13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- Figure 7 : SEM micrograph of the outer specimen surface away from the fracture surface after specimen failure; oxidized carbides surrounding nitrides are seen Figure 8 : Schematic representation of the fracture mechanisms for crack initiation from a surface particle [11] 3.3.2. Stage I crack initiation on large grains via persistent slip bands (PSB) For the larger grain size material 1, the fatigue crack initiation corresponded to typical Stage I crack initiation along persistent slip bands as shown in Figure 9 for two specimens. The stage I crack initiation takes place over several grains, it occurs from the larger grains of material 1. This type of crack initiation was already observed in a previous study [9, 11] for larger mean grain size material, about 50µm. Figure 9 : Fatigue crack initiation sites on microstructure 1 with the biggest mean grains size (11 µm) : Stage I crack initiation across an area of several grains of (a) 70µm of diameter and (b) 150µm of diameter Figure 10 : Schematic representation of the fatigue crack initiation and propagation mechanisms for stage I initiation (for the material with larger grains in the present study) A probabilistic life model was described by F. Alexandre [11] for this mechanism. The number of crack initiation cycles has been assessed using a Tanaka-Mura [16] type formulation with a modification to account for grain size effect.
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