采用挤压AZ31B镁合金薄壁圆筒试样, 分别进行了单轴和多轴加载下的对称应变控制疲劳实验,研究了不同加载路径对疲劳寿命的影响. 单轴加载包括对称拉压和扭转路径, 多轴加载包括45°比例加载和90°非比例加载路径. 结果表明, 在加载的等效应变幅值为0.3%-0.55%附近, 4种加载路径下的应变-寿命曲线均出现了不连续的拐点;比例加载路径在等效应变幅大于0.45%时疲劳寿命最高, 拉压路径在等效应变幅小于0.45%时疲劳寿命最高; 非比例加载路径的疲劳寿命最低. 使用基于临界平面法的多轴疲劳模型FS,SWT以及修正SWT分别预测了各个路径加载下的疲劳寿命. 预测结果表明,  SWT模型对于拉压和循环扭转加载下寿命预测结果误差较大; FS模型与修正SWT模型可以较好地预测挤压AZ31B镁合金各个路径加载下的疲劳寿命.

Magnesium alloy components were widely used in automobile and aircraft industries,due to their light weight, high specific strength, stiffness, damping capacity, machinability, and recyclability. Engineering components subjected cyclic loading inevitably and led to fatigue failure. Most studies on magnesium alloy were focus on uniaxial fatigue, very limited work has been done of magnesium alloys under multiaxial loading. In this study, strain–controlled multiaxial fatigue experiments were conducted on extruded AZ31B magnesium alloy using thin–walled tubular specimens in ambient air. Four loading paths, including fully reversed tension–compression, cyclic torsion, 45^? in–phase axial–torsion and 90? out–of–phase axial–torsion, were adopted in the fatigue experiments. It is observed that the strain–life curve displays a distinguishable kink under each loading path at the equivalent strain amplitude around 0.3% to 0.55%. The fatigue life under the proportional loading path is the highest when equivalent strain amplitudes higher than 0.45%, and the fatigue life under the tension–compression loading path is the highest when equivalent strain amplitudes lower than 0.45%. For the same equivalent strain amplitude, fatigue life under nonproportional loading resulted in the shortest fatigue life. Three critical plane multiaxial fatigue criteria were employed to predict fatigue life. Predictions by Smith–Waston–Topper (SWT) parameter do not agree well with the fatigue life for the tension–compression and cyclic torsion loading, and 76% predicted results are within factor–of–five boundaries. The Fatemi–Socie (FS) parameter and a modified SWT parameter are found to be able to predict fatigue lives reasonably well for all loading paths, and 95% predicted results are within factor–of–five boundaries. In addition, crack initiation of extruded AZ31B and AZ61A magnesium alloy based on experimental observation were discussed to explain prediction results vary much for the same multiaxial fatigue criterion between the two materials. It was demonstrated that AZ31B magnesium alloy and AZ61A magnesium alloy has different damage mechanism due to different microstructures. Optical microscopy observations exhibited lamellar twinning exist in a little big elongated grains with an average grain size of 50 μm in extruded AZ31B magnesium alloy at strain amplitude of 1%. In the same situation, mechanical twins were observed in almost every equiaxed grain with an average grain size of 20 μm in extruded AZ61A magnesium. Ex–situ SEM microscopic observation of the microstructure evolution showed fatigue micro–cracks were at the grain boundaries or slip bands in extruded AZ31B magnesium alloy, while at twin boundary in extruded AZ61A magnesium alloy.