FIGURE 3 The distribution of fatigue lifeand stress amplitude.
3.3 | Fracture observations
Fatigue failure of all the specimens occurred from the surface scratches in this experiment, verified by the observation of SEM. And no facets were observed in the initiation area of all the specimens.
Figs. 4A-B show two typical examples of fatigue crack initiated from the root of scratch. It can be observed that scratch in Fig. 4A is distributed along the circumferential direction and the one in Fig. 4B is along the axial direction of the specimens. Both the two cases have the same failure mode.
Surface scratches can serve as stress concentrators and produce localized stress concentration at the surface. Micro scratch can push crack to the stage of propagation stage in a short time. Small crack propagation can be ignored. Thus, it can be regarded as the equivalent initial crack. At these positions around scratches where cyclic plastic deformation is higher than the average, which caused fatigue failure.30