The development of layered material systems for aerospace components (including laser-deposited materials, multi-layer damping treatments and a variety of protective coatings) requires methods for predicting interfacial fatigue crack growth under sustained mixed-mode loading.  To this end, a dissipated energy theory of fatigue crack growth for homogeneous materials under mode I loading has recently been extended to a general bimaterial specimen configuration under sustained mixed-mode I/II loading. The dissipated energy theory allows the prediction of fatigue crack growth rates based on numerical results for the plastic dissipation per cycle in conjunction with a substantially reduced test matrix of cyclic constitutive and monotonic fracture properties, without the need for extensive crack growth testing.  As such, it provides a method for rapid evaluation of prospective new material systems, as well as insight into the design of fatigue-resistant interfaces.  
    An inherent assumption of the authors' prior work is that a perfectly sharp interface exists between the top and bottom layers, when in reality a grading of mechanical properties can exist across the interface.  As such, the current work extends the approach of the previous studies to incorporate a grading of yield strength between the two layers through parametric finite element modeling with ABAQUS.  Results suggest that incorporation of a graded layer acts to increase the overall amount of plastic dissipation, which is dominated by the weaker (i.e., lower yield strength) material.  However, this increase is bounded between the numerical results for a perfectly sharp interface (no graded layer) and those for a homogeneous specimen of the weaker material (no strength mismatch), and is small relative to the effect of applied mode-mix ratio. Overall, results suggest that while the presence of a graded layer can have a measurable effect on the plastic dissipation, the assumption of a perfect interface is reasonable for typical material systems.