Damage modeling of gear contact fatigue considering surface property gradients

An elastic-plastic damage model is developed to investigate gear contact fatigue accounting for surface property gradients in mining transmission systems. The material response is described using a damage-coupled constitutive formulation incorporating nonlinear kinematic hardening and isotropic softening. Material and damage parameters are calibrated experimentally, and the elastic-plastic damage model is validated with good accuracy. In this study, the macro crack is assumed to nucleate when the local damage achieves the critical value, D_C = 0.95 in this study, and the evolution of elements with D = D_C is interpreted as the progression of crack propagation. The gear is deemed to have failed by pitting when the number of failure elements in any hardened layer exceeds 4% of the total number of elements on the working tooth flank. The results demonstrate that local degradation of load-bearing capacity induces significant stress redistribution, and stress concentrations appear at the boundaries between zones at different damage levels. The transmission error amplitude may increase by approximately 4.7% once the failure threshold is reached. During crack propagation, damage accumulation increases the effective stress through the reduction of the load-bearing capacity, thereby activating plastic deformation under nominally elastic contact conditions and leading to kinematic hardening and isotropic softening behaviors. This work provides a systematic framework for predicting crack initiation and pitting evolution in gears with surface property gradients, offering potential insights for gear contact fatigue assessment and life prediction.