13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- (a) 40µm (50X) (b) 100µm (20X) Figure 12. Optical photographs for cross section through the cracks and secondary cracks (a) –without hydrogen & (b) –hydrogen environment With hydrogen environment, fracture is sometimes initiated from secondary cracks and very close to the surface due to pitting under hydrogen attack as shown in Figure 12.b. The crack stress field will produce a strain magnification in adjacent micro-cracks and penetration of hydrogen atoms will result in further cracking of ferrite phase at the crack tip. The crack cross the perlitic phase and will propagate in the straight direction, facilitating hydrogen propagation and weeping. 5. Discussion It can be seen that critical effective Tef,c-stresses have the influence on crack paths. Pipe submitted to internal pressure has higher negative critical value of the T-stress, Tef,c, range and consequently lower constraint in comparison with bending specimens. SENT and CT specimens have also a high negative value of the Tef,c-stress for cracks emanating from notch tip but lower than a notched pipe. For these specimens and pipe submitted to internal pressure, crack extension is observed along x direction, i.e. perpendicular to the principal tensile stress (Fig. 13.a). DCB specimens exhibits a particular positive Tef,c-stress values ( Fig. 13.b). In this case, crack bifurcation appears after fracture initiation. Figure 13. Kinking direction for (a) SENT and CT specimen, (b) DCB specimen. Crack extension in API 5L X52 steel is governed by ductile failure mechanism, namely, nucleation of microvoids, growth and coalescence of these microvoids. However, pipe under internal pressure, CT, and SENT specimens, voids close to the notch are not elongated along the direction of loading but are elongated along shearing stress direction component which induced failure by mode II superimposed with mode I. Therefore, crack extension has a zigzag path characteristic of mixed mode (I + II failure mode). This zigzag mechanism can be seen on Figure 14.a with and without the hydrogen effect.
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