Micromechanical analysis of gear fatigue-ratcheting damage considering the phase state and inclusion

Despite the fact that high-quality steels are manufactured in recent years, non-metallic inclusions such as Al₂O₃ are still inevitable during steel manufacturing process. Early failures of bearings and gears are widely associated with the non-metallic inclusions, especially in some heavy duty machines such as wind turbines, ships and high-speed rails. Inclusions commonly act as a stress raiser, forming the micro-crack at the matrix material, which severely limits the service performance of heavy-duty components. Besides, the phase component of such steels represents a typical multiphase state, which makes the inclusion-induced failure mechanism still remain unclear. In this study, the gear rolling contact fatigue (RCF) performance is evaluated on the micro level by considering the effect of the inclusion and different phase states. The crystal plasticity framework is implemented to model different phase states, describing the stress-strain response on slip systems. Fatigue damage and ratcheting damage are both considered in a local area surrounding the subsurface inclusion. The influence of different phases, including martensite and austenite, around the inclusion on the damage accumulation are also investigated. The simulation results reveal that the fatigue damage and the ratcheting damage surrounding the inclusion occur at different areas. The inclusion induces the ratcheting damage at the neighboring matrix within a highly localized area. The higher ratcheting damage is mainly found following the direction of approximately 45° with respect to the rolling direction. Besides, the different contributions of subsurface inclusions and grain boundaries on the ratcheting damage are found.