Master thesis and internship[BR]- Master's thesis : FEM Analysis of Braided Composite Materials[BR]- Integration internship

Abstract

These past decades, braided composites have been used in a variety of industries including aerospace, automotive, sporting, medical and marine applications. Their use in more advanced fields requires robust models to represent their mechanical behavior. However, due to their geometric complexity, braided composite are difficult and costly to model.

The goal of this thesis is to study the mechanical response in stiffness and strength of 2D triaxially braided composites thanks to different numerical approaches. This work is inscribed in the framework of the ViBra (Virtual Braiding) project. It is performed in collaboration with GDTech engineering and the LTAS-Computational & Multiscale Mechanics of Materials (CM3) laboratory which is part of the Aerospace and Mechanical Engineering Department at the University of Liège.

First, two existing methods, the 4-plies and Representative Unit Cell (RUC) approaches, are presented, tested and validated thanks to examples found in the literature. The 4-plies approach represents the tows and matrix of the braid as UD plies and retrieves the braid stiffness properties thanks to the Classical Laminate Plate Theory. The RUC approach is a geometrical based approach. It is able to generate a RUC and perform a 2-step homogenization to retrieve the braid stiffness properties. Qualitatively, the two methods have shown good consistency with experimental data.

Secondly, a third method, the 3-plies approach, has been developed to build a Finite Element Model (FEM). The 3-plies approach is based on the 4-plies approach and allows to represent the triaxial braids as three UD plies corresponding to the different tows orientations instead of four with a pure matrix ply. This approach is more convenient for the FEM and allows to avoid premature failure due to a pure matrix ply. The resulting plies stiffness properties are used as inputs in the FEM. The FEM analysis has been conducted thanks to the Siemens NX software with the SAMCEF solver. Its results have been validated thanks to physical tests data found in the literature.

Finally, a Progressive Damage Model (PDM) has been integrated in the FEM to study the mechanical response in strength and the failure mechanisms of braided composites. Its results were compared to physical tests results. It was found that the failure predictions were not satisfactory. Therefore, the material model used to represent damage needs to be improved in future works.