Abstract: On the basis of the continuum fatigue damage theory, a new nonlinear fatigue damage cumulative model is proposed by the defined fatigue damage variable by means of the varying behavior of the material ductility in the process of fatigue damage. The fatigue limit, mean stress and the unseparable characteristic for the damage variables and the loading parameter are considered by the proposed fatigue damage cumulative model, and the effect of the loading sequence is also included. The characteristic of the nonlinear accumulation for fatigue damage may be reflected in this model. A fatigue damage evolution equation is derived, which can describe the fatigue damage evolution behaviour well. The recurrence formula of the fatigue damage model is derived under multilevel loading, and the loading case below the fatigue limit is also considered. The fatigue test data for three kinds of material were used to verify the model proposed in this paper. The results show that good agreement is achieved with this model to predict fatigue life, compared with that of Miner's rule.

Key words: nonlinear cumulat ive fat igue damage, material duct ility, damage calculat ion, lifep redict ion