Mixed-mode Cohesive-zone Models for Delamination and Deflection in Composites

Cohesive-zone models for interfaces incorporate both strength and energy parameters. Therefore, they provide a natural bridge between strength-based models and energy-based models for fracture, allowing delamination to be described by a single framework that covers a range of applications for which the strength or energy criteria alone might not be sufficient. In this paper, the relationships between cohesive-zone models and fracture models based on strength or energy are discussed. A mixed-mode formulation of cohesive-zone models has been used to investigate a number of issues related to the delamination of interfaces. It has been shown that linear-elastic fracture mechanics (LEFM) provides an excellent description of mixed-mode delamination, beyond the limits where LEFM would usually be thought to be appropriate. In particular, the concept of a nominal phase angle, calculated from stress-intensity factors is very robust. Compressive normal stresses on interfaces can be accommodated by the finite thickness of cohesive-zone models, resulting in increased levels of nominal toughness. The length scale naturally associated with cohesive zones allows them to describe mixed-mode fracture of interfaces with a modulus mismatch across them. Cohesive-zone models have also been used to explore the phenomenon of crack deflection at interfaces. The results of these calculations appear to be more consistent with strength-based models of deflection, rather than with energy-based models. They indicate that the strength ratio between the cohesive strengths of the interface and substrate play an important role in determining whether crack deflection or propagation occurs.