13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- roughness of the crystallographic surface [1, 13] III. Vacuum stage II propagation (little striations) The second area inside the fish-eye presents a classic stage II fracture surface, but the striations are not marked. This can be explained by the fact that this propagation step still occurs in vacuum [13]. Utilisation of Pedron’s data [25] is certainly suitable in this step of the propagation. This step propagation stops when the crack reaches the specimen surface. Figure 13 : SEM fractography of the stage II fatigue a) vacuum crack propagation in the fish eye b) air crack propagation beyond the fish-eye. IV. Crack reaches specimen surface air propagation with marked striations When the fish-eye crack reaches the specimen surface, the ligament between the penny-shaped fish-eye crack and the specimen surface will break very quickly due to the high local stress intensity factor. In this step the surface crack can approximately be considered as semi-elliptical. The propagation occurs in air: I d F K π σ = (3) Where σ is applied stress, d the crack depth from the surface and FI the correction factor depending on the geometry of the specimen and crack. Values of this factor FI can be found in handbooks [22]. The last step on the fracture mode corresponds to final ductile fracture. In conclusion, 3 different crack initiation scenarios have been identified for Inconel 718. The likelihood of the occurrence of each of these scenarios depends on the microstructural heterogeneities, such as e.g. grain size distributions, particle size distributions and their spatial distributions. The link between the probability to encounter each crack initiation scenario and the total fatigue life of the specimens is schematically sketched in Figure 14. Further analysis of the resulting distributions in fatigue life is under progress. Figure 14 : Schematic representation of the relationship between probability to encounter each crack initiation scenario function with the total fatigue life of the specimen for a specific loading value (the relative position of the stage I initiation curve and the internal nitride initiation curve is dependent of a scale effect. These curves can be translated.)
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