13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- The longitudinal cross-section of specimen S1550.02 is shown in Fig. 4. Cracks initiated from the surface and extended into the specimen interior. The density of the surface cracks decreased as a function of distance from the fracture surface. Fig. 4(b) shows the detailed view in the interior of the specimen. There were small round voids, but no cracks. A magnified view of the region near the fracture surface (Fig. 4(c)) indicates that there were small holes near the fracture surface. As these sections were taken at random, it is concluded that there were no cracks in the interior of the specimens, apart from the small round voids. 3.4. SEM fractography Figure 5. Specimen tested at an applied stress rate of 0.02 MPa s-1: (a) overview of fracture surface of specimen A.02c, (b) a magnified view of cone part of specimen A.02c; and (c) brittle facets steps on the fracture surface of Socp.02c. The fracture surface of specimen A.02c tested in air (Fig. 5) presented a cup and cone fracture with secondary cracks up to 140 µm in size, and showed significant ductility. These secondary cracks were surrounded by dimples, and had a small number of orientations. A magnified view of the fracture surface shown in Fig. 5 (b) indicates that the fracture surface comprised dimples, due to ductile microvoid coalescence (MVC) and secondary ductile cracks. All the secondary cracks were ductile and were surrounded by dimples. The fracture surfaces of other specimens tested in air were similar to specimen A.02c. Similar to that obtained in air, the fracture surface of specimens Socp.02a and Socp.02b tested at Ecorr showed a cup and cone fracture with a big secondary crack about 270 µm in size, and small secondary cracks less than 100 µm in size. However, one specimen Socp.02c tested at Ecorr showed dimples and secondary cracks as well as some brittle facets, as shown in Fig. 5(c). Those brittle facets built up horizontal steps; dimples were like a linker between those steps. These brittle facets covered a substantial area in the middle of the fracture surface. Nevertheless, they were entirely surrounded by microvoid coalescence. The fracture surface of specimen S950.02 tested at -950 mVAg/AgCl at a rate of 0.02 MPa s -1 showed a cup and cone fracture as illustrated in Fig. 6. Fig. 6(b) presents a magnified view of region A, which comprised dimples and some secondary cracks. Region B at the edge, shown in Fig. 6(c), comprised brittle facets at the edge and shear dimples, i.e. dimples which had formed in the shear lips of the (a) (b) (c)
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