13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- (a) (b) (c) Figure 6 (a) AFM scanned image of an area near to crack initiation for the specimen failed at 3x108 cycles (b) height profile through the area (from one corner to the other corner) showing significant PSBs and dislocations (c) 3D surface topography of the region The process of utilization of additional fatigue cycles for the creation and enhancement of PSBs explains the large difference in fatigue failure cycles for very small decrease in the stress levels. At lower stress levels, development of PSBs and their subsequent enhancement required significantly higher fatigue cycles. However, for higher slope S-N curves, only small subsurface region is affected from the localized plasticity, hence relatively smaller difference in the fatigue cycles are usually required for any small decrease in the stress value. FCC materials are known to show PSBs under loading [19]. The main slip system for the material was FCC structure (111) [18]. When the material was fatigue loaded, the crystal plane (111) with higher Schmid factor suffered from the higher degree of plastic deformation. The strains get localized due to the dislocation accumulation at grain boundaries of the material [11]. The interaction of precipitates at the grain boundaries of the material having higher Schmid factor with the dislocations started to occur [9, 15]. The partial surrounding of the grains at the leading surface of specimen [9] and this interaction in combination, not only generated the PSBs and local yielding
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