Effect of periodic overloads on threshold fatigue crack growth in Al-alloys

Experiments were performed on 10 mm thick C(T) specimens cut from Al-alloys to assess fatigue crack growth behavior under periodic overloads by comparison to constant amplitude loading. The tests were performed at a sufficiently high stress ratio to avoid crack closure, whose absence was confirmed from Load-COD response. Delta K-th values were determined under these loadings in air, salt water, and vacuum. Even though closure was absent, Delta K-th shows a systematic dependence on overload plastic zone ratio. Similar results were obtained in both air and salt water. In vacuum however, near threshold crack growth is accelerated by comparison to no-overload conditions, while higher growth rates see a reversal in this trend. These results appear to support the possibility that residual stress moderates crack-tip surface chemistry in near-threshold fatigue, an effect that ceases in vacuum.