Issue34

B. Schramm et alii, Frattura ed Integrità Strutturale, 34 (2015) 280-289; DOI: 10.3221/IGF-ESIS.34.30 281 enable a large application spectrum and meet different local demands such as absorbability, abrasion and fatigue of structures. However, for these inhomogeneous material regions the concepts of LEFM are not directly applicable. In this context, the influence of a material gradation on the crack growth as well as the possibility to extend the concepts of LEFM for the application on structures with material properties varying from location and direction have to be analyzed. The investigations were carried out by the Collaborative Research Centre (SFB/TRR 30) of the German Research Foundation and involved among other things a structure (Fig. 1a, [1] ) which combines a graded microstructure with a practice-oriented geometry. The structure consists of a ferritic-perlitic base material of the heat treatable steel 51CrV4 within the unformed region of the shaft and a martensitic microstructure within the formed flanged. The different microstructures are characterized by different fracture mechanical properties in form of threshold value  K th , fracture toughness K IC , crack velocity da/dN (Fig. 1b, [2] ). The elastic properties (e.g. Young’s modulus E and Possion’s ration  ) are not affected by the production process. a) b) Figure 1: a) Metallographic micrograph of the flanged shaft [1], b) crack velocity curves of different microstructures [2]. The main aim of these investigations is to understand the processes which control the crack growth in fracture mechanical graded structures, especially the influence of the material gradation on the limits of fatigue crack growth, the crack growth rate, the crack propagation direction and the lifetime of graded components. In this context the following questions are of interest:  Under which condition is crack growth possible?  How fast does the crack grow?  In which direction does the crack grow in case of stable crack growth?  When does the unstable crack growth start?  Which residual life time can be expected for the cracked structure? T HEORETICAL I NVESTIGATIONS OF F RACTURE M ECHANICAL G RADED M ATERIALS asic theoretical investigations are carried out to understand and predict the crack propagation behavior in fracture mechanical graded structures. The focus is especially on the influence of the material gradation on the limits of fatigue crack growth (stable and unstable crack growth), on the crack growth velocity da/dN as well as on the crack propagation direction. Fig. 2 shows a structure with different fracture mechanical materials M1 and M2. In Fig. 2a the initial crack is situated in certain distance from the material gradation. Hence, there is only homogeneous and isotropic material behavior around the crack tip. Accordingly, the crack growth occurs in dependency of the present loading situation (Mode I, Mode II or Mixed Mode) for plane structures. In this case, the crack propagation concepts for homogeneous and isotropic materials, such as the MTS-concept of Erdogan and Sih [3], can be applied. In Fig. 2b the crack tip is within the material transition and sees two fracture mechanical different microstructures. Hence, a change in the crack growth rate da/dN and possibly of the direction of crack propagation might occur due to the fact that the crack takes the path of least resistance. The B

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