Stephen D. Hart, Alexandria, Virginia

Stephen D. Hart, Alexandria, Virginia

16C H A P T E R

Ultrasonic Testing Glossary

From Nondestructive Testing Handbook, Third Edition: Volume 7, Ultrasonic Testing © 2007. Reprinted with permission of The American Society for Nondestructive Testing Inc.

IntroductionMany of the definitions in this glossaryare adapted from the second and thirdedition of the Nondestructive TestingHandbook.1-16 The definitions have beenmodified to satisfy peer review andeditorial style. For this reason, referencesat the end of this glossary should beconsidered not attributions butacknowledgments and suggestions forfurther reading.

The definitions in this NondestructiveTesting Handbook volume should not bereferenced for tests performed accordingto standards, specifications or contracts.Written procedures should refer todefinitions in standards.

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

Aacceptance standard: Document defining

acceptable discontinuity size limits.7See also standard.

acoustic emission: Transient elastic wavesresulting from local internalmicrodisplacements in a material. Byextension, the term also describes thetechnical discipline and measurementtechnique relating to thisphenomenon.16

acoustic impedance: Material propertydefined as the product of soundvelocity and density of the material.The relative transmission andreflection at an interface are governedin part by the acoustic impedances ofthe materials on each side of theinterface.7,22 See also characteristicacoustic impedance; specific acousticimpedance.

acoustic microscopy: General termreferring to the use of high resolution,high frequency ultrasonic techniquesto produce images of features beneaththe surface of a test object.7

AE: Acoustic emission method ofnondestructive testing.

amplitude linearity: See linearity,amplitude.

amplitude, echo: Vertical height of areceived signal on an A-scan,measured from base to peak for avideo presentation or from peak topeak for a radio frequencypresentation.7

analog-to-digital converter: Circuitwhose input is analog and whoseoutput is digital.16

angle beam: Ultrasound beam travelingat an acute angle into a medium. Theangle of incidence or angle ofrefraction is measured from thenormal to the entry surface.7,22

angle of incidence: Included anglebetween the beam axis of the incidentwave and the normal to the surface atthe point of incidence.7,21

angle of reflection: Included anglebetween the beam axis of the reflectedwave and the normal to the reflectingsurface at the point of reflection.7,21

angle of refraction: Angle between thebeam axis of a refracted wave and thenormal to the refracting interface.7,21

angle beam testing: Technique ofultrasonic testing in whichtransmission of ultrasound is at anacute angle to the entry surface.7,21

angle beam transducer: Transducer thattransmits or receives ultrasonic energyat an acute angle to the surface. Thismay be done to achieve special effectssuch as setting up transverse or surfacewaves by mode conversion at aninterface.7,21

anisotropy: Condition in whichproperties of a medium (velocity, forexample) depend on direction in themedium.

anomaly: Variation from normal materialor product quality.10

antinode: Point in a standing wave wherecertain characteristics of the wave fieldhave maximum amplitude.7,21

area linearity: See linearity, area.array: Group of transducers used for

source location.16

array transducer: Transducer made up ofseveral piezoelectric elementsindividually connected so that thesignals they transmit or receive maybe treated separately or combined asdesired.7 See also phased array.

artificial discontinuity standard: Seeacceptance standard.

556 Ultrasonic Testing

PART 1. Terms


A-scan: One-dimensional display ofultrasonic signal amplitude asfunction of time or depth in testobject.

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

ASNT: American Society forNondestructive Testing.

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.7,23 See also ultrasonicabsorption.

attenuation: (1) Decrease in acousticenergy over distance. This loss may becaused by absorption, leakage,reflection, scattering or other materialcharacteristics. (2) Decrease in signalamplitude caused by acoustic energyloss or by an electronic device such asan attenuator.10,16,21

attenuator: Device for varying the signalamplitude on an ultrasonicinstrument. Usually calibrated indecibels.7,21

Bback reflection: Signal received from the

far boundary or back surface of a testobject.7,21

background noise: Extraneous signalscaused by random signal sourceswithin or exterior to the ultrasonictesting system, including the testmaterial.7,21 It has electrical,mechanical or chemical origins.16

Sometimes called grass or hash.baseline: Horizontal trace across the

A-scan display. It represents time andis generally related to materialdistance or thickness.7

beam exit point: See probe index.beam spread: Widening of the sound

beam as it travels through amedium.21 Specifically, the solid anglethat contains the main lobe of thebeam in the far field.7

bel (B): See decibel.boundary echo: Reflection of an

ultrasonic wave from an interface.7,22

brittleness: Material characteristic thatleads to crack propagation withoutappreciable plastic deformation.10

broad band: Having a relatively widefrequency bandwidth. Used to describepulses that display a wide frequencyspectrum and receivers capable ofamplifying them.7

B-scan: Data presentation techniquetypically applied to pulse echotechniques. It produces atwo-dimensional view of a crosssectional plane through the testobject. The horizontal sweep isproportional to the distance along thetest object and the vertical sweep isproportional to depth, showing thefront and back surfaces anddiscontinuities between.7,22

bubbler: See water column.burst, forging: See crack, forging.

Ccalibration, basic: Procedure of

standardizing an instrument by usinga reference standard.

calibration reflector: Reflector with aknown dimensioned surface in aspecified material, established toprovide an accurately reproduciblereference level.7 See also referencestandard.

cathode ray: Stream of electrons emittedby a heated filament and projected ina more or less confined beam underthe influence of a magnetic or electricfield.7,22

cathode ray tube: Vacuum tubecontaining a screen on whichultrasonic scans may be displayed.Used for A-scans or B-scans in thetwentieth century.7

certification: With respect tonondestructive test personnel, theprocess of providing written testimonythat an individual has met therequirements of a specific practice orstandard. See also certified andqualified.

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

characteristic acoustic impedance:Acoustic impedance typical orcharacteristic of a particular material.See acoustic impedance; specific acousticimpedance.

compensator: Electrical matchingnetwork to compensate for electricalimpedance differences.7,22

compressional wave: Wave in whichparticle motion in the material isparallel to the wave propagationdirection. Also called longitudinalwave.7

contact technique: Testing technique inwhich the transducer face makes directcontact with the test object through athin film of couplant.7,22

contact transducer: Transducer used inthe contact technique.7

557Ultrasonic Testing Glossary


continuous wave: Wave of constantamplitude and frequency.

contracted sweep: Misnomer that refersto extending the duration of thesweep to permit viewingdiscontinuities or back reflectionsfrom deeper in the test object. Thesweep appears to be compressed.7

corner effect: Strong reflection obtainedwhen an ultrasonic beam is directedtoward the intersection of two or threeintersecting surfaces.7,22

couplant: Substance used between thetransducer and the contacting surfaceto permit or improve transmission ofultrasonic energy into or from the testobject.7,22

crack: (1) Break, fissure or rupture,sometimes V shaped in cross sectionand relatively narrow. By convention,a discontinuity is called a crack if it isat 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).10

crack, cold: Crack that occurs aftersolidification, because of high stressesfrom nonuniform cooling.10

crack, cooling: Crack resulting fromuneven cooling after heating or hotrolling. Cooling cracks are usuallydeep and lie in a longitudinaldirection but are usually not straight.10

crack, fatigue: Progressive growth of acrack that usually develops on thesurface and is caused by the repeatedloading and unloading of the object.10

crack, forging: Crack developed byforging at too low a temperature,resulting in rupturing of thematerial.10 Also called burst.

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

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

critical angle: Incident angle of theultrasound beam where the refractedbeam is parallel to the surface andabove which a specific mode ofrefracted energy no longer exists.7,21

cross coupling: Cross talk.cross noise: Cross talk.cross talk: Unwanted signal leakage

(acoustical or electrical) across anintended barrier, such as leakagebetween the transmitting andreceiving elements of a dualtransducer.7,22 Also called cross noiseand cross coupling.

CRT: See cathode ray tube.

crystal: See transducer element.crystal, X-cut: Cut with face

perpendicular to the X-direction of thepiezoelectric crystal.7 In a quartz sliceso cut, a thickness mode of vibrationoccurs when the slice is electricallystimulated in the X direction.7,22

crystal, Y-cut: Piezoelectric crystal whosecut face is perpendicular to theY direction. In quartz, a transversemode of vibration is obtained whenthe slice is electrically stimulated inthe Y direction.7,22

crystal mosaic: Multiple crystals mountedin the same surface on one holder andconnected so as to cause all to vibrateas one unit.7,22

C-scan: Presentation technique applied toacoustic data and displaying an imageof two-dimensional test object withscaled grays or colors representing theultrasonic signals. The amplituderepresented in each pixel may be apulse echo, through-transmission orpitch catch value calculated from eachA-scan datum.

cutoff frequency: Upper or lower spectralresponse of a filter or amplifier, atwhich the response is a specifiedamount less (usually 3 or 6 dB) thanthe maximum response.

Ddamping: (1) Limiting the duration or

decreasing the amplitude ofvibrations, as when damping atransducer element.22 (2) Deliberateintroduction of energy absorbers toreduce vibrations.7

damping capacity: Measure of the abilityof a material to dissipate mechanicalenergy.7,23

damping material: Highly absorbentmaterial used to cause rapid decay ofvibration.7

damping, transducer: Material bonded tothe back of the piezoelectric elementof a transducer to limit the duration ofvibrations.7,21

damping, ultrasonic: Decrease or decayof ultrasonic wave amplitudecontrolled by the instrument ortransducer.

dead zone: Interval following the initialpulse at the surface of a test object tothe nearest inspectable depth.21 Anyinterval following a reflected signalwhere additional signals cannot bedetected.7



decibel (dB): Logarithmic unit forexpressing relative signal power, suchas the loudness of a sound, inproportion to the intensity of areference signal. One tenth of a bel.Decibel in signal amplitude is twicethat in signal power.16 One decibelequals ten times the base tenlogarithm of the ratio of two powers.

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.10,18 Some discontinuitiesdo not exceed an accept/rejectcriterion and are therefore not defects.Compare crack; discontinuity;indication.10

delamination: Laminar discontinuity,generally an area of unbondedmaterials.7

delay line: Material (liquid or solid)placed in front of a transducer tocause a time delay between the initialpulse and the front surfacereflection.7,22

delayed sweep: See sweep delay.delayed time base: See delayed sweep.delta effect: Reradiation or diffraction of

energy from a discontinuity.22 Thereradiated energy may include wavesof both the incident mode andconverted modes (longitudinal andtransverse).7

delta t ( t): Duration measured betweentwo points in time. Also called timedifferential.

depth compensation: See distanceamplitude correction.

depth of field: Focal zone.depth of focus: Focal zone.detectability: A measure of the ability to

detect signals from small reflectors.Limited by the signal-to-noise ratio.

diffraction: Deflection of a wavefrontwhen passing the edge of anultrasonically opaque object.7,22

diffuse reflection: Scattered, incoherentreflections from rough surfaces.7,21

discontinuity: Interruption in thephysical structure or configuration of atest object. After nondestructivetesting, a discontinuity indication canbe interpreted to be a flaw or a defect.10

Compare defect; indication.dispersion: In acoustics, variation of wave

phase with frequency.7dispersive medium: Medium in which

the propagation velocity depends onthe wave frequency.7

distance amplitude correction:Compensation of gain as a function oftime for difference in amplitude ofreflections from equal reflectors atdifferent sound travel distances.7Refers also to compensation byelectronic means such as swept gain,time corrected gain, time variable gainand sensitivity time control.7,22

divergence: Term sometimes used todescribe the spreading of ultrasonicwaves beyond the near field. It is afunction of transducer diameter andwavelength in the medium.7 See beamspread.

double-crystal technique: See pitch catchtechnique.

dual transducer: See send/receivetransducer.

dynamic range: Ratio of maximum tominimum reflective areas that can bedistinguished on the display at aconstant gain setting.7,19

Eecho: Reflected acoustic energy or signal

indicating such energy.7effective penetration: In a material, the

maximum depth at which a test signalcan reveal discontinuities.

electrical noise: Extraneous signalscaused by external sources or electricalinterferences within an ultrasonicinstrument.21 A component ofbackground noise.7

electromagnetic acoustic transducer:Transmitting transducer based on theforce exerted on a current flowing in amagnetic field. A receiving transducerthat detects the current produced bymoving a conductor in a magneticfield.7

EMAT: See electromagnetic acoustictransducer.

evaluation: Process of deciding theseverity of a condition after anindication has been interpreted.Evaluation determines if the testobject should be rejected or accepted.7See also indication and interpretation.

expanded sweep: Short durationhorizontal sweep positioned to allowclose examination of a signal.7



Ffalse indication: See indication, false.far field: Zone beyond the near field in

front of a plane transducer in whichsignal amplitude decreasesmonotonically in proportion todistance from the transducer.7 Alsocalled the fraunhofer zone.

filter: (1) Electrical circuit that leaves asignal unaffected over a prescribedrange of frequencies and attenuatessignal components at all otherfrequencies.10,20 (2) Data analysisprocess for treating data files.

flat bottom hole: Type of reflectorcommonly used in referencestandards. The end (bottom) surface ofthe hole is the reflector.7

flaw location scale: Specially graduatedruler that can be attached to an anglebeam transducer to relate the positionof an indication on the display to theactual location of a discontinuitywithin the test object.7

flaw: Unintentional anomaly orimperfection. See also defect anddiscontinuity.10

focal zone: Distance before and after thefocal point in which the intensitydiffers a specified amount (usually6 dB) from the focal intensity.7 Alsocalled depth of field or depth of focus.

focused beam: Sound beam thatconverges to a cross section smallerthan that of the element.

focused transducer: Transducer thatproduces a focused sound beam.7

fraunhofer zone: See far field.frequency, fundamental: In resonance

testing, the frequency at which thewavelength is twice the thickness ofthe test material.7,22

frequency, pulse repetition: Number ofpulses per second.7

frequency, test: Nominal ultrasonic wavefrequency used.7,22

frequency: Number of complete wavecycles passing a given point persecond or the number of vibrationsper second.7

fresnel field: See near field.fresnel zone: See near field.front surface: First surface of the test

object encountered by an ultrasonicbeam.7

Ggate: (1) Electronic device for selecting

signals in a segment of the trace on anA-scan display. (2) The interval alongthe baseline that is monitored.7

general examination: In personnelqualification, a test or examination ofa person’s knowledge, typically (in thecase of nondestructive test personnelqualification) a written test on thebasic principles of a nondestructivetest method and general knowledge ofbasic equipment used in that method.(According to ASNT’s guidelines, thegeneral examination should notaddress knowledge of specificequipment, codes, standards andprocedures pertaining to a particularapplication.)10

ghost: Aliasing indication arising fromcertain combinations of pulserepetition frequency and time basefrequency.7,23 See also wrap around.

grass: See background noise.grinding crack: Shallow crack formed in

the surface of relatively hard materialsbecause of grinding heat. Grindingcracks typically are 90 degrees to thedirection of grinding.10

group velocity: Speed at which theenvelope of an ultrasonic pulse (manyfrequencies) propagates through themedium.7

Hhardness: Resistance of metal to denting,

to plastic deformation by bending orto mechanical deformation byscratching, abrasion or cutting.Typically measured by indentation.

harmonic: Vibration frequency that is anintegral multiple of the fundamentalfrequency.7,21

hash: See background noise.heat affected zone: Base metal that was

not melted during brazing, cutting orwelding but whose microstructure andphysical properties were altered by theheat.10

hertz (Hz): Measurement unit offrequency, equivalent to one cycle persecond.10,17

horizontal linearity: Measure ofproportionality between positions ofindications on the horizontal traceand the positions of theircorresponding reflectors.



Iimmersion technique: Test technique in

which the test object and thetransducer are submerged in a liquid(usually water) that acts as thecoupling medium.22 The transducer isnot usually in contact with the testobject.7

impedance, acoustic: See acousticimpedance.

indication: Nondestructive testequipment response to a reflector,requiring interpretation to determineits relevance. Compare crack; defect;discontinuity; indication, false.10

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

indication, false: Test indication thatoriginates where no discontinuityexists in the test object. Comparedefect; indication, nonrelevant.10

indication, nonrelevant: Indicationpossibly caused by an actualdiscontinuity that does not affect theusability of the test object (a change ofsection, for instance) or that is smallerthan a relevant indication. Compareindication, false and indication,relevant.10

indication, relevant: Indication from adiscontinuity (as opposed to anonrelevant indication) requiringevaluation by a qualified inspector,typically with reference to anacceptance standard, by virtue of thediscontinuity’s size, shape, orientationor location. Compare indication,nonrelevant.10,19

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 acousticpulse.7

insonification: Irradiation with sound.7interface: Physical boundary between two

adjacent media.7,21

interface synchronization: See interfacetriggering.

interface triggering: Triggering the sweepand auxiliary functions from aninterface echo occurring after theinitial pulse.7 Also called interfacesynchronization.

interpretation: Determination of thesource, significance and relevance oftest indications.10

isotropy: Condition in which significantmedium properties (sound speed, forexample) are the same in alldirections.7

Llamb wave: Type of ultrasonic wave

propagation in which the wave isguided between two parallel surfacesof the test object. Mode and velocitydepend on the product of the testfrequency and the thickness. Platewave.7

linearity, amplitude: Constantproportionality between the signalinput to the receiver and theamplitude of the signal appearing onthe display of the ultrasonicinstrument or on an auxiliarydisplay.7,19 Also called vertical linearity.

linearity, area: Constant proportionalitybetween the signal amplitude and theareas of equal discontinuities locatedat the same depth in the far field.Necessarily limited by the size of theultrasonic beam and configuration ofthe reflector.7

logarithmic decrement: Naturallogarithm of the ratio of theamplitudes of two successive cycles ina damped wave train.7

longitudinal wave: Wave in which pointsof same phase lie on parallel planesurfaces. 7,23

loss of back reflection: Absence orsignificant reduction of an indicationfrom the back surface of the testobject.7,21

Mmain bang: See initial pulse.manipulator: In immersion testing, a

device for angular orientation of thetransducer7,24 and for scanningmotion in three axes.

markers: Series of indications on thehorizontal trace of the display screento show increments of time ordistance.7,21

material noise: Random signals caused bythe material structure of the testobject.7,21 A component of backgroundnoise.

mechanical properties: Measurableproperties of a material related to itsbehavior, such as toughness, hardnessand elasticity. Compare physicalproperties.

mode conversion: Change of ultrasonicwave propagation mode uponreflection or refraction at an interface.7

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.



mode of vibration: Manner in which anacoustic wave is propagated, ascharacterized by the particle motion inthe wave21 (transverse, lamb, surfaceor longitudinal).7

model, analytical: Mathematicalrepresentation of a process orphenomenon.

multiple back reflections: Repetitiveechoes from the far boundary of thetest object.7,21

multiple-echo technique: Techniquewhere thickness is measured betweenmultiple back reflections, minimizingerror from coatings or from changes intemperature or contact pressure.

Nnarrow band: Relative term denoting a

restricted range of frequencyresponse.7,22

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

(2)Nondestructive examination.NDI: Nondestructive inspection.NDT: 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.21

neper: Natural logarithm of a ratio of twoamplitudes (equal to 8.686 dB) used asa measure of attenuation. Power ratiosare expressed as half the naturallogarithm.7

nodal points: In angle beam testing, thelocation of reflections at oppositesurfaces as a wave progresses along atest object.7

noise: Undesired or unintended signalsthat tend to interfere with normalreception or processing of a desiredsignal. The origin may be an electricor acoustic source, smalldiscontinuities or abrupt changes inthe acoustic properties of the testmaterial.7,22 See also signal-to-noise.

noncontact transducer: In ultrasonictesting, a sensor designed for wavepropagation through gas.

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

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

nondestructive examination (NDE):Another term for nondestructivetesting. In the utilities and nuclearindustry, the word examination issometimes preferred because testingcan imply performance trials ofpressure containment or powergeneration systems.10

nondestructive inspection (NDI):Another term for nondestructivetesting. In some industries (utilities,aviation), the word inspection oftenimplies maintenance for a componentthat has been in service.10

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

nonrelevant indication: See indication,nonrelevant.

normal incidence: (1) Condition inwhich the axis of the ultrasonic beamis perpendicular to the entry surface ofthe test object. (2) Condition wherethe angle of incidence is zero.7

Ooptimum frequency: Probe frequency

that provides the highestsignal-to-noise ratio compatible withthe detection of a specificdiscontinuity. Each combination ofdiscontinuity type and material mayhave a different optimumfrequency.7,22

orientation: Angular relationship of asurface, plane, discontinuity or axis toa reference plane or surface.7,21

oscillogram: Common term for a recordor photograph of data displayed onscreen.7,22

Pparasitic echo: See spurious echo.particle motion: Movement of particles

of material during wave propagation.7penetration, ultrasonic: Propagation of

ultrasonic energy into a material.7 Seealso effective penetration.

period: Value of the minimum durationafter which the same characteristics ofa periodic waveform or a periodicfeature repeat.10



phantom: Reference standard used toverify the performance of diagnosticultrasound systems.7

phase velocity: Velocity of a singlefrequency continuous wave.7

phased array: Mosaic of transducerelements in which the timing of theelements’ excitation can beindividually controlled to producecertain desired effects, such as steeringor focusing the beam.

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

piezoelectric effect: Ability of certainmaterials to convert electrical energy(voltage) into mechanical energy(stress) and vice versa.7,22

pitch catch technique: Ultrasonic testtechnique that uses two transducers,one transmitting and the otherreceiving on the same or oppositesurface.7,21,22 Also called double-crystaltechnique or two-transducer technique.

plane wave: See longitudinal wave.plate wave: See lamb wave.point of incidence: Point at which the

axis of the sound beam leaves thewedge of an angle beam transducerand enters the test object.7,22 See alsoprobe index.

poling: Process of reorienting crystaldomains in certain materials byapplying a strong electric field atelevated temperatures, inducingmacroscopic polarization andpiezoelectric behavior.7

presentation: Technique used to showultrasonic information. This mayinclude A-scans, B-scans or C-scans,displayed on various types of recordersor display instruments.7,21

primary reference response level:Ultrasonic response from the basicreference reflector at the specifiedsound path distance, electronicallyadjusted to a specified percentage offull screen height.7

probe: See sensor; transducer.probe index: Point on a transverse wave

or surface wave transducer throughwhich the emergent beam axispasses.7,23 See also point of incidence.

propagation: Movement of a wavethrough a medium.7,21

pulse: Transient electrical or ultrasonicsignal that has a rapid increase inamplitude to its maximum value,followed by an immediate return.16

pulse echo technique: Ultrasonic testtechnique in which discontinuities aredetected by return echoes from thetransmitted pulses.7

pulse length: Measure of pulse durationexpressed in time or number ofcycles.7,22

pulse repetition frequency: See repetitionrate.

pulse tuning: Control of pulse frequencyto optimize system response.7

Qqualification: Process of demonstrating

that an individual has the requiredamount and the required type oftraining, experience, knowledge andabilities.10 See also certification andqualified.

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

quality assurance: Administrative actionsthat specify, enforce and verifyquality.10

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

Rradian (rad): Measurement unit of plane

angle subtending, in a circle, an arcequal in length to the radius.

radio frequency display: Presentation ofunrectified signals.7,22 See also videopresentation.

range: Maximum ultrasonic path lengththat is displayed.7 See also sweeplength.22

rarefaction: Thinning or separation ofparticles in a propagating medium dueto the decompression phase of anultrasonic cycle. Opposite ofcompression. A compressional wave iscomposed of alternating compressionsand rarefactions.7,21

rayleigh wave: 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.7 Also calledsurface wave.



receiver: (1) Section of the ultrasonicinstrument that amplifies echoesreturning from the test object.(2)Transducer that picks up theechoes.7

recommended practice: Set of guidelinesor recommendations.10

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

reference standard: (1) Test objectcontaining known reflectorsrepresenting accept or reject criteria.(2) Sample test object selected forreference.

refracted beam: Beam transmitted in thesecond medium when an ultrasonicbeam is incident at an acute angle onthe interface between two mediahaving different sound speeds.7,22

refraction: Change in direction of anacoustic wave as the ultrasonic beampasses from one medium into anotherhaving different acoustic speeds. Achange in both direction and modeoccurs at acute angles of incidence. Atsmall angles of incidence, the originalmode and a converted mode may existsimultaneously in the secondmedium.7

refractive index: Ratio of the speed of theincident wave to that of a refractedwave. It is known as the refractiveindex of the second medium withrespect to the first.7

reject: Minimize or eliminate lowamplitude signals (such as electrical ormaterial noise) so that other signalsmay be further amplified. This controlcan reduce vertical linearity.7 Alsocalled suppression.22

rejection level: Level above or belowwhich a signal is an indication of arejectable discontinuity.7,22

relevant indication: In nondestructivetesting, an indication from adiscontinuity requiring evaluation.7

repetition rate: Number of pulsesgenerated or transmitted per unit oftime (usually seconds).7

resolving power: Measure of the ability ofan ultrasonic system to separate twosignals close together in time ordistance.7,21

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.7,21

resonance technique: Method using theresonance principle for determiningspeed, thickness or presence oflaminar discontinuities.7

resonant frequency: Frequency at whicha body vibrates freely after being set inmotion by some outside force.7,21

RF display: See radio frequency display.ringing signals: (1) Closely spaced

multiple signals caused by multiplereflections in a thin material.(2) Signals caused by continuedvibration of a transducer.7,22

ringing technique: Test technique forbonded structures in which unbondsare indicated by increased amplitudeof ringing signals.7,22

ringing time: Time that the mechanicalvibrations of a transducer continueafter the electrical pulse hasstopped.7,22

roof angle: In a dual-element delay linetransducer, the tilt angle by which thetransducer elements of the delay lineare oriented to direct the beams of thetwo elements to intersect at a specifiedzone in the medium.7

SSAM: scanning acoustic microscope.saturation: Condition in which high

amplitude signals on a display screendo not increase with increased gainand appear flattened.7

scanning: Movement of the transducerover the surface of the test object in acontrolled manner so as to achievecomplete coverage. May be either acontact or immersion technique.7

scattering: Uncontrolled reflection ofultrasonic waves by smalldiscontinuities or surfaceirregularities.7

schlieren system: Optical system used forvisual display of an ultrasonic beampassing through a transparentmedium.7,22

search unit: See transducer.self-coupling transducer: Contact

transducer that allows testing with aliquid couplant.

send/receive transducer: Transducerconsisting of two piezoelectricelements mounted side by sideseparated by an acoustic barrier. Oneelement transmits; one receives.7,21

sensitivity: Ability of signal to changewith small changes of measuredquantity.

sensor: Device that detects a materialproperty or mechanical behavior (suchas radiation or displacement) andconverts it to an electrical signal.Probe; transducer.



shadow: Region in a test object thatcannot be reached by ultrasonicenergy traveling in a given direction.Shadows are caused by geometry orthe presence of intervening largediscontinuities.7

shear wave: See transverse wave.shoe: Device used to adapt a straight

beam transducer for use in a specifictype of testing, including angle beamor surface wave tests and tests oncurved surfaces.7,22 See also wedge.

SH wave: Transverse horizontal wave.SI (International System of Units):

Measurement system using decimals inwhich the following seven units areconsidered basic: meter, mole,kilogram, second, ampere, kelvin andcandela.10,17

signal: Physical quantity, such aselectrical voltage, that containsinformation.10,20

signal-to-noise ratio: Ratio of signalamplitude (responses that containinformation) to baseline noiseamplitude (responses that contain noinformation). See also noise.10,20

skip distance: In angle beam tests of plateor pipe, the distance from the soundentry point to the exit point on thesame surface after reflection from theback surface.7 Also called V path.22

Snell’s law: Physical law that defines therelationship between the angle ofincidence and the angle of refraction.

SNR: See signal-to-noise ratio.SNT-TC-1A: See Recommended Practice

No. SNT-TC-1A.specific acoustic impedance: Acoustic

impedance in a particular test objector a defined volume of a specifiedmaterial. See also acoustic impedance;characteristic acoustic impedance.

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

spectrum: Amplitude distribution offrequencies in a signal.7

spectrum response: Amplification (gain)of a receiver over a range offrequencies.7

spherical wave: Wave in which points ofthe same phase lie on surfaces ofconcentric spheres.7,23

spurious echo: General term denotingany indication that cannot beassociated with a discontinuity orboundary at the location displayed.7Also called parasitic echo.

squint angle: Angle by which theultrasonic beam axis deviates from theprobe axis.7

squirter: See water column.

standard: (1) Reference object used as abasis for comparison or calibration.(2) Concept established by authority,custom or agreement to serve as amodel or rule in the measurement ofquantity or the establishment of apractice or procedure.7,22

standing wave: Wave in which theenergy flux is zero at all points. Suchwaves result from the interaction ofsimilar 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.7,21,22

stiffness: Slope of curve of stress to strain,described by Young’s modulus ofelasticity. Compare hardness.

straight beam: Ultrasonic wave travelingnormal to the test surface.7,22

suppression: See reject.surface wave: See rayleigh wave.SV wave: Shear vertical wave.sweep: Uniform and repeated movement

of a spot across the display screen toform the horizontal baseline.7 Alsocalled time base.

sweep delay: (1) Delay in time of startingthe sweep after the initial pulse.(2)Control for adjusting the time.7,22

Also called time delay.sweep length: Length of time or distance

represented by the horizontal baselineon an A-scan.7,22

Ttest surface: Surface of the test object at

which the ultrasonic energy enters oris detected.7

threshold: Voltage level setting of aninstrument that causes it to registeronly signals greater or less than aspecified magnitude.10,20 Thisthreshold may be adjustable, fixed orfloating.16

through-transmission technique: Testtechnique in which ultrasonic energyis transmitted through the test objectand received by a second transduceron the opposite side. Changes inreceived signal amplitude are taken asindications of variations in materialcontinuity.7

time base: See sweep.time delay: See sweep delay.time differential: See delta t.time of flight: Time for an acoustic wave

to travel between two points. Forexample, the time required for a pulseto travel from the transmitter to thereceiver via diffraction at adiscontinuity edge or along the surfaceof the test object.7



tone burst: Wave train consisting ofseveral cycles of the same frequency.7

transducer: (1) Device that convertsmechanical energy to electrical outputor vice versa. (2) Piezoelectric devicethat converts attributes of thestress-strain field of an acoustic waveinto an electrical signal of voltageversus time. Sensor; probe.16

transducer, differential: Piezoelectrictwin-element or dual-pole transducer,the output poles of which are isolatedfrom the case and are at a floatingpotential.16

transducer element: In an ultrasonictransducer, the piezoelectric crystal tobe coupled to the test surface. Alsocalled the crystal.

transducer, flat response: Transducerwhose frequency response has noresonance or characteristic responsewithin its specified frequency band.16

transducer relative sensitivity: Responseof the transducer to a given source.16

transducer, resonant: Transducer thatuses the mechanical amplification dueto a resonant frequency (or severalclose resonant frequencies) to givehigh sensitivity in a narrow band,typically ±10 percent of the principalresonant frequency at the –3 dBpoints.16

transducer, single-ended: Piezoelectricsingle-element transducer, the outputpole of which is isolated from thecase, the other pole being at the samepotential as the case.16

transducer, wide band: Transducer whoseresponse to surface displacements isflat over a wide frequency range.

transfer function: Description of changesto the waves arising as they propagatethrough the medium or, for atransducer, the relationship betweenthe transducer output signal and thephysical parameters of the acousticwave at the transducer.16

transmission angle: Incident angle of atransmitted ultrasonic beam. It is zerodegrees when the beam isperpendicular (normal) to the testsurface.7,21

transmission characteristics: Test objectcharacteristics that influence thepassage of ultrasonic energy, includingscattering, attenuation or surfaceconditions.7

transmission technique: Seethrough-transmission technique.

transmitter: (1) Transducer that emitsultrasonic energy. (2) Electrical circuitsthat generate the signals emitted bythe transducer.7

transverse horizontal (polarized) wave:Transverse wave in which the particlevibration is parallel to the incidencesurface.7

transverse vertical (polarized) wave:Transverse wave in which the plane ofvibration is normal to the incidencesurface.7

transverse wave: Type of wave in whichthe particle motion is perpendicular tothe direction of propagation.7,22 Alsocalled shear wave.

transverse wave transducer: Transducerthat generates transverse waves in atest object.

two-transducer technique: See pitch catchtechnique.

Uultrasonic absorption: Damping or

dissipation of ultrasonic waves as theypass through a medium.7,21 See alsoattenuation coefficient.

ultrasonic spectroscopy: Analysis of thefrequency content of an acousticwave. Generally performedmathematically using a fast fouriertransform.7

ultrasonic spectrum: Usually thefrequency range from 20 kHz to50 MHz but may extend much higherin special applications.7

ultrasonic: Of or relating to acousticvibration frequencies greater thanabout 20 kHz.7,22

ultrasonic testing: Method ofnondestructive testing, using acousticwaves at inaudibly high frequencies atthe interrogating energy.

UT: Abbreviation for the ultrasonicmethod of nondestructive testing.7,22

Vvertical limit: Maximum useful readable

level of vertical indication on anA-scan.7

vertical linearity: See linearity, amplitude.video presentation: Display presentation

in which radiofrequency signals havebeen rectified and usually filtered.7,22

V path: See skip distance.



Wwater column: Tube filled with water and

attached to the front of a transducerto couple an ultrasonic beam to a testobject. A delay line between the initialpulse and the front surface signal. Alsoserves as a coupling device.7 See alsodelay line.

water jet: Unsupported stream of watercarrying ultrasonic signals between thetransducer and the test object surface.7Also called a squirter or water column.

water path: In immersion testing or witha water column, the distance from thetransducer face to the test object’sfront surface.7,22

wave interference: Production of a seriesof maxima and minima of soundpressure as a consequence of thesuperposition of waves havingdifferent phases.7,22

wave train: Series of waves or groups ofwaves passing along the same courseat regular intervals.7

wavefront: In a wave disturbance, thelocus of points having the samephase.7,22

waveguide: Device to transmit elasticenergy from a test object to a remotetransducer — for example, a wirejoined at one end to a test object andat the other end to a transducer.

wavelength: Distance needed in thepropagation direction for a wave to gothrough a complete cycle.7,21

wear face: Protective material on the faceof a transducer to prevent wear of thepiezoelectric element.7,22

wedge: Device used to direct ultrasonicenergy into a test object at an acuteangle.7,22 See also shoe.

wheel transducer: Device that couplesultrasonic energy to a test objectthrough the rolling contact area of awheel containing a liquid and one ormore transducers.7,22

wrap around: Display of misleadingultrasonic reflections from apreviously transmitted pulse becauseof excessive pulse repetitionfrequency.7,24 See also ghost.



c = speed of sound in air (meter persecond)

D = diameter (meter)dB = decibel

E = Young’s modulus of elasticityf = frequency (hertz)

R, r = reflection coefficientT = transmission coefficientt = time (second)

V = voltv = velocity (meter per second)Z = impedance (pascal second per

meter)= attenuation= strain= wavelength (meter)= angle (radian)= density (kilogram per cubic meter)= Poisson’s ratio; stress (pascal)


PART 2. Symbols


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

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

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

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

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

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

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

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

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

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

11. Nondestructive Testing Handbook, thirdedition: Vol. 1, Leak Testing.Columbus, OH: American Society forNondestructive Testing (1998).

12. Nondestructive Testing Handbook, thirdedition: Vol. 2, Liquid Penetrant Testing.Columbus, OH: American Society forNondestructive Testing (2000).

13. Nondestructive Testing Handbook, thirdedition: Vol. 3, Infrared and ThermalTesting. Columbus, OH: AmericanSociety for Nondestructive Testing(2001).

14. Nondestructive Testing Handbook, thirdedition: Vol. 4, Radiographic Testing.Columbus, OH: American Society forNondestructive Testing (2002).

15. Nondestructive Testing Handbook, thirdedition: Vol. 5, Electromagnetic Testing.Columbus, OH: American Society forNondestructive Testing (2004).

16. Nondestructive Testing Handbook, thirdedition: Vol. 6, Acoustic EmissionTesting. Columbus, OH: AmericanSociety for Nondestructive Testing(2005).

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

18. API RP 5A5, Recommended Practice forField Inspection of New Casing, Tubing,and Plain End Drill Pipe, sixth edition.Washington, DC: American PetroleumInstitute (1999).

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

20. 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).

21. 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.

22. 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.


References


23. “Ultrasonic Flaw Detection.” TheGlossary of Terms Used in NondestructiveTesting. British Standard 3683, Part 4.London, England: British StandardsInstitute (1985). Superseded by EN1330-4, Nondestructive Terminology:Part 4, “Terms Used in UltrasonicTesting.” Brussels, Belgium: EuropeanCommittee for Standardization (2000).

24. MIL-STD-371, Glossary of Terms andDefinitions for Ultrasonic TestingProcedures. Washington, DC: UnitedStates Department of Defense, UnitedStates Army (October 1987).



Stephen D. Hart, Alexandria, Virginia

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