Fatigue life prediction under conditions where cyclic creep–fatigue interaction occurs

A stress-based model is proposed for correlating data and predicting fatigue life under conditions where cyclic creep–fatigue interaction occurs. This model is an extension of the Basquin’s stress–life relation and considers that both the fatigue strength coefficient σf′ and fatigue strength exponent b are sensitive to mean stress, both decreasing as an exponential function of the ratio of mean stress-to-stress amplitude. By combining the respective mean-stress functions with the Basquin relation, an equivalent stress–equivalent life relation is derived which is very similar to the original Basquin equation. Thus, the Basquin equation is a particular case of the present model for the case where mean stress is zero. The model depends on a single parameter λ, known as the mean stress sensitivity factor to cyclic loading, and depends on material and probably on test conditions as well. The mean stress sensitivity factor may be determined from experimental data at fixed ratio of mean stress-to-stress amplitude, other than zero, or by trial-and-error method to fit experimental mean-stress data onto the S–N curve due to zero mean stress. The model is tested on published creep–fatigue data of copper, steels and β-Ti-alloy and agreement is found to be very good.