Analysis of the effect of using two different strain rates on the acoustic emission in bone

,

~,omcchanzcsVol

19, hi0 2. pp 119-127. 1986

W:l-9290 5

Pnnted m Great Bnram

86 13 00 + .oO

1986 Pergamon Prcrs Ltd

ANALYSIS OF THE EFFECT OF USING TWO DIFFERENT STRAIN RATES ON THE ACOUSTIC EMISSION IN BONE RICHARD

A. FISCHER,STEVENW. ARMS, MALCOLMH. POPE and DAVID SELIGSON

Department of Orthopaedics and Rehabilitation, University of Vermont College of Medicine, Burlington, VTO5405, U.S.A.

Abstract-To study the effect of strain rate on the acoustic emission amplitude signature of bone, bovine cortical bone was milled into standard tensile specimens which were tested at two different strain rates while being monitored with acoustic emission equipment. It was demonstrated that the amplitude distribution of the acoustic events in bone is dependent on strain rate. Greater numbers of events occurred with the slower strain rate (0.0001 SK’), but these events were of lower amplitude than those emitted during the more rapid strain rate (0.01 s- ‘). The plot of the cumulative event amplitude distribution followed the power-law model, and the slope of this output, the b-value, represented a signature of the amplitude distribution. The mechanical test results were consistent with the behavior of a viscoelastic multi-phase composite material.

Acoustic emission (AE) may be defined as transient elastic stress waves that stem from sudden energy release in a material (ASTM, 1981). This energy may involve plastic deformation of a material (e.g. dislocation movements) or crack initiation and propagation. Precise detection and amplification equipment is required to detect these stress waves. iosef Kaiser (1950) began the scientific study of AE using electronic equipment capable of detecting non-audible signals. Today AE monitoring is extensively used commercially as a means for nondestructive assessment of the structural integrity of industrial materials. The AE signal is a damped sinusoidal wave whose amplitude represents the strength of the signal. The acoustic response of a material may be characterized as either a low amplitude, continuous form of emission, or a high amplitude, burst-type of emission. These two varieties of emissions represent the extreme conditions on a continuum, which can be expressed graphically by the Cumulative Event Amplitude Distribution (CEAD). The low amplitude, continuous form of emission is associated with the summation of many dislocations (shifts of atomic planes) within a material’s structure. High amplitude, burst-type emissions are associated with sudden energy release, such as fibers breaking in boron-epoxy composites (FitzRandolph et a/., 1972) or sudden fracture of brittle inclusions in metal (Bianchetti et al., 1976). The relationship between AE response and mechanical behavior in composite materials has been studied, (Awerbuch, 1982; Carlyle, 1978; Henneke et al., 1975; Kelly et al., 1976; Rotem and Baruch, 1974; Rotem and Altus, 1979; Weir, 1973).

Received 29 November 1982;in revised fom 30 August 1985.

Presented at the Sixth Annual Meeting of the American Society of Biomechanics, Seattle, Washington, 13-l 5 October 1982.

AE has also been used to study the material properties of bone (Thomas et al., 1977; Wright et al., 1981; Hanagud et al., 1977; Nicholls and Berg, 1981; Hanagud et al., 1975; Yoon et al., 1980; Krauya and Lyakh, 1978; Hanagud el al., 1973; Netz, 1979; Knet-s et al., 1975). Hanagud et al. (1973) using bovine femora first demonstrated detectable acoustic emission from bone. In 1975, Hanagud et 01. showed that oven dried bone specimens had a greater acoustic emission rate than moist bone specimens, when loaded in three point bending at 0.05 in. min - ’ (0.127 mm min - ’ ). In 1977, Hanagud et al. demonstrated that soft tissue of 2-9 mm thickness overlying bone does not affect the bone’s acoustic emission response to stress. In that study, three point bending at 0.05 in. min-’ (0.127 mmmin- ’ ) was performed on whole rabbit tibiae and porcine ribs. Netz (l979), examined the AE response of canine femora in torsion at 6 degrees s- ‘. In that study, the acoustic events were demonstrated to occur in the nonlinear, plastic portion of the load-deflection curve. Nicholls and Berg (1981) monitored the acoustic response of rabbit tibia callus, in shear, and found no acoustic emissions until 507; of load to failure. Wright et al. (1981) performed uniaxial tensile tests on machined specimens of normal, decalcified and deproteinized bone from bovine femora. A strain rate of 0.025 s-’ was utilized. AE from the control specimens began just prior to yield, with a rapid increase in events prior to failure. The initial acoustic emissions at yield were of the low amplitude, continuous type, whereas the emissions just prior to fracture were of the high amplitude, burst-type variety. The decalcified specimens exhibited low amplitude emissions prior to fracture. The deproteinized specimen exhibited no AE except at fracture. Nicholls and Berg (198 1) have proposed the use of AE for the monitoring of fracture healing. It has been argued that because the initiation of AE does not 119

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R. A. FISCHER,S. W. ARMS, M. H. POPEand D. SELIGSON

typically occur in bone until plastic deformation occurs, that this clinical use of AE cannot be considered nondestructive or safe (Peters, 1982). Nicholls and Berg (1981) demonstrated that AE in rabbit fracture callus began at approximately 50% of yield strength, indicating that initial AE in callus occurs early enough in a loading test to be considered safe. However, the application of AE technology to a clinical situation in which a fracture is loaded and monitored for acoustic events requires the arbitrary selection of a rate of loading. It has been shown in materials other than bone that the release rate of acoustic energy is dependent on the strain rate and the volume of material (Hamstad, 1974; Pollock, 1979). Bone is a viscoelastic substance (Burstein and Frankel, 1968; Evans, 1973; Gottesman and Hashin, 1980; Hayes, 1978; Lakes et a[., 1979), therefore its mechanical behavior is dependent on strain rate. Rauber (1876),McElhaney (1966), Pope and Outwater (1972) Crowninshield and Pope (1974), Sammarco et al. (1971), Panjabi er a[. (1973), Wright and Hayes (1976), Carter and Hayes (1977), and Hayes (1978) established that the faster bone is loaded, the greater the modulus of elasticity and ultimate strength becomes. Precise information on the effect of strain rate on the appearance and characteristics of acoustic signals in bone is not known. Therefore the present study was undertaken to examine the effect of two different strain rates on the amplitude distribution of the acoustic emission response in bone. MATERIALS AND .METHODS

Specimens of cortical bone were obtained from fresh, mature bovine metatarsals and were machined into tensile test specimens. The flat, plantar portion was then milled into the final specimen shape, using a slow milling rate. During the entire process of specimen production, the bone was constantly kept cool and wet with lactated Ringer’s solution according to the methods described by Sedlin and Hirsch (1966). Each metatarsal yielded only one specimen, and this was machined from the same longitudinal position in each case. Specimen dimensions are shown in Fig. 1. The specimens were wrapped in moist absorbent paper, sealed and stored at - 20°C until tested. Tensile testing was performed using an MTS (Materials Test System) (model 810.11). All testing was performed in one session, to minimize variation due to repeated calibrations, temperature and humidity. The specimens werecompletely thawed and were randomly assigned to the different strain rates. Parallel, selftightening wedge grips (MTS TG28) were used to grip the ends of the specimens in the machine. An extensometer was then clipped to the specimen for the accurate determination of strain. A Dunegan/Endevco 3000 series acoustic emission system was used to obtain data on total emission counts, location and amplitude distribution. Machined

-w-

T

Fig. 1. Dimensions of the tensile test specimens used in this study. L= 13.97cm. W = 3.18cm. T = 3.18mm. S = 3.18 mm, R = 1.905cm, G = 3.18cm.

aluminum C-clamps were utilized to compress two piezoelectric AE transducers (Dunegan/Endevco model SlOOOB.M.) against the surface of the bone specimen. The transducers were separated by a distance of 7.1 cm measured from transducer center to transducer center. The amplitude threshold detector was set at 30 dB and gains were set at 50 dB per channel for all tests involving normal cortical bone. The envelope used for these tests was 10ms. Only those acoustic events emanating from the area of the fracture were included in the distribution data. Signals occurring from outside the gauge length of the specimen (i.e. the grip-bone interface, extensometer-bone interface or machinery noise) were automatically cancelled. A 5 V piezoelectric pulser connected to the 920 distribution analyzer was utilized to ‘map’ the appropriate window on each specimen. The amplitude distribution of the AE was determined using both lineardifferential plots and log-summation (log-sum) plots. Amplitude distributions may be expressed as F(V) = F( V,)( v/v,)-b. In this formula, V is the highest voltage attained by an AE event, or the peak amplitude of an AE event. V. is the detection threshold. F(V) is the number of events whose amplitude exceeds V. Utilizing the distribution analyzer in the log-sum mode, a log-log plot of amplitude distribution is obtained (Pollock, 1978). The slope of this straight line (if the Cumulative Event Amplitude Distribution, CEAD, follows the power-law relationship) is represented by -b. The slope of this line (or b-value) is a characteristic of a material, as well as of the mechanism of deformation of that material (Pollock, 1981). The b-value was measured by a voltmeter connected to the slope output of the 920 distribution analyzer.

Fig.2.TWO specimens of normal cortical bone. The lower specimen has already been subjected to a tensile test.

121

123

Acoustic emission tn bone The reading from the slope meter (S ) is related to the b-value through the expression b = 0.9S[~m.J( vm,,-

Vo)l

The higher the b-value, the greater the percentage of Lowamplitude events. Ten samples of cortical bone were tested at two different strain rates. Five samples were tested in tension at strain rates of 0.2 mmmin-’ (0.0001 s- ‘) and five samples were tested at a strain rate of 2Ommmin-’ (0.01 s-l). The length of the extensometer and the cross-sectional area of the specimens were measured with calipers. Then the specimens were preloaded 13.6 kg to further tighten the finger trap-like action of the grips and ameliorate the problem of initial slippage. In addition to the X-Yrecorder plotting total AE counts vs time, and load vs extensometer deflection, the linear voltage display transformer load vs crosshead displacement was also plotted. From this data, the occurrence of the initial AE along the stress vs strain curve was determined. Finally a single specimen of normal bone was tested in tension at a rate of 4 mm min - ’ (0.002 s- I). When the initial acoustic event was recorded, the MTS machine was switched to hold, thereby changing from a dynamic to a static load. The specimen was monitored for any continuation of acoustic signal after cessation of dynamic load. All data was subjected to statistical analysis utilizing Mann-Whitney rank sum analysis.

RESULTS

The mechanical behavior of the tensile specimens is summarized in Table I. The modulus of elasticity, ultimate strength and ultimate strain values were determined from the load vs deflection curves. The initial slope of the load vs deflection curve was utilized for calculation of the elastic modulus. The cortical bone behaved in a fashion typical of compact bone subjected to tensile stress. The bone specimens tested at a greater strain rate (0.01 s-l) displayed a greater ultimate stress than those tested at a slower rate (0.0001 s- ‘), (p = 0.004). The modulus of

elasticity was greater in the specimens tested at 0.01 s-l, however this increase was not statistically significant (p = 0.155). These specimens also exhibited a greater ultimate strain when tested at the greater strain rate. This difference was statistically significant (p = 0.004). All specimens were carefully machined and measured with calipers. The mean cross-sectional area of the specimens tested at a strain rate of 0.0001 s-l was 10.29 mm’ (SD. of 0.21 mm’). The mean crosssectional area of the specimens tested at a strain rate of 0.01 s- ’ was also 10.29 mm* (SD. of 0.72 mm’). All tensile test specimens fractured within the gauge length, well away from the fillets (Fig. 2). The acoustic emission response in these specimens was affected by the strain rate (Table 2). The specimens tested at a slower rate (0.0001 s-‘) exhibited more acoustic events than the specimens tested at the faster (0.01 s- ’ ) strain rate (p = 0.004). The amplitude distribution for these specimens, when plotted in the log-sum mode, occurred in accordance with the power-law relationship. The slope of this plot (the b-value) was a straight line (Fig. 4). The results of the amplitude distributions for these specimens are summarized in Table 3. The specimens tested at the greater strain rate had higher amplitude emissions, as demonstrated by the lower b-value, than the specimens tested at the slower strain rate (p = 0.004). Because of the lower number of events obtained at the higher strain rate, the mean amplitude was derived from the linear/differential plot of the amplitude distribution for each test. These results are summarized in Table 4. The initial AE occurred well into the plastic region of the stress-strain curve near the point of failure of the tensile specimens (Table 4, Fig. 3). The final specimen of bone subjected to tensile testing exhibited the first acoustic event at 88”, of the ultimate stress. At this point, the strain rate was changed from 0.02 s- ’ to 0 s- ‘. During the hold phase of this test, 20 more acoustic events were recorded. This occurred over the next 7.2 s until finally the specimen fractured in the middle of its gauge length. The b-value for this specimen of normal bone \vas 0.78, intermediate between that of the specimens tested at the faster strain rate of 0.01 s-’ (b = 0.67) and the

Table I Strain rate 0.001 s-’

0.01 s-’

P

2.75 x lo’ 1.10 Y IO’O

4.98 x IO4 3.52 x 10”

0.155

113.00 6.89

156.00 14.7

0.004

I.09 0.20

1.70 0.36

0.004

elasticity (MN m-*)

Modulus Mean S.D.

of

Ultimate Mean S.D.

stress (MN m-‘)

Ultimate Mean S.D.

strain

(“.)

124

R. A.

FISCHER,

S. W. ARMS,M. H. POPE and D. SELIGSON

Table 2. Number of events and b-value (log-summation

specimens

tested

at

0.01 s- ’

0.001 s-’

46 17

b-value Mean S.D.

1.01 0.13

4 3

0.004

0.67 0.29

0.004

(

0.016

61 (20)

43 (1.58)

MN/m2)

-1 *

160

P

0.01 s- ’

0.001 S-I

STRESS

of 0.0001 s-’

.Ol

AND CONCLUSIONS

The results of the mechanical tests were consistent with those of other researchers. These specimens of bovine bone (metatarsals) demonstrated an ultimate strain of from 1.09 to 1.70%. Evans (1973) stated that the ultimate strain of specimens from bovine femora, in tension, is from 0.75 to 0.9 %. Wright et al. in 198 1 found ultimate strains of 2 %, and Crowninshield and Pope (1974) showed ultimate strains of 2.54.5 “/,, both using bovine femora. The values for ultimate strength and modulus of elasticity are also in the range demonstrated by Wright and Hayes (1976)and Wright et al. (1981). The ultimate strain increased from 1.09 % at a strain rateof0.0001 s-l to 1.70%atastrainrateof0.01 s-t. This disagrees with the results obtained by Crowninshield and Pope (1974), Saha and Hayes (1976). and Wright and Hayes (1976) for bone tested in tension, as well as the results of McElhaney (1966) for bone tested in compression. Our specimens, unlike those in other studies, were of large size to minimize the stress concentration effects of normal bone architecture. These specimens were meticulously prepared

Table 3. Mean amplitude Iinear-ditTerential plot

Mean (S.D.)

rate

P value DISCUSSION

Number of events Mean S.D.

a strain

(6 = 1.01).

plot)

SEC

140,

Table 4 Strain rate (s- ‘) 1

2

3 STRAIN

4

(X)

Fig. 3. Composite stress vs strain curves for normal bovine bone specimens tested in tension at 0.0001 s-’ (S) and 0.01 s- ’ (F). The asterisk (*) marks the onset of AE.

Mean ( %I

Percentage of ultimate strain at which initial AE occurred 94 0.07 0.001 97 0.03 0.01 Percentage of ultimate stress at which initial AE occurred 99 0.001 0.45 98 2.00 0.01

1000

I

CUMULATIVE

EVENT

SD.

AMPLITUDE

DISTRIBUTION

V(dB)

Fig. 4. Log-sumdisplayofthecumulativeevent amplitudedistributionofbone testedat0.001 s-l. Ve = the detection threshold, Y= the peak amplitude of an AE event and F( V’)= the number of events whose amplitude exceeds V. The slope of the best fit line from V, to V, corresponds to the b-value.

Acoustic emission in bone

125

value provides the investigator with a useful tool for and milled to exact dimensions. All specimens fraccharacterizing the AE amplitude distribution of bone. tured in either a short oblique or transverse fashion Other researchers have demonstrated a relationship well away from the large diameter fihets (see Fig. 2), but between the amount of calcification in bone and AE this situation only occurred after multiple other comcounts (Hanagud er al., 1975; Wright rc al., 1981). binations of specimen dimensions were tried and Wright rr al. (1981) also stated that the AE from the rejected. We attempted to make our specimens as free decalcified specimens in their study was in the form of of iatrogenic stress concentrations as possible. Currey low amplitude emissions. The b-value may prove to be (1970) points out that stress concentrators in a specia tool for the determination of the degree of calcifimen are particularly deleterious in high speed loading. Sammarco et al. (1971) and Panjabi et al. (1973) both cation of bone. showed that in torsional loading of whole bones (i.e. The fewer acoustic events that were recorded during the more rapid strain rate may be a result of the presumably free of iatrogenic stress concentrators), a positive correlation exists between increasing strain refractory period inherent in the distribution analyzer. rate and deformation of the bone prior to failure. A Any acoustic event will trigger the reset clock of the specimen of bone that is free of stress concentrations distribution analyzer, whether that event emanates from inside or outside the locate window. During this may demonstrate higher ultimate strain, especially refractory period (dead time) significant acoustic when tested at a higher strain rate. events from within the location window will be lost. Another possible explanation for the observed The large size of our specimen was chosen so that the higher ultimate strains at the greater strain rate could transducers could be placed directly on the bone, not be a peculiarity of bovine metatarsal cortical bone. The the grips. This helps to eliminate deleterious backpossibility of grip slippage artifact was eliminated by an extensometer attached directly to the bone. ground noise. The locate mode of the distribution The AE response in cortical bone was dependent on analyzer is inaccurate when the transducers are less strain rate. When bone was loaded in tension at a than 5 cm apart (Miller, 1982; Awerbuch, 1982; greater strain rate, both the quantity and amplitude Pollock, 1982). The large size of our specimens imdistribution of the acoustic events changed. The acproved the accuracy of the location mode of the AE equipment. oustic signals emitted during the tests with greater In order to accurately locate the source of AE in strain rate were of higher amplitude and were fewer in number than those emitted at a slower strain rate, bone, reflections of the AE signal bouncing back to the confirming that the acoustic response of bone to stress transducers must be eliminated. To do this, the largest is strain rate dependent. We only evaluated two strain envelope on the 920 analyzer (10 ms) was necessary. rates in our study, therefore conclusions regarding a This was also found to be true by Awerbuch (1982) in his work with composites. Unfortunately, the inbroader range of strain rates are not possible. Research into the magnitude of difference between strain rates creased dead time, which improves the accuracy of the locate mode, negatively influences the recording of necessary to effect significant variation in acoustic valuable AE events during a rapid test. emission data would be useful data for the application Currey (I 964) has compared bone to a viscoelastic of AE technology on a clinical basis. composite material such as fiberglass. In this model, Higher amplitude acoustic emissions are generally the bone is a two phase material consisting of a matrix characteristic of brittle materials. In other words, with a low modulus of elasticity, with crystals of high lower b-values are associated with more brittle matemodulus of elasticity embedded in this matrix. Katz rials (Pollock, 198 1). Ductile materials are characterized by higher b-values (i.e. lower amplitude distriand Ukraincik (1972),Katz (1980). Wright and Hayes (1980) and Gottesman and Hashin (1980) have debutions). The highest b-values are characteristic of scribed bone as being a composite material in which plastic zone growth prior to crack extension, whereas the lowest b-values are found for discontinuous crack the interstices between closely packed osteons are filled with matrix (cement lines) and irregularly shaped growth processes in brittle metals of high strength (Pollock, 1981). The fact that the log-sum output of fragments of bone. Others have investigated the mechthe acoustic signal amplitude distribution follows a anical properties of the constituents of bone (Ascenzi and Bonucci, 1976; Strandh, 1960; Fung, 1968; straight line, means that the AE source mechanism in bone in tension follows the power-law model first Gottesman and Hashin. 1980; Katz and Ukraincik, 1971; Sanjeevi Edal., 1982). developed in seismology by Ishimoto and Iido (1939), The AE response in industrial composite materials and Gutenberg and Richter (1949). Mogi (1962). Scholz (1968), Nakamura et al. (1972), Hardy (1972) has been investigated by Weir (1973). Henneke (1975), and Pollock (1981) have extended the power-law Kelly et al. (I 976) and Carlyle (1978). Rotem and Altus model into the AE evaluation of other materials. (1979) classified the mode of fracture in composites by Previously reported b-values in bone tested in comanalyzing the b-value ‘signature’ of the amplitude distribution. Rotem and Baruch (1974) demonstrated pression range from 0.7 (normal bone) to 1.0 (healing that during axial loading of fiber epoxy composite bone with callus) (Pollock, 1981). The b-values of this study determined in tension, ranged from 0.67 to 1.01, specimens, the acoustic emission output continued when the load was changed from dynamic to static. In depending on the strain rate. The utilization of the b-

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R. A. FISCHER,S. W. ARMS,M. H. POPEand D. SELIGSON

this type of composite material, failure occurs by the summation of individual fiber failures. Because the epoxy matrix is viscoelastic, the relaxation of stress occurs when loads are held constant. Therefore, during a hold period, the matrix releases in a viscoelastic manner and transfers increasing stress to the fibers, which fracture (Rotem. 1964). When a tensile test specimen was loaded in tension to 88 y; of its ultimate stress (the presence of the first event) and then the load level was held constant, acoustic emissions continued to be emitted for 7.2 s and despite no further increase in load, the bone failed. This behavior is consistent with that of the fiber epoxy composites tested in a similar fashion. Pope and Outwater (1972) showed that the mode of failure in bone (and wood) varies according to strain rate. At lower strain rates, the fracture front follows an inter-lamellar path preferentially, whereas at faster strain rates the crack front moves through all the constituents of the composite structure of bone. The fast strain rate failure in bone, characterized by higher amplitude AE, may correspond to the rapid loading of viscoelastic fiber epoxy composites, where insufficient time for matrix relaxation results in crack initiation in the matrix. Ultrastructural study of bone specimens whose loading history was controlled by AE could resolve important controversies regarding the behavior of the components of bone and its composite structure. No acoustic events were observed either during the preload, or during the elastic region of the tests. This is in agreement with the results from Wright et al. (1981). However, in their study AE initiated just prior to the yield point, whereas in our study AE did not occur until failure was imminent. This would indicate that AE technology is not suitable for evaluating the integrity of bone. However, we feel that our samples were free of stress concentrations, and that we were careful to eliminate extraneous acoustic activity both because of our technique, and the use of a large enough bone specimen to take full advantage of the AE equipment’s locate mode. Thomas et al. (1977) demonstrated that adefect in bone will result in initial AE activity much earlier along the stress-strain curve. A fracture in bone would serve as such a stress concentrator. Indeed, Nicholl’s study of the AE activity in callus supports the idea that healing fractures can be more safely monitored acoustically than can whole bone. It is evident from this study that if AE technology is to be utilized clinically for the assessment of fracture healing careful selection of, and control of, the rate of loading of the bone will be necessary.

Acknowledgements-This work was possible only through the generous donation of acoustic equipment by Dunegan/Endevco. The authors would also like to acknowledge the contributions by Stuart Benway and Robert Eck, Department of Mechanical Engineering, University of Vermont, and Solomons’ Wholesale Beef Inc., Burlington, VT.

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