Critical factors for determining a first estimate of fatigue limit of bearing steels under rolling contact fatigue

This article investigates the variables that influence the fatigue limit of bearing steels. The experimental evaluation of the fatigue limit of rolling element bearings is a difficult, time consuming and an expensive process. Hence, an analytical method is desirable. Modern bearing steels have a complex microstructure with various non-metallic inclusions, which act as local stress risers and may cause the matrix to accumulate micro-plastic strain. This eventually results in the nucleation of fatigue micro-cracks and ultimately bearing failure. Therefore, one interpretation of the fatigue limit is as a macro-scale stress below which no local yielding occurs. Such a fatigue limit will be dependent upon the mechanical properties of both the matrix and the inclusions as well as the inclusion geometry and distribution. A finite element analysis (FEA) based study was conducted to determine the critical factors that have the largest impact on the fatigue limit of bearing steels during rolling contact fatigue. The effects considered include static vs rolling contact, inclusion geometry (size, shape, orientation, and distribution), the temperature dependence of the mechanical properties of both the matrix and the inclusion (elastic modulus, thermal expansion coefficient, and yield strength), and the possibility of an inclusion becoming debonded from the matrix. The results of this study showed that a static contact model instead of rolling contact model is sufficient to capture the fatigue limit in the case of hard, stiff inclusions. The stress concentration due to debonded inclusions was much more dramatic than any of the other terms considered followed by the change in the elastic modulus due to temperature. The effect of the geometry (size, shape, and orientation) of an individual stiff inclusion on the fatigue limit was found to be relatively minor.