13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- (a) strong negative T stress (b) low negative T-stress (c) positive T-stress Figure 14. Proposed mechanisms for ductile crack extension under negative or positive ef c T , -stresses. For DCB specimen, the T-stress component is positive and voids are elongated closed to shearing direction. Crack path is linear along a direction which is close to pure mode II bifurcation angle (70°) (see Fig. 14.b). Therefore, ductile crack extension is governed by the intensity of shearing mode induced by the T-stress. Thus, the following model of crack extension can be assumed to describe crack paths as a function of the critical effective T-stress. Due to hard particles inside voids (these particles promote voids nucleation by stress concentration), voids cannot be closed by compressive (negative) T-stress and crack extension is then stable in notch direction according to scheme in [12]. If the T-stress is positive and higher than opening stress at some distance ahead of the notch tip, void extension then occurs in x direction which is corresponding to the maximum Tef,c-stress direction. In this case, crack extension is made by bifurcation. 6. Conclusion Fracture toughness is not really intrinsic to material but depend on constraint. The critical notch stress intensity factor versus the effective T stress build up a master curve which coupled the fracture driving force gives the critical conditions for any kind of geometry and loading mode. For the pipe steel API X52,the master curve exhibits a linear decreasing behavior. Slope and value at origin are few affected by hydrogen embrittlement. Crack path after fracture initiation is affected by constraint evaluated by T stress. A loss of constraint corresponds to high negative T effective value. This situation occurs in notched pipe submitted to internal pressureand crack path remains normal to maximum principal stress. This is unchanged after hydrogen embrittlement but fracture appearance is then modified. Over a short distance ahead of notch tip , there is a mixture of dimples and brittle facets. Fracture of DCB specimens occurs under high constraint characterized by a positive value of T stress. Crack kinking occurs due to mixed mode I +II induced by superposition of T stress. It has been seen on CT specimen with a lower constraint than DCB specimen that hydrogen embrittlement promotes this crack kinking. Further works are required to understand the shift of critical T effective value to promote crack kinking to lower value. The assumption of decreasing shearing mode cohesion energy with hydrogen will be investigated. Following the same idea, evolution of ratio of shearing and opening mode cohesion energy will be determined by appropriate experiments.
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