Crystal plasticity modelling of strain incompatibility and localized damage in fretting fatigue

Although Inconel 718 superalloy has excellent mechanical properties, its damage resistance under fretting contact stress is largely determined by localized defect accumulation at the tribological interface. Among the most important interfacial variables, surface topographical asperities and macroscopic friction tractions decisively influence the spatial damage distribution and dislocation evolution during fretting. To simulate this microstructural damage development, a crystal plasticity finite element model was developed, which explicitly incorporates the macroscopic surface roughness profile. The results show that the macroscopic contact stress field is significantly modulated by local microstructural heterogeneity. As a result, fatigue damage does not exactly follow the classical Hertzian prediction, but accumulates preferentially at soft-hard grain boundaries due to strong kinematic incompatibility. Higher surface roughness drives the local accumulation of geometrically necessary dislocations (GNDs), while higher macroscopic frictional tractions induce extensive intragranular multiplication of statistically stored dislocations (SSDs). Furthermore, the local reduction and relaxation of plastic strain gradient in surface grains leads to the complete exhaustion of local strain-hardening capacity. Together with the rapid increase in local SSDs, these micromechanical findings establish a fundamental thermodynamic criterion for predicting the transition from subsurface fatigue to the early detachment of wear particles in demanding tribological systems.