Unit-cell geometry and mechanical properties of three-dimensional seven-directional braided composites

Three theoretical microstructural models were constructed based on manufacturing three-dimensional seven-directional (3D7d) braided preforms, and their relationships with braiding angles, fiber volume fractions, and yarn filling factor were investigated. The influences of braiding parameters, loading directions, and braiding structure on the performances of 3D7d braided composites were predicted in combination with a bridging model. The results concluded that the longitudinal modulus and tensile strength decreased with increasing braiding angles, while the transverse modulus and strength were opposite, and the performances of composites were better at higher fiber volume fractions. Compared with 3D4d braided composites, the transverse strength of 3D7d braided composites improved significantly. Furthermore, the three microstructural models were damaged in the order of interior, surface, and corner unit cell models during tensile loading. Especially, the fifth and seventh yarns within unit cell models were damaged firstly under longitudinal and transverse loads, respectively. This method adopted in this work provides a basis for predicting the mechanical properties of 3D7d braided composites.