Finite Element Modelling of Deep Zone Residual Stresses in Rolling Contact Bearing Elements.

Abstract

Bearing elements during service are subjected to rolling contact fatigue (RCF) resulting in bearing material evolution and development of residual stresses under the contact track with progressive stress cycles. The evolved bearing material contains different microstructural features, termed as dark etching regions (DERs) and white etching bands (WEBs); visible after Nital etching. Intensive work has been carried out in the past few decades to define these microstructural changes along with mechanical properties evolution, however, its formation mechanism and relation to residual stresses are still debatable and require further understanding. Current research aims to develop a finite element model (FEM) to investigate deep zones residual stresses incorporating a semi- empirical material model. The cyclic hardening parameters are determined from spatial and temporal analysis of localised hardness change in evolved bearing material microstructure. This micromechanical response is incorporated into a non-linear isotropic kinematic (NIKH) hardening model as an input parameter for FEM. To understand the formation mechanism and development of microstructural alterations in bearing elements, a systematic approach has been used to conduct the RCF test. The bearing elements are RCF tested in a high-speed rotary tribometer under varying operating conditions. State-of-the-art methodologies have been employed to deliver novel outcomes to bridge the gaps in knowledge within the context of DERs/WEBs formation and in- depth understanding of its complex multidisciplinary mechanisms in terms of tribo- mechanics, thermo-mechanical and chemical subsurface interactions. The post-RCF subsurface investigations have been carried out with optical microscope, white light 3D interferometer, scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS) and Nanoindentation. Results from FEM and post RCF analysis have been compared to correlate the progression of residual stresses with microstructural alterations. The extensive microscopic results of DERs are quantified in terms of 3D DER% Maps as a combined function of temperature, contact stress, and rolling cycles. Moreover, the microstructural features i.e., lower angle bands (LABs) and higher angle bands (HABs) have been characterised and respective formation mechanism has been discussed. It is proposed that the carbon migration via dislocation motion cannot anticipate the development of microstructural changes and that the effect of thermal diffusion and cyclic plasticity should be included when modelling the formation of DERs/WEBs. These novel findings constitute significant contributions to new knowledge and will be impactful for wider bearing manufacturing industries.