Environment-sensitive closure and fatigue crack propagation behavior of Al 2090

Abstract

Fatigue crack propagation (FCP) behavior of Al-Li alloys is largely influenced by extrinsic crack closure. The studies on closure have been confined to the relatively benign moist air environment. The synergistic interaction between the aggressive environment and closure on FCP, particularly that in the near-threshold ΔK regime, has often been neglected due to the difficulties in closure measurement in aqueous environments. In the present study, FCP behavior of Al 2090 was examined in moist air, vacuum, and aqueous 3.5 pct NaCl at different R ratios with an emphasis on the understanding of environment-sensitive crack closure mechanisms. The closure in Al 2090 was strongly affected by applied ΔK, R ratio, and environment. The increase in near-threshold da/dN in aqueous 3.5 pct NaCl over that in moist air mostly disappeared after crack closure correction, indicating that environment-sensitive closure was a major determining factor for the FCP rates of high-closure materials. Interestingly, Al 2090 showed a lower closure level in vacuum than in moist air, despite the enhanced fracture surface roughness and reduced FCP rates in low and intermediate ΔK regimes. The increasing R ratio tends to reduce closure contribution and eventually increase the FCP rates of Al 2090 for each environment. The trends observed in the present study show that closure is determined by complex interactions between the environment and R ratio, particularly in a low stress intensity regime.