13th International Conference on Fracture June 16–21, 2013, Beijing, China e) f) Fig.8 intergranular and transgranular crack at initiation area Fracture surface gives more interesting results. In the initiation area, the grains trace is visible (Fig.8a and Fig.8b). Three types of pattern were found: one intergranular and two transgranulars. The intergranular pattern is seen in fig8b and 8d. This intergranular crack appears not smooth, but presents a very fine linear structure on the grain surface which believes related to the dislocation slips. Moreover, two lines of hollows separated by around 2 m are visible (Fig. 8d). The first transgranular crack is shown in Fig. 8a and Fig.8c. The fracture surface through the grain is developed as a staircase. The distance between two stairs is about 2 m. It suggests the same mechanism that those forming the stair structure in the surface as blue arrows in Fig.7. The second pattern of transgranular crack is shown Fig.8e or Fig.8f (where a transition from an intergranular crack to the second transgranular pattern is seen). In this transgranular case, the fracture surface is composed of longisectional pipes arranged side by side. The diameter of the pipes is 0.6m in average. It may be correspond to PSB on the surface indicated by red arrows in Fig.7. Now, it is interesting to relate the fracture surface pattern with the subscale damage produced by PSB as Micron Abreast Pipes seen in Fig.6. An illustration of this fracture mechanism is schematically reported on the Fig. 9. The staircase structure is probably a fracture occurring partially on abreast pipes, and crossing over to abreast pipe just below or above. The distance between two stairs is about 2 m, in good agreement with the distance between two abreast pipes. In the case of second transgranular pattern, the crack path is entirely on abreast pipes in the same dislocation slip plane. The characteristic dimension of this pattern is pipe diameter about 0.5 m. The line of hallows (separated in distance 2 m) seen on the intergranular surface Fig 6d are probably the end of abreast pipes at grain boundary (in good agreement with the distance between two neighbored pipes). However, abreast pipes sometime may linked through the nanoscale wall between pipes to form subscale 2 dimensions planar default as small facets denoted by yellow arrows in Fig 8c. As shown previously (Fig.6 and 8), irreversible slip band not only take place on the surface but also at subsurface (with a minor density). The internal irreversible plastic deformations will potentially facilitate the fatigue crack initiation. Then it can conclude here that very high cycle fatigue crack initiate not only by extrusion and intrusion, but also due to irreversible slip bands in the subsurface. It starts from the irreversible slip bands in the surface, which induces stress
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