13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Figure 3. S-N curve for the material 3.2 Fractography Scanning Electron Microscope (SEM) was used to observe the fatigue crack initiation sites and the fracture surfaces. It was observed that for all smooth and notched specimens, the cracks initiated from the surface of specimens, even at VHCF cycles. Fig. 4 (a) shows the cross-section of the fracture surface of the specimen failed at 7.78x108 cycles when loaded at 520MPa. It can be seen that the appearance of fracture was different at different locations and based on the appearance, four different regions were visible. Radial patterns of striations were observed having 110 to 130um spacing between each other. The initiation site was a localized region on the surface of the specimen. Fig. 4 (b) shows the crack initiation region and crack propagation direction from the surface of specimen. The striations started from the crack initiation site and extended to more than three-quarter of the surface of specimens. The details about the regions are discussed elsewhere [18]. Cleavage fracture was observed with many secondary cracks and higher surface roughness. (a)
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