13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- After burst tests, fracture surfaces were examined with a Scanning Electron Microscopy (SEM) for determination of fracture initiation points and also for checking of eventual fracture surface modification under influence of hydrogen. It has been found for hydrogen tests, inner fracture surface is characterized by an array of surface cracks (see Fig. 7-a). Careful examination shows that, surface cracks density is higher and average crack length is smaller. View of surface cracks under hydrogen conditions is given in Fig. 7.b at magnification X250. Figure 7. (a) surface cracks mesh at the edge of fractured notch for test in hydrogen. (b) magnification X250. Examination of the notch bottom shows that fracture is originated at centre of notch ( mm 1 at 2 ± ).The mechanism of fracture initiation is developing of micro cracks from notch bottom (Fig. 7.a). Generally fracture surface consist of two parts. First part is a flat surface created by micro cracks growth from notch bottom and second part is final failure surface, which is made by shear mechanisms. This second part prevails at the inner surface of tube along notch and it may be considered as final ductile failure (Fig. 8.a). For test in hydrogen, the following aspect was observed. There is alternation of brittle and ductile sites of fracture character (Fig.8.b). The depth of particular surface aspect doesn’t exceeds a distance of about 50 μm from notch bottom. No crack bifurcation was observed with or without hydrogen and this fact is considered as the result of fracture occurring under low constraint. Figure 8. (a) Micro cracks at the notch bottom as source of fracture initiation for test in hydrogen. (b) fracture character at the inner surface. 4.2 Fracture under high constraint Fracture tests have been made on DCB specimen made in X52 steel. Geometry and dimensions of specimen is aregiven in figure (9).
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