13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- Fig. 10 Shear stress acting at the outer node close to the interface crack tip for the three initial states Fig. 11 Film stress at the instant t=1.03μs for the three initial states For the moment, these results may be theoretically explained by the fact that the action direction of the residual stress is parallel to the interface. Therefore, the load component contributing to the interface separation developed by the in-plane residual stress would be slight when the deflection of the coating is small as considered in the present article. Even so, we hope more physical experimental outcome can be obtained to further support the present cognizance. Actually, a group of relevant tests are undertaken though much difficulty arises in the many sides of it, especially at the quantitative definition of the residual stress and the observation of the interface crack state. It is also worthwhile to note that these theoretical results are basically drawn from linear dynamic analysis. Such linear modeling could not include the large deformation of the coating, which may lead to more obvious effect of the in-plane residual stress on the interface stress and therefore the interface crack behavior. Moreover, as reveled by Wu [3], the residual stress would influence greatly the total in-plane stress in the coating, which will affect the cracking of the coating under impact. Once the coating fractures around the impact region edge, the interface crack would be arrested around the edge. In comparison, if the coating can endure the impact and maintain intact, the partially released residual stress would affect greatly the interface crack behavior. 4. Conclusions The model was set up to investigate the behavior of interface crack between the film and substrate subjected to stress wave, in which the crack surface contacting was considered. The dynamic deformation, stress and strain energy versus different crack lengths are calculated for the cases of three initial stress states, that is, initially stress free film, initially compressed film and initially tensioned film. The history of the normal stress acting at the interface center node indicate that the interface crack permits the transmission of compressive stress while denies the transmission of tensile stress. The dynamic open displacement at the inner node 5μm away from the crack tip further reveals that the crack surfaces seldom contact after separating by the first compressive pulse. The initial stress state would almost not influence the strain energy release rate of the interface crack or the interface stress around the crack tip if only small strain is taken into account in the modeling. Anyway, the film stress will be greatly changed by the residual stress, which will ultimately determine the fracture of the film.
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