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Tribology International 42 (2009) 236–242

Contents lists available at ScienceDirect

Tribology International

0301-67

doi:10.1

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Septem� Corr

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journal homepage: www.elsevier.com/locate/triboint

The relationship between AE and dissipation energy for fretting wear$

Satoshi Ito a,�, Masayuki Shima b, Tatsuhiro Jibiki b, Hideki Akita c

a Graduate School, Tokyo University of Marine Science and Technology, 1-6, Etchujima 2 cho-me, Koto-ku, Tokyo 135-8533, Japanb Tokyo University of Marine Science and Technology, 1-6, Etchujima 2 cho-me, Koto-ku, Tokyo 135-8533, Japanc Hitachi Construction Machinery Co. Ltd, 650 Kandatsu, Tsuchiura, Ibaraki 300-0013, Japan

a r t i c l e i n f o

Article history:

Received 29 August 2007

Received in revised form

31 March 2008

Accepted 12 June 2008Available online 15 August 2008

Keywords:

Fretting wear

Dissipation energy

Acoustic emission

9X/$ - see front matter & 2008 Elsevier Ltd. A

016/j.triboint.2008.06.010

sented at the 34th Leeds-Lyon Symposium o

ber 2007.

esponding author. Tel./fax: +813 52457435.

ail address: d072015@kaiyodai.ac.jp (S. Ito).

a b s t r a c t

This paper describes the behavior of acoustic emission (AE), especially the correlation between the AE

output and the dissipation energy under the fretting conditions. Fretting tests are conducted with a ball

contacting with a flat disc in air. The specimens used are a bearing steel for a ball, and a bearing steel or

an aluminum alloy for a flat disc. The results show that the behavior of the AE output and the

dissipation energy during each fretting cycle is not so similar to each other throughout the test, but the

total AE root-mean-square and the total dissipated energy during the test have a good correlation

between them.

& 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Recently, acoustic emission (AE) methods have been applied toa lot of tribological problems. These include prediction of rollingcontact fatigue, abnormal sliding wear, and fracture mechanics ofstructures. From one such study, Wada has argued that the AEmethod can be an effective technique in evaluating the extent ofsliding wear [1]. The authors have already established that there isgood correlation between the total amount of wear and the totalenergy dissipated during fretting. These experiments employed abearing steel plate in contact with a bearing steel ball andexamined a wide range of applied loads and strokes [2]. The sameresults have also been reported by Fouvry et al. [3].

Considering a system with relatively low rigidity to supportfretting contacts, such that the relative slip amplitude fluctuatesmore or less with changes in friction force during fretting action;such a relationship is very useful in evaluating fretting wearproperties. Moreover, such low rigidity systems often appear inpractical applications.

In order to calculate the energy dissipation during fretting,both the tangential force and the relative slip amplitude mustsimultaneously be measured over the fretting cycles. Conse-quently, this technique is difficult to apply to the evaluation offretting wear for machine elements in practical use.

In this paper, the relationship between AE outputs and theenergy dissipation during fretting is examined with a view topredicting the extent of fretting wear by monitoring AE outputs

ll rights reserved.

n Tribology, Lyon, 4th to 7th

generated during fretting in practical machines. Needless to say,AE sensors are not so difficult to incorporate into such machines.Also, the outputs can easily be detected over time. Based on theresults, the applicability of AE measurement to fretting wearproblems is subsequently discussed.

2. Experimental apparatus

The experimental system is shown in Fig. 1. The parametersmeasured were tangential force T, relative slip amplitude D (peak-to-peak), and AE. The AE was measured using a sensor mountedon the sphere holder; T was measured using a load sensorcomprising four strain gauges; and D was measured using an eddycurrent-type gap sensor.

T and D were passed through a 100 Hz low-pass filter andrecorded on a data-logger with a sampling time of 20ms. The AEsignals, after first being passed through a 50 kHz high-pass filter,were then transformed into enveloped signals and recorded witha 100 dB gain on the data-logger. This allowed the sampling speedof the data-logger to be reduced. The data-logger was activatedevery 50 fretting cycles and 65,000 data points were recordedrepeatedly.

The dissipated energy E is defined as the area under the slipcurve (T–D curve), and was calculated by numerically integratingusing T and D. This was the average value over 5 cycles. The RMSvalue, AErms, was then obtained from

AErms ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

N

XN

i¼1

AE2i

vuut (1)

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Fig. 1. Apparatus (left), cross-section (right).

S. Ito et al. / Tribology International 42 (2009) 236–242 237

where N is the total number of samples over 5 cycles and AEi is theAE voltage at each sampling time.

The contacting members used were a ball and a flat disc. Theball was 9.525 mm in diameter and made of bearing steel (AISIE52100/ HV 770). Two flat discs were used; one made of bearingsteel (AISI E52100/ HV 780) and the other made of aluminum alloy(AISI 2017/ HV 90). The flat discs were polished to a maximumpeak height of less than 0.1mm.

A normal force of 39.2 N for the bearing steel disc and 19.6 Nfor the aluminum alloy disc was applied using a dead weight. Theoscillation amplitude D was set in the range 40–200mmnominally. The mean Hertzian contact pressure for the bearingsteel disc (206 GPa Young’s modulus, 0.3 Poisson’s ratio) was1083 MPa and for the aluminum alloy disc (75 GPa Young’smodulus, 0.3 Poisson’s ratio) was 58 MPa.

Fig. 2. Results for m, E and AErms (bearing steel disc).

3. Results

3.1. Bearing steel disc

Typical results for the bearing steel disc are shown in Fig. 2.The friction coefficient m and energy dissipation E are similarlylow in the early stages of fretting. In contrast, AErms is low up toabout 200 fretting cycles, then drastically increases and fluctuatesup to 6000 cycles, subsequently settling to a steady value.

The T–D curves and the AE outputs during 5 fretting cycles areshown in Figs. 3 and 4. These correspond to the arrows A and B inFig. 2. The arrows in the T–D curve indicate the cyclic direction.The two AE outputs, (a) and (b), correspond to the upper andlower paths of the T–D curve.

In Figs. 3 and 4, the T–D curves are similar to each other inspite of the different stages of fretting. In addition, they do notexhibit stick–slip behavior in the gross-slip region. However, theAE outputs differ markedly. More specifically, the AE outputsgenerated in the early stages of fretting (Fig. 3) were very high,with values going out of range at over 11 V. The correspondingfrequencies were also high. Conversely, in the steady state (Fig. 4)the AE outputs become rather weak with low correspondingfrequencies. One commonality is that, for both fretting stages,intense AE outputs do not occur in the partial-slip region.

Fig. 5 shows the relationship between the cumulative values ofE and AErms. The number of fretting cycles is also indicated. Thecurve approximates to two straight lines of differing gradient.These intersect at around 5000 cycles (indicated by the boldarrow in Fig. 5).

Fig. 6 shows the relationship between total AErms and total E upto 20,000 cycles for set displacements of 40–200mm. Therelationship is clearly linear. In the case of the set displacement

of 40mm, gross-slip does not occur. However, partial-slip, calledMindln-slip, does occur during fretting. Consequently, total E isvery low at about 0.13 J. The corresponding total AErms is nearlyequal to the value DAE, which corresponds to the backgroundnoise that is always present during fretting.

3.2. Aluminum alloy disc

Typical results for the aluminum alloy disc are shown in Fig. 7.On the whole, the behavior is close to that obtained for thebearing steel disc. However, the value of E/fretting cycle is lowerthan that for the bearing steel disc because of the difference in theapplied load. In contrast, the value of AErms/fretting cycle is higherthan that obtained for the bearing steel disc for all fretting cycles.Also, it tends to fluctuate, even during the steady state frettingstage. Fig. 8 shows the relationship between cumulative E andcumulative AErms. In comparison with Fig. 5, though the gradientis higher, the behavior is very similar. The relationship between

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Fig. 3. T–D curves and AE outputs (corresponding to arrow A in Fig. 2). Fig. 4. T–D curves and AE outputs (corresponding to arrow B in Fig. 2).

S. Ito et al. / Tribology International 42 (2009) 236–242238

total E and total AErms up to 20,000 cycles is shown in Fig. 9. Theresults also reveal a linear relationship, though the gradient isgreater than that for the bearing steel disc. In addition, thevalue of the intercept, DAE increases by about 2.5 kV. This may bedue to differences in the background noise levels during theexperiments.

Fig. 5. Cumulative E vs. cumulative AErms (bearing steel disc).

4. Discussion

4.1. Behavior of AE generated during fretting

Sun et al. [4] have presented results showing that the frictioncoefficient under sliding contact conditions is directly related tothe AE generated. However, Figs. 2 and 7 show that, under frettingcontact conditions, the friction coefficient is not always related toAE.

Possible reasons are as follows. The amount of wear debrisgenerated in the first 50 cycles is relatively small, as shown inFig. 10. However, as fretting continues, damage accumulates onthe surfaces and in the subsurfaces. Consequently, a lot of debriscan easily be generated. This process could lead to the increasedAE output.

In the steady state, accumulated wear debris lying between thecontacting surfaces can interfere with metal-to-metal contact. Inspite of the high friction coefficient, this would result in relativelylow values of AErms compared with the early stages of fretting.

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Fig. 6. Total E vs. total AErms (bearing steel disc).

Fig. 7. Results for m, E and AErms (aluminum alloy disc).

Fig. 9. Total E vs. total AErms (aluminum alloy disc)

Fig. 8. Cumulative E vs. cumulative AErms (aluminum alloy disc).

S. Ito et al. / Tribology International 42 (2009) 236–242 239

In order to confirm this hypothesis, the wear debris lying onthe contacting surfaces was removed and fretting restarted. Theresults, shown in Fig. 11, clearly demonstrate that, as a result ofremoving the wear debris, the value of AErms sharply increases.Though not shown herein, this intense AE does not occur unlessthe debris is removed.

Fig. 12 shows the wear debris that was carefully collected usingan adhesive tape. Though only 50 fretting cycles had been

completed (10,000–10,050th), lots of debris was generatedcompared to that generated from the earliest stage of fretting.This suggests that intense AE occurs during the generation of weardebris.

The generation of intense AE and wear debris can be explainedin terms of the development of tribologically transformedstructure (TTS) layer. Sauger et al. [5] have investigated therelationship between the dissipated energy and development of aTTS layer in fretting. Their research indicated that the accumula-tion of a specific amount of dissipated energy and development ofa TTS layer are necessary for the development of wear. It ispresumed that such TTS layer was sufficiently developed ataround 10,000 fretting cycles that can explain the following fact.

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Fig. 10. Wear debris generated in the first 50 fretting cycles (aluminum alloy disc),

(wear debris collected on adhesive tape).

Fig. 11. Results for m, E and AErms (aluminum alloy disc).

Fig. 12. Wear debris generated during 50 fretting cycles (10,000–10,050th) (wear

debris collected on adhesive tape).

S. Ito et al. / Tribology International 42 (2009) 236–242240

The rapid increase in the value of AErms after removing the debris,compared with that of the earliest stage of fretting as well as therapid generation of wear debris.

4.2. Relationship between total AErms and total E

Figs. 6 and 9 clearly show good correlation between total AErms

and total E for both the bearing steel and the aluminum alloy

discs. In contrast, Figs. 2 and 7 show that the curves for E vs.fretting cycles and AErms vs. fretting cycles differ markedly. Inorder to further examine this result, a series of values for AErms/cycle and E/cycle were calculated for the entire fretting process.The results are shown in Fig. 13. The data scatter is mainly due toAErms fluctuations in the early stages of fretting (0–5000 cycles).Though correlation between instantaneous results is poor, thecumulative values, shown in Fig. 6, have a clear linear relationship.

Now consider the reason for the scatter described above.Development of TTS layer is affected by the accumulation ofdissipated energy, and a certain amount of cumulative dissipatedenergy sets off rapid wear [5]. In fact, the intensity of AErms

generated during the first few hundred cycles is low, then itdrastically increases even if the dissipated energy is not sodifferent from the initial stage. These phenomena result in thescatter of AErms.

In the steady-state stage (5001–20,000 cycles), TTS layer maybe protected by loosed wear debris which exist between thecontacting surfaces, so the wear rate becomes low and nearlyconstant. This is the reason why the decrease in scatter of AErms iscaused in the steady-state stage of fretting, as well as decrease ina0 AErms.

4.3. Relationship between wear damage and AErms and E

Fig. 14 shows how total AErms and total E vary with the wearvolume (Vf) of the flat disc, for various slip amplitudes. The wearvolume was calculated using

V f ¼p4

a4

R(2)

R ¼ða2 þ d2

Þ

2d

where a is half the width of the wear scar, d is the wear depth andR is the wear scar’s radius of curvature.

Good correlation was found between total AErms and wearvolume. This result suggests that for relatively large slipamplitudes (where measurable amounts of wear occur), measure-ment of AErms would be useful in estimating the extent of frettingwear damage.

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Fig. 13. E vs. AErms (bearing steel disc).

Fig. 14. Total E and total AErms vs. wear volume (Vf) (bearing steel disc).

S. Ito et al. / Tribology International 42 (2009) 236–242 241

The relationship between total AErms and wear volume, andtotal E and wear volume, may easily be extrapolated to predict theamount of fretting wear. The dissipated energy E correlates with

the worn volume Vw (Eq. (3)). Noting that total AErms has a linerrelationship with total E (Eq. (4)), the worn volume Vw can bepredicted from Eq. (5):

Vw ¼ f ðEÞ (3)

AErms ¼ gðEÞ (4)

VW ¼ FðAErmsÞ (5)

However, it should be noted that the quantitative relationshipsdepend on the contact geometry and materials.

4.4. Difference in behavior of AErms and E between the bearing steel

and aluminum alloy discs

The behavior of AErms/cycle and that of E/cycle for the bearingsteel disc is close to that for the aluminum alloy disc. However, thevalue of E/cycle is lower for the aluminum alloy disc than that forthe bearing steel disc. In contrast, the value of AErms/cycle ishigher than that for the bearing steel disc, even if the applied loadis halved. This is due to the low strength of the aluminum alloydisc. More specifically, local fractures occur more easily inaluminum alloy than in bearing steel. Furthermore, aluminumalloy is more adhesive than steel. This results in the generation ofmore frequent and more intense AE. Hence, though on the whole,the behaviors are alike, from Figs. 6 and 9 it is clear that thegradients differ greatly.

4.5. Influence of relative humidity

It is well known that fretting wear is greatly influenced by therelative humidity of the air [6]. So, further experiments wereconducted with low relative humidity to examine the relationshipbetween total AErms and total E.

The relation between total AErms and total E obtained in highhumidity correspond well with those obtained in low humidity,shown in Fig. 15. Though the relationship is unaffected by therelative humidity, the actual values of total AErms and total E dodiffer when compared for the same fretting stroke D. The airhumidity greatly affects the friction coefficient, which is lower inhigh humidity. This results in a change in the dissipated energy.

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Fig. 15. The influence of relative humidity on the relationship between total E and

total AErms (bearing steel disc).

S. Ito et al. / Tribology International 42 (2009) 236–242242

Waterhouse pointed out that water vapor in air has a lubricatingaction and reduces damage [6]. It is considered that AErms and E

are similarly influenced by air humidity. Therefore, measurementof AE can be effective in estimating fretting wear, even if the airhumidity changes.

4.6. Applicability of AE detection to fretting

The fretting mechanism comprises a variety of problems.Above all, practical detection of fretting cracks and fretting wear iscritical. The detection of crack initiation and propagation duringfretting has been investigated in other research using intensitydetection [7]. The method employed herein remains difficult toapply to the detection of crack initiation and propagation at thistime. This study focused mainly on the mean AE value AErms.

Consequently, signals generated by a crack only result in increasesto the AErms value. However, this may equally be the result ofwear.

To resolve this problem, other approaches are needed, e.g.,analyzing the AE frequency spectrum [8].

5. Conclusions

This research represents the first step in the development of apractical method employing AE to evaluate fretting problems. Therelationship between AE output and dissipation energy duringfretting was studied. This employed a bearing steel ball acting oneither aluminum or bearing steel flat plate. Though manyproblems remain to be solved to develop the method intopractical equipment, the following conclusions were drawn:

(1)

There is a good correlation between total dissipated energyE and total AErms.

(2)

Both the generation of wear debris, and the accumulationthereof between the frictional surfaces, plays a significant rolein varying the intensity of AE.

(3)

Variations in relative humidity do not influence the relation-ship between total AErms and total dissipated energy.

Acknowledgment

This research was financially supported by Taiho KogyoTribology Research Foundation (TTRF). We deeply appreciateTTRF.

References

[1] Wada M. Tribologists 1990;35(4):228.[2] Fouvry S, Vincent L, Kapsa P. Wear 1996;200:186.[3] Shima M, Sato J. Tribologists 1986;31(7):507.[4] Sun J, Wood RJK, Wang L, Care I, Powrie HEG. Wear 2005;259:1482.[5] Sauger E, Fouvry S, Ponsonnet L, Kapsa Ph, Martin JM, Vincent L. Wear

2000;245:39.[6] Waterhouse RB. Fretting corrosion. Oxford: Pergamon Press; 1972. p.126.[7] Cadario A, Alfredsson B. Eng Fract Mech 2005;72:1664.[8] Warren AW, Guo YB. Int J Fatigue 2007;29:603.