Subsurface cracks in a homogeneous material and those on the interface of coated materials are analyzed with the aid of the finite element method.
Two loading cases are considered. The first case is a subsurface crack problem where a moving concentrated load is applied on the surface of the coating layer. The fracture parameters for the subsurface crack the faces of which are possibly under partly frictional contact, are evaluated numerically for various cases such as different combination of materials of the coating layer and the substrate, the change in the ratio of the subsurface crack length to the thickness of the coating layer and the change in the coefficient of friction both on the crack surface and on the surface of the coating layer. The effects of the combination of materials, the geometry of the subsurface crack and the magnitude of the coefficient of friction on the fracture parameters are discussed. It is found that the stress intensity factor becomes larger as the crack length to coating thickness ratio and Dundurs' parameter α becomes larger and the coefficient of friction on the crack surface becomes smaller. It is also found that the magnitude of friction on the surface has negligible effect on the variation of the stress intensity factor. The conditions for the subsurface crack to propagate along the interface or to kink out of the interface are also examined.
The second case is a problem wherein a out-of-plane concentrated load is applied to the surface of the coating layer. A finite element equation for the problem subjected to out-of-plane loading is firstly derived. The stress intensity factors and the energy release rates for this problem are calculated for various cases such as different combination of materials and the change in the geometry of the subsurface crack. Also the energy release rates for the kinked subsurface crack under the same loading conditions are calculated. It is found that the energy release rate for the subsurface crack in a homogeneous material gets its maximum value when the crack tip kinks toward the surface, and the energy release rate for the subsurface crack in a coated material gets its maximum value when the crack tip kinks into the substrate.
