Estimation of Unnotched Low-Cycle Fatigue Life of a Medium Carbon Steel Through Observation of Microcrack Initiation and Propagation

In order to investigate low-cycle fatigue life on the basis of serial observation of microcrack initiation and propagation, cyclic strain controlled-low-cycle fatigue tests were carried out on unnotched specimens. Either constant or two step strain amplitude was selected in experiments. It was found that the unnotched low-cycle fatigue process is dominated by the microcrack growth. The microcrack growth rate is proportional to a crack length. From those experimental evidence, the Manson-Coffin relation is derived using microcrack propagation law. Moreover, in case of two step strain cyclig, the prior fatigue history hardly gives effect on the subsequent microcrack growth. Combining such results as mentioned above, the Miner rule is found another indication of the microcrack propagation law.