Fretting fatigue damage nucleation under out of phase loading using a continuum damage model for non-proportional loading

Damage nucleation involves creation of micro cracks, which are discontinuities in a material considered as continuous at a larger scale. Continuum Damage Mechanics (CDM) approach provides a tool to study damage nucleation under plane and fretting fatigue conditions. Under fretting fatigue conditions, the loading sequence may produce non-proportional stresses. This paper aims to investigate the effect of non-proportional loading on damage nucleation. For this purpose, a CDM based damage model for non-proportional loading is developed and applied to cylindrical pad and flat specimen configuration. The numerical results are also compared with experimental results from literature. It is found that, taking into account the triaxiality function variation in damage law, improves crack initiation lifetime estimation. In addition, a sensitivity analysis is performed by varying stress range and triaxiality function. It is found that for in phase loading only the stress range affects the initiation life, whereas, for out of phase loading both stress range and triaxiality function affect initiation life, especially at higher ranges.