Sandwich test specimens for measuring interface crack toughness

A crack lying along one interface on an elastic sandwich structure is analyzed. When the thickness of the middle layer is small compared with the other length scales of the structure, a universal relation is found between the actual interface stress intensity factors at the crack tip and the apparent mode I and mode II stress intensity factors associated with the corresponding problem for the crack in the homogeneous material. Therefore, if the apparent stress intensity factors are known, for example calculated from the applied loads as if the structure was homogeneous, this information can be immediately converted into the interface stress intensity factors with the universal relation. This observation provides the theoretical basis for developing sandwich specimens for measuring interface crack toughness. The universal relation reveals the extent to which the asymmetry inherent to a bimaterial interface induces asymmetry in the near tip crack field. In particular, the result of the study can be used to infer whether stress intensity factors for a homogeneous body can be used with good approximation in place of the actual interface stress intensity factors. A proposal for simplifying the approach to interfacial fracture is made which plays down the role of the so-called oscillatory interface singularity stresses.