13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Fig. 3. Fatigue crack initiation in high cycle or very high cycle regime, (a). Alloy 690, quasi-cleavage, σa=140MPa, Nf=1.61x10 8, (b). Ti6Al4V, grain boundary, σa=280MPa, Nf=3.0x10 7, (c). Martensitic ferritic steel (MFS), facet, σa=460MPa, Nf=1.2x10 6, (d). Martensitic austenitic steel (MAS), lath, σa=600MPa, Nf=3,2x10 8. 3.2 Strain localization or plasticity exhaustion in very high cycle regime In order to study the material damage mechanism in the VHCF regime, a special specimen was prepared by polishing the SNDFCO or fish eye. The local strains were then measured and evaluated using the EBSP technique. The local strain of each individual grain could be mapped through the image analysis of all the grains [12]. Fig. 4 shows the relative strains (strain contouring) for the specimens within SNDFCO or fish eye from Alloy 690, Ti6Al4V and MFS materials. A common character for these three specimens is that strain is very concentrated or localized (red color in the figures). Table 2 shows a comparison of the maximum strain and the average strain in SNDFCO or the views observed. The maximum strain is much higher than the average values. For MFS material, the maximum strain can be eight times higher than the average value. This indicates that the strain under SNDFCO is highly localized in this material. (d) (c)
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