Overview

In Chapter 8, the essential framework and methodology for quantifying the influences of environment on crack growth was described. Here, environmentally assisted fatigue crack growth (or corrosion fatigue) in gaseous and aqueous environments, and its conjoint action with stress corrosion cracking, are considered. Illustrations (constrained by the “windows of opportunity” to a large extent) are drawn from research in the author's laboratory, and will highlight aluminum alloys, titanium alloys, and high-strength steels. The approach follows that used for stress corrosion cracking, and focuses on coordinated experiments and analyses that probe the underlying chemical, mechanical, and materials interactions for crack growth. Linkage of the fracture mechanics based approach to the traditional stress-life (S-N) approach is made to provide a “common basis” for the interpretation and utilization for fatigue data in design, and to address “key (physically based) sources” for variability in S-N data. The various processes, and their inter-relationships, are depicted in the schematic diagrams shown previously in Fig. 8.7. Their incorporation into models for fatigue crack growth, however, is different, and is presented in Section 9.2.

It should be noted that, in corrosion fatigue, manifestations of environmental effects are reflected in a frequency dependence that gives rise to increase in fatigue crack growth rate (per cycle) with decreasing loading frequency that cannot be attributed to concomitant stress corrosion cracking.