13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- Figure 5. PSBs and dislocations on the surface of specimen failed at 3x108 cycles (520MPa) The surface parallel to the loading direction of other specimens were also investigate and it was found that the PSBs started to emerge out after 106 cycles on the surface of specimens. The density of PSBs continuously increased with further fatigue cycles [11]. With increase in the fatigue cycles, the topography of the PSBs enhanced. Longer, deeper and broader slip bands were obtained for higher fatigue cycles which increased the surface roughness of the specimen. The surface roughness of the specimens was measured as 178nm and 247nm for the specimen failed at 6.45x107 and 7.78x108 cycles respectively. This showed that the additional fatigue cycles after 106 cycles were utilized in the creation of PSBs at the surface of the specimen. The topography of PSBs was investigated in detail through AFM. PSBs were present in the form of shallow intrusions and extrusions. Fig. 6 (a) shows 90um x 90um size AFM scanned image of a near fracture region in specimen failed at 3x108 cycles. Fig. 6 (b) shows the 3D image of the area. The topography of intrusions and extrusions was like shallow scratches and waves, respectively [15-17]. Fig. 6 (c) shows the height profile data from the top corner of the area through the slip bands to the other corner of the area (shown by the red line in Fig. 6 (a)) showed many different crust and troughs. Several combinations of intrusions and extrusion, parallel to each other, were observed [16-17]. The distance between intrusions and extrusions was very small and maximum depth of the slip bands reached to around 250nm in some regions.
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