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

The approach of microcrack growth law-aided low-cycle fatigue life evaluation, derived on a cyclically stable material in a previous paper, was applied to cyclic hardening and softening types of materials. Cyclic strain controlled low-cycle fatigue tests were conducted on WT 80 and SCM 435 H steels (cyclic softening type) and SUS 304 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 μm in diameter were used as well as unnotched specimens. It was found that the previous method is essentially applicable for all materials tested. That is, the microcrack growth rate is proportional to crack length and fatigue life predicted on the basis of the microcrack growth law agrees fairly well with the measured value. Cyclic softening and hardening behaviours which afftect the cyclic stress-strain relationships, as well as cyclic strain-induced martensitic transformation observed on SUS 304 steel, hardly influence the microcrack growth.