Effect of local yield strength gradients on fatigue crack propagation

Abstract

A new crack propagation effect is discussed analytically and demonstrated experimentally. It is shown that local one-dimensional variations in yield strength (strength gradients) can effect a major change in the rate of crack growth under conditions of constant amplitude cycled stress intensity at the crack tip as determined by linear elastic fracture mechanics. Rice's path independent integral is combined with certain fracture mechanics arguments to derive a crack propagation law which predicts that crack extension per cycle is modulated by strength gradients if they are present, in proportion to their slope. Experimental data are developed for two aluminum alloys which demonstrate the effect conclusively. The importance of these results as they apply to real engineering components where strength gradients are known to occur because of casting, forging, rolling, joining, etc. is discussed. The strength gradient effect is shown to be discrete and separate from other effects including the influence of local levels of strength.