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