Prediction of failure of structures and materials remains an important topic in engineering design. In that respect, cohesive laws, describing the mechanical behaviour of a failure process zone, have shown to be useful tools in numerical simulation of crack growth [
], in particular in structures in which large-scale bridging develops [
]. The modelling has been rather advanced and is now reasonably well established. However, the determination of cohesive laws, which in most case can be regarded as material properties [
], still remains an unresolved issue. This is particular true for mixed mode cracking. Mixed mode cohesive laws are sometimes interpolated from measurements of pure mode I and pure mode II tests [
]. Furthermore, the normal stress is often assumed to depend on the normal crack opening, but not on the tangential crack opening displacements. It is difficult to evaluate the validity of such assumptions.