Effect of cycle mean strain on small crack growth in Alloy 718 at elevated temperatures: Rosenberger, A. H. and Ghonem, H. Fatigue Fract. Eng. Mater. Struct. (Nov. 1992) 15 (11) 1125–1139

Abstract

High frequency (80 Hz) fatigue tests have been conducted at ∼22°C on aged 8090 AlLi alloy plate in the as-received T8771 condition. Single edge notched specimens were used with a T-L orientation. Crack propagation rates were studied at an R-ratio of 0.5, under rising ΔK conditions, while testing in desiccated air and three freely corroding aqueous environments; distilled water, 0.6 M NaCl, and 1.0 M AlCl3. Particular attention was directed towards behavior near the cyclic crack rate threshold and in stage 1. The fatigue tests were supplemented by electrochemical potential studies and detailed fractographic observations using optical microscopy and scanning electron microscopy techniques. The major role of the aqueous environments was to promote S-L splitting (delamination) at grain boundaries, with a subsequent effect on the stress state at the crack tip. The lowestΔKth, 1.05MPa m12, was obtained in the AlCl3 solution and this environment caused sufficient splitting at the threshold that plane stress conditions played an important role in events at the crack tip. In all the other test environments, the ΔKth was ∼1.7 MPa m12, no splitting was present at the threshold and the crack tip was under plane strain conditions. Increasing the ΔK affected splitting in all the aqueous environments, producing a peak in the number of splits between 5 MPa m12and 6 MPa m12, and causing a sufficient amount of splitting that the crack tip was placed under plane stress conditions throughout Stage 2. Consistent with this, Stage 2 cyclic crack rates, da/dN, were very similar in all the aqueous environments, where they obeyed a power law relationship, da/dN∝(ΔK)4. In contrast, splitting effects were insignificant in desiccated air and the crack tip remained under plane strain conditions throughout the test, leading to out of plane cracking and ridge formation in the mid-thickness of the specimen, and resulting in severe growth retardation at ΔK values above ∼3.0 MPa m12. Analysis of the observations led to the conclusion that the S-L splitting phenomenon is associated with both localized anodic dissolution processes and hydrogen embrittlement effects, with solute segregation probably playing a contributory role.