Mapping of the rate-controlling steps for environment-assisted fatigue crack growth applied to the aluminium alloy 6013 T6

A formalism for the identification and mapping of the rate-controlling steps of environment-assisted growth of long fatigue cracks is presented. The formalism is based on the assumption that the slowest one of several successive steps determines the crack growth rate. The method is worked out for the special case of a number of three potential rate-controlling steps characterized by physical quantities of postulated physical dimensions. Asymptotic analytical expressions for the dependence of the crack growth rate on stress intensity factor range, ΔK, and frequency, f, are derived essentially by means of dimensional analysis. This analysis depends on a basic understanding of the underlying processes but it works without full knowledge of the details. A map in the f–ΔK plane displaying the regions of rate control of the individual steps is obtained by comparison of the analytical expressions. The map is fitted to the experimental data measured for the thin sheet aluminium alloy 6013 T6 in an aqueous NaCl solution. The physical relevance of the quantities specified by the fitting procedure and the potential predictive capability of the model are discussed.