13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- value cannot be indicated. As shown in Figure 6, for tests conducted at 450°C, the small grain size material achieves significantly higher fatigue lives than the larger grain size material. The fatigue life of material 2 is at least twice superior to that of the material 1. 0.095 0.096 0.097 0.098 0.099 0.1 0.101 0.102 0.103 0.104 0.105 1.0E+05 1.0E+06 1.0E+07 Nf Δεnormalisé Material 2 [12] Material 1 [12] Material 2 - Present study Material 1 - Present study Figure 6 : Repartition of the fatigue life at 450°C, Comparison of the two microstructures. 3.3. Discussion of the fatigue crack initiation scenarios and crack propagation steps In this section the different crack initiation and propagation scenarios are discussed in detail and compared with the literature to gather the elements necessary for a quantitative study of the contribution of each crack progression mechanism to the total fatigue lifetime. Inconel 718 DA presents at least two known modes of crack initiation [9, 11] modes depending of the microstructure for smooth round bars: 1. Crack initiation on the surface particles for the lowest fatigue life 2. Stage I crack initiation for the longest fatigue life. In this study, a supplementary crack initiation mode is observed: 3. Internal crack initiation on nitrides leading to fish-eye cracks. This third crack initiation mode appears for long fatigue lifes and will be discussed in detail. 3.3.1. Crack initiation on surface particle This type of fatigue crack initiation phenomenon is not shown here but it was clearly observed in the F. Alexandre’s work [9, 11] for larger strain amplitude and temperature. These brittle particles may fracture during loading. In addition, the niobium carbides particles on the surface are strongly oxidized and transformed into Nb2O5 type oxide [9, 15], as shown in Figure 7. The increased volume of carbides and the stress concentration associated with the oxidation of carbides may lead to early crack initiation. The surface crack initiation probability is dependent of the probability to encounter a large oxidized particle on the specimen surface. This probability increases for large specimens. The fatigue life is then mainly spent in Stage II propagation from an initial crack size equal to the size of the larger grain.
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