13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- temperature versus number of cycles. Figure 9 is an enlargement of the end part of Figure 2, where the different points are reported. As previously published [5,9] for subsurface observations, the number of cycles of crack initiation Ni, corresponds to the weak temperature increase (point 1). The number of cycles between point 1 and 2 (Figure 9) corresponds to the crack initiation stage. Between point 2 and 3, the fast short crack propagation takes place. After the point 3, the plastic zone ahead the crack tip (figure 9) produces a great lot of PSB and then a great number of specific heat sources (PSB formation which are only visible on the specimen surface). The intrinsic dissipation d1 and so the temperature grows rapidly until fracture. Figure 9: End of the temperature evolution versus number of cycles curve in relation with the number of cycles in each stage 6. Conclusion In this paper, temperature follow-up on the surface of flat specimen for a ferrite Armco iron in the gigacycle fatigue domain is compared to the fracture surface observations made by SEM. The good agreement between thermal and SEM results allows to drawing the conclusions below: - the crack initiation at the number of cycles Ni is due to a rapid coalescence of the PSB on the specimen surface (point 1 and 1bis). Then, the crack initiation spreads over the thickness of the specimen (coalescence of PSB between point 1 and 1bis in specimen volume) to achieve ΔK threshold (point 2) of about 4 MPa√m. In this test, more than 99% of the total life is devoted to the crack initiation. - when the crack propagation (stage II) occurs, the temperature at the crack tip rapidly increases, firstly in short crack propagation regime (point 2→3), and then in a long crack propagation regime (point 3→5). In the long crack propagation regime, the number of PSB is very important, and so the specific heat sources and the temperature increase rapidly. ACKNOWLEDGEMENTS This research was supported by the grant from the project of Microplasticity and energy dissipation in very high cycle Fatigue (DISFAT, project No. ANR-09-BLAN-0025-09), which funded by the National Agency of Research, France (ANR). Aknowledgements are due to F. Garnier (Ecole Centrale de Paris) for the SEM observations.
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