13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- 2) ( 1 p StageI i A d N εΔ = (1) where Ni is the number of cycles to initiation, d is the grain size (here, 7µm), Δεp is the strain plastic amplitude (in our study, the macroscopic plastic strain is almost equal to zero), and AstageI a constant depending on temperature. This model accounting for grain size effect is consistent with the findings of our study because crack initiation occurs on the largest grains for the material with the larger grains. This stage I crack initiation is known to be sensitive to the size of the γ’’ precipitates [17]. Indeed the γ’’ precipitates influence the homogenization on the slip bands. Reducing the risk of crack initiation via PSBs [18].When the grain size is small, the size of the γ’’ precipitates increases and the slip bands are more homogeneously distributed in the grains. So this theory may explain why stage I crack initiation occurs on the biggest grain of the material. The dislocation structure and the precipitate size have an effect on the fatigue life [19]. 3.3.3. Internal crack initiation on particles leading to a fish-eye crack This last mechanism is observed in material 2 with smaller grains; crack initiation occurs on an internal particle (nitrides) leading to a fish-eye crack. The mechanism was widely observed for other materials, like in steels for very cycle fatigue [20, 21]. Only a semi-quantitative discussion of crack initiation and propagation steps is presented here. Figure 11 : SEM micrography of a fish-eye crack with a dark nitride particle in its centre for the small grain material 2 and the schematic representation SEM observations (Figure 11) show an example of a fish-eye pattern formed on the fracture surface and the nitride at the centre of the fish eye. The difference of contrast between the three zones observed on the fracture surface suggests different crack growth mechanisms. At the centre of the fish-eye mark, a bright-facet area was found around the inclusion at the fracture origin as shown in Figure 12. The bright-facet area reveals a very rough topology compared to the area inside the fish-eye. The propagation mechanism can be classified in five steps: I. fracture of the large nitride during loading (or during forging) II. Near threshold micropropagation with crystallographic character, under vacuum III. vacuum stage II propagation (few striations), IV. Air propagation with marked striations when the crack reaches specimen surface, V. final ductile fracture All these steps will be studied to gather elements to better understand the contribution of the
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