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Predicting white etching matter formation in bearing steels using a fretting damage parameter

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Abstract

White etching cracks (WECs) are deleterious artifacts found in fatigue damaged bearings. The formation of WECs has been attributed to the frictional energy dissipation at subsurface cracks during cyclic rolling contact. This study introduces a novel Ruiz fretting damage parameter (FDP) implementation to predict the frictional energy dissipation at subsurface cracks. A parametric study using a finite element model explores the role of crack length, orientation, depth, and coefficient of friction (COF) on FDP. The FDP is maximized at a normalized length of 0.50a and depth of 0.78a, between the orientation of ± 30°, at a COF of μ = 0.3, in agreement with experimental observations. These results provide strong evidence of the FDP as a predictor of WEC formation.

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... The importance of steel microstructure, heat treatment, lubricant composition, and electrical current is also becoming more understood (Ščepanskis, Gould, and Greco 2017;Gould and Paladugu et al. 2019;Roy et al. 2019;Gould et al. 2021). Recent evidence does indicate bearing failures due to WECs can be reduced through implementation of several countermeasures (Jensen, Heuser, and Petersen 2021); however, design life calculations are still under development and require further verification Leung, Voothaluru, and Neu 2021;Natarajan et al. 2021). Pertinent research questions are: ...
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A common cause for premature bearing failures in wind turbine gearboxes are the so-called White Etching Cracks (WEC). These undirected, three-dimensional cracks are bordered by regions of altered microstructure and ultimately lead to a cracking or spalling of the raceway. An accelerated WEC test was carried out on a FE8 test rig using cylindrical roller thrust bearings made of martensitic 100Cr6 steel. The resulting WECs were investigated with several characterisation techniques. Ultrasonic measurements showed the WEC were mainly located in the region of the overrolled surface in which negative slip occurs, which agrees with hypotheses based on an energetic approach for a prognosis. SEM orientation contrast imaging of the area around WEC revealed an inhomogeneous structure with varied grain sizes and a large amount of defects. Microstructure characterization around the WEA using EBSD showed significant grain refinement. Atom probe tomography showed the microstructure in the undamaged zone has a plate-like martensitic structure with carbides, while no carbides were detected in the WEA where the microstructure consisted of equiaxed 10 nm grains. A three dimensional characterisation of WEC network was successfully demonstrated with X-ray computerized tomography, showing crack interaction with unidentified inclusion-like particles.
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Subsurface cracks decorated with white etching areas are one of the failure marks in bearings subjected to rolling contact fatigue. In the literature, multiple mechanisms were proposed to explain this phenomenon, one of which stated that the white etching matter formed around a subsurface crack is a symptom of crack faces rubbing which leads to local changes in the material microstructure. The current work presents a theoretical study on white etching area formation by the modelling of frictional interaction at crack faces in mode-II, caused by cyclic shearing and compression – the conditions typical for rolling contact. Based on energy considerations, the feasibility of white etching area formation along subsurface cracks in bearings is theoretically investigated.
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White etching cracks (WEC) can lead to premature rolling contact fatigue. Possible drivers of WEC can be mixed friction, sliding between rolling elements and raceways, electrical current, critical additives, and water-contaminated lubricant. With respect to WEC failures induced by sliding between rolling elements and raceways under mixed friction, an approach is presented that can explain the experimentally observed failure characteristics of cylindrical roller thrust bearings. Variants of the bearing were tested using a WEC-critical lubricant. The tests showed that not only the contact pressure and sliding between rolling elements and raceways but also the lubrication conditions (specific film thickness) and the frequency of the contact load cycles have an influence on WEC life. These influences are reflected best by a newly introduced characteristic parameter termed friction energy accumulation. As far as WEC failures induced by sliding under mixed friction are concerned the friction energy accumulation could be used for a comparative assessment of the WEC risk of arbitrary rolling bearing applications. A link between the friction energy accumulation and the absorption of hydrogen is discussed and can provide further explanations for the susceptibility of bearing components to WEC formation.
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Among the prevalent tribological failures affecting rolling element bearings, an unconventional rolling contact fatigue mode has been identified as white etching cracks. Those correspond to three-dimensional branching crack networks partially bordered by white etching microstructure, eventually leading to premature and unpredictable failure. Recent work supports that this failure mode may be associated with various combinations of operating conditions depending on the application or test rig, but that all seem to converge towards similar tribological drivers related to surface-affected hydrogen evolution at asperity scales, which is known to embrittle the bearing steel. Nevertheless, as white etching cracks remain delicate to reproduce without artificial hydrogen charging, the underlying formation mechanisms remain unsettled. The present work aims to better understand how some of the main tribomechanical and tribochemical drivers may trigger white etching cracks and premature failures. In this study drivers such as sliding kinematics, water contamination, and electrical potential and lubricant additives are progressively transposed on a twin-disc machine that provides an enhanced control of contact parameters. Various attempts advocate that the tested drivers are not self-sufficient to reproduce the failure mode in such apparatus, but confirm that specific lubricant additives may reduce the fatigue life by promoting surface-initiated embrittled cracking similar to white etching cracks. A local criterion accounting for the local sliding frictional power dissipation and the lubrication regime is further proposed to assess the risk of white etching cracks based on the analysis of various reproduction and occurrences.
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The actual service life of wind turbine gearboxes is often well below the desired 20 years. One of the prevalent failure modes in gearbox bearing raceways is white structure flaking (WSF) in as little as 6–24 months of operation by the formation of axial cracks and white etching cracks (WECs) with associated microstructural change called white etching areas (WEAs). Despite these failures having been observed for two decades in various industries, the drivers and mechanisms for their formation are still highly contested. Discussed in this review are methods for searching and analysing WECs, mechanisms for WEA microstructural change, WEC initiation and propagation theories, WSF formation drivers and finally technologies and processes offering resistance to WSF. This updated review serves as a recap, comprehensive update on findings, current focus areas and remaining challenges. This paper is part of a Themed Issue on Recent developments in bearing steels.
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Non-metallic inclusions such as sulfides and oxides are byproducts of steel manufacturing process. When a component is subjected to repetitive loading, fatigue cracks can emanate from these inclusions due to stress concentrations that happen because of mismatch in elastic–plastic properties of inclusions and matrix. In certain applications such as gears and bearings, crack initiation from inclusions is accompanied with microstructural alteration. This paper employs a numerical as well an experimental approach to investigate these microstructural changes which are so-called “butterfly wings”. A 3D finite element model was developed to obtain the stress distribution in a domain subjected to Hertzian loading with an embedded non-metallic inclusion. A formerly introduced 2D model based on continuum damage mechanics (CDM) was developed to simulate the butterfly wing formation in 3D. Wingspan-to-inclusion ratios were observed at different cross sections following an analytical serial sectioning procedure. A closed form solution was suggested for the wingspan-to-observed-inclusion-diameter ratio and the results were corroborated with the data available in the open literature. On the experimental front, nonmetallic inclusions inside a sample made of bearing steel was detected using ultrasonic inspection method. Rolling contact fatigue (RCF) tests were run on the specimen and post-failure serial sectioning was conducted to understand the 3D shape of butterflies formed around an inclusion detected by ultrasound. Comparison of experimental and numerical serial sectioning of the wings showed a close correlation in the butterfly wings geometry.
Article
For high-strength steels, the crack initiation of very-high-cycle fatigue (VHCF) is commonly at the interior of material with fish-eye (FiE) morphology containing a fine-granular-area (FGA) surrounding an inclusion as crack origin, and FGA is regarded as the characteristic region of crack initiation. Here, we carefully examined the micro-morphology of FGA and FiE for two high-strength steels. The results revealed that the microscopic nature of FGA is a thin layer of nanograins. Then we proposed the formation mechanism of FGA: Numerous Cyclic Pressing (NCP) between originated crack surfaces, which causes grain refinement at originated crack wake and therefore the formation of FGA. The results of second set experiment showed that the cases with negative stress ratios exhibit the prevalence of nanograin layer in FGA region and the nanograin layer vanishes for the cases with positive stress ratios, which is a verification of the proposed NCP model.
Article
The formation of white etching area/WEC that were developed in wind turbine gearbox bearings from service, large scale transient gearbox test bearings (non-hydrogen charged), and hydrogen-charged laboratory-scale test rollers were studied by a serial sectioning process to map their features and elucidate initiation/propagation mechanisms. A large-scale transient test rig designed to simulate conditions to wind turbine gearbox bearing operation was used to test four high-speed wind turbine gearbox spherical roller bearings. The oil used was a commercially available ISO VG 320 water-soluble PAG fully formulated wind turbine gearbox oil. WEC initiated subsurface at inclusions. Crack initiation predominated at small length/diameter inclusions (∼ 3-20 μm). It is proposed that one mechanism of WEC formation is due to multiple linking of small WEC to form larger WEC networks that eventually propagate to the surface resulting in WSE. In the hydrogen-charged laboratory-scale tests, WEC formation was highly sensitive to the concentration of diffusible hydrogen.
Article
The formation of white etching features that were created in wind turbine gearbox bearings from service, large scale transient gearbox test bearings and hydrogen charged lab test rollers were investigated by a serial sectioning process to map their features and elucidate initiation/propagation mechanisms. FIB and TEM were also used as advanced analysis techniques. • Strong evidence was found that WEA/WECs could be initiated at inclusions. Crack initiation predominated at small length/diameter inclusions (∼3 - 20 μm in length/diameter) and evidence suggested that butterflies could initiate WECs. • It is proposed that one mechanism of WEC formation is due to multiple linking of small WECs to form larger WEC networks that eventually propagate to the surface resulting in WSF. • WEC formation was found to be highly sensitive to the concentration of diffusible hydrogen in hydrogen charged RCF test rollers. • Evidence for a void/cavity coalescence mechanism for butterfly crack formation was observed. In addition formation of superfine nano-grains intersecting dissolving spherical M3C carbides was found as part of the WEA formation mechanism in butterfly wings.
Article
The mechanical properties of hardened AISI 52100 bearing steel such as flexural strength, microhardness and Young’s modulus are considerably influenced by the austenite content retained in the microstructure. A microstructure-sensitive finite element simulation approach is presented which considers the effect of retained austenite to estimate the mechanical properties. The austenite grain size is derived as a function of austenitising temperature and holding time using a modified Arrhenius type equation. The simulation strategy involves division of the two-dimensional domain using triangular elements such that a group of six neighbouring triangular elements represented a hexagonal grain of calculated size. Material inhomogeneity is introduced by enforcing austenite properties to a fraction of the elements equal to the volume percent of retained austenite in the steel. The predictions from the simulation approach for 8% and 20% retained austenite volume fractions matched well with earlier experimental results.
Article
Despite constant expansion and engineering progress, wind turbines still present unexpected failures of heavy duty mechanical components drastically affecting the cost of energy. Among the most prevalent tribological failures in wind turbine rolling element bearings, a peculiar rolling contact fatigue mode has been associated to broad subsurface three-dimensional branching crack networks bordered by white etching microstructure, and thus named White Etching Cracks (WEC). Compared to conventional microstructural alterations, WECs tend to develop at moderate loads and cycles eventually leading to premature failures that remain unpredictable using fatigue life estimations. Far from being generic to specific manufacturers, WECs occur in various industrial applications, for various bearing types, components, lubricants, steels grades and heat treatments. As WEC occurrences present no common evident denominator, they remain delicate to reproduce on laboratory test rigs without prior artificial hydrogen charging, so that no consensus on WEC formation mechanisms have been confirmed yet. In this study, a thorough tribological analysis of WEC formation mechanisms has been led. Expertise protocols have been established to best reveal and observe WECs that commonly develop at unconventional locations versus the contact area. First analysis of WEC reproductions on standard rolling element bearings either hydrogen precharged or kept neutral have signified that artificial hydrogen charging, commonly employed to apprehend the failure mode, results in similar WEC morphologies but tends to alter WEC tribological initiation. In consequence, WEC reproductions in remarkably different configurations but without hydrogen charging have been compared in order to propose a better understanding of WEC surface-affected formation mechanisms: first, initiation via tribochemical hydrogen permeation at nascent steel surfaces formed either directly at the raceway or at surface microcracks flanks and second, propagation by local hydrogen embrittlement at crack tips function of the stress state. An extensive root cause analysis have then been led suggesting that WEC may be associated to various combinations of macroscopic operating conditions that often interact and come down to similar tribological parameters including high sliding energy thresholds, specific lubricant formulations and tribochemical drivers such as water contamination and/or electrical potentials. Further investigations on a minimalist twin-disc fatigue tribometer have provided additional evidence that WEC influent drivers are non-self-sufficient, supporting that WEC formation mechanisms rely on a subtle equilibrium between tribo-material, tribo-mechanical and tribo-chemical drivers that all should be mastered to design efficient and durable countermeasures.
Article
White structure flaking (WSF) as a premature wear failure mode in steel rolling element bearings is caused by white etching cracks (WECs) that form in the 1 mm zone beneath the contact surface. Hydrogen diffusion into raceways and transient operating conditions have been suggested as drivers of WSF. The initiation and propagation mechanisms for WEC formation are not well understood. This study elucidates WEC initiation processes in hydrogen charged 100Cr6 bearing steel by carrying out two-roller RCF testing. The application of serial sectioning enabled mapping and modelling of entire independent WECs and revealed critical information about initiators. The application of FIB tomography verified initiation mechanisms. The results show strong evidence for subsurface initiation of WECs from inclusions.
Article
White structure flaking (WSF) as a premature wear failure mode in steel rolling element bearings is caused by white etching cracks (WECs) formed in the 1 mm zone beneath the contact surface. Hydrogen release and diffusion into the bearing steel during operation and transient operating conditions have been suggested as drivers of WSF. The presence of diffusible hydrogen in steel under rolling contact fatigue (RCF) has been shown to strongly promote the formation of WEA/WECs. However, the initiation and propagation mechanisms, as well as the thresholds for WEC formation, are not well understood. This study uses hydrogen charging of 100Cr6 bearing steel rollers prior to testing on a two-roller RCF rig to enable WEA/WEC formation under service realistic loading. This study focuses on the influence of the concentration of diffusible hydrogen, the magnitude of the contact load and the number of rolling cycles on the formation of white etching features (butterflies, WEA/WECs) which are determined by a serial sectioning process. The formation of butterflies was found to be independent of concentration of diffusible hydrogen with the test parameters used, but dependent on contact pressure and number of rolling cycles up to a threshold. WEA/WEC formation thresholds were found at certain values of the concentration of diffusible hydrogen, contact pressure and number of rolling cycles. The results also show evidence for a subsurface initiation mechanism of WECs from non-metallic inclusions. It is proposed that one mechanism of WEC formation is due to multiple linking of extended butterflies or small WECs in the subsurface to form larger WEC networks that eventually propagate to the surface resulting in WSF.
Article
The formation of white etching cracks in the 1mm zone beneath the contact surface in steel rolling element bearings causes a premature wear failure mode called white structure flaking. The formation drivers of white etching cracks are contested, as are the initiation and propagation mechanisms of the cracks. Hydrogen diffusion into bearing steel sourced from the hydrocarbon lubricant or water contamination and transient operating conditions have been suggested as formation drivers. Extensive work has been conducted at Southampton to further understanding of white structure flaking and this paper summarises these evidences and the conclusions made. Serial sectioning has been used to map subsurface wear volumes of wind turbine gearbox bearings from service and large-scale test rigs, test specimens/bearings from laboratory under hydrogen charged conditions and non-hydrogen charged conditions. The process involves polishing of cross sections of test specimens/bearings at approximate to 3-5 mu m material removal intervals typically over hundreds of slices, and this was used to map white etching cracks in their entirety for the first time. Serial sectioning has allowed a comprehensive investigation of the initiation and propagation mechanisms of white etching cracks and thresholds for their formation with respects to concentration of diffusible hydrogen, contact pressure and number of rolling cycles. From these studies it has been found that white etching cracks can form by subsurface crack initiation at inclusions under hydrogen charged and non-hydrogen charged conditions; hence it has been confirmed that this is one mechanism of WEC formation. Small/short sized sulfide inclusions, globular manganese sulfide+oxide inclusions and small globular oxide inclusions between approximate to 1 mu m and 20 mu m in diameter/length predominated as crack initiators. In addition, detailed focused ion beam/transmission electron microscopic studies have been conducted to enhance the understanding of butterfly crack and white etching area formation mechanisms.
Article
As the mechanism of fretting fatigue is very complex, there is not a satisfactory fretting fatigue damage criterion at present. In this paper, a fretting related damage (FRD) parameter was proposed based on fretting fatigue mechanism and experimental results. Analysis result proves that tangential force can be used to quantify the effect of fretting. The fatigue lives predicted by FRD parameter are in close agreement with the experimental data. This parameter is succinct and has a determinate physical meaning. The proposed method can make full use of the plain fatigue research results for fretting fatigue life prediction.
Article
Sub-surface cracks can be initiated due to surface fatigue, which eventually reach the surface, leading to pitting, spalling and removal of material. In this investigation, a new approach based on shear stress reversal at the crack tips is implemented to propagate a subsurface initiated crack under fretting load. Hertzian line contact geometry is used to investigate the effects of different factors such as Hertzian pressure, coefficient of friction, displacement amplitude, and depth of the initial crack. Crack propagation paths and propagation life of the cracks under the surface are investigated in detail. Once the crack reaches the surface, it is assumed that the material enclosed is spalled and thus removed. Wear volumes and wear rates are calculated and effect of each variable on wear is analyzed. Wear rates increase with increase in maximum Hertzian pressure, displacement amplitude, coefficient of friction and depth of initial crack. The wear rates are also correlated to crack propagation rates and are in agreement with the dissipated energy wear coefficients obtained from literature.
Article
A three-dimensional subsurface penny-shaped crack in an elastic half-space subjected to a compressive moving load is analyzed using the finite element method. The compressive load is applied through a spherical asperity, which moves from left to right on the top surface of the half-space. Normal contact between the crack faces of the penny-shaped crack is modeled using the classical Lagrange multiplier method for constraint enforcement; the tangential contact between the crack faces is assumed to exhibit frictional behavior. Therefore, although the present analysis is limited to a purely linear elastic quasistatic approach, the analysis results show the loading path dependence caused by the frictional contact. Based on linear elastic fracture mechanics, stress intensity factors along the crack front of the penny-shaped crack are evaluated as functions of the crack-front angle, frictional coefficient, normalized load position, and the ratio of the crack depth to the crack length. Finite element analysis shows that shearing-mode failure rather than tearing-mode failure is the dominant cracking mechanism of the penny-shaped crack. This shearing-mode failure tends to occur in the direction of the loading path.
Article
Butterflies are microscopic damage features forming at subsurface material imperfections induced during rolling contact fatigue (RCF) in rolling element bearings. Butterflies can lead to degradation of the load bearing capacity of the material by their associated cracks causing premature spalling failures. Recently, butterfly formation has been cited to be related to a premature failure mode in wind turbine gearbox bearings; white structure flaking (WSF). Butterflies consist of cracks with surrounding microstructural change called ‘white etching area’ (WEA) forming wings that revolve around their initiators. The formation mechanisms of butterflies in bearing steels have been studied over the last 50 years, but are still not fully understood. This paper presents a detailed microstructural analysis of a butterfly that has initiated from a void in standard 100Cr6 bearing steel under rolling contact fatigue on a laboratory two-roller test rig under transient operating conditions. Analysis was conducted using focused ion beam (FIB) tomography, 3D reconstruction and transmission electron microscopy (STEM/TEM) methods. FIB tomography revealed an extensive presence of voids/cavities immediately adjacent to the main crack on the non-WEA side and at the crack tip. This provides evidence for a void/cavity coalescence mechanism for the butterfly cracks formation. Spherical M3C carbide deformation and dissolution as part of the microstructural change in WEA were observed in both FIB and STEM/TEM analyses, where TEM analyses also revealed the formation of superfine nano-grains (3–15 nm diameter) intersecting a dissolving spherical M3C carbide. This is evidence of the early formation of nano-grains associated with the WEA formation mechanism.
Article
Premature wind turbine gearbox bearing failures in the form of white structure flaking (WSF) can occur in as little as 6–24 months of operation. WSF is not fully understood but is thought to be due to hydrogen release and diffusion into the bearing steel and/or transient operating conditions not fully understood. The initiation mechanisms of white etching cracks (WECs) are contested, where amongst others mechanisms, subsurface initiation at non-metallic inclusions (perhaps associated with extension of butterfly cracks) and surface crack initiation are cited. For the first time this study applies serial sectioning to map WEC networks in wind turbine gearbox bearings to elucidate WEC initiation mechanisms. A comparison is made between WEC data for inner rings of an industrial transient test gearbox bearing and a planet bearing that spalled in service. It is proposed that one mechanism of WEC formation in wind turbine gearbox bearings is due to subsurface WEC initiation from inclusions, either in a butterfly manner or non-butterfly manner; where these small WECs link together to form larger WEC networks, these eventually propagating to the surface resulting in WSF. Small size/length inclusions were found to be likely WEC initiators, therefore the data suggests that steel cleanliness standards analysing inclusion density (as opposed to maximum inclusion lengths) are more relevant in understanding butterfly/WEC initiation in wind turbine gearbox bearings. However standards used should be able to differentiate pure sulfides from sulfides+oxide encapsulations and record inclusions that are only a couple of mircometer’s in length/diameter.
Article
The problem of a plane incomplete frictional contact, subject to a constant normal force, together with periodically varying tangential and additional normal forces, is studied. The evolution of the stick-slip patterns, together with the frictional energy dissipation, both pointwise and summed over the whole contact, is found for a wide range of load cases, for two example geometries. viz. a Hertzian contact and a contact having a central flat face and rounded edges. The results are useful both in determining the damping properties of the contact and in assessing the localisation of surface damage which may give rise to fretting damage and possible crack nucleation.
Article
The current state of knowledge of the prediction of fretting fatigue is reviewed and a summary given of the way fracture mechanics may be used to analyse a propagating crack. Recent experimental work on an aluminium alloy, HE15-TF, which discovered a distinct size effect (indicating that the absolute size of the contact has a marked effect on fatigue life), is reported and mechanical aspects of the problem, i.e. interfacial traction distribution, stress-state-induced and surface finish effects are discussed in depth. It is concluded that fretting fatigue is an initiation-controlled process and a tentative explanation of the size effect is proposed.
Article
When contacting elastic systems are subjected to periodic loading, frictional slip occurs resulting in energy dissipation. Here we investigate the effect of the relative phase of harmonically varying tangential and normal loads on the frictional dissipation in a very simple uncoupled frictional system. We demonstrate that this effect is substantial when the system experiences periods of separation, but more modest when contact is continuous. The maximum dissipation occurs when the normal and tangential loads are approximately π/2 out of phase.