13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- (blue lines) - When the number of cycles is below 3.2086x107 cycles, the temperature is the same on the overall width of the specimen. The PSB formation on specimen surface are scarce and located on both side of the middle section of the specimen [7,8]. The intrinsic dissipation d1 (specific heat source due to PSB) measured from the temperature measurement on the specimen surface is about 5°C/s [8]. There is no crack on the specimen surface. - At the number of cycles N = 3.2086x107, the temperature profile on the specimen width shows a point (point 1, figure 3 and 7a) with a higher temperature. At this number of cycles, there is a rapid coalescence of PSB on specimen surface. Probably, the microcrack (due to the PSB) is also opened on the surface thickness of the specimen (point 1bis, figure 7a), and then, there is a coalescence between microcracks on the two surfaces (width and thickness). These points (1 and 1bis) correspond to the transition between crack initiation stage and short crack propagation stage (figure 6b).On the figure 7a, according to the all SEM observations (not presented here), this transition on the fracture surface has been reported. From the point 1, the temperature maximum increases, but this point does not move on the specimen width. This due to the formation of the crack initiation area between point 1 and 1bis When the crack spreads from point 1 to point 1bis, the ΔK threshold is reached. The calculation according to the blue line is 4.1 MPa√m, which correspond to a plastic zone size of ∼ 47μm. The number of cycles N = 3.2086x107 is the number of cycles at the crack initiation Ni. This allows to calculate the ratio Ni/Nf equal to 0.9978. So, 99.8% of the total life is devoted to the initiation of the crack. As previously reported for the subsurface crack initiation in the gigacycle fatigue domain [4-5, 9-11], the major part of the total life is devoted to the crack initiation. - Between point 1 / 1bis and 3, it is the fast short crack propagation. The length crack on the surface goes from 294 to 882 μm. At the point 3, the ΔK is 10.7 MPa√m, which correspond to a plastic zone size of ∼317 μm, and the d1 about 20°C/s [8]. - From the point 3, the long crack propagation begins, with the occurrence of striations. From this point (figure 8), the crack length and the temperature increase favors related to the high dislocation density the formation of many PSB in the plastic zone ahead the crack tip (figure 8). - Then, the fatigue crack propagation goes on to the point 5. At this point, the d1, reachs around 400°C/s (unpublished results). Figure 9: SEM observations of the high density PSB in the long crack propagation stage From each point on the figure 3, the number of cycles can be calculated and reported on the curve 3
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