Keywords: delamination growth, decohesion elements, quasi-static and dynamic analysis.

Delamination is one of the predominant forms of failure in laminated composites when subjected to transverse loads and due to the lack of reinforcement in the thickness direction. Delamination can cause a significant reduction in the compressive load-carrying capacity of a structure. Decohesion elements are widely used at the interface between solid finite elements to predict and to understand the damage behaviour in the interfaces of different layers in composite laminates. However, numerical instabilities frequently occur when using the decohesion interface model to simulate the interface damages.

In this paper, a new decohesion element model is developed and presented to stabilize the finite element simulations of delamination propagation in composite laminates and to overcome these numerical instabilities. In this model, a pre-softening zone is proposed ahead of the existing softening zone. In this pre-softening zone, the initial stiffness and the interface strength are gradually decreased. The onset displacement corresponding to the onset damage is not changed in the proposed model. Moreover, the critical energy release rate of the materials is kept constant. Also, the traction based model includes a cohesive zone viscosity parameter to vary the degree of rate dependence and to adjust the peak or maximum traction.

The proposed decohesion element is implemented using the LS-DYNA finite element code as a user defined material (UMAT) using the standard library eight-node solid brick element. This approach for the implementation requires modelling the resin rich layer as a non-zero thickness medium. In fact, this layer has a finite thickness and the volume associated with the decohesion element can in fact be set to a very small value by using a very small thickness (e.g. 0.01 mm). To verify these procedures, the crack growth along the interface of a double cantilever beam (DCB) is studied. The two arms are modelled using the standard LS-DYNA eight-node solid brick elements and the interface elements are developed in a FORTRAN subroutine using an adaptive algorithm. The LS-DYNA code calculates the strain increments for a time step and passes them to the UMAT subroutine at the beginning of each time step. The material constants, such as the stiffness and strength, are read from the LS-DYNA input file by the subroutine. The current and maximum relative displacements are saved as history variables which can be read in by the subroutine. Using the history variables, material constants, and strain increments, the subroutine is able to calculate the stresses at the end of the time step by using the form of constitutive equations. The subroutine then updates and saves the history variables for use in the next time step, and outputs the calculated stresses.

The numerical simulation and the experimental results of the DCB in Mode-I are presented and compared to illustrate the validity of the new model in both quasi-static and dynamic analysis. It is anticipated that the proposed model brings stable simulations and can be widely used in quasi-static, dynamic and impact problems. Furthermore, the new model can be effectively used for a range of different element size (reasonably coarse mesh) and can save a large amount of computation.

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