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

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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