A Study On The Low-cycle Fatigue Failure Behaviors Of AZ31B Magnesium Alloy Under Asymmetric Loadings

Due to the high specific strength and fatigue strength, and fine cutting performance, etc, Magnesium alloys are widely used in the automobile, aviation, electronics, and aerospace industries. During the actual service process, magnesium alloys and their structural components are inevitably subjected to the cyclic loadings. When materials and structures are subjected to a cyclic loading with the high non-zero mean stress, a cyclic accumulation of inelastic deformation will occur, which is called ratcheting. Ratcheting effect produces not only undesirably large deformations, but also fatigue damage in the material. It is significant to investigate the fatigue behavior of magnesium alloy for life assessment and reliability design. In this dissertation, uniaxial ratcheting and low-cycle fatigue failure behaviors of the hot-rolled AZ31B magnesium alloy were studied. A constitutive model was develeped to describe the uniaxial ratcheting behavior of AZ31B magnesium alloy at room temperature. The failure mechanism under the combined function of the ratcheting effect and low cycle fatigue is analyzed. The results and achievements of the dissertation are as follows:(1) 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. Results show that when the mean stress or stress amplitude is increased, the ratcheting strain and its rate rapidly increase and the fatigue failure life is reduced. The low-cycle fatigue failure life is significantly affected by the ratcheting strain. Loading histories have great influence on the ratcheting respond and fatigue life of the studied magnesium alloy, the previous stress cycling with high mean stress or stress amplitude restrains the ratcheting strain in the subsequent cycling with low mean stress or stress amplitude. 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.(2) Within the framework of unified viscoplasticity, a model is established on the Ohno-Wang II kinetic hardening rule to describe the uniaxial ratcheting behavior of AZ31B magnesium alloy at room temperature. A modified model is proposed to improve the prediction accuracy. In the modified model, the parameter mi of the Ohno-Wang â…¡ kinematic hardening rule is improved as a function of the maximum equivalent stress. The predicted results by the modified model well agree with experiment results.(3) The failure mechanism under the combined function of the ratcheting effect and low cycle fatigue is analyzed through microstructure analysis experiment. Result show that the twinning can decrease the fatigue resistance. When the mean stress and stress amplitude are increased, the number of twins in large grain increases. So, the fatigue failure life is reduced. The stress rate has little effect on the twinning. A new model is established to describe the relationship between the linear density of twins and the fatigue parameter, the absolute average error is only6.93%.(4) Considering the combined effects of ratcheting damage and fatigue damage on the material, 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. 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. The value of the standard deviation (SD) of prediction errors for the developed stress-based fatigue life model is less than half of other models.