13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- ϕ0<0 for KII>0, ϕ0>0 for KII<0 and KI always larger than 0. ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ + + + = 2 II I II II I II 0 K K K B K K K A μ ϕ (7) In this empirical formula the parameters have to be set to: A = 155,5° and B = -83,4°. 2.2. Comparison of the theoretical concepts and experimental findings Experimental determined kinking angles ϕ0 for different almost isotropic materials (for example PMMA, Araldit B, PVC, AlZnMgCu) are shown in Fig. 2. The kinking angle doesn’t depend on the material. However the KII/KI-ratio is important. Furthermore the kinking angles determined by different fracture criteria (for example Erdogan and Sih, Richard) are presented in dependency of the Mixed Mode ratio KII/(KI+KII). It can be seen that these crack propagation criteria are able to predict the crack kinking angle for isotropic and nearly isotropic material sufficiently exact [7]. For more information about experimental investigations see [5, 8]. Figure 2. Kinking angle from criteria for 2D Mixed Mode cracks in comparison to experimental data Fig. 3 shows aluminum alloy specimens which vary in their rolling direction. The specimen in Fig. 3a was rolled in crack direction, in Fig. 3b diagonal and in Fig. 3c perpendicular to the crack direction. After the production and rolling process these specimens were investigated experimentally. The resulting kinking angles ϕ0 of these anisotropic structures are presented in Fig. 4. The results show that the crack propagation concepts are not able to predict the kinking angles of the rolled aluminum alloy specimens very well. The concepts are not able to consider the predominant direction due to the rolling process.
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