An augmented cohesive element for coarse meshes in delamination analysis of composites

Numerical analysis of delamination failure in composite laminates is very often carried out with cohesive interface element in a finite element framework, due to their robustness and ease of use. Explicit time integration is often preferred, to overcome material and damage non-linearity and impact loading scenarios. However, a very fine mesh is needed to correctly represent the nonlinear cohesive zone ahead of the numerical crack tip to achieve a stable and mesh size insensitive global failure response. This is especially true for mode I delamination cases, where the cohesive zone is extremely small, owing to a low critical energy release rate. This mesh size requirement limits the applicability of cohesive zone models largely to coupon scale problems. The current work augments cohesive elements with additional rotational degrees of freedom at the nodes, which enables a higher order displacement approximation and allows relatively coarser mesh to be used. The present formulation is applied to a mode I delamination benchmark problem, which demonstrates the superiority of the augmented element over the traditional cohesive element in terms of coarse-mesh accuracy and computational time saving.