2018 年 104 巻 11 号 p. 689-698
Fatigue tests were performed to obtain co-planar and branch crack growth rates on rail and wheel steel under non-proportional mixed mode I/III cycles. In the experiments, sequential and overlapping mode I and mode III cycles that simulated the load experienced by the rolling contact fatigue cracks were applied to the crack in cylindrical specimens made of rail and wheel steel. Experiments showed that a long co-planar crack could be produced under certain loading conditions. Based on the fracture surface observations by SEM and the results of FEA, the long co-planar crack growth is thought to be driven mainly by mode III loading and the role of mode I is an assistant, keeping the crack face opened. It was observed that the cracks were apt to branch when the strength of the material increased. It was also observed that the crack branched when the degree of overlap between the mode I and mode III cycles increased. We proposed the equivalent stress intensity factor range for branch crack that can consider the crack face contact and successfully regressed the crack growth rate data. Comparing the fracture surfaces and the co-planar crack growth rates data under non-proportional mixed mode I/III loading with that under I/II loading, it is found that the mechanism of shear mode crack growth is essentially the same regardless of whether the main driving force is in-plane shear or out-of-plane shear.
