The evaluation of surface residual stress in aeronautic bearings

using the Barkhausen noise effect

S. Desvauxa,b, M. Duquennoya,*, J. Gualandrib, M. Ouraka

aIEMN, Departement OAE (UMR CNRS 8520), Universite de Valenciennes et du Hainaut Cambresis, B.P. 311,

Valenciennes cedex 9, 59313, FrancebSNFA, Z.I. n82 Batterie 900 Rouvignies, Valenciennes 59309, France

Received 2 December 2002; revised 4 March 2003; accepted 17 March 2003

Abstract

Bearings in aeronautic engines are subject to heavy mechanical demands. The bearing raceways withstand levels of mechanical stress

capable of causing metal fatigue that can lead to bearing malfunction, which in turn may cause engine failure mid-flight. For this reason,

regular verifications of engine bearings to gauge the degree of metal fatigue are essential. Such verifications require knowledge of the pre-

stress state of the bearing raceways through use of surface residual stress (SRS) estimates. In this paper, we present a non-destructive method

for estimating SRS, based on the Barkhausen noise (BN) effect. This method was validated on several different batches of bearings. Our

investigations have shown this method to be rapid, well suited to industrial imperatives connected to on-line measurement and easily adapted

to the circular geometries of the bearings rings. In addition, we have shown the efficiency of the BN effect for estimating the SRS of bearing

raceways after engine operation, in order to perform necessary bearing maintenance.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Residual stress; The Barkhausen noise effect; Bearings

1. Introduction

Bearings in aeronautic engines are subject to extremely

severe running conditions. The contact areas between the

balls or rollers and the raceways (both the inner and outer

rings) sustain mechanical stress leading to a hertzien

fatigue process. As a result of this fatigue, micro-cracks

may appear on the contact area. Without intervention,

these cracks can develop until they cause bearing

malfunction, which may lead to engine failure. It is

therefore indispensable to ensure that the contact areas of

aeronautic engine bearings are in optimal condition,

paying particular attention to the metallurgical aspect of

the metal (evidence of grinding abuse, for instance) and to

the metals stress level for any bearing-loading zone by

targeting compressive residual stress. Both the validation

of a method capable to introduce residual stresses and the

development of a non-destructive method that can ensure

the follow-up of associated residual stresses has become a

necessity. This paper presents a non-destructive method

for identifying the surface stress of the contact areas

between the balls or rollers and their raceways. This

method is based on the phenomenon of Barkhausen noise

(BN), and is adapted to industrial imperatives connected

to on-line measurement. It is rapid, requires no direct

contact and is suitable for the circular geometry of the

bearing rings. Using X-ray diffraction as our method of

reference, we have shown the efficiency of BN in

estimating the surface residual stress (SRS) of raceways

following grinding operations, specific pre-stress treat-

ments and engine operation. Given the success of our

current research, we have begun to consider the possibility

of transferring this method to an industrial environment.

2. Barkhausen effect

BN, as it is often called, was discovered in 1919 by

H. Barkhausen. He put the first in evidence the brutal and

discontinuous character of the movement of magnetic

0963-8695/03/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/S0963-8695(03)00046-X

NDT&E International 37 (2004) 917

www.elsevier.com/locate/ndteint

* Corresponding author. Fax: 33-03-27-51-11-89.E-mail address: marc.duquennoy@univ-valenciennes.fr (M.

Duquennoy).

domain walls. The discontinuous character of magnetic

domain wall movement can be explained if one considers

the crystal in the scale of the wall and takes into account

the number of surface imperfections (precipitate, vacancy,

dislocations, grain boundaries) which can mark the

perfection of its crystalline network. Confronted with a

defect, the domain wall movement stops abruptly,

remaining blocked at this point until the field reaches a

certain value. If the value of the applied field continues to

increase, the domain wall movement suddenly resumes,

moving on to another defect. The blocking capacity of the

crystalline defects depends on the interaction energy

existing between the defect and the domain wall. An

abrupt domain wall movement provokes an elementary

Barkhausen event; the sum of these events constitutes BN.

Since Weiss domain walls are blocked by defects in the

material, BN allows the measurement of any modifi-

cations in the magnetic microstructure and consequently

any modifications in the metallurgic microstructure. Fig. 1

shows the evolution of BN during a hysteresis loop. In

addition to the electromagnetic disruption, some types of

domain wall movement will provoke a magnetostriction

phenomenon. This change in dimension generates an

elastic wave, called acoustic BN or Broadcast

magnetoacoustics.

The works of Tiitto [1] in 1977 are the principal

starting point for research concerning the use of BN as a

method of non-destructive control of the metallurgic

states, particularly stress. Under the influence of an

applied magnetic field, domains whose magnetization

parallels that of the magnetic field increase, and domains

whose magnetization is perpendicular to the field decrease

until they finally disappear. Similarly, when a material

undergoes tensile stress, the domains whose magnetization

parallels the direction of the tensile stress increase at the

expense of the other domains, gradually taking over the

total available volume. On the other hand, domains whose

magnetization is perpendicular to the direction of

compressive stress increase and eventually cancel out

the other domains. Thus, as Tiitto showed, the level of the

BN is influenced both by stress and by applied magnetic

fields. If the stress and the magnetic field generate the

same type of change in domain configurations,

the cumulative effect causes elevated levels of BN. On

the other hand, if the stress and the magnetic field cause

conflicting effects on wall movement, the level of BN is

reduced. This phenomena of BN and the influence of

stress on the latter have been well described by many

authors [1,2].

3. Use of Barkhausen noise to estimate stress

Many authors [36] have shown that it is possible to

use BN to characterize traction or compression stress.

Pasley [7] was one of the first to use BN to quantify stress

measurements. He studied the evolution of the BN

amplitudes as a function of to the level of stress on a

test tube that was being bent. For a traction load, Pasley

observed an increase in the BN amplitude in the elastic

domain, followed by a zone of saturation when moving

into the plastic domain. For a compression load, he

detected the reduction of the BN amplitude in the elastic

domain, followed once again by noise saturation as the

domain changes to plastic. Langman [8] studied the

influence of stress on the shape of the hysteresis loop on

annealed condition mild steel. He showed that the

hysteresis loop narrows when the measurement of

Fig. 1. Barkhausen noise and the associated hysteresis loop.

S. Desvaux et al. / NDT&E International 37 (2004) 91710

the field parallels the applied stress, and becomes larger,

though with a smaller amplitude, when the measurement

of the field is perpendicular to the stress. Gilbert et al. [9]

tried to link the applied stress level to the shape of the BN

signal envelope. He applied uniaxial pressure to two of

the magnetically soft steels (with a significant density in

pure iron) used in electric motors. The author noted the

temporal Barkhausen signal, transverse to the uniaxial

pressure, as indicated by the envelope for three loads 200,

400 and 600 MPa, and he noticed an increase in the

Barkhausen signal in terms of the applied stress. However,

the most prevalent representations give the maximum

Barkhausen signal envelope versus the applied stress [4,6,

912] or applied strain [13].

Other studies have been done to determine stress using

the BN level of pieces tempered at different temperatures

and having different degrees of hardness [14], or on

materials with different micro-structures [15]. Overall

results are highly diverse, as could be expected given the

variety of microstructures present in the samples and the

sensitivity of BN to a wide selection of metal properties.

For example, different BN levels have been observed for

pieces with the same level of stress but different

metallurgic microstructures [6]. In addition, whatever of

the initial metallurgic microstructure of the material,

magnetically soft steels and low stress levels due to

traction or compression (2500 and 1000 MPa) producesharp noise level variations that are nearly linear. On the

other hand, sensitivity to stress appears to decrease at high

stress levels, and a phenomenon of saturation has been

observed [6].

The evaluation of SRS has been the object of several

publications. The research of Moorty and Nierlich [16,17]

concerning carburising steels EN36 and high bearing steel

100Cr6, has shown that the BN level allows the quantifi-

cation of SRS. Evaluation of SRS using BN has also proved

very useful for the testing pipes where some defects due to

tensile surface stress can appear either on the pipe surface or

on the welds. Tonshoff and Lindgren [18,19] have shown

that comparing BN parameters to X-ray diffraction

measurements, make it possible to control SRS levels

using BN on soft steel for stress values situated between

2400 and 400 MPa.Different parameters were exploited to establish a

linear relation between SRS and the BN. Tonshoff

considered the amplitude, the positions of the amplitude

of the temporal signal and the maximal height of the BN

impulse height distribution [20]. Theiner, on the other

hand, used the amplitude of the Barkhausen signal and the

value of the coercive field [21]. Studies have indicated

that 2D mapping of SRS using BN could limit defects

during the shaping of steel plate [22,23]. For stress

between 2150 and 300 MPa, the mapping is done bymeasuring the BN level versus the micro-deformations in

the longitudinal and transversal directions.

4. Determination of surface residual stress using

Barkhausen noise

In 1957, Biorci and Pescetti [24] proposed a very simple

mathematical formulation for modelling a Barkhausen

impulse in pure iron. He verified that this formulation was

well adapted by verifying that the sum of impulses

permitted the original shape of the BN to be reconstituted.

In 1997, Saquet [25] reactualised this study, representing an

elementary Barkhausen event in temporal terms and from a

Fourier transformation in terms of frequency. Several

authors [26,27] have mentioned the attenuation phenom-

enon resulting when an electromagnetic field is propagated

in matter, as well as its consequences on the representation

of the BN frequency spectrum.

Because the elementary impulse, and therefore the BN, is

attenuated both by frequency and by the depth of the

elementary impulse (i.e. the distance between the surface

where the receiving wire has been placed and the domain

wall that moves), we can use this attenuation phenomenon

to obtain information about surface stress. Given that the

high-frequency components of the Barkhausen signal are

rapidly attenuated as the wave progresses through the

material, by choosing to analyse the components with

the highest frequency, we are able to study the part of the

Barkhausen signal that is typical of the matters surface

state, leading in turn to information about the surface stress.

In order to establish the relationship between BN and

surface stress, it was first necessary to establish reference

samples. For this purpose, we used several M50 steel

samples, each with a different level of SRS. These samples

were characterized using X-ray diffraction. For each of

these samples, we determined the BN and calculated its

corresponding spectrum. Among the different spectrum

analysis parameters, we chose the area under the section

being analysed.

BN covers a large frequency bandwidth, from about

100 Hz to several MHz. Two filtering procedures are

necessary: a low-pass filter to eliminate high-frequency

interferences and a high-pass filter to free from the slow

variations in the magnetic flux through the section of the

coil of the probe. Our frequency analysis of the BN signal

limited the frequency range between 20 kHz and 1 MHz.

This bandwidth corresponds to that of the receiving coil

used in this study, and in addition to limiting interference, it

allows calculation times to be reduced. Within the 20 kHz

1 MHz bandwidth, we had to select the frequency analysis

band characteristic of the SRS in order to obtain the

relationship between the BN spectrum of each reference

sample and the levels of surface stress as determined by the

X-ray diffraction method.

Since high frequency components were used, the upper

bound of the section is determined by the bandwidth of the

receiving wire: 1 MHz. Then, the lower bound of the

analysis section had to be determined. With this in mind,

we first took 10 BN measurements for each sample. Then,

S. Desvaux et al. / NDT&E International 37 (2004) 917 11

a frequency band was chosen for the high-frequency

analysis [F1;F2] where F2 was equal to 1 MHz. The area

was then calculated for the 10 spectrum measurements. We

averaged these area calculations for each sample, and

connected them to their level of residual surface stress.

Then, we looked for the best regression curve (linear,

exponential) that would allow us to characterize the

evolution of the BN parameter (area) as a function of

the surface stress. Finally, we shifted the lower bound of the

analysis frequency range, and we repeated all these steps

until we obtained the optimum correlation coefficient. A

computerized search using an appropriate frequency range

allowed us to demonstrate a linear relationship between the

SRS and the BN parameter. For reasons of confidentiality,

we cannot specify the value of lower bound F1. Generally,

increasing compression stress causes a reduction of the area

under the amplitude spectrum (Fig. 2). The error margin for

measurements of the area under the amplitude spectrum in

the frequency range [F1, F2] was a maximum of 5%. This is

represented by the vertical bars in Fig. 2. The horizontal

bars show the corresponding error margin for stress

measurements using X-ray diffraction.

5. Validation of the method

In order to validate the method, we used BN to estimate

the different levels of SRS on five bearing rings that had

undergone a variety of shot peening operations. These five

rings, respectively, B1, B2, B3, B4 and B5, were made of

M50 steel, whose characteristic structure is a tempering

martensitic matrix composed of ferrites and carbides. The

mechanical properties of M50 steel are primarily deter-

mined by thermal treatments, and the principal chemical

constituents are summarized in Table 1.

Given that the estimation of the SRS is obtained from a

parameterof theBN spectrum, it is essential tocalibrate before

trying to estimate the stress itself. This calibration consists of

creating a table giving the stress versus the value of area under

the section being analysed (analysis frequency range [F1,

F2]). Because BN is influenced by many material character-

istics and properties, a table had to be created for each type of

bearing, taking into consideration every slight difference in

material and every manufacturing process (i.e. heat treat-

ments, chemical treatments and mechanical treatments). For

our study, we created a table using four reference samples.

Fig. 2. Parameters of Barkhausen noise versus the residual surface stress where R2 is the correlation coefficient.

Table 1

Micrograph and the mechanical properties of bearings

S. Desvaux et al. / NDT&E International 37 (2004) 91712

Once the calibration phase is achieved, we estimated the

SRS on five rings, B1, B2, B3, B4 and B5, first using BN and

then using X-ray diffraction. In Fig. 3, we have presented

the SRS measured by X-ray diffraction versus those

estimated using BN. The stress error measured by X-ray

diffraction was obtained by moving the diffraction peak,

which is a function of the metallurgical and geometrical

characteristics of the sample. The stress error estimated by

BN was obtained statistically with a confidence limit of

95%. These experimental results show a clear parallel

between the measurements of SRS using X-ray diffraction

and those estimated using BN; the correlation coefficient of

the linear regression is 99.28%. Given that the X-ray

diffraction method is the calibrated method of reference, the

systematic concurrence of the stress estimates of the two

methods confirms the pertinence and the reliability of the

Barkhausen method for the evaluation of SRS of any given

material.

6. Estimation of surface residual stress on bearings,

following engine operation

This study aims to test the material fatigue of bearings

after engine use, by studying the stress level of the bearings.

The estimation of the SRS after engine operation using BN

is a question of enormous importance for the aircraft

industry of the future. Indeed, the cost of plane maintenance

operations, particularly engine maintenance, continues to

climb due to the longer life expectancy of the planes and the

increasingly stricter safety requirements. For this reason,

some companies plan to do systematic bearing overhauls

following engine operation. These overhauls would include

a visual examination of the components, followed by the re-

grinding or super finishing of the bearing raceways. This

new approach to engine maintenance will require

taking material fatigue into account. Depending on

the programmed flight plan, the engine bearings and

materials will sustain supplementary fatigue, which will

lead again to modifications of both the materials micro-

structure and its level of stress. These overhauls will be

limited to those bearings exhibiting a fatigue level under

pre-defined limits, since it would obviously be too

dangerous to repair bearings whose raceways have already

sustained heavy material fatigue.

The level of the material fatigue is currently character-

ized by measuring the materials residual internal stress.

Doing so requires a non-destructive testing method whose

stress estimates are reliable enough to clearly reject all the

parts, which should not be repaired without eliminating

parts that are still in satisfactory condition. But this method

has to be cheap enough to keep the cost of controlling and

overhauling under the cost of simply replacing the parts.

Both of these conditions are respected by the magnetic

method using BN presented in this article. The SRS

measurements obtained with this method have been

validated by comparison with those measurements obtained

by X-ray diffraction, and if we verify the levels of

compressive residual stress on the total surface of the

bearing rings after engine operation, the cost is 40 times less

than that of the X-ray diffraction method. In addition, the

mapping of 200 measurement points is 60 times as fast. The

measurements of surface stress were made on a batch of 20

bearings with the same reference. The inner rings of these

bearings are made of 52,100 steel, a material characterized

by a compound spheroid structure of ferrites and carbides.

The mechanical properties of 52,100 steel are summarized

in Table 2.

After creating the table which allows the estimation of

surface stress on these inner rings using BN, we estimated

the SRS on all 20 rings and compared the estimations using

BN with the measurements obtained with the X-ray

diffraction method. The results, shown on Fig. 4, indicate

a clear parallel between the estimations using BN and

Fig. 3. Surface residual stress measured by X-ray diffraction versus surface residual stress estimated by Barkhausen noise.

S. Desvaux et al. / NDT&E International 37 (2004) 917 13

the measurements using X-ray diffraction, with the excep-

tion of two inner rings (90,343 and A244). To understand

why the X-ray diffraction measurements and BN esti-

mations of the SRS differed for both rings 90,343 and A244,

we verified the homogeneity of the stress over the width of

the raceway. The difference in the results obtained with

these two methods could be explained by the fact that the

volumes inspected were not the same for both methods (Fig.

5(a) and (b)). For the X-ray diffraction, the measurement

was localized only a few mm3 were irradiated and X-rays

were focused in the middle of raceway. For the BN method,

on the other hand, the volume of material contributing to

measurement is several mm3. Therefore, of the stress

evolves along the transversal direction of the raceway

(following the direction z in Fig. 6), the estimated SRS were

different for the two methods.

In order to control the homogeneity of the stress over

the width of the raceway, we did several mappings using

BN on the load zones of three inner rings, one ring where

the X-ray and Barkhausen measurements concurred (ring

A79) and two others, the rings 90,343 and A244. These

mappings were obtained by moving the position of

Fig. 4. Surface residual stress estimations using Barkhausen noise on 20 inner rings of 52,100 steel compared to the X-ray diffraction measurements.

Table 2

Micrograph and the mechanical properties of bearings

Fig. 5. Bulk inspected (a) by X-rays diffraction and (b) by Barkhausen

noise.

S. Desvaux et al. / NDT&E International 37 (2004) 91714

the magnetic sensor along the raceway in the transversal

direction, at the same time causing the ring to rotate on its

axis (Fig. 6). These mappings correspond in the end to

those developed for the stress levels of the bearing

raceways, similar to the C-scan of ultrasonic methods.

Obtaining the same kind of mapping with the X-ray

diffraction method would require approximately 200

measurements, which explains the higher cost and the

longer time needed for this method, as opposed to the BN

method.

The three mappings discussed above are presented in

Figs. 79. For reasons of confidentiality, BN levels versus

the spatial position on the raceway are given rather than

the stress levels. Despite this presentation, the homo-

geneities and the heterogeneities of the stress appear

clearly, given that the BN levels are directly proportional

to the levels of SRS.

Fig. 7 represents the mapping of raceway A79. Clearly,

the level of BN is homogeneous and uniform throughout the

raceway. The observed stress level is the result of the initial

stress (prior to engine operation) and of the stress introduced

by the loads appearing between the loading zones of the

inner ring and the rolling elements. This homogeneity

means that both the pre-stress treatments prior to engine

operation and the load distribution on the inner ring during

engine operation were homogeneous.

Figs. 8 and 9 represent the mappings of inner rings

90,343 and A244 ,respectively. The level of BN, and

consequently the stress level, is neither homogeneous nor

uniform in the transverse direction of the raceways of these

Fig. 6. Mapping of surface loading zone inspected using Barkhausen noise.

Fig. 7. Mapping of the Barkhausen noise level on the loading zones of A79

bearing inner rings after engine operation.

Fig. 8. Mapping of the Barkhausen noise level on the loading zones of

90,343 bearing inner rings after engine operation.

S. Desvaux et al. / NDT&E International 37 (2004) 917 15

two rings. This lack of homogeneity and uniformity

probably comes from a heterogeneous distribution of the

loads on these inner rings during engine operation. On

mapping in Fig. 8, the load must have been located

essentially at the top of raceway, between 10 and 13 mm,

whereas mapping in Fig. 9 shows the load situated between

2 and 5 mm. This second load, however, must have been

less important than the previous one, given that the

Barkhausen level is higher. Clearly, the loads on the ring

introduce compressive stress, which has the effect of

reducing the level of BN.

These mappings allowed us to discover a significant

asymmetry in the surface stress distribution of the loading

zone, which explains the difference between the localized

X-ray diffraction measurements and the Barkhausen

estimations presented in Fig. 4. In addition, these mappings

highlight the importance of producing complete maps of

raceway stress distribution, because incomplete mappings

can hide possible asymmetries in the stress field.

7. Conclusion

In this article, we have presented a non-destructive

method based on the phenomenon of BN to identify SRS in

the contact zones between ball or roller bearings and their

raceways. Using X-ray diffraction as the method of

reference, we have shown the efficiency of BN for

estimating the fields of SRS on raceways after specific

pre-stress treatments and after engine operation. Some

mappings of the loading zones were created using BN.

These mappings show an uncentered load on the raceway of

the inner ring after engine operation. This observation is

especially important since it will not only allow the rejection

of a certain number of bearings during the maintenance

cycle (i.e. during renovation), but also encourage a review

of the conditions which led to these poorly distributed loads.

Finally, this magnetic BN method has the advantages of

being rapid, suitable for the circular geometry of the rings,

and requiring no direct contact. Given the progress of our

research, we hope in the near future to move this method out

of the laboratory and into the industrial environment.

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The evaluation of surface residual stress in aeronautic bearings using the Barkhausen noise effectIntroductionBarkhausen effectUse of Barkhausen noise to estimate stressDetermination of surface residual stress using Barkhausen noiseValidation of the methodEstimation of surface residual stress on bearings, following engine operationConclusionReferences