Effect of cyclic crack opening displacement rate on corrosion fatigue crack velocity and fracture mode transitions for Al–Zn–Mg–Cu alloys

• Time-domain analysis of aluminum alloy CF data using a cyclic COD-rate equation.
• Cyclic COD-rate shown to be the rate controlling mechanical variable.
• Cycle-dependent TCF and cycle-time-dependent C-SCC components identified.
• Fracture mode transitions modeled using a critical hydrogen model.

A time-domain analysis of corrosion fatigue data obtained by Holroyd and Hardie (1983) on Al–Zn–Mg–Cu alloy 7017-T651 shows that there is a correlation between Stage II crack velocity and cyclic crack opening displacement (COD)-rate. This correlation holds for all cyclic frequency-ΔK data combinations and fracture modes, suggesting that cyclic COD-rate, which is a function of both frequency and ΔK, is a more fundamental crack driving force variable. A diffusion-limited “critical hydrogen” embrittlement model is developed to rationalize effects of cyclic frequency on corrosion fatigue crack velocity and fracture mode transitions.