Numerous studies have been carded out in recent years to gain a better understanding of the structural integrity of the interfaces in modern composite systems. These problems are complicated by the fact that at the bimaterial interface crack tip, the tensile and shear effects are coupled, and the fracture resistance of the interface will generally depend on the degree of this mode mixity. Several test specimens have been proposed (see, e.g., Hutchinson and Suo [1]) for determining the interface fracture toughness and crack propagation characteristics of dissimilar isotropic materials. In the study of the fracture behaviour of layered materials, however, beam-type specimens, such as the double cantilever beam (DCB) specimen and the end-loaded-split (ELS) specimen are usually used. These two specimens are commonly employed for the determination of the mode I and mode II fracture toughness of, for example, composite laminates. In these cases, homogenised in-plane elastic constants are generally assumed, and calibration expressions giving the strain energy release rates for completely homogeneous, isotropic as well as orthotropic specimens have been obtained [2]- [5].Recently, the present authors [6] have obtained, using the Boundary Element Method (BEM), calibration results for the interface crack between two orthotropic layers of thickness h in DCB and ELS specimen set-ups. They relate the stress intensity factors, K 1 and K 2, and the strain energy release rates, G, to the normalised crack-length a/h. The material system treated was one in which an orthotropic, transversely isotropic layer was bonded to another layer of equal thickness made of the same material, but the latter being in the plane of transverse isotropy. The effects of the material properties were analysed with respect to two parameters, rll and r12, which correspond to the purely imaginary roots of the characteristic equation for the material. These parameters are related to the commonly used engineering constants as follows:(1) Int Journ of Fracture 72 (1995)