An elasto-plastic damage accumulation model for fatigue life predication of ductile metals at the yield stress

From the analysis of massive fatigue test data, we find a mismatch between the fatigue life predictions done by stress-life method (SN) and those by strain-life method (epsilon N) around the yield stress of ductile metals. Since the SN and epsilon N methods are widely used in engineering applications, this work aims to explain such mismatch and thereby to address the fatigue life prediction at material's yield stress, at which the material's elastic damage and plastic damage are comparable. Based on a normalized damage concept, we propose an elasto-plastic damage accumulation model, a data-driven approach, to evaluate the fatigue damage at the yield stress. By differentiating the damage caused by the elastic from the plastic, the damage of each loading cycle is formulated as a function of both stress and strain amplitudes to accurately capture the material's response state. With introducing the strain-energy-density based weighting factor, the proposed model can accord well with the classical methods from low-cycle fatigue to high-cycle fatigue. When it comes to the yield stress, the fatigue life estimated by the proposed model compares favorably with the fatigue test data. Therefore, beyond clarifying the mismatch between the classical approaches, the proposed model is expected to improve the accuracy in fatigue damage evaluation of ductile metals at the yield stress.