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Simultaneous monitoring of acoustic emission and ultrasonic attenuation during fatigue of 7075 aluminium
Simultaneous monitoring of acoustic emissioa and ultrasonic attenuation during fatigue of 7075 aluminium J. C. D u k e , Jr. a n d R. E. G r e e n , Jr.
Although ultrasomc attenuatJon momtonng has been demonstrated to be capable of providing a quantatatlve assessment of integrity during fatigue, much less success has been achieved m developmg the corresponding capabil2ty using acoustic ennsszon momtormg. A thorough review of the previous work employing either techmque for the exatmnatlon of alumzmum during fatague is presented In adchtion, results from simultaneous momtoring of ultrasomc attenuation and acoustzc emzssion during the fatague of 7075 alutrumum demonstrate the feasibility for utzhzang acoustic ermssion as a means of continually surveying the structural integrity of a piece It Is evident from the results that, due to the passlve nature of the acoustlc erms~on momtormg, sensor location relative to failure location is not critical, thus allowing surveillance of structures with irregular geometries However, the utzhzauon of hlgh frequency sound m ultrasomc attenuation momtonng makes th~s techmque for inspectmg known problem areas extremely useful m the presence of intense enw_ronmental noLse The overall findings indicate the capablhty of both techmques for assessmg structural integrity during fatlgue while prowdmg complementary practlcal advantages There are four different basic ultrasomc techmques which have been used to detect the onset of fatigue damage m cyclically loaded structural materials body wave reflection, surface wave reflection, ultrasomc attenuation, and acoustic emission At present, ultrasonic attenuation and acoustic emission measurements are superior to all other ultrasonic techniques for the early detection of fatigue damage Several studies have been performed utilizing either one or the other of these techniques The material receiving the most attention has been alumimum, particularly the alummmm alloy 7075 Principally because of its extensive use m aircraft structures which are constantly subject to a fatigue environment
UL TRASONIC A TTENUA TION The first ultrasonic technique used to study the development of fatigue damage during stress cycling was the ukrasonic attenuation technique As early as 1956, Truell and Hikata 1 observed changes in ultrasonic attenuation in the early stages of fatigue cycling on a polycrystallme alummium specimen In subsequent publications, Truell and coworkers 2-5 reported remits of experiments where ukrasonic attenuation measurements were made dunng fatigue tests up to fracture of the test specimens They used cychng rates of 400 to 1900 cycles per minute and in every case the attenuation changed as a function of the number of cycles of loading For tests carned out to fracture there was a marked increase in attenutation lust prior to failure In 1962, Ponomarev6 measured the ultrasonic attenuation m metallic and nonmetalhc cylindrical samples during repetitive loading The metallic samples, steel, copper, and duralumin were subjected to torsional loading, whde the nonmetallic samples, glass, plemglass, quartzite, and quartz were subjected to periodic hmpact loads He established that the moment of fadure onset of a part due to fatigue damage can be determined from the character of the change in the ultrasonic attenuation curve
Pawlowslu 7,8 m 1963, measured ultrasomc attenuauon dunng fatigue cycling of specimens of low carbon steel, an alloy steel, an alummmm alloy, and grey cast iron Using a cyclic loading rate of 25 cycles per second he found that all the materials tested showed an increase m attenuation with increasing number of faugue cycles His change in attenuauon versus number of cycles curves exhibited two distinct regions The first region was nearly linear and showed a relatively gradual increase m attenuation with increasing number of cycles The second regi0n Which occurred in the final stage of the fatigue life of the specimen showed a more rapid increase of attenuation with increasing number of cycles and terminated with fracture of the specnnen. Pawlowskl concluded that an irreversible increase in attenuation can be regarded as an indicator of damage accumulation developing gradually in the process of fatigue These irreversible changes he attributed to microcrack formation which coalesce to form maerocracks and eventual fracture In 1971, Moore et al9 attempted unsuccessfully to relate ultrasonic surface wave measurements of attenuation and velocity to fatigue damage m five specimens of 1100 alumimum which had undergone faugue deformation for 30, 50, 70, 80 and 90% of the specimen life. A possible reason for their lack of success was apparent interference effects between some unspecified wave and the surface wave of interest Then in 1972 Joshi and Green 10 reported expenments which used the change m ultrasomc attenuation measurements as a continuous momtor of fatigue damage during cyclic testing of 6061-T6 alumhmum and steel specunens Each spechmen was fatigued as a cantilever beam in reverse bendmg at 30 Hz with a vibraUonal amplitude peak-to-peak set at a fixed value in the range 7 5 to 15 mm In a typical test, the ultrasonic attenuation hmtially remained constant, increased slowly, and then increased catastrophically just prior to fracture of the test specimen All experiments performed on both alurmmum and steel specimens at
0142--1123/79/030125--08 $02 00 © IPC Business Press L,m,ted 1979
INT J FATIGUE July 1979 125
various vibrational amplitudes yielded snnilar results in that ultrasonic attenuation served as a very sensitive indicator of fatigue damage and in every case indicated that failure was imminent several hours before conventional ultrasomc testing could detect an additional echo caused by waves reflected from a crack Panowlcz, 11 in 1974, examined the effects of latent defects on the fatigue hfe of 1100 alumlmum with this technique The defects were sunulated by rejoining saw cuts by plastic welding It was concluded that the specimen lifetime could be predicted p n o r to the onset of failure modes detectable by conventional NDT techniques Most recently Mlgnogna et a112 have extended the work mentioned earher 10 to continuous measurement of changes in ultrasonic attenuation in plain and 'rivet simulated' 1 59 m m thick sheets of 7075-T6 aircraft a l u m m m m alloy during fatigue cycling in reverse bending at 1725 cycles per minute The attenuation typically remained constant initially, increased slowly, and then increased rapidly lust prior to fracture o f the test specimen These results demonstrate that ultrasonic attenuation monitoring can indicate fatigue damage in a l u m m m m alloy sheets much earlier than the appearance of an additional pulse echo resulting from the reflection of energy by a crack as observed in a normal A-scan
A COUSTIC EMISSION Even though Katser 13 is generally credited wtth discovering acoustic emission in 1950, it was not until 1954 when Dunegen et a114 imtiated work in the ultrasonic range of frequencues that acoustic emission actually became a scientific and engineering tool Even though the amount of work performed in this area since that time is extensive, only a few investigators have m o m t o r e d acoustic emission during fatigue tests The main reason for this is that acoustic emission detection systems o f current design normally operate in the frequency range from 20 kHz to 1 MHz with their peak response being about 100 kHz and acoustic emission signals are random and transient in nature Unfortunately, the noise generated by extraneous sources during fatigue testmg is predominant in this same frequency range and, therefore, difficulty is encountered in separating the acoustic emission signal from the background noise In 1967 and 1968, Hartbower et a115-17 monitored acoustic emission associated with slow crack growth during low-cycle fatigue testing of precracked steel spe~mens possessing different heat treatments and o f a l u m m m m and titanium alloys F o r this low-cycle fatigue work an accelerometer in conlunction with a filter apparently elumnated the noise problem Hutton, 18-2° in 1969, reported measuring acoustic emission during high cycle t e n s i o n - t e n s i o n fatigue testing of notched a l u m m m m and carbon steel specimens The latter test in particular gave evidence that macrocrack nucleation caused sharp increases m acoustic emLssion prior to detection of a visible crack The emission data showed a gradual increase during the miUal stages Also m 1969, Dunegan et al21-23 proposed a m e t h o d of detecting fatigue damage b y intermittently stopping the fatigue cycling and p r o o f testing the specumen by overstressing while m o m t o n n g the acoustic emission. This m e t h o d has the advantage o f elumnation o f most of the background noise, but the disadvantage that ~t is not a continuous on-hne technique Nevertheless, it is mformauve
126
INT J FATIGUE July 1979
to note that for steel and a l u m m m m alloy test specnnens and for steel and a l u m m m m pressure vessels snmlar experimental results were obtained In all cases, both the total acoustic emission count and the counts per cycle increased slowly with the increasing number of fatigue cycles and then increased rapidly just prior to failure The same year, Moore et al 9 monitored acoustic emission counts during fatigue cycling of 1100-0 alumlmum specimens They observed that the cumulative number of acoustic emission counts changed slowly in the early part of the faugue process and then changed by several orders of magmtude A relationship was observed between the significant change in the slope of acoustic emission curve and the percent o f faugue hfe The change was observed to occur at less than 50% of the fatigue life and as such offered a possible early fatigue damage warning Kusenberger et al24 in 1972, monitored crack growth in stress cycled specimens of 4340 steel and 7075-T5 a l u m m m m Periodically, cycling was stopped, the crack length at the surface was measured, and a constant p r o o f load was applied to the specimen During proof loading, the specimen was monitored for acoustic emission The results obtained with the 4340 steel specimens were puzzling in that essentially no acoustic emission above the nominal background level was recorded until the last cycle prior to failure Such results were obtained even though on several occasions proof loads were raised nearly to yield The results of the experiments with the 7075-T5 alummmm specimens indicated a strong dependence of the acoustic emission on the state of stress A much lower count was obtained for plane strain than for plane stress states In 1973 Egle et 8]25-27 reported an acoustic emission system for monitoring high cycle fatigue crack growth They used a conventional acoustic emission system with a high pass filter between the transducer and pre-amphfier and a second high pass filter between the pre-amphfier and digital counter In addition, an electromagnetic transducer was used to gate the counter every 10 or 100 load cycles F o r low cycle fatigue of a notched a l u m m m m plate the number of acoustic emission counts per load cycle first decreased and then increased slowly lust prior to failure F o r high cycle fatigue of an a l u m m m m specimen m a displacement controlled cantilever beam tester, the emission exhibited a relatively high count at the start of the loading followed by a sharp decrease and then an increase F o r specimens tested with higher m a x i m u m displacements and shorter lives, the emission continued to increase until failure F o r specunens tested with lower m a x i m u m displacements and longer lives, the emission tended to level off and then to decrease Egle attributes these results to the fact that the rate o f crack propagation decreased for the lower m a m m u m displacements because of the displacement control on the fatigue tester Harris and Dunegan 28 continuously monitored fatigue crack growth in 7075-T6 a l u m m m m and 4140 steel by acoustic emission techmques The acoustic emission rate per cycle was consistently observed to be cyclic in nature, le,the rate was observed to be high for a few cycles of loading followed by a period of reactivity This was regarded as strong evidence that crack growth in these materials does not occur by a uniform growth per cycle, but by a burst of growth followed by a stationary period Morton e t a/29 performed sumlar investigations m which they monitored acoustic emission and quanutatwely correlated it with crack growth rate and the applied range
of stress intensity for high cycle fatigue of 2024-T851 alummium alloy They concluded that the acoustic emissions observed dunng fatigue are more closely related to the crack tip plaint volume than to crack extension Bailey and Pless3° used acoustic emLmon techniques and sound travel tLme determinations to locate cracks in a structural fatigue test specrmen The specimen was subjected to a spectrum of cychc load conditions which simulated a total of 150 000 aircraft flight hours The sloecunen was momtored for acoustic ermss~on approx~nately 10% of the total fatigue test tL,ne It was found, m agreement w~th the results of other investigators, that crack growth at a given location was not continuous throughout the fatigue tests. Cracks were observed to grow durmg some test passes, but apparently remamed stationary dunng other passes Some cracks did not even appear to grow contmuously during a single pass Crack growth was reported to be rapid or slow for given cracks at different tLmes A total of 18 crack locations were identified by acoustic emission momtormg, 15 of which were verified during metaUurglcal investigation Vary and Kluna 31 reported the results of a prelL,nmary investigation to assess the feaslhihty of continuously morntonng acoustic emLssion s~gnals from fatigue cracks durmg cychc bend tests Plate specunens of 6A1-4V titanium, 2219-T87 alummlum, and 18-N1 maragmg steel were tested w~th and w~thout crack starter notches The investigation indicated that it was possible to extract meanmgful acoustic emlsmon s~gnals m a cyclic bend machine enwronment and that such ~coumc emission momtonng is a potentially useful tool for investigating fatigue cracl~ng Ultrasonic crack growth momtonng was apphed s~multaneously w~th acoustic emission momtonng for a few specimens Although abrupt increases in the acoustic ermsslon count rate were observed, the ultrasonic signals indicated only a smooth increase in crack size Pless and Badey52 used an acoustic emission monitormg system to detect crack extensions induced m a large aircraft using structure through cyclic loading The crack extensions were induced by tension-tension loadmgs at specified percentages of design hmtt load acting on fourinch long crack-starter sawcuts made at well-defined locations on the specimen Since both audible and inaudible fracture increments were detected, ]t was concluded that the tests proved the feas~bthty of the acoustic emission momtormg technique However, even though the signal-to-noise ratio was high enough to v~tually exclude false alarms during these tests, the tests could prowde no information about acoustic and other transients resulting from active flight systems Hatano 35 measured acoustic emission dunng highcycle fatigue testing of pure alumhmum specimens He detected intermittent burst-type emission in the early stage of fatigue testing before observation of macroscopic crack growth He also noted that most of the acoustic emission signals were detected dunng that portion of the fatigue cycle where the cyclic load was a mammum In 1976, I~sh] et a154 studied the acoustic emis~on generated dunng cyclic deformation of pure alummlum and reported an acoustic emission peak accompanied by a Bauschlnger effect This peak was observed w~th the appearance of irreversible plastic strain and the peak height decreased w~th increase in the number of cycles up to a constant value at the saturation stress Badey and colleagues55-58 expandea upon their earlier work 50,52 on assessing the posslbdlty of usmg acoustic
ermsslon for in-flight momtoring of aL,xn~.ft structures. Their Fhght Structural Momtonng System (FSMS) was designed to detect the large transient acoustic sound generated by stogie large crack extensions rather than the lower level signals generated by fatigue crack initiation. Using the FSMS they succ,esdully momtored crack extensions in fullsize complex structural components dunng fatigue testing by a combmauon of sKjnal processing techmques mvolwng spatial discrimination, frequency filtering, and sagnal amphtude discrimination Cracks were initiated and grown on a wing test arucle at selected fastener holes from sharpnotch sawcuts by subjecting the specimen to a flight-byt'hght load spectrum typical of a military transport aircraft Acoustic emission was detected from 12 of the 15 test holes momtored which experienced crack growth and three of six extraneous acoustic emission sources were traced to loose fasteners For three of the cracks monitored, the acoustic enusmon counts showed reasonable correlation w~th the amount of crack growth. The greatest detractor to correlatable data was random noise Horak and Weyhreter59 reported the development of an acoustic emission system for momtonng components and structures m a severe fatigue noLse environment. Their system utilized master-slave discrimination, rise time discrLmmatlon, and coincidence detection to identify and reject unwanted noise signals from computer processing Among the dLscnmmauon concepts which they found to be unsatisfactory were frequency filtering, computerized data reduction, and mechanical dampening. Since Horak and Weyhreter found that the wave forms of the vanous transient noises contributed most to false data generation, the only acoustic emimon signals rehably detected were those which occurred dunng the time penods between the tran~ent fatigue noises Use of the system on a fatiguing part which had not begun to crack, 10 false locations were indicated m 16 hours compared x'nth 56 false locations m 15 seconds indicated without their discrimination system Crack propagation was not always continuous and sometimes stopped completely for several hundred cycles Although crack propagation is generally considered to occur during the maximum stress apphcation of the fatigue cycle, their results showed that acoustic emLsslon resulting from crack extension occurs dunng low, medium, and high parts of the loadmg curve These data led them to caution that the exclusive use of time gates to ehmmate extraneous noise signals might ehrnmate a large portion of the acoustic emission signals also As a result of the various types of defects and loads involved, crack lmtiatlon was detected and located from several minutes to several hours prior to ultimate failure Since crack ruination occurred within the material, crack lmtiation s~ze was not determined Although ultrasoinc attenuation momtonng has been demonstrated to be capable of prowding a quantitative assessment of integrity dunng fatigue, much less success has been achieved in developmg the corresponding capablhty using acoustic emission monitoring Through sLmultaneous momtonng of ultrasomc attenuation and acoustic emismon during the fatigue of alummlum, the feas~hthty for utihzmg acoustic emission as a means of continually surveying the structural lntegnty of a piece can be assessed The importance of such an assessment can be reahzed by recognizing that the passive acoustic ermssion momtonng technique could be utihzed to examine pieces of Lrregular geometry which, because of inaccessibility, could not be examined ultrasonically
INT J FATIGUE July 1979 127
EXPERIMENTAL Continuous s~multaneous momtonng of ultrasonic attenuation and acoustic emission was performed on 7075 alummmm alloy specnnens subjected to reverse benchng cychc fatigue at 1725 cycles per minute The peak to peak amphtude was maintained at 9 mm, which corresponded to a maximum tensile fibre stress of 255 MN/m 2, for all the tests Specnnens m the form of rectangular bars 17 8 mm (0 7 in) wide, 304 8ram (12m) long, and 12 7mm (0 5m) thick were tested in two heat treated conditions 7075-T651, as received, and 7075, solution treated The 7075-T651 condition was obtained by solution treating the 7075 alummmm alloy at 466-477°C, quenchlng it in water at 60-71°C, stretching it between 1 and 3%, and then artificially ageing it at 1 1 6 - 1 2 7 ° C for 2 3 - 2 8 hours By concluding the treatment after the water quench, the 7075 solution treated condmon was obtained Fig 1 depicts the specimen geometry as well as its testing configuration An effort was made to acousUcally insulate the sample from the fatigue machine in order to mmnmze any mechanical noise because of the nature of the monitoring procedure A specially designed acoustic ermsslon system permltted reliable acoustic emission measurement~ to be made despite the high level background noise from the fatigue testing machine and associated grips This system,* combined a unique transducer design w~th a multi-shielded Mu-metal and copper screen cable and other special features for shielding against electromagnetic interference
The maximum sensltiwty of the system was at 300 kHz These special features yielded an acoustic emission system W-lth a signal-to-noise ratio at ]east three orders of magnitude better than conventional acoustic emission systems which use regular piezoelectric transducers, pre-amphflers, and eoamal cables The output of the acoustic emission system was passed through a true root-mean-square voltmeter (Bal]entree mode] 320) and recorded on one channel of a multichannel stnp chart recorder (Fig 2) The ultrasonic attenuation measurement system consisted of a conventional 10 MHz longitudinal mode transducer (Aerotech Gamma), tunable pulser-receiver (Matec model 6600), and an automatic attenuation recorder (Matec model 2407A), as shown in Fig 3 An automatic gain control In the attenuation recorder maintained the lmt]al u]trasomc pulse voltage at a constant value, and permitted consistent ultrasonic attenuation readings to be continuously recorded on a second channel of the multichannel strip chart recorder The side-by-side presentauon of ultrasonic attenuation and acoustic emission data enabled easy comparison of the experimental results In order to facilitate the combined testing it was advantageous to synchronously trigger the ultrasonic pulse (Fig 4) Using a light detecting sensor stunulated by a reflective region of the rotating cam of the fatigue machine, the ultrasonic system could be triggered at any point in the fatigue cycle By trlggenng at a fixed point in the fatigue cycle a constant path length for the ultrasound pulse could Transducer
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I N T J F A T I G U E July 1979
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oe assured For this study the system was triggered at the undeflected portion of the cycle Adeht~onally, because of the extreme sensmvtty of the acoustic emission detection system it was necessary to operate the uhrasomc system well below the maxhmum capability Despite reducing the power output of the ultrasonic system the ultrasonic pulse was st,ll detectable by the acoustic emlsmon system Because of the method of momtonng the true root mean square voltage output of the acoustic emission detection system, this detected ultrasonic signal appeared only as an increase in the background noise level Although the maximum capability of neither techtuque, if apphed separately, could be exploited, considerable information was obtained by optimizing the combined monitoring methods
RESULTS Figs 5 and 6 display the results of typical fatigue tests on 7075-T651 as received, and 7075 solution treated specimens, respectively In Fig 5 the change in ultrasonic attenuation exhibits no appreciable alteration unttl 80% (1 5 x 106 cycles = 100%) of the faugue life at which point it changes rapidly going off scale at approxzmately 97% of the fatigue hfe To achieve high sensitiwty, a fine scale of monitoring the change in attenuation was used Had a coarser scale been used, the attenuation would have been seen to continue to increase until failure The true root mean square (rms) voltage output of the acoustic emission signal, which was RMS (. . . .
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amplified by 70 dB, was seen to fluctuate at the beginning of the cychc deformation, level off, and begin to steadily increase at 80% of the fatigue life, with a more rapid increase occurnng after 95% of the fatigue life Unlike the smooth, nearly monotonic behavzour of the change in ultrasonic attenuation, the rms curve is seen to have local maxima and rmnima The peaks, m actuality, consist of many mdlwdual burst type indications occurnng over an extended period of time, and their appearance is due to the condensation of considerable data into the presented plots Very slmdar behavlour is seen in Fig 6 which is typical of the solution treated specimens The change in ultrasonic attenuation shows a small but steady increase until 90% of the fatigue life (7 4 x 106 cycles = 100%) where it begins to increase rapidly, going off scale at 94% of the fatigue life Slrrularly the rms curve after a peak at the beginning of the deformation levels off until around 78% of the fatigue llfe where the overall rms curve begins to increase slowly with several large peaks superimposed on overall increasing trend A rapid, overall increase began at 87% of the fatigue life and levelled off at an elevated value, at 95% of the fatigue life The fatigue tests in general appear very similar with the change in ultrasonic attenuation, exhibiting little variation until between 90% and 95% of the fatigue life where a rapid increase occurred The rms curves compared well, fluctuating at the start of the fatiguing, levelling off for a major port~on of the fatigue life, and finally increasing rapidly between 85% and 95% of the fatigue life Figs 7, 8 and 9 are photographs of scanning electron micrographs of typical fatigue fracture surfaces Fig 7 pictures at 3 x 105 ongmal magnification fatigue striations
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Fig 6 Change in ultrasonic attenuation and true root mean square voltage of acoustic emission detected m relation to percentage of fatigue life (7 4 x 106 cycles) 7075 alummlum, solution treated (maximum tens,le fibre stress 255 MN/m 2)
F=g 7 Scanning electron mlcrograph of fatigue strmtJons o f 7075-T651 alum=mum, as recewed The arrow indicates a fractured inclusion particle
INT J FATIGUE
July 1979
129
in 7075-T651 along with a fractured inclusion particle which can be seen to lead to a crack running out into the matrix material Fig 8 pictures snnilar striations at 2 x 103 ongmal magnification, in a solution treated sample along with a fractured inclusion particle, the striations are noticeably larger in this more ductile material Fig 9 shows, in addition to fatigue striations, at 2 x 103 original magnification, in a solution treated sample, two intermetallic particles which have remained contiguous
DISCUSSION Although the intent of the combined monitoring of fatigue of the 7075 specimens was to establish whether or not acoustic emission m o m t o n n g could serve in a mmilar capacity to ultrasonic attenuation monitoring for providing early warning of fatigue failure, much more m a y be learned from this study Initially,however, it should be strongly emphasized that acoustic emission m o m t o r m g clearly exhibits the capability of giving early warning of fatigue failure Unlike ultrasonic attenuation (Figs 5 and 6) which remains relatively unchanged until the onset of failure, the acoustic emission exhibits considerable activity during the early stages of deformation This is somewhat expected in light of the explanation of Duke 4° regarding the acoustic emission occurnng d u n n g elastic tensile deformation of 7075 alumimum Acoustic emission occurring prior to yield was described as the result of the breakaway of piled up dislocations This breakaway takes place as elastic strain energy, supplied by tensile loading, provides the necessary activation energy for the dislocations to surmount their barriers The activation energy necessary to cause breakaway
F=g 9 Scanning electron mlcrograph of fat=gue strlat=ons of 7075 alummlum, solution treated The arrow md=cates an intermetalhc particle
of piled up dislocations, in this case, would be supplied by the cyclic deformation A slight change in attenuation* is observed prior to the rapid change near failure with no corresponding effect in the acoustic emission behaviour This is thought to be due to the senmtNity of the ultrasonic attenuation to moving dislocations, which would occur throughout the specimen, while the acoustic emission would be sensitive to dislocation breakaway or dislocation creation, which would occur In localized regions at any one time Therefore, moving dislocations would cause an increase in the ultrasonic attenuation but would not affect the acoustic emission behaviour ** This dislocation break.away responsible for the initial acoustic emission behavlour occurring on a localized scale would only comcldentally interact with the ultrasonic wave and even then due to the slow system response would not result in a change in the attenuation measured This situation serves to show the complementary nature of the two techniques in that the passive acoustic emission technique is sensitive to localized transient phenomenon while the active ultrasonic attenuation monltorlng technique is affected by the more steady state behaviour of the entire material it surveys Furthermore, as failure becomes evident, both techniques are seen to undergo rapid increases Whereas the change in ultrasonic attenuation increases monotonically,
Fig 8 Scanning electron mlcrograph of fatigue striations of 7075 alumlmum, solution treated The arrow indicates a fractured inclusion particle
130 INT J FATIGUE July 1979
* Most noticeable in the solution treated specLmens * * The effect being more pronounced m the solution treated specimens might be expected realizing that the heat treatment has removed the malor~ty of d~slocatlon pinning points, the precipitate particles Because of this, dislocations which break away from pileups during the initial portion of cycling may move with few Mmpedlments triggering additional break-aways, until finally becoming strongly pinned
the acousUc ermss~on is seen to undergo fluctuations as it increases This is thought to be a result of an increase m the amount of emission which occurs when a crack is formed, or as the crack extends, and as the new surfaces abrade against one another dunng subsequent fatigue cycles As cycling continues, the abrasion Ls smoothed out due to plastic flow, causing a decrease in emissions from this source Since fadure does occur, other sources of acoustic emission might be expected to contribute, but any such contribution was not discermable m the present study Scanning electron mlcrographs (Figs 7 and 8) prov]de evidence to support the contention that inclusion particles fracture and cause microcracks to form that link up with others to cause the final fracture However, the lmtiation and progression of this series of events, as well as the effect of the lntermetalhc particles, Fig 9, under the influence of cyclic deformation are not as distract as during pure tension Although the emissions from different sources might be expected to be different, the momtonng technique utthzed in this study would not differentiate if indeed chfferences were present The results of the fatigue study descnbed here clearly establishes the fact that the distract rmcroscopic deformation mechanisms which give rise to acoustic emission during tensile deformation of 7075 a l u m z n m m reported by Duke 40 do not remain distinct during fatigue
part by the Air Force Office of Scientific Research (AFSC), United States Air Force, a special note of appreciation is clue to Mr Wflham J Walker and Lt Col J D Morgan, III in thLs regard
REFERENCES 1 2 3
Truell, R , Ch,ck, B,, Picker, A and Anderson, G. "The use of ultrason,c methods to determine fatigue effects in metals', WADC Techmcal Report 59-389 (1959)
4
Truell, R , Chick, B , Anderson, G , Elbaum, C. and Fmdley, W 'Ultrasonic methods for the study of stress cycling effects in metals', WADD Techmcal Report 60 920 (1961)
5
Chick, B , Hikata, A , Anderson, G , Flndley, W, Elbaum, C and Truell, R 'Ultrasonic methods m the study of fatigue and deformation m single crystals', WPAFB Report No ASD-TDR62-186 Part II, AD No 408704 (1963)
6
Ponomerev, P V 'Ultrasonic control of fatigue damage to matermls', Zavodskaya Laboratorlya 28 (1962) 1345-1346, Enghsh translation In Ind Lab 28 (1963) 1429-1431
7
Pawlowskl, Z 'Internal friction of metals and the problem of damage cumulation with static and variable Ioadmgs', Proc of Vibration Problems (Warsaw) 4 (1963) 43--64
8
PawlowJkl, Z 'Ultrasonic attenuation during cyclic straining', Proc Fourth International Conference on Nondestructive Testing (1963) (Butterworths, London, 1964) pp 192--195
9
Moore, J F , Tseng, S and Martin, G 'The early detection of fatigue damage', Techmcal Report AFML-TR-71-185, AD 730 348 (1971)
CONCL USIONS 1 Acoustic emission momtormg may be used to provide early detection of fatigue failure in both as received 7075T651, and solution treated alummium specuuens 2 Ultrasonic attenuation and acoustic emission momtonng appear to be sensitive to different dislocation mechanisms, both of which are active dunng fatigue, ie ultrasonic attenuation is affected by moving dislocations, while acoustic emissmns are primarily caused by dislocation breakaway and creation 3 Ultrasonic attenuation momtormg takes into account the entire area surveyed by the ultrasonic beam, wknle acoustic
10
Josh,, N. R and Green. R E Jr 'Ultrasonic detection of fatigue damage'. Engr Frect Mech 4 (1972) pp 577--583
11
Panowlcz, W V, 'Ultrasomc detectmn of fatigue damage m aluminum specimens containing reduced latent defects', Master's Thesis (Dept of Mechanics and Materials Science, The Johns Hopkins Umverslty, Baltimore, Maryland, 1975)
12
Mzgnogna, R B , Duke, J C. Jr and Green, R. E. Jr. 'Early detection of fatigue cracks in aircraft aluminum alloy sheets', Paper presented at National Fall Conference o f ASNT, Detroit, Michigan (October 1977) (submitted for publication m Materials Evaluatmn)
13
Kmser, J 'Untersuchungen uber des auftreten Gerauschen belm Zugversuch'. PhD Thesis (Techmsche Hochschule, Mumch, 1950J, Arklv fur des Eisenhuttenwesen AR EIA 24 (January-February 1953) pp 43--45
14
Dunegan, H L , T a t r o , C A andHama, D O 'Acoustic emission research', Lawrence Rad/a~on Laboratory, L/vermore, Cahfomm Report UCID-4868, Rev 1 (1964)
15
Gerbermh, W W and Hertbower, C E 'Some observations on stress wave emission as a measure of crack growth'. IntJ Fract Mech 3 (1967) pp 185--192
16
Hartbower, C E ,Gerbermh, W W and Llebowitz, H 'Investigation of crack-growth stress-wave relationships', J Engng Fract Mech 1 (1968) pp 291--308
17
Hartbower, C E , Gerberzch, W. W and Cnmmma, P. P 'Momtormg subcrltzcal crack growth by detection of elastic stress waves'. The Welding Journal WEJUA 47 (1968) pp 1--18s
18
Hutton, P H 'Use of acoustic emission to study failure mechanics m metals', ASME Paper No 69-Met-8 (1969)
19
Hutton, P H 'Acoustic emission -- a new tool for evaluating structural soundness', Proc Seventh SFmposlum on Nondestructive Evaluation of Components and Materials m
emission momtormg responds only to mechanisms capable of re]easing sufficient elastic energy to be detected above the electronic background noise of the system 4 Acoustic emissmn activity early in the fatigue deformation of both 7075-T651 and 7075 solution treated alummmm is consistent with the argument of Duke40 which views the breakaway of piled up dislocations upon the application of stresses below yield as a possible source of acousuc emission 5 The fluctuations in the acoustic emission behaviour near failure in both types of specimens tested are due in part to the mechanical abrasion of cracked surfaces 6 SLmflar sources are believed to be responsible for the acoustic emission dunng fatigue, as those in the case of studies involving quas~-static tensile deformation 7 Since ultrasonic attenuation is known to increase pnnclpally as the result of both scattenng from cracks and absorption by vibrating and moving chslocations it was not possible to clearly determine the mechanisms responsible for the observed behaviour
A CKNOWL EDGEMENTS Sincere appreciation is expressed to Mr Don Bzrchon and Dr Robert Dukes of the Adrmralty Marine Technology Establishment for the loan of the Low Noise Acoustic Emission System Additionally, this work was supported in
Truell, R end Hikata, A. 'Fatigue m 2S alum=mum as observed by ultrasomc attenuation methods', Watertown Arsenal Techmca/Report N o WA L 143/14-47 (1956) TrueU, R. end Hikata, A 'Fatigue and ultrasonic attenuation', ASTM STP 213 (1957)
Aerospace Weapons Systems and Nuclear Applications, San Antomo. Texas (April 1969) pp 165--171 20
Hutton, P H 'Acoustic emission applied to determination of structural integrity', Automotive Engmeenng 79 (1971 ) pp 3 3 - 3 7
INT. J. FATIGUE July 1979 131
Dunegan, H. L., Harris, D. O. and Tetelman, A. S. 'Detection of fatigue crack growth by acoustic emission techniques', Proc Seventh Symposmm on Nondestructive Evaluauon o f Components end Materials m Aerospace Weapons Systems and Nuclear Apphcatlons, San Antomo, Texas (April 1969), pp 20-31, Materials Evaluation 28 (1970) pp 221--227
32
Pleu, W M and Bailey, C. D. 'Detection of large crack extensions in aircraft wing structure by acoustic peak detectors', Proc Tenth Symposium on NDE, San Antomo, Texas (April 1975) pp 30--43
33
Hatano, H. 'Acoustic emission during high cycle fatigue testing', J Soc Matenals Scmnce, Japan 24 (1975) pp 4 8 - 5 3
22
Harris, D O , Dunegan, H L, and Tetelman, A. S. 'Prediction of fatigue lifetime by combined fracture mechanics and acoustic emission techniques', Lawrence Radletson Laboratory Report, UCR L 71760 (October 1969)
34
Kmhi, T , Obata, Y , Tanaka, H , Sakaklbara, Y., Hormchl, R and Aokl, K 'Acoustic emission peak under cyclic deformation', J Japanese Inst of Metals 40 (1976) pp 492--498
23
Harris, D O. and Dunagan, H L. "Verification of structural integrity of pressure vesselsby acoustic emission and periodic proof testing', Dunegan Research Corporation, Livermore, Cahfomla, Technical Report DRC-71-2 (May 1971 )
35
Bailey, C D 'Acoustic emission for m-fhght monitoring on aircraft structures', Materials Evaluation 34 (1976) pp 165--171
36
Kusenberger, F N., Lankford, J J r , Francis, P. H. and Barton, J R 'Nondestructive evaluation of metal fatigue', AFOSR-TR-72-1167 (1972)
Bailey, C D., Hamilton, J. M and Plato, W M 'AE monitoring of rapid crack growth m a production-size wing fatigue test article', NDT International 9 (1976) pp 298--304
37
Egle, D M., Mitchell, J R , Bergey, K. H. and Appl, F. J. 'Acoustic emission for monitoring fatigue crack growth', Instrum Soc Am Trans 12 (1973) pp 368-374
Balley, C D and Pies, W M 'Acoustic emission structureborne background noise measurements on aircraft during flight', Matenals Evaluation 34 (1976) pp 189--195, 201
38
Pless,W M , Bailey, C. D end Hamilton, J M 'Acoustic emission detection of fatigue crack growth m a production-size aircraft wing test article under simulated flight loads', Materials Evaluation 36 No 5 (1978) pp 41--48
39
Horak, C R and Weyhreter, A F 'Acoustic emission system for monitoring components and structures in a severe fatigue noise environment', Materials Evaluation 35 No 5 (1977) pp 59--63, 68
40
Duke, J C Jr 'Nondestructive .nvest.gat=on of the mechanical deformation of 7075 aluminum', Doctoral Dissertation (Dept of Mechanics and Materials Science, The Johns Hopkins University, Baltimore, Maryland, 1978)
21
24
25
26
Egle, D. M. 'Detecting high cycle fatigue crack growth using acoustic emission', Presented at the Fracture and Flaws Symposium, Albuquerque, New Mexico (March 1973)
27
Mitchell, J R., Egle, D. M. and Appl, F J 'Detecting fatigue cracks with acoustic emission', Proc Okla Acad Scl 53 (1973) pp 121-126
28
Harris, D O. and Dunegan, H. L. 'Continuous monitoring of fatigue crack growth by acoustic emission techmques', Dunegan-Endevco Technical Report DE-73-2 (February 1973), Exp Mech 14 (1974) pp 71-81
29
Morton, T M , Herrmgton, R. M and Bleletmh, J G 'Acoustic emissions of fatigue crack growth', Engr Fract Mech 5 (1973) pp 691--697
30
Badey, C D. and Plees, W M. 'Acoustic em=sslons used to nondestructwely determine crack locations in aircraft structural fatigue specimen', Proc Ninth Symposlurn on NDE, San Antomo, Texas (April 1973) pp 224-232
31
Vary, A. and Khma, S. J. 'A potential means of using acoustic emission for crack detection under cychc-load conditions', Proc Ninth Symposium on NDE, Sen Antumo, Texas (April 1973) pp 258--266
132
I N T J. F A T I G U E J u l y 1979
A U THO RS John C Duke, Jr is with the Engineering Science and Mechamcs Department, College of"Engineering, Virginia Polytechnic Institute and State Umvers~ty, Blacksburg, Vzrgm~a 24061, USA and Robert E Green, Jr ~s with the Mechamcs and Materials Science Department, The Johns Hopl~ns University, Baltunore, Maryland 21218, USA Enqmmes should be directed mmally to Professor Duke
Although ultrasomc attenuatJon momtonng has been demonstrated to be capable of providing a quantatatlve assessment of integrity during fatigue, much less success has been achieved m developmg the corresponding capabil2ty using acoustic ennsszon momtormg. A thorough review of the previous work employing either techmque for the exatmnatlon of alumzmum during fatague is presented In adchtion, results from simultaneous momtoring of ultrasomc attenuation and acoustzc emzssion during the fatague of 7075 alutrumum demonstrate the feasibility for utzhzang acoustic ermssion as a means of continually surveying the structural integrity of a piece It Is evident from the results that, due to the passlve nature of the acoustlc erms~on momtormg, sensor location relative to failure location is not critical, thus allowing surveillance of structures with irregular geometries However, the utzhzauon of hlgh frequency sound m ultrasomc attenuation momtonng makes th~s techmque for inspectmg known problem areas extremely useful m the presence of intense enw_ronmental noLse The overall findings indicate the capablhty of both techmques for assessmg structural integrity during fatlgue while prowdmg complementary practlcal advantages There are four different basic ultrasomc techmques which have been used to detect the onset of fatigue damage m cyclically loaded structural materials body wave reflection, surface wave reflection, ultrasomc attenuation, and acoustic emission At present, ultrasonic attenuation and acoustic emission measurements are superior to all other ultrasonic techniques for the early detection of fatigue damage Several studies have been performed utilizing either one or the other of these techniques The material receiving the most attention has been alumimum, particularly the alummmm alloy 7075 Principally because of its extensive use m aircraft structures which are constantly subject to a fatigue environment
UL TRASONIC A TTENUA TION The first ultrasonic technique used to study the development of fatigue damage during stress cycling was the ukrasonic attenuation technique As early as 1956, Truell and Hikata 1 observed changes in ultrasonic attenuation in the early stages of fatigue cycling on a polycrystallme alummium specimen In subsequent publications, Truell and coworkers 2-5 reported remits of experiments where ukrasonic attenuation measurements were made dunng fatigue tests up to fracture of the test specimens They used cychng rates of 400 to 1900 cycles per minute and in every case the attenuation changed as a function of the number of cycles of loading For tests carned out to fracture there was a marked increase in attenutation lust prior to failure In 1962, Ponomarev6 measured the ultrasonic attenuation m metallic and nonmetalhc cylindrical samples during repetitive loading The metallic samples, steel, copper, and duralumin were subjected to torsional loading, whde the nonmetallic samples, glass, plemglass, quartzite, and quartz were subjected to periodic hmpact loads He established that the moment of fadure onset of a part due to fatigue damage can be determined from the character of the change in the ultrasonic attenuation curve
Pawlowslu 7,8 m 1963, measured ultrasomc attenuauon dunng fatigue cycling of specimens of low carbon steel, an alloy steel, an alummmm alloy, and grey cast iron Using a cyclic loading rate of 25 cycles per second he found that all the materials tested showed an increase m attenuation with increasing number of faugue cycles His change in attenuauon versus number of cycles curves exhibited two distinct regions The first region was nearly linear and showed a relatively gradual increase m attenuation with increasing number of cycles The second regi0n Which occurred in the final stage of the fatigue life of the specimen showed a more rapid increase of attenuation with increasing number of cycles and terminated with fracture of the specnnen. Pawlowskl concluded that an irreversible increase in attenuation can be regarded as an indicator of damage accumulation developing gradually in the process of fatigue These irreversible changes he attributed to microcrack formation which coalesce to form maerocracks and eventual fracture In 1971, Moore et al9 attempted unsuccessfully to relate ultrasonic surface wave measurements of attenuation and velocity to fatigue damage m five specimens of 1100 alumimum which had undergone faugue deformation for 30, 50, 70, 80 and 90% of the specimen life. A possible reason for their lack of success was apparent interference effects between some unspecified wave and the surface wave of interest Then in 1972 Joshi and Green 10 reported expenments which used the change m ultrasomc attenuation measurements as a continuous momtor of fatigue damage during cyclic testing of 6061-T6 alumhmum and steel specunens Each spechmen was fatigued as a cantilever beam in reverse bendmg at 30 Hz with a vibraUonal amplitude peak-to-peak set at a fixed value in the range 7 5 to 15 mm In a typical test, the ultrasonic attenuation hmtially remained constant, increased slowly, and then increased catastrophically just prior to fracture of the test specimen All experiments performed on both alurmmum and steel specimens at
0142--1123/79/030125--08 $02 00 © IPC Business Press L,m,ted 1979
INT J FATIGUE July 1979 125
various vibrational amplitudes yielded snnilar results in that ultrasonic attenuation served as a very sensitive indicator of fatigue damage and in every case indicated that failure was imminent several hours before conventional ultrasomc testing could detect an additional echo caused by waves reflected from a crack Panowlcz, 11 in 1974, examined the effects of latent defects on the fatigue hfe of 1100 alumlmum with this technique The defects were sunulated by rejoining saw cuts by plastic welding It was concluded that the specimen lifetime could be predicted p n o r to the onset of failure modes detectable by conventional NDT techniques Most recently Mlgnogna et a112 have extended the work mentioned earher 10 to continuous measurement of changes in ultrasonic attenuation in plain and 'rivet simulated' 1 59 m m thick sheets of 7075-T6 aircraft a l u m m m m alloy during fatigue cycling in reverse bending at 1725 cycles per minute The attenuation typically remained constant initially, increased slowly, and then increased rapidly lust prior to fracture o f the test specimen These results demonstrate that ultrasonic attenuation monitoring can indicate fatigue damage in a l u m m m m alloy sheets much earlier than the appearance of an additional pulse echo resulting from the reflection of energy by a crack as observed in a normal A-scan
A COUSTIC EMISSION Even though Katser 13 is generally credited wtth discovering acoustic emission in 1950, it was not until 1954 when Dunegen et a114 imtiated work in the ultrasonic range of frequencues that acoustic emission actually became a scientific and engineering tool Even though the amount of work performed in this area since that time is extensive, only a few investigators have m o m t o r e d acoustic emission during fatigue tests The main reason for this is that acoustic emission detection systems o f current design normally operate in the frequency range from 20 kHz to 1 MHz with their peak response being about 100 kHz and acoustic emission signals are random and transient in nature Unfortunately, the noise generated by extraneous sources during fatigue testmg is predominant in this same frequency range and, therefore, difficulty is encountered in separating the acoustic emission signal from the background noise In 1967 and 1968, Hartbower et a115-17 monitored acoustic emission associated with slow crack growth during low-cycle fatigue testing of precracked steel spe~mens possessing different heat treatments and o f a l u m m m m and titanium alloys F o r this low-cycle fatigue work an accelerometer in conlunction with a filter apparently elumnated the noise problem Hutton, 18-2° in 1969, reported measuring acoustic emission during high cycle t e n s i o n - t e n s i o n fatigue testing of notched a l u m m m m and carbon steel specimens The latter test in particular gave evidence that macrocrack nucleation caused sharp increases m acoustic emLssion prior to detection of a visible crack The emission data showed a gradual increase during the miUal stages Also m 1969, Dunegan et al21-23 proposed a m e t h o d of detecting fatigue damage b y intermittently stopping the fatigue cycling and p r o o f testing the specumen by overstressing while m o m t o n n g the acoustic emission. This m e t h o d has the advantage o f elumnation o f most of the background noise, but the disadvantage that ~t is not a continuous on-hne technique Nevertheless, it is mformauve
126
INT J FATIGUE July 1979
to note that for steel and a l u m m m m alloy test specnnens and for steel and a l u m m m m pressure vessels snmlar experimental results were obtained In all cases, both the total acoustic emission count and the counts per cycle increased slowly with the increasing number of fatigue cycles and then increased rapidly just prior to failure The same year, Moore et al 9 monitored acoustic emission counts during fatigue cycling of 1100-0 alumlmum specimens They observed that the cumulative number of acoustic emission counts changed slowly in the early part of the faugue process and then changed by several orders of magmtude A relationship was observed between the significant change in the slope of acoustic emission curve and the percent o f faugue hfe The change was observed to occur at less than 50% of the fatigue life and as such offered a possible early fatigue damage warning Kusenberger et al24 in 1972, monitored crack growth in stress cycled specimens of 4340 steel and 7075-T5 a l u m m m m Periodically, cycling was stopped, the crack length at the surface was measured, and a constant p r o o f load was applied to the specimen During proof loading, the specimen was monitored for acoustic emission The results obtained with the 4340 steel specimens were puzzling in that essentially no acoustic emission above the nominal background level was recorded until the last cycle prior to failure Such results were obtained even though on several occasions proof loads were raised nearly to yield The results of the experiments with the 7075-T5 alummmm specimens indicated a strong dependence of the acoustic emission on the state of stress A much lower count was obtained for plane strain than for plane stress states In 1973 Egle et 8]25-27 reported an acoustic emission system for monitoring high cycle fatigue crack growth They used a conventional acoustic emission system with a high pass filter between the transducer and pre-amphfier and a second high pass filter between the pre-amphfier and digital counter In addition, an electromagnetic transducer was used to gate the counter every 10 or 100 load cycles F o r low cycle fatigue of a notched a l u m m m m plate the number of acoustic emission counts per load cycle first decreased and then increased slowly lust prior to failure F o r high cycle fatigue of an a l u m m m m specimen m a displacement controlled cantilever beam tester, the emission exhibited a relatively high count at the start of the loading followed by a sharp decrease and then an increase F o r specimens tested with higher m a x i m u m displacements and shorter lives, the emission continued to increase until failure F o r specunens tested with lower m a x i m u m displacements and longer lives, the emission tended to level off and then to decrease Egle attributes these results to the fact that the rate o f crack propagation decreased for the lower m a m m u m displacements because of the displacement control on the fatigue tester Harris and Dunegan 28 continuously monitored fatigue crack growth in 7075-T6 a l u m m m m and 4140 steel by acoustic emission techmques The acoustic emission rate per cycle was consistently observed to be cyclic in nature, le,the rate was observed to be high for a few cycles of loading followed by a period of reactivity This was regarded as strong evidence that crack growth in these materials does not occur by a uniform growth per cycle, but by a burst of growth followed by a stationary period Morton e t a/29 performed sumlar investigations m which they monitored acoustic emission and quanutatwely correlated it with crack growth rate and the applied range
of stress intensity for high cycle fatigue of 2024-T851 alummium alloy They concluded that the acoustic emissions observed dunng fatigue are more closely related to the crack tip plaint volume than to crack extension Bailey and Pless3° used acoustic emLmon techniques and sound travel tLme determinations to locate cracks in a structural fatigue test specrmen The specimen was subjected to a spectrum of cychc load conditions which simulated a total of 150 000 aircraft flight hours The sloecunen was momtored for acoustic ermss~on approx~nately 10% of the total fatigue test tL,ne It was found, m agreement w~th the results of other investigators, that crack growth at a given location was not continuous throughout the fatigue tests. Cracks were observed to grow durmg some test passes, but apparently remamed stationary dunng other passes Some cracks did not even appear to grow contmuously during a single pass Crack growth was reported to be rapid or slow for given cracks at different tLmes A total of 18 crack locations were identified by acoustic emission momtormg, 15 of which were verified during metaUurglcal investigation Vary and Kluna 31 reported the results of a prelL,nmary investigation to assess the feaslhihty of continuously morntonng acoustic emLssion s~gnals from fatigue cracks durmg cychc bend tests Plate specunens of 6A1-4V titanium, 2219-T87 alummlum, and 18-N1 maragmg steel were tested w~th and w~thout crack starter notches The investigation indicated that it was possible to extract meanmgful acoustic emlsmon s~gnals m a cyclic bend machine enwronment and that such ~coumc emission momtonng is a potentially useful tool for investigating fatigue cracl~ng Ultrasonic crack growth momtonng was apphed s~multaneously w~th acoustic emission momtonng for a few specimens Although abrupt increases in the acoustic ermsslon count rate were observed, the ultrasonic signals indicated only a smooth increase in crack size Pless and Badey52 used an acoustic emission monitormg system to detect crack extensions induced m a large aircraft using structure through cyclic loading The crack extensions were induced by tension-tension loadmgs at specified percentages of design hmtt load acting on fourinch long crack-starter sawcuts made at well-defined locations on the specimen Since both audible and inaudible fracture increments were detected, ]t was concluded that the tests proved the feas~bthty of the acoustic emission momtormg technique However, even though the signal-to-noise ratio was high enough to v~tually exclude false alarms during these tests, the tests could prowde no information about acoustic and other transients resulting from active flight systems Hatano 35 measured acoustic emission dunng highcycle fatigue testing of pure alumhmum specimens He detected intermittent burst-type emission in the early stage of fatigue testing before observation of macroscopic crack growth He also noted that most of the acoustic emission signals were detected dunng that portion of the fatigue cycle where the cyclic load was a mammum In 1976, I~sh] et a154 studied the acoustic emis~on generated dunng cyclic deformation of pure alummlum and reported an acoustic emission peak accompanied by a Bauschlnger effect This peak was observed w~th the appearance of irreversible plastic strain and the peak height decreased w~th increase in the number of cycles up to a constant value at the saturation stress Badey and colleagues55-58 expandea upon their earlier work 50,52 on assessing the posslbdlty of usmg acoustic
ermsslon for in-flight momtoring of aL,xn~.ft structures. Their Fhght Structural Momtonng System (FSMS) was designed to detect the large transient acoustic sound generated by stogie large crack extensions rather than the lower level signals generated by fatigue crack initiation. Using the FSMS they succ,esdully momtored crack extensions in fullsize complex structural components dunng fatigue testing by a combmauon of sKjnal processing techmques mvolwng spatial discrimination, frequency filtering, and sagnal amphtude discrimination Cracks were initiated and grown on a wing test arucle at selected fastener holes from sharpnotch sawcuts by subjecting the specimen to a flight-byt'hght load spectrum typical of a military transport aircraft Acoustic emission was detected from 12 of the 15 test holes momtored which experienced crack growth and three of six extraneous acoustic emission sources were traced to loose fasteners For three of the cracks monitored, the acoustic enusmon counts showed reasonable correlation w~th the amount of crack growth. The greatest detractor to correlatable data was random noise Horak and Weyhreter59 reported the development of an acoustic emission system for momtonng components and structures m a severe fatigue noLse environment. Their system utilized master-slave discrimination, rise time discrLmmatlon, and coincidence detection to identify and reject unwanted noise signals from computer processing Among the dLscnmmauon concepts which they found to be unsatisfactory were frequency filtering, computerized data reduction, and mechanical dampening. Since Horak and Weyhreter found that the wave forms of the vanous transient noises contributed most to false data generation, the only acoustic emimon signals rehably detected were those which occurred dunng the time penods between the tran~ent fatigue noises Use of the system on a fatiguing part which had not begun to crack, 10 false locations were indicated m 16 hours compared x'nth 56 false locations m 15 seconds indicated without their discrimination system Crack propagation was not always continuous and sometimes stopped completely for several hundred cycles Although crack propagation is generally considered to occur during the maximum stress apphcation of the fatigue cycle, their results showed that acoustic emLsslon resulting from crack extension occurs dunng low, medium, and high parts of the loadmg curve These data led them to caution that the exclusive use of time gates to ehmmate extraneous noise signals might ehrnmate a large portion of the acoustic emission signals also As a result of the various types of defects and loads involved, crack lmtiatlon was detected and located from several minutes to several hours prior to ultimate failure Since crack ruination occurred within the material, crack lmtiation s~ze was not determined Although ultrasoinc attenuation momtonng has been demonstrated to be capable of prowding a quantitative assessment of integrity dunng fatigue, much less success has been achieved in developmg the corresponding capablhty using acoustic emission monitoring Through sLmultaneous momtonng of ultrasomc attenuation and acoustic emismon during the fatigue of alummlum, the feas~hthty for utihzmg acoustic emission as a means of continually surveying the structural lntegnty of a piece can be assessed The importance of such an assessment can be reahzed by recognizing that the passive acoustic ermssion momtonng technique could be utihzed to examine pieces of Lrregular geometry which, because of inaccessibility, could not be examined ultrasonically
INT J FATIGUE July 1979 127
EXPERIMENTAL Continuous s~multaneous momtonng of ultrasonic attenuation and acoustic emission was performed on 7075 alummmm alloy specnnens subjected to reverse benchng cychc fatigue at 1725 cycles per minute The peak to peak amphtude was maintained at 9 mm, which corresponded to a maximum tensile fibre stress of 255 MN/m 2, for all the tests Specnnens m the form of rectangular bars 17 8 mm (0 7 in) wide, 304 8ram (12m) long, and 12 7mm (0 5m) thick were tested in two heat treated conditions 7075-T651, as received, and 7075, solution treated The 7075-T651 condition was obtained by solution treating the 7075 alummmm alloy at 466-477°C, quenchlng it in water at 60-71°C, stretching it between 1 and 3%, and then artificially ageing it at 1 1 6 - 1 2 7 ° C for 2 3 - 2 8 hours By concluding the treatment after the water quench, the 7075 solution treated condmon was obtained Fig 1 depicts the specimen geometry as well as its testing configuration An effort was made to acousUcally insulate the sample from the fatigue machine in order to mmnmze any mechanical noise because of the nature of the monitoring procedure A specially designed acoustic ermsslon system permltted reliable acoustic emission measurement~ to be made despite the high level background noise from the fatigue testing machine and associated grips This system,* combined a unique transducer design w~th a multi-shielded Mu-metal and copper screen cable and other special features for shielding against electromagnetic interference
The maximum sensltiwty of the system was at 300 kHz These special features yielded an acoustic emission system W-lth a signal-to-noise ratio at ]east three orders of magnitude better than conventional acoustic emission systems which use regular piezoelectric transducers, pre-amphflers, and eoamal cables The output of the acoustic emission system was passed through a true root-mean-square voltmeter (Bal]entree mode] 320) and recorded on one channel of a multichannel stnp chart recorder (Fig 2) The ultrasonic attenuation measurement system consisted of a conventional 10 MHz longitudinal mode transducer (Aerotech Gamma), tunable pulser-receiver (Matec model 6600), and an automatic attenuation recorder (Matec model 2407A), as shown in Fig 3 An automatic gain control In the attenuation recorder maintained the lmt]al u]trasomc pulse voltage at a constant value, and permitted consistent ultrasonic attenuation readings to be continuously recorded on a second channel of the multichannel strip chart recorder The side-by-side presentauon of ultrasonic attenuation and acoustic emission data enabled easy comparison of the experimental results In order to facilitate the combined testing it was advantageous to synchronously trigger the ultrasonic pulse (Fig 4) Using a light detecting sensor stunulated by a reflective region of the rotating cam of the fatigue machine, the ultrasonic system could be triggered at any point in the fatigue cycle By trlggenng at a fixed point in the fatigue cycle a constant path length for the ultrasound pulse could Transducer
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RESULTS Figs 5 and 6 display the results of typical fatigue tests on 7075-T651 as received, and 7075 solution treated specimens, respectively In Fig 5 the change in ultrasonic attenuation exhibits no appreciable alteration unttl 80% (1 5 x 106 cycles = 100%) of the faugue life at which point it changes rapidly going off scale at approxzmately 97% of the fatigue hfe To achieve high sensitiwty, a fine scale of monitoring the change in attenuation was used Had a coarser scale been used, the attenuation would have been seen to continue to increase until failure The true root mean square (rms) voltage output of the acoustic emission signal, which was RMS (. . . .
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amplified by 70 dB, was seen to fluctuate at the beginning of the cychc deformation, level off, and begin to steadily increase at 80% of the fatigue life, with a more rapid increase occurnng after 95% of the fatigue life Unlike the smooth, nearly monotonic behavzour of the change in ultrasonic attenuation, the rms curve is seen to have local maxima and rmnima The peaks, m actuality, consist of many mdlwdual burst type indications occurnng over an extended period of time, and their appearance is due to the condensation of considerable data into the presented plots Very slmdar behavlour is seen in Fig 6 which is typical of the solution treated specimens The change in ultrasonic attenuation shows a small but steady increase until 90% of the fatigue life (7 4 x 106 cycles = 100%) where it begins to increase rapidly, going off scale at 94% of the fatigue life Slrrularly the rms curve after a peak at the beginning of the deformation levels off until around 78% of the fatigue llfe where the overall rms curve begins to increase slowly with several large peaks superimposed on overall increasing trend A rapid, overall increase began at 87% of the fatigue life and levelled off at an elevated value, at 95% of the fatigue life The fatigue tests in general appear very similar with the change in ultrasonic attenuation, exhibiting little variation until between 90% and 95% of the fatigue life where a rapid increase occurred The rms curves compared well, fluctuating at the start of the fatiguing, levelling off for a major port~on of the fatigue life, and finally increasing rapidly between 85% and 95% of the fatigue life Figs 7, 8 and 9 are photographs of scanning electron micrographs of typical fatigue fracture surfaces Fig 7 pictures at 3 x 105 ongmal magnification fatigue striations
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Fig 6 Change in ultrasonic attenuation and true root mean square voltage of acoustic emission detected m relation to percentage of fatigue life (7 4 x 106 cycles) 7075 alummlum, solution treated (maximum tens,le fibre stress 255 MN/m 2)
F=g 7 Scanning electron mlcrograph of fatigue strmtJons o f 7075-T651 alum=mum, as recewed The arrow indicates a fractured inclusion particle
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in 7075-T651 along with a fractured inclusion particle which can be seen to lead to a crack running out into the matrix material Fig 8 pictures snnilar striations at 2 x 103 ongmal magnification, in a solution treated sample along with a fractured inclusion particle, the striations are noticeably larger in this more ductile material Fig 9 shows, in addition to fatigue striations, at 2 x 103 original magnification, in a solution treated sample, two intermetallic particles which have remained contiguous
DISCUSSION Although the intent of the combined monitoring of fatigue of the 7075 specimens was to establish whether or not acoustic emission m o m t o n n g could serve in a mmilar capacity to ultrasonic attenuation monitoring for providing early warning of fatigue failure, much more m a y be learned from this study Initially,however, it should be strongly emphasized that acoustic emission m o m t o r m g clearly exhibits the capability of giving early warning of fatigue failure Unlike ultrasonic attenuation (Figs 5 and 6) which remains relatively unchanged until the onset of failure, the acoustic emission exhibits considerable activity during the early stages of deformation This is somewhat expected in light of the explanation of Duke 4° regarding the acoustic emission occurnng d u n n g elastic tensile deformation of 7075 alumimum Acoustic emission occurring prior to yield was described as the result of the breakaway of piled up dislocations This breakaway takes place as elastic strain energy, supplied by tensile loading, provides the necessary activation energy for the dislocations to surmount their barriers The activation energy necessary to cause breakaway
F=g 9 Scanning electron mlcrograph of fat=gue strlat=ons of 7075 alummlum, solution treated The arrow md=cates an intermetalhc particle
of piled up dislocations, in this case, would be supplied by the cyclic deformation A slight change in attenuation* is observed prior to the rapid change near failure with no corresponding effect in the acoustic emission behaviour This is thought to be due to the senmtNity of the ultrasonic attenuation to moving dislocations, which would occur throughout the specimen, while the acoustic emission would be sensitive to dislocation breakaway or dislocation creation, which would occur In localized regions at any one time Therefore, moving dislocations would cause an increase in the ultrasonic attenuation but would not affect the acoustic emission behaviour ** This dislocation break.away responsible for the initial acoustic emission behavlour occurring on a localized scale would only comcldentally interact with the ultrasonic wave and even then due to the slow system response would not result in a change in the attenuation measured This situation serves to show the complementary nature of the two techniques in that the passive acoustic emission technique is sensitive to localized transient phenomenon while the active ultrasonic attenuation monltorlng technique is affected by the more steady state behaviour of the entire material it surveys Furthermore, as failure becomes evident, both techniques are seen to undergo rapid increases Whereas the change in ultrasonic attenuation increases monotonically,
Fig 8 Scanning electron mlcrograph of fatigue striations of 7075 alumlmum, solution treated The arrow indicates a fractured inclusion particle
130 INT J FATIGUE July 1979
* Most noticeable in the solution treated specLmens * * The effect being more pronounced m the solution treated specimens might be expected realizing that the heat treatment has removed the malor~ty of d~slocatlon pinning points, the precipitate particles Because of this, dislocations which break away from pileups during the initial portion of cycling may move with few Mmpedlments triggering additional break-aways, until finally becoming strongly pinned
the acousUc ermss~on is seen to undergo fluctuations as it increases This is thought to be a result of an increase m the amount of emission which occurs when a crack is formed, or as the crack extends, and as the new surfaces abrade against one another dunng subsequent fatigue cycles As cycling continues, the abrasion Ls smoothed out due to plastic flow, causing a decrease in emissions from this source Since fadure does occur, other sources of acoustic emission might be expected to contribute, but any such contribution was not discermable m the present study Scanning electron mlcrographs (Figs 7 and 8) prov]de evidence to support the contention that inclusion particles fracture and cause microcracks to form that link up with others to cause the final fracture However, the lmtiation and progression of this series of events, as well as the effect of the lntermetalhc particles, Fig 9, under the influence of cyclic deformation are not as distract as during pure tension Although the emissions from different sources might be expected to be different, the momtonng technique utthzed in this study would not differentiate if indeed chfferences were present The results of the fatigue study descnbed here clearly establishes the fact that the distract rmcroscopic deformation mechanisms which give rise to acoustic emission during tensile deformation of 7075 a l u m z n m m reported by Duke 40 do not remain distinct during fatigue
part by the Air Force Office of Scientific Research (AFSC), United States Air Force, a special note of appreciation is clue to Mr Wflham J Walker and Lt Col J D Morgan, III in thLs regard
REFERENCES 1 2 3
Truell, R , Ch,ck, B,, Picker, A and Anderson, G. "The use of ultrason,c methods to determine fatigue effects in metals', WADC Techmcal Report 59-389 (1959)
4
Truell, R , Chick, B , Anderson, G , Elbaum, C. and Fmdley, W 'Ultrasonic methods for the study of stress cycling effects in metals', WADD Techmcal Report 60 920 (1961)
5
Chick, B , Hikata, A , Anderson, G , Flndley, W, Elbaum, C and Truell, R 'Ultrasonic methods m the study of fatigue and deformation m single crystals', WPAFB Report No ASD-TDR62-186 Part II, AD No 408704 (1963)
6
Ponomerev, P V 'Ultrasonic control of fatigue damage to matermls', Zavodskaya Laboratorlya 28 (1962) 1345-1346, Enghsh translation In Ind Lab 28 (1963) 1429-1431
7
Pawlowskl, Z 'Internal friction of metals and the problem of damage cumulation with static and variable Ioadmgs', Proc of Vibration Problems (Warsaw) 4 (1963) 43--64
8
PawlowJkl, Z 'Ultrasonic attenuation during cyclic straining', Proc Fourth International Conference on Nondestructive Testing (1963) (Butterworths, London, 1964) pp 192--195
9
Moore, J F , Tseng, S and Martin, G 'The early detection of fatigue damage', Techmcal Report AFML-TR-71-185, AD 730 348 (1971)
CONCL USIONS 1 Acoustic emission momtormg may be used to provide early detection of fatigue failure in both as received 7075T651, and solution treated alummium specuuens 2 Ultrasonic attenuation and acoustic emission momtonng appear to be sensitive to different dislocation mechanisms, both of which are active dunng fatigue, ie ultrasonic attenuation is affected by moving dislocations, while acoustic emissmns are primarily caused by dislocation breakaway and creation 3 Ultrasonic attenuation momtormg takes into account the entire area surveyed by the ultrasonic beam, wknle acoustic
10
Josh,, N. R and Green. R E Jr 'Ultrasonic detection of fatigue damage'. Engr Frect Mech 4 (1972) pp 577--583
11
Panowlcz, W V, 'Ultrasomc detectmn of fatigue damage m aluminum specimens containing reduced latent defects', Master's Thesis (Dept of Mechanics and Materials Science, The Johns Hopkins Umverslty, Baltimore, Maryland, 1975)
12
Mzgnogna, R B , Duke, J C. Jr and Green, R. E. Jr. 'Early detection of fatigue cracks in aircraft aluminum alloy sheets', Paper presented at National Fall Conference o f ASNT, Detroit, Michigan (October 1977) (submitted for publication m Materials Evaluatmn)
13
Kmser, J 'Untersuchungen uber des auftreten Gerauschen belm Zugversuch'. PhD Thesis (Techmsche Hochschule, Mumch, 1950J, Arklv fur des Eisenhuttenwesen AR EIA 24 (January-February 1953) pp 43--45
14
Dunegan, H L , T a t r o , C A andHama, D O 'Acoustic emission research', Lawrence Rad/a~on Laboratory, L/vermore, Cahfomm Report UCID-4868, Rev 1 (1964)
15
Gerbermh, W W and Hertbower, C E 'Some observations on stress wave emission as a measure of crack growth'. IntJ Fract Mech 3 (1967) pp 185--192
16
Hartbower, C E ,Gerbermh, W W and Llebowitz, H 'Investigation of crack-growth stress-wave relationships', J Engng Fract Mech 1 (1968) pp 291--308
17
Hartbower, C E , Gerberzch, W. W and Cnmmma, P. P 'Momtormg subcrltzcal crack growth by detection of elastic stress waves'. The Welding Journal WEJUA 47 (1968) pp 1--18s
18
Hutton, P H 'Use of acoustic emission to study failure mechanics m metals', ASME Paper No 69-Met-8 (1969)
19
Hutton, P H 'Acoustic emission -- a new tool for evaluating structural soundness', Proc Seventh SFmposlum on Nondestructive Evaluation of Components and Materials m
emission momtormg responds only to mechanisms capable of re]easing sufficient elastic energy to be detected above the electronic background noise of the system 4 Acoustic emissmn activity early in the fatigue deformation of both 7075-T651 and 7075 solution treated alummmm is consistent with the argument of Duke40 which views the breakaway of piled up dislocations upon the application of stresses below yield as a possible source of acousuc emission 5 The fluctuations in the acoustic emission behaviour near failure in both types of specimens tested are due in part to the mechanical abrasion of cracked surfaces 6 SLmflar sources are believed to be responsible for the acoustic emission dunng fatigue, as those in the case of studies involving quas~-static tensile deformation 7 Since ultrasonic attenuation is known to increase pnnclpally as the result of both scattenng from cracks and absorption by vibrating and moving chslocations it was not possible to clearly determine the mechanisms responsible for the observed behaviour
A CKNOWL EDGEMENTS Sincere appreciation is expressed to Mr Don Bzrchon and Dr Robert Dukes of the Adrmralty Marine Technology Establishment for the loan of the Low Noise Acoustic Emission System Additionally, this work was supported in
Truell, R end Hikata, A. 'Fatigue m 2S alum=mum as observed by ultrasomc attenuation methods', Watertown Arsenal Techmca/Report N o WA L 143/14-47 (1956) TrueU, R. end Hikata, A 'Fatigue and ultrasonic attenuation', ASTM STP 213 (1957)
Aerospace Weapons Systems and Nuclear Applications, San Antomo. Texas (April 1969) pp 165--171 20
Hutton, P H 'Acoustic emission applied to determination of structural integrity', Automotive Engmeenng 79 (1971 ) pp 3 3 - 3 7
INT. J. FATIGUE July 1979 131
Dunegan, H. L., Harris, D. O. and Tetelman, A. S. 'Detection of fatigue crack growth by acoustic emission techniques', Proc Seventh Symposmm on Nondestructive Evaluauon o f Components end Materials m Aerospace Weapons Systems and Nuclear Apphcatlons, San Antomo, Texas (April 1969), pp 20-31, Materials Evaluation 28 (1970) pp 221--227
32
Pleu, W M and Bailey, C. D. 'Detection of large crack extensions in aircraft wing structure by acoustic peak detectors', Proc Tenth Symposium on NDE, San Antomo, Texas (April 1975) pp 30--43
33
Hatano, H. 'Acoustic emission during high cycle fatigue testing', J Soc Matenals Scmnce, Japan 24 (1975) pp 4 8 - 5 3
22
Harris, D O , Dunegan, H L, and Tetelman, A. S. 'Prediction of fatigue lifetime by combined fracture mechanics and acoustic emission techniques', Lawrence Radletson Laboratory Report, UCR L 71760 (October 1969)
34
Kmhi, T , Obata, Y , Tanaka, H , Sakaklbara, Y., Hormchl, R and Aokl, K 'Acoustic emission peak under cyclic deformation', J Japanese Inst of Metals 40 (1976) pp 492--498
23
Harris, D O. and Dunagan, H L. "Verification of structural integrity of pressure vesselsby acoustic emission and periodic proof testing', Dunegan Research Corporation, Livermore, Cahfomla, Technical Report DRC-71-2 (May 1971 )
35
Bailey, C D 'Acoustic emission for m-fhght monitoring on aircraft structures', Materials Evaluation 34 (1976) pp 165--171
36
Kusenberger, F N., Lankford, J J r , Francis, P. H. and Barton, J R 'Nondestructive evaluation of metal fatigue', AFOSR-TR-72-1167 (1972)
Bailey, C D., Hamilton, J. M and Plato, W M 'AE monitoring of rapid crack growth m a production-size wing fatigue test article', NDT International 9 (1976) pp 298--304
37
Egle, D M., Mitchell, J R , Bergey, K. H. and Appl, F. J. 'Acoustic emission for monitoring fatigue crack growth', Instrum Soc Am Trans 12 (1973) pp 368-374
Balley, C D and Pies, W M 'Acoustic emission structureborne background noise measurements on aircraft during flight', Matenals Evaluation 34 (1976) pp 189--195, 201
38
Pless,W M , Bailey, C. D end Hamilton, J M 'Acoustic emission detection of fatigue crack growth m a production-size aircraft wing test article under simulated flight loads', Materials Evaluation 36 No 5 (1978) pp 41--48
39
Horak, C R and Weyhreter, A F 'Acoustic emission system for monitoring components and structures in a severe fatigue noise environment', Materials Evaluation 35 No 5 (1977) pp 59--63, 68
40
Duke, J C Jr 'Nondestructive .nvest.gat=on of the mechanical deformation of 7075 aluminum', Doctoral Dissertation (Dept of Mechanics and Materials Science, The Johns Hopkins University, Baltimore, Maryland, 1978)
21
24
25
26
Egle, D. M. 'Detecting high cycle fatigue crack growth using acoustic emission', Presented at the Fracture and Flaws Symposium, Albuquerque, New Mexico (March 1973)
27
Mitchell, J R., Egle, D. M. and Appl, F J 'Detecting fatigue cracks with acoustic emission', Proc Okla Acad Scl 53 (1973) pp 121-126
28
Harris, D O. and Dunegan, H. L. 'Continuous monitoring of fatigue crack growth by acoustic emission techmques', Dunegan-Endevco Technical Report DE-73-2 (February 1973), Exp Mech 14 (1974) pp 71-81
29
Morton, T M , Herrmgton, R. M and Bleletmh, J G 'Acoustic emissions of fatigue crack growth', Engr Fract Mech 5 (1973) pp 691--697
30
Badey, C D. and Plees, W M. 'Acoustic em=sslons used to nondestructwely determine crack locations in aircraft structural fatigue specimen', Proc Ninth Symposlurn on NDE, San Antomo, Texas (April 1973) pp 224-232
31
Vary, A. and Khma, S. J. 'A potential means of using acoustic emission for crack detection under cychc-load conditions', Proc Ninth Symposium on NDE, Sen Antumo, Texas (April 1973) pp 258--266
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A U THO RS John C Duke, Jr is with the Engineering Science and Mechamcs Department, College of"Engineering, Virginia Polytechnic Institute and State Umvers~ty, Blacksburg, Vzrgm~a 24061, USA and Robert E Green, Jr ~s with the Mechamcs and Materials Science Department, The Johns Hopl~ns University, Baltunore, Maryland 21218, USA Enqmmes should be directed mmally to Professor Duke