Abstract: The whole-life ratchetting and fatigue failure behavior of cast AZ91D magnesium alloy, along with their dependence on stress level, are investigated through a series of uniaxial symmetric strain-controlled and asymmetric stress-controlled fatigue tests at room temperature. The experimental results indicate that the AZ91D alloy exhibits significant cyclic hardening with a higher cyclic hardening observed at higher strain amplitude. During the stress-controlled cyclic loading, the AZ91D alloy presents pronounced ratchetting. At higher mean stress, the ratchetting is strongly influenced by both the mean stress and stress amplitude, whereas at lower mean stress, it is only dependent on the mean stress. Furthermore, an increase in mean stress or stress amplitude leads to a decrease in fatigue life. However, when the peak stress remains constant, the fatigue life increases with increasing stress ratio. Meanwhile, the investigation on the ratchetting-fatigue interaction of AZ91D alloy reveals that the ratchetting could promote fatigue damage, resulting in a reduction in fatigue life. However, the failure of the alloy is primarily governed by fatigue damage, as the ratchetting strain at fracture is not sufficiently excessive to induce significant ratchetting damage.

Key words: cast magnesium alloy, ratchetting, stress level, low-cycle fatigue, failure mechanism