Masuda Kenichi, Ishihara Sotomi, Oguma Noriyasu, Ishiguro Minoru, Sakamoto Yoshinori
Department of Mechanical Engineering, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan.
National Institute of Technology, Toyama College, Toyama 939-8630, Japan.
Materials (Basel). 2022 Jan 19;15(3):755. doi: 10.3390/ma15030755.
Fatigue crack growth (FCG) experiments were performed using a low-temperature extruded magnesium alloy AZ31 with texture. Under a constant maximum stress intensity factor (K), the stress ratio R was changed from 0.1 to -1 during the fatigue crack growth process, and the FCG behavior before and after the R change was investigated. As a result, tensile twins were generated owing to the fatigue load on the compression side of R = -1, and the FCG velocity was accelerated. In addition, when the maximum compressive stress at R = -1 (|(σ)|) exceeded the compressive yield strength of the material (σ), the FCG velocity after R fluctuation greatly accelerated. On the other hand, under the condition |(σ)| < σ, the degree of acceleration of the FCG velocity due to R fluctuation was small. In either case, the degree of acceleration in the FCG increased as the K value increased. The above FCG acceleration mechanism due to the R fluctuation was considered based on the observation of the deformation and twinning states of the fatigue crack tip, the fatigue crack closure behavior, and the cyclic stress-strain curve of the fatigue process. The FCG acceleration mechanism was as follows: First, the driving force of the FCG increased owing to the increase in crack opening displacement due to the generation of tensile twins. Second, the coalescence of the main crack and a plurality of microcracks were generated at the twin interface. The elasto-plastic FCG behavior after the stress ratio fluctuations is defined by the effective J-integral range ΔJ.
采用具有织构的低温挤压镁合金AZ31进行疲劳裂纹扩展（FCG）实验。在恒定的最大应力强度因子（K）下，疲劳裂纹扩展过程中应力比R从0.1变化到-1，并研究了R变化前后的FCG行为。结果，由于R = -1时压缩侧的疲劳载荷产生了拉伸孪晶，且FCG速度加快。此外，当R = -1时的最大压缩应力(|(σ)|)超过材料的压缩屈服强度(σ)时，R波动后的FCG速度大幅加快。另一方面，在|(σ)| < σ的条件下，R波动引起的FCG速度加速程度较小。在这两种情况下，FCG的加速程度都随着K值的增加而增大。基于对疲劳裂纹尖端的变形和孪晶状态、疲劳裂纹闭合行为以及疲劳过程的循环应力-应变曲线的观察，考虑了上述R波动引起的FCG加速机制。FCG加速机制如下：首先，由于拉伸孪晶的产生导致裂纹张开位移增加，从而使FCG驱动力增大。其次，在孪晶界面处产生主裂纹与多个微裂纹的合并。应力比波动后的弹塑性FCG行为由有效J积分范围ΔJ定义。
