Strength-probability-time diagrams using power law and exponential kinetics models for fatigue

Abstract

Strength-Probability-Time (SPT) diagrams provide an intuitively pleasing method for presenting reliability data based on extrapolations from accelerated fatigue testing data. If power-law crack growth kinetics are assumed the calculations required to generate the SPT diagram are particularly simple. However, if exponential or other more complex forms are used, this is not the case. If the accelerated data are for dynamic fatigue measurements (strength as a function of stressing rate) the SPT diagram can only be determined after numerical integration of the crack growth equations, followed by non-linear regression to the fatigue data. However, we have developed software to perform this task. In this paper we describe the methods used and show sample results of lifetime predictions using SPT diagrams. Also, the effect of using different crack growth kinetics models on predicted lifetimes is discussed.