The effects of cohesive strength and toughness on mixed-mode delamination of beam-like geometries

Cohesive-zone models have been used to study the effects of strength and toughness on the delamination of beam-like geometries. The conditions under which linear-elastic interfacial mechanics provide a good frame-work for predicting failure of such systems have been studied. It has been determined that the phase angle derived from LEFM calculations provides an excellent description of the partitioning between the mode-I and mode-II energy-release rates over a wide range of fracture-length scales. In particular, the nominal phase angle can be a useful parameter, even when the fracture-length scale is so large that the interface stresses do not exhibit the expected inverse-square-root dependence. The analysis has also shown that nominal phase angles with a magnitude greater than 90° can have physical significance, provided the interface layer is thick enough to accommodate compression without crack-surface contact. Finally, the role of modulus mismatch has been studied. A length scale introduced by the cohesive strength allows a crack-tip phase angle to be established, when LEFM predicts oscillating stress fields at the crack tip. This crack-tip phase angle is shifted from the nominal phase angle based on a characteristic geometrical length by an amount that depends on the cohesive parameters of the interface and the modulus mismatch. It has been shown that, as a result of this shift, both modulus mismatch parameters can influence the strength of an interface.