Re-examination of fatigue crack propagation mechanism under cyclic Mode II loading

The essence of rolling contact fatigue is the so-called "Mode II fatigue crack propagation." However, its understanding has not progressed as much as that of Mode I. We think that this cause is the name: "Mode II fatigue crack propagation." Originally, Modes I and II represented the loading modes for still cracks in fracture mechanics, and not the fatigue crack propagation mechanism. There are many cases where the knowledge of fatigue crack propagation under Mode I loading is applied to that under Mode II loading without consideration. Moreover, in the rolling contact fatigue region where the fatigue crack propagates under Mode II loading, a large plastic deformation is caused by the rolling contact load. Therefore, it is necessary for the test method to reproduce the effects of an actual machine to test materials that exhibit large plastic deformation. Therefore, in this study, we aim to classify the fatigue crack propagation phenomena, regardless of Mode I and II loadings, and re-examine the mechanisms. To that end, we developed a novel test method that enables pure Mode II loading. We used a micro-thin film disc as a specimen, making it possible to cut out and test a part subjected to a large plastic deformation from the actual machine. By observations of the crystallographic structure before the fatigue test and the successive observation of fatigue crack propagation behavior, we propose a crack propagation mechanism, namely, damage accumulation type fatigue crack propagation under Mode II loading, which is different from the opening type fatigue crack propagation.