13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- The enlargement of the plane strain fatigue crack (Figure 5a) shows two main zones separated by the line at the point 3. The crack initiation site is located in the lower edge. In the stage I (Figure 5b), the trace of the grains in which Persistant Slips Bands (PSB) have appeared as previously shown [7] are visible. After the point 3, the stage II of long crack propagation begins with striations. At the beginning of this stage II, the striations are not well established (Figure 5c), whereas after the point 4 (and the corresponding line), the striations are well defined (Figure 5d). The lower side of the specimen has been more particularly observed (Figure 6a,b,c). The figure 6c is the picture around the point 3 where a change in fracture surface can be observed. After the point 3, striations are present and correspond to the crack propagation as previously reported. Between the specimen corner and the point 3, another fracture surface change appears at the point 1. On the right side of the photo, the grain traces are clearly observed, whereas after the point 1 (Figure 6b), the fracture surface appearance change, change which is attributed to the transition from the crack initiation stage (stage I) to the short crack propagation. From these observations, the locations of the different points can be measured. The results are: point 1 : ∼ 294 μm, point 3 : ∼ 882 μm, point 4 : ∼ 1294 μm, point 4 : ∼ 1629 μm. Figure 5 a)b)c)d) : SEM plane strain fatigue crack surface and details of each stage d) a) b) c) 3 4
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