13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- Figure 2. II I K K ∆ ∆ ratios for left and right crack-tip for different crack lengths From the experimental investigation initiation of the crack kinking takes place for a crack length a about ( ) 65 70 mm ∼ , in an angle 0ϕ (Figure 1) of 45 5± . Two different crack propagation criteria have been examined to estimate which one should be used in outer to model crack kinking in cross linked PVC sandwich foam cores. At first the Energy density factor criterion [14] is used. According to this criterion, crack initiation occurs in the radial direction in which the local energy density possesses a stationary value. The energy W per unit area A, is expressed by: , r S dA dW = (2) where r is the radial distance from the crack tip and S is the strain energy density factor which represents a material element in the distance r from the crack tip. The distance r from the crack tip is assumed to be small compared to the crack length a, and S is given by: 2 22 12 2 11 2 II I II I S a K a K K a K + = + (3) where
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