Cumulative damage laws for fatigue crack initiation and stage i propagation

Abstract

A simple model for the demarcation of the initiation and stage I fatigue crack growth phases is introduced. Experimental cumulative damage data are used to determine the phase boundary between these two régimes, and an exponential accumulation of damage theory is invoked for each; here, damage is equated to crack length. Using both the model and the data, predictions are made of lifetime in double, treble and multilevel strain range tests. The theory successfully accounts for the load sequence effect, which is a consequence of the initiation period, and the effect of the difference between successive strain range levels. It is suggested that the Palmgren-Miner expression be replaced by individual expressions for the accumulation of damage in the separate phases of initiation and stages I and II fatigue crack propagation.