13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- among other things absorbability, abrasion and fatigue of structures. The material gradation has a remarkable influence on the crack propagation behavior which is why the development of appropriate concepts is important. The material gradation can be realized by different properties. On the one hand the gradation can be defined by different fracture mechanical properties, on the other side by differences in the elastic parameters. Furthermore a combination of the fracture mechanical gradation and the elastic gradation is imaginable. The following considerations deal with a fracture mechanical gradation. 3.1. Influence of a fracture mechanical material gradation on crack propagation A fracture mechanical material gradation exists for example within the flanged shaft, Fig. 5a, which is used as a demonstrator object by the collaborative research centre Transregio 30 with the title “functionally graded materials in industrial mass production”. Besides the development and manufacturing of these innovative materials another important task is the characterization of the mechanical properties of these materials regarding the remarkable influence of the gradation on the fracture mechanical behavior and hence on the life time of such a graded structure [9]. Due to a thermo-mechanical production process the unformed region of the shaft shows ferritic-perlitic base material of the heat treatable steel 51CrV4, whereas the formed flange consists of a martensitic structure. Fig. 5b clarifies that the fracture mechanical properties of these microstructures (i.a. Threshold value ΔKth, fracture toughness KIC, crack velocity da/dN) differ extremely from each other. The elastic properties (Young’s modulus E, Poisson’s ratio ν) are not affected by the production process. Furthermore there is a defined distinctive transition zone between the mentioned microstructures which is neglected in this contribution. base material pure martensite a) b) Figure 5. a) Metallographic micrograph of the flanged shaft [9], b) crack velocity curves for the steal 51CrV4 with different microstructures [10] 3.1.2. Influence on the limits of fatigue crack growth Fig. 6 shows the correlation between cyclic stress Δσ and crack length a in dependency of a fracture mechanical gradation for a Griffith crack (geometry factor Y = 1) [10, 11]. Below the Threshold value curves a crack is not able to grow, whereas the unstable crack growth is above the fracture limit curves. The region of stable fatigue crack growth is between both material curves. The crack starts in base material and reaches the martensitic microstructure at the crack length a = 6mm. It can be seen that the region of stable fatigue crack growth is more distinctive for the base material than for martensite. Furthermore there is no real overlapping at the transition, hence the
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