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

微裂纹扩展低辅助低周疲劳寿命评价方法是先前在循环稳定材料上得到的，现在应用于循环软化和硬化类型的材料。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。实验中选择恒定应变幅或两步应变幅。为了模拟微裂纹的扩展，我们使用了直径为40 m的小孔的缺口试样和未缺口试样。结果表明，当应变幅不变时，微裂纹扩展速率与裂纹长度成正比，根据微裂纹扩展曲线预测的疲劳寿命与实测值吻合较好。在SUS304的两步加载试验中，微裂纹扩展速率受先验应变历史的影响。为了更有效地观察微裂纹的萌生和扩展，研制了一种图像处理分析仪辅助裂纹观测系统。利用该系统对两步应变过程中微观组织变化对微裂纹扩展的影响进行了统计检验。最后，结合细观研究了裂纹萌生、扩展和组织变化的Miner规律

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