Predicting the constitutive behavior of biaxial braided composites using beam unit cells

Braided composites have a complex internal geometry with two or more sets of intertwined and undulated yarns. The internal geometry strongly depends on the shape of the desired component and on process parameters. This increases the demand for material characterization and constitutive modeling for structural analysis. This paper presents an approach for the prediction of the nonlinear material behavior of biaxial braided composites using finite element (FE) unit cells. A repeating unit cell (RUC) with beam elements representing the yarns is used for the prediction of the constitutive behavior. The approach, based on Cox's binary model [1], yields several advantages: a fast and automated model generation and meshing process in combination with numerical efficiency due to the decreased number of degrees of freedom (DOFs). Main goal of the unit cell calculations is to predict the influence of the internal geometry of the braided composite onto the elastic, nonlinear and failure behavior. Two biaxial braided composites with 45° and 60° braiding angle are modeled in this paper. Elastic and nonlinear predictions match the experimental results and the stress field obtained correlates well with a classical continuum unit cell.