Delamination

Helius PFA provides a powerful User-Defined Material Subroutine for modeling cohesive element-based delamination.

This cohesive User Material Subroutine provides several commonly used constitutive laws for delamination initiation and progression. However, these constitutive laws are treated with the same convergence-enhancing technology that Helius PFA has previously applied to intra-laminar material failure. Consequently, Helius PFA provides unprecedented robustness for modeling the simultaneous evolution of intra-laminar and inter-laminar (delamination) failure of composite materials and structures. This section of the Theory Manual discusses the specific details of the cohesive element-based delamination capability. For specific instructions for utilizing the cohesive element-based delamination capability, please see the Helius PFA User's Guide.

There are two commonly used methods in finite element analysis for simulating delamination, namely, the Virtual Crack Closure Technique (or VCCT), and the Cohesive Zone Method. The Virtual Crack Closure Technique is a global node-based method that utilizes node reaction forces and displacements to estimate energy release rates. The Cohesive Zone Method is most often implemented as an element-based approach that relies on interface constitutive relations to predict and simulate delamination. The cohesive element-based formulation is used in Helius PFA for a number of different reasons, listed below.

1. The cohesive element-based formulation predicts both delamination initiation and delamination propagation, while the VCCT predicts only propagation of existing delaminations.
2. The cohesive element-based formulation does not require the imposition and management of no-penetration boundary conditions for delaminated surfaces since these are handled directly by the element's constitutive relations. The VCCT requires considerable additional logic for managing the imposition of no-penetration boundary conditions for delaminated surfaces.
3. The cohesive element-based formulation is completely compatible with any other forms of nonlinearity that are simultaneously present in the solution. In contrast, the VCCT assumes linear elastic fracture behavior. Thus, any other nonlinearities present must be fixed (held constant) at any particular point in time where the energy release rate is computed and crack advancement is considered.