Entwicklung robuster Interface-Elemente für die Simulation von Delaminations-Prozessen in CFK-Laminaten

• Applicant: Professor Dr.-Ing. Werner Wagner
• Subject Area: Mechanics ; Applied Mechanics, Statics and Dynamics
• Term: from 2010 to 2014
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 181611900

Delamination is a commonly observed failure mode in composite materials. Due to stiffness changes and the lack of reinforcement in thickness direction, delamination already occurs by low loads and can yield total structural collapse. For instance, interface elements are used to simulate thin resin layers between lamina, providing the opportunity for an adequate simulation of crack initiation and propagation. Typically, linear and exponential cohesive laws are used to simulate the softening behavior. The drawback of the cohesive zone model is the requirement of a correct reproduction of the stress distribution ahead of the crack tip. This means that very fine meshes, due to high stress gradients in the delamination process zone and the associated partial bonding in one element, are needed. Thus, the element size along the delamination part is defined by the interface elements and not as usual by the adjacent shell elements. The purpose of this research is to improve the robustness of discontinuous interface elements with different numerical techniques. In the application for research the results of the past work is summarized and later on providing prospective works are presented. In the research done so far, the interface element has been improved by a multi hybrid formulation from Hu-Washizu with bilinear and special developed shape functions for stresses and strains whereby the element size could be reduced by 50 percent. High order and adapted integrations schemes were investigated to describe the total stress distribution in one element. Finally the bilinear element kinematic has been enhanced to quadratic and serendipity shape functions to give a better approximation of the displacements within the delamination process zone. In the subsequent research an additional mixed IF-Formulation by Hellinger-Reissner (HR) will be developed and implemented into the element algorithm. Providing free shape functions for stresses, the mixed HR-Formulation offers a good approximation of the stress distribution ahead of the crack front and can advance the robustness. Further investigations in the field of enhanced kinematics shall soften the stiffness and avoid oscillation in the load-displacement curve. Beside the current nodal displacement degrees of freedom at each node, additive rotational degrees of freedom and additive virtual element degrees of freedom are to be implemented in the interface formulation. Afterwards, the additional degrees of freedom will be integrated in the developed hybrid interface. Finally, focusing on the improvement of robustness the cohesive law will be amplified. In the current state of work the exponential cohesive law is based on relative displacements. The contact formulation and the mixed mode problem, including a cohesive law depending on the interlaminar fracture toughness, are to be reviewed in collaboration with the DLR and the RWTH Aachen.

• DFG Programme: Research Grants