13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- existing fracture criteria for 3D-Mixed-Mode-loading conditions. Experimental investigations on spatial Mixed-Mode-loading are also necessary, in order to get a solid basis, on which, on the one hand, the existing fracture criteria for 3D-Mixed-Mode could be enhanced, on the other hand, new fracture criteria or hypotheses including the desired understanding could be established. 2. Mixed-Mode-criteria for 3D-loading conditions For a complete prediction of crack growth behavior under combined loading not only the crack growth direction is required, but also the determination of comparative stress intensity factors, like KV or ∆KV, see Eq. 2 and Eq. 3. Compared to characteristic fracture mechanical values, e.g. threshold value or fracture toughness value, conclusions could be drawn on crack growth behavior. In this purpose some criteria for characterizing the crack growth under spatial Mixed-Mode-loading were established: • Crack growth criterion by POOK [2-4] • σ1'-criterion by SCHÖLLMANN et al. [5, 6] • Criterion by DHONDT [7] • 3D-criterion by RICHARD [1] A comparison of these criteria is given in e.g. [1]. 3. Experiments on cracks under general loading In order to understand the 3D-fracture-process completely not only further theoretical, but also experimental investigations have to be performed. Currently several types of specimens are available for experimental investigations of fatigue crack growth and fracture under various Mixed-Mode-loading conditions [8-17]. None of these specimens enables investigating the full range of all basic fracture modes or any combinations thereof. But the AFM-specimen with the corresponding loading device and the so-called CTSR-specimen (Compact-Tension-Shear-Rotation-specimen) in combination with the special loading device fulfill these high requirements. Some of the experimental results of both specimen types are shown below. 3.1. Experiments on AFM-specimen As already mentioned the AFM-specimen, developed by RICHARD, allows the investigation of crack problems under general loading conditions by using a simple uniaxial testing machine [18]. In the past experiments under static load were performed in order to determine the fracture limit surface and the crack deflection angles φ0 and ψ0 [19], see Fig. 3. For fatigue tests this loading device is less suitable, due to its high weight and deformation. Only low test frequency, which leads to high test duration, can be realised for fatigue experiments. Making use of this background the CTSR-specimen (Fig. 4a) with the corresponding loading device (Fig. 4b) was developed.
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