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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]

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: 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: 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]