13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- the fatigue crack extends more rapidly in the vertical direction than in the horizontal direction. This effect may be related to an inhomogeneous phase distribution in the specimen. This will be clarified by a phase analysis of the fracture surface which is under way. a) side view with edge-notch in unloaded state b) front view with edge-notch in unloaded state c) side view with fatigue crack after 45.000 load cycles d) front view with fatigue crack after 45.000 load cycles Figure 14. Data of the test setup gained from the long-distance microscopes with an edge-notched micro-specimen Figure 15. Local crack growth rate of the fatigue crack in horizontal direction (blue) and vertical direction (red) 5. Summary and outlook The fatigue damage accumulation process in a flange shaft made by a thermo-mechanical forming is dominated by single cracks which are initiated in the soft ferritic phase. Even though in those cases where these cracks are very small they do not propagate as microscopically small cracks as the microstructure is greatly refined due to the forming process with grain sizes of the order of 1 micron or even less. However there is a strong influence of the texture of the material on the crack path leading to pronounced deviations from straightforward mode-I crack propagation. A significant retardation in the crack growth rate was observed when the crack crosses the transition zone between the ferritic and the martensitic phases. This effect will be studied by using different horizontal direction vertical direction
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