Prediction of S-N curves at various stress ratios for structural materials

Predictive models in the past for stress ratio effect on S-N fatigue life have been subject to modifications since the 19th century. Many of them have been developed with limited experimental verifications or beyond the theoretical limit. Also, they have employed weak S-N curve models for the development, resulting in inefficiency for applications. There has been, as such, a gap in the development of predictive models between S-N curve behaviour and stress ratio effect. In this paper, an efficient procedure is proposed for predicting S-N curves at different stress ratios. The characteristics of the constant fatigue life (CFL) diagram were clarified for theoretical capability and limitations, and dependence of experimental fatigue behaviour. The Kim and Zhang S-N curve model was dovetailed with the linear CFL lines of the four segments for predicting S-N curves for the whole range of stress ratios. With the benefits of damage parameters in the Kim and Zhang S-N curve model, analytical expressions for CFL as functions of two independent variables (i.e. applied stress and stress ratio) were derived for theoretical predictions. The theoretical predictions were found to be in agreement with the experimental results.