Wang Xi-Shu, Tan Chang-Hao, Ma Juan, Zhu Xiao-Dong, Wang Qing-Yuan
School of Mechanical Engineering, Chengdu University, Chengdu 610106, China.
Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China.
Materials (Basel). 2018 Sep 13;11(9):1700. doi: 10.3390/ma11091700.
The low cycle fatigue tests on the crack initiation and propagation of cast magnesium alloys with two small holes were carried out by using in-situ scanning electron microscope (SEM) observation technology. The fatigue crack propagation behaviors and fatigue life, which are affected by two small artificial through holes, including the distances between two holes and their locations, were discussed in detail based on the experimental results and the finite element analysis (FEA). The results indicated that the fatigue multi-cracks occurred chiefly at the edges of two holes and the main crack propagation was along the weak dendrite boundary with the plastic deformation vestiges on the surface of α-Mg phase of cast AM50 and AM60B alloys. The fatigue cracking characteristics of cast AZ91 alloy depended mainly on the brittle properties of β-MgAl phase, in which the multi-cracks occurred still at the edges of two holes and boundaries of β-MgAl phase. The fatigue crack initiation position of cast magnesium alloys depends strongly on the radius of curvature of through hole or stress concentration factor at the closed edges of two through holes. In addition, the fatigue multi-cracks were amalgamated for the samples with titled 45° of two small holes of cast Mg-Al alloys when the hole distance is less than 4D (D is the diameter of the small hole).
采用原位扫描电子显微镜（SEM）观察技术，对带有两个小孔的铸造镁合金的裂纹萌生与扩展进行了低周疲劳试验。基于实验结果和有限元分析（FEA），详细讨论了两个小人工通孔（包括两孔之间的距离及其位置）对疲劳裂纹扩展行为和疲劳寿命的影响。结果表明，疲劳多裂纹主要出现在两个孔的边缘，主要裂纹扩展沿着铸造AM50和AM60B合金α-Mg相表面带有塑性变形痕迹的薄弱枝晶边界。铸造AZ91合金的疲劳开裂特性主要取决于β-MgAl相的脆性，其中多裂纹仍出现在两个孔的边缘和β-MgAl相的边界处。铸造镁合金的疲劳裂纹萌生位置强烈依赖于通孔的曲率半径或两个通孔封闭边缘处的应力集中系数。此外，当铸造Mg-Al合金两个小孔呈45°倾斜且孔间距小于4D（D为小孔直径）时，疲劳多裂纹会合并。
