Experimentally characterized critical interfacial fracture energy is often written as an explicit trigonometric function of mode-mixity and is used to determine whether an interfacial crack will propagate or not under given loading conditions for an application. A different approach to assess whether an interfacial crack will propagate is to employ a failure locus consisting of the critical fracture energies corresponding to different fracture modes, represented by an implicit formulation. Such a failure locus can be linear, elliptical, among other shapes. As it is nearly impossible to obtain isolated GIc or GIIc values through experimentation, extrapolations are used to determine these two extreme values based on intermediate experimental data. However, the magnitude of these extreme values as well as the shape of the two forms of failure curves are at risk of being inconsistent should proper care not be taken. An example of such an inconsistency would be to use a trigonometric formulation to obtain the extreme values through extrapolation and then employ those values in simulation through an elliptical failure. In this work, we have employed a series of commonly used interfacial fracture energy measurement techniques over a range of mode-mixities for a metal/polymer interface to demonstrate the potential discrepancy in the two approaches and to underscore the need for a consistent approach in evaluating interfacial crack propagation.