Stress-based fatigue life prediction models for AZ31B magnesium alloy under single-step and multi-step asymmetric stress-controlled cyclic loadings

The uniaxial ratcheting and low-cycle fatigue failure behaviors of the hot-rolled AZ31B magnesium alloy are studied by the asymmetric cyclic stress-controlled experiments at room temperature. The effects of loading histories with the variable mean stress, stress amplitude and stress rate on the ratcheting respond and fatigue life of the studied magnesium alloy are discussed. Considering the combined effects of ratcheting damage and fatigue damage on the material failure, the stress-based fatigue life prediction models are developed to evaluate the low-cycle fatigue life under the single-step and multi-step cyclic loadings. Results show that (1) the prior stress cycling with high mean stress or stress amplitude restrains the ratcheting strain in the subsequent cycling with low mean stress or stress amplitude. (2) Due to the rate-independent property of the studied material at room temperature, the effects of the loading history with variable stress rate on the ratcheting behavior are not obvious. (3) The comparisons between the measured and predicted results confirm that the developed model can give an accurate estimate of the low-cycle fatigue life for the hot-rolled AZ31B magnesium alloy under the single-step and multi-step asymmetric stress-controlled cyclic loadings.