Some fractographic contributions to understanding fatigue crack growth

Abstract Some fractographic and metallographic contributions to understanding fatigue crack growth in metallic materials are reviewed, with an emphasis on environmentally assisted fatigue. The formation of ductile and brittle striations at intermediate-to-high ΔK, where each stress cycle produces a striation, is reasonably well understood at the microscopic level, but the details of the dislocation activity involved are still controversial. The evidence suggests that, in fairly benign environments, egress of dislocations around crack tips predominates, whereas environmentally assisted fatigue crack growth of many materials in liquid-metal, hydrogen, and aqueous environments involves an adsorption-induced dislocation-emission (AIDE) process at crack tips. There is also fractographic evidence that nanovoids often form just ahead of crack tips at intermediate-to-high ΔK in all environments. The effects of variables such as solution composition, electrode potential, and cycle frequency can be qualitatively explained largely in terms of surface-reaction kinetics and adsorbed-hydrogen coverage in many cases, but a quantitative understanding is lacking. Fatigue crack growth at low ΔK, even in inert environments, is not well understood, and only speculative explanations can be offered.