Comparison of the growth of individual and average microcracks in the fatigue of Al-SiC composites

In reliability predictions, the driving force for crack growth is often taken to be proportional to the product of the applied stress and crack length to the one-half power as in the Paris equation. On the scale of microcracks, the Paris equation appears to persist, at least in the short term, for the growth of individual microcracks. Yet, when evolving distributions of microcrack lengths are analyzed, the fact that the lognormal distribution best fits those of the smaller crack lengths suggests that the growth of all microcracks cannot be described by the Paris equation. These aspects of the microcrack growth problem are explored in this study of what controls the growth behavior of fatigue microcracks on the surface of smooth specimens of a 2124 Al-20 vol pct SiCw composite. Experimental observations of individual microcrack growth are reported and correlated with the distribution data. A Monte Carlo simulation constructed to replicate the observed distributions is presented to assist quality assurance inspection scheduling and provide insight from a probabilistic standpoint. This work indicates that conventional notions of long crack growth cannot be applied directly to microcracks due to the intermittent pattern of their growth. The growth behavior of the averaged crack, however, can be described by a single Paris equation when arrest and coalescence are separately considered. As a result, for reliability purposes, it appears feasible to base microcrack growth predictions on the growth of the average crack and the dispersion of the experimental distribution,