13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- crack becomes unstable immediately after reaching the martensitic structure. Figure 6. Δσ-a-diagram for a fracture mechanical gradation with a sharp transition at crack length a = 6mm 3.1.3. Influence on the crack growth velocity Fig. 7a shows schematic crack velocity curves for two materials characterized by different fracture mechanical properties [10, 11]. At the beginning crack growth is starting within material 2. At reaching the other material a change to the crack velocity curve of material 1 occurs. Hence, the crack grows faster and reaches the fracture toughness much earlier as if it grows further within material 2. The worst imaginable case is that at the change to curve 1 the cyclic stress intensity factor ΔK is already larger than the cyclic fracture toughness ΔKC1 of material 1 leading to the immediate failure of the structure. In Fig. 7b the gradation is oriented opposite. The crack starts in the material with the worse fracture mechanical properties, material 1, and reaches material 2 afterwards. Due to the change the crack velocity slows down and more load cycles are tolerated until final fracture. At the best crack arrest occurs if the cyclic stress intensity factor ΔK is smaller than the Threshold value ΔKth2 of material 2 at the material change. As can be seen, a positive or a negative impact on the prospective lifetime is connected with the gradation constellation. a) b) Figure 7. Schematic crack velocity curves: a) transition from material 2 to material 1, b) transition from material 1 to material 2
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