Wear, 130 (1989)
361 - 379
AN INVESTIGATION OF ACOUSTIC EMISSION IN SLIDING FRICTION AND WEAR OF METALS S. LINGARD and K. K. NG university of Hong Kong, ~ep~r~rne~~ [Hong Kong] (Received July 6,1988;revised
of ~ec~ani~a~ E~~.~eering, Pokfulam Road
accepted October 21,1988)
Summary Measurements of acoustic emission (AE) in severe sliding of metallic specimens were performed with a view to determining relationships, if any, between AE and wear-friction parameters. It was found that AE is readily observed in dry sliding and that emission rates and cumulative count data are sensitive to the external variables which influence tribological contact conditions. A relationship between cumulative AE count and frictional work is proposed and possible reasons for the form of the relationship are discussed. Emissions did not appear to be directly dependent on rtites of wear but the possibility that AE-wear correlations exist should not be precluded in view of recent work in other laboratories using si~ific~tly different apparatus and instruments.
1. Introduction Acoustic emission technology has been extensively applied to monitoring the integrity of structures and to the investigation of material response to macroscopic mechanical strain fields. Acoustic emission (AE) is a phenomenon arising from a rapid release of strain energy produced by deformation, part of which radiates from the source in the form of elastic waves which can be detected by a suitable transducer at the surface. The sources probably include crystal defects and dislocations, grain boundary movements of polycrystallines, crushing of voids or inclusions and the initiation and growth of cracks. A number of reviews have surveyed the field of AE [ 1 - 31 and a range of publications have dealt with the monitoring of AE in crack formation and fracture. Relatively little has been published on AE in tribological studies although Kannatey-Asibu and Domfeld [4,5] have investigated AE in orthogonal metal cutting and in particular its relationship to tool wear. Research has also been undertaken into the use of AE techniques in detecting and @ Elsevier Sequoia/Printed
in The Netherlands
locating the source of rolling contact fatigue failures in rolling element bearings [6,7]. Recognition that sliding friction and wear are processes involving deformation and fracture, and giving rise to AE, raised the questions, amongst others, as to how significant the emission from sliding pairs is and how it relates to parameters governing the behaviour of a wearing system. The investigation reported here was an attempt to obtain at least preliminary answers to the most basic questions and the following objectives emerged. (1) To detect and quantify the AE signals in wear-friction tests. (2) To investigate the effects of loading, sliding speed and material combinations on AE in a sliding wear system. (3) To investigate the relationships between AE signals and the rate of wear, and between AE signals and the coefficient of friction.
The experiments were conducted using a twodisc wear machine (Amsler type A135) with a stationary upper disc and a rotating lower disc loaded together in edgewise sliding contact. Attached to the upper disc by a stiff linking plate, and maintained in contact with the linking plate via a coupling medium, was a resonant AE transducer (Bruel and Kjaer type 8314, resonant frequency about 800 kHz). The transducer signals were channelled through a preamplifier and a wide-band conditioning amplifier to an AE pulse analyser. The conditioning amplifier handles a frequency range 0.1 Hz - 2 MHz and its amplification can be adjusted in 1 dB increments from 0 to 60 dB. The pulse analyser was operated in a “weight” mode which produces an output count based on the time periods during which the AE signal exceeds four preset trigger levels. The instrument characteristics are such that the analyser output count is approximately proportional to the area under the AE curve and the count rate and cumulative count data are therefore directly indicative of the energy levels of AE activity. AE activity consists of short bursts, typically lasting about 100 /..Ls,giving signal levels of 10 PV to 1 mV at transducer output, and most easily detectable in the frequency range 100 kHz - 1 MHz. The arrangement of the apparatus is shown, diagrammatically, in Fig. 1. Six different materials were used for the manufacture of the test specimens, which were discs of either 72 or 44 mm diameter. The thickness of the discs, and hence the contact width, was usually 10 mm, although a limited number of tests were done with discs 3 and 7 mm thick. The rubbing surfaces were finished to a nominal surface roughness of 0.4 pm R,. Four groups of tests were conducted, each for a particular material combination, as indicated in Table 1. Prior to each run the specimens were subjected to a consistent cleaning and degreasing procedure. Tests were run at constant load and speed for a suitable length of time during which the following parameters were recorded at regular intervals: time; number of revolutions;
AE TRANSDUCER UPPER
AE PULSE ANALYSER LOWER (ROTATING1 SPECIMEN
Fig. 1. Experimental
Material of upper (stationary) specimen
Material of lower (rotating) specimen
Medium carbon steel (En24)
friction torque; wear scar dimensions; cumulative frictional work; cumulative AE count. It was necessary to select an appropriate amplification setting for each test depending on the AE intensity level. The settings used varied from 10 to 30 dB. A filter was employed to exclude AE signal frequencies outside the range 100 kHz - 2 MHz.
3. Results Initial tests were performed with rotating mild steel discs rubbing on stationary aluminium disc specimens (a combination known to give relatively high but fairly consistent and reproducible wear rates , and with which the wear behaviour is not affected by complex back transfer processes). It was rapidly established that significant AE signal levels could be recorded.
A body of data was established with the intention of investigating the relationship between the AE signals and the rate of wear of the specimens. Covering a variety of loads, speeds, and running times and hence providing a range of total wear volumes and rates, the results showed no readily discernible relationship between wear and AE. In fact, plots of the wear rates and wear volumes against the AE count rates and cumulative AE counts produced only widely scattered points which appeared random in distribution. At that point it was recognized that such a result is unsurprising in view of the Archard theory of adhesive wear, which indicates that only a very small proportion of asperity interactions or events produces a wear particle, whereas all such events contribute to frictional forces. Furthermore, recent developments in the theory of unlubricated friction associate frictional forces with the bulk deformation of surface and subsurface material under the action of adhesion and ploughing by asperities which interact with it . It would therefore be expected that the AE output would be more likely to relate to frictional forces rather than to wear parameters and subsequent analysis of the data confirmed this view. A selection of the experimental results is shown in Figs. 2 - 7 in the form of log-log plots of the cumulative AE output against the frictional work done. It will be seen that although there is considerable scatter in the data, all the experiments show a systematic relationship between CC and Wr, a relationship which is approximately linear on the log-log graphs. Over the range of investigation of the appropriate variables, the cumulative AE count can be expressed
Fig. 2. Cumulative AE count against frictional work, aluminium 50 N, u = 0.43 m s-l, gain 10 dB. Experimental point symbols different tests.
(1) on mild steel (2), N = indicate the results of six
Id Wt INm)
Fig. 3. Effect of materials, N = 50 N, u = 0.43 m s-l, gain 20 dB: X, aluminium (1) on mild steel (2); 0, brass (1) on cast iron (2); 0, aluminium (1) on bronze (2); 0, bronze (1) on hardened steel (2). IO7
Wf t Nml
Fig. 4. Effect of loading, u = 0.43 m s-l, A,125N;o,lOON;X,75N;m,50N.
2x = b(W,)=
gain 20 dB, bronze
(1) on hardened
The value of the multiplier b is arbitrary, depending on the particular transducer characteristics and the level of amplification selected. Values of exponent a were found to be predominantly in the range 1.2 - 1.5, and for
IO3 Wf INm)
Fig. 5. Effect of speed, N = 50 N, gain 20 dB, bronze 1.41 m s-l; x, u = 0.70 m s-‘; 0, u = 0.43 m s-l.
(1) on hardened
0, w =
Fig. 6. Effect of loading, N; x, 75 N; o, 50 N.
u = 0.43 m s-l,
gain 10 dB, aluminium
(1) on steel (2): A, 100
the arrangement of mild steel sliding on aluminium, a was about 1.3, as shown in Table 2. A power law relationship between AEa .M.s. and the rate of frictional working given by Diei and referenced by Jiaa and Dornfeld [lo] has close similarities with eqn. (l), although direct comparison is difficult at this stage. It appears probable, however, that the two expressions have the same physical roots.
Fig. 7. Effect of speed, s-‘;o,u = 0.86ms-‘;x,u
IO) W, I Nm)
N = 50 N, gain 10 dB, aluminium = 0.70 ms-‘;A,u = 1.41 ms-I.
(1) on steel (2):
2 of a and b
10 10 10 10 10 10 20
0.31 0.47 0.37 0.26 0.55 0.71 2.85
1.33 1.25 1.26 1.37 1.29 1.32 1.25
50 75 100 125 50 50 50
0.43 0.43 0.43 0.43 0.70 0.86 1.41
20 20 20 20 20 20 20
0.40 0.12 0.04 0.03 1.39 1.39 0.24
1.30 1.44 1.51 1.40 1.46 1.16 1.68
Sliding speed (m s-l)
50 75 100 50 50 50 50
0.43 0.43 0.43 0.70 0.86 1.41 0.43
Brass on cast iron
q, u = 0.43 m
4. Discussion The implication of the relationship given in eqn. (l), which was somewhat unexpected, is that the emission rate increases continuously as the test proceeds. This trend was consistently observed in all the experiments conducted and, in order to investigate it further, consideration had to be given to variables which were likely to change with time in the course of an individual experiment. The processes of wear, friction and AE under dry sliding are, of course, inherently unsteady and are governed by brief asperity interactions, lasting perhaps 1 ms, which cause bursts of AE that endure for even shorter times. However, for periods of, say, a few seconds, events occur sufficiently frequently that their external effects merge to produce quasisteady levels of frictional forces, wear rates and contact temperatures [S]. The variations in frictional forces, wear rates and bulk specimen temperatures were investigated to determine how they behaved in the longer term. Generally, it was found that immediately after the start of a test, there was a short period when the wear rate and frictional force, both initially high, settled down to levels which remained reasonably constant for the rest of the run. It should be noted that for this particular set of experimental conditions with testing periods not exceeding a few minutes, substantial short term variations of the contact temperature and the coefficient of friction occur at relatively high frequency (greater than 20 Hz). However, the average contact temperatures and average coefficients of friction, as measured on instruments with time constants around 1 s, remain sensibly steady after the first few seconds of running [ 111. The experimental configuration and procedure ensured that the controlling independent variables of the normal load, sliding speed and initial specimen surface conditions remained constant. The principal dependent variables of the average friction, wear and temperature remained approximately constant, as described above. However, with the test configuration used, the wear scar on the stationary disc gradually increases in length, resulting in a changing apparent area of contact. Assuming that a correlation exists between AE count rate and the apparent area or contact length it can be seen that, using eqn. (1) AE count rate C = ab(Wf)"' dt = ab(/.LAy
Also if Q = constant then qas Hence if
u = constant then C cc q(a-l)
For two discs of the same diameter (when wear takes place only on the stationary specimen) *=t
and to a first approximation 40:i3
It follows from eqns. 14) and (5) that & a (j3)@-1)
As mentioned earlier, the value of a for the combination of mild steel sliding on aluminium is about 1.3, so in that case C would be expected to be approximately proportional to I. It also follows that since Q,equals a constant there is no systematic re~tion~ip between. ZC or C and Q. Furthermore, there can be no fundamental reason why C should be related to material already removed; hence the apparent contact area and the length of the wear scar are the only remaining external variables likely to influence the AE count rate. In order to separate the effects of the apparent area and the wear scar length, tests were conducted with specimen widths of 10,7 and 3 mm under otherwise identical conditions. Figure 8 shows the AE count rate, expressed
30 E 1
Fig. 8. Dependency s-l, gain 10 dB: 0,
of AE count rate on apparent contact w = 3 mm; X, w = 7 mm;n, w = 10 mm.
N = 50 N, u = 0.43 m
per unit sliding distance, against the length of the wear scar. Within the limits of experimental error and the intrinsic scatter of the AE measurements, the results suggest an approximately linear relationship between the count rate and the wear scar length except for the region corresponding to the initial (running-in) period. The dependence of the emission rate on the wear scar length (rather than contact area) corresponds, at least qualitatively, with the work of Kannatey-Asibu and Dornfeld , who found a relationship, albeit nonlinear, between the square of the R.M.S. AE signal and the contact length of the chip on an SAE 1018 tool in orthogonal metal cutting. It is interesting, but necessarily speculative, to consider possible reasons for the dependence of the emission rate on the contact length. 4.1. The Coulomb friction model For contact of two cylinders with full sliding, the pressure distribution and the surface shear stress distribution (for half the contact zone) may be represented in the simplified form shown in Fig. 9. Frictional stress on the surface of the rotating cylinder is given by 7XZ= IJP and approximating 7XZ= up,
the normal stress distribution
by a power law,
(7) i 11 The area of contact also varies with the contact pressure and rises from zero at contact entry, eventually reaching a point x = 1’/2 where contact is complete and the frictional stress is equal to the shear strength of the material. Putting
Fig. 9. Idealized
and shear stresses.
and noting that at x = 1’12 the area of contact reaches the apparent area dA, then
dF, = r,, dAi = 1 ‘I2
F, = 2
WWWm+ l) + 1) + 1) 1”
If, for simplicity, linear relationships between p, dA 1 and x are assumed, then F, =
At x = 1’12, bulk deformation of the material begins and from eqn. (7)
From eqns. (8) and (9)
and the rate of working in region 1
The rate of working in region 2 is simply ti, = TkW(l_ 1’)U and the total rate of working for the contact is @~,+$,-
wTku 3 (31--- 21’)
Arguing that frictional work is largely dissipated in plastic deformation and that the proportion of it which appears in AE is constant, then the AE rate would be expected to be a function of the contact length. 4.2. Asperity contact model Alternatively, it may be assumed that the real area of contact comprises a number of discrete asperity contacts, each contributing plastic work to the net energy balance. Associated with the deformations will be an AE output.
The number and size distribution of the asperities is unknown but it is reasonable to postulate that only a proportion of the (larger) asperities generate AE above the detectable threshold level. It would then be expected that either of the two following situations might pertain: (a) the number of large asperities in the contact at any given time depends on the contact length and that number controls the rate of emission; or (b) the contact time of an individual asperity depends on the contact length and the number of such asperities is so small that the contact time is the factor which determines the rate of emission.
5. Conclusions (1) AE is readily detectable in the dry sliding of metals. (2) Emission rates vary systematically with loads, speeds and material combinations, but for the experimental conditions employed here, no consistent relationship between the AE and the absolute wear rate was found. (3) A correlation exists between the cumulative AE count and the total frictional work done. (4) The rate of AE increases with the length of apparent area of contact in the direction of sliding. Why this should be so is not clear, since in the experiments reported the frictional work rate remained essentially constant; it therefore appears that the energy dissipated in AE is not a constant proportion of the input energy. It is possible that the effects observed are influenced by the frequency response of a system of changing physical dimensions, but that seems unlikely in view of the magnitudes of the output signal frequencies. The nature of the emission spectra associated with tribological processes and their relationships with contact phenomena should be further investigated with a view to assessing their relevance to the fundamental theory of friction and wear.
References 1 P. H. Hutton and R. N. Ord, Acoustic emission. In R. S. Sharpe (ed.), Research Techniques in Nondestructive Testing, Academic Press, London, 1970. 2 A. A. Pollack, Acoustic emission. In R. W. B. Stephens and H. G. Levinthall (eds.), Acoustic and Vibration Progress, Chapman and Hall, London, 1974. 3 R. V. Williams, Acoustic Emission, Adam Hilger, Bristol, 1980. 4 E. Kannatey-Asibu and D. A. Dornfeld, Quantitative relationships for acoustic emission from orthogonal metal cutting, Trans. ASME, 103 (1981) 330 - 340. 5 E. Kannatey-Asibu and D. A. Dornfeld, A study of tool wear using statistical analysis of metal-cutting acoustic emission, Wear, 76 (1982) 247 - 261. 6 L. Rogers, The application of vibration signature analysis and acoustic emission source location to on-line condition monitoring and anti-friction bearings, Tribol. Int., lZ(2) (1979) 51 - 59. 7 T. Yoshioka and T. Fujiwara, A new acoustic emission source locating system for the study of rolling contact fatigue, Wear, 81 (1982) 183 - 186.
8 S. Lingard, K. H. Fu and K. H. Cheung, Some observations on the wear of aluminium rubbing on steel, Wear, 96 (1984) 85 - 98. 9 K. L. Johnson, Aspects of friction. In D. Dowson, C. M. Taylor, M. Godet and D. Berthe (eds.), Proc. 7th Leeds-Lyon Symp. on Tribology, Leeds, September 9 - 12, 1980, Westbury House, Guildford, 1981. 10 C. L. Jiaa and D. A. Dornfeld, Experimental studies of sliding friction and wear via acoustic emission signal analysis, Wear, to be published. 11 S. Lingard, Estimation of flash temperatures in dry sliding, Proc. Inst. Mech. Eng., London, 198C (8) (1984) 91 - 97.
constants depending on AE instrumentation and material characteristics of the contacting pair real contact area (m*) apparent contact area (m*) cumulative AE count count rate (s-l) disc diameter (m) friction force (N) contact area constant apparent contact length in sliding direction (m) power law exponent power law exponent normal load (N) contact pressure (N m-*) maximum contact pressure (N m-*) wear volume (m3) wear volume per unit sliding distance sliding distance (m) sliding speed (m s-l) contact width perpendicular to sliding direction (m) frictional work (J) rate of working (J s-l)
coefficient of friction shear stress (N m-*)
F K 1 m ;
P PO 9 Q s U W u(9 Wf