13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- (a-1) Crack initiation site (b-1) Crack initiation site (a-2) Inner area (~200μm from surface) (b-2) Inner area (~200μm from surface) Figure 7. Fracture surfaces of SA-P steel tested in (a) RH 25% and (b) RH 85% Similar fatigue morphology can be found in DA-U steel tested in high humidity, as shown in Fig. 8. It is hard to distinguish any difference in fracture mechanism between the SA-P and the DA-U steels failed in high humidity, depending on the analysis of the results shown in Figs. 7 and 8. Therefore, it may be induced that the propagation of small cracks was accelerated by embrittlement due to hydrogen generated in cathode reaction. In addition, most of fracture surfaces in the region beyond crack initiation are occupied by martensite lath cracking regardless of humidity and aging condition, which may explains that little difference in growth rate of larger cracks is recognized between different aging conditions and humidity, as seen in Fig. 5. (a) Crack initiation site (b) Inner area (about 200μm from surface) Figure 8. Fracture surfaces in DA-U steel at RH 85%
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