Behavior of plasticity-induced crack closure and roughness-induced crack closure in aluminum alloy

For center-cracked tension specimens subjected to constant amplitude loading, fatigue crack closure and crack growth behavior in an aluminum alloy 7050-T7451 were investigated. The specimens were tested with stress ratios of −1.0, 0.0, 0.1 and 0.3 under constant amplitude loading. Plasticity-induced crack closure and roughness-induced crack closure were found to be the main tools for explaining the crack growth behavior under plane strain and plane stress conditions. From a measurement of roughness height distribution of fractured surfaces using a surface profiler, the roughness-induced crack closure was evaluated as the main factor at low stress intensity factor range ΔK. This closure phenomenon was qualitatively predicted using a single asperity model. On the other hand, the plasticity-induced crack closure was the decisive factor at high ΔK and could be predicted by the modified strip-yield model based on a Dugdale model. In the case of concurrence of two crack closures, therefore, the total crack closure behavior with ΔK level for four stress ratios could be described effectively by two closure models, a single asperity model and a strip-yield model.