A two-scale method for predicting the mechanical properties of 3D braided composites with internal defects

Pore defects have significant effect on the mechanical properties of 3D braided composites. Two-scale finite element models of the fiber tows and the braided composites are developed to predict the elastic properties and the micro stress of 3D braided composites. Two basic types of pore defects, the voids in resin matrix and the dry patches in fiber tows, have been taken into account. Periodical boundary conditions are applied on the two-scale FE models by the coupling and constraint equation defining commands in ANSYS. The predicted effective elastic constants agree well with the available experimental data, demonstrating the validity of the two-scale FE models. The effects of braiding angle and fiber volume fraction on the engineering elastic constants are discussed. Furthermore, the effects of two types of defects on the effective elastic properties of 3D braided composites are also discussed in detail. Some useful conclusions are drawn herein. From simulation, dry patches in fiber tows have more significant effect on the elastic properties than voids in matrix.