In the analysis of composite delamination problems, the magnitudes of Mode I and Mode II stress intensity factors or energy release rates are typically not unique, due to oscillatory behavior of near-tip stresses and displacements. This behavior currently limits the ability to consistently apply interfacial fracture mechanics to predict composite delamination. The virtual crack closure technique (VCCT) is a method used to extract Mode I and Mode II components of energy release rates from finite element fracture models. Energy release rate components extracted from an oscillatory delamination model using the VCCT are dependent on the virtual crack extension length, Δ.
In this paper, a recently-developed modified VCCT from the literature is used to extract Δ- independent energy release rate quantities for composite delamination problems where the delamination occurs between two plies or ply groups modeled as in-plane orthotropic materials. Numerical cases studied are taken from existing work in the literature. Energy release rate ratios are compared to those obtained using other methods proposed in the literature for the analysis of oscillatory delamination problems. Unlike other methods, the method of mode separation used in this work does not involve altering the fracture model to eliminate its oscillatory behavior. Instead, it is argued that consistent, Δ-independent energy release rate quantities can be extracted directly from oscillatory solutions. For the cases studied, results show that mode mix values resulting from application of the modified VCCT are comparable to those obtained via existing methods. Some mode mix predictions obtained (in the literature) using existing methods lie outside the range of reasonable values for such problems, however. The Δ-independent energy release rate quantities extracted using the modified VCCT can serve as guides for testing the convergence of finite element models. This technique also has potential as a consistent method for extracting energy release rate quantities from numerical models of composite delamination.