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Tribology International 42 (2009) 807–815

Contents lists available at ScienceDirect

Tribology International

0301-67

doi:10.1

� Corr

E-m

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

Incipient damage detection and its propagation monitoring of rolling contactfatigue by acoustic emission

Ziaur Rahman a,�, Hiroaki Ohba a, Takeo Yoshioka b, Takashi Yamamoto b

a Technical Research Center, Toyo Electric Mfg. Co., Ltd., 3-8 Fukuura, Kanazawa-ku, Yokohama, Kanagawa 236-0004, Japanb Department of Mechanical Systems Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan

a r t i c l e i n f o

Article history:

Received 25 December 2007

Received in revised form

30 July 2008

Accepted 28 October 2008Available online 11 November 2008

Keywords:

Rolling element

Acoustic emissions

AE source locator

Incipient damage diagnosis

Condition monitoring

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

016/j.triboint.2008.10.014

esponding author. Tel.: +8145 785 3702; fax:

ail address: ziasaga@hotmail.com (Z. Rahman

a b s t r a c t

The acoustic emission (AE) technique was applied to rolling contact fatigue tests using a test-rig running

under constant load and speed for detecting the incipient damage and damage location. This

incipiently-damaged roller was investigated in detail and monitored by further running to determine

the damage severity and to understand the surface damage propagation process by applying the AE

techniques. The conventional AE parameters and AE signal features were studied, and their relation with

the AE source locator hit count rate were correlated. The results demonstrated the successful use of the

AE measurement unit, which is principally, consists of the AE data analyzer and the AE source locator as

a new system for detecting incipient damage produced by fatigue. Moreover, the system is able to

forecast the position of the damage in the roller, capable of providing an indication of the severity of

damage i.e. damage size, and thus it could allow the user to monitor the rate of further degradation of

the rolling elements.

& 2008 Elsevier Ltd. All rights reserved.

1. Introduction

In railway industry, the challenge of today is to better controlthe performance of rolling contact elements such as gears,bearings etc. in driving devices of electric railcars in order tosuccessfully avoid disturbances, failures, breakdowns and un-planned shutdowns. In the industry, clients prefer long intervalsbetween inspections and long running periods of cars with nomaintenance. Well-developed condition monitoring techniquesgive us the possibility to monitor the present status of movingparts in driving devices online, and help us by avoidingcatastrophic in-service failure. The trend towards increasingpower, weight-to-load factor, complexity and efficiency in rotatingmachinery pushed to rolling elements design limits, and theirfault identification requires improved detection ability of incipi-ent damage mechanism which is one of the major concerns in theindustry. The mechanical deterioration in rotating machinescaused by wear is almost always noticeable itself by means ofchanges in the levels of vibrations and the acoustic emission (AE)at different points of the machine. One of the advanced faultidentification procedures commonly used is the condition-basedvibration or AE signature analysis. Since the sensing frequency ismuch higher in AE sensor based testing comparing with linearresponse accelerometers in vibration analysis, the AE technique

ll rights reserved.

+8145 785 3053.

).

can detect the incipient damage on a micro-scale, such as fatigue,stress corrosion, fretting wear and sliding friction [1]. Therefore,the AE technique was able to detect smaller damages incomparison with the well-established vibration measurement[1–3]. However, the application of the AE technique to monitorthe damage formation and failures is comparatively recentand is still at an early stage in condition monitoring for rollingelements [4,5].

The AE is defined as the transient elastic waves generated bythe rapid release of strain energy from localized sources withinthe material due to the changes in the internal structure [6]. TheAE technique can detect crack initiation and growth, while thevibration accelerometer can only detect the appearance of spalling[7], or the crack growth at a later stage of the crack growth process[8]. Siores et al. [9] concluded that the AE results could becorrelated to various damage conditions. However, to date, mostresearchers have performed the trial tests with the ‘artificial’ or‘seeded’ damages in rolling elements. This damage or defect wasartificially introduced by scratching the surface, introducingdebris into the lubricant of machine with an electrical dischargeor spark erosion method etc. [10–14]. Williams et al. [15]published a report on lifetime test with the undamaged bearingthrough to failure; however defect determination was analyzed byconventional vibration metrics. Furthermore, Yoshioka et al.[16,17] introduced a method for damage diagnosis and measure-ment of fatigue crack propagation by time intervals of the AEgeneration and observing the effecting area of the loading zone byan AE location method for life tests of radial ball bearing.

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Z. Rahman et al. / Tribology International 42 (2009) 807–815808

The objectives of this study were to detect and locate the non-artificially induced (natural) damage of the rollers at its incipientstage, and also damage growth monitoring to understand itsevolvement and propagation behavior with the relationship of AEparameters. The study involved in the monitoring of sound rollingelements (new undamaged rolling elements with no simulated orseeded damage) running in contacts through its incipient damageevolvement by collecting the AE signals from their healthconditions. The roller with the incipient damage was furtherfatigue tested up to its final damaged stage as the spallingoutbreak.

2. Test procedures

2.1. Test rollers

Two sound rollers made of conventional gear and pinionmaterials were used for the tests as follower and driver,respectively. The geometry of the rollers and their contact positionare shown in Fig. 1.

The case hardened low alloy steel roller (SNCM420) withcurvature surface was used as a higher velocity rotating roller isthe driver, and the induction hardened steel roller (S40C) with flatsurface was used as a lower velocity rotating roller is the follower.The curvature was fixed at a radius of 25 mm to avoid the edgeeffect arising in the line contact tests. Initial surface roughness forthe driver and follower was Rz 1.0–2.0mm.

2.2. Test arrangement and AE data acquisition

Fig. 2 illustrates a data acquisition system. The system iscomposed of three units. These are AE measurement unit, datalogger unit and a computer. The AE measurement unit consists ofAE sensor, preamplifier, AE signal processor, AE data analyzer,AE signal to pulse converter, AE source position sensor andAE source locator.

∅16

∅

16

8

R25

∅30

∅30

Follo

wer

D

river

Fig. 1. Geometry of two rollers in contact.

The AE sensor used in the tests is a wide band type sensor witha relatively flat response between 50 kHz and 1 MHz. The AEsensor was cemented to the bottom of the oil bath where thedriver got the lubricant to obtain the consistent AE measurementsthroughout the test. The distance between the driver surface andthe AE sensor was taken as minimum as 1.75 mm to minimize theeffects of varying impedance as the AE signal is traveling throughdifferent bodies. Moreover, the output of the AE sensor is fed intothe preamplifier for data preamplification at 40 dB. The AE signalsfrom preamplifier are first filtered at a range of high pass filter tolow pass filter of 100 and 400 kHz, respectively, and thencompared with the threshold in the AE signal processor. Byapplying the threshold level of 45 dB in the AE signal processor,the AE signal-to-noise-ratio was greatly improved. The AE hitsignals, which exceed the threshold, are enveloped and convertedinto a pulse signal by an AE signal to pulse converter with aduration of 500ms as shown in Fig. 3. This conversion can beuseful not only to locate the AE sources but also to deriveinformation about the AE sources. This pulse signal is fed into theAE source locator.

The AE source locator receives both signals from the AEsignal processor and the AE source position sensor, and decides acontact position of the test rollers at the moment of AE gene-ration as a position of AE source. The AE source position sensorwas worked by the mechanism of induction displacement. Thesensor was triggered by the measurement range of 0–1 mm andwhich have a responsive frequency of 18 kHz (�3 dB). To detectthe position of sources, the AE source position sensor wascemented on the test rig frame facing the position marker.The output of the AE source position sensor is fed into the AEsource locator.

Fig. 4 shows how to estimate the damage position correspond-ing to AE sources by the software of the AE source locator.The electric accuracy of the AE source locator sensor is withinthe one address in a 100 division of rotary synchronizedsignal. The distance between the AE source position sensor andposition marker was set as close as possible at a distance of0.64 mm to maintain the accuracy for damage location positionidentification in every single division. The AE source locatoralways monitors the rotary synchronized signal from the AEsource position sensor and makes starting the roller division scalewhenever the AE source locator receives the rotary synchronizedsignal. The roller division corresponds to the position on thecircumference of the test roller, and the roller division scaleincreases from 0 to 100 divisions with the rotational angle of theroller to the AE position sensor. The AE source locator estimatesthe roller division at the moment when the locator receives the AEhit signal as AE source position. In this study, the valuerepresented the incipient damage is the accumulation of ‘30pulse signals of continuous AE hits’ in 2 min for a particular AEsource location. For an example in Fig. 4, the position No. 3 of theroller division scale is the AE source position that corresponds to apossible incipient damage location. This value represents theincipient damage occurrence status as ‘preset level’ that causesthe test to stop. The one division on the roller division scale isequal to one-hundredth of the circumference of the test roller,which is about 0.9 mm.

The AE data acquisition unit is connected to a computer toanalyze real time pulse signal data with the software forcontinuous monitoring of the AE source location. The analyzeddata was displayed on the computer screen online showing the hitcount rate from the different positions of rolling elementscontacts.

The data logger unit collects load, torque, vibration accelera-tion and oil temperature, and feeds their data into a computer asshown in Fig. 2.

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AE signal to Pulse converter

(Envelope)

AE sensor

AE source position sensorTest

rollers

Pre-amp (40dB)

AE pulse signal

Position marker

Rotary synchronized signal

Signal to stop the test

Temperature

Data

logger

Torque

Load

Vibration acceleration

AE output signal

AE source locator

AEdata analyzer

AE signal processor

Computer

High pass filter: 100kHzLow pass filter: 400kHzThreshold: 45dB

Fig. 2. Data acquisition system.

Sig

nal v

olta

ge

1pulse

Threshold

Time

Envelopecurve

AE signal processorInput

1hit

AE signal to pulse converter

Output

Fig. 3. Definition of AE hit signal and pulse signal.

Estimated position of incipient damage

1

Rotary synchronized signal

Pulse signal due to AE hit

AE source position in roller

division scale 00199893211 cycle of AE source locator

AE source position in roller division scale

Pul

se s

igna

l cou

nts

Pre-set level: 30 hit counts/ 2 min (max)

0

(Accumulation for 2 min)

30

[1 revolution of test roller: 94mm]

0019989321

Fig. 4. The AE source locator data acquisition and estimation of damage position.

Z. Rahman et al. / Tribology International 42 (2009) 807–815 809

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Z. Rahman et al. / Tribology International 42 (2009) 807–815810

2.3. Rolling contact fatigue (RCF) tests

The test-rig for RCF tests was equipped with load sensor,torque sensor, vibration accelerometer, oil temperature monitor-ing sensor, revolutions counter, AE sensor and AE source positionsensor. The AE measurement conditions are shown in Table 1.

Fig. 5 shows the schematic diagram of the two-roller test-rig.The tests were carried out under a constant Hertzian stress of3.5 GPa by a spring loading system. The rollers were rotating atabout 800 and 730 rpm for driver and follower, respectively,which produced a slip ratio of 9% fixed by gears. An additive-freegear oil API: GL-5 was used for the tests. The minimum oil filmthickness hmin calculated from the Hamrock–Dowson equation[18] was less than 0.4mm at 50 1C. The tests were first conductedby sound rollers. All the tests were run until the two-roller test-rigstopped automatically due to the presence of an incipient damageon the roller. The AE source location was divided into 100divisions corresponding to the circumference of 94.25 mm for aprecise monitoring of the AE source location. Attempts were madeto detect and identify any damage in the roller by fatigue at itsearly stage as an incipient damage. The incipient damage istermed as micro-damage before any metal loss is evident in thelubricating system. In this study, the incipient damage wasconsidered as an early damage on roller before spalling. Tworelay circuits were designed to stop the rig when it reached themaximum cumulative revolution of 20 million cycles, or exceededa preset level of hit count per minutes to detect incipient damageappearance in the roller. After some initial trial run-in tests, 30 hitcounts/2 min received by the AE locator in a particular division on

Table 1AE measurement conditions.

High pass filter 100 kHz

Low pass filter 400 kHz

Threshold 45 dB

Preamplifier 40 dB

Sample rate 2 M-samples/s

Preset level (auto-stop level) for incipient damage 30 hit counts/2 min

AE sensor

Position sensor

Load

Oil out

Driver

Follower

Oil in

Fig. 5. Schematic diagram of two-roller test machine and arrangement of two

sensors.

the roller was determined as the preset level which correspondsto the incipient damage on the roller.

Further study was carried out after the occurrence of incipientdamage on the roller to investigate the damage propagationbehavior and its relationship with the AE parameters. Therefore,as the second step, the incipiently-damaged roller was set to thetest-rig again and further tested according to the RCF testprocedure except setting a preset level of the incipient damagedetection. For a certain cycles after running of the incipiently-damaged roller, the test was stopped and damage progress and/orseverity was observed with optical microscopy, and analyzed withtest run cycles after the incipient damage and the AE locator hitcount rate. A total of four after-tests were run further until thefinal stage of the fatigue damage i.e. spalling occurred on the samedivision on the roller where incipient damage was originallyoccurred. The four tests of the incipiently-damaged roller weretaken for 1700, 4200, 4700 and 16,400 number of cycles whichwere corresponding to the individual elapsed time of 2.1, 5.3, 5.9and 20.5 min, respectively.

3. Results and discussions

3.1. AE hit count rate

The typical AE features were employed to identify and classifyAE signatures. Fig. 6(A) displays the AE hit count rate (number ofAE hit signals per 1 min) with time during the RCF test andmultiple after tests of incipiently-damaged roller. In the begin-ning, plastic deformation and interference of the sharp edges ofthe initial surface roughness occurred by continued rolling contactwhich yields some higher AE hit count rate before the steady statecommences. The AE hit count rate showed a constant steady statestage for a period of 156 h (N ¼ 6.3�106 cycles) which was 88% ofthe total test hours. Following the steady state stage, AE hit countgradually increased until the incipient damage occurred. The stagecomprising with the increasing tendency of the AE hit count ratecould be termed as ‘surface degradation stage’. At the end of thisstage the AE hit count rate from a particular division of thefollower reached the preset level of incipient damage status, andthe test was ended at 175 h 37 min (Nf ¼ 7.1�106 cycles). Thesteady state persistency varied to some extent for other testsbecause of the surface properties of the rollers. However, it did notaffect the detection process of the incipient damage occurrence.

3.2. AE signal and AE source location

The AE signatures and their corresponding frequency spectrawere recorded throughout the tests. The typical AE signatureamplitude levels varied from 45 dB (0.017 V) to 78 dB (0.80 V)throughout the tests. In Fig. 6(A), there was a decrease in AE hitcounts per minute at approximately 115 h and again at 150 h. Thissubsequent drop might be attributed to healing [19] which maybe resulted from smoothing of the sharp edges of the surfaceroughness due to wear by continued rolling contact. Moreover,Fig. 6(B) shows that the hit count rate in the AE source locator didnot accumulate greatly at any particular division of the roller andthus did not reach the preset level (30 hit counts) after 2 min datasummation after 114 h. The frequent AE hit counts came from allthe divisions of the rollers in contact, which represented thefrictional hit count in the ‘steady state’. Some AE hit counts mightbe caused by entrapped wear particles in the contact interface.Figs. 6(C) and (D) are considered from the ‘surface degradationstate’ to estimate the correlation between the AE hit counts ratewith the AE locator data accumulation to identify the incipient

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0

25

50

75

100

Hit

coun

t

0

200

400

600

800

1000

0.02

Time, h

AE

hit

coun

t/min

175.6

a)

b)

c)d)

Incipient damage test after damage test

ii)

iii)

iv)

0

25

50

75

100

Hit

coun

t0

25

50

75

100

1

Roller division

Hit

coun

t

176.0175.82.55 45.0 67.4 89.9 112.4 135.0 157.5 10 19 28 37 46 55 64 73 82 91 100

1

Roller division10 19 28 37 46 55 64 73 82 91 10

01

Roller division10 19 28 37 46 55 64 73 82 91 10

0

Fig. 6. (A) Rate of AE hit counts during of a test and after incipiently damaged test, and locator data of (B) 2 min after 114 h, (C) 2 min after 166 h, and (D) last 2 min at

incipient damage.

0

2

4

6

8

10

0Frequency, kHz

FFT

mag

nitu

de, V

-0.2

-0.1

0

0.1

0.2

0

Am

plitu

de, V Threshold

level

Time, μs100 200 300 400 500 200 400 600 800 1000

Fig. 7. A single AE hit signature with corresponding frequency spectrum of an incipient damage on roller.

Z. Rahman et al. / Tribology International 42 (2009) 807–815 811

damage occurrences. Analyzing the highest level of AE hit countdata accumulation after 166 h shown in Fig. 6(C), even thesummation of 2 min data did not reach the preset level. Therefore,incipient damage was not occurred at this stage though itproduced several peaks in AE hit counts rate. Sudden increase inAE hit count attributed to any possible breaking or deformation ofcontact surfaces could be responsible for the surface degradation.

Fig. 6(D) represents the last 2 min data summation of the‘surface degradation state’. In this stage, the AE source locator hitcount data was accumulated in some divisions, and at a particulardivision No. 19, it showed the increase of maximum hit count datasignificantly. Even analyzing the last one minute of test data, itshowed that the AE source locator data accumulations at divisionNo. 19 represented the incipient damage (see top right figure inFig. 11) as it reached the preset level of 30 hit counts and forcedthe test to stop. However, when examining the historical data andobserving the AE signatures before the roller failed due to

incipient damage, there were no sign of strong periodic impulses,and its corresponding AE source locator data accumulation in anyparticular division showed no roller surface degradation (forexample see Fig. 6(B)).

Fig. 7 represents the burst AE waveform and its frequencyspectrum collected from the tests at the last ‘surface degradationstate’ at the point in Fig. 6(D) which exceeds the threshold level of45 dB greatly after an extended stable ‘steady state’. Theexamination on the AE signal waveforms showed that thewaveforms corresponding to jumps in the AE activities weredistinctly different from those corresponding to the typical AEsignature of ‘steady state’. Moreover, at this point, the periodicimpulse features in the AE signals were clearly found in the AEsource locator data through online monitoring. In addition, the AEhit count started to accumulate in a particular division of theroller, which showed the strong evidence of surface degradationas an incipient damage was evolving. This accumulation of the AE

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Z. Rahman et al. / Tribology International 42 (2009) 807–815812

locator data in a particular point was significantly different fromthe other states. The signal exhibited strong impulse periodicitybecause of the crack propagation around the incipient damage atcontact between test rollers and the impacts generated betweenthe driver and the incipient damage in the follower. Only a singleincipient damage occurred on the S40C induction hardeningmaterials of the lower velocity rotating roller (follower) specimenfor all tests. The presence of ‘incipient damage’ (see top left offigure in Fig. 11) on the roller surface was evidently found fromvisual examination at division No. 19, which reached the presetlevel after 1 min of the AE source locator data accumulation.Moreover, though the position of the AE sensor was submergedinto the lubricating oil and the AE signals travels through differentbodies of varying impedance and wave velocities, the system candetect the incipient damage with the higher accuracy of itsposition. The summarized data output of the system confirmedthe point of damage position accurately in the roller as theimpedances of AE traveling different bodies did not contrast withthe incipient damage identification results [20]. The cross-sectional view of the crack propagation behavior of the incipientdamage was also examined, and confirmed the existence of thesub-surface incipient damage but no occurrence of completespalling on the roller.

-10

-8

-6

-4

-2

0

inicipientdamage

After incipient damage, cumulative cycles

Rel

ativ

e w

eigh

t los

s, m

g

1700 5900 10600 27000

Fig. 8. Weight loss behavior of further multiple tests of incipiently-damaged roller.

0

200

400

600

800

1000

1200

After incipient damage, cumulative cycles (×100)

AE

rise

time,

μs

Before incipient damage, 7 min2.1 min5.3 min5.9 min20.5 min

0 17 59 106 270

Fig. 9. Relationship between AE risetime and AE duration

From the above discussion, it was found that the new AEmeasurement unit, which was effectively a combination ofsimultaneous use of the AE data analyzer and the AE sourcelocator, could be a better condition monitoring tool for detectingand locating the incipient damage in rolling elements in contacts.

3.3. Incipient damage propagation and severity of damage

In the second step, crack propagation behavior after theincipient damage before spalling occurrences was investigated.The relationship between severity of damage in terms of weightloss of the follower and test cycles was estimated. The weight lossof 0.2570.05 and 0.3070.05 mg were found for driver andfollower, respectively, in case of ‘incipient damage’ detection testin the first step. Fig. 8 represents the relative weight loss of theroller (follower) from the incipient damage test and the furthermultiple tests of the incipiently-damaged roller for correspondingdamage propagation cycles.

The surface initiated spalling fatigue failures are usuallyoccurred under partial EHL or boundary lubrication condition[21]. In this study, the roughness of the contact rollers were1–2mm and the average minimum contact thickness is 0.4mm atan average experimental temperature of 48 1C. Therefore, the

0

200

400

600

800

1000

1200

After incipient damage, cumulative cycles (×100)

AE

dur

atio

n, μ

s

Before incipient damage, 7 min2.1 min5.3 min5.9 min20.5 min

0 17 59 106 270

with contact cycles before and after incipient damage.

y = 4.5252x + 1436.3

0

2500

5000

7500

10000

12500

15000

17500

0Max hit counts/damage division

Cyc

les

afte

r inc

ipie

nt d

amag

e

500 1000 1500 2000 2500 3000 3500

R2 = 0.996

Fig. 10. Relationship between AE hit count and contact cycle numbers after

incipient damage.

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Z. Rahman et al. / Tribology International 42 (2009) 807–815 813

specific film thickness lies in the boundary lubrication conditionas the film thickness was very small and which was less than thecomposite surface roughness of the contacting rollers. During thetests of incipiently-damaged roller, the incipient damage initiatedat the surface; which propagated into the surface and converted

1

Hit

coun

ts

Rotationdirection

200μm

200μm

200μm

200μm

Hit

coun

ts

1

Hit

coun

ts

1

Hit

coun

ts

1

Hit

coun

ts

200μm

Fig. 11. Incipient damage propagation be

into a complete spalling by a peeling action at the surface as thefurther test-run continued. It was found that the completespalling outbreak resulted greater weight loss of the roller,however with the incipient damage it was found the minimum.Therefore, from the view point of weight loss of the roller, it was

0

200

400

600

800

000

0

25

50

75

100

1Roller division

0

200

400

600

800

000

0

200

400

600

800

000

0

200

400

600

800

000

Hit counts at incipient damage

Cycles after incipient damage: 1700

Cycles after incipient damage: 5900

Cycles after incipient damage: 10600

Total cycles: 7116300

Total cycles: 7114600

Total cycles: 7118800

Total cycles: 7119300

Max hit count/div = 175

Max hit count/div = 479

Max hit count/div = 724

Max hit count/div = 30

Cycles after incipient damage: 27000

Total cycles: 7131000Max hit count/div = 3319

Cycles after incipient damage: 0

10 19 28 37 46 55 64 73 82 91100

1Roller division

10 19 28 37 46 55 64 73 82 91100

1Roller division

10 19 28 37 46 55 64 73 82 91100

1Roller division

10 19 28 37 46 55 64 73 82 91100

havior with the rate of AE hit count.

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0

20

40

60

80

100

inicipientdamage

1700

After incipient damage, cumulative cycles

Rat

io o

f spa

lling

are

a, %

5900 10600 27000

Fig. 12. Relationship between ratio of spalling area and contact cycles before and

after incipient damage.

Z. Rahman et al. / Tribology International 42 (2009) 807–815814

estimated that this system could detect the incipient damageacceptably, and as the tests of incipiently-damaged rollerprogressed, the weight loss of the roller increased.

3.4. Incipient damage propagation behavior with AE duration

From the multiple after tests of incipiently-damaged roller,damage propagation with the degree of damage was characterizedby the differences in AE risetime and AE durations. Fig. 9 showsthe influence of the incipiently-damaged state propagation andthe damage size increasing on the AE risetime and the AEduration. The change in AE risetime was profound than that of theAE duration as amplitudes took a comparably longer time to reachits crest as the tests continued. However, the damage propagationincluding crack propagation around the damage after the incipientdamage was evidently different from each other because of thevariation in dynamic response and impact between damage areaof the follower and the contact surface of the driver. This implied amore clear evidence of the degree of damage by the AE risetimedata characteristics. Therefore, stable and unstable damagepropagation mechanisms were believed to be responsible for thedifferences in AE risetime and AE duration with each other testresults. A higher value of AE risetime and AE duration at the finaltest of the incipiently-damaged roller showed the spallingoccurrence as the severity of damage generated from thepropagating incipient damage. The higher intensity of AE para-meters (i.e. AE risetime, AE duration, AE hit count etc.) wasinstigated from the heavily damaged division on roller as a severespalling where the incipient damage existed before.

3.5. Incipient damage propagation behavior with AE hit count rate

The maximum AE hit counts at a particular damaged divisionduring the tests of incipiently-damaged roller are plotted in Fig. 10against the elapsed cycles after the incipient damage up to thesevere spalling occurrence at the division No. 19 on the rollersurface. A line of the best fit was obtained, as well as theconfidence (R2

¼ 0.996), of the line fit to the data. A linearrelationship between AE hit count and contact cycles numberafter incipient damage was identified. This relationship illustratedthe effectiveness of the AE source locator use in this test fordamage identification and its propagation monitoring in rollingelements in contact.

Moreover, Fig. 11 shows the micrographs of the rolling contactsurface of the incipient damage, and incipient damaged propaga-tion behavior with its corresponding AE hit count rate for definednumber of cycles. For all the tests of the incipiently-damagedroller, the AE hit count was mostly developed at the division No.19 of the roller where incipient damage was occurred during theRCF test. Therefore, the accumulation number of the AE hit countdepended on the degree of damage i.e. damage size. It was alsofound that the AE hit count data accumulation developed in twodivisions (Nos. 18 and 19), which represented to the larger damageon the roller surface and provided information of spallingoutbreak. The tests relationships between the AE hit count rateand the incipient damage propagation rate suggested thepossibility of predicting the remaining fatigue life of a rollingelement, based on short term AE monitoring of rolling contact.

The damage sizes were estimated by the ratio of spalling areawhich was defined as total spalling area to micrograph area.Fig. 12 shows the ratio of spalling area (damage size) with thecorresponding rolling contact cycles before and after the incipientdamage. The damage sizes evidently correlated with the amountof weight loss of the roller, and signify damage propagationbehavior from the incipient damage.

4. Conclusions

In this study, a new detective system by the AE measurementunit was used to identify and detect any damage in rollingelements during their contacts. The AE measurement unitconsisted of AE sensor, preamplifier, signal processor, AE dataanalyzer, AE signal to pulse converter, AE source position sensorand AE source locator. The new unit is capable of providing ‘real’damage/crack information at its very initial stage of birth. It wasfound that the combination of the AE source locator and the AEdata analyzer could be used as a useful tool for characterizing theincipient damage and its propagation behavior at the position ofdamage occurrence. The AE hit count rate is an importantparameter to define an incipient damage, its detection and thesize of the damage during the damage propagation.

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