A general damage equation for low cycle fatigue life prediction at elevated temperature

Abstract

In addition to this improved technique for predicting hold time and frequency effects, which is similar in approach to existing theories, a completely new approach to low cycle fatigue failure prediction at elevated temperature is developed. A differential equation representation of fatigue damage is proposed for uniaxial conditions such that cyclic life can in principle be predieted for arbitrary strain (stress) controlled fatigue cycling.

A new method is developed for predicting the effect of hold time and frequency on elevated temperature fatigue life. This follows from a stress-strain damage function which is proposed as a new measure of fatigue damage. This damage function is extended to predicting the effeot of hold time and frequency through a variation of Coffin's frequency modified approach. It is shown to have good success in correlating available strain controlled fatigue test results.

Initial comparisons between the failure relations developed from this damage equation and available experimental fatigue data are good. The damage equation is shown to be capable of including the important elevated temperature effects of mean strain (stress), strain (stress) rate, and hold time.