Application of Microcrack Growth Law-Aided Low-Cycle Fatigue Life Evaluation to Cyclic Softening and Hardening Type of Materials

将微裂纹扩展低辅助低周疲劳寿命估算方法应用于循环软化硬化型材料。对WT80、SCM435H钢（循环软化型）和SUS304钢（循环硬化型）进行了循环应变控制的低周疲劳试验。实验中选用了恒应变幅和两步应变幅两种应变幅。为了模拟微裂纹的生长，使用了具有直径为40 m的小钻孔的缺口试样以及无缺口试样。结果发现，在恒定的应变幅的情况下，微裂纹扩展速率成正比的裂纹长度和疲劳寿命的基础上预测的微裂纹扩展低同意相当好的测量值为每种材料。针对SUS304不锈钢两步加载试验中微裂纹扩展速率受应变历史影响的问题，为了更有效地观察微裂纹的萌生和扩展过程，研制了一套图像分析仪辅助裂纹观察系统。利用该系统对两步应变过程中微观结构变化对微裂纹扩展的影响进行了统计检验。最后，结合裂纹萌生、扩展和组织变化的微观方面，对Miner规则进行了研究

The approach of microcrack growth low-aided low-cycle fatigue life evaluation, having been previously derived on a cyclically stable material, was applied to cyclic softening and hardening type of materials. Cyclic strain controlled low-cycle fatigue tests were conducted on WT80 and SCM435H steels(cyclic softening type) and SUS304 steel(cyclic hardening type). Either constant or two step strain amplitude was selected in experiments. To simulate microcrack growth notched specimens with a small drilled hole of 40 m in diameter was used as well as unnotched specimen. It was found that in case of the constant strain amplitude, the microcrack growth rate is proportional to a crack length and the fatigue life predicted on the basis of the microcrack growth low agrees fairly well with the measured value for each material. In the two step loading test of SUS304, the microcrack growth rate was affected by the prior strain history.In order to make observation of microcrack initiation and propagation more efficiently, a image processing analyser-aided crack observation system was developed. The effect of the micro-structure change on the microcrack growth during two step straining was statistically examined using that system. Finally, the Miner rule was investigated in conjunction with the microscopic aspects on crack initiation and propagation and structure change