A STATISTICAL MODEL OF INTERMITTENT SHORT FATIGUE CRACK GROWTH

The stochastic nature of the growth of short surface fatigue cracks under uniform cyclic loading can be attributed to their interaction with a stochastic microstructure. In this paper, a computational scheme is proposed to describe the statistical effects of the temporary arrest of short cracks by grain boundaries, and the modification of rates of propagation across grains by various effects of grain boundary constraint. The fact that a crack tip may be in one of two states, either temporarily arrested at a grain boundary or actively propagating, is treated explicitly by considering probability densities evolving in complementary state spaces corresponding to each case. The evolution of the densities during fatigue is described by conservation equations, which are solved numerically. It is shown how engineering quantities, such as the time to initiation of a macroscopic crack, can be derived from the calculated densities. The explicitness with which the characteristics of short crack growth are treated greatly enhances the potential accuracy of predictions based on this approach. The theory is constructed to allow considerable flexibility in the microstructure-related mechanisms assumed to control growth rates in any given material. However, as it is stated here, the theory is restricted to Mode I (tensile opening) transgranular crack growth under constant amplitude cyclic loading. Examples are given of the various effects that can be treated, as well as those that would require modification of the structure of the computational procedure. The theory is illustrated by an application to data for Al 2219-T851.