticparticle testing, magnetization of a testobject by passing electric powerdirectly through the test object.Common current flow techniquesinclude prods and the head shottechnique. Compare field flowmagnetization.

current induction technique: See inducedcurrent magnetization.

cutoff frequency: In ultrasonic testing,upper or lower spectral response of afilter or amplifier, at which theultrasonic response is a specifiedamount less (usually 3 or 6 dB) thanthe maximum response.

cycle: (1) Interval of time during which aprocedure occurs. For example, ademagnetization cycle. (2) A singlecomplete period of a waveform orother variable.

DD* (detectivity star): In infrared and

thermal testing, sensitivity figure ofmerit of an infrared detector.Detectivity is expressed inversely sothat higher D*s indicate betterperformance. D* is taken at specifictest conditions of chopping frequencyand information bandwidth anddisplayed as a function of spectralwavelength. D* is the detectivityscaled to the unit sensitive detectionarea, with detectivity corresponding tothe inverse of the noise equivalentflow.

damping: (1) Limiting the duration, ordecreasing the amplitude ofvibrations, by introducing anabsorbent material or throughinstrument or transducer control. (2)Deliberate introduction of energyabsorbers to reduce vibrations.

damping capacity: Measure of the abilityof a material to dissipate mechanicalenergy.20

damping material: Highly absorbentmaterial used to cause rapid decay ofvibration.

dark adaptation: Process by which theeye becomes accustomed to lowluminance levels. For example, anadjustment to less than approximately0.034 cd·m–2 for scotopic vision.19 Indark adaptation, the pupils dilate andthe two types of photoreceptors in theretina change chemical balance. Aftera finite amount of time, possibly10 min, vision will change fromphotopic vision to mesopic or scotopiclow illumination vision.

dark adapted vision: See accommodation;mesopic vision; scotopic vision.

dead zone: In ultrasonic testing, intervalfollowing the initial pulse at thesurface of a test object to the nearestinspectable depth.18 Any intervalfollowing a reflected signal whereadditional signals cannot be detected.

decay curve: In radiographic testing,graph showing activity as a function oftime for an isotope. Decay curves areused in determining exposure times.

decibel (dB): Logarithmic unit forexpressing relative acoustic signalpower, such as the loudness of asound, in proportion to the intensityof a reference signal. Decibel in signalamplitude is twice that in signalpower. Twenty decibels iscommensurate with ten-fold voltageamplification.

defect: Discontinuity whose size, shape,orientation or location (1) makes itdetrimental to the useful service of itshost object or (2) exceeds anaccept/reject criterion of an applicablespecification. Some discontinuities donot exceed an accept/reject criterionand are therefore not defects.Compare anomaly; crack; discontinuity;indication. See also flaw.

deformation: Change in dimensions, dueto stress or strain. Deformation may becompletely reversible (elasticdeformation) or permanent (plasticdeformation). See also creep.

degreasing fluid: Chemical cleaningagents used to remove contaminantsfrom test surfaces before inspection.See also cleaning, chemical.

delay line: In ultrasonic testing, material(liquid or solid) placed in front of atransducer to cause a time delaybetween the initial pulse and the frontsurface reflection.15

delta effect: In ultrasonic testing,reradiation or diffraction of energyfrom a discontinuity.15 The reradiatedenergy may include waves of both theincident mode and converted modes(longitudinal and transverse).

delta (t): In acoustic emission testing,time interval between the detectedarrival of an acoustic emission wave attwo sensors. Also called time differentialor difference in time of arrival.

demagnetization cycle: Reduction ofresidual magnetism to an acceptablelevel, generally less than 0.2 to 1.0 mT(2 to 10 G). See also coil, demagnetizingand direct current downcycledemagnetization.

demagnetizing factor: See self-demagnetizing factor.



demodulation: In electromagnetictesting, process wherein a carrierfrequency modulated with a signal oflower frequency than the carrierfrequency is converted to a closerepresentation of the originalmodulating signal.24

density: (1) In radiographic testing,degree of X-ray film darkness as aresult of exposure as measured with adensitometer. (2) Material property ofmass per unit volume.

depth compensation: See distanceamplitude correction.

depth of field: See depth of focus.depth of focus: The region in front of

and behind the focused distancewithin which objects still may beresolved. In a fixed focus system, thisparameter is sometimes called depth offield. See also transducer, focused.

depth of penetration: See skin effect;effective depth of penetration; standarddepth of penetration.

descaling: Removal of a thick layer ofhigh temperature oxides from ametallic surface through mechanicalcleaning or chemical cleaning means.

detectivity star: See D*.detector coil: See coil, sensing.detector probe: In leak testing, an

adjustable or fixed device throughwhich air and/or tracer gas is drawninto the leak test instrument and overthe sensing element or detector. Alsocalled a sampling probe or a sniffer probe.

detector probe test: In leak testing,pressure leak test in which the leakageof a component, pressurized with atracer rich mixture, is detected byscanning the test object boundarysurface with a detector probeconnected to an electronic leakdetector. Leakage tracer gas is pulledfrom the leak through the probe inletto the sensing element to cause avisible or audible signal on theindicator of the leak test instrument.

detergent remover: See emulsifier,hydrophilic.

developer: (1) In liquid penetrant testing,a material that is applied to thesurface after excess liquid penetranthas been removed and that is designedto enhance the liquid penetrantbleedout to form indications. May be afine dry powder, a solution that driesto form a dry powder or a suspension(in solvent or water) that dries leavingan absorptive film on the test surface.(2) In radiography, a chemical solutionthat reduces exposed silver halidecrystals to metallic silver.14

developer, dry: In liquid penetranttesting, a finely divided dry powderthat is applied to the surface afterexcess liquid penetrant is removed andthe surface is dried in order to increasethe bleedout by capillary action.

developer, nonaqueous wet (NAWD): Inliquid penetrant testing, fine particlessuspended in a volatile solvent. Thevolatile solvent assists bleedout bydiluting the penetrant. Sometimescalled solvent developer.

developer, soluble: In liquid penetranttesting, fine particles completelysoluble in its carrier that dries to forman adsorptive coating.

developer, suspendible: In liquidpenetrant testing, the developer thatconsists of fine particles suspended inwater and that dries to an absorptivecoating. Applied to the part afterremoval of excess liquid penetrant andbefore drying.

developing time: In liquid penetranttesting, the elapsed time necessary forthe developer to absorb and showindications from penetrantentrapments.

dewetting: Flow and retraction of liquidon a surface, caused by contaminatedsurfaces or dissolved surface coatings.Compare water break free.

diamagnetic material: Substance with amagnetic permeability less than 1 thatweakly repels an external magneticfield. Compare ferromagnetic material;paramagnetic material.

differential amplifier: Amplifier whoseoutput signal is proportional to themathematical difference between twoinput signals.16

differential measurement: Inelectromagnetic testing, themeasurement of system imbalance byusing differential coils, in contrast toabsolute and comparativemeasurements.23

differentiated signal: An output signalproportional to the input signal’s rateof change.23

diffraction: (1) In radiographic testing,special case of scatter, wherecoherently scattered (full intensity isretained) photons undergointerference or reinforcement,resulting in patterns indicative of thescattering medium. See also X-raydiffraction. (2) In ultrasonic testing,deflection of a wavefront whenpassing the edge of an ultrasonicallyopaque object.15

diffuse indication: In magnetic particletesting, particle cluster not clearlydefined — for example, an indicationfrom a subsurface discontinuity.

diffuse reflection: Scattered, incoherentreflections from rough surfaces. Seealso lambertian; matte.18



diffusion: Process by which moleculesintermingle as a result of concentrationgradients or thermal motion. Spreadingof a gas through other gases or solidswithin a volume.

diffusion, thermal: Process by whichthermal energy is transferred from hotor cold regions and finally is spreadout. See also conduction and thermaldiffusivity.

diffusivity, thermal (α): Speed at whichheat diffuses through an object.Expressed as the rate α of temperaturechange with time. Each material hasits own characteristic value ofdiffusivity, combining the overallinfluence of thermal conductivity k,density ρ and specific heat Cp:

In a practical sense, thermal diffusivitydetermines how fast a material willheat up or cool down. The rate oftemperature change with time is morerapid in a material with a highthermal diffusivity (for example,metals) and slower in a material with alower diffusivity (for example,plastics).

dilation: In image processing, thecondition of a binary image where thepixel in the output image is a 1 if anyof its eight closest neighbors is a 1 inthe input image. See also closing;erosion; and opening.

diopter: In optics, term used to identifythe refractive (light bending) capacityor resolving power of a lens. Equal tothe inverse of the length (in meters) ofthe optical axis.

direct current (DC): Electricity that flowscontinuously in one direction througha conductor. The only true source ofdirect current is a battery, althoughsome rectified power waveforms mayresemble direct current. See also full-wave current and half-wave current.Compare alternating current.

direct current downcycledemagnetization: In magneticparticle testing, massaging themagnetism of a component down toan acceptable level through a 25-stepto 30-step process, where thepersistence of one polarity is overcomein decreasing steps by a field reversingat each step.

directional lighting: Lighting providedon the work plane or objectpredominantly from a preferreddirection.19

direction of view: See angle of view.direct photometry: Simultaneous

comparison of a standard lamp and anunknown light source.19

direct viewing: (1) Viewing of a testobject in the viewer’s immediatepresence. The term direct viewing isused in the fields of robotics andsurveillance to distinguishconventional from remote viewing.(2) Viewing of a test object duringwhich the light image is not mediatedthrough a system of two or morelenses (as in a borescope) ortransduced through an electronicsignal (as with a charge coupledcamera). The term direct viewing is usedin some specifications to meanviewing possibly with a mirror ormagnifier but not with a borescope.Compare indirect viewing; remoteviewing.

direct vision instrument: Device offeringa view directly forward. A typicalscene is about 20 mm (0.75 in.) wideat 25 mm (1 in.) from the objectivelens. See also borescope.

discontinuity: Interruption in thephysical structure or configuration of atest object. After nondestructivetesting, a discontinuity indication maybe interpreted as a defect.25 Compareanomaly; defect; indication.

discontinuity, artificial: Referenceanomaly such as hole, indentation,crack, groove or notch introduced intoa reference standard to provideaccurately reproducible indications fordetermining test sensitivity levels.

discontinuity, inherent: Materialanomaly originating fromsolidification of metal. Pipe, bandingand nonmetallic inclusions are themost common inherentdiscontinuities and can lead to othertypes of discontinuities infabrication.22

discontinuity inversion: Inelectromagnetic testing, technique formeasuring some dimension(s) of adiscontinuity by the application of amathematical algorithm to themeasured test data.

discontinuity, primary processing:Discontinuity produced from the hotor cold working of an ingot intoforgings, rods, bars and other shapes.22

discontinuity, secondary processing:Discontinuity produced duringmachining, grinding, heat treating,plating or other finishing operations.22

discontinuity, service induced:Discontinuity caused by the intendeduse of the part.

dispersion: In acoustics, variation of wavephase with frequency. See alsorefraction.

dispersive medium: Medium in whichthe propagation velocity depends onthe wave frequency.

αρ

== kCp



dissipation: In infrared and thermaltesting, generation of heat by plasticdeformation.

distal tip: In a manipulative orinterrogating system, of or pertainingto the end opposite from the eyepieceand farthest from the person using thesystem. The steel end portion of aborescope insertion tube where theimage bundle, light guides andchannel (if applicable) terminate. Itoften has three or four holes: one forthe image bundle, two for the lightguides and possibly a fourth hole as aworking channel. See also objective.

distance amplitude correction (DAC):Compensation of gain as a function oftime for difference in amplitude ofreflections from equal reflectors atdifferent sound travel distances. Refersalso to compensation by electronicmeans such as swept gain, timecorrected gain, time variable gain andsensitivity time control.15

divergence: In ultrasonic testing, termsometimes used to describe thespreading of ultrasonic waves beyondthe near field. It is a function oftransducer diameter and wavelengthin the medium. See beam spread.

domain: Macroscopic dipole substructurewithin a ferromagnetic materialpermanently magnetically saturated.Domains are randomly oriented in ademagnetized material, but theirorientation may be preferentiallyrotated through the application of anexternal magnetic field.

dose rate: In radiographic testing, ionizingradiation delivered per a specified unitof time and measured, for instance, insievert per minute (or in rem perhour). See also absorbed dose.14

dosimeter: Device that measures ionizingradiation dose, such as a film badge orionization chamber.14

downcycle: See direct current downcycledemagnetization.

dragout: In liquid penetrant testing,carryout or loss of liquid penetrantmaterials as a result of their adherenceto objects dipped into the materials.

drain time: In liquid penetrant testing,portion of dwell time during whichthe excess liquid penetrant, emulsifier,detergent remover or developer drainsoff an object.

drying oven: In liquid penetrant testing,oven used for increasing theevaporation rate of rinse water or ofan aqueous developer vehicle fromparts.

dry powder: (1) For magnetic particletesting, see magnetic particle, dry.(2) For liquid penetrant testing, seedeveloper, dry.

dry technique: Magnetic particle testtechnique, generally used withportable equipment, where theferromagnetic particles are applied aspowder.

ductile crack propagation: Slow crackpropagation that is accompanied bynoticeable plastic deformation andthat requires energy to be suppliedfrom outside the body. See also fatiguecrack propagation. Compare brittle crackpropagation.

ductility: Ability of a material to undergoplastic deformation without fracture.Compare brittleness.

dwell time: In liquid penetrant testing,the time when a penetrant oremulsifier is in contact with the testsurface. Compare soak time.

dynamic range: In ultrasonic testing,ratio of maximum to minimumreflective areas that can bedistinguished on the display at aconstant gain setting.1

Eecho: In ultrasonic testing, reflected

acoustic energy or signal indicatingsuch energy. See also pulse echotechnique.

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

eddy current testing (EC):Nondestructive test method in whicheddy current flow is induced in thetest object. Changes in the flow causedby variations in the specimen arereflected into a nearby coil, coils, halleffect device or other magnetic fluxsensor for subsequent analysis bysuitable instrumentation andtechniques. See also electromagnetictesting.23

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.23

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

effusivity, thermal: Ability of heat toescape from a body, expressed as acharacteristic of that body. Square rootof the product of thermalconductivity, mass density and specificheat.



elastic deformation: Temporary changein shape linearly proportional to theamount of applied force. Elasticallydeformed material returns to itsoriginal size and shape after the load isremoved. Elastic deformation is thestate in which most components areused in service. Compare creep;modulus of elasticity; plastic deformation.

elasticity: Ability of a material to regainits former shape after removal ofapplied stress.

electric field: Vector field of either theelectric field intensity (V·m–2) or of theelectric flux density (C·m–2).

electrical center: In electromagnetictesting, center established by theelectromagnetic 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.23

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

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.23

electronvolt (eV): Kinetic energy acquiredby an electron in passing through apotential difference of 1 V in vacuum;1 eV = ~1.60 J. The electronvolt iscommonly used to express the energyof gamma rays and X-rays.

electrostatic spraying: Technique ofapplying a uniform surface coating,wherein the material being sprayed isgiven a high electrical charge(potential) while the test piece isgrounded.

emissivity: Variable ratio of the totalenergy radiated by a given surface at agiven temperature to the total energyradiated by a blackbody at the sametemperature. Emissivity can be total,directional or hemispherical. Emissivityis a surface phenomenon dependingon surface condition and composition.Smooth materials have loweremissivities than matte or corrodedmaterials. Emissivity values rangebetween 0 for a perfect reflector to 1.0for a blackbody.

emissivity, effective (ε*): In infrared andthermal testing, the measuredemissivity value of a particular surfaceunder existing measurementconditions (rather than the generictabulated value for the surfacematerial) that can be used to correct aspecific measuring instrument toprovide a correct temperaturemeasurement.

emulsifier: In liquid penetrant testing,liquid that mixes with an oily liquidpenetrant such that the mixture canthen be washed from the surface withwater. See also soak time.

emulsifier, hydrophilic: In liquidpenetrant testing, water based liquidthat interacts with the liquidpenetrant oil in the manner of adetergent, allowing the liquidpenetrant to be washed from thesurface with water.

emulsifier, lipophilic: In liquid penetranttesting, oil based liquid that mixeswith liquid penetrant oil to form anemulsion that can be removed fromthe surface with water.

end effect: In bar and tube testing, edgeeffect. See also coil technique.

endoscope: See borescope.equivalent 20/20 near vision acuity:

Vision acuity with remote viewing orother indirect viewing thatapproximates 20/20 direct viewingclosely enough to be considered thesame for visual testing purposes.

erosion: (1) Loss of material ordegradation of surface quality throughfriction or abrasion from movingfluids, made worse by solid particles inthose fluids or by cavitation in themoving fluid. See wear. (2) In imageprocessing, condition of a binaryimage where the pixel in the outputimage becomes a 1 if each of its eightneighbors is a 1 in the input image.See also closing, dilation and opening.

erosion-corrosion: Simultaneousoccurrence of erosion and corrosionleading to an accelerated loss ofmaterial.

etch crack: Shallow crack in hardenedsteel containing high residual surfacestresses, produced in an embrittlingacid environment.22

etching: (1) In liquid penetrant testing,chemical cleaning process for thecontrolled removal of surface materialby chemical agents before inspection.(2) Subjecting the surface of a metal topreferential chemical or electrolyticattack to reveal structural detailsbefore viewing under a microscope.



evaluation: Process of determining themagnitude and significance of adiscontinuity after the indication hasbeen interpreted as relevant.Evaluation determines if the testobject should be rejected, repaired oraccepted. See also indication;interpretation.

expanded sweep: In ultrasonic testing, ashort duration horizontal sweeppositioned to allow close examinationof a signal.

exposure factor: In X-radiography, thequantity that combines sourcestrength (milliampere), time (usuallyminute) and distance. It is the productof milliamperage and time divided bydistance squared and determines thedegree of film density.

evaluation: Process of deciding theseverity of a condition after anindication has been interpreted, todetermine whether it meetsacceptance criteria.

eye sensitivity curve: Graphic expressionof vision sensitivity characteristics ofthe human eye to monochromaticlight wavelengths. In the case of aphysical photometer, the curve shouldbe equivalent to the standard observer.The required match is typicallyachieved by adding filters between thesensitive elements of the meter andthe light source. See photopic vision.

Ffalse brinelling: See wear, fretting.fahrenheit: Disused scale for temperature

(T) based on 32 °F as the freezingpoint of water and 212 °F as theboiling point of water at standardatmospheric pressure; a relative scalerelated to the rankine scale.0 °F = 459.67 °R; 1 °F ΔT = 1 °R ΔΤ .

far field: In ultrasonic testing, zonebeyond the near field in front of aplane transducer in which signalamplitude decreases monotonically inproportion to distance from thetransducer. Also called the fraunhoferzone. Compare near field.

false indication: See indication, false.farsightedness: Vision acuity functionally

adequate for viewing objects at adistance, generally farther than arm’slength. Also called hyperopia. Comparenearsightedness.

far vision: Vision of objects at a distance,generally beyond arm’s length.Compare near vision.

fatigue crack propagation: Progressivefracture of a material that begins at adiscontinuity and increases underrepeated cycles of stress. Thephenomenon leading to fractureunder repeated or fluctuating stresseshaving a maximum value less than thetensile strength of the material. Seealso ductile crack propagation. Comparebrittle crack propagation.

feature extraction: From an enhancedimage, derivation of some featurevalues, usually parameters fordistinguishing objects in the image.

felicity effect: In acoustic emissiontesting, appearance of significantacoustic emission at a load (orpressure) level below the previousmaximum applied.

felicity ratio: In acoustic emission testing,measurement of the felicity effect.Defined as the ratio between (1) theapplied load (or pressure) at whichacoustic emission reappears during thenext application of loading and (2) theprevious maximum applied load.

ferrite: Form of pure iron that has a bodycentered cubic structure stable below910 °C (1670 °F). Solid solution of oneor more other elements in alpha iron.Also called alpha iron. (2) Inelectromagnetic testing, any of severalmagnetic substances that consistessentially of an iron oxide combinedwith one or more metals (such asmanganese, nickel or zinc) havinghigh magnetic permeability and highelectrical resistivity.

ferromagnetic material: Material such asiron, nickel or cobalt whose relativepermeability is considerably greaterthan unity, depends on themagnetizing force and often exhibitshysteresis. Materials that are moststrongly affected by magnetism arecalled ferromagnetic. See also domain;curie temperature. Compare diamagneticmaterial; paramagnetic material.

fiber optics: Technology of efficienttransmission of light throughtransparent fibers such as glass, quartzand plastic by means of total internalreflection. Groups of fibers carryinglight to the distal end are in randomorder, while the image bundle carryingthe image back to the eyepiece iscoherent. See also borescope; criticalangle.

fiberscope: See borescope, fiber optic.



field flow magnetization: In magneticparticle testing, imparting a magneticfield within a component by using atleast a portion of the test object tocomplete the magnetic circuit. Fieldflow magnetization may impartlongitudinal, circular or toroidalmagntization depending upon thetools and test configuration. Commonfield flow magnetization tools includeelectromagnetic yokes or contour probes,ferromagnetic cores, rigid or flexibleencircling coils, iron core induction coilpole extenders, permanent magnetsand internal conductors. Comparecurrent flow magnetization.

fiberscope: Jargon for fiber opticborescope.

field of view: Range or area where thingscan be seen through an imagingsystem, lens or aperture. See also angleof field. Compare depth of field.

field of vision: Range or area wherethings can be perceived by eyesight ata point in time, assuming the eye tobe immobile.

filled crack: Cracklike discontinuity, opento the surface but filled with someforeign material, such as oxide orgrease, that tends to prevent liquidpenetrants from entering.

fill factor: (1) In magnetic particletesting, convenient quantity forcharacterizing how closely the outsidediameter of a specimen matches theinside diameter of the magnetizingcoil. With a high fill factor, the ratioof the cross sectional area of the coildivided by the cross sectional area ofthe specimen is less than 2;intermediate, 2 to 10; low, greaterthan 10. See also coil clearance, annular.(2) For encircling coil electromagnetictesting, the ratio of the cross sectionalarea of the test object to the effectivecross sectional core area of the primaryencircling coil (outside diameter ofcoil form, not inside diameter that isadjacent to the object).23,13 Forinternal probe electromagnetic testing,the ratio of the effective crosssectional area of the primary internalprobe coil to the cross sectional area ofthe tube interior.23

fill factor effect: In electromagnetictesting, effect of fill factor on couplingbetween coil and test object. Seecoupling coefficient.4

film badge: In radiographic testing,package of photographic film worn asa dosimeter badge by radiographicpersonnel and workers in the nuclearindustry to measure exposure toionizing radiation. Absorbed dose canbe calculated by the film densitycaused by irradiation.

film speed: In radiographic testing,relative exposure required to attain aspecified film density.14

filter: (1) Electrical circuit or physicaldevice that leaves a signal unaffectedover a prescribed range of frequenciesand attenuates signal components atall other frequencies. Common filtertypes include neutral density, lowpass, band pass and high pass. (2) Dataanalysis process for reducing data files.

filtering: (1) Network that passeselectromagnetic wave energy over adescribed range of frequencies andattenuates energy at all otherfrequencies.23 (2) Processing device orfunction that excludes a selected kindof signal or part of a signal. (3) Inradiography, the thickness ofabsorbing material placed in a primaryradiation beam to selectively removelonger wavelength radiation, therebyadjusting the quality of theradiographic image.

finite element analysis; finite elementmodeling (FEA; FEM): Numericalmodeling technique for the analysis ofa continuous system whereby thatsystem is decomposed into acollection of finite sized elements. Seealso model, analytical.

fit up: In manufacturing, to secure one ormore joint members into properposition with special external fixturingin order to prevent movement duringwelding.22

fixing: In radiographic testing, procedureused in radiographic film processingthat removes undeveloped silver saltsin the emulsion from the surface ofthe film, leaving only the developedblack silver of the image on the film.

flakes: Short discontinuous internalfissures in ferrous metals attributed tostresses produced by localizedtransformation and/or decreasedsolubility of hydrogen during coolingusually after hot working. Flakesappear as bright silvery areas (fish eyes)on an otherwise ductile fracturesurface; flakes appear as short,discontinuous cracks on a polishedand etch cross section.22

flammability: Tendency to combust,considered to be characteristic ofliquids having flash point below 60 °C(140 °F) and a vapor pressure notexceeding 275 kPa (40 lbf·in.–2) at37.8 °C (100 °F).

flash magnetization: See capacitordischarge technique.

flash point: Lowest temperature at whicha substance will form an ignitablemixture in air. The value varies withcircumstances.



flat bottom hole: In ultrasonic testing,type of reflector commonly used inreference standards. The end (bottom)surface of the hole is the reflector. Seealso calibration reflector.

flaw: Anomaly or unintendeddiscontinuity. See also defect. Comparediscontinuity.

flaw location scale: In ultrasonic testing,specially graduated ruler that can beattached to an angle beam transducerto relate the position of an indicationon the display to the actual location ofa discontinuity within the test object.

flexible laminated strip: In magneticparticle testing, a shared flux indicatorin the form of a thin ferromagneticshim containing a series oflongitudinal artificial discontinuities,which is used only to verify thedirection of magnetic induction.

fluorescence: Phenomenon of absorptionof electromagnetic radiation and itsreemission at a lower energy (longer)visible light wavelength. Fluorescencein NDT may be a material’s responseto ultraviolet or ionizing radiation.The emission ceases as soon as theexciting energy is removed. Differsfrom phosphorescence, whichcontinues to emit after excitationenergy is removed. See alsospectrofluorometer.

flux: See magnetic flux.focal plane array (FPA): Linear or

two-dimensional matrix of detectorelements, typically used at the focalplane of an instrument. Inthermography, rectangular focal planearrays are used in staring(nonscanning) infrared imagers.

focal spot: (1) Point at which theinstrument optics image the infrareddetector at the target plane. In aradiation thermometer, this point iswhere the spot size is the smallest. In ascanner or imager, this point is wherethe instantaneous field of view (IFOV)is smallest. (2) In radiographic testing,area on target that receivesbombardment of electrons. See alsoeffective focal spot.

focal zone: In infrared and thermaltesting, distance before and after thefocal point in which the intensitydiffers a specified amount (usually6 dB) from the focal intensity. Alsocalled depth of field or depth of focus.

focus: Position of a viewed object and alens system relative to one another tooffer a distinct image of the object asseen through the lens system. Seeaccommodation; depth of field.

focusing, automatic: (1) Feature of acamera whereby the lens systemadjusts to focus on an object in thefield of view. (2) Metaphoricalattribute of a borescopic instrument’sdepth of field (the range of distance infocus). The depth of field is so great inthe case of video borescopes thatfocusing is unnecessary for mostapplications. Despite the name, nomechanism is actively adjusted. Thedepth of field is large both because ofthe small diameter of the lens apertureand because of the proximity of thelens to the charge coupled device.

focusing, primary: Focusing by the lensof the image onto a fiber optic bundleat the tip of a probe.

focusing, secondary: Focusing at theeyepiece of a borescope or otheroptical instrument, specifically themanual refocusing needed when theviewing distance changes.

fog: Increase of film density caused bysources other than from the intendedprimary beam exposure. Heat,humidity, pressure and scatterradiation can all cause fogging of thefilm.

footcandle (ftc or fc): Disused unit ofilluminance, where1 ftc = 1 lm·ft–2 = 10.76 lx.

footlambert (ftl): Disused unit ofluminance, where 1 ftl = 3.426 cd·m–2.

fovea centralis: A small pit in the maculalutea that contains the largestconcentration of cone cells in the eyeand is responsible for central, highresolution vision. See also cone; maculalutea; photopic vision; rod.

foveal vision: See photopic vision.FPI: Fluorescent penetrant inspection. Seefluorescent penetrant testing.

fracture: Break, rupture or crack largeenough to cause a full or partialseparation.

fracture mechanics: Field of solidmechanics that deals with behavior ofcracked bodies subjected to stress andstrain.

frame: Complete raster scan or bitmappedimage projected on a video screen.There may be 24, 25 or 30 frames persecond, depending on the videostandard used. See also field.

fraunhofer zone: See far field.frequency (f, ν): Number of times per

second that a cyclical waveformrepeats. The unit of frequency is hertz(Hz).

frequency, fundamental: In resonancetesting, the frequency at which thewavelength is twice the thickness ofthe test material. See also harmonic.15

fretting wear: See wear, fretting.friction oxidation: See wear, fretting.fresnel field: See near field.



front surface echo: In ultrasonic testing,first surface of the test objectencountered by an ultrasonic beam.Compare back surface echo.

full-wave rectified alternating current(FWRAC): Single-phase or three-phasealternating current converted toproduce unidirectional current.Rectified current contains moreamplitude variation, or ripple, thandirect current from a battery.

furring: In magnetic particle testing,buildup of dry magnetic particles atmagnetic poles resulting fromovermagnetization of the test object.

Ggalling: Surface damage more severe than

fretting, caused by friction betweenhigh spots leading flaking due tosubsurface fatigue. See also galling.Compare wear, fretting.

galvanic series: List of metals, alloys andgraphite (a nonmetal) in sequencewith the most anodic (easily corroded)in liquids at one end of the list andthe most cathodic (least easilycorroded) at the other end. Forpractical reasons, this sequence iscompiled using seawater as theelectrolyte — 3 to 5 percent sodiumchloride and other salts dissolved inwater.

gamma iron: see austenite.gamma ray: High energy, short

wavelength electromagnetic radiationemitted by the nucleus of a radioactiveisotope. Energies of gamma rays areusually between 0.01 and 10 MeV. X-rays also occur in this energy rangebut are of nonnuclear origin. Comparealpha ray; beta ray; X-ray.14

gas ballast: Gas (air) admitted into thepumping chamber of a mechanicalpump and inhibiting condensation ofvapors in the chamber.

gasket seal: Resilient ring, usually virginpolytetrafluoroethylene (PTFE), in apiping or tubing connection. Compareinterference sealing thread.

gate: (1) Electronic device for selectingsignals in a segment of the trace on anA-scan display. (2) The intervalmonitored along the baseline.

gauss (G): Disused CGS unit of magneticflux density denoting one flux line ormaxwell, passing through one squarecentimeter. The preferred unit of fluxdensity is the tesla (T), where 1 T = 104

G.gauss meter: See tesla meter.

general examination: In personnelqualification, a test or examination ofa person’s knowledge, typically (in thecase of nondestructive testingpersonnel qualification) a written teston the basic principles of anondestructive test 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 particular application.)Compare practical examination andspecific examination.

geometrical optics: Mathematical studyof how light rays are reflected andrefracted and practical techniquesbased on such understanding,including the transmission of imagesby lenses and mirrors. Also called lensoptics.

geometric unsharpness: In radiographictesting, fuzziness or lack of definitionin a radiographic image resulting fromthe source size, object-to-film distanceand the source-to-object distance.14

getter: Reactive material that traps gasand removes it from a vacuumchamber. Several metals such astitanium, zirconium and tantalum canform getters for gases.

ghost: In ultrasonic testing, aliasingindication arising from certaincombinations of pulse repetitionfrequency and time base frequency.20

See also wrap around.glare: Excessive brightness (or brightness

varying by more than 10:1 within thefield of view that interferes withobservation or interpretation of a testresponse. Glare may be absolute orblinding (dazzle), disability ordiscomfort depending upon intensity.Often caused by reflection, whetherspecular (smooth surface) or diffuse(rough surface), of light or radiationsources.

gloss meter: Reflectometer used tomeasure specular reflectance.19

gnomon: Artifact intended to cast ashadow. The shadow may be used tomeasure time or distance (an examplewould be the indicator on a sundial).

gouge: Surface indentation caused byforceful abrasion, impact or flamecutting. Also called nick. Compare toolmark.

grain: Individual crystal in apolycrystalline material. See also grainboundary.

grain boundary: Interface that formsbetween grains of solidifying metal asthe random oriented crystal latticesmeet.



graininess: Film characteristic that resultsfrom improper film processing andthat consists of the grouping orclumping together of many smallsilver grains into masses visible to thenaked eye or with slightmagnification. Compare mottle.14

gray (Gy): SI unit for measurement of thedose of ionizing radiation absorbed perunit mass at a specified location.Replaces the rad where rad denotesradiation absorbed dose, not radian.1 Gy = 1 J·kg–1 = 100 rad.

gray body: In physics, a theoretical objectwhose spectral absortivity andemissivity are constant for allwavelengths. Compare blackbody.

gray level: Integer number representingthe brightness or darkness of a pixelor, as a composite value, of an imagecomprised of pixels.

green rot: Form of high temperatureattack on nickel chromium and nickelchromium iron alloys. Degradation isdue to precipitation and subsequentoxidation of chromium carbideparticles. Common to furnaceenvironments.

grinding burn: Surface anomaly causedby improper steel machiningparameters. Term describes the etchedappearance of localized regions ofuntempered and self-temperedmartensite caused by excessiveheating. See also crack, grinding; tarasovetching technique.

group velocity: Speed at which theenvelope of an ultrasonic pulse (manyfrequencies) propagates through themedium.

guide tube: Cable connected to isotopicradiographic source and used to movethe source in and out of the exposuredevice. See also control cable; pigtail;pill.

Hhalation: In radiographic testing,

spreading of light around a brightimage on a fluorescent screen ordeveloped film.

half-wave rectified alternating current:Power waveform rectified fromsingle-phase alternating current toproduce a pulsating unidirectionalfield.

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

hall effect detector: Semiconductorelement that produces an outputelectromotive force proportional tothe product of the magnetic fieldintensity and a biasing current. Suchsensors are available commercially inaxial and transverse form. See alsotesla meter.

halide: Compound of two or moreelements, one of which is a halogen.

halogen: Any of the nonmetallicelements — fluorine, chlorine, bromineand iodine — or any gaseous chemicalcomponent containing one or more ofthese elements.

hardness: In materials science, theresistance of a material todeformation, scratching, abrasion orcutting. See also brinell hardness;rockwell hardness.

harmonic: Vibration frequency that is anintegral multiple of the fundamentalfrequency. See also frequency,fundamental.18

heading: Upsetting wire, rod or bar stockin dies to form parts having some ofthe cross sectional area larger than theoriginal. Example products are bolts,rivets and screws.

headstock: In magnetic particle testing,one of two points on a wet horizontalunit, often equipped with a pneumaticram, which contacts and supports thetest object during current flowmagnetization in the head shottechnique.

head shot technique: In magneticparticle testing, imparting circularmagnetization in a component bypassing current directly through it. Seealso current flow magnetization;headstock; shot.

heat: Energy associated with the randomand chaotic motions of the atomicparticles from which matter iscomposed. All materials (hot or cold)contain heat and radiate infraredenergy. The unit for measuring heat isthe joule (J), equal to about0.24 calorie (cal) or 9.481 × 10–4

British thermal units (BTUs). Compareinfrared radiation; temperature.

heat affected zone (HAZ): Portion of basemetal not melted during brazing,cutting or welding but withmechanical properties altered by theheat.

heat checking: Surface cracking causedwhen metal rapidly heated (or cooledand heated repeatedly) is preventedfrom expanding freely by colder metalbelow the surface. Friction mayproduce the heat. See also grindingburn; crack, grinding.



heat treatment: Heating and cooling ametal or alloy in such a way as toobtain desired conditions orproperties. Heating for the solepurpose of hot working is excludedfrom the meaning of this definition.

hemispherical properties, radiation:Radiation properties (emissivity,absorptivity, reflectivity) as referencedto all directions of hemisphericalspace.

hermetic seal: Fusion seal that is leaktight.

hertz (Hz): Measurement unit offrequency, equivalent to one cycle persecond.

hit lockout time: In acoustic emissiontesting, time interval set to suppresslate arriving parts of an acousticemission signal.

horseshoe magnet: U shaped bar magnet.See also keeper.

hot tear: Crack formed in a cast metalduring solidification and due toexcessive tensile stress associated withhindered contraction duringvolumetric shrinkage. Hot tears oftenoccur where areas of differentthicknesses adjoin.

hot thermionic ionization gage:Absolute pressure gage that monitorsion current proportional to gas densityat pressures less than 0.1 Pa (1 mtorr).Electrons produced by a heatedfilament (usually of tungsten oriridium and often thorium coated)ionize the gas and produce a positiveion current that flows to a wirecollector. This current is proportionalto gas density over the absolutepressure range below 100 mPa(1 mtorr) for a given gas composition.

hot working: Deforming metal plasticallyat temperature and rate such thatstrain hardening does not occur. Lowtemperature limit is recrystallizationtemperature. Compare cold working.

hsu-nielsen source: See pencil break source.hue: Characteristic of light at a particular

bandwidth; the degree to which avisual stimulus can be described interms of primary colors (red, green,blue and yellow).

human factors: The mental and physicalmake of the individual, theindividual’s training and experienceand the conditions under which theindividual must operate that influencethe ability of the NDE system toachieve its intended purpose. Humanfactors is one of the principal elementsaffecting the reliability ofnondestructive tests.

hyperopia: See farsightedness.

hysteresis: In magnetic andelectromagnetic testing, apparentlagging of the magnetic effect whenthe magnetizing force acting on aferromagnetic body is changed;phenomenon exhibited by a magneticsystem wherein its state is influencedby its previous history. See also coerciveforce; magnetic saturation.

hysteresis loop: In magnetic andelectromagnetic testing, curve showingflux density B plotted as a function ofmagnetizing force H as H is increasedto the saturation point in bothnegative and positive directionssequentially. The curve forms acharacteristic loop.

IIACS: International Annealed Copper

Standard. See percent InternationalAnnealed Copper Standard.

illuminance: Intensity of visible light perunit area (density of luminous flux) ona surface. Illuminance is measured inlumens per square meter (lm·m–2) orlux. Compare luminance.

illuminate: Cast light on (something).Compare illuminance.

image: Reproduction of an objectproduced by light rays. An imageforming optical system gathers a beamof light diverging from an object pointand transforms it into a beam thatconverges toward another point, thusproducing an image.

image enhancement: Any of a variety ofimage processing steps, used singly orin combination to improve thedetectability of objects in an image.

image bundle: The main component of afiber optic borescope, comprised of agroup of optical fibers carrying theimage to the eye. The image bundleaverages between several thousand totens of thousands of individual fibersarranged in numerical order on thedistal and eyepiece ends (coherent).Fiberscope resolution depends on thesize, quality and configuration of thefibers. The fiber diameter ranges from6 µm to 10 µm — smaller diameterfibers demonstrate finer resolution.Compare light guide bundle.

image orthicon: Television tube that usesthe photoemission method. Comparevidicon tube.

image processing: Actions applied singlyor in combination to an image, inparticular the measurement andalteration of image features bycomputer. Also called picture processing.



image quality indicator (IQI): Inradiographic testing, strip of materialthe same composition as that of thematerial being tested, representing apercentage of object thickness andprovided with a combination of steps,holes or slots or alternatively made asa series of wires. When placed in thepath of X-rays, its image provides acheck on the radiographictechnique.14

imager, infrared: In thermal and infraredtesting, an instrument that collects theinfrared radiant energy from a targetsurface and produces an image inmonochrome (black and white) orcolor, where the gray shades or colorhues correspond respectively to targetexitance.

image segmentation: In imageprocessing, technique in which theimage is partitioned into regions, eachhomogeneous.

immersion technique: In ultrasonictesting, technique in which the testobject and the transducer aresubmerged in a liquid (usually water)that acts as the coupling medium.Compare contact technique.15 Thetransducer is not usually in contactwith the test object.

impedance: In electromagnetic testing,opposition that a circuit presents tothe flow of an alternating current,specifically the complex quotient ofvoltage divided by current.23

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 object.23

impedance plane diagram: Inelectromagnetic testing, graphicalrepresentation of the locus of pointsindicating the variations in theimpedance of a test coil as a functionof a parameter, such as conductivity orliftoff. See also argand diagram; complexplane diagram.

impedance plane diagram, normalized:In electromagnetic testing, diagram inwhich the impedance of the probe inair is a reference value to whichimpedance values in other conditionsare compared. Usually the plotted dataare (1) the measured reactance dividedby the reactance of the coil in airversus (2) the measured resistance lessthe resistance in air divided by the coilreactance in air.

impurity: Element or compound whosepresence in a material is unintentionalor unwanted.

incandescence: Emission of visibleradiation as a result of heating. Seealso Planck’s distribution law.

inclusion: In manufacturing, foreignparticles or impurities, usually oxides,sulfides, silicates and such, that areretained in metal (welds or castings)during solidification or that areformed by subsequent reaction of thesolid metal.

incomplete fusion: In welding, thefailure of a weld bead to joincompletely with the base metal orpreceding bead. Also called lack offusion.

incomplete penetration: In welding, rootpenetration less than complete orfailure of a root pass and a backingpass to fuse with each other. Alsocalled lack of penetration.

in control: Of a measureable feature ofinterest, stable between the upper andlower bounds as plotted on a controlchart. See also statistical process control.

index of refraction: Ratio of velocity oflight in a vacuum to velocity of lightin a material. See also Snell’s law.

indication: Nondestructive test responsethat requires interpretation todetermine its relevance. Comparedefect; discontinuity; indication, false;indication, nonrelevant.

indication, false: (1) Test indication thatcould be interpreted as originatingfrom a discontinuity but that actuallyoriginates where no discontinuityexists in the test object. (2) Indicationdue to misapplied or improper testing.Compare nonrelevant, nonrelevant;defect.

indication, nonrelevant: Indication thathas no relation to a discontinuity thatmight constitute a defect. Testresponse caused by geometry or by aphysical condition that is not adiscontinuity (a change of section, forinstance).

indication, relevant: Indication from adiscontinuity (as opposed to a falseindication) requiring evaluation by aqualified inspector, typically withreference to an acceptance standard,by virtue of the discontinuity’s size orlocation.

indirect viewing: Viewing of a test objectduring which the light image ismediated through a system of two ormore lenses (as in a borescope) ortransduced through an electronicsignal (as with a charge coupledcamera). Compare direct viewing; remoteviewing.

indium antimonide (InSb): Materialfrom which fast, sensitivephotodetectors used in infraredscanners and imagers are made. Suchdetectors usually requiring coolingwhile in operation. Operation is in theshort wave band (2 to 5 µm).



induced current magnetization: Inmagnetic particle testing, noncontactmeans for testing delicate ring shapedobjects for circumferentialdiscontinuities. The technique is basedon the fact that a time varying currentpassing through an internalconductor, often a soft iron orlaminated core, self-induces anencircling magnetic field. This timevarying magnetic field will induce asecondary current circling through thering. This secondary current then self-induces the toroidal magnetic fieldused for testing. See also right handrule.

inductance: Property of electric circuit, bywhich current in it or in a nearbycircuit creates magnetic flux in theother circuit. Inductance is measuredin henries, where one henry equalsone weber per ampere(1 H = 1 Wb·A–1). See alsoself-inductance.

inductor: In magnetic andelectromagnetic testing, deviceconsisting of one or more associatedwindings, with or without a magneticcore, which impedes the flow ofcurrent.

infrared and thermal testing:Nondestructive testing that uses heatdiffusion and infrared radiation asinterrogating energy.

infrared radiation (IR): Electromagneticenergy with a wavelength between750 nm (400 THz) and 1 mm(300 GHz). Compare visible light;ultraviolet radiation.19

infrared thermography: Imaging of atemperature field through the emittedinfrared radiation. Techniqueincorporates the use of an instrumentor system that converts incominginfrared radiant energy from a targetsurface to a thermal map, orthermogram, on which color hues orgray shades can be related to thetemperature distribution on thatsurface. See infrared radiation.

initial pulse: Pulse applied to excite thetransducer. It is the first indication onthe screen if the sweep is undelayed.Also called the main bang. May refer toan electrical pulse or an acoustic pulse.See also dead zone. Compare echo; backreflection; front surface reflection.

insonification: In ultrasonic testing,irradiation with acoustic energy, suchas ultrasound.

interface: Physical boundary between twoadjacent media.18

interface triggering: In ultrasonic testing,triggering the sweep and auxiliaryfunctions from an interface echooccurring after the initial pulse. Alsocalled interface synchronization.

interference: Production of a series ofmaxima and minima of a wave(electromagnetic or standing sonicwaves) as a consequence of thesuperposition of waves havingdifferent phases.15

interference objective: In a microscope, asmall, metallized glass mounted incontact with the test object andadjustable for tilt to control fringespacing.

interference sealing thread: Piping sealusing a tapered connection made upunder great pressure, forcing themating surfaces together more tightlythan possible with a spiral thread.Compare gasket seal.

interlaced scanning: Process originallydeveloped for cathode ray tubetechnology whereby the pictureappearing on a video screen is dividedinto two parts. Interlaced scanningreduces flicker by increasing theelectron beam’s downward rate oftravel so that every other line is sent.When the bottom is reached, thebeam is returned to the top and thealternate lines are sent. The odd andeven line scans are each transmitted at1/60 s, totaling 1/30 s per frame andretaining the standard rate of30 frames per second. The eye’spersistence of vision allows the oddand even lines to appear as a singleimage without flicker. Compareprogressive scanning.

internal conductor: In magnetic particletesting, rod of conductive materialthreaded through a hole in acylindrical test object to inducecircular magnetic flux. An internalconductor may be centered in the hole(a central conductor) or be offset nearor touching one side of the cylinder’sinside surface.

internal conductor technique: Inmagnetic particle testing, circularmagnetization technique that uses aninternal conductor.

interpretation: Determination of thecause, significance and relevance oftest indications.

inverse square law: Physical law for apoint source of energy. The quantityor strength is inversely proportional tothe square of the distance from theorigin.

inversion: See discontinuity inversion.ion current: In leak testing, current that

flows at all times from the positiveemitter (heater) to the negativecathode collector of the heated anode(alkali ion) halogen vapor detector.This current increases in the presenceof halogenated gases.



ionizing radiation: Form ofelectromagnetic radiation that candisplace orbital electrons from atoms.Types include X-rays, gamma rays andparticles such as neutrons, electrons(beta particles) and alpha particles.

ionization gage: High vacuum gage thatdepends on the measuring of electricalcurrent resulting from ionization ofgas. Examples include thermionicionization gages (bayard-alpert), coldcathode gages (penning or philip) andalphatron gages.

iris: Ring of variable area around thepupil and in front of the lens of theeye. The surface area of the iris adjustsspontaneously to change the amountof light entering the eye.

irradiance: Total radiant power, in wattsper square meter (W·m–2), falling upona known surface area at a given angle.Compare radiance. See also radiometer.

irradiance, spectral: Measure of energyemitted by a radiation source asfunction of wavelength. Units ofspectral irradiance are watts per squaremeter (W·m–2) and are often plottedversus wavelength. See also spectral.

IshiharaTM plate: Trade name for a kindof pseudoisochromatic plate used forcolor differentiation vision testing.

isobaric: Having constant barometricpressure.

isotherm: In infrared and thermal testing,locus or pattern superimposed on athermogram or on a line scan thatincludes or highlights all points thathave the same apparent temperature.

isotropy: Condition of material whoseproperties are independent of test axiswith respect to coupon orientation.Compare anisotropy.

Jjaeger eye chart: Eye chart used for near

vision acuity examination.joint: (1) Part of the casting mold where

the cope and cheek, cope and drag orcheek and drag come together. (2) Partof weld where two welded parts meet.

joint efficiency: Strength of a weldedjoint expressed as a percentage of thestrength of the unwelded base metal.

Kkaiser effect: In acoustic emission testing,

absence of detectable acousticemission until the previous maximumapplied stress level has been exceeded.

keeper: Ferromagnetic material placedacross the pole faces of a permanenthorseshoe magnet to reduce thereluctance of the gap and to preventloss of magnetism.

kelvin: Absolute temperature scale relatedto the celsius (or centigrade) relativescale. The kelvin unit is equal to 1 °C;0 kelvin = –273.16 °C; the degree signand the word degree are not used inexpressing kelvin temperatures.

ketos ring: See test ring.kinematic viscosity: Ratio of absolute

viscosity divided by the liquid’sdensity. Kinematic viscosity is oftenreported in centistokes.

kinetic vision acuity: Vision acuity witha moving target. Studies indicate that10 to 20 percent of visual efficiencycan be lost by target movement.

Kirchoff’s law: Principle that thesummation of all flux exchanges(absorbed, reflected, transmitted) on asemitransparent object equal unity.

known discontinuity standard (KDS):Part containing artificial anomalies ofa desired size and location used toperform system performance checks orto classify test materials. A set of KDSsamples or panels may bemanufactured to be twins of eachother.

known discontinuity standard, nickel-chrome (Ni-Cr): Set of two twinpanels used to evaluate liquidpenetrant material or processsensitivity. The brass Ni-Cr panel twinsare 35 mm wide by 100 mm long andhave a pattern of 10, 20, 30 or 50 µmdeep cracks across their width.Compare penetrant system monitor.

knudsen number: In leak testing, theratio of mean free path tocharacteristic dimension of thesystem.23

Llack of fusion: See incomplete fusion.lack of penetration: See incompletepenetration.

lambert cosine law: See cosine law.lambertian: Having a surface that reflects

light or acoustic energy diffusely in alldirections rather than specularly. Seealso matte. Compare specular.

laminar flow: Class of viscous flow wherevelocity distribution of fluid in a crosssection of a tube is parabolic.

lamination: Planar discontinuity rolledinto plate, sheet or strip caused bypipe, inclusions or blowholes in theoriginal ingot or by rollover duringrolling mill operations. After rollforming of the product, laminationsare usually flat and parallel to theoutside surface.

lap: In NDT, a surface imperfection thatappears as a seam and is caused byfolding and then by rolling or forgingmetal without actually joining.



laser: Acronym (light amplification bystimulated emission of radiation). Adevice that produces a high powermonochromatic and coherent (spatialand temporal) beam of radiation.

L·D–1 ratio: Convenient means forexpressing the shape of a test object interms of length L divided by diameterD. In magnetic particle testing, ratioused to judge whether a test object isappropriate for coil techniquemagnetization or demagnetizationalone or whether pole extensions orstacking is required. See alsoself-demagnetizing factor.

leakage field: See magnetic flux leakagefield.

leakage rate: In leak testing, quantity ofleakage fluid per unit time that flowsthrough a leak at a given temperatureas a result of a specified pressuredifference across the leak.

leaked visible light: In fluorescentnondestructive testing,electromagnetic radiation with awavelength between 380 and 780 nmthat is generated by a UV-A source butnot filtered out of the emissionspectrum. Leaked visible light isgenerally violet and not accuratelymeasured using a photometric sensor.See also light contamination; photometer;radiometer; UV-A; UV-A filter; visiblelight.

leak testing (LT): Nondestructive testingmethod for detecting, locating ormeasuring leaks or leakage inpressurized or evacuated systems orcomponents. Leaks are sought bylooking (bubble or dye tests), sniffing(gas or tracer detection) or by listening(ultrasonic test). See also manifold; massflow rate; response time.

leak testing, acoustic: Technique thatmonitors for elastic waves resultingfrom the flow of fluids through leaks,generally in the frequency range 30 to100 kHz. See also choked flow;transition flow.

leak testing, bubble: Technique in whicha leak in a pressurized component isindicated by the formation of bubblesof escaping gas. Methods includeimmersion, vacuum box and bubblesolution tests. See also accumulationtest technique; air flow; alkali ion diode;soak time.

leak testing, dynamic: Technique inwhich the system under test ispumped continuously. See also leaktesting, hood test.

leak testing, foam: Bubble leak testingtechnique in which the tracer gasblows a hole through a blanket offoam covering the test object, thusindicating the location of the leak.

leak testing, halogen detector probe:Pressure leak technique in which theleakage of a component, pressurizedwith a halogen rich mixture, isdetected by scanning over the testobject boundary surface with a probeconnected to a halogen leak detector.Halogen gas is pulled from the leakthrough the probe inlet to the sensingelement to cause a visible or audiblesignal on the indicator of the leak testinstrument. See also clean up time;detector probe; ion current; response factor.

leak testing, hood test: Quantitativetechnique in which a test object undervacuum test is enclosed by a hoodfilled with tracer gas so as to subject allparts of the test object to examinationfor leakage at one moment. A form ofdynamic leak testing in which theentire enclosure or a large portion ofits external surface is exposed to thetracer gas while the interior isconnected to a leak detector, with theobjective of detecting leakage ormeasuring its total rate. See also cleanup time; leech box.

leak testing, integrated leakage rate test:Technique performed for an entiresystem or component by pressurizingthe system to the calculated peakcontainment internal pressure relatedto the design and determining theoverall integrated leakage rate.

leak testing, optical: Technique that usesa visual means of leak detection, suchas holographic laser interferometry.Optical leak testing is used formicroelectronic and pharmaceuticalpackaging. See also hermetic seal;standard leakage rate.

leak testing, pressure: Technique of leaktesting objects pressurized with a tracergas with the subsequent detection andlocation of any existing leaks with asampling probe (a qualitative test).Tests performed by increasing thepressure inside a test boundary to alevel greater than the surroundingatmosphere and detecting leakage bysystematic examination of the outsideof the test surface. Leaks are located attime of detection; however, it isimpossible to accurately determine atotal leakage rate for the object beingpressure tested. See also soak time.

leak testing, radioactivity: Technique ofusing a radioactive tracer gas, such askrypton-85, to detect leaks because ofits radioactivity.

leak testing, ultrasonic: Leak test thatdetects ultrasound in the 40 kHz rangefrom gas flowing through the leakpath. See also leak testing, acoustic.



leech: In magnetic particle testing,permanent magnetic orelectromagnetic accessory used toensure adequate electrical contactduring current flow magnetization.Sometimes spelled leach.

leech box: In leak testing, the doublecompartmented box of which the outercompartment is evacuated and thenthe inner compartment is pressurizedto produce a pressure differential acrossthe test boundary under the innercompartment.

lens: Transparent object, whether artificialor natural, that refracts light passingthrough it in order to focus the light.

lens optics: See geometrical optics.lifting power: In magnetic particle

testing, the mass of a ferromagneticbar that a yoke can suspend throughattraction. Often this mass is aminimum that the yoke must meet orexceed.

liftoff: In electromagnetic testing,distance between the probe coil andthe test object.

liftoff effect: In electromagnetic testing,the change in system responseobserved because of a change incoupling between a test object and aprobe whenever the distance betweenthem is varied.

light: Electromagnetic radiation that canexcite the retina and produce a visualsensation. The visible portion of theelectromagnetic spectrum extendsfrom 380 to 780 nm.

light adapted vision: See photopic vision.light contamination: In fluorescent

nondestructive testing, unwantedvisible light present in darkened testarea. Sources may include gaps incurtains, leaked visible light from theUV-A source or fluorescence from theinspector’s clothing.

light guide bundle: Bundle of filaments,usually glass, that carries noncoherentlight (optical fibers are arranged inrandom order) from a high intensitysource through a fiber optic borescopeto illuminate an object. Contrast withimage guide bundle.

lighting, back: Placement of light sourceand image sensor on opposite sides ofthe test object, used when thesilhouette of a feature is important.Example back lighting applicationswould be optical profile projectors andindustrial optical comparators.

lighting, front: Placement of light sourceand image sensor on the same side ofthe test object.

lighting, strobe: Lighting that flashesintermittently at a rate that may beadjusted and is often perceived as aflicker, used to image moving objectsor still objects with potentialmovement.

lighting, structured: Combining a lightsource with optical elements to form alight pattern at a know angle. Thistechnique can be useful for imaging oracquiring dimensional information.

light meter: See photometer. Compareradiometer.

limited certification: Of a person,certified only for specific operations;usually called limited Level I or II ordesignated as having limitedcertification because they are notqualified to perform the full range ofactivities expected of personnel at thatlevel of qualification, for a givenmethod.

linearity, area: In ultrasonic testing,constant proportionality between thesignal amplitude and the areas ofequal discontinuities located at thesame depth in the far field. Necessarilylimited by the size of the ultrasonicbeam and configuration of thereflector.

linearity, horizontal: In ultrasonictesting, measure of proportionalitybetween positions of indications onthe horizontal trace and the positionsof their corresponding reflectors.

linearity, vertical: In ultrasonic testing,constant proportionality between thesignal input to the receiver and theamplitude of the signal appearing onthe display of the ultrasonicinstrument or on an auxiliary display.1Also called amplitude linearity.

line pair: Pair of adjacent, parallel linesused to evaluate the resolution of aspecific imaging system. Minimum linepair is a measure of system resolutionand refers to the smallest distance thata specific imaging system can resolvebetween a line pair.

lines of force: See magnetic flux.liquid crystals: In infrared and thermal

testing, thermochromic (change colorwith temperature) chemicalcompounds with the mechanicalproperties of a liquid and the opticalproperties of a solid. Liquid crystal(some combination of cholestericand/or chiral nematic compounds)optical properties cause them to reflectvivid spectral colors for temperaturechanges. Their adjustable response issensitive and can be made to changefrom red to blue over a temperaturegradient as small as 1 K (1 °C = 1.8 °F).

liquid penetrant, dual-response: Liquidpenetrant that produces discontinuityindications visible under eitherultraviolet radiation or visible light.

liquid penetrant, fluorescent: Highlypenetrating liquid used in theperformance of liquid penetranttesting and characterized by its abilityto fluoresce under ultravioletradiation.



liquid penetrant testing, fluorescent:Inspection technique that uses a dyedliquid that is usually green in colorand fluoresces brilliantly underultraviolet radiation. The sensitivity ofa fluorescent penetrant depends on itsability to form indications that appearas small sources of light against a darkbackground. Also known as fluorescentpenetrant inspection (FPI).

liquid penetrant leak testing: Techniqueof penetrant testing in which thepenetrant is applied to one surface of atest material while the opposite surfaceis tested for indications that wouldidentify a leak or void passing throughthe material thickness.

liquid penetrant, liquid oxygen (LOX)safe: Liquid penetrant material orsystem specifically designed to becompatible with or nonreactive inpresence of liquid oxygen.

liquid penetrant, postemulsifiable:Liquid penetrant that requires theapplication of a separate emulsifier torender the excess surface liquidpenetrant water washable. See alsoemuslifier; soak.

liquid penetrant testing (PT):Nondestructive testing method using aliquid that can enter discontinuitiesopen to the test surface. When drawninto a layer of developer, the liquid ishighly visible in small traces.Fluorescent liquid penetrants fluorescebrightly under ultraviolet radiation,whereas visible dye penetrants areintensely colored to be readily visibleon developer backgrounds whenilluminated with visible light. See alsoliquid penetrant testing, fluorescent;visible dye penetrant.

liquid penetrant, water washable:Liquid penetrant with built inemulsifier that makes it directly waterwashable.

location plot: In acoustic emissiontesting, a spatial representation ofacoustic emission sources calculatedby using an array of transducers.

logarithmic decrement: In ultrasonictesting, the natural logarithm of theratio of the amplitudes of twosuccessive cycles in a damped wavetrain.

longitudinal: Direction parallel to thelong axis of an object andperpendicular to its radius — forexample, down the length of acylinder. Compare circumferential;radial; transverse.

longitudinal magnetic field: Active orresidual magnetization oriented alongthe longest axis of the part. See alsolongitudinal magnetization.

longitudinal magnetization: Result ofmagnetic field flow magnetizationwhere induced magnetic flux linesflow parallel to the long axis of thecomponent. Longitudinalmagnetization occurs within anencircling coil, between the poles ofan electromagnetic yoke or contourprobe, between iron core inductioncoil pole extenders or between a pairof permanent magnets. See also fieldflow magnetization.

lot tolerance percent defective (LTPD):The poorest quality, in percentdefective, individual lot that isacceptable in a sampling plan. LTPD isthe percent defective that will beaccepted by the sampling plan at most10 percent of the time. With such aplan, the producer agrees to supplyjust enough nonconforming productsuch the consumer will accept the lotusing the agreed to sampling plan andacceptable quality level. Compareacceptable quality level.

low pass filtering: In image processing,linear combination of pixel values tosmoothen abrupt transitions in adigital image. Also called smoothing.

lumen (lm): SI photometric unit ofluminous flux, weighted according tothe photopic vision response. Onelumen equals the light emitted by onecandela (cd) point source into onesteradian (sr) solid angle(1 lm = 1 cd·sr–1).

lumen method: In visual testing, lightingdesign procedure used forpredetermining the relation betweenthe number and types of luminaires(lamps), the room characteristics andthe average illuminance on the workplane. It takes into account both directand reflected flux. Also called fluxmethod.19

luminance: Photometric brightness of alight source defined by the density ofits luminous intensity, measured asluminous flux per unit solid angle perunit area in a given direction.Reported in candela per square meter(cd·m–2). Compare illuminance.

luminous efficacy; luminous efficiency:Ratio of the total luminous flux of alight source to the total radiant flux orto the power input.

luminous exitance: luminous flux perarea, emitted or reflected from acertain location on a surface.Measured in lumens per square meter(lm·m–2). Compare radiant exitance.

luminous flux: Luminous energy per unittime as measured in lumens. Compareradiant flux.

luminous intensity: Luminous flux perunit solid angle in the direction ofinterest. Measured in candela.Compare luminance; radiant intensity.



lux (lx): SI unit of illuminance, equal toone lumen per square meter(1 lx = 1 lm·m–2).

luxmeter: Device used to measureilluminance. See photometer.

Mmachine vision: Automated system

function of acquiring, processing andanalyzing images to evaluate a testobject or to provide information orinterpretation for humaninterpretation. A typical machinevision system consists of a lightsource, a video camera, a videodigitizer, a computer and an imagedisplay.

macula lutea: Oval, highly pigmentedyellow spot near the center of theretina of the human eye. Diffuse ringof yellow pigment which partlyoverlaps the fovea and surrounds itout to around 10 degrees and whichabsorbs blue light, thus changing thecolor of the ligzht reaching receptorsbeneath. See also fovea centralis.

magnetic circuit: Path followed by fluxlines that may include the test object,any air gaps and an electromagnetic orpermanent magnet yoke.

magnetic field: Energy vector fieldsurrounding a magnet or electriccircuit.

magnetic field indicator: In magneticparticle testing, small, hand helddevice used to display the intensity ofuniform external magnetic flux asangular deflection of a display needle.The device contains a permanentreference magnet coupled to amovable, field sensing magnet, andsome units may be calibrated. Oftencalled a pocket field indicator.

magnetic field intensity (H): Magnitudeof the vector field surrounding amagnetic dipole,26 in ampere permeter. Often called magnetic fieldstrength.

magnetic field, tangential: Magneticfield at an object’s surface parallel tothe surface. The tangential field iscontinuous (equal on either side) withthe interface of material to air.Measurement can be influenced byexternal fields.

magnetic flow magnetization: See fieldflow magnetization.

magnetic flux: Convenient concept forvisualizing the vector field of magneticinduction that comprises a magneticfield. Flux lines form closed loops thatdo not cross. Magnetic flux isgoverned by the density of flux lines.The number of flux lines is expressedin weber (Wb), where 1 Wb = 108

maxwell (Mx). The density of fluxlines is expressed in tesla (T), where1 T = 104 gauss (G).

magnetic flux density (B): Amount ofmagnetic induction passingperpendicularly through a given area,measured in tesla.

magnetic flux indicator: See flexiblelaminated strip; shared flux indicator.Compare magnetic field indicator.

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

magnetic flux leakage testing (MFL):Nondestructive test method whereinduced magnetism in a ferromagneticobject forms localized poles at surface.Near-surface discontinuities areindicated by a signal in an inductioncoil or hall element. Comparemagnetic particle testing.

magnetic flux meter: Device thatmeasures total change in magneticflux density by monitoring the voltageinduced in a coil.27 See also teslameter.

magnetic gradient: Change in magneticfield intensity with distance, in ampsper square meter (A·m–2).

magnetic particle: In magnetic particletesting, finely divided ferromagneticpowder of proper size, shape, relativepermeability, visibility and retentivityfor use in a test.

magnetic particle, dry powder: Inmagnetic particle testing,ferromagnetic particles, larger thanthose used in wet suspensions,introduced to the test object surfaceby dusting or puffing. See also powderbulb; powder blower.

magnetic particle; dual-use: In magneticparticle testing, particle coated withpigment that provides contrast whenviewed under controlled levels ofambient white light but that alsofluoresces under ultraviolet radiation.Testing with fluorescent particles isperformed under low ambient lightingand controlled ultraviolet radiation.

magnetic particle; fluorescent: Inmagnetic particle testing, a particlecoated with pigment that fluoresceswhen excited with UV-A radiation.Testing with fluorescent particles isperformed under low ambient lightingand controlled ultraviolet radiation.



magnetic particle testing (MT):Nondestructive testing method whereinduced, or residual, magnetism in aferromagnetic test object formslocalized poles at surface and near-surface discontinuities indicated by afinely divided iron based powder.Compare magnetic flux leakage testing.

magnetic particle, visible: In magneticparticle testing, common termdescribing finely divided powder fornonfluorescent magnetic particle tests.The particles may be their naturalcolor or may be coated to enhancecontrast. Testing using visible particlesis performed under a controlled levelof ambient lighting and typically doesnot need any ultraviolet irradiation.

magnetic pole: One of two opposite endsof a dipole where flux enters or leavesa magnetized object. Any locationwhere flux enters or leaves a testobject.

magnetic rubber: In magnetic particletesting, replica casting mediumcontaining magnetic particles, whichwhen cured and removed from aproperly magnetized recess, provides apermanent mold with visibleindications.

magnetic saturation: Result of completedomain alignment where an increasein the coercive field H produces nochange in flux density B. See alsohysteresis loop.

magnetic sector: In leak testing,permanent magnet that separates theion species in the spectrometer tube ofthe helium mass spectrometer.

magnetic stripe card: In magneticparticle testing, a credit card sizeddevice with encoded magneticreversals of varying strength forregular evaluation of bath sensitivity.See also particle concentration. Comparesettling test.

magnetic writing: In magnetic particletesting, nonrelevant indication thatmay be caused when two magnetizedobjects come into contact.

magnetization: (1) Induced dipolemoment per unit volume of a solid.(2) Act of inducing a magnetic field ina ferromagnetic object.

magnetizing force: Magnetomotive forceper unit length of a magnetic circuit.Measured in ampere turns per meter(At·m–1).

magnetomotive force: Magnetic fieldintensity, measured in air or vacuumin ampere turns.

magnetometer: In magnetic particletesting, device for measuring thestrength of magnets or the intensity ofmagnetic fields. See pocket fieldindicator.

magnification: The ratio of apparentimage size of an object viewedthrough an optical system to its actualsize.

main bang: See initial pulse.manifold: In leak testing, a collection of

vacuum hardware such a valves,piping and chambers connectedtogether to form a test system.

manipulator: In the immersiontechnique of ultrasonic testing, adevice for angular orientation of thetransducer28 and for scanning motionin three axes.

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

marker: In ultrasonic testing, series ofindications on the horizontal trace ofthe A-scan display screen to showincrements of time or distance.18

martensite: Generic term for a rapiddiffusionless phase transformationthat deforms the parent phase andmay also change its volume. Whilecommon to many metals and alloys,martensite commonly refers to a hardmetastable phase of steel.

mask: (1) A spatial filter in the sensingunit of a surface inspection systemthat ensures image quality. (2) Ann × n square matrix with differentvalues 0that serves as a filter in imageprocessing. (3) Covering of a portionof a test object or film so as to preventtracer gas from entering leaks thatmay exist in the covered section.(4) In radiographic testing, a selectiveradiation filter. (5) In radiography, acover with an aperture to view aspecific area.

mass flow rate: In leak testing, weight,moles or number of molecules passingthrough a system as function of time.

mass spectrometer leak detector: Devicethat measures the mass-to-charge ratioand has design factors optimized toproduce an instrument that has highsensitivity to a single tracer gas.

mass-to-charge ratio: Ratio of mass(kilogram) to electrical charge(coulomb) of a molecule or the atomicmass of the molecule divided by theatomic charge of the molecule.23

match bend effect: Optical illusionwhereby an area of uniform brightnessappears to be nonuniform because ofcontrast with the brightness of anadjacent area.

material safety data sheet: Documentthat contains information relative tosafety and health in handling anddisposal of chemicals. Manufacturersof liquid penetrant materials arerequired to provide material safetydata sheets to users in accordance withthe OSHA Hazard CommunicationStandard.19



mathematical morphology: Imageprocessing technique of expandingand shrinking. The basic operators inmathematical morphology are dilation(expanding), erosion (shrinking),opening and closing.

matte: Having a surface that reflects lightdiffusely rather than at an angle equalto the angle of incidence; not shiny. Ifreflection from a surface is completelyisotropic, it is lambertian. The termmatte is generally applied to smoothsurfaces or coatings. Compare specular.

Maxwell’s equations: Fundamentalequations of electromagnetic fieldtheory:

where B is magnetic flux density, D iselectric flux density, E is electric fieldintensity, H is magnetic field intensity,J is current density, t is time, ρ isvolume charge density and � is the deloperator.

measurement spatial resolution,IFOVmeas: In infrared and thermaltesting, smallest target spot size onwhich an infrared imager can producea measurement, expressed in terms ofangular subtense (mrad per side). Theslit response function (SRF) test is usedto measure IFOVmeas.

mechanical properties: Properties of amaterial that reveal its elastic andinelastic behavior where force isapplied, thereby indicating itssuitability for mechanical applications(for example, modulus of elasticity,tensile strength, elongation, hardnessand fatigue limit).

medium; transmitting medium:Composition of the measurement pathbetween a target surface and themeasuring instrument through whichthe radiant energy propagates. Thiscan be vacuum, gaseous (such as air),solid, liquid or any combination ofthese.

mercury cadmium telluride (HgCdTe):Material used for fast, sensitiveinfrared photodetectors used ininfrared sensors, scanners and imagersthat requires cooled operation.Operation is in the long wavelengthregion (8 to 12 µm).

mesopic vision: Vision adapted to a levelof light between photopic (greaterthan 3 cd·m–2) and scotopic (less than0.01 cd·m–2). An official definition ofthe human eye’s mesopic compositespectral response has not yet beendeveloped. See also dark adaptation.Compare photopic vision; scotopic vision.

metallography: The study of thestructure of metals and alloys byvarious methods including optical andelectron microscopy.

metallurgy: The science and technologyof metals and their alloys. Ametallurgist may focus on the miningand processing of ores into usefulform (extractive metallurgy), focus onthe physical or mechanical propertiesthat vary with composition, thermalhistory or environment (physicalmetallurgy) or focus on a material’sresponse to applied forces (mechanicalmetallurgy).

microporosity: Porosity visible only withaid of a microscope.

microscope: Instrument that providesenlarged images of small objects.There are many types of microscopes,only some of which are optical innature. Some optical microscope typesinclude binocular (stereo), confocal,inverted and compound. Other typesinclude acoustic microscopes andelectron microscopes.

microsegregations: (1) Segregation withina grain, crystal or small particle. Alsocalled coring. (2) Narrow cracks,usually long and straight, on thesurfaces of highly finished wroughtmetals. Often very shallow, theiridentity must be established to ensurethat indications are not fromdetrimental cracks, deep laps or longinclusion stringers.

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.

MIG welding: See welding, gas metal arc.minimum line pair: Closest distance that

a specific imaging system can resolvebetween a pair of adjacent, parallellines (line pair) used to evaluatesystem resolution.

mode conversion: Change of ultrasonicwave propagation mode uponreflection or refraction at an interface.

mode converted signal: Unintendedsignal from mode conversion ofprimary test angle, due to interactionwith component geometry such as thesignals after back wall signal whentesting a long narrow bar.

∇ × − ∂∂

EB

=t

∇ × ∂∂

+HD

J=t

� =∇ ⋅D ρ

� = 0∇ ⋅ B



mode of vibration: Manner in which anacoustic wave is propagated, ascharacterized by the particle motion inthe wave18 (transverse, lamb, surfaceor longitudinal).

model, analytical: Mathematicalrepresentation of a process orphenomenon.

modulation: (1) In infrared and thermaltesting, changes in one wave traincaused by another. (2) In thermalscanning and imaging, imageluminant contrast(Lmax – Lmin)·(Lmax + Lmin)–1.

modulation transfer function: Ininfrared and thermal testing, measureof the ability of an imaging system toreproduce the image of a target. Aformalized procedure is used tomeasure modulation transfer function.It assesses the spatial resolution of ascanning or imaging system as afunction of distance to the target. Seealso slit response function.

modulus of elasticity: Measure of amaterial’s rigidity or stiffness, relatedto the slope of the stress-versus-straincurve within the linear elasticdeformation range. Measured inmegapascals (MPa). Also called Young’smodulus.

molecular flow: In leak testing,phenomenon occurring when meanfree path length of gas molecules isgreater than the largest cross sectionaldimension of a leak or the tubethrough which flow is occurring.

molecular weight: For a gas, the mass of22.4 L (0.8 ft3) at standard conditions.

monochromatic: Of a single wavelengthor color.

monochromator: Device that uses prismsor gratings to select and separate asingle wavelength of theelectromagnetic spectrum. Amonochromator is often used totransmit a desired narrow band oflight or energy.

morphology: See mathematicalmorphology.

mottle: (1) Apparently randompositioning that creates an accidentalpattern. (2) In radiographic testing,nonuniform density where it shouldbe uniform, resulting from scatteredradiation, secondary radiation,forward scatter and film irregularities.Often confused with graininess.

multidirectional magnetization: Inmagnetic particle testing, two or moremagnetic fields in different directionsimposed on a test object sequentiallyand in rapid succession through phasecontrol of the supplied current. Seealso phase and swinging fieldmagnetization.

multifrequency: In electromagnetictesting, two or more frequenciesapplied sequentially or simultaneouslyto the test coil.

multifrequency technique: Inelectromagnetic testing, use of theresponse of a test specimen to morethan one frequency, usually toseparate effects that would beindistinguishable at a single frequency.

multiparameter; multivariable: Inelectromagnetic testing, of orpertaining to a test system havingmany parameters that affect theresponse. These parameters can oftenbe distinguished with amultifrequency technique.

multiple-echo technique: In ultrasonictesting, technique where thickness ismeasured between multiple backreflections, minimizing error fromcoatings or from changes intemperature or contact pressure.

mutual inductance: Property of twoelectrical circuits whereby a voltage isinduced in one circuit by a change ofcurrent in the other circuit. See alsocoupled.16

myopia: See nearsightedness.

Nnarrow band: Relative term denoting a

restricted range of frequency response.Compare broad band.15

NDE: (1) Nondestructive evaluation.(2) Nondestructive examination. Seenondestructive testing.

NDI: Nondestructive inspection. Seenondestructive testing.

NDT: See nondestructive testing.near field: Distance immediately in front

of a plane transducer in which theultrasonic beam exhibits complex andchanging wavefronts. Also called thefresnel field or fresnel zone. Compare farfield.18

near ultraviolet radiation: See UV-A.near vision: Vision of objects nearby,

generally within arm’s length.Compare far vision.

nearsightedness: Vision acuityfunctionally adequate for viewingobjects nearby, generally within arm’slength. Also called myopia. Comparefarsightedness.

near-surface discontinuity: Subsurfaceinterruption in the physical structureor configuration of a test object that isclose to, but not breaking, the testobject’s surface. (This sense of nearsurface differs from that in methodsthat distinguish a test object’s nearsurface from its far surface, adistinction rarely made in magneticparticle testing.)



necking down: Localized reduction inarea of a specimen or structuralmember during tensile deformation.

negative sliding: Rolling and sliding ofmeshing gears or rollers when therolling and sliding are in oppositedirections. Compare positive sliding.

neper (Np): Disused unit of physical fieldand power quantities; the naturallogarithm of a ratio of two amplitudes(equal to 8.686 dB) used as a measureof attenuation. Power ratios areexpressed as half the naturallogarithm.

neural acuity: Ability of the eye andbrain together to discriminate patternsfrom background. Discrimination isinfluenced by knowledge of the targetpattern, by the scanning techniqueand by the figure-to-groundrelationship of a discontinuity. Thefigure/ground relationship can bedescribed as having a level of visualbackground noise.

neutron: Uncharged elementary particlewith mass nearly equal to that of theproton.14

neutron fluence: Integrated exposure(product of current and time) ofneutrons per unit area.

neutron flux: Neutron current; quantityof neutrons passing through a unitarea per unit time.

nick: See gouge. Compare tool mark.NIST: Acronym for the National Institute

of Standards and Technology(formerly National Bureau ofStandards), United States Departmentof Commerce, Gaithersburg,Maryland.

NIST traceability: Property of the resultof a measurement, or the value of astandard; instruments, calibrationreports and laboratories are nottraceable. Traceability can be related tostated references or standards, throughan unbroken chain of comparisons allhaving stated uncertainties.

nit (nt): Disused unit for measuringluminance, equivalent to one candelaper square meter.

noble metals: Cathodic metals (such asgold, platinum and silver), whichstrongly resist corrosion.

nodal point: In ultrasonic testing anglebeam inspections, the location ofreflections at opposite surfaces as awave progresses along a test object.Compare antinode.

noise: Component of physical quantity,such as voltage, that providesnonrelevant information. Comparesignal.

noise equivalent temperaturedifference: In infrared and thermaltesting, temperature difference equalto the noise signal; a measure ofthermal resolution, but not taking intoaccount characteristics of the displayand the operator’s subjectiveinterpretation.

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 nondestructivetesting. In research and academiccommunities, the word evaluation isoften preferred because it emphasizesinterpretation by knowledgeablepersonnel.

nondestructive examination (NDE):Another term for nondestructivetesting. In the utilities and nuclearindustry, examination is sometimespreferred because testing can implyperformance trials of pressurecontainment or power generationsystems.

nondestructive inspection (NDI):Another term for nondestructivetesting. In some industries (utilities,aviation), the word inspection oftenimplies maintenance for a componentthat has been in service.

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). Sometimes callednondestructive evaluation, nondestructiveexamination or nondestructive inspection.

nonferromagnetic material: Material notmagnetizable and essentially notaffected by magnetic fields. Compareferromagnetic material.

normal incidence: (1) In ultrasonictesting, condition in which the axis ofan ultrasonic beam is perpendicular tothe entry surface of the test object.(2) Condition where the angle ofincidence is zero.

normalize: Adjust a display or graph suchthat the maximum value is unity.

null: In electromagnetic testing, to adjusta bridge circuit so that the test sampleand reference arms produce equal andopposite currents through thedetector.



null signal: In electromagnetic testing,fixed component of the test coil signalthat is subtracted from the outputsignal leaving only that part of thesignal that varies with the test objectconditions; it reduces dynamic rangerequirements.

nonrelevant indication: See indication,nonrelevant.

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.

Oobjective: In discussion of a lens system

(camera, borescope, microscope ortelescope), of or pertaining to the endor lens closest to the object ofexamination — at the end oppositefrom the eyepiece. See also distal.

oersted (Oe): Disused CGS measurementunit of magnetizing force, or magneticfield intensity. Replaced in SI byampere per meter, or ampere turns permeter: 1 Oe = 79.57747 A·m–1.

ohm (Ω): Measurement unit of electricalresistance.

oil country tubular goods (OCTG):Hollow cylindrical components, suchas pipes, used in petroleum wells tocase the hole and to convey petroleumand related products. See also casing.

opaque: In infrared and thermal testing,impenetrable to radiant energy. Inthermography, an opaque material isone that does not transmit thermalinfrared energy.

opening: In image processing, theoperation of erosion followed bydilation. A single opening eliminatesisolated single pixels. Compare closing.

opsin: See visual purple.optic disk: Area in the retina through

which the fibers from the variousreceptors cross the inner (vitreoushumor) side of the retina and passthrough it together in the optic nervebundle. This transitional area iscompletely blind.

optics: Physical science of thetransmission of radiation, especially oflight. See also geometrical optics.

optimum frequency: Test frequency thatprovides the highest signal-to-noiseratio compatible with the detection ofa specific discontinuity. Eachcombination of discontinuity type andmaterial may have a differentoptimum frequency.

organoleptic: Relying on or using senseorgans, such as the human eye.

orthicon: See image orthicon.

oscillogram: Common term for a recordor photograph of data displayed on anoscilloscope screen.

outgassing: Forms of gas coming frommaterial in a vacuum system. Includesgases adsorbed on the surface,dissolved in material and trapped inpockets and those due to evaporation.

Pparafoveal vision: See scotopic vision.parallax: Apparent difference in position

of an imaged point according to twodifferently positioned sensors.

parallel magnetization: In magneticparticle testing, the dubious practice ofimparting circular magnetization in asample near a current carryingconductor. Compare internal conductor.

paramagnetic material: A material thathas a relative permeability slightlygreater than unity and is practicallyindependent of the magnetizing force.Compare diamagnetic material;ferromagnetic material.

parameter distribution: In acousticemission testing, display of thenumber of times an acoustic emissionparameter falls between the values xand x + δx as a function of x. Typicalparameters are amplitude, rise timeand duration.

parasitic echo: See spurious echo.particle concentration: In magnetic

particle testing, amount of powdersuspended within a known samplevolume of bath. Typically measuredwith a settling test or throughevaporation and weighing. See alsocentrifuge tube.

parting line: In manufacturing, mark lefton the die casting where the diehalves meet. Also, the surface betweenthe cover and ejector portions of thedie.

pascal (Pa): An SI derived unit ofpressure, stress, modulus of elasticityand tensile strength. Pressure is forceper unit area, and a pascal is definedas one newton per square meter.

pass: In welding, a single bead of weldmetal along the entire joint or theprocess of laying down that bead. Seealso weld, multi-pass.

parts per million (ppm): Concentrationof a substance in a mixture. Forexample, the amount of solvent vaporin a working environment that is ahealth and safety hazard. The ratio ofparts per million is expressed in units— for example, μL·L–1 or µg·g–1.

peak hold: Feature of an instrumentwhereby an output signal ismaintained at the peak instantaneousmeasurement for a specified duration.Compare valley hold.



pearlite: Most often, a lamellar structureof cementite and ferrite in some steelsand cast irons; sometimes, a lamellaralpha and beta structure in nonferrousalloys.

peening: Mechanically working a surfaceto induce a compressive residual stressthrough the use of impacting metallicshot, hammer blows or laser pulses.Compare mechanical cleaning; plasticdeformation.

pencil break source: In acoustic emissiontesting, an artificial source using thefracture of a brittle graphite orequivalent cylinder in a suitable fittingto simulate an acoustic emissionsignal. Also called hsu-nielson source.

penetrability: Ability of a material to bepenetrated by an energy or liquid. Forexample, the ability to allow a liquidinto very fine openings such as cracks,or the ability of an X-ray beam to passthrough a material because ofkilovoltage.

penetrameter: See image quality indicator.penetrant: See liquid penetrant.penetrant system monitor (PSM):

Stainless steel panel that is used forregular liquid penetrant systemverification to show that testsensitivity has not degraded with time.The penetrant system monitor panel is100 mm (4 in.) wide × 150 mm (6 in.)long × 1.16 mm (0.085 in.) thick andis divided into two vertical halves.One half is chrome plated, which mayor may not be media blasted, andcontains five radial cracks ofincreasing size. The other half is mediablasted and acts as a liquid penetrantremovability tool. This panel is notdesigned to be a liquid penetrantsensitivity check. Also called star burstpanel or TAM panel. Compare knowndiscontinuity standard; nickel-chromeknown discontinuity standard.

percent International Annealed CopperStandard (%IACS): Traditionalmeasurement of conductivity σ as apercentage of the conductivity of purecopper, arbitrarily rated at 100 percent.In SI, conductivity is measured insiemens per meter (S·m–1). See alsoconductivity.

peripheral vision: Seeing of objectsdisplaced from the primary line ofsight and outside the central field ofvision.19

permanent magnet: Material with highretentivity, which maintainsmagnetization after a coercive fieldhas been removed. In magneticparticle testing, permanent magnetyokes must also have a high coercivity.Compare electromagnet.

permeability (µ): (1) Ability of a materialto be magnetized, measured asincrease in flux density. (2) Ratio ofmagnetic induction B overmagnetizing force H. Absolutepermeability in SI units is measured inhenries per meter (H·m–1). The valueand dimension of absolutepermeability depend on the system ofunits used. In anisotropic media,permeability is a matrix. Comparereluctance. See also hysteresis loop;permeability of free space; permeability,relative.

permeability, incremental: Inelectromagnetic testing, ratio of thechange in magnetic induction to thecorresponding change in magnetizingforce.

permeability, initial: 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).

permeability of free space (µ0):Calculation constant describing theratio of magnetic induction B tomagnetizing force H within a vacuum.1 µ0 = 4 × 10–7 H·m–1.

permeability, relative (µr): Unitless ratioof a material’s permeability to thepermeability of free space.

pH: A measure of the acidity or alkalinityof a solution. Negative of log C, whereC is the concentration of hydrogenions. Values lower than 7.0 are acidic;values equal to 7.0 are neutral; valueshigher than 7.0 are alkaline.

phantom: In ultrasonic testing, referencestandard or realistic model used toverify the performance of diagnosticultrasound systems.

phase: (1) A circuit conductor carryingalternating current of a givenfrequency, as in one-phase or three-phase power. (2) Point on a 360-degreeharmonic power waveform (thyristors,for example, vary total power outputthrough phase control). (3) Inmetallurgy, a physically homogeneousportion of a material system,specifically the portion of an alloycharacterized by its microstructure at aparticular temperature during meltingor solidification.

phase analysis: In electromagnetictesting, 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.23



phase angle: In electromagnetic testingand magnetic particle testing, angularequivalent of the time displacementbetween corresponding points on twosine waves of the same frequency.23

phased array: In ultrasonic testing,mosaic of transducer elements inwhich the timing of the elements’excitation can be individuallycontrolled to produce certain desiredeffects, such as steering or focusing thebeam. See also transducer, array.

phase detection: In electromagnetictesting, derivation of a signal whoseamplitude is a function of the phaseangle between two alternatingcurrents, one of which is used as areference.23

phase diagram: In materials science,graph showing the temperature andcomposition limits of phase fields in amaterial system under specific heatingor cooling conditions.

phase sensitive system: Inelectromagnetic testing, system whoseoutput signal depends on the phaserelationship between the voltagereturned from a pickup or sensing coiland a reference voltage.23

phase shift: In electromagnetic testing,change in the phase relationshipbetween two alternating quantities ofthe same frequency.23

phase velocity: In ultrasonic testing,velocity of a continuous acoustic waveat a particular frequency.

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

photochromic lens: Eyeglass materialthat automatically darkens to reducelight transmission when exposed toultraviolet radiation.

photoconduction: Method by which avidicon television camera tubeproduces an electrical image, in whichthe conductivity of the photosensitivesurface changes in relation to theintensity of the light reflected fromthe scene focused onto the surface.Compare photoemission.

photodetector; photon detector: Ininfrared and thermal testing, type ofinfrared detector that has fast response(on the order of microseconds),limited spectral response and usuallyrequires cooled operation.Photodetectors are used in infraredradiation thermometers, scanners andimagers because, unlike thermaldetection, direct photon interactionobviates external heating of thedetector for the signal to be sensed.Compare pyroelectric detector.

photoelasticity: Effect of a material’selastic properties on the way that itrefracts or reflects light. Thisphenomenon is commonly used toestimate the magnitude anddistribution of stress in a componentthrough the use of either a transparentmodel of a part or a thin layer ofphotoelastic material bonded to acomponent.

photoelectric effect: Emission of freeelectrons from a surface bombarded bysufficiently energetic photons. Suchemissions may be used in anilluminance meter, calibrated in lux.1Interaction of photons with atoms inwhich the full energy of the photon isabsorbed by an orbital electron,removing the electron from the atom.

photoemission: Method by which animage orthicon television camera tubeproduces an electrical image, in whicha photosensitive surface emitselectrons when light reflected from aviewed object is focused on thesurface. Compare photoconduction.

photometer: Device used to measureluminance or illuminance.Illuminance photometers are oftencalled lux meters. Photometer sensorsare filtered such that their responsivityclosely matches the spectralresponsivity curve of the human eye.Compare radiometer.

photometric brightness: See luminance.photometry: Study and measurement of

electromagnetic radiation withapproximate wavelengths between 380and 780 nm, which are within thehuman eye’s spectral responsivity. Seealso photometer; photopic vision; relativephotometry. Compare radiometry.

photon: Particle of light, hypothesized toexplain those behaviors of light inwhich its behavior is corpuscularrather than wavelike.

photopic vision: Average spectralresponsivity curve of the human eyewhen adapted to well lit conditions(greater than 0.034 cd·m–2). Thephotopic spectral luminous efficiencyresponse curve is governed by anaveraged retinal cone response withsensitivity peaks centered at about555 nm. Also known as foveal visionand light adapted vision. Comparemesopic vision; scotopic vision.19

photoreceptor: Light sensor.physical properties: Nonmechanical

properties such as density, electricalconductivity, heat conductivity andthermal expansion.

picture element: See pixel.



pie gage: In magnetic particle testing, onetype of shared flux indicator in the formof a handle mounted disk comprisedof ferromagnetic wedges surroundedby a copper matrix. When properlydemagnetized before use the spacebetween wedges provides artificialdiscontinuities at 0, 45 and 90 degreesand provides verification of magneticflux direction during dry powdertesting. Compare berthold penetrameter.

piezoelectric effect: Ability of certainmaterials to convert electrical energy(voltage) into mechanical energy(stress) and vice versa.15

pigtail: In gamma radiography, flexiblecable to which an isotope bearing pillmay be attached for movement in andout of a shielding container. See alsocontrol cable; guide tube.

pill: In gamma radiography, capsulecontaining isotopic source ofradiation. See also control cable; guidetube; pigtail.

pipe: (1) Longitudinal centerlinediscontinuity inherent in ingots orimparted to some rolled metal andconsisting of a concavity or voids.May also be called worm holes. (2) Castor wrought tubular product.

pirani gage: In leak testing, a wheatstonebridge circuit that measures the effectof gas thermal conductivity changescorresponding to pressure variations.Measures pressure from atmosphericdown to 0.1 Pa (1 mtorr).

pitch catch technique: Ultrasonic testtechnique that uses two transducers,one transmitting and the otherreceiving on the same or oppositesurface.15,18 21 Also called double-crystaltechnique or two-transducer technique.Compare multiple echo technique; pulseecho technique.

pitting: Forming of small cavitydiscontinuities in a surface bycorrosion, wear or other degradation.See also cavitation errosion.

pixel: Single addressable point in a rasterdigital image. The image from aconventional computer is an array ofpixels, and each has a numericalvalue. Formerly called picture element.

plane of focus: See focus, principalplane of.

Planck’s distribution law: Fundamentallaw that relates the emitted energyspectral radiance to wavelength and toemitted surface temperature. See alsoblackbody; incandescence;stefan-boltzmann law; Wien’s law.

plane wave: See wave, longitudinal.plastic deformation: Permanent

distortion due to an applied stressabove a material’s elastic limit. See alsodissipation. Compare elasticdeformation.

plate wave: See wave, lamb.

pocket field indicator: See magnetic fieldindicator.

point of incidence: In ultrasonic testing,point at which the axis of a soundbeam leaves the wedge of an anglebeam transducer and enters the testobject.15 See also probe index.

pole: See articulated pole piece; magneticpole.

poling: Process of reorienting crystaldomains in certain materials byapplying a strong electric field atelevated temperatures, inducingmacroscopic polarization andpiezoelectric behavior.

pooling: In liquid penetrant testing,collection of excessive amounts ofliquid penetrant, emulsifier, water ordeveloper in an incompletely drainedarea of a part.

pores: (1) Small voids within a metal. Seealso porosity. (2) Minute cavities,sometimes intentional, in a powdermetallurgy compact. (3) Minuteperforations in an electroplatedcoating.

porosity: Discontinuity in metal resultingfrom the creation or coalescence ofgas. Very small pores open to thesurface are called pinholes.22

positive sliding: Rolling and sliding ofmeshing gears or rollers when thedirections of rolling and sliding arethe same. Compare negative sliding.

postemulsification: Liquid penetrantremoval step that uses a separateemulsifier applied over the surfaceliquid penetrant to render itremovable by water spray. See alsoprerinse technique.

poultice corrosion: See corrosion, poultice.powder blower: In magnetic particle

testing, compressed air device used todeliver a cloud of dry magneticparticles to the surface of a test object.

powder bulb: In magnetic particle orliquid penetrant testing, pneumaticdevice compressed by hand to delivera cloud of dry magnetic particles ordry powder developer to the surface ofa test object.

practical examination: In certification ofnondestructive testing personnel, ahands-on examination using testequipment and sample test objects.Compare general examination; specificexamination.

precipitation hardening: Hardening inmetals caused by the formation(precipitation) of a constituent from asupersaturated solid solution. Methodcommonly applied to many aluminumalloys to increase strength. See alsoaging.



prerinse technique: In liquid penetranttesting, postemulsifiable penetrantremoval step in which major portionof a nonwater washable liquidpenetrant is mechanically removedwith a water spray before applicationof emulsifier. Sometimes calledprewashing. See also postemulsification.

pressure proof testing: In leak testing,test of system at pressure considerablyabove the allowable working pressureto demonstrate structural capability.

primary radiation: Radiation emittingdirectly from the target of an X-raytube or from a radioactive source.14

primary reference response level:Ultrasonic response from the basicreference reflector at the specifiedsound path distance, electronicallyadjusted to a specified percentage offull screen height. See also distanceamplitude correction.

probability of detection (PoD): Theprobability of finding an anomaly ofgiven characteristics, under preciseconditions, while using a specific testprocedure.

probe: See sensor; transducer.probe index: Point on a transverse wave

or surface wave transducer throughwhich the emergent beam axispasses.20 See also point of incidence.

process: Repeatable sequence of actions tobring about a desired result.

process control: See statistical processcontrol.

prod: In magnetic particle testing,handheld pair of electrodes fortransmitting magnetizing current froma portable power source to the testobject during the prod magnetizationtechnique. See also leech.

prod magnetization technique: Inmagnetic particle testing, impartingcircular magnetization in acomponent by passing current directlythrough it via a prod. See also currentflow magnetization.

progressive scanning: Display methoddesigned for liquid crystal displays andother new video technologies whereeach row of an image is refreshed insequential order. This method is lessprone to jaggedness or flicker and isbetter suited for viewing fine details.Compare to interlaced scanning.

propagation: Advancement of energy or acrack through a medium.

pseudocolor: Image enhancementtechnique wherein colors are assignedto an image at several gray scaleintervals.

pseudoisochromatic plate: Image usedfor color vision examinations. Eachplate bears an image which may bedifficult for the examinee to see if hisor her color vision is impaired. Seealso Ishihara™ plates.

psychophysics: Interaction betweenvision performance and physical orpsychological factors. One example isthe so-called vigilance decrement, thedegradation of reliability based onperforming visual activities over aperiod of time. See also human factors.

PT: Liquid penetrant testing.pulse: In ultrasonic testing, transient

electrical or ultrasonic signal that hasa rapid increase in amplitude to itsmaximum value, followed by animmediate return. Compare burst.

pulse echo technique: Ultrasonic testtechnique in which discontinuities aredetected by return echoes from thetransmitted pulses. Compare multipleecho technique.

pulse technique: In electromagnetictesting, multifrequency technique inwhich a broadband excitation such asan impulse is used. Either thefrequency components are extractedand analyzed or the interpretation isbased directly on characteristics of thetime domain waveform.

pulse tuning: Control of ultrasonicultrasonic pulse frequency to optimizesystem response.

pump, adsorption: Pump that creates avacuum by collecting gas on theinterior surfaces of the pump.Pressures of 2 Pa (20 µbar) are readilyattained. The pump has a finitecapacity but may be regenerated foradditional use. See also backstreaming;baffle.23

pump, cryogenic: Pump that condenseschamber gas on a cold surface of 4 to80 K (–269 to –194 °C). Cooling isprovided by liquid gas such as liquidhelium or by refrigeration. See alsobackstreaming; baffle.23

pump, diffusion: High vacuum pumpwith no moving mechanical parts thatuses a vapor jet to sweep gas from thevacuum chamber and achievepressures as low as 1 nPa (10 ptorr).23

pump, displacement: Mechanical pumpthat physically sweeps gas out of avolume and creates a vacuum. Rotarypiston and rotary vane pumps are twoexamples. A displacement pump canachieve pressures in the 0.1 to 1.0 Pa(10 to 1 mtorr) range. See alsobackstreaming; baffle.

pump, fore: Mechanical pump in ahelium mass spectrometer thatperforms initial evacuation of a systemto a pressure of 0.1 Pa and thenaccepts the exhaust from the highvacuum pump such as a diffusionpump. The forepump lowers pressureto less than 10 kPa into which thediffusion pump can exhaust its gas.



pump, ion: Pump that combines electricand magnetic fields to ionize gas andtrap the gas inside the pump, thusremoving it from the vacuumchamber. See also ionization gage.23

pump, mechanical: Mechanical devicewith pumping fluid and seals thatphysically removes a portion of thegas from a system with eachrevolution of the armature. Amechanical pump can pump achamber down to about 0.1 Pa(1 mtorr). See also gas ballast; rootsblower.

pump, sorption: Pump consisting of asieve and liquid nitrogen with abilityto pump to 0.1 Pa (1 mtorr).23

pump, turbomolecular: Molecularturbine that drives gas out of avacuum chamber, achieving a highvacuum pressure in the 10 nPa(0.1 ntorr) range.

pupil: Black aperture in the center of theiris, through which light enters thelens to impinge on the retina.

pyroelectric detector: Type of thermalinfrared detector that acts as a currentsource with its output proportional tothe rate of change of its temperature(heating or cooling of pyroelectricmaterial creates charge accumulation).Compare photodector.

pyroelectric vidicon: Video camera tubewith its receiving element fabricatedof pyroelectric material and sensitiveto wavelengths from about 2 to20 µm; used in infrared thermalviewers. Sometimes called pyrovidicon.Compare vidicon tube.

pyrometer, laser: Infrared radiationthermometer that projects a laserbeam to the target, uses the reflectedlaser energy to compute targeteffective emissivity and automaticallycomputes target temperature(assuming that the target is a diffusereflector). Not to be confused withlaser aided aiming devices on someradiation thermometers.

pyrometer, ratio: Infrared thermometerthat uses the ratio of incominginfrared radiant energy at twonarrowly separated wavelengths todetermine a target’s temperatureindependent of target emittance; thisassumes graybody conditions and isnormally limited to relatively hottargets, above about 420 K(150 °C ≅ 300 °F).

pyrometry: Measurement of fire or of hotobjects, such as the monitoring offurnace or foundry conditions.

QQ of a coil: Quality factor of an

electromagnetic testing coil; related tothe ratio of maximum energy stored tothe total energy lost per period.

quadrature: Relation between twoperiodic functions when the phasedifference between them is 90 degrees(that is, the time delay is one-fourth ofa period).

qualification: Process of demonstratingthat an individual has the requiredamount and the required type oftraining, experience, knowledge andabilities.19 See also certification andqualified.

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

quality: Ability of a process or product tomeet specifications or to meet theexpectations of its users in terms ofefficiency, appearance, reliability andergonomics.19

quality assurance: Administrative actionsthat specify, enforce and verifyquality.19

quality control: Physical andadministrative actions required toensure compliance with a qualityassurance program. Quality controlmay include nondestructive testing inthe manufacturing cycle.19

quality factor (Q): Of a coil used inelectromagnetic testing, the ratio ofreactance to resistance defined at theoperating frequency.

quantitative quality indicator (QQI): Inmagnetic particle testing, a shared fluxindicator containing an artificialdiscontinuity held in intimate contactwith a test object’s surface duringactive magnetization to ensure thatproper magnitude and direction ofmagnetic induction have beenobtained for testing. The artificialdiscontinuity may be circular or linearand is defined in terms of percent oftotal shim thickness.

quick break: In magnetic particle testing,sudden cessation of magnetizingcurrent. A quick break is needed whenusing three-phase full-wave rectifiedalternating current during coil orinduced current magnetization. Therapid change in current producesstrong magnetic induction duringtoroidal magnetization and reducesthe disturbing flux near poles forsensitive testing of the test object’sends during coil magnetization.



Rrad: Radiation absorbed dose. Unit ofabsorbed dose of ionizing radiation. Onerad is equal to the absorption of 10–5 J(100 erg) of ionizing radiation energyper gram of matter.14 Replaced by thegray (Gy).

radial: Of or pertaining to direction fromcenter of a circle (or a sphere or crosssection of a cylindrical object) to itssurface and perpendicular to its axis. Aradial pattern appears to radiate froma point, like the spokes from the hubof a wheel. Compare circumferential;longitudinal; transverse.

radian: Angle equal to 180·π–1 degrees or57.29578 angular degrees.

radiance: A measure of radiant fluxdensity (per unit projected area) perunit solid angle. Radiance isindependent of distance, is measuredin watts per square meter steradian(W·m–2·sr–1) and may describe emittedor received energy. Compare irradiance.

radiant energy: Total energy, in joules, ofelectromagnetic radiation emitted by asource. Radiant energy is determinedby integrating radiant flux withrespect to time.

radiant exitance: Radiant power per area,emitted or reflected from a certainlocation on a surface. Measured inwatts per square meter (W·m–2). Ininfrared and thermal testing,sometimes called radiosity. Compareluminous exitance.

radiant flux: Radiant energy’s rate offlow, measured in watts or joules persecond (J·s–1). Compare luminous flux.

radiant intensity: Electromagnetic fluxemitted per unit solid angle in a givendirection from the source. Measured inwatts per steradian (W·sr–1). Compareluminous intensity.

radiant power: See radiant flux.radiation: The transfer of energy through

propagation of electromagnetic waves.See radiant energy. Compare conduction;convection.

radiation reference source: In infraredand thermal testing, blackbody or othertarget of known temperature andeffective emissivity used as a referenceto obtain optimum measurementaccuracy, ideally, traceable to theNational Institute of Standards andTechnology.

radiation safety officer: Individualsupervising program to provideradiation protection. Therepresentative appointed by thelicensee for liaison with the applicableregulatory agency.14

radio frequency (RF) display: Inultrasonic testing, presentation ofunrectified signals.15 See also A-scan;video presentation.

radiographic screens: In radiographictesting, thin sheets used to intensifythe effect of radiation on films.14 Thescreens can be made of a fluorescentmaterial or a metal such as lead.Metallic screens absorb secondary andscattered radiation, which helps toimprove image quality.

radiographic testing: Use of penetratingradiant energy in the form of X-rays,gamma rays or neutrons fornondestructive testing of objects toprovide images of the objects’interiors. Also called radiography;radiologic testing.

radiography: See radiographic testing.radiological testing: Disused term forradiologic testing.

radiologic testing: Another term forradiographic testing. Compare radiology.

radiology: Study of ionizing radiationand its interaction with material.Compare radiologic testing.

radiometer: Device used to measureirradiance. In nondestructive testing,radiometers are used to measure UV-Aoutput or visible light in watts persquare meter (W·m–2). Used influorescent liquid penetrant andmagnetic particle testing to measureoutput of excitation sources. See alsoirradiance. Compare photometer.

radiometry: Study and measurement ofelectromagnetic radiation emitted by asource or falling upon a surface.Compare photometry.

radiosity: See radiant exitance.range: In ultrasonic testing, maximum

ultrasonic path length that isdisplayed. See also sweep length.15

rankine: Disused scale for absolutetemperature and related to thefahrenheit relative scale. The rankineunit (°R) is equal to 1 °F;0 °R = –459.72 °F.

rarefaction: Of particles in a propagatingmedium, thinning or separation dueto the decompression phase of anultrasonic cycle. Opposite ofcompression. A compressional wave iscomposed of alternating compressionsand rarefactions.18



raster: Repetitive pattern whereby adirected element (a robotic arm or aflying dot on a video screen) followsthe path of a series of adjacent parallellines, taking them successively in turn,always in the same direction (from topto bottom or from left to right),stopping at the end of one line andbeginning again at the start of thenext line. Following a raster patternmakes it possible for electron beams toform video pictures or frames and fora sensor bearing armature to cover apredetermined part of the surface of atest object.

rat’s tooth principle: (1) The tendencyfor hard material on a tooth’s frontsurface to wear more slowly than softmaterial on the back surface, keepingthe edge sharp. (2) Mechanism of wearwhereby adjacent hard and softsurfaces wear at different rates,producing a self-sharpening edge.

rebleed technique: See bleed backtechnique.

receiver: (1) Section of an ultrasonicinstrument that amplifies echoesreturning from the test object. (2) Inultrasonic testing, transducer thatpicks up the echoes.

recommended practice: Set of guidelinesor recommendations. Compare code;standard.19

Recommended Practice No. SNT-TC-1A,Personnel Qualification and Certificationin Nondestructive Testing: Set ofguidelines for employers to establishand conduct a nondestructive testingpersonnel qualification andcertification program. SNT-TC-1A wasfirst issued in 1968 by the Society forNondestructive Testing (SNT, nowASNT) and has been revised every fewyears since.

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

rectified alternating current: See half-wave current and full-wave current.

red mud: Debris (usually oxides of thecontacting metals) of fretting wear,mixed with oil or grease and retainedat or near the site of its formation. Seealso cocoa; wear, fretting.

reference junction: In a thermocouple,the junction of the dissimilar metalsbut not the measurement junction.The reference junction is normallymaintained at a constant referencetemperature.

reference number: In electromagnetictesting, number associated with theimpedance of a coil adjacent to a testsample.

reference standard: (1) In NDT, an objectcontaining known discontinuities atknown distances and used to establisha baseline for comparison andstandardization of nondestructive testinspection equipment. (2) Standard,generally having the highestmetrological quality available at agiven location or in a givenorganization, from whichmeasurements made there are derived.Compare working standard.

reflectance; spectral reflectance: Ratio ofwave energy (radiant flux) reflectedfrom a material to incident waveenergy (incident radiant flux) per unitarea. Compare reflectivity.

reflection: General term for the processby which the incident energy leaves asurface or medium from the incidentside, without change in frequency.Reflection is usually a combination ofspecular and diffuse reflection.19

reflection probe: Coil system that usesboth an excitation and a detection orsensing coil on the same side of thesample.23

reflectivity: Ability of a surface to reflectradiation, expressed as the ratio ρ ofthe intensity of the total energyreflected from a surface to totalradiation on that surface:

For a perfect mirror, reflectivity ρapproaches 1.0; for a blackbody thereflectivity is 0. Compare reflectance;reflection.

reflectometer: Instrument used forquantitative analysis of surfacereflectance and appearance bydescribing surface reflectanceproperties like gloss, roughness andrefractive index.

refracted beam: Beam transmitted in thesecond medium when an ultrasonicbeam is incident at an acute angle onthe interface between two mediahaving different sound speeds. See alsoSnell’s law.15

refraction: Deflection of a wave due to achange in its speed as it passes fromone material to another. For ultrasonicenergy, a change in both direction andmode occurs at acute angles ofincidence. At small angles ofincidence, the original mode and aconverted mode may existsimultaneously in the second medium.See also Snell’s law.

refractive index: Ratio of the speed of theincident wave to that of a refractedwave. It is known as the refractive indexof the second medium with respect tothe first.

ρ ε τ= 1 −− −−



refractometer: Device that measures therefractive index of a liquid. This valuevaries with concentration and hencehas been used to measure hydrophilicremover, coolant or detergentconcentrations.

reject: In ultrasonic testing, minimize oreliminate low amplitude signals (suchas electrical or material noise) so thatother signals may be further amplified.This control can reduce verticallinearity. Also called suppression.15

rejection level: Level above or belowwhich a signal is an indication of arejectable discontinuity.15

relative humidity: Ratio (in percent) ofthe water vapor content in the air tothe maximum content possible at thattemperature and pressure.

relative measurement: Evaluation of aproperty that is based upon somevariable rather than a calibrationstandard. For example, relativeirradiance would evaluate oneradiation source based upon theemission of another radiation source,while absolute irradiance would be acalibrated measurement. Compareabsolute measurement; comparativemeasurement.

reluctance: Resistance of a material tochanges in magnetization. Reciprocalof magnetic permeability.

rem: Roentgen equivalent man. Disusedunit of absorbed radiation dose inbiological matter. It is equal to theabsorbed dose in rads multiplied by thequality factor of the radiation.Compare sievert.14

remanent magnetism: See residualmagnetic field.

remote viewing: (1) Indirect viewing of atest object far from the viewer’simmediate presence — for example,viewing with telemetry or crawlers.The term remote viewing is used in thefields of robotics and surveillance todistinguish conventional from distantviewing tasks. (2) Viewing of a testobject during which the light image ismediated through a system of two ormore lenses (as in a borescope) ortransduced through an electronicsignal (as with a charge coupledcamera). This use of the term remoteviewing in some specifications is amisnomer, intended merely todistinguish borescopy from directviewing. Compare direct viewing;indirect viewing.

repeatability: Ability to reproduce aresult, for example a detectableindication, in separate processings andtests from a constant source.

repetition rate: In ultrasonic testing,number of pulses generated ortransmitted per unit of time (usuallyseconds).

replica: Piece of malleable material, suchas polyvinyl or polystyrene plasticfilm, molded to a test surface for therecording or analysis of the surfacemicrostructure.

replication: Method for copying thetopography of a surface by making itsimpression in a plastic or malleablematerial.

residual magnetic field: Magnetizationremaining in a ferromagnetic materialafter magnetizing force H is reduced tozero.

residual technique: In magnetic particletesting, testing procedure used onlywith highly retentive materials wherea remnant magnetic field is relied onto attract magnetic particles. Comparecontinuous technique.

resistance, electrical (R): Opposition totransmission of electric currentthrough material; ratio of voltage tocurrent. Measured in ohms (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).

resistance, thermal (R): Measure of amaterial’s resistance to the flow ofthermal energy, inversely proportionalto its thermal conductivity k, wherek = 1·R–1.

resistance temperature device: Sensorthat measures temperature by achange in resistance as a function oftemperature.

resistivity (ρ): 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):

resolution: A system’s ability to depicttwo objects or signals in closeproximity as separate from oneanother. Resolution, or resolvingpower, varies with size, distance,sensor characteristics, object shape,object color and contrast. See also linepair.

RG

LA

= =1 ρ

ρσ

=1



resolving power: Ability of detectionsystems to separate two points in timeor distance. Resolving power dependson the angle of vision and thedistance of the sensor from the testsurface. Resolving power in visionsystems is often measured usingparallel lines. Compare resolution.

resonance: Condition in which thefrequency of a forcing vibration(ultrasonic wave) is the same as thenatural vibration frequency of thepropagation body (test object),possibly resulting in large amplitudevibrations.18

resonance technique: In ultrasonictesting, method using the resonanceprinciple for determining speed,thickness or presence of laminardiscontinuities.

resonance testing, process compensated(PCRT): Inspection method, generallycoupled with computer basedresonance spectroscopy, for detectingdiscontinuities based on a change inthe resonance of a component.Resonance changes with mass, shapeand material properties.

resonant frequency: Frequency at whicha body vibrates freely after being set inmotion by some outside force.18

response time: In leak testing, the timerequired for a leak detector signal toreach a specified value after theapplication of a step input.23,18 Thesignal reaches 63 percent of final valuein one time constant.

response factor: In leak testing, responseof a halogen leak detector to3 × 10–7 Pa·m3·s–1 (3 × 10–6 std cm3·s–1)of tracer refrigerant-12 or less, dividedby the response to the same quantityof another tracer gas. Thus, the actualleakage rate of a detected leak willequal the indication of the detectormultiplied by the response factor ofthe specific halogen tracer gas used.The response factor of a mixture oftracer and nontracer gases will be theresponse factor of the tracer divided bythe fraction of tracer gas in the test gas(by volume).

response function: In electromagnetictesting, ratio of response to excitation,both expressed as functions of thecomplex impedance.16

retentivity: Material’s ability to maintainremnant magnetism in the absence ofa coercive field.

retina: In the eye, the tissue on back,inside surface of the eyeball, oppositethe pupil, where light sensitive rodsand cones sense light. See also cone;fovea centralis; iris; pupil; rod.

retinene: See visual purple.rhodopsin: See visual purple.

right hand rule: In magnetic particletesting, a technique for visualizing therelationship between a flowing currentand its induced magnetic field. Whenthe right hand is closed in a fist withthe thumb extended and when currentflows out along the thumb, the fingerspoint in the direction of theself-induced magnetic field.

ringing signals: (1) In ultrasonic testing,closely spaced multiple signals causedby multiple reflections in a thinmaterial. (2) In ultrasonic testing,signals caused by continuedmechanical vibration of a transducer.15

ringing technique: In ultrasonic testing,test technique for bonded structures inwhich unbonds are indicated byincreased amplitude of ringingsignals.15

ringing time: In ultrasonic testing, timethat the ringing signal of a transducercontinues after the electrical drivingforce behind the initial pulse has beenremoved.

rinse: In liquid penetrant testing, processof removing liquid penetrant testingmaterials from the surface of a testobject by means of flooding withanother liquid, usually water. Alsocalled wash when performed afteremulsification.

rise time: In magnetic particle testing, theduration of time for a current sourceto reach its set point.

rockwell hardness testing: Evaluationmethod for determining the hardnessof a material by forcing an indenterinto it under specified conditions.Conditions and indenter type and sizevary with the rockwell hardness scalechosen. Indention depth is related tohardness, and the result is reported asthe material’s rockwell hardness.Compare brinell hardness testing.

rod: Retinal receptor that responds at lowlevels of luminance even down belowthe threshold for cones. At these levelsthere is no basis for perceivingdifferences in hue and saturation. Norods are found in the fovea centralis.19

Concentrated toward the outer regionof the retina, rods assist with mesopicvision and are responsible for scotopicnight vision. Compare cone.

roentgen (R): Disused unit formeasurement of ionizing radiationintensity; amount of ionizingradiation that will generate oneelectrostatic unit in 1 cm–3 of air atstandard atmospheric conditions. Theroentgen has been replaced by an SIcompound unit, coulomb perkilogram (C·kg–1).



roof angle: In a dual-element delay linetransducer for ultrasonic testing, thetilt angle by which the transducerelements of the delay line are orientedto direct the beams of the twoelements to intersect at a specifiedzone in the medium.

root mean square (RMS): Statisticalmeasure of the magnitude of a varyingquantity. Also known as quadraticmean, root mean square is especiallyuseful when data varies betweenpositive and negative (for example, asinusoidal wave).

roots blower: Blower that uses two lobedrotors mounted on parallel shafts withmechanical pumps to obtain greaterpumping speeds and lower pressures.23

rotameter: Meter that uses a float and atapered glass bore to measure flow.23

rusting: Formation of hydrated ironoxides on the surface of a ferrous (forexample, iron or steel) component. Seealso corrosion. Compare scaling.

SSAM: Acronym for scanning acousticmicroscope.

sample and hold: Feature of aninstrument whereby an output signalis maintained at an instantaneousmeasurement value for a specifiedduration after a trigger or until anexternal reset is applied.

sampling, partial: Testing of less than100 percent of a production lot.

sampling, random partial: Partialsampling that is fully random.

sampling, specified partial: Partialsampling in which a particularfrequency or a sequence of sampleselection is prescribed. An example ofspecified partial sampling is the testingof every fifth unit.

saturation: (1) In nondestructive testing,signal amplitude at or above sensor’smaximum capability. (2) In magneticparticle testing, that degree ofmagnetization where a further increasein magnetizing force produces nosignificant increase in magnetic fluxdensity in an object. (3) In visualtesting, relative or comparative colorcharacteristic resulting from a hue’sdilution with white light.

scan angle: For a line scanner, the totalangular scan possible at the targetplane, typically 90 degrees.

scanning: Movement of a sensor over thesurface of a test object in a controlledmanner so as to achieve completecoverage.

scan position accuracy: For a linescanner, the precision with whichinstantaneous position along the scanline can be set or measured.

scalar quantity: Quantity completelyspecified by a single number and unit.Example scalar quantities includemass, charge, temperature, electricpotential at a point inside a mediumand the distance between two pointsin three-dimensional space. Comparevector quantity.

scale: (1) Layer of adherent oxidationproduct on the surface of a metal,caused by elevated temperatureexposure to an oxidizing atmosphere.(2) Layer of insoluble constituents ona metal surface, as in cooling tubesand water boilers.22 Compare corrosion;rusting.

scattering: Random reflection andrefraction of energy caused byinteraction with material it strikes orpenetrates. See also backscatter;compton scatter; fogging; mottling.

schlieren system: In ultrasonic testing,optical system used for visual displayof an ultrasonic beam passing througha transparent medium.15

scintillation: In radiographic testingemission of light of specificfrequencies after the absorption ofelectromagnetic radiation, such asX-rays or gamma rays.

scoring: (1) Formation of deep scratchesin the direction of sliding.(2) Reducing the thickness of a partalong a line to weaken it purposely ata specific location.22

scotopic vision: Dark adapted vision,using only the rods in the retina,where differences in brightness can bedetected but differences in huecannot. Vision is wholly scotopicwhen the luminance of the testsurface is below 3 × 10–5 cd·m–2

(2.7 × 10–6 cd·ft–2). Also known asparafoveal vision. See also darkadaptation. Compare mesopic vision;photopic vision.

scuff mark: Area covered by finescratches, usually due to rubbing ofone piece against another.

seam: Linear surface discontinuity, oftenoriented parallel to the component’saxis and produced during metalrolling. Seams can originate fromingot blowholes, cracks or improperforging.

secondary magnetic flux: Inelectromagnetic testing, magnetic fluxdue to induced flow of eddy currents.

sector: For a line scanner, a portion of thetotal scan angle over whichmeasurement is made at the targetplane.

seebeck effect: See thermoelectric effect.segregation: In manufacturing,

nonuniform distribution of alloyingelements, impurities or microphases.



selectivity: In electromagnetic testing,characteristic of a test system, ameasure of the extent to which aninstrument can differentiate betweenthe desired signal and disturbances ofother frequencies or phases.23

self-demagnetizing factor: Estimate ofthe resistance of a test object tomagnetization, the resistance beingdue to the proximity of magneticpoles of opposite polarity. For coilmagnetization, the internalmagnetization within a low L·D–1 ratiotest object is opposite of the coil’smagnetic field and a lower distancebetween poles results in a greaterinternal resistance.

self emulsifiable: See penetrant, waterwashable.

self-inductance: Ratio of magnetic fluxformed around a conductor to theamount of current passing through astraight or coiled conductor. Self-inductance is measured in henries,where one henry equals one weber perampere (1 H = 1 Wb·A–1). See alsoinductance.

sensitivity: Ability of a sensor or systemto distinguish a signal or indicationfrom background noise. See alsoprobability of detection.

sensitization: (1) In materials science,precipitation of chromium carbides inthe grain boundaries of a corrosionresistant alloy, resulting inintergranular corrosion that wouldotherwise be resisted. (2) Inradiographic testing, condition ofexposed silver halide emulsion inradiographic film before development.

sensor: Device that detects a materialproperty or mechanical behavior (suchas radiation or displacement) andconverts it to an electrical signal orphysical change.

settling test: In magnetic particle testing,one technique for determining theconcentration of magnetic particles ina new bath or to check forcontamination or other bathproblems. See also centrifuge tube;particle concentration. Comparemagnetic stripe card.

shadow: In ultrasonic testing, region in atest object that cannot be reached byultrasonic energy traveling in a givendirection. Shadows are caused bygeometry or the presence ofintervening large discontinuities.

shadow casting: In visual testing,technique of vapor depositing a thinmetal film onto a replica at an obliqueangle in order to obtain a micrographof a test surface of an opaque testobject.

shared flux indicator: In magneticparticle testing, device held inintimate contact with test objectduring active magnetization to showthe direction of magnetic induction.Examples include the bertholdpenetrameter, flexible laminated strip andpie gage. See also quantitative qualityindicator.

shear wave: See wave, transverse.shielding: (1) In radiographic testing,

material or object used to reduceintensity of or exposure to penetratingradiation. (2) In electromagnetictesting, conducting or magneticmaterial (or a combination of both)placed so as to decrease susceptibilityto interference.

shoe: In ultrasonic testing, device used toadapt a straight beam transducer foruse in a specific type of testing,including angle beam or surface wavetests and tests on curved surfaces.15

See also wedge.shot: In magnetic particle testing, the

period of time when current is flowingthrough the test object or inductioncoil. Shot duration and the number ofshots required for testing may bevaried. See also coil technique; currentflow technique.

shoulder: In manufacturing, cylindricalmetal component (pipe) surface,machined to receive threadingindentations but in fact not threaded,where the thread stops on the outsidesurface of the pipe.

shrink: In nondestructive testing, internalrupture occurring in castings due tocontraction during cooling, usuallycaused by variations in solidificationrates in the mold. Includes shrinkagesponge, small voids (stringers orbunches) or a fingerprint pattern ofsemifused seams. Also applied tosurface shrinkage cracks.

SH wave: See wave, transverse horizontal.siemens per meter (S·m–1): SI unit ofconductivity.

sievert (Sv): SI unit for measurement ofexposure to ionizing radiation; replacesrem. 1 Sv = 1 J·kg–1 = 100 rem.

SI (International System of Units):International measurement system inwhich the following seven units arebasic: meter, mole, kilogram, second,ampere, kelvin and candela.

signal electrode: In visual testing,transparent conducting film on theinner surface of a vidicon’s faceplateand a thin photoconductive layerdeposited on the film.

signal processing: Acquisition, storage,analysis, alteration and output ofdigital or analog data.



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

signal: Physical quantity, such as voltage,that contains relevant information.

silicon controlled rectifier (SCR): Solidstate electronic component used tovary power output in an arclessmanner. The power waveform from asilicon controlled rectifier will containspikes: conversion between peak, rootmean square and average is notstraightforward across the outputrange.

skin depth: See standard depth ofpenetration.

skin effect: In magnetic particle testingand electromagnetic testing, term usedto describe the penetration ability ofcyclical current or magnetization as afunction of frequency, conductivityand relative permeability. In magneticparticle testing, skin effect refers toalternating current’s inability topenetrate deeper than 1 to 3 mm(0.04 to 0.12 in.) with typical testingvariables.25 See also skin depth.

skip distance: In angle beam tests of plateor pipe in ultrasonic testing, thedistance from the sound entry pointto the exit point on the same surfaceafter reflection from the back surface.Also called V path.15

slag: Nonmetallic product resulting fromthe mutual dissolution of flux andnonmetallic impurities in smelting,refining and welding.

slit response function: In infrared andthermal testing, measure of themeasurement spatial resolution(IFOVmeas) of an infrared scanner orimager. See modulation transfer function.

smoothing: In image processing, linearcombination of pixel values to smoothabrupt transitions in a digital image.Also called low pass filtering.

Snell’s law: In optics and acoustics, thephysical law that defines therelationship between the angle ofincidence and the angle of refraction.The relationship’s numeric expressionis the index of refraction.

SNT-TC-1A: See Recommended Practice No.SNT-TC-1A.

soak time: In liquid penetrant testing,period of time when the emulsifierremains in contact with the liquidpenetrant on the surface of the testobject. Soak time ceases when theliquid penetrant emulsifier is quenchedwith water or completely removed bywater rinsing. Also called emulsificationtime. Compare dwell time. (2) In leaktesting, the period of time betweenwhen the system or componentreaches test pressure and either when

the leak detector solution is applied tothe surface or when the leak detector isused to scan that surface.18

solvent: (1) In chemical cleaning, avolatile liquid with the ability todissolve another material. (2) In liquidpenetrant testing, the liquidsometimes used to preclean and/orremove excess liquid penetrant fromthe specimen. See also solvent removal;visible dye penetrant testing.

solvent removal: In liquid penetranttesting, process of removing excessliquid penetrant from the surface of atest object by hand wiping with asolvent dampened cloth. See also visibledye penetrant testing.

spalling: Cracking or flaking of smallparticles of metal, usually in thinlayers, from the surface of an object.

spatial resolution: Spot size in terms ofworking distance. In an infraredradiation thermometer, spot size isexpressed in milliradians or as a ratioof the target spot size (containing95 percent of the radiant energy, byconvention) to the working distance.In scanners and imagers it is mostoften expressed in milliradians.

specification: Set of instructions orstandards invoked to govern theproperties, results or performance of aspecific set of tasks or products.Compare code; recommended practice;standard.19

specific examination: In certification ofnondestructive testing personnel, awritten examination that addresses thespecifications and products pertinentto the application. Compare generalexamination; practical examination.

specific gravity: Unitless ratio of thedensity of a material divided by thedensity of water. Water has a densityof about 1 g·cm–3, or 1000 kg·m–3, at15.6 °C (60 °F).

spectral: Prefix used to denote a variablethat changes as a function ofwavelength.

spectral power distribution: Radiantpower per unit wavelength as afunction of wavelength. Also knownas spectral energy distribution, spectraldensity and spectral distribution. SeePlanck’s law.

spectral reflectance: Radiant fluxreflected from a material divided bythe incident radiant flux.

spectral responsivity: Measure of aphotometric or radiometric sensor’ssensitivity over a wavelength range ofinterest, often presented as percentversus wavelength. Photometricsensors should exhibit a bell shapedspectral responsivity curve over thevisible light range, whereasradiometric sensors may exhibit a flator other response curve.



spectral transmittance: Fraction ofincident radiant flux of a givenwavelength that passes through amedium. See also spectrophotometer.

spectrofluorometer: Instrument capableof determining the fluorescentexcitation and fluorescent emissionspectra of a sample.

spectrometer: Device used to characterizethe emission spectrum of a source ofelectromagnetic radiation in countsper integration time, relativeirradiance or absolute irradiance versuswavelength or frequency.

spectrometer, mass: Instrument capableof measuring the mass-to-charge ratio ofa charged particle. The device firstionizes the particle (imparts a positivecharge) and then measures theaccelerated particle’s deflection as itpasses through a known magneticfield. In leak testing, a commoninstrument encountered is a heliummass spectrometer.

spectrophotometer: Instrument capableof measuring the amount of visiblelight reflected from or transmittedthrough a sample. Aspectrophotometer may, for example,be used to measure the spectraltransmittance of an optical filter.

spectrophotometry: Quantitativemeasurement of visible light reflectedfrom or transmitted through a sampleas a function of wavelength.

spectroradiometer: Instrument used tomeasure the spectral powerdistribution of a radiation source.Common spectroradiometers observethe ultraviolet, visible light andinfrared wavelengths.

spectroradiometry: Quantitativemeasurement of electromagneticradiation. Spectroradiometryencompasses absolute radiometricmeasurements of any wavelength,including visible light.

spectroscopy: The study of how radiantenergy and a medium interact withrespect to wavelength or frequency.See also spectrofluorometer;spectrophotometer; spectroradiometer.

spectrum: (1) Amplitude distribution offrequencies in a signal.(2) Representation of radiant energy inadjacent bands of hues in sequenceaccording to the energy’s wavelengthsor frequencies. A rainbow is a wellknown example of a visible spectrum.

spectrum response: In infrared andthermal testing, amplification (gain) ofa receiver over a range of frequenciesor wavelengths.

specular: Pertaining to a mirrorlikereflective finish, as of a metal.Compare lambertian.

specular reflection: When reflectedwaves and incident waves form equalangles at the reflecting surface.Compare diffuse reflection.

speed of light: Speed of all radiantenergy, including light, is2.997925 × 108 m·s–1 in vacuum(approximately 186 000 mi·s–1). In alltransparent materials the speed is lessand varies with the material’s index ofrefraction, which itself varies withwavelength.19

speed of vision: Reciprocal of theduration of the exposure time requiredfor something to be seen.19

spot size: (1) In infrared and thermaltesting, instantaneous size (diameterunless otherwise specified) of the areaat the target plane that is beingmeasured by the instrument. (2) Ininfrared thermometry, the valuespecified by most manufacturers tocontain 95 percent of the radiantenergy of an infinitely large target ofthe same temperature and emissivity.

spot size, effective focal: In radiographictesting, size and geometry of focal spotafter target interaction. Viewed fromalong the primary beam central axis atthe target the effective focal spotwould appear nearly square andsmaller than the actual focal spot areacovered by the electron stream.

spurious echo: In ultrasonic testing,general term denoting any indicationthat cannot be associated with adiscontinuity or boundary at thelocation displayed. Also called parasiticecho.

SQUID: Acronym for superconductingquantum interference device, asensitive detector of magnetic fieldsusing quantum effect.

squint angle: In ultrasonic testing, angleby which an ultrasonic beam axisdeviates from the probe axis. Comparedivergence.

squirter: See water column.standard: (1) A physical object with

known material characteristics used asa basis for comparison or calibration.(2) A concept established by authority,custom or agreement to serve as amodel or rule in the measurement ofquantity or the establishment of apractice or procedure. (3) Document tocontrol and govern practices in anindustry or application, applied on anational or international basis andusually produced by consensus. Seealso acceptance standard, workingstandard and reference standard.1



standard atmospheric conditions:Standard temperature and pressure.Atmospheric pressure of 101.325 kPa(14.6959 lbf·in.–2). Temperature of20 °C (293.15 K, 68 °F or 527.67 °R).The density of dry air at theseconditions is 1.2041 kg·m–3

(0.07517 lbm·ft–3).standard depth of penetration: In

electromagnetic 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.

standard leakage rate: In optical leaktesting of hermetically sealed packages,the quantity of dry air at 25 °C (77 °F)flowing (in atmospheric cm3·s–1)through a leak or multiple leak pathswhen the high pressure side is at100 kPa (1 atm or 760 torr absolute)and the low pressure side is at pressurenot greater than 100 Pa (1 torrabsolute).29 An equivalent standardleakage rate of a given sealed package,with a measured leakage rate, is theleakage rate, of the same package withthe same leak geometry, that wouldexist under the standard leakage rateconditions.

standard: Object, document or conceptestablished by authority, custom oragreement to serve as a model or rulein the measurement of quantity or theestablishment of a practice orprocedure.15 See also reference standardand acceptance standard.

standardization, instrument: Adjustmentof instrument readout before use to aspecified reference value. Comparecalibration; verification.

standard observer response curve: Seeeye sensitivity curve.

star burst panel: See penetrant systemmonitor.

statistical process control: Application ofstatistical methods such as controlcharts, continuous improvement anddesigned experiments to manage andobserve the outcome of a repeatedprocess.

steel: Iron alloy, usually with less thantwo percent carbon.

stefan-boltzmann law: Relationshipgoverning the wavelengthindependent rate of emission ofradiant energy per unit area. The lawrelates the total radiation intensity tothe fourth power of absolutetemperature and emissivity of thematerial surface. For example,intensity (heat flow) from a copperblock at 100 °C (212 °F) is 300 W·m–2

(95 BTU·ft–2·h–1). (The stefan-boltzmann constant for photonemission = 1.52041 × 1015

photon·s–1·m–2·K–2). See also blackbody;Planck’s law; Wien’s distribution law.

step wedge; stepped wedge: (1) Inradiographic testing, device with stepsof various thicknesses in the range oftested parts’ thicknesses, for theradiographic testing of parts havingthickness variations or complexgeometries. The stepped wedge mustbe made of material radiographicallysimilar to that of the radiographic testobject and may include penetrametricfeatures (such as calibrated holes) inany or all steps. (2) In ultrasonictesting, device made from a materialacoustically similar to the test object,with steps of various thicknesses thatare used to standardize or calibrate aflaw detector or thickness gage.

stereo photography: Photographictechnique involving the capture andviewing of two binocular images ofthe same object to reconstruct itsthree-dimensional image.

stick welding: See welding, shielded metalarc.

stiffness: The ability of a structure orshape to resist elastic deformation. Fora given shape the stiffness scales withits moment of inertia (varies withcross sectional dimensions). Comparehardness; modulus of elasticity.

straight beam: Ultrasonic wave travelingnormal to the test surface. Compareangle beam.15

strain: Deflection or alteration of theshape of a material by external forces.

stress: (1) In physics, the action in amaterial that resists external forcessuch as tension and compression.(2) Force per unit of area.

stress concentration: Region where forceper unit area is elevated, often becauseof geometric factors or cracks. Alsoknown as a stress raiser.

stress raiser: Contour or property changethat locally increases stress magnitude.

stress riser: See stress raiser.stringer: In wrought materials, an

elongated configuration ofmicroconstituents or foreign materialaligned in the direction of working.Commonly, the term is associatedwith elongated oxide or sulfideinclusions in steel.



substrate: Layer of metal underlying acoating, regardless of whether thelayer is base metal.

subsurface discontinuity: Discontinuitynot open to the surface. See alsonear-surface discontinuity.

subsurface fatigue: Fatigue cracking thatoriginates below the surface. Usuallyassociated with hard surfaced or shotpeened parts but may occur any timesubsurface stresses exceed surfacestresses.

suppression: See reject.surface tension: Characteristic of liquids

where the outer surface contracts tothe smallest possible area.

surface wave: See wave, rayleigh.survey meter: In radiographic testing,

portable instrument that measures rateof exposure dose or ionizing radiationintensity.14

SV wave: Shear vertical wave.sweep: In ultrasonic testing, uniform and

repeated movement of a spot acrossthe display screen to form thehorizontal baseline. Also called timebase.

sweep delay: (1) In ultrasonic testing,delay in time of starting the sweepafter the initial pulse. (2) Control foradjusting the time.15 Also called timedelay.

sweep length: In ultrasonic testing,length of time or distance representedby the horizontal baseline on anA-scan.15

swinging field magnetization: One formof multidirectional magnetization wheretwo time-varying magnetic fields arecombined such that the resultantmagnetization vector rapidly rotatesthrough an angle.

system: (1) A combination of testmaterials, such as a liquid penetrantand an emulsifier, that are furnishedby the same manufacturer and arequalified together. (2) Device or set ofdevices used for a test.

TTAM panel: See penetrant system monitor.tape head probe: In electromagnetic

testing, head of a tape recorder used asan eddy current coil; a type ofhorseshoe coil.

tarasov etching technique: Visual testtechnique for detection of adhesivewear and grinding burn in hardenedsteels. The tarasov technique usesspecific acid etching solutions.

temperature: Measure of the intensity ofparticle motion in degrees celsius (°C),degrees fahrenheit (°F) or, in theabsolute scale, kelvin (K) or degreesrankine (°R). An increment of 1 K = 1°C = 1.8 °R = 1.8 °F. Compare heat.

tempering: In materials science, a processof heating a metal, alloy or glass toalter its properties. Hardened steel isoften tempered to improve ductility,and aluminum is tempered to increasestrength, whereas glass is tempered tobalance internal stresses.

tesla (T): SI derived unit of measure formagnetic flux density. 1 T = 1 Wb·m–2

= 104 G.tesla meter: Magnetometer used to

measure active or residual magneticinduction in the location anddirection of interest. Tesla metergenerally use hall effect sensors, whichmay be transverse or axial in type. Seealso magnetic flux meter; hall effect.

test object, test surface: Physical part orspecimen subject to nondestructivetesting.

thermal: Physical phenomenon of heatinvolving conduction, convection orradiation.

thermal conductivity vacuum gage:Instrument that operates on principlethat as gas molecules are removedfrom a system, the amount of heattransfer by conduction is reduced. Thisrelationship is used to indicateabsolute pressure. See also conductivity,thermal.

thermistor: Temperature detector, usuallya semiconductor, whose electricalresistance decreases predictably andnonlinearly with increasingtemperature. The coefficient ofelectrical resistance with temperatureis typically on the order of –4 percentK–1. Compare thermocouple.

thermistor bolometer, infrared:Thermistor configured to collectradiant infrared energy; a type ofthermal infrared detector.

thermocouple: Device for measuringtemperature based on the fact thatopposite junctions between certaindissimilar metals develop an electricalpotential when placed at differenttemperatures. See also thermoelectriceffect. Compare thermistor.

thermoelectric effect: Phenomenon thatexplains the operation ofthermocouples; that in a closedelectrical circuit made up of twojunctions of dissimilar metalconductors, a direct current will flowas long as the two junctions are atdifferent temperatures. Thephenomenon is reversible; if thetemperatures at the two junctions arereversed, the flow of current reverses.Also called seebeck effect.



thermography: In infrared and thermalinspection, technique that usesinfrared radiation to seekdiscontinuities in materials,components and structures.Thermography may be active (pulsedthermography or thermosonics) orpassive (thermal wave imaging orinfrared thermography.

thermogram: In infrared and thermaltesting, thermal map or image of atarget where the gray tones or colorhues correspond to the distribution ofinfrared thermal radiant energy overthe surface of the target (qualitativethermogram); when correctlyprocessed and corrected, a graphicrepresentation of surface temperaturedistribution (quantitativethermogram).

thermoluminescent dosimetry: Inradiographic testing, means ofmeasuring ionizing radiation dose byusing a material that stores energy dueto irradiation, which energy can bemeasured as light emission when thematerial is heated.

thermomechanical coupling: Interactionbetween mechanical and thermalbehaviors of materials. For examplethe temperature of the alloy maychange with applied force, or themechanical response may change withalloy temperature.

thermometer: Any device used formeasuring temperature.

thermopile: Device constructed by thearrangement of thermocouples inseries to add the thermoelectric effectvoltage. A radiation thermopile is atype of thermal infrared detector, athermopile with junctions arranged tocollect infrared radiant energy from atarget.

three-way sort: In electromagnetictesting, a sort based on a test objectsignal response above or below twolevels established by three or morecalibration standards. Compare two-way sort.23

threshold: (1) A value in a phenomenonwhere a large change of output occurs.(2) Setting of an instrument thatcauses it to register only those changesin response greater or less than aspecified magnitude. See thresholding.

thresholding: Digital data processingtechnique that reduces a gray levelimage into a binary (black and white)image.

thresholding, adaptive: Threshold valuevarying with inconstant backgroundgray level.

threshold, resolution: Minimumdistance, expressed in minutes of arc,between a pair of points or parallellines when they can be distinguishedas two, not one,. Vision acuity, in sucha case, is the reciprocal of one-half ofthe period expressed in minutes.19

through-transmission technique: (1) Inultrasonic testing, a test technique inwhich ultrasonic energy is transmittedthrough the test object and receivedby a second transducer on theopposite side. Changes in receivedsignal amplitude are taken asindications of variations in materialcontinuity. Compare angle beam;straight beam. (2) Of or pertaining toelectromagnetic techniques where theexcitation field penetrates the testobject so that the detected signal isresponsive to features external to oron the opposite surface.

thyristor: See silicon controlled rectifier.time constant: (1) Time it takes for any

sensing element to respond to 63.2percent of a step change at the targetbeing sensed. (2) In infrared sensingand thermography, the time constantof a detector is a limiting factor ininstrument performance, as it relatesto response time.

time of flight: In ultrasonic testing, timefor an acoustic wave to travel betweentwo points. For example, the timerequired for a pulse to travel from thetransmitter to the receiver viadiffraction at a discontinuity edge oralong the surface of the test object.

tone burst: In ultrasonic testing, wavetrain consisting of several cycles of thesame frequency.

tool mark: Shallow indentation or groovemade by the movement ofmanufacturing tools over a surface.Compare gouge; nick.

toroidal magnetization: See field flowmagnetization; induced currentmagnetization.

trace: Line formed by electron beamscanning from left to right on a videoscreen to generate a picture.

tracer: In leak testing, a gas that is sensedas it penetrates an aperture.

transducer: (1) In ultrasonic testing andacoustic emission testing, a device thatconverts mechanical energy toelectrical output and vice versa.(2) Piezoelectric device that convertsthe physical parameters of an acousticwave into an electrical signal ofvoltage versus time. May also be calledsensor or probe.

transducer, air-coupled: See transducer,noncontact.



transducer, angle beam: In ultrasonictesting, a probe that transmits orreceives ultrasonic energy at an acuteangle to the surface. This may be doneto achieve special effects such assetting up transverse or surface wavesby mode conversion at an interface.18

transducer, array: Ultrasonic transducermade up of several piezoelectricelements individually connected sothat the signals they transmit orreceive may be treated separately orcombined as desired. See also phasedarray.

transducer, contact: In ultrasonic testing,transducer used in the ultrasoniccontact technique.

transducer damping: In ultrasonictesting, material bonded to the back ofa piezoelectric element of anultrasonic transducer to limit theduration of vibrations.18

transducer, differential: In acousticemission testing, piezoelectric twin-element or dual-pole transducer, theoutput poles of which are isolatedfrom the case and are at a floatingpotential.

transducer, electromagnetic acoustic(EMAT): In electromagnetic testingand ultrasonic testing, electromagneticdevice using lorentz forces andmagnetostriction in conductive andferromagnetic materials to generateand receive acoustic signals forultrasonic nondestructive tests.

transducer element: In an ultrasonictransducer, the piezoelectric crystal tobe coupled to the test surface. Alsocalled the crystal.

transducer, flat response: In acousticemission testing, acoustic transducerwhose frequency response has noresonance or characteristic responsewith its specified frequency band (thebandwidth to –3 dB being defined)and the ratio between the upper andlower limits of the frequency bandbeing typically not less than 500 kHz.

transducer, focused beam: In ultrasonictesting, immersion transducerproducing a sound beam thatconverges to a cross section smallerthan that of the transducer element.Focused beam transducers may bespherically (point) or cylindrically(line) focused and have varying focaldistances.

transducer, immersion: In ultrasonictesting, transducer type used in a watercolumn, water jet or the ultrasonicimmersion technique. Immersion probesmay be planar or focused beam.

transducer, noncontact: In ultrasonictesting, a sensor designed for wavepropagation through gas as opposed topropagation through couplant or water.Such transducers, with frequenciesbetween 50 and 400 kHz, are usefulfor inspection of water incompatiblematerials or for proximity sensing.

transducer, pulser: In acoustic emissiontesting, transducer used as an artificialsource, introducing a repeatable,transient signal to calibrate and verifyan acoustic emission processor.

transducer relative sensitivity: Inultrasonic testing and acousticemission testing, responsivity of anacoustic transducer to a given source.

transducer, resonant: Specialized form ofundamped transducer that usesmechanical amplification due to aresonant frequency (or several closeresonant frequencies) to give highsensitivity in a narrow band, typically±10 percent of the principal resonantfrequency at the –3 dB points. Suchtransducers have high output andlonger ringing time.

transducer, single-ended: Piezoelectricsingle-element transducer, the outputpole of which is isolated from thecase, the other pole being at the samepotential as the case.

transducer, send/receive: Transducerconsisting of two piezoelectricelements mounted side by sideseparated by an acoustic barrier. Oneelement transmits; one receives.18

transducer, wheel: Device that couplesultrasonic energy to a test objectthrough the rolling contact area of awheel containing a liquid and one ormore transducers.15

transducer, wideband: Transducer whoseresponsivity to surface displacementsis flat over a wide band.

transfer calibration: In infrared andthermal testing, technique forcorrecting a temperature measurementor a thermogram for various errors byplacing a radiation transfer standardadjacent to the target.

transfer function: In acoustic emissiontesting, description of changes to thewaves arising as they propagatethrough the medium or, for atransducer, the relationship betweenthe transducer output signal and thephysical parameters of the wave at thesource.

transfer standard: Precision radiometricmeasurement instrument with NISTtraceable calibration used to calibrateradiation reference sources.

transient heat flow: Heat flow occurringduring the time it takes an object toreach thermal equilibrium or steadystate.



transition flow: Phenomenon that occurswhen the mean free path of gas isabout equal to the cross sectionaldimension of a leak or the tubethrough which flow is occurring.Compare choked flow.

transmission angle: In ultrasonic testing,incident angle of a transmittedultrasonic beam. It is zero degreeswhen the beam is perpendicular(normal) to the test surface.18

transmission characteristics: Inultrasonic testing, test objectcharacteristics that influence thepassage of ultrasonic energy, includingscattering, attenuation or surfaceconditions.

transmission technique: Seethrough-transmission technique.

transmissivity: In infrared and thermaltesting, proportion τ of infraredradiant energy impinging on anobject’s surface, for any given spectralinterval, that is transmitted throughthe object:

where τ is transmissivity, ε isemissivity and ρ is reflectivity. For ablackbody, transmissivity = 0.Transmissivity is the internaltransmittance per unit thickness of anondiffusing material. See alsotransmittance, spectral.

transmitter: (1) Transducer that emitsultrasonic energy. (2) Electrical circuitsthat generate the signals emitted bythe transducer.

transverse: Oriented at a right angle(normal) to the long axis. Comparecircumferential; longitudinal; radial.

troland: Unit of retinal illuminance equalto that produced by a surface whoseluminance is 1 cd·m–2 when the pupilmeasures 1 mm2. Unit is convenient asa method for correcting photometricmeasurements of luminance valuesimpinging on the human eye byscaling them by the pupil size.

tubing string: Pipe with which oil or gashas contact as it is brought to theEarth’s surface. See also casing; casingstring.

tungsten inert gas (TIG) welding: Seewelding, gas tungsten arc.

two-color pyrometer: See pyrometer, ratio.two-transducer technique: See pitch catchtechnique.

two-way sort: Electromagnetic sort basedon a test object signal response aboveor below a level established by two ormore calibration standards. Comparethree-way sort.23

Uultrasonic absorption: Damping or

dissipation of ultrasonic waves as theypass through a medium.18 See alsoattenuation coefficient.

ultrasonic spectroscopy: Analysis of thefrequency content of an acousticwave. Generally performedmathematically using a fast fouriertransform.

ultrasonic spectrum: Acoustic frequencyrange, usually from 20 kHz to 50 MHzbut sometimes much higher in specialapplications.

ultrasonic: Of or relating to acousticvibration frequencies greater thanabout 20 kHz.15

ultrasonic testing (UT): Method ofnondestructive testing, using acousticwaves at inaudibly high frequencies atthe interrogating energy.

ultraviolet borescope: See borescope,ultraviolet.

ultraviolet radiation (UV):Electromagnetic radiation withwavelengths between 40 and 400 nm.See also irradiance; UV-A.

ultraviolet radiometer: See radiometer.undercut: In welding, undesirable groove

left unfilled by weld metal, createdduring welding and located in baseplate at the toe of a weld.

Unified Numbering System (UNS):Alphanumeric system for identifyingalloys according to a registrymaintained by ASTM Internationaland SAE International.

unity: In mathematics, a value ofone (1.0).

U-shaped coil: See coil, horseshoe.UV-A: Electromagnetic radiation with

wavelengths between 315 and400 nm. Fluorescent nondestructivetesting has historically used ultravioletenergy centered at 365 nm. See alsoirradiance; radiometer.

UV-A filter: Device used to modify theemission spectrum from an ultravioletradiation source by eliminating mostvisible light and all higher energyultraviolet radiation (UV-B and UV-C).

Vvalley hold: Feature of an instrument

whereby an output signal ismaintained at the lowestinstantaneous measurement for aspecified duration; opposite of peakhold.

τ ε ρ= 1 −− −−



vector quantity: Any physical quantitywhose specification involves bothmagnitude and direction and thatobeys the parallelogram law ofaddition. Example vector quantitiesinclude: displacement, force, velocity,acceleration and momentum.Compare scalar quantity.

verification: To check for discrepanciesbetween a standard and the unit andto adjust the device so that readingsfall within tolerance limits. Comparecalibration.

vertical limit: In ultrasonic testing,maximum useful readable level ofvertical indication on an A-scan.

vertical linearity: See linearity, amplitude.video: Pertaining to the transmission and

display of images in an electronicformat that can be displayed on amonitor or screen.

video presentation: In ultrasonic testing,display presentation in whichradiofrequency signals have beenrectified and usually filtered. Compareradio frequency presentation.15

videoscope: Jargon for video borescope. Seeborescope, video.

vidicon tube: Analog television tube thatuses the photoconduction method.Compare image orthicon; pyroelectricvidicon.

vigilance decrement: Degradation ofreliability during performance ofvisual activities over a period of time.See also human factors; psychophysics.

viscosity: The resistance of a fluid todeformation by shear or tensile stress.Lower viscosity equates to greaterfluidity.

viscous flow: In leak testing, flow of gasor gas mixtures through a leak or ductunder conditions such that the meanfree path is smaller than the crosssection of the leak or opening. Viscousflow may be either laminar orturbulent and is most likely to occurduring leak tests at atmospheric orhigher pressures. With vacuumconditions, the flow of tracer gases tothe leak detector element is usually bydiffusion, resulting in slow response toleaks being probed by a tracer jet.

visibility: Quality or state of beingperceivable by the eye. Outdoors,visibility is usually defined in terms ofthe distance at which an object can bejust perceived by the eye. Indoors,visibility usually is defined in terms ofthe contrast or size of a standard testobject, observed under standardizedview conditions and having the samethreshold as the given object.19

visible dye penetrant: Liquid penetrantcharacterized by its intense visiblecolor, usually red. Also called colorcontrast or nonfluorescent penetrant.Compare liquid penetrant, fluorescent.30

visible light: Any radiant energy with awavelength between 380 and 780 nm.Compare white light.

vision: Perception by eyesight. See darkadaptation; far vision; machine vision;mesopic vision; near vision; peripheralvision; photopic vision; scotopic vision;speed of vision.

vision acuity: Ability to distinguish finedetails visually at a given distance.Quantitatively, it is the reciprocal ofthe minimum angular separation inminutes of two lines of widthsubtending one minute of arc whenthe lines are just resolvable asseparate.19

visual acuity: See vision acuity.visual angle: Angle formed by lines

drawn from center of eye subtendedby an object or detail at the point ofobservation. It usually is measured inminutes of arc.19

visual efficiency: Reliability of a visualsystem. The term visual efficiency uses20/20 near vision acuity as a baselinein the United States for 100 percentvisual efficiency.

visual field: Locus of objects or points inspace that can be perceived when headand eyes are fixed. The field may bemonocular or binocular.19

visual perception: Interpretation ofimpressions transmitted from theretina to the brain in terms ofinformation about a physical worlddisplayed before the eye. Visualperception involves any one or moreof the following: recognition of thepresence of something (object,aperture or medium); identifying it;locating it in space; noting its relationto other things; identifying itsmovement, color, brightness orform.19

visual performance: Quantitativeassessment of the performance of avisual task, taking into considerationspeed and accuracy.19

visual purple: Chromoprotein calledrhodopsin, the photosensitive pigmentof rod vision. The mechanism ofconverting light energy into nerveimpulses is a photochemical process inthe retina. Chromoprotein istransformed by the action of radiantenergy into a succession of products,finally yielding the protein called opsinplus the carotenoid known as retinene.

visual task: Appearance and immediatebackground of those details andobjects that must be seen for theperformance of a given activity. Theterm visual task is a misnomer becauseit refers to the visual display itself andnot the task of extracting informationfrom it. See visual field.



visual testing (VT): Method ofnondestructive testing usingelectromagnetic radiation at visiblefrequencies.

voids: Hollow spots, depressions orcavities. See also discontinuity; porosity.

volt (V): Measurement unit of electricpotential.

V path: See skip distance.

Wwater break free: Surface that is

chemically and physically clean, uponwhich applied water will momentarilyform an even, continuous film. Seealso clean.

water column: In ultrasonic testing, tubefilled with water and attached to thefront of a transducer to couple anultrasonic beam to a test object. Delayline between an initial pulse and afront surface signal. See also delay line.

water jet: In ultrasonic testing,unsupported stream of water carryingultrasonic signals between thetransducer and the test object surface.Also called a squirter or water column.

water path: In immersion testing or witha water column in ultrasonic testing,the distance from the transducer faceto the test surface.15

wave, compressional: Wave in whichparticle motion in the material isparallel to the wave propagationdirection. Also called longitudinal wave.

wave, continuous: In ultrasonic testing, awave of constant amplitude andfrequency.

wavefront: In ultrasonic testing, a wavedisturbance or the locus of pointshaving the same phase.15

waveguide: Device to transmit elasticenergy from a test object to a remotetransducer. For example, a wire joinedat one end to a test object and at theother end to a transducer.

wave, lamb: Type of ultrasonic wavepropagation in which the wave isguided between two parallel surfacesof the test object. Mode and velocitydepend on the product of the testfrequency and the thickness. Alsocalled plate wave.

wavelength: Distance between repeatingvalues of a wave. For example, thedistance from one peak to the nextpeak on a sine wave. Comparefrequency.

wave, longitudinal: Wave in whichpoints of same phase lie on parallelplane surfaces.20

wave, rayleigh: Ultrasonic wave thatpropagates along the surface of a testobject. The particle motion is ellipticalin a plane perpendicular to thesurface, decreasing rapidly with depthbelow the surface. The effective depthof penetration is considered to beabout one wavelength. Also calledsurface wave.

wave, spherical: In ultrasonic testing andleak testing, acoustic wave in whichpoints of the same phase lie onsurfaces of concentric spheres.20

wave, standing: Acoustic wave in whichthe energy flux is zero at all points.Such waves result from the interactionof similar waves traveling in oppositedirections as when reflected wavesmeet advancing waves. A particularcase is that of waves in a body whosethickness is an integral multiple ofhalf-wavelengths, as in resonancetesting.18,21

wave train: In ultrasonic testing, series ofwaves or groups of waves passingalong the same course at regularintervals.

wave, transverse: In ultrasonic testing,type of wave in which the particlemotion is perpendicular to thedirection of propagation.15 Also calledshear wave.

wave, transverse horizontal (polarized):In ultrasonic testing, transverse wavein which the particle vibration isparallel to the incidence surface.

wave, transverse vertical (polarized): Inultrasonic testing, transverse wave inwhich the plane of vibration is normalto the incidence surface.

wear: See erosion; rat’s tooth principle; wear,adhesive; wear, fretting.

wear face: In ultrasonic testing, protectivematerial on the face of a transducer toprevent wear of the piezoelectricelement.15

wear oxidation: See wear, fretting.wear, adhesive: Degradation of a surface

by microwelding and consequentfracture due to the sliding of onesurface against another. See alsotarasov etching technique. Comparefretting.

wear, fretting: Surface degradation bymicrowelding and microfracturescaused over extended periods by lightloads and vibration without materialdeformation. Also called chafing,friction oxidation and wear oxidation.See also cocoa; red mud. Comparebrinelling; galling; spalling.

wedge: In ultrasonic testing, device usedto direct ultrasonic energy into a testobject at an acute angle.15 See alsoshoe. Compare delay line.

weld, arc: General term for joining ofmetals by heating them to the pointof melting with an electric arc.



weld, butt: Weld that joins the edges oftwo work pieces in the same plane.

weld, fillet: Weld of approximatelytriangular cross section at the vertex orcorner of two surfaces, other than anedge, butt or spot weld. See also weldthroat.

welder’s flash: Clinical condition,specifically keratoconjunctivitis,commonly caused by overexposure toultraviolet radiation emitted by awelding arc. Compare blue haze; bluelight hazard.

welding, gas metal arc (GMAW): Inertgas shielded metal joining process thatuses a continuous and consumablewire electrode. Also called MIG (metalinert gas) welding.

welding, gas tungsten arc (GTAW): Inertgas shielded metal joining process thatuses a nonconsumable tungstenelectrode. Filler material, whenneeded, is manually fed into themolten weld puddle. Also calledtungsten inert gas (TIG) welding.

welding, shielded metal arc (SMAW):Joining of metals by heating themwith an electric arc betweenelectrode(s) and the work piece, usingan inert gas to shield the electrode(s).

welding, submerged arc (SAW): Joiningprocess in which the electrical arcbetween the continuously fedconsumable electrode and theworkpiece is protected by a fusiblegranular flux. The thick flux layerprotects the molten weld and protectsthe welder from ultraviolet radiationfrom the arc.

weld, multipass: Weld made by manypasses, one pass at a time.

weld size: Thickness of weld metal — in afillet weld the distance from the rootto the toe of the largest isosceles righttriangle that can be inscribed in across section of the weld. Compareweld throat.

weld throat: Distance from the root of afillet weld to its face.

weld throat, actual: Shortest distancefrom the root of a fillet weld to itsface, as opposed to theoretical throator weld size.

weld throat, effective: In fillet welds, theweld throat including the amount ofweld penetration but ignoring excessmetal between the theoretical face andthe actual face.

weld throat, theoretical: Distance fromthe beginning of the root of a filletweld perpendicular to the hypotenuseof the largest right triangle that can beinscribed within the cross section ofthe fillet weld. Compare weld size.

wet horizontal unit: In magnetic particletesting, stationary equipment used inthe wet technique, which supplies ameasured amount of electric currentto a headstock and tailstock, allowscarrier fluid application and may beequipped with a rigid multiple-turnencircling coil.

wet technique: In magnetic particletesting, technique in which magneticparticles are suspended in a wellagitated bath of carrier fluid. The wettechnique may be incorporated intothe continuous technique or the residualtechnique. Compare dry technique. Seealso centrifuge tube.

wetting action: Action of liquid inspontaneously spreading over andadhering to solid surfaces. See alsowater break free.

wheatstone bridge: General circuitconfiguration that uses the balancebetween two bridge circuit legs tomeasure an unknown electricalproperty such as resistance,capacitance, inductance or impedance.

white light: Light combining allfrequencies in the visible lightspectrum (wavelengths from 380 to780 nm) and in equal proportions.

Wien’s displacement law: In infrared andthermal testing, method fordetermining the wavelength ofmaximum emittance for a blackbody.See also blackbody; Planck’s law; stefan-boltzmann law.

wipe-off technique: See bleed backtechnique.

wobble: In electromagnetic testing, aneffect 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.23

work hardening: Increase in hardnessaccompanying plastic deformation ofa metal. Usually caused in a metal byrepeated impacting, bending orflexing. See also peening; plasticdeformation.

working distance: (1) Distance from asource of electromagnetic radiation tothe specimen. (2) Distance from thetarget to the instrument, usually tothe primary optic.

working standard: Standard that is lowerin quality and cost than a referencestandard against which it is calibratedand that is routinely used to calibrateor check material measures, measuringinstruments or reference materials.Compare reference standard; standard.



wrap around: In ultrasonic testing,display of misleading ultrasonicreflections from a previouslytransmitted pulse because of excessivepulse repetition frequency.28 See alsoghost.

XX-ray: Penetrating electromagnetic

radiation emitted when the innerorbital electrons of an atom areexcited and release energy. Radiation isnonisotopic in origin and is mostoften generated by bombarding ametallic target with high speedelectrons. Compare alpha ray; beta ray;gamma ray.

X-ray diffraction (XRD): Radiographictesting technique used for materialcharacterization, based on change inscattering of X-radiation as a result ofinteraction with test material. See alsodiffraction.

X-ray fluorescence (XRF): Radiographictesting technique used for surfacematerial characterization, based onwavelengths of fluorescence frommaterial irradiated by X-rays.

Yyoke: In magnetic particle testing,

portable U shaped electromagnet orpermanent magnet that induceslongitudinal magnetization in theregion of the test object between itsmagnetic poles. See also articulated polepieces; contour probe; field flowmagnetization; lifting power.

Young’s modulus: See modulus ofelasticity.

Zzone: In line scanners for infrared testing,

a scanned area created by thetransverse linear motion of theproduct or process under ameasurement sector of the scanner.



1. ASTM E 1316, Standard Terminology forNondestructive Examinations.West Conshohocken, PA:ASTM International (2007).

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

3. Nondestructive Testing Handbook, thirdedition: Vol. 1, Leak Testing.Columbus, OH: American Society forNondestructive Testing (1998).

4 Nondestructive Testing Handbook, thirdedition: Vol. 2, Liquid Penetrant Testing.Columbus, OH: American Society forNondestructive Testing (1999).

5. Nondestructive Testing Handbook, thirdedition: Vol. 3, Infrared and ThermalTesting. Columbus, OH: AmericanSociety for Nondestructive Testing(2001).

6. Nondestructive Testing Handbook, thirdedition: Vol. 4, Radiographic Testing.Columbus, OH: American Society forNondestructive Testing (2002).

7. Nondestructive Testing Handbook, thirdedition: Vol. 5, Electromagnetic Testing.Columbus, OH: American Society forNondestructive Testing (2004).

8. Nondestructive Testing Handbook, thirdedition: Vol. 6, Acoustic EmissionTesting. Columbus, OH: AmericanSociety for Nondestructive Testing(2005).

9. Nondestructive Testing Handbook, thirdedition: Vol. 7, Ultrasonic Testing.Columbus, OH: American Society forNondestructive Testing (2007).

10. Nondestructive Testing Handbook, thirdedition: Vol. 8, Magnetic Testing.Columbus, OH: American Society forNondestructive Testing (2008).

11. Nondestructive Testing Handbook, thirdedition: Vol. 9, Visual Testing.Columbus, OH: American Society forNondestructive Testing (2010).

12. CRC Handbook of Chemistry andPhysics, 91st edition. Baton Rouge, LA:CRC Press (2010).

13. Mordfin, L., ed. Handbook of ReferenceData for Nondestructive Testing.WestConshohocken, PA: ASTMInternational (2002).

14. NDT Terminology.Wilmington, DE:E.I. du Pont de Nemours & Company,Photo Products Department (n.d.).

15. TO33B-1-1 (NAVAIR 01-1A-16)TM43-0103, Nondestructive TestingMethods.Washington, DC: UnitedStates Department of Defense, UnitedStates Air Force (June 1984): p 1.25.

16. IEEE Standard Dictionary of Electricaland Electronic Terms. New York, NY:Institute of Electrical and ElectronicsEngineers, distributed byWiley-Interscience, a division ofJohn Wiley and Sons (1984).

17. Shull, P. Nondestructive Evaluation:Theory, Techniques, and Applications.New York, NY: Marcel Dekker (2002).

18. Weismantel, E.E. et al. “Glossary ofTerms Frequently Used inNondestructive Testing.” MaterialsEvaluation. Vol. 33, No. 4. Columbus,OH: American Society forNondestructive Testing (April 1975):p 42A-44A.

19. IES Lighting Handbook: ReferenceVolume. New York, NY: IlluminatingEngineering Society of North America(1984).

20. British Standard 3683, The Glossary ofTerms Used in Nondestructive Testing.Part 4, “Ultrasonic Flaw Detection.”London, United Kingdom: BritishStandards Institute (1985). Supersededby EN 1330-4, NondestructiveTerminology: Part 4, Terms Used inUltrasonic Testing. Brussels, Belgium:European Committee forStandardization (2000).

21. O’Hanlon, J.F. A User’s Guide to VacuumTechnology, second edition. New York,NY: John Wiley and Sons (1989).

22. EPRI Learning Modules. Charlotte, NC:Electric Power Research Institute (n.d.).

23. ASTM E 268-81, Definitions Approvedfor Use by Agencies of the Department ofDefense as Part of Federal Test MethodStandard No. 151b and for Listing in theDoD Index of Specifications andStandards.West Conshohocken, PA:ASTM International (1981).

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

25. Lovejoy, D. Magnetic Particle Inspection:A Practical Guide. New York, NY:Chapman & Hall (1993).

26. Jiles, D. Introduction to Magnetism andMagnetic Materials. New York, NY:Chapman & Hall (1998).


References


27. Betz, C.E. Principles of Magnetic ParticleTesting. Chicago, Illinois: Magnaflux(1967).

28. MIL-STD-371, Glossary of Terms andDefinitions for Ultrasonic TestingProcedures.Washington, DC: UnitedStates Department of Defense, UnitedStates Army (October 1987).

29. MIL-STD-883, Test Method StandardMicrocircuits. Revision D, Method1014. Washington, DC: Department ofDefense (1995).

30. Nondestructive Testing: Liquid Penetrant,fourth edition. ProgrammedInstruction Handbook PI-4-2. FortWorth, TX: General Dynamics,Convair Division (1977).




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Nondestructive Testing Glossary