SEPARATION OF CRACK EXTENSION MODES IN ORTHOTROPIC DELAMINATION MODELS

In modeling a crack along a distinct interface between dissimilar elas tic materials, the ratio of mode I to mode II stress intensity factors or energy release rates is typically not unique, due to oscillatory b ehavior of near-tip stresses and displacements. Although methods have been developed for comparing mode mixes for isotropic interfacial frac ture problems, this behavior currently limits the applicability of int erfacial fracture mechanics in predicting delamination in layered mate rials without isotropic symmetry. The virtual crack closure technique (VCCT) is a method used to extract mode I and mode II energy release r ate components from numerical fracture solutions. Energy release rate components extracted from an oscillatory solution using the VCCT are n ot unique due to their dependence on the virtual crack extension lengt h, Delta. In this work, a method is presented for using the VCCT to ex tract Delta-independent energy release rate quantities for the case of an interface crack between two in-plane orthotropic materials. The me thod does not involve altering the analysis to eliminate its oscillato ry behavior and it is similar to existing methods for extracting a mod e mix from isotropic interfacial fracture models. Knowledge of near-ti p fields is used to determine the explicit Delta dependence of energy release rate parameters. Energy release rates are then defined that ar e separated from the oscillatory dependence on Delta. A modified VCCT using these energy release rate definitions is applied to results from finite element analyses, showing that Delta-independent energy releas e rate quantities result. The modified technique has potential as a co nsistent method for extracting a mode mix from numerical solutions. Th e Delta-independent energy release rate quantities extracted using thi s technique can also aid numerical modelers, serving as guides for tes ting the convergence of finite element models. Direct applications of this work include the analysis of planar composite delamination proble ms, where plies or debonded laminates are modeled as in-plane orthotro pic materials.