When mechanical structures such as aircraft are used over long periods, random loading can cause crack initiation, potentially leading to fatigue failure. Accurate prediction of fatigue life under random loading is therefore essential for structural design and maintenance scheduling. In this study, we attempted to improve the accuracy of fatigue life prediction while maintaining simplicity by calculating crack propagation life from crack growth rate curves and incorporating crack propagation mechanisms into an equation based on the linear damage accumulation rule. This method utilizes crack opening stress, which is influenced by the loading history of past stress amplitudes, making its accurate estimation crucial for fatigue life prediction. Accordingly, we investigated the effect of compressive stress—applied during aircraft takeoff and landing—on crack opening stress using aluminum alloy A2024-T4, commonly used in aircraft. Crack opening stress was examined through constant amplitude tests and two-step variable amplitude tests. In the constant amplitude tests, it was confirmed that the opening stress decreased when the stress ratio exceeded a certain value. In the two-step variable amplitude tests, when the insertion ratio of compressive stress was 1/11, the behavior was similar to that under constant amplitude with compressive stress; when the insertion ratio was 1/101, the behavior resembled that without compressive stress.
Kobayashi et al. (Wed,) studied this question.