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Review 249

Acoustic emission for physical examinationof metalsby H. N. G. Wadley, C. B. Scruby, and J. H. Speake

Acoustic -emission techniques are beginningto be frequently used as a means of detectingand locating deformation and fractureprocesses in both metallic and non-metallicengineering structures. This has happeneddespite a relatively poor understanding ofthe basic physical processes involved in thegeneration, propagation, and detection ofacoustic -emission signals, and of theinfluence in metals of factors such ascomposition and microstructure, upon theacoustic emission from different types ofdeformation and fracture processes. Themeasurement of acoustic emission, poten-tially, could yield much information aboutdynamic aspects of these processes. How-ever, it is shown that the limitations ofexisting acoustic-emission recordinginstrumentation have enabled only qualitativeinformation to be obtained to date. Even thisdata, however, has demonstrated that metal-lurgical variables greatly affect the acoustic-emission response of metals, and simplemodels to account for this are discussed.

Acoustic emission is the name given both to theelastic waves generated within a solid as a con-sequence of deformation and fracture processes,and to the techniques employed for their measure-ment. Acoustic emission from metals has beenstudied for some 30 years now, but serious interestand development of the technique did not take placeuntil the 1960s. At that time it was heralded as themost significant test technique to appear for manyyears, and extravagant claims were made about itsability to detect, locate, and identify flaws in struc-tures, and also about its potential for studying thedynamics of deformation and fracture processes.Some of this earlier optimism has. subsequentlybeen shown to be unfounded, partly because verylittle attention was paid to the fundamental aspectsof the emission process, and also because thequalitative nature of the measurements has maderesults from different workers very difficult tocompare. Commercial pressures to obtain resultsin the field on full-scale structures must be heldpartly responsible for the lack of fundamental

Dr H. N. G. Wadley is with the Metallurgy Divisionand Dr C. B. Scruby the Materials Physics Division,Atomic Energy Research Establishment, Harwell,Oxfordshire. Dr J. H. Speake was with the MaterialsPhysics Division, AERE; he is now with PantatronSystems Ltd, Motherwell, Scotland.

studies, but it is also true that the necessary wide-band recording facilities have only recently be-come available. In recent years, an increasingeffort has been devoted to the understanding of thefundamental aspects of acoustic emission, such asthe nature of the source, and the way in which theelastic waves are propagated and detected, etc.Thus, it is an opportune moment to review thesubject of acoustic emission, both as a laboratoryinstrument and as a viable means of monitoringstructural integrity.

This review has four principal sections: thefirst deals with the nature of the acoustic -emissionsource, and the problems of detection and interpre-tation after propagation of the signal through thematerial of interest to the detector; the seconddeals with the emission arising during plastic de-formation; the third with emission from the pro-pagation of cracks; and the fourth with theapplication of acoustic emission to the detection,location, and assessment of flaws in structures.The consequences of the use of acoustic emissionfor monitoring the structural integrity of metallicstructures are discussed in the last section.

DETECTION AND INTERPRETATION OFACOUSTIC EMISSION

In this section the limited theoretical understand-ing of the emission -source function, the propaga-tion of elastic waves through the specimen, and thedetection and interpretation of the emission signalsare highlighted. In addition, the practical problemsinherent in many of the techniques currently beingused for measurement of acoustic emission arediscussed.

The source

Acoustic emission is generated during transientchanges in the local stress and strain fields withina material. Such changes accompany deformation,fracture, or phase-transformation processes. It ishoped that analysis of the acoustic-emission wave-forms will provide new information about thedynamic features of such processes. The energyof the elastic waves which constitute the emissionwaveform is provided by the work done by theexternal load on the testpiece.

A number of attempts have been made to modelacoustic -emission sources which can be broadlyclassified into two general types. One type con-siders the source as a generator of elastic radia-tion energy and uses macroscopic parameterssuch as bulk stress and strain to obtain a static

International Metals Reviews, 1980 NO.2 41

partitionprocess

42 Wadley et al.: Acoustic emission

solution to the problem. This approach ignoreshow the source relaxations ;are,r.elated to thedeformation process itself. The other, and inmany ways more difficult, approach involves usingthe local time-varying stress and strain fields inthe vicinity of the source to calculate the dynamicchanges associated with operation of the source.

Static-source models

This approach considers an energy partition pro-cess at the source, 1 Fig. 1. The stored energy,available to a source for generating elastic waves,is not totally converted into acoustic -emissionradiation: some is converted into surface energy(owing to the creation of new crack faces and/orsurface slip steps), some into the energy of adislocation network, and some into heat as aresult of plastic deformation. Only a fraction ofthe available energy is radiated as transient elasticwaves which constitute acoustic emission, andwhich, ultimately, are converted to thermal energy.

If the energy of the acoustic emission couldbe measured and the partition function determined,then it might be possible to estimate the energyof the source event. This would provide a meansfor relating fracture mechanics to microscopicfracture processes. However, this proves ratherdifficult in practice. First, the fraction of storedenergy radiated as elastic waves depends criticallyboth on the nature of the source mechanism and onthe properties of the surrounding material. Inparticular, the rate of energy release can have agreat influence upon the partition of energy amongthe dissipative processes, Le. if it is rapid, the

,Istored lattice-

~II lstrain energy

I II I

II II I

I III Inew fracture-

I /1 /~source I / .....-/// ,surface energyevent release of: / //// ::::::---. • t:. I

e.g.crack strain "........................:growth energy \ Ithermal" ..•.•....•.... ~

'\, :energy" I, I

" I", I

'\ :elastic..:wave'-......lenergyI

1 illustration of partition of elastic energyduring a crack -growth event: stored elastic-strain energy is released in form of newsurface energy, thermal energy, and a transientelastic waveform called acoustic emission;after Birchon 1

International Metals Reviews, 1980 JNo. 2

material surrounding the emission source mayhave insufficient time to absorb the energy, and soa substantial fraction may be radiated as elasticwaves.2 Thus, the partition function is itself avariable for different sources, and unless it canbe determined for each source it is impossible torelate the energy of the elastic waves to the totalenergy of the source.

A second, and equally important, drawback tothis approach is that even if the partition functionwere known, the fraction of elastic-wave energydetected by the transducer could vary dependingupon the position of the source. This arises be-cause longitudinal and shear waves propagate in ametal at different velocities .while surface wavesare dispersive and are subject to frequency-dependent attenuation. The introduction of surfacescan also change the effective source-to-detectordistance making it very difficult to account for theeffects of attenuation and velocity differences. Inaddition, the energy radiated in a specified direc-tion from the source is a function of sourcegeometry. Thus, the proportion of the emittedenergy actually reaching the sensor (and thus theestimated source energy) is a complex functionof the configuration of the source, testpiece, andsensor properties.

Despite these drawbacks a number of authorsdeveloped static -source models to explain theirexperimental measurements: James and Carpen-ter3 consider the breakaway of dislocations frompinning points as a model of the emission. sourcein LiF, while both Pascual4 and Sedgwick 5 con-sider the multiplication process as the source inaluminium alloys and alkali halides, respectively.Fisher and Lally6 consider individual cooperativeslip events as the source, and attempt to estimatedislocation velocities. Both Kiesewetter andSchiller7 and MasonS attempt to relate acousticemission to the operation of Frank-Read sources.Gillis9, 10 and Gillis and Hamstad11 have discussedthe relationships required to provide new insightinto energy-release process. However, the quanti-tative relationships required to provide newinsight into deformation and fracture processeshave yet to be calculated, principally for thereasons already discussed in this section.

Dynamic-source models

Pollock 12 has developed a more dynamic-sourcemodel based on a mass supported between twosprings. When the spring constant of one springwas instantaneously -changed, the resulting shift tothe new equilibrium was considered as theemis-sion -generating process. While his model servesto emphasize that acoustic -emissions are gener-ated by relaxation processes, it is much :too sim-plified for modelling the source -functions fordeformation or fracture processes in metals.

A more fundamental approach to modellingdislocation sources of acoustic emission has beenattempted by Kiesewetter and Schiller 7 and by

Wadley et al.: Acoustic emission 43

to the case where it is moving at constant velocityalong a line inclined to the surface. His resultsshow that for subsonic velocities the generatedelastic waves differ only in amplitude from thoseof a stationary source. These effects could be-come important for rapidly growing cracks.

2 Model used by Pekeris and Lifson 16 to calcu-late surface displacements consisted of a aforce monopole F at vertical depth h with bH( t) time variation; c at epicentre E verticalcomponent of surface displacement consistedof a sharply rising wave step caused byarrival of compressional wave at h/vL,

followed at h/v s by shear wave leading to aconstant, ~inite displacement in direction of F

( 1)

+gzwOu~

O::UI----

65 H(t)If)l.L (b)

TIME-.

Fd = k Jlk

sourceevent

Elastic -wave propagation

During the operation of a source, elastic wavesare generated and it is important to understandtheir propagation behaviour within a solid. Pekerisand Lifson 16 have calculated both the vertical andhorizontal components of the surface displace-ments at various positions on the surface of a half-space, for a monopole source with Heaviside-function time dependence. Their analysis showsthat the initial vertical displacement at theepic entre (the position on the surface verticallyabove the source, Fig. 2) consists of a step functionwhich has the same time dependence as the source,arrives at a time h/vL, where h is the depth of thesource and v L the longitudinal-wave speed, and hasan amplitude d related to the source-force mag-·nitude F by

where Jl is the shear modulus and k can be calcu-1ated from the material elastic constants. Follow-ing the longitudinal-wave arrival, the surfacecontinues to rise until the time hlvs when theshear wave reduces the velocity of the surfaceeventually to zero, to give a permanent displace-ment. This important result shows that it ispossible to relate the time variation of a surface-displacement waveform (Le. an acoustic-emissionsignal) to the time dependence of the force relaxa-tion of the source event. Breckenridge et al.2 4 andlater Scruby et al.2 5 have used the fracture ofglass capillaries and pencil leads to simulate amonopole source with Heaviside time variation.The waveforms so obtained are in remarkably goodagreement with theoretical prediction. Thus, if an

Mirabile, 13 who apply the work of Eshelby14 ondislocation motion. Eshelby treats the radiationfrom moving dislocations in an analogous mannerto the electromagnetic radiation from an accelerat-ing electric charge, and calculCjltes the acousticenergy radiated from an oscillating kink into thesurrounding medium as a function of dislocationparameters. 14 Kiesewetter and Schiller7 apply theresult to the Frank-Read source, to obtain corre-lations with dislocation -line length which agreewith their experimental observations. However, itis not clear how justified they are in their sim-plifying assumptions about oscillation frequency inthis particular application.

A recent approach, which is already offeringsome success, has been to model the acoustic-emission source in an analogous manner to thatused by seismologists for modelling earthquakes .15The simplest model16 for an acoustic-emissionsource is the point-force monopole, whose timevariation is a Heaviside function. The physicalpicture of such a source function is the final, rapidfracture of a tensile specimen, when the force ona very small area perpendicular to the force direc-tion suddenly falls from some approximately con-stant value to zero. The important advantage ofsuch a model is that it is possible to evaluate thesurface displacements of the specimen due to theoperation of the source. The earliest formulationof the problem, for a surface source was byLamb, 17 but Pekeris 18,19 solved the problem fora point source buried within the bulk of an elasticsolid (the more usual case for acoustic emissionduring deformation).

It is possible to extend the analysis to sourceshaving non-zero rise times and which may be re-presented by force dipoles, quadrupoles, or com-binations of multipoles with variable orientations,in order to describe more realistically the forcerelaxations occurring in the vicinity of some dis-location motion or crack-growth event. Neverthe-less, these point-force models pf the source areinadequate for a complete description of, forinstance, a crack front of given length and pre-scribed shape, which starts to grow at constantvelocity and then stops. In addition to the magni-tudes of the force relaxations, and their timedependence, it is also important to take into accountthe extended nature of the source. The generalcase is extremely complex and has yet to be solved,but Burridge and Willis20 and Willis21 have ob-tained solutions for expanding self-similar cracks,while Simmons and Clough22 have examined theeffects of interference of elastic waves generatedat the extremities of extended sources. Work is inprogress at a number of laboratories to apply thistype of analysis specifically to acoustic -emissionsources, and the results are awaited with interest.

To date, little consideration has been given tomodelling a dynamic -source event, in particularwhen the source velocity approaches the shear-wave velocity of the material. Roy23 has extendedthe existing theory for a stationary point source

International·Metals Reviews, 1980 NO.2


acoustic-emission waveform could be measured atthe epicentre, the source function could be deducedusing the theoretically calculated or an experi-mentally measured, transfer function.

A second feature of Pekeris and Lifson'scalculation is that the shape of the surface-dis-placement waveform is very sensitive to thedistance of the observation position from the epi-centre, Fig. 3. When this distance is large, thesurface-displacement waveform is dominated bysurface waves and it is not clear, at present, if themeasurement of these can give information aboutthe source. Certainly, seismologists can inferdetails about the mechanism of earthquakes frommeasurements at large epic entre distances.

Recently, two groups of workers have solvedthe problem of elastic -wave propagation in an -infinite plate.2 6,2 7 They calculated the displace-ments of the surfaces of an infinite plate inresponse to a point source either on the surface orwithin the plate. This development should nowenable properties of the source to be determined in

bounded plate-like bodies in addition to the infinitehalf-space, and prove important for the practicalapplication of acoustic emission.

Although the simple source used by Pekerisand Lifson accounts for the surface-displacementwaveforms from breaking glass capillaries (Fig. 4),it does not fully account for the surface displace-ments of acoustic -emission waveforms from frac-ture events in metals25;' source models based onmultipoles are required for this.

An additional effect arises through the neces-sity for using practical specimens. In semi-infinite specimens, the wavefront from a sourcereaches the surface at the epic entre in a formclosely related to that generated by the source,Fig. 5. In a bounded specimen, however, the initialwavefronts reaching a point on the surface mayhave undergone multiple reflections, interference,and mode conversions (Fig. 6), so that the surface-displacement waveform bears little resemblanceto the waveform generated by the source. Further-more, the later portions of the waveform will

1·4I

H=O

J:.=1000H

1:...=100H

I iL=40H

1-1 1-2 1-3 1-4

R

S

0.9 lO 1·1 1·2 1-31~lri

--~ H"=5

~ IH=10; ~I ir i

~ "H=20

{/\

;J(S

5·04·03·02·0lOo05

0·5 0'6 07 0·8O~ P, i I I

-0'5C 'spor...- P+ I I

-0'5t 'spO~ P, i I

-0'5[ 'SP

t1'0~ P-. °E +~ fsp......2'0~ I

~ 1·0 P

~ 0 'w 'SPU

:5 7·00....if) 6-0°5'0<i 4·0~ 3·0~ 2-0~ lO

o

1-5ii'

r=0'5H

r=0-25H

1:0·5 lO'N'

~

3 Vertical displacement Wat surface for different epicentral distances r: H is depth of source, T

is ct/(r2 + R2) 1/2, C is shear velocity; P, S, and R indicate arrival of compressional, shear, andRayleigh waves, respectively; SP indicates a diffracted wave which starts as S and is convertedinto P wave at surface; after Pekeris and Lifson16

International Metals Reviews, 1980 No.2


4 Oscillogram showing vertical displacement atepic entre on one surface of an AI-alloy blockresulting from a step load applied to oppositesurface; source was breaking of 0.1 mm dia.glass capillary and detector a capacitancetransducer; after Breckenridge et al.24

piezoelectrictransducer

I!L I-~~~-Isource functione.g. force-pulse dipole

surface displacementcomplex series ofreflections followedby specimen ringing

Conventional specimen geometry modifieselastic waveform generated by source byconverting energy into normal modes ofspecimen; it is then extremely difficult torecover original source information

6

o0-1-•...

z~100tj 90<t-I0..(j)

o-I<tUi=D:::W> 10~ 0·.············· .

~-IWD:::

5 Semi -infinite specimen: arrival of elasticwaves from source cause epicentral surfacedisplacements at epic entre closely related tosource

represent the response of the normal modes of thespecimen to a transient-pulse stimulation withinthe mate'rial and therefore provide more informa-tion about specimen geometry than the sourcefunction.

(2)y(t) == Z(t) * S(t)

where * stands for convolution. Taking the Fouriertransform of equation (2), the source spectrumz( v) is given by

z( v) == y( v) (3)s( v)

There have been a number of approaches tothe determination of source properties frommeasurements of acoustic-emission waveforms inbounded media. First, provided that the conditionsof time invariance and linearity can be satisfied(and this is often not the case) the problem can beconsidered in terms of convolution. If Z( t) is thesource function, S (t) the impulse response of thespecilnen, and y(t) the actual displacement at apoint on the surface

and Z( t) can be obtained from this by taking theinverse Fourier transform. Thus, if the acoustic-emission spectrum is divided by the transferfunction of the system it should, in principle, bepossible to evaluate Z(t). This approach, Le.deconvolution, has been developed by a number ofworkers,2S-30 with limited success. One pro-blem, to which little attention appears to havebeen paid, is the effect of 'noise', which can leadto large errors. 31 It is surprising therefore thatauthors make no comment on the validity of theirresults, and do not discuss· the frequency-depen-'dent errors which are introduced by deconvolution.It also proves difficult in practice to measure thetransfer function of the specimen.

A second approach is to compare values ofY( t) directly (without deconvolution) and seekempirical differences which can be related to

specimensurface

epicentral surfacedisplacementlongitudinal pulsereproduces sourcefunction

)acoustic-emissionsource

(F~F)(

source functione.g. force- pulse dipole



changes in source mechanism. In practice, thishas generally been performed in the frequencydomain, 32-3 5 when it is known as spectral analy-sis. However, even in this approach uncontrollablevariables can make it difficult to assess thevalidity of a result. Probably the most serious ofthese is that the transfer function, and hence theresponse of the normal modes of the specimen, isitself a function of the source position x within thedeformed material, so that equation (2) becomes

The detector

The surface -displacement waveform produced byan acoustic-emission source is detected with atransducer. The transducer converts mechanicalvibrations of the specimen into electrical signalswhich can then be amplified and processed to pro-vide information about the source.

Detection by a transducer of a surface-dis-placement waveform causes the surface displace-ment Y(t, x) to be convolved with the transducerresponse function T(t)

where V(t, x) is the electrical output of the trans-ducer. The ideal transducer should measure hori-zontal and vertical components of surface displace-ment (or velocity) at a point on the surface (sincethese are what are calculated from theory) and·convert these linearly into electrical signals over abandwidth covering the full spectrum of the surfacedisplacements. This bandwidth could extend up to,or even above, 100 MHz. For example, consider thecreation of a 30 Jlm long crack in steel, growing atabout 10%of the shear-wave velocity, Le. at300 ms -1, then the source lifetime would be about10-7 s and elastic waves with frequencies up to andabove 10 MHz would be generated. Higher velocitiesand/or shorter cracks could generate much higherfrequencies. In addition, the transducer must havesufficient sensitivity to detect the small displace-

Metals attenuate ultrasonic waves and it isnecessary to consider the effects of materialattenuation upon the propagation of acoustic-emission waveforms. The degree of attenuationof an ultrasonic waveform depends upon bothmaterial properties (elastic constants, grain size)and wave properties, e.g. mode of propagation (L,S, or surface wave) and frequency. Papadakis36,37and Latiff38 have tabulated attenuation coefficientsfor bulk waves in a range of materials and havediscussed the changes in these coefficients as thefrequency is varied. In the majority of experimentsthe measurable effect of material attenuation isreduction in amplitude of the high -frequency com-ponents of the waveform. Thus, the source -to-detector distance should be minimized to reduce theeffect, or the frequency dependence of the attenua-tion should be measured and taken into consideration.

ments caused by the arrival of the acoustic-emission pulses, which are usually ~10-10 m. Sucha transducer does not, at present, exist. Themajority of acoustic-emission signals are detectedwith piezoelectric sensors (often constructed fromlead zirconate titanate), which have a high sensi-tivity (detecting displacements ;::.10-14m), butwhich only respond over a narrow band about theirresonance frequency, which is usually in the range50-1000 kHz. All transducers impose limitationsupon the amount of source information that can beretrieved from the detected signal and these arenow considered.

Piezoelectric transducers with resonancesbelow about 1 MHz, have sensitivities which droprapidly at higher frequencies, even when damped.This makes it impossible to cover fully the spec-trum of Y(t, x) and hence the source spectrum.Therefore, their use should be restricted todetecting and locating the positions of weakemission sources. Attempts have been made touse them in a standardized manner and to com-pare qualitatively the activity from different pro-cesses by, for example, amplitude- or energy-distribution analysis. When this is carried out, .great care has to be taken because only a fractionof the frequency spectrum is monitored and thedistribution of energy within a source spectrumcan vary between events so that the energymeasured in a narrow frequency band is not aknown constant fraction of the total. True quan-titative comparisons of energy are only feasible ifthe full frequency bandwidth of the source iscovered by the detector.

It is important to note that even over itsoperating bandwidth the mode of operation of apie zoelectric transducer may change, with a partof its operating range being displacement sensi-tive and other parts being velocity or perhapsacceleration sensitive,3 9 depending on the fre-quency and width of its resonance. In principle, itdoes not matter which of the three quantities,displacement, velocity, or acceleration, is actuallymeasured in an acoustic -emission experiment,provided it is known, because they are all inter-related by simple differentiation and integrationoperations. However, there are often practicallimitations which decide the form of detector tobe used. For example, there is a lower frequencylimit for accurate integration and similarly anupper frequency limit for differentiation. H mean-ingful comparisons between different experimentsare to be made, it is necessary that transducerswith a standard operation mode should be univers-ally used.

Although it is possible to standardizemeasurements with piezoelectric transducers itis not possible to perform an absolute calibrationon them at present, Le. the voltage output of thetransducer cannot be absolutely related to thesurface motion. Capacitance transducers develop-ed over the last five years at the US NationalBureau of Standards24 and at the Atomic Energy

(5)

(4)

V(t , x) = Y(t, x) * T(t)

Y(t, x) = Z(t) * S(t, x)


( 6)

Research Establishment, Harwell, 40 do not sufferfrom this drawback. They can be constructed tobe displacement sensitive over a frequency rangeextending from dc to more than 50 MHz with acalibrated response. They are, however, lesssensitive than a resonance device although stillable to detect displacements ~ 10-12 m. Becauseof this limitation, capacitance transducers arerestricted to monitoring the more energeticacoustic sources. This can prove a seriouslimitation in some low-strength materials, butless so during fracture in high -strength or brittlesteels.

Signal processing and interpretation

Bolin 41 has produced a bibliography listing recentpapers dealing with both the signal processing andinterpretation of acoustic -emission signals, whileStone and Dingwall 42 have discussed critically thesignificance of many of the commonly measuredparameters of such signals. Consequently thesewill only be considered briefly here. Befo~e doingthis, however, there are some general points whichshould be borne in mind when attempting to measureacoustic emissions.

In any acoustic-emission test it is importantto maximize the signal-to-noise ratio, and if thisis to be done without reducing bandwidth then thatmeans reducing the background -noise level whosechief origin is electromagnetic interference andamplifier white noise. Sometimes, spurious signalsare recorded, e.g. from friction rubbing at thegripping points on a tensile specimen. This prob-lem can be overcome by the careful choice ofspecimen test geometry and, if need be, by the useof several transducers not only to detect but alsoto locate the origin of each emission. Anothermajor problem is the very wide dynamic range ofacoustic-emission signals, whose displacementamplitudes may be less than 10-15 m and some-times more than 10-9 m; a range of 106• In addi-tion, the rate of generation of acoustic -emissionsignals can vary considerably. In some experi-ments, so-called 'continuous' emission has beenobserved where the low-amplitude acousticemissions are generated at such a high rate that~hey overlap in time to produce the effect of ahigh white -noise level. On the other hand, otherexperiments have generated perhaps only three orfour detectable so-called 'burst' emissions (high-amplitude discrete events) during a test lastingseveral hours.


Counting

Counting the number of times the signal amplitudeexceeds a present threshold during an experiment(the cumulative ringdown count) or its time deriva-tive (the ringdown-count rate) constitutes thecommonest method of displaying an acoustic-emission result.43 A lesser used technique is tocount the number of acoustic-emission events.The number of ringdown counts for an event de-tected by a transducer is44

vR VoN=-ln -{3 Vt

where v R is the resonant frequency of the trans-ducer, {3the logarithmic dec"rement, V0 the initialvoltage, and l't the threshold voltage.

The limitation of this technique is that theresults are strongly influenced by the geometryof the specimen, the properties of the detector andits bonding to the specimen, the precise detectionthreshold, and the performance of amplifiers andfilters. These combine to introduce both a degreeof irreproducibility in measurements and greatdifficulty in standardization of method which is soimportant for systematic studies. It is also quiteimpossible to relate ringdown -count measure-ments to properties of the source function and itis hoped therefore that the decline in the use ofsuch techniques for fundamental studies willcontinue.

Energy analysis

The acoustic-emission energy is generally assum-ed to be proportional to the integral of the squareof the transducer output voltage. The energy isusually measured after amplification of 80-100 dB,and over a limited bandwidth of typically 1 MHz.The commonly measured root mean square (RMS)voltage is closely related to the energy rate (Le.acoustic -emission 'power '). It is not possible torelate the measured energy to the acoustic -waveenergy for a variety of reasons (see, for example,Curtis 45). Two of the most important of theseare uncertainty in the mode of transducer opera-tion and only partial coverage of the source band-width by the detector (discussed above).

If a velocity-senSitive device is used (outputV:0ltage V proportional to surface velocity u), thenthe electrical energy Ee i$ proportional to ~the~coustic energy incident on the transducer Ea,I.e.

00 00

Ee a: J [V(t) ]2dt a: J [u(t) ]2dt a: Ea

In the frequency domain, using Rayleigh'stheorem 46

These properties of acoustic emission meanthat it is difficult to use a single, all encompassingparameter to describe an experimental resultuniquely. The most common ways in which signalshave been processed in the past are:

-00 -00

(7)

(i)(ii)(iii)(iv)

countingenergy analysisamplitude analysisfrequency analysis.

00

Ee a: J [U( v) ]2dv-00

where U(v) is the Fourier transform of u(t).

(8)



A relationship between the energies can be derivedin the frequency domain because46

E = 2A2T2 [sin(1TVoT~2 AT . (12)e TTVOT J

However, for a displacement transducer (Vproportional to surface displacement x) Ee is nolonger proportional to Ea since

Thus, no quantitative relationship can be drawnbetween electrical and acoustic -emission energyunless the pulse duration and bandwidth are known.

The advantage of using the RMS transducervoltage or power measurement is that it gives acontinuous measurement of a parameter of theemission which can be standardized and used forcomparative experiments. It must be emphasized,however, that extreme precautions have to be takenif this is to be related to some source property.

In this section the limitations of existinginstrumentation for the measurement of acousticemission have been highlighted and an attempt

Frequency analysis

Some of the implications and limitations of fre-quency analysis have already been discussed abovewhere it was shown that frequency analysis couldyield information about source rise time andfracture type. However, unless extreme pre-cautions are taken the analysis is limited to theobservation of changes in the frequency spectrumwhich it is hoped can be correlated with differenttypes of deformation process. The most commonlyemployed method of extracting frequency infor-mation from emission is a digital one"in which theemission waveform after amplification is passedinto a transient recorder to digitize the waveformfor subsequent access into a small computer.Standard Fourier transform routines then permitfrequency analysis. This digital approach requiresa digitization frequency of at least twice that ofthe maximum expected frequency and so evenbroad bandwidth systems are currently limited,by existing technology, to about 50· MHz becausethe maximum digitization rate is about 100 MHz.However, almost all frequency analysis has beendone with narrower bandwidth systems with anupper frequency limit of about 5 MHz. Digitaltechniques require a lot of instrumentation andmuch of the early work was done by direct obser-vation or by photographic recording and visualanalysis of shape change. Mullin and Mehan 49applied frequency analysis to boron - and carbon-reinforced epoxy systems and by analysing thepulse shape changes of emissions photographed onan oscilloscope showed that different frequ"encieswere associated with different failure processesin the composite. Unfortunately, their work wasnot rigorous and it is difficult to know how muchreliance to place on their results. Graham andAlers 50 applied frequency analysis to emissionsproduced during deformation and fracture of apressure-vessel steel. They found that in theirfrequency range (up to 2 MHz) two distinct typesof spectra were observed; one which was associ-ated with plastic flow and the other with crackextension. Rather surprisingly,. in view of thelimited bandwidth and the bounded system, theyfound the results were independent of the type oftest and sample geometry. Speake and Curtis 51using a limited bandwidtp detection system butusing constant -geometry specimens and varyingonly the fracture process, were able to show thatchanges in frequency content of the detectedemissions could be correlated with different fail-ure processes in carbon-fibre reinforced plastics.This significant result showed that different typesof failure process could be identified by frequencyanalysis as long as other variables were con-trolled. This finding has also been supported byOno and Ucisik,52 who analysed the emission fromcompact tension specimens of three high -strengthaluminium alloys.

(9)

(10)

(11)

(13)

U( v) = 21TivX( v)

-00

00

Ee ex: I [x(t) ]2dt

Hence

E Joo [U( v)]2 de ex: 2 V

-00 v

Comparison with equation (8) shows that the energyspectrum now contains a frequency-dependentterm. This frequency dependence of the measuredenergy can be eliminated by differentiatj()n of thevoltage signal before squaring and integrating.

The effect of bandwidth on measured energyis illustrated by considering a rectangular pulseof amplitude A and duration T, and system band ....width ~ T,about the frequency vo. Then

where V is the amplitude of a transducer voltage,Vo the lowest detectable amplitude, and b is thedistribution characteristic.

The b-value has been used to characterizeemission from different processes. Typically, bis in the range 0.2-0.4; small b-values beingindicative of a high proportion of energetic relaxa-tion events. There is, however, no fundamentalimportance in the precise b-value measured in anexperiment, but it can serve empirically to charac-terize different forms of emission.48

A11zplitude analysisIn this technique, which On047 .has recently re-viewed, the amplitudes of the voltage signals froma piezoelectric transducer are plotted as distri-butions and then compared. It has commonly beenfound that the number of signals exceeding a speci-fied level is given by

N( a) = ( v:) -b



•....WI-<!

~ 40::.10 I-

Z::>oU

zoU5(j)

1032:wI

U~(j)

::>oU<!X

102 <02·52·0

acousticemission

E: = 2·8 X 10-5 S-1

E:: = 5·5 X 10-5 S-1-~-

0·5 1-0 1-5STRAIN, 010

Dependence of acoustic -emission count rate onstrain and strain rate for eu single crystalorientated for single slip; after Jax andEisenblatier60

o

lO

0·8

•....(j)(j)

W0::tn 0-4

8

NIEz2:0·6

Jax el ale 60,61 have observed the emissioncount rate and RMS voltage from single-slip-oriented crystals of copper, both as a function ofstrain and strain rate. During constant strain-rate deformation they observed a sudden rise inemission count rate during stage I, followed by asteady decrease as the work -hardening rate in-creased, Fig. 8. The emission rate at a particular

105

A number of studies proposed models ofacoustic -emission sources based upon specificdeformation mechanisms, e.g. Frank-Read-sourceoperation 5,58 or the unpinning of dislocations. 6,59For both mechanisms the source of emission isenvisaged as the rapid collective motion of a largenumber of dislocations similar to the model pro-posed by Fisher and Lally. 6 Let us now proceedto build a more complete description of the sourcesof acoustic emission during deformation and, inparticular, account for the dependence of emissionactivity upon microstructure variables such asgrain size and composition. Throughout it must beremembered that the measured emission bearslittle resemblance to the elastic waves generatedat the source and consequently much of the ensuingdiscussion is, necessarily, qualitative.Single crystals

Single crystals are the best metallurgically de-fined materials, and a considerable understandinghas been gained of their deformation mechanisms.The emission from face-centred cubic singlecrystals is usually of the so-called continuous typeand consequently individual events cannot be ob-served. The inference of the source mechanismrests on a comparison of the strain and strain-rate dependence of the emission and deformationbehaviour.

acoustic emission

-.....-,/

// ill/'

/ /\hypothetical/ stress - strain

//

/

/IT/-_/

I

made to emphasize the approach required if fun-damentally significant results are eventually tobe obtained. Nevertheless, many studies have beenreported where results have been obtained usingless than ideal measurement techniques. Thesestudies have revealed much about the mechanismsof deformation and fracture which are the sourcesof detectable emissions in metals.

7 Dependence of acoustic -emission count rate onstrain for Cu single crystal from Fisher andLally,6 reported by Ying53

o 2 4

The mechanisms of deformation and fracturewhich generate detectable acoustic emission areonly now becoming understood. The aim in thissection is to clarify the understanding of thesource of emission in terms of dislocation dynam-ics in the light of these recent studies. Theexperiments reported up to about 1973 have beenextensively reviewed by others, 53- 56 and it wasabundantly clear that the emission was generatedby the motion of dislocations but that the mechan-ism of deformation could have a profound effectupon the observed form of the emission. Ying,53in his review, attempted to relate the strain de-pendence of the acoustic-emission count rateobserved by Fisher and Lally 6 from copper singlecrystals oriented for single slip to simultan€ouschanges in total dislocation density, Fig. 7. Althougha fair correlation after stage I was found, it wasclear that yield and stage I deformation generatedconsiderably more emission than the model pre-dicted. Attempts have been made by others torelate parameters of acoustic-emission activity57

(e.g. count rate) to the mobile component of thedislocation density, both in 7075-T6 (Al-Zn-Mg)aluminium alloy58 and alkali halide crystals.3However, the assumption that all mobile disloca-tions contribute equally to the observed emissionalso appears to be a gross oversimplification.

ACOUSTIC EMISSION DURING PLASTICDE FORMA TION

1(/)800w-~0::

~600::>oUz0400U5(j)

~wu200~::>o~



where Am is the mobile dislocation line length, bis the Burgers vector, and v the mean dislocationvelocity (a constant for changes of strain ratesince the flow stress did not vary significantly),they considered the emission increase with strainrate to be due to an increase in moving disloca-tion line length. This postulate was also con-sistent with the observation that the mean square

strain also increased with increasing strain rate,presumably only because the rate of sourceactivation had also increased, rather than theinitiation of a new strain -rate-sensitive emissionsource. Jax et ale postulated the emission sourceto be the unpinning of dislocations resulting in thepropagation of groups of dislocations across aslip plane. The strain dependence of the emissionwas attributed to a strain dependence of the meanfree path of unpinned dislocations which is amaximum at the start of deformation anddecreases during stage II deformation.

Kiesewetter and Schiller 7 reported a similarstrain and strain -rate dependence for the -emis-sion from aluminium single crystals, Fig 9. Un-fortunately, the differences in apparent emissionbehaviour between aluminium and copper crystalsof similar orientation cannot be directly attributedto changes in deformation behaviour because ofthe absence of a standardized measurement tech-nique. Kiesewetter and Schiller observed, likeJax, that the mean square emission voltage wasproportional to strain rate E. Since the strainr ate is given by

E = Am b v

12

10>~

( 14)

emission was proportional to crystal length at aparticular flow stress and strain rate.

Using Eshelby's theory14 of the elastic radia-tion from an oscillating screw dislocation, theyproposed that the emission dependence. upon strainresulted from a reduction in the size to whichdislocation loops (generated from Frank-Readsources) could expand. Unfortunately, their supportfor this model was based upon the assumption thatthe mean dislocation velocity did not vary duringdeformation from yield to fracture. This is anoversimplification; Gorman et al.62 have shownthat dislocation velocity is proportional to flowstress in aluminium and it is quite possible forthe mean speed of dislocations to vary by as muchas a factor of 100 during a test.

A weak aspect of all these theories of acoustic-emission sources has been the lack of informationabout dislocation motion occurring during an ex-periment. Imanaka et al.63 extended normalmeasurements of acoustic emission and stress-strain behaviour. In a novel experiment theysimultaneously monitored the stress, strain,acoustic-emission amplitude and count rate, andthe ultrasonic attenuation of 27 MHz longitudinalwaves during compression deformation of coppersingle crystals. They deduced that the deformationsequence during this experiment started with dis-locations detaching themselves from impurity-atompinning points at low stre.ss levels. As the stressrose, the free dislocation seg~ent length increas-ed toward a critical value of 10-6 m. Once thecritical length was reached rapid dislocationmultiplication occurred by the activation of Frank-Read sources. They suggested that the emissionsource during this sequence of events was thesudden (timescale less than 4 x 10-6 s) increasein the mobile -dislocation density associated withFrank-Read-source operation.

Dependence of RMS voltage of· acoustic -emission signals on strain from an A1 singlecrystal; after Kiesewetter and Schiller 7

The origin of the emission in both copper andaluminium appears the same; namely, the suddenrelease of dislocations from a source followedby their propagation as a group over an appreci-able fraction of the crystal glide plane. Emissionmay, as mentioned above be the Eshelby radiationaccompanying dislocation accelerations, or it maybe just the re -arrangement of the elastic strainenergy field following an abrupt local plastic re-laxation. However, it is clear that not everymotion of each dislocation is detected in a test.Those events incorporating the greatest numberof dislocations moving over the largest fractionof the glide plane will be most easily detected.Less energetic motions could generate signalsbelow the limit of detection and consequentlywould fail to contribute to the emission signalfrom the transducer. Processes which restrictthe magnitude of slip events, for instance theforest interaction, cause a reduction in acoustic-emission activity, and are the origin. of the lossof detectable emission after d~formation to highstrains.

If)2:0::

6

stress ...J«z(9U)zQIf)If)-- ~--/-- If

/ -- Uacoustic f=emission ~

ou«

2 3 4 5STRAIN, °/0

o

N 8IEz2:v)'6 /~ /f= /If) 4 I

/II

9

International Metals Reviews, 1980 NO.2


was least localized by high stacking fault energy.Siegel67 in a related study with polycrystals, re-ported a strong dependence of emission activityupon stacking fault energy during unloading. Herelated the increased emission from low stackingfault energy metals to the larger reverse plasticstrains.

A number of important experiments have notbeen performed to date. Temperature variations,despite their well known effects upon dislocationmotion, have not been studied, presumably because

, of simultaneous variations in transducer / couplantefficiency. In addition, there is a need for a seriesof systematic experiments to study the influence ofcrystal structure, composition, and microstructure.

The strain dependence of acoustic-emissionactivity would be expected to be dependent uponcrystal orientation, stacking fault energy, com-position, and test temperature, since each influen-ces the magnitude of slip events.

The effect of crystal orientation upon thestrain dependence of the emission voltage has beenreported by Kuribayashi et al.64 They' observed theacoustic emission from 99. 9% aluminium crystalswith four orientations, which in all cases was simi-lar to that observed by Kiesewetter and Schiller. 7

However, the degree of emission at a fixed strainwas dependent upon crystal orientation. The great-est activity was observed at strains of O.5-0.7/0

from crystals with orientations either close to themiddle of the (001)1(111> boundary or at the centreof the (100)(110>(111>stereographic triangle. The Fcc polycrystalsleast activity was observed from the crystal with One way of testing the models used to account foran orientation about halfway along the (100)/ (110) the emission from single crystals is to study theooundary. However, these differences became less emission activity, when one component of the micro-apparent after deformation to high strains. Although structure (which affects the source process) isthere have been detailed studies of the orientation systematically varied, e.g. the grain size. Onedependence of slip processes65 the results of effect of reducing the grain size is to restrict theKuribayashi et ale are apparently not simply inter- area of individual slip events which should leadpreted. In particular, the observation of similar therefore to a reduction in emission activity.levels of emission from crystals with single- andmultiple-slip orientations would not have been Bill68 has reported the effect of grain sizeexpected and remains unexplained. upon the acoustic -emission count for both alum in-

ium and copper. For aluminium, little emissionChanges of stacking fault energy are expected was observed for strains ~5 x 10-4 and no grain-

to have a significant effect upon the emission size dependence was found. However, when theactivity because it is this parameter which deter- metal was strained to 2/

0a strong grain-size de-

mines the ease of cross-slip, and the degree of pendence was measured (Fig. 10), which was muchstrain localization within a slip band. Hatan066 greater than the corresponding yield-stresshas used wide-band recording techniques (0.1- dependence. Bill suggested that the emission was4 MHz) to monitor the acoustic-emission power generated by the operation of 'tlnstable' disloca-accompanying the deformation of a range of tion sources located near grain boundaries. Suchmaterials with different stacking fault energies. sources were considered to be activated duringThe peak acoustic-emission power recorded during the progression of slip from grain to grain. Thesystematic testing of the materials is shown in grain -size in<;lependenceof the emission activityTable 1, and it is clear that the emission decreases during microstrain was attributed to an emissionrapidly with decreasing stacking fault energy. This source controlled by the forest-dislocation dis-is consistent with the emission increasing with tribution (grain -size independent).increasing ease of cross-slip. However, the resultsneed to be treated with caution because other metal- The results obtained by Bill and their interpre-lurgical variables, which can also· modify the tation demonstrate the severe problems encoun-acoustic emission, were uncontrolled in these ex- tered in the interpretation of continuous emissionperiments. Somewhat surprisingly, acoustic measured by a counting technique. Kiesewetter andemission was most energetic when deformation Schiller7 have measured the mean square voltage

Table 1 Acoustic -emission power and stacking fault energy for some fcc metals

Material

Stacking fault

energy, IlJcm-2

Acoustic-emission power,pW

Single crystals Polycrystals

Cu 410

Cu-O. 34Al 390

Cu-1.15Al 330

a-brass (Cu-30Zn) 140

Stainless steel(18Cr-8Ni) 100

4

1.20.08

1.3

0.06

0.00



1000 500 333 250 166 143 125AVERAGE GRAIN SIZE, I-lm

10 Grain-size dependence of acoustic-emissioncount (cumulated up to a stress of 8.3 MNm-2for each test) for pure AI; after Bill68

activity of the two specimens with largest grain sizewere similar, but reduction of grain size led toan increase in emission activity. According to thesimple model discussed above, the emissionactivity should decrease with decreasing grainsize. The discrepancy between model predictionand experimental observation could be a conse-quence of the higher impurity concentration inKishi et ale 's material. This could serve to re-strict cooperative dislocation slip processes tospecimens with the highest flow stresses (smallestgrain size) due to the drag of the Cottrellatmospheres. Further work is required to re-solve this inconsistency.

The errors that can be introduced throughequipment limitations and which are rarely dis-cussed in the literature, have recently been high-lighted by a systematic study of the grain -sizeand frequency dependence of the acoustic emissionfrom 99.99/0 purity aluminium reported byFleischmann et al.7o They measured the acoustic-emission energy from deforming polycrystallinealuminium at three frequencies (0.94, 1. 56, and2.17 MHz). They observed, at each frequency, asimilar dependence of acoustic -emission activityupon strain to that reported by Kiesewetter andSchiller 7 with a maximum just after yield. How-ever, at constant grain size, the position of theemission maximum was shifted to higher strainsand the breadth of the peak widened as the obser-vation frequency increased. In addition, when thegrain size was varied, the position of the peak atfixed frequency moved to higher strains withincreasing grain size. These results were inter-preted by recourse to a model in which the life-time of each slip event generating an acoustic-emission signal was proportional to the obstaclespacing (sessile dislocations or grain boundaries).As the obstacle spacing decreased· the lifetime ofeach emission event was reduced and led to anincrease in the higher frequency components ofthe emission. Thus, restriction of the frequencywindow through which measurements are madecan lead to considerable errors which casts adegree of doubt on the validity of all narrow-bandresults.

The origin of acoustic emission from singlecrystals appears with only small modification toaccount for some of the measurements made onfine-grain polycrystals. Major .ambiguities re-main however, and it is important that the truegrain -size dependence for fcc metals be measuredaccurately to confirm the results of either Bi1l68

or Kiesewetter and Schiller. 7

Non-ferrous alloys

Another microstructural parameter that willmodify the sources of emission is material puritysince it can affect both the magnitude and timedependence of slip events.

The most frequently reported experimentshave been performed with aluminium alloys and,

o

20

gg·9g%AI0·2mm.grain size

gg·9g%AIunannealedcondition

gg·9g%AI2mm grain size

gg'9g%AIsingle crystal

8-3 MN m-2 applied tensile stress0'2V trigger-level setting

o

o

z0if) 600 0 0(j)

~ 00 0 0

W400I 0

U 0

~ 200 0::>0U 0«

I-Z::>oU800

when the grain size of aluminium was varied inthe range 0.05-2 mm. The emission from eachspecimen showed a similar flow-stress dependencebut in contrast to Bill's work, the emission activity'increased with increasing grain size approachingthat of a single crystal, Fig. 11. These resultswere interpreted by considering the systematicchanges in slip increment accompanying changesof grain size. For single crystals, typical slipincrements were in the range 5-20 /lm, the lowestvalues being observed after extensive work hard-ening, the highest values being found at yield. Thedisplacement of the characteristic curve of poly-crystals below that of the single crystal was thenconsidered to reflect the reduction in slip areawhen grains with several intersecting slip systemsare deformed.

Kishi et ale 69 measured the RMS emissionusing a resonant transducer during the deforma-tion of polycrystalline aluminium. They testedfour specimens of 99. 9/0 purity aluminium withgrain sizes of 30,43,60, and 94 /lm. The emission

5 10 15STRESS, MN m-2

11 Stress dependence of mean square voltage ofacoustic-emission signals from AI specimenswith a range of grain sizes; after Kiesewetterand Schiller 7

t:§ 2·00::W";""o...(/)

(\J

6~7'5if)~wLI-W ~ 7-0~z1--tf)«::>0::01-U tf) 0·5« 9g·5% AI 0-5 mmtf) grain sizeL

International Metals Reviews, 1980 NQ.2


o

15

o 20 30 50AGING TIME, h

13 Effect of isothermal tempering at 170°C onyield-region acoustic-emission energy ofquenched Al-4Cu; after Wadley and Scruby2

only returning when the material had been over-aged at higher temperatures. EisenbHitter et al.61

have reported some of the work of Zeittler, whoobserved similar trends in yield -emission activityduring the deformation of isothermally agedAICuMg2 alloys, Fig. 12. Wadley and Scruby2 andFenici et ale 74 have independently made studiesof the effect of aging on the acoustic emissionfrom high -purity AI-4Cu and obtained similarresults. In the former study solution -treatedmaterial was systematically aged at 170°C forperiods up to 48 h. There was a pronouncedincrease in yield -emission activity after thequenched material has been aged for short periodsof time. However, the detected emission energydepended critically on aging time, and increasedto a maximum after about 2 h before decreasingagain, Fig. 13. This peak in activity, whichoccurred a long time before peak hardness (2-4 dat 170°C),was observed when the microstructurecontained a fine dispersion of Guinier-Preston(GP) zones, Fig. 14. A simple model was proposedto account for the aging peak, based on the changesin deformation mechanism as aging progressed.In the quenched condition, deformation was homo-geneous and continuous in as much as there werecomparatively few obstacles that could temporarily

14 Transmission electron micrograph of micro-structure of AI-4Cu after quenching and aging1 h at 170°C;small (100 A dia.) GP zones on{lOO}planes; after Wadley and Scruby2

2·0 10(a)1-6 stress 8

1-2 495°C, 2h, H20 quench 6180°C, 336 h, H20 quench

4

2>o E

(b) stress10 -l

<t:C\l z'E 1-6 495°C, 2h, H20 quench 8(9z 180°C, 24h, H20 quench if)2:

6 6ro- 1-2(j) if)(j) (j)

~ 08 4 2:f- W(j)

I

2 uf-(j)

=>00

(c) 10~

1-6 8

1·2 6

4

495°C, 2h, H20 quench180°C, 0'5h, H20 quench 2

00 40 50

to a lesser extent, brass. In most tests two com-ponents of acoustic emission (burst and continuous)are observed, but the proportion of each and thetotal of both, is often very dependent upon the de-tailed composition and heat treatment of the testmaterial. 57,66

A great diversity in acoustic-emissionactivity has been observed during the deformationof aluminium alloys. During constant strain -rateexperiments most acoustic activity is usually(but not always) observed in the vicinity ofyield. 4,60,62,70-78 Two types of model have beenused in an attempt to explain the source process.

Dislocation models

Agarwal et al.59 have reported the acoustic-emission activity associated with the deformationof solution -treated and room -temperature aged2024 (AI-Cu-Mg) aluminium alloys. They ob-served a rapid decrease in acoustic-emissioncount as the aging time was increased, the count

12 Variation of voltage of acoustic-emissionsignals with time during constant extension-rate tests of AlCuMg2 alloy in three heat-treated conditions; from Zeittler, reported byEisenbHitter et al.6 1



0·15

0·2

temperature aging, and also the increase in acti-vity in the overaged condition because of an in-crease in interparticle spacing to 2-3 /lm. Thetwo models are complementary, that of Wadleyand Scruby2 being applicable to early stages inaging and that of Agarwal et ale 59 being applicableto later stages.

Other workers, including EisenbHitter et al.61

Hatano,66 and Hamstad and Mukherjee 72 have re-ported similar aging effects in 2024 (Al-Cu-Mg)and 7075 (AI-Zn-Mg) aluminium alloys, togetherwith the appearance of a second peak in emissionin the plastic region. This second peak, which canbe seen in Fig. 12, has not yet been explained; itmight be caused by an interaction between disloca-tions and solute atoms (Le. dynamic strain aging)but further work is required before any firm con-elusions are drawn.

Inclusion and precipitate fracture model

Copious burst emission, observed in the yieldregion of some aluminium alloys, has been attri-buted to the fracture of the inclusions and largeprecipitates that are always found in commercial-purity aluminium alloys. 75-77 Although the frac-ture of brittle inclusions cannot account for manyof the effects of aging on acoustic -emissionactivity, it is possible, under certain circum-stances, for it to be a significant component of thetotal emission from a commercial-grade alumin-ium alloy. Bianchetti et al.75 have observed aburst component of emission from 7075-T6, onlywhen the batch of material contained large (20-60 /lm) inclusions which underwent fractureduring testing, Fig. 15. The fracture of these in-elusions, which were rich in chromium and con-tained traces of iron, magnesium, titanium, andcopper, was considered to be the source of manyhigh -amplitude burst emissions. Brittle -inclusionfracture has also been considered to be a sourceof emission in 7075-T651 aluminium alloys. 72Wadley and Scruby77 have examined the emissionfrom commercial-purity AI-4Cu and AI-4. 5Mgalloys and observed many cracked inclusions afterthe fracture of specimens that exhibited burstactivity in the yield region. However, further,unpublished, work to relate the rate of particlefracture to the strain dependence of the acoustic-emission power has still not established completelythat cracking is the dominant source of acousticemission; most emission occurred in the vicinityof yield, but many inclusions fractured during thequiet period when the specimen had started toneck down. The cracking of inclusions in alumin-:-ium alloys may be one source of emission inmaterials containing large brittle inclusions. How-ever, inclusion fracture alone is incapable ofaccounting for all the emission activity. The ex-tent to which the emission activity is governed byinclusion cracking must depend upon the compo-sition and shape of the inclusions, their volumefraction, and the mechanical properties of thematrix. None of these factors as yet, have beensystematically examined.

>

0·10

(a)

inhibit the motion of dislocations other than Cott-rell atmospheres. As aging progressed the ob-stacle (GP(I) zones) strength increased and so thenumbers of dislocations that could be held in apile -up increased. As the stress then rose in thevicinity of yield it was conjectured that the dis-locations could break through some of the obstaclesleading to local precipitate reversion and a suddenstrain increment thus generating acoustic emis-sion. Continued aging led to an increase in thestrength of the obstacles and an inability in theyield region for dislocations to break through andreversion to occur.

Agarwal et ale 59 also attributed the emissionsource during yield to the operation of dislocationsources, by estimating both the minimum disloca-tion length that could act as a dislocation sourceand the slip distance that produced a detectabledisplacement at the transducer. During aging theaverage interparticle spacing decreased until atpeak hardness it was about 0.05 /lm. Taking thisvalue also to be the free-dislocation length, thestrains in peak -hardened material were shownto be below the sensitivity limit of the detectionsystem. This model adequately explains theirobserved decrease in activity during room-

4 6 12STRAIN, 010

15 Dependence of RMS voltage of acoustic -emis-sion signals on strain during deformation of7075-T6 AI alloy a one batch of material con-tained large inclusions and these generated ban additional component of burst emissions;alter Bianchetti et al.75

-S0·05<!z(.9li)zoU)(j)

2 0'8wI

UI-(j)

6 0'6u<!(j)

2: 0040:::



scale12505-1

trigger level60l-lV

scale12505-1

IIItrigger level 181-lVIIIIIII

STRAIN --.

trigger level181-lV

trigger level 60\...t.Vscale

1505-1

tf)tf)W0:::r-tf)

However, Tandon and Tangri 82 have recentlyinvestigated the sources of acoustic emissionduring the tensile deformation of Fe-3Si withvariable grain size. Using conventional countingtechniques combined with extensive etch -pitstudies of specimens tested to different strains,they attempted to compare the emission-strainrelationship (Fig. 16), with the strain variation ofgrain -boundary dislocation sources and alsoexamine the effect of grain size. During the micro-yield region about 100 bursts of emission wereobserved. Etch-pit studies revealed the presenceof dislocation pile-ups within grains close to thesurface of the specimen; these pile -ups appeared tohave been generated by grain -boundary sources.They considered the origin of emission to be thecooperative activation of perhaps 300 such sourcesto generate a single emission pulse. As the speci-men was further deformed a yield drop was ob-served. This was associated with the formation of aLuders band and an accompanying high emissionrate, which was attributed to the operation of dis-location sources. The band propagated along thelength of the specimen at approximately constantvelocity and gave less emission activity than theyield did. This, they attributed to the localizationof deformation in a band about 25 grains across.Once the band had propagated the length of thespecimen the operation of grain -boundary sourceswas inhibited so that the emission rate droppedduring work hardening. The effect of grain sizeupon the emission count during microyield is

i

a 35 j.1m average grain size; b 97 Jlm averagegrain size

16 Effect of threshold level on acoustic -emissioncount rate for two specimens of Fe-3Si withdiffering grain sizes; after Tandon and Tangri82

(14)

The fracture of brittle inclusions in aluminiumalloys has been extensively examined by van Stoneet al. 78 Using stop tests they examined the straindependence of inclusion fracture in 2014,2024,2124 (AI-Cu) and 7075 and 7079 (AI-Zn) alumin-ium alloys and found that the fraction of crackedinclusions was proportional to strain, the constantof proportionality varying from alloy to alloy. Inaddition, they found that their data fitted therelation

where uf is the stress at which a particle of dia-meter d, Young's modulus E, and surface energy y,fractures. The stress-concentration factor at theparticle was q. Thus, the largest inclusions shouldfracture first during a tensile test, to generatelarge -amplitude acoustic -emission transients.The fracture of inclusions seems likely to be asource of acoustic emission in commercial alloys,but further confirmatory work is needed.

Two aspects of the emission behaviour ofnon-ferrous alloys have been reported recentlybut, as yet, have received little systematic study.The first is the observation of the second acoustic-emission peak associated with the plastic regionin some alloys. 60,71 The second is the emissionassociated with dynamic strain aging (Portevin-Le Chatelier effect).4,61,70,76,78,79 Pascual4 hasobserved an increase in enlission before theoccurrence of a type A serration load drop duringserrated yielding, and an increase in emissionduring the occurrence of a type B serration loaddrop. He proposed that the emission source isassociated with increases in the mobile-dislocationdensity. Hatan080 has observed that the details ofthe emission during serrated yielding change asthe strain rate changes but has not attempted torelate this to dislocation behaviour. The suddenrelease of dislocations during the deformation ofmaterials exhibiting the Portevin-Le Chateliereffect, and the strain increment this causes wouldseem ideal sources of acoustic emission, and itwould be enlightening to ascertain the effects ofgrain size or of changing solute upon the emission.

Iron-base alloys

A full understanding of the emission from ferriticsteels should start with a detailed study of theemission from pure iron. There had been, in fact,little work reported on the sources of acousticemission in pure body-centred cubic metals in-cluding iron. The closest approach has been astudy by Higgens and Carpenter, 81 who examinedthe sources of acoustic emission in pure ironcontaining between O. 005 and O. 017/0 carbon. Theyobserved emission in both the elastic and plasticportions of the stress-strain curves. The formerthey attributed to stress-induced motion of mag-netic domain walls, and the latter to the breakawayof dislocations. One metallurgical variable, thegrain size, which would be expected to affect sucha process significantly, was not studied.



1000rNMy = total counts up

to 90% CJyiQld800 •

>- 600L

Zw

400

1-5

1·0zw<l

too~0·5

trigg<2r l<2v<21 181lVb.r N = total counts up to strain of 2·5%

- total counts up to 90% CJyiQld

•o 80 160 240 320 400 480 1260AVERAGE GRAIN SIZE, 11m

17 Effect of grain size on acoustic-emissioncount from Fe-3Si (cumulated up to 90~o ofO"yield); after Tandon and Tangri 82

200 •

• ••

o 100 200 300 400 1260AVERAGE GRAIN SIZE, 11m

18 Effect of grain size on acoustic -emissioncount from Fe-3Si (cumulated up to 90~o of~ield and a strain of 2. 5~o); after Tandon andTangri82

shown in Fig. 17 and during the interval between90'10of yield stress and 2. 5%strain in Fig. 18.The emission increased with increasing grainsize during microyield but like Bill's observationwith aluminium 68 decreased with increasing grainsize during post-yield deformation.

There have been a number of recent studiesof the emission from ferritic steels. Threedeformation processes have been proposed toaccount for the majority of the observations.

Lftders - band propagationCopious yield -region emission from mild steelshas been associated with the initiation and pro-pagation of Liiders bands.33,60,82 Again, mostactivity is observed during initiation of the band.Little systematic work has been reported, andone can only speculate that the emission sourceis similar to that proposed by Tandon and Tangrifor Fe-3Si. A similar source of emission in mildsteels, deformed in the temperature range 200°-300°C, is observed during serrated yielding, 60, 82 •

and is related to the propagat~on of groups ofdislocations in a cooperative manner.

Carbide fracture

Several investigations have proposed the fractureof carbide platelets as the source of acousticemission from pressure-vessel steels.60,83,84

This emission source is still in some doubt, sinceQno et al.3 5 have been unable to identify carbidefracture categorically as the emission source in4130 ferritic-pearlitic steel while Bentley andBirchon 83 considered the cracking of carbidesto be definitely undetectable. Clearly, furtherwork is required to reduce these inconsistencies.

Decohesion/fracture of inclusions

The brittle fracture of MnS inclusions has beenpostulated as a source of acoustic emission incommercial steels. 3 5,82 The emission activitydepends upon sulphur content and the orientationof the specimen with respect to the rolling axis,

Table 2 Relative acoustic -emission activities of 9 commercial steels. during crack propagation atroom temperature

IDtimateYield stress, tensile stress,

Designation Type MNm-2 MNm-2

~ 300-M High-strength steel 1615* 1960~....•:::- BS 1501-261 Mo-B steel 405 623....•~C)cod BS 1501-271 Mn-Cr-Mo-V steel 515 638bD~ BS 1501-151 Semi -killed C-Mn steel 232 431....•tI)codQ) BS 1501-213 Semi-killed, Nb-treated C-Mn steel 410 572~C)Q) BS 1501-224 Fully killed, Al grain -refined C-Mn steel 284 466Q

j, X60 C-Mn pipeline steel 405* 626

BS 1501-821 Austenitic stainless steel 342* 620

BS 1501-622 2. 25Cr-lMo steel 363* 543

*0.2% proof stress.


gain 99dB50 cross- h(Zad sp(Z(Zd 0-2mm min-1 E 10

g(Zn(Zral yi(Zld E 9 zz 40 load load •. 20z 8 0~ )._- -.., I 180 7 Vi

0"'30 _ // total ' ;<' ~~~6 ~« T (Zmission '.... 12 w 5 w~9 20 / count ,........... '........ 10~ 4 U$2

I "....... '1 8 w 3 ~ r.

10 I " c1(ZavaQ(Z-crack I 6 ~ 2 If)~I •. " (Zxt(Znslon l 4 u 6 zI ....•<d~til(Z-crack (Zxt(Znsion - 2 ~ 1 u5o u 0 «uo 2 4 6 8 10 12 14 16

CROSS-HEAD DISPLACEMENT, mm19 Total acoustic -emission count, load, and

extension as function of cross -head displace-ment for a C-Mn pressure-vessel steel pulledto failure at room temperature; after Palmer90

which suggests that decohesion of the inclusion/matrix interface may be a major source of emis-sion. Further work is required to relate the strain(and stress) dependence of inclusion fracture/deco-hesion to the strain dependence of detectableemissions. Systematic studies are required toisolate the role of inclusion aspect-ratio andorientation upon the magnitude of the emissionsignals.

ACOUSTIC EMISSION DURING CRACK GROWTH

The sources of acoustic emission during thedeformation of a uniaxial tensile specimen dis-cussed in the previous section can be considerablymodified in the multi -axial stress state that ex-ists in the vicinity of a crack, and may even beexhausted during the final stages of fracture. Theaddition of several modes of fracture, and theirsubtle dependence on detailed composition, heattreatment, and testing conditions all lead to agreat diversity in acoustic-emission activity.Clearly, a study of the emission characteristics(count rate, cumulative count, power, amplitudedistribution, etc.) during the early part of a tensiletest would alone be unable to predict the detailsof the mechanism of fracture. In this section anattempt is made to ascertain the processes ofdeformation and fracture that are sources ofacoustic emission in a crack-propagation experi-ment, in particular concentrating on the quietductile -crack problems, 85,86 before moving on todiscuss the sources of emission during environ-mentally assisted fracture and fatigue fracture.

Ductile -crack propagation

Copious acoustic -emission activity during crackgrowth might be expected because of the manylarge strain -energy relaxation events involved.However, the many studies of acoustic emissionduring ductile-crack growth (e.g. Refs. 85-101),reveal a wide range of activity: some materialsemit large amounts of emission, but others arealmost completely quiet. Ingham et al.8 5 have


2

o 1 2 3 4CLIP-GAUGE DISPLACEMENT

(arbitrary units)t , I , • I , I ! Io 1 234 5 6 789

ACOUSTIC-EMISSION COUNT RATE, 103 s-'20 Force (as function of clip-gauge displacement)

for a 300-M steel; acoustic -emission countrate is plotted as a function of force ratherthan of displacement as in Fig. 19; afterIngham et al.86

made a comparative study of the relative acoustic-emission activity during crack propagation in arange of steels with yield strengths between 230 and1620 MNm-2 (Table 2). The relative activity in-c reased with increasing yield stress and decreas-ing ductility. Steels undergoing ductile fracturecan (with only a few exceptions87,88) be classifiedbroadly into one of two categories.85,89-100 In thelower strength, but tough, steels the majority ofthe emission occurs during the period of plastic-zone expansion, and the increments of crackadvance appear quiet, 89-91 Fig. 19. However, whenhigh -strength steels are tested, much of the acti-vity occurs during the last 20~o of the test and theprocesses associated with plastic-zone expansionno longer dominate the total emission, 92 -100

Fig. 20. This lack of emission during crack growthin low-strength ductile materials has been termedthe ductile-crack problem and has been a majordrawback to the widespread application of acousticemission to engineering structures (see section on'Applications' below).

Plastic-zone expansionThe acoustic emission during plastic - zone expan-'sion in a precracked specimen has been linkedwith the fracture of brittle inclusions and/or theirdecohesion from the matrix, carbide cracking, andrather poorly defined dislocation processes. Noneof the proposed sources has as yet been conclu-sively shown to generate the major component ofthe emission during zone expansion.

The rate of acoustic-emission activity from atypical low-strength precracked steel specimen



where k is the applied stress intensity, L thedistance between the grips, W the specimen width,B the specimen thickness, Y a function of a/W(a is the crack length), and m the proportionality'constant between stress-wave amplitude and theload drop per unit thickness.

Work by Scruby et al.2 5,102 also shows alinear relationship between emission amplitudeand crack increment area, but only when thesurface displacement is measured at the epicentre.In their broad -band study the specimen wasdesigned to minimize distortion of the elasticwaves which are released by a crack-propagationevent. The source was restricted in position sothat it was vertically beneath the transducer, atadepth h. A capacitance transducer was then ~sed

Incremental crack growth

Many of the high -strength steels, and in particular,those with a low work-hardening capacity, generateappreciable emission during crack growth.92-101The emission has often been observed as high-amplitude bursts generated during load drops in aconstant extension-rate test (the period when crackadvance occurs).94, 96-98,100,101 Kishi et al.101

have divided the macroscopic area of fracture bythe number of emissions generated during crackgrowth and obtained an average microcrack sizeof approximately the martensitic-Iath packet sizein a Ti-6AI-4V alloy. Attempts have been madeto relate areas of crack advance to some para-meter of the emission.93,96,97 Gerberich et al.,96for instance, have proposed a relationship betweenacoustic -emission amplitude A and crack incre-ment area ~ of the form

usually increases, and then decreases as it isloaded, and attempts have been made to relate thecumulative count to the plastic-zone volume,89ignoring the source process. Such relationshipsare highly speculative in view of the uncertaintyof the emission source and the crude methodsemployed for monitoring the emission. Considerthe series of events associated with the loadingof a precracked steel specimen: as the specimenis loaded a stress concentration develops at thetip of the crack and when the source activationstress (or strain) for one of the processes des-cribed above is reached acoustic emissions aregenerated. The rate cf generation of theseemissions is then determined by the volume rateat which the activation stress propagates throughthe specimen. This model then 'needs furthermodification because experiments performed usingtensile specimens (see above) have clearly shownthat emission sources operate over a range ofstresses (or strains) and the assumption of asingle activation stress is too simple. The emis-sion rate is affected both by the rate of increaseof the plastic - zone volume and by the stress

. dependence of source activation.

to record the vertical displacement of the speci-men surface and this was compared with theoreti-cal predictions based upon work by Willis21 andPekeris and Lifson.16, 18 They proposed ,that for amonople source the amplitude of the leading edgeof an acoustic -emission signal d could, usingequation (1), as a first approximation "be relatedto the incremental crack area !::J. by the relationship

( 16)o. 0470'1

d = -Il-h-- A

where 11 was the shear modulus and 0'1 the stressin the vicinity of the crack event. The result is notapplicable to most conventional experiments be-cause d is never measured with a displacement-s ensitive calibrated transducer, h is usually anunspecified variable, and the transducer is oftennot situated at the epic entre .

Despite these reservations concerning quanti-fication of data, it is clear that in some materialscrack growth generates copious bursts of emission.A problem that is yet to be solved is why somematerials generate detectable acoustic emissionduring ductile-crack propagation while others do not .

An important factor is the yield stress of thematerial. It is clear from equation (16) that if themechanism of emission were unchanged theamplitude of the emission would be proportionalto the local stress, and since acoustic -emissiondetection systems have a sensitivity threshold, itis conceivable that the elastic waves releasedduring crack gr'owth in low yield-strength materialsmight go undetected. Another possibility is that thefracture mechanisms in high -strength materialsare different from those in low-strength materials.Thus, the incidence of quasicleavage has been pro-posed as a possible emission source in somematerials8 5,94 while 'pop-in' appears to generatediscrete emissions in mar aging steels.100

Clark and Knott 100 have proposed a mechan-ism of emission for materials with a low work-hardening capacity (Oy/OUTS~ 0.8) that may accountfor the high activity in some low-alloy (and possiblymaraging) steels. They have examined the emissionduring crack growth in three steels and observedmost emission in material with least work-harden-ing capacity and have attributed the emission to theonset of fast shear of interinclusion ligaments.Such a fracture mode has been observed as a zig-zag fracture topography, 102 the individual shearfacets being up to 1 mm in length. 103 The fastshear of an inter inclusion ligament (which can belikened to the fracture of a sma~l tensile specimen)at the head of a crack would, because of the suddenstrain relaxation, be expected to be an energeticemission source. It is not clear at present, how-ever, which of the detailed processes taking place 'during such a fracture is the source of the detec-table emission. One possibility is the intense sheardeformation when local strains ~1 can be produ,ced,but emission could also be associated with themicrovoid~linkagemechanism wheI:lfinal S'epara-tion occurs, or some complex combination of both.

(15)2 5 n2kW1/2

A ~ . YLB ~


Nevertheless, it does appear that this fast-shear mechanism acts as a dominant source duringductile fracture, whereas the classical void-coalescence process appears to generate littledetectable emission. These findings are consistentwith both the broad102 and narrow-band104 resultsof Wadley et ale using a quenched and systematic-ally tempered, low-alloy steel, although here theemission source during local shear appeared toextend over areas of several hundred squaremicrometres rather than the area of the shearwall (up to several square millimetres).

Environmental fracture

Acoustic emission has from a early stage in itsdevelopment appeared a useful technique fordetecting the onset of cracking during hydrogenembrittlement or stress corrosion cracking, first,because both phenomena often induce a brittlemode of fracture in an otherwise ductile material,and secondly, because high-strength steels areparticularly prone to such phenomena. Duneganand Tetelman105 have used acoustic emission tomonitor the onset of cracking in hydrogen -chargedAISI 4340 steel and proposed a relationship be-tween count rate (dN/dt.), and fracture toughness Kof the form

~~ == 6. 7 x 10- 5 K 5 (17)

Such a specific relationship between the emission-count rate and the stress intensity is somewhatcontroversial and, bearing in mind the crudeness ofthe emission -monitoring technique used, must beregarded as a purely empirical finding of veryrestricted applicability.

It was clear from experimental studies, e.g.Chaskelis et al., 106 that the emission rate duringhydrogen embrittlement did increase with increas-ing stress intensity during aqueous stress corro-sion cracking of 4340 steel. However, the emissionrate was also sensitive to the microstructure ofthe steel. When other conditions were kept con-stant the emission decreased" as the temperingtemperature of the steel increased, (perhaps as a


consequence of the restoration of work-hardeningcapacity).

The use of acoustic-emission techniquesduring environmentally assisted fracture is provingto be a useful, empirical indicator of the initiationof cracking,106-108 but as yet has made little con-tribution to the understanding of the micromechan-isms of fracture. Wadley and Scruby 102 in anexploratory study have applied broad -band tech-niques to the fracture of cathodically pre chargedlow-alloy steel that had controlled microstructures,in an attempt to examine the relationships betweenemission rise time and amplitude and the mechan - .ism of fracture. During hydrogen-assisted inter-granular fracture a range of rise times wasobserved which was consistent with fracture by aseries of jumps covering areas from one to fivegrain -boundary facets. However, much work stillneeds to be done to quantify the data further.

McIntyre and Green 109 have studied theacoustic-emission activity of three steels, 817 M40,897 M39, and AISI 4340 during stress corrosioncracking in 3. 5%NaCI solution and during gaseoushydrogen embrittlement. The acoustic -emissionactivity in each steel was related to the area ofcrack advance, but they found that the averageacoustic-emission energy (measured over 26 dBdynamic range) per unit crack extension (dE/dA)was dependent not only upon steel composition andmicrostructure but also crack environment(Table 3).

The intergranular mechanisms of fracturegenerated more acoustic emission in the dynamicrange employed than the transgranular mechan-isms, suggesting that individual emission -generat-ing events during intergranular fracture cover, onaverage, larger areas compared with transgranu-lar fracture. Increasing the grain size duringintergranular fracture also led to a proportionalincrease in emission activity, again as a possibleconsequence of increased crack increment area.Finally, tests performed in hydrogen gas (asopposed to NaCI solution) led to larger emissionevents, possibly because of the higher availabilityof atomic hydrogen.

Table 3 Effect of metallurgical and environmental variables upon acoustic-emission energy per unitarea of crack advance during fracture of three steels109

Steel

817 M40

897M39

AISI4340

dE/dAGrain size, Ilm Fracture path Environment lO-2Jmm-2

17 Intergranular 3.5ioNaCI 9100 Inte rgranular 3.5%NaCI 108~7 Inte rgranular H2 at 200 torr 31

1.0 Transgranular 3.5%NaCl 510 Transgranular H2 at 190 torr 210 Transgranular H2 at 76,0torr 1.5

11 Intergranular 3. ,5%NaCl 3120,0 Intergranular 3.5%NaCl 210

11 Intergranular H2 at 200 torr 37


60 ~Vadley et al.: Acoustic emission

Fatigue-crack: growth

Acoustic -emission techniques can be extremelysensitive indicators of the onset of cracking, andhave been applied to fatigue-cracking studies by anumber of workers.ll0-113 The high sensitivitycan lead to problems in interpreting the emissionand unless great care is taken the data may havelittle direct relation to growth of fatigue cracks. 113Thus, there have been reports of spurious emis-s ions due to both frictional effects at the grips andmachine noise.110 Perhaps more importantly,acoustic -emission signals have commonly beenobserved during crack closure and have beenattributed to crack-face rubbing, 110 and thissource of emission could mask that due to incre-mental crack growth unless care is taken to ignoreemissions during crack closure.113

If suitable precautions are taken it wouldappear that the technique can give qualitativeadditional information about the initiation of crackgrowth in favourable materials. 112 The extensionof the technique to the measurement of crackadvance has, to date, met with limited success,110and requires systematic work to relate emissionparameters to crack advance. Despite a numberof detailed studies, the technique has not provideda great deal of additional insight into the mechan-isms of fatigue-crack propagation.

APPLICATIONS OF ACOUSTIC EMISSION

No attempt is made here to review the wide rangeof technological applications to which acoustic-emission monitoring has been applied (for reviewsof these see Refs. 1,56,114-123). Rather the aimis to discuss the consequences of the eariier find-ings for current techniques as reliable monitorsof structural integrity. In general, technologicalapplications have concentrated upon threeproblems:

(i) detecting the presence of flaws

(ii) locating the position of flaws

(iii) assessing the significance of flaws.

In common with other non-destructive testingtechniques the first and second problems havebeen tackled with some success, whereas the thirdis turning out to be extremely complex.

Detecting presence of flaws

It was shown above that. a range of deformationand fracture processes in metals emit acoustic-emission signals in laboratory experiments and ifthese could be detected in an engineering structureeither during its fabrication or preservice prooftesting, or in-service use, they would provide avaluable warning of possible failure. However, thebackground -noise environment, against which theemission signals must be detected can be so highthat only the more energetic ones are detect-able. 1,124,125 Thus, materials which fail by brittlemodes (e.g. cleavage,hydrogen embrittlement,


stress corrosion cracking, or temper embrittle-ment) may well generate detectable acousticemission which can provide some warning ofimpending failure. However, great care is takennowadays to avoid failure by these processes, andthe majority of engineering steels and aluminiumalloys are used in a tough condition where thenormal failure mode would be by the slow ductilegrowth of pre-existing or creep-fatigue-inducedflaws. Laboratory experiments have shown thatsuch crack-growth processes can, in some circum-stances, generate very-high-amplitude acoustic-emission signals (e.g. high -strength materials withlow work-hardening capacity), but often, andespecially when crack growth occurs by slow voidcoalescence, little or no detectable emission hasbeen detected. Thus, while crack growth in somepressure vessels constructed from maragingsteels 126 has been reliably detected by acoustic-emission techniques, pressure vessels constructedfrom lower strength C-Mn mild steels have failedwith no release of detectable emission duringcrack growth.86 The phenomenon of the quietductile crack throws considerable doubt upon theviability of acoustic emission as a technique formonitoring structural integrity in tough materials.If acoustic emission is to be applied to suchmaterials considerable care will need to be ex-ercised to optimize detection techniques, so thatlow-amplitude emissions can be detected againstthe background noise. This means using narrowbandwidth, highly sensitive piezoelectric trans-.ducers which are closely spaced on the structureso as to minimize the effect of signal attenuation.It is also important, as Birchon has reported, 1 tochoose carefully the observation frequency in orderto minimize background interference. These re-quirements for detectability have (see below) adetrimental effect upon the ability to assess defectsignificance.

Locating position of flaws

Details of a number of successful flaw-locationtechniques have been published recently .125,127-132It is clear that if a flaw emits a sufficiently ener-getic .acoustic emission then the position of theflaw can be estimated by measuring the timedelays between detection at an array of transdu-cers covering a structure. However, for accuratelo?ation a knowledge is required of, among otherthIngs, the velocity with which the wave travelledfrom the source to the transducer. Although wavevelocities are well characterized in elastic semi-infinite solids they can often depend upon geometryfor the types of engineering structures of interest.The normal procedure, therefore, is to calibratethe surface-wave velocity by pulsing a transducerso that it simulates acoustic -emission signals,and then to measure delay times. However, sourceloc.ation .by this method is restricted to the positionof Its epIcentre, and no information is recordedwhich enables the source depth to be measured.One possible approach may be to measure thedelay in arrival between the longitudinal and shear

components of the emission provided they can bedetected, since this is proportional to the source-to-transducer distance, as opposed to the epi-centre-to-transducer distance measured forsurface waves.

Assessing significance of flaws

When an acoustic -emission signal is detected by atransducer and its position within a structurecomputed, a judgement has to be made regardingthe significance of the event. There are a numberof possibilities: (a) did the event come from someflaw which is about to cause structural failure, (b)did it come from an inclusion failure or disloca-tion motion associated with subcritical crackgrowth, or (c) was it spurious (e.g. surface oxidelayer cracking)? This question is important be-cause many acoustic-emission signals are pro-duced during the testing of a structure, and only afew may be associated with its failure, so that itwould be uneconomical to close down a structureeach time an event was detected. This is theproblem which at present is proving extremelydifficult to solve. One indication of defect severityis given by·observing the spatial distribution ofevents, 125 since a number of events clustered atone point may be indicative of a major growingflaw. But this method is unreliable. Two tech-niques which have been proposed for character-izing the severity of a flaw by analysis of theemission waveform are amplitude-distribu-tion12,133 and spectral-frequency26,30-32 analysisdiscussed above.

Amplitude-distribution analysis

It has been shown that for laboratory experimentsthe amplitude distribution of acoustic -emissionsignals changes as the mode of deformation/fracture changes. 99,100 When deformation occursby a few discrete steps a peak is observed athigh amplitudes which is absent when the deforma-tion occurs by a large number of small processes.Thus, it is usually argued, measurement of theamplitude distribution on an engineering structurewill give a method of determining the mode offlaw growth.

However, the problems with this technique are:first, that the amplitudes of signals detected withnarrow-band transducers depend upon the fre-quency spectrum of the acoustic wave (which canvary from event to event), and secondly, that elasticwaves detected on the surface of engineeringstructures are subject to attenuations which varydepending upon the source-to-transducer distanceand orientation relationship. These attenuationsoriginate from material attenuation/scattering andgeometrical spreading of the elastic waves. Inaddition, as Pekeris and Lifson 16 have shown, thesurface-wave -amplitude even for a simple pointforce buried within an infinite half -space can varybecause of diffraction effects, and these are par-ticularly important close to the epicentre (whenother sources of attenuation are minimized).


~requency analysis

Deformation and fracture events have character-istic lifetimes; for instance, the growth of a 30 /lmlong cleavage microcrack in iron at a velocity ofabout a tenth the shear -wave velocity lasts for""100 ns. Thus, the elastic waves emanating fromthe crack will have frequencies extending wellabove 10 MHz. If, however, the crack length wereincreased (perhaps by increasing the grain size)so that the crack event lasted longer, one wouldexpect a shift to lower frequency in the energyspectral-density function. It has been suggested31that different types of acoustic-emission sourcewill generate different spectra and that critical-flaw growth might be characterized by its spectral'fingerprint'. As shown above some success hasbeen claimed for this approach in laboratorytests.28,32 but it requires the use of broad-band(and hence insensitive) transducers combined withspecimens of relatively simple geometry. For thegeneral case of an engineering structure of com-plex geometry where ambient-noise levels requiremeasurements over narrow frequency ranges, con-volution of the source function with the transferfunctions of the structure and transducer is likelyto make it extremely difficult to observe thesefingerprints~

So, what should be done to characterizecritical-flaw growth? The answer is not clear atpresent because the relationships between themeasured parameters of acoustic-emission sig-nals and the dynam ic properties of source eventsare not known. Theoretical and experimentalstudies at the US National Bureau of Standards,Cornell University, and AERE, Harwell, are aimedat this problem, but to date offer no simple prac-tical solution.

CONCLUDING REMARKS

A considerable amount of work in the field ofacoustic emission has been published in thescientific and engineering literature of the last20 years. While only a selection from the litera-ture has been included, every attempt has beenmade to make this as representative as possible.An attempt has been made in this review toemphasize a number of important aspects. First,the basic physical processes involved in thegeneration, propagation, and detection of acousticemission have been discussed. Secondly, sourcesof error in existing interpretations of acousticemissions were highlighted. Thirdly, the originsof acoustic -emission signals during the deforma-tion and fracture of metals have been criticallydiscussed.

As a consequence of this, a number of recom-mentations have been made concerning the prac-tical application of acoustic emission; the mostsignificant of these being the use of narrow-bandhigh -sensitivity techniques to detect the presenceof defects and separate, sophisticated, wide-bandtechniques to assess their significance.



The great majority of laboratory studies have,up to now, been essentially qualitative, mainlybecause of the use of uncalibrated, poorly defineddetection procedures. Facilities now exist for atleast partial calibration of detection systems andit is to be hoped that future work will be stan-dardized to facilitate exchange and comparison ofthe results of different workers. Of equal impor-tance has been the gradual trend toward controland systematic variation of microstructure andtesting variables.

Given the task of relating the features of themeasured acoustic emission from an experiment,to the properties of the deformation or fractureprocess by which it is generated, it is now evidentthat a number of requirements have to be fulfilled:

1. Detection systems must be able to cover themajority of the frequency spectrum of thesource. This requires developments at highand low frequencies

2. Errors in acoustic -emission measurementsdue to instrumentation need to be measuredand corrected

3. A clear understanding of elastic-wave pro-pagation in the specimen of interest isrequired

4. An adequate theory needs to be developed toallow the interpretation of the properties ofan acoustic -emission source to be inferredfrom acoustic-emission measurements.Great advances in the interpretation of earth-quake seismogram s suggest that this ispossible for acoustic -emission measurements.

Acoustic emission still affords the opportinity togain an exciting new insight into the dynamicbehaviour of deformation events in bulk specimensundergoing deformation and fracture. In addition,because of its passive nature, the monitoring ofacoustic emission does not alter the mechanismof deformation. No other existing technique offersthese possibilities. However, for this potential tobe fully realized the requirements presented abovehave to be fulfilled. This has not yet happened andthe application of acoustic emission as a non-destructive testing (NDT) technique for monitoringstructural integrity of metallic structures has,consequently, had mixed success to date. However,it is probable that acoustic emission will have arole to play in future years in the battery of NDTtechniques which may be brought to bear onimportant engineering problems.

ACKNOWLEDGMENTS

The authors would like to record their apprecia-tion of the many helpful discussions with theircolleagues at Harwell during the compilation ofthis review. In addition, to thank Dr G. Clark andDr J. Knott of Cambridge University for the useof their unpublished literature survey.


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99. M. Arii, H. Kashiwaya, and T. Yanuki: Eng. acoustic emission as an on line monitoringFract. Mech., 1975,7, 55!. system for nuclear reactors', 1974.

100. G. Clark and J. F.Knott:Met. Sci., 1977,11, 126. A. C. E. Sinclair: CEGB Report RD/B/N406,53I. 'Acoustic emission techniques for inservice

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107. C. E. Hartblower, W. W. Gerberich, and P. P. 133. M. P. Kelly, D. O. Harris, and A. A. Pollock:Crimmins: Weld. J., 1968,47, (1), !. 'Monitoring structural integrity by acoustic

108. D. G. Chakrapant and E. N. Pugh: Metall. emission', STP 571, 221; 1975, Philadelphia,Trans., 1975, 6A, 1155. Pa,ASTM.

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