Uniaxial Tension And Compression Ratcheting Behavior Of Magnesium Alloys Under Cyclic Load

The use of magnesium alloys in transportation vehicles involves unavoidably cyclic deformations and fatigue as the parts in the vehicles undergo cyclic stresses. From the deformation perspective, the mean stress in stress-controlled fatigue induces ratcheting, which results from the accumulation of plastic strain in the low to intermediate cycle fatigue regime.In this work, AZ31 Mg alloy samples with different microstructures and static mechanical properties were obtained using extrusion and annealing treatment. The effects of extrusion ratio and annealing time on the ratcheting behavior of AZ31 Mg alloy were investigated under asymmetrical uniaxial tension-compression cyclic loading, such as the mean stress is 40MPa, while the stress amplitude is 100MPa. The ratcheting behavior of AZ91D Mg alloy was also studied.The results shown:(1)The mean grain size decreased and the yield strength increased significantly with the extrusion ratio until the extrusion ratio reached 16, and thereafter the extrusion ratio had little effects. The mean grain size reached a minimum value and the yield strength reached a maximum value when the extrusion ratio was 16. The yield strength of AZ31 Mg alloy decreased when the annealing treatments were performed on the alloy.(2)Both of these Mg alloys presented the following characteristic behavior with increasing number of loading cycles:first an apparent cyclic softening was observed, then a cyclic hardening occurred, and finally a stable state was reached. The ratcheting strain of AZ31 magnesium alloy and AZ91D magnesium alloy increased with mean stress and stress amplitude. The load history influenced the ratcheting behaviors of these Mg alloys.(3)The extrusion ratio did not influence the cyclic softening/hardening behavior or the final ratcheting strain variation trend of the extruded AZ31B Mg alloy with the mean stress and the peak stress. However, the extrusion ratio influenced the final ratcheting strain variation trend of the extruded AZ31B Mg alloy with the stress amplitude. Increasing the extrusion ratio also reduced the ratcheting strain and the effects of the load history on the ratcheting behavior of the extruded AZ31B Mg alloy.(4)The ratcheting behaviors of these Mg alloys depended on not only the static mechanical properties, but also the homogeneity of grain size. Higher yield strength and more homogenous will decrease the ratcheting strain. Influences of the annealing treatments influenced the ratcheting behaviors of AZ31 Mg alloy were very slightly.(5)Constitutive equations between the final ratcheting strain and the applied loading were built with the least-square method. The predicted agreed with the experimental.