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Pitting is frequently encountered in machine elements, such as bearings, cams and gears, containing parts in relative sliding-rolling motion. Most of investigations on rolling contact fatigue have been carried out by means of numerical/analytical models in the frame of LFEM, but the interpretation of the numerical results is still controversial. The SIFs history is highly non-proportional and the intensity of the peaks is not expected to cause propagation, since they do not exceed the fatigue threshold usually found for long cracks in steels and cast irons. To shed light on this issue, this work is intended to reproduce experimentally the SIF-cycles, estimated by our previous model of RCF crack, by using an alternative non-standard procedure. The novel experimental apparatus is based on the exploitation of a kinked edge crack subjected to a compressive load distributed through the thickness. Changing the load position and intensity permits to induce different values and ratios of SIFs. It has been found that lubricant is strictly necessary to trigger crack propagation, which can be in form either of coplanar extension followed by branching or of branching only. Coplanar extension occurs when mode II SIF range exceeds a certain threshold, well below the typical fatigue threshold. Empirical evidences are in accordance with the tendency of the RCF cracks to do coplanar extension and to branch into the bulk material and toward the surface. Coplanar extension is more pronounced in the interior of the sample, while branching occurs only at the edges. The results indicate that our previous modelling of RCF crack is likely to underestimate the effective SIFs. Thus this work would open the way to improve the actual understanding and modelling of RCF. [ABSTRACT FROM AUTHOR]

• A Voronoi finite element model for crack initiation life of RCF is built. • The coupling damage model considers the microstructure of material. • The impact mechanism of grain characteristics and material defects is revealed. The material microstructure of plays an important role in crack initiation of rolling contact fatigue. To address crack initiation of tapered roller bearings, the inhomogeneous characteristic of material is considered. The random distribution of material grains is simulated with Voronoi graph, and a Voronoi finite element model of rolling contact fatigue is given. Numerical analysis of damage-life constitutive relation is carried out with finite element analysis. The effect of material microstructure inhomogeneity on initiation life of fatigue crack is studied. Researches show that the smaller the grain size, the more difficult the fatigue crack appearing. The larger the difference of elasticity modulus between non-metallic inclusions and matrix material, the greater the impact on fatigue life of crack initiation. The angular and sharp non-metallic inclusions reduce fatigue life more than rounded shape inclusions. Initiation life of fatigue crack has a minimum value on a critical depth of nonmetallic inclusion. The longer distance between two laterally arranged inclusions, the greater it affect the initiation life of fatigue crack. The effect of distance between two vertically arranged inclusions on fatigue crack initiate depends on the depth of inclusions. [ABSTRACT FROM AUTHOR]

Rolling contact wear is a complex phenomenon that might involve different wear mechanisms (adhesive, abrasive, fatigue and corrosive) and different stress concentration mechanisms (asperity, dent, debris, inclusions, etc.). The interactions among these mechanisms might accelerate or decelerate the overall wear progress. Therefore, it is complicated to model and monitor the fluctuations of wear progress. The current descriptive models are either describing individual physical phenomena within rolling contact wear or describing a specific stage of wear progress. Thus, the interactions among different wear mechanisms and the transition events among different stages of wear progress are not sufficiently addressed. Therefore, the purpose of this paper is to propose a descriptive model of the wear evolution process in rolling bearings over the whole lifetime. The descriptive model utilises a wide range of empirical findings in the literature to describe the wear interactions and evolution in the five-stage scenario: running-in, steady-state, defect initiation, defect propagation, and damage growth. The new descriptive model provides the most probable scenario of wear evolution in rolling bearings, which is useful for modelling and monitoring the wear progress. It illustrates the wear evolution stages, the involved wear mechanisms in each stage, the interaction among wear mechanisms in each stage, the surface topology changes and the influencing factors within each stage. For design, condition monitoring and prognosis purposes, these aspects are significantly important to understand, model, test and monitor the wear evolution process. [ABSTRACT FROM AUTHOR]

Abstract: Current work presents Finite Elements study of hydrogen diffusion into a metal substrate loaded by rolling contact. The main goal of the presented study is to investigate stress effect on hydrogen transport in rolling contact conditions. In this work, rolling contact is defined by the Hertzian pressure, moving along the bearing ring. Although, hydrogen diffusion with respect to bearing is studied here, very similar approach can be used for other components of machinery subjected to rolling contact. Governing equations are given in the current study in dimensionless form, and parametric study, analyzing influence of different dimensionless parameters on subsurface concentration of hydrogen, is presented. [Copyright &y& Elsevier]

• The wear performance of the defected rail materials under various slip ratios and contact stresses was investigated. • The surface damage of the defected rail materials under various slip ratios and contact stresses was studied. • The effect of slip ratio and contact stress on crack propagation mechanisms near the dent was discussed. The defects on rail surface may have negative effects on the rolling contact fatigue (RCF) performance of the rail materials. This investigation researched on the RCF behaviors of the defected rail materials under various slip ratios and contact stresses. The results show that both the wear loss and the average crack length increase if the slip ratio or the contact stress increases. Moreover, the surface damage characteristics change from simple adhesion spots to significant fatigue crack grouping when slip ratio and contact stress rise. Finally, the experimental results also highlight that the slip ratio provides the deformation area for crack initiation whereas the contact stress accelerates the crack propagation by emphasizing the geometric effects. [ABSTRACT FROM AUTHOR]

Abstract: This paper proposes the use of the weight function technique to calculate mode I and mode II stress intensity factors for a shallow surface crack typical of rolling contact fatigue, that experiences the detrimental effects of the pressurization of lubricating fluid. The weight function technique was able to easily take the pressurization into account just by adding the pressure term to the nominal stress component which is then integrated. The crack closure was also modeled by introducing an assumption for the distribution of the contact pressure on the crack face. The results were validated with the literature data and finite element analyses. Parametric simulations were performed showing that mode I and mode II stress intensity factors strongly depend on the crack angle with respect to the surface, and almost linearly on the size of the crack. In addition, the proposed algorithm was able to include any residual stress distribution. Compressive residual stress hinders pressurization and promotes the crack closure. This effect was parametrically investigated and it was found that cracks, and especially small ones, can even remain closed, with the opening effect of the pressurization completely suppressed. [Copyright &y& Elsevier]

Abstract: Rolling contact fatigue cracks in railway track called squats are studied in this paper. In the first part, the effects of an elastic foundation (sleepers and the ballast) on stress intensity factors obtained at a crack tip are studied. A simplified finite element model (FEM) and an extended finite element model (XFEM) were created to investigate these effects, the XFEM model being limited in geometrical size, but more able to model crack growth. Both FEM and XFEM confirmed that an elastic foundation leads to an additional bending stress which increases the crack growth rate significantly. Field results also authenticate that squat form cracks appear on timber sleepers more commonly than on concrete ones. These results indicate that considering these bending stresses in a FE model, is important to achieve a more realistic model of squat development. In the second part, a short crack of 250μm length is simulated to investigate how variations of traction ratio (TR), friction coefficient between the crack faces (FC) and the crack angle affect SIFs when the rail is mounted on an elastic foundation. Simulations show that a crack on a rail laid on elastic foundation (clips, sleepers and ballast) can lead to significantly higher SIFs in many conditions and consequently raises crack growth rate. This indicates that foundation stiffness is as important as water entrapment and friction coefficient between the crack faces. [Copyright &y& Elsevier]

Abstract: The purpose was to develop a fracture mechanics based method for determining the life of surface initiated rolling contact fatigue or spalling. The life simulations were based on the asperity point load mechanism, a mode I crack growth assumption and LEFM. The life prediction was verified against the spalling life in some gear teeth, which had been measured for the simulation data. The computational tool required an equivalent mixed-mode life parameter. Such are suggested in the literature and some of these were evaluated. Also, the work required material properties for crack growth at stress cycles with highly compressive minimum loads. An experimental series was performed for crack growth at R <0. Negative crack closure limits K I,cl were suggested by the compliance but not the crack growth rate. Simulations with small negative closure limits predicted the spalling life in the gears. It was however noted that the life predictions depended more on K I,cl than the equivalent mixed-mode life parameter. [Copyright &y& Elsevier]

Abstract: Harfacing welding is a widely used method on severe worn, corroded or oxidized surfaces to regain its functionality. For metal-to-metal sliding or rolling contact applications, in which oxidative wear, subsurface fatigue and adhesive wear are the main wear mechanisms, the materials usually selected are steels with a carbon contents between 0,1 and 0,7% and up to 20% of alloy content, such as martensitic tool steels. Among them, H13 tool steel has a great number of applications. Variables such as sliding speed, load, or contact stress may have decisive influence on wear rates. Laboratory tests like Pin-On-Disk (POD) are frequently used to evaluate the tribologic behaviour of different pairs of material. The objective of this work was to study the influence of the pin geometry (flat and spherical) in a POD test on the wear resistance of a weld-deposited hardfacing of H13 modified steel against low carbon steel AISI 1020 under different load and sliding speed conditions. It was observed an influence of the pin geometry on the wear rates, especially at high sliding speeds. Under light loads, the worn volume of the flat pin was greater, while under heavier loads it is the spherical pin the one with greater worn amount. [Copyright &y& Elsevier]

Abstract: The crack path of surface initiated rolling contact fatigue was investigated numerically based on the asperity point load mechanism. Data for the simulation was captured from a gear contact with surface initiated rolling contact fatigue. The evolvement of contact parameters was derived from an FE contact model where the gear contact had been transferred to an equivalent contact of a cylinder against a plane with an asperity. Five crack propagation criteria were evaluated with practically identical crack path predictions. It was noted that the trajectory of largest principal stress in the uncracked material could be used for the path prediction. Different load types were investigated. The simplified versions added some understanding but the full description with cylinder and asperity pressures was required for accurate results. The mode I fracture mechanism was applicable to the investigated rolling contact fatigue cracks. The simulated path agreed with the spall profile both in the entry details as in the overall shape, which suggested that the point load mechanism was valid not only for initiation but also for rolling contact fatigue crack growth. [Copyright &y& Elsevier]

Abstract: A novel experimental method, previously developed by the authors, has been applied in order to promote Mode III co-planar crack propagation in a bearing steel. The method consists in out-of-phase multiaxial fatigue tests, adopting a stress history typical of sub-surface RCF, onto specimens containing micro-notches that were previously subjected to Mode I pre-cracking at R =−2. Three defect sizes, expressed in terms of Murakami’s √area parameter, equal to 630μm, 315μm and 220μm were considered. The experimental data clearly show that the threshold conditions for out-of-phase tests is lower than in simple torsional tests, thus promoting the shear growth. This enhanced damage mechanism, which cannot be described neither in terms of fatigue criteria nor in terms of crack driving force at the crack tip, is characterised by intense plastic deformation and rubbing of the crack lips with an opening of the crack faces. In order to compare the multiaxial and torsional fatigue test results the effect of this type of opening on the friction of the crack faces will be investigated. [Copyright &y& Elsevier]

Abstract: Uncertainty analysis and parametric studies are presented for estimating the fatigue failure probability of surface cracks in silicon nitride ball bearings subjected to rolling contact fatigue. Uncertainty quantification of input parameters are presented first based on experimental data, inspection capability, and geometric reasoning. Surrogate models for equivalent stress intensity factors are then used for uncertainty propagation, which are built upon high fidelity finite element modeling with half-penny-shaped surface cracks. Instead of black-box type surrogate modeling, physical observations are employed to decompose the high dimensional surrogate model into multiple one-dimensional models. The cross-validation technique is used to find the best surrogate that has the smallest prediction variance. The probability of failure is estimated using Monte Carlo simulation and surrogate models. The parametric studies show that reducing the maximum crack size (by limiting inspection threshold) and increasing the fatigue threshold (by improving fracture toughness of a material) are the most effective ways of reducing the probability of failure. For example, decreasing the maximum crack size by 4.4% and increasing the lowest fracture threshold by 2.8% results in the reduction of probability of failure by 40%. Ball survivability increases with decreasing ball diameter, for a given peak Hertzian stress. In order to apply the current study to hybrid ball bearing design, the survivability results are generalized through non-dimensionalization. [Copyright &y& Elsevier]

Abstract: Rolling contact and cavitation testing were integrated in a testing methodology to study their combined effect on silicon nitride rolling elements. This testing methodology is well suited to perform controlled cavitation and rolling contact experiments. Acoustic cavitation method and a rotary tribometer were utilized to achieve this objective. Silicon nitride degradation initiated with the formation of blisters in rolling contact. This blisters underwent cracking in the subsequent testing, thereby allowing the lubricant to squeeze in due to rolling contact. Erosion pits formed and their density increased along with their growth by detaching grains and bridging into adjacent pits. As the erosion severity in the rolling contact increased the material eventually failed in a short period of time with respect to the testing conditions. The effect of cavitation and rolling contact parameters on material damage is detailed in this paper thereby allowing an evaluation of material systems for cavitation-rolling contact fatigue (RCF) conditions. [Copyright &y& Elsevier]

Abstract: A comprehensive approach is developed for studying the fatigue phenomena (crack initiation and propagation) induced by repeated rolling or rolling–sliding contacts between wheel and rail. Cracks initiate and propagate in the rail head in a complex varying multiaxial stress regime due to Hertzian or non-Hertzian contacts generating 3D residual stress pattern. This paper presents the main steps of such an approach devoted to the modeling of defects induced in the rails by the traffic. Special attention is paid to some of the principal difficulties met as well as to the proposed solutions. Examples of applications for the prediction of initiation as well as propagation of some defects are presented. It is shown that numerical simulations predict very well the locus of crack initiation as well as its propagation in the rail. Our approach presents at least three main originalities: first, it is a global approach starting from the evaluation of the initial state of the rail to the simulation of the crack propagation under complex loading including multiaxial residual stresses. Second, special and original numerical methods for the evaluation of the initial states, the overloads and the elastoplastic state under service loading have been developed. Third, a new concept based on a “structural Paris law” has been developed and used in the crack propagation simulations. [Copyright &y& Elsevier]

Abstract: Rolling contact fatigue crack growth continues to affect many railways worldwide. It is most often controlled through rail grinding in a preventive maintenance strategy, but to plan the required frequency and depth of grinding, prediction of crack growth rates has a vital role. This paper presents crack growth rate results from a new three-dimensional rail model containing an inclined surface breaking rolling contact fatigue crack. The calculations are based on a shear mode of crack growth, driven by the Hertzian contact pressure on the railhead and moderated by friction between the crack faces (“crack face friction”). The results from the model show good correlation with those from the previously published work in the area, with particularly good agreement at higher levels of surface friction coefficient. Applying the new model to a range of surface and crack face friction coefficients predicted that crack growth rate will rise with reduced internal crack face friction at all crack sizes. For small cracks (2 and 5mm radius) rates were predicted to rise with increased surface traction, but this trend was reversed at larger crack sizes (10 and 19mm radius). Identical trends were found when the modelling was repeated using the previously developed half-space based “2.5d” model, indicating that although this older model cannot represent the rail geometry its high speed means it remains a useful tool for investigating the effects of contact parameters on rail rolling contact fatigue. The next study in this area could therefore consider if there is a uniform or crack size related ratio for mapping 2.5d results to three-dimensional rail geometry to produce closer agreement in crack growth rates as well as trends. For the three-dimensional model, consideration of alternative crack morphologies and movement of the contact running band away from the rail centreline would provide additional data on the effect of rail grinding and re-profiling on rolling contact fatigue. [Copyright &y& Elsevier]

Abstract: There has been a considerable body of research concerned with predicting the service life of rolling contact components. It is well known that the prevailing cause of failure in rolling contact components is fatigue crack. Although the trend has been toward the use of surface integrity in fatigue life prediction, one aspect missing in most models has been a consideration of the residual stress scatter in the subsurface. Since the fatigue life of a manufactured component depends on its weakest point, the local extreme is most relevant for the fatigue life prediction. This study thus incorporates the residual stress scatter into the rolling contact fatigue life model to predict the fatigue life closer to the experimental life. It is noted that residual stresses vary significantly at the same depth. After the residual stress scatter is incorporated into the model, the predictions agree well with the experimental lives. [Copyright &y& Elsevier]