Crack closure effects on fatigue damage ahead of crack tips

Elber's assumed long ago that the effective stress intensity factor (SIF) range Delta K-e(ff )= K-max- K-op is the actual driving force for fatigue crack growth (FCG), where K op is the SIF that fully opens the crack, and his idea still is widely used to predict residual lives of cracked components. However, although crack closure can affect the FCG process, the Delta K-e(ff) idea cannot explain many of its peculiarities. To try to understand why this happens, the actual K-op role in FCG is questioned comparing Delta K-e(ff)-based predictions with similar predictions obtained using an alternate model that estimates crack increments assuming they are caused instead by the accumulated damage ahead of the crack tip. To be fair, this damage is calculated by the very same strip-yield mechanics used to calculate K-op and Delta K-e(ff), i.e. the deformations predicted by the strip-yield model are used to describe the cyclic strain field ahead of the crack tip as well. Hence, the main goal of this exercise is to compare two different hypotheses for the actual FCG driving force using the same formulation basis. Both models are tested for different materials, constraint factors, and stress to yield strength ratios combinations, considering and neglecting the effect of crack closure. This exercise indicates that the effects of crack closure predicted by the critical damage model can be significantly lower than those predicted by the Delta K-e(ff) model.