13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- being initially biaxial tensioned to 300MPa; CRS corresponds to the case that the film being initially biaxial compressed to -300MPa. 3. Results and discussion The interface normal stress acting at the center point of the impacted region is shown in Fig. 3, which is named as SY_I and the symbol ‘+’ and ‘-’ represent the film side of the interface crack surface and the substrate side, respectively. It is indicated that the interface crack would permit the transmission of compressive stress pulse while deny the transmission of tensile stress. It is also shown that the normal stress is almost continuous across the interface, as one can see that the two curves corresponding to the results of the two nodes across the interface almost identical. Fig. 3 Normal stress acting at the interface center point The interface normal stress acting at the third interface node numbered from the interface crack tip is shown in Fig. 4, which is denoted by SY_INTE2 with the symbols ‘+’ and ‘-’ of the same meaning as aforementioned. Great tensile stress pulse appears after the reflection of the transmitted part of the input stress pulse at the free surface of the film. This is thought to be the principle driving force for the interface crack extension. Fig. 4 Normal stress acting at the outer node close to the interface crack tip The normal stress and shear stress contours for the time point t=1.03μs are shown in Figs.5 and 6,
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