镁合金作为21世纪"绿色"高强轻质结构材料，其多轴应力作用下循环变形和疲劳失效机制的研究几乎是空白。本项目针对镁合金进行多轴疲劳与损伤演化的试验与理论研究。以典型挤压镁合金为研究对象，通过系统的多轴疲劳试验，考察镁合金不同热处理状态下的疲劳和循环变形行为。结合开发的原位在线观测技术及多种测试分析方法，探讨材料宏观循环变形行为与微观结构演化之间的关系，阐明镁合金多轴应力下的疲劳损伤演化机理和循环变形机制。采用BP神经网络建立疲劳失效分析模型，将宏观表象与微观机理联系起来，揭示微观结构对疲劳失效的影响。基于临界面法，考虑微观结构效应，建立适用于镁合金的多轴疲劳损伤模型，实现疲劳寿命及裂纹行为的预测。该研究成果为提高镁合金的疲劳性能提供可靠的试验数据，推进对密排六方晶体结构材料的循环变形与疲劳失效机制的认识，更为镁合金在工程实际中的可靠使用及其结构件的安全设计提了必要的理论依据和试验基础。

To reduce fuel consumption and greenhouse gas emissions, lightweight magnesium alloys with high specific strength and specific stiffness are potential structural materials for the next generation of transportation vehicles in the automotive and aerospace industries. Structural components the aircraft and automobiles usually undergo multiaxial loading. However, multiaxial cyclic deformation and fatigue damage mechanism of magnesium alloys have not been well studied. The overall goal of the proposed project is to experimentally investigate the multiaxial cyclic deformation and fatigue damage evolution of magnesium alloys at both macro and micro scales, and to theoretically develop multiaxial fatigue criteria to predict fatigue life and cracking behavior. This study not only has important theoretical significance, but also a vital practical value. .The proposed research will explore the multiaxial cyclic deformation and the fatigue damage evolution of magnesium alloys through experimental investigation and theory analysis. Fatigue experiments will be conducted on magnesium alloys AZ31 and AM30 using smooth thin-walled tubular specimens and plate specimens in ambient air. The research project has the following major tasks:.Ⅰ. Experimental exploration of the effects of loading path, loading magnitude, and high-low sequence loading on cyclic deformation, fatigue life, and cracking behavior. The basic loading paths include fully reversed strain-controlled tension-compression, cyclic torsion, proportional axial-torsion, 45 out-of-phase, and 90-degree out-of-phase axial-torsion. .Ⅱ. Experimental exploration of the microscopic mechanisms governing cyclic deformation and fatigue processes. The slip patterns, the formation and evolution of mechanical twins and dislocation substructures and their interaction will be observed by in-situ monitoring technique, OM, SEM, AFM and TEM. EBSD and XRD will be conducted for the analyses of the evolution of texture and misorientation distribution due to cyclic deformation. The results will be used to characterize and clarify the microscopic mechanisms governing macroscopic cyclic deformation behavior. .Ⅲ. Influence of microstructure on cyclic deformation and fatigue of magnesium alloys. Different heat treatment processes, such as solid solution, quenching, and ageing, will be further adopted to investigate the influence of heat treatment and the associate microstructure on cyclic deformation and fatigue of magnesium alloys. An artificial neural network (BP model) is trained to predict macroscale parameters (fatigue life) for any given microstructure and evaluate the effect of microstructures on fatigue damage. .Ⅳ. Development of multiaxial fatigue damage criteria based on the critical plane approach to predict fatigue life and cracking orientation. A general "unified" multiaxial fatigue criterion will be developed to consider the influence of microstructure on fatigue life.