Nonlinear Fatigue Life Prediction Based on a Plastic Strain Amplitude Assumption

Variable amplitude fatigue design is commonly performed by using S-N curves and the linear damage rule known as Palmgren-Miner’s rule. The problem with this methodology is that the linear damage rule does not consider the loading sequence effect. Although a number of theories and models for fatigue damage accumulation has been proposed, the linear damage rule is still widely used in the standards. Hence, further research towards fatigue damage modelling is of high importance to predict the fatigue capacity of structures subjected to variable amplitude more accurately. Herein, it will be shown that the exponent of a previously proposed nonlinear fatigue damage model, based upon the applied stress amplitude and fatigue limit can be considered as the plastic strain amplitude of the surface material. An improvement of the model is thereafter proposed, where the fatigue limit plastic strain is also considered. Subsequently, an initial verification of the proposed improvement was performed, and a discussion regarding further research is presented.