Delamination between layers with different fiber orientations is an important problem in applications of fiber composites. Although finite element analysis is widely used to calculate energy release rates, results in the literature and in the present work show that the individual Mode I and Mode II energy release rates do not converge, and this is typical of interface cracks at bimaterial interfaces. This problem was addressed by noting that delaminations occur at the resin-rich layer between layers with different orientations, and refining the analysis to include this feature. As has been noted by others, enclosing the delamination crack in an homogeneous layer removes the difficulties associated with interface cracks, and convergence of Mode I and Mode II energy release rates is achieved. Results are given that establish the effect of the thickness and material properties of the interface layer on the Mode I and Mode II energy release rates. Calculations for beam-type geometries that are representative of double cantilever beam and end-notched flexure specimens are presented, that illustrate the effect of material properties, loading, and the compliance of the delaminated sections on the mode mixity.
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