Contribution to the modeling of braided composites

Abstract

During the past decades, the applications of braided composites have increased rapidly in a wide variety of sectors including aerospace, automotive, and marine industries. Their intensive use in engineering applications has inevitably created the need to build models in order to determine their mechanical properties. However, the beneficial qualities of braided composites come at a cost in terms of analysis: their mechanical behavior is significantly more difficult to model because of the intrinsic complexity of their architecture. The present work is carried out in collaboration with GDTech engineering and the University of Liège within the framework of the ViBra (i.e. Virtual Braiding) project, whose main goal is to set up reliable numerical simulation tools for the study of braided composites. Specifically, this thesis focuses on the evaluation of the effective elastic mechanical properties of two-dimensional triaxial braided composite materials through finite element analysis. The proposed approach is a homogenization-based multi-scale modeling procedure with a focus made on the meso-scale level. It requires the development of a robust and fully-parametrized model capable of generating the Representative Unit Cell (RUC) of any braided composite material. The model can generate lots of different braided architectures: in addition to the basic geometrical parameters, the model can adapt the tows cross-sectional shape, the undulation path of the tows, or the braiding pattern (i.e. diamond or regular). Material properties were assigned to each constituent of the RUC, taking into account the variation of local orthotropic direction of the fibers inside the tows. Combined with Periodic Boundary Conditions (PBC), homogenization simulations were performed and effective elastic properties were extracted. The methodology developed in the thesis is then validated by making a comparison between an article of the literature and results coming from the present model where a good agreement is achieved. Subsequently, a parametric study is performed to study the influence of the braiding angle on the effective elastic properties. The study is carried out on both diamond and regular braids, with a braiding angle varying from 15° to minimum 70°.