13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- prepare samples for EBSD observations. Aim of these observations is to determine how lattice misorientation in ferrite phase evolves when fatigue goes. As well-known, cyclic hardening / softening behavior has an impact on dislocation structure[8-9]. This type of experiments can be considered as a first step to evaluate dislocation changes, before conducting direct observation of dislocations. For each sample, two distinctive map scans of a 200x180 µm2 area have been conducted. Misorientation curves presented in Figs. 10 and 11 are the average of these two scans. Figure 10(a) reveals that misorientation distribution does not significantly change between stages 5 and 25% of Nf for 20 Hz tests. This is less the case of ultrasonic tests, according to Fig. 10(b), even though dramatic change cannot be seen between these three fatigue stages. However, main result that has to be pointed out here is the drift of distribution into lower misorientation values for fatigued specimens, compared to the condition before fatigue test. (a) Case of 20 Hz loading frequency (b) Case of 20 kHz loading frequency Fig.10. Change of misorientation distribution for Sw+30 stress level Fig.11. Comparison of misorientation distributions for 20 Hz and 20 kHz conditions at N=25% Nf Such a result is confirmed in Fig. 11, where results at N=25% Nf for both stress levels studied are presented. It is found at a same frequency, distribution from Sw+10 and Sw+30 are similar. However, mode of distributions from 20 kHz tests is obviously lower than 20 Hz. This significant change of mode is certainly linked with dislocation structure changes. More precisely, such a result is an
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