Key issues in microstructure modeling of 3D braided composites

Numerical analysis based on micromechanics is an effective method to study the mechanical properties of three-dimensional (3D) braided composites, in which the establishment of micromechanics model is the basis of mechanical analysis. The direction cosine of fiber yarns and the characterization of geometric parameters of microstructure are two key issues in the modeling of microstructure. Focusing on these two key issues, the three-cell model of 3D four-directional braided composites are studied to comprehensively analyze various spatial directions of fiber yarns in the 45° division and horizontal division of unit cell, so as to obtain the detailed fiber yarn direction cosine. Aiming at the cross sections of three types of fiber yarns, the complex relationship between the braiding process parameters and the geometric parameters of the unit cell is analyzed and deduced. The unit cell structure is quantitatively characterized by using the braiding process parameters as the initial parameters, and the calculation methods of the volume fraction of fiber yarns and yarn filling coefficient are obtained. Finally, the predicted elastic constants by stiffness-volume averaging method are compared with experimental results, demonstrating the analysis results of fiber yarns directions. This paper can provide a theoretical reference for the microstructure modeling of 3D multi-directional braided composites, and has certain practical value in engineering.