13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- harder surface can be obtained by U2 treatment, the fatigue strength mainly depends on the level of surface stress. For the N48 samples, U1 process also introduced a higher compressive residual stress but the sub-surface fatigue crack initiation induced by inclusions hinders the improvement of fatigue strength. Figure 9. The fatigue crack initiating from surface defects (U2, 562MPa, 5.4x106, left; N48U2, 974MPa, 6.46x104, right) Figure 10. The sub-surface fatigue crack induced by inclusions (N48U1, 974.2MPa, 1.3x105) (Left); EDX data on the inclusion. The observations of the fatigue fractures were shown in Figure 9 and Figure 10. It obviously can be seen that the fatigue crack inducing the material’s failure can be divided into two modes: one is the surface crack initiation induced by the persistent slip bands (PSBs) or surface defects; another is the sub-surface crack induced by the inclusions within materials. After plasma nitride, the surface of S45C steel can be enhanced effectively and the fatigue crack initiating from sample’s surface becomes difficult. With the high cycle loading the inclusion within the sub-surface can induce a stress concentration and a fine granular area (FGA) (Figure 10, left), which was composed by the aluminum oxide, calcium oxide and magnesium oxide or other hard phases (Figure 10, right), was produced and expand to fracture. The mechanism of the formation of FGA is not clear and there are many theories explaining this phenomenon, for example, the theory of hydrogen diffusion, the dispersive decohesion of carbide and the grain refinement induced by stress concentration [9-11]. From the observations of fractures results, the UNSM treatment not only can improve the fatigue strength but also has an influence on the fatigue crack initiation. For the un-treated samples, the surface cracks transfer form surface crack induced by PSBs to the crack induced by the UNSM process marks (Figure 9, left) which we also can found in the surface morphology results. So we can also find that with increase of the test loading the fatigue life decreases as the reason of relaxation of residual stress and the stress concentration induced by the surface defects. For the nitriding samples, also were surface cracks induced by surface defects (Figure 9, right) found and the fatigue strength decreases with the increase of striking number (U2) as the reason of decrease of surface residual stress and more surface defects induced by the UNSM treatment after U2 process. We can also find more surface fatigue crack initiations for N48 samples with U1 treatment than U2
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