Shear Failure Analysis of 3D Seven-Directional Braided Composites

According to the “four-step” braiding technique, this paper analyzed the movement laws of internal space yarns in the projection plane and in space, and established a 3D seven-direction braided composite material solid finite element model by combining the observed yarn directions on the surface and side of the preform. Based on the 3D Hashin criterion, a progressive damage failure analysis method was adopted to study the shear failure mechanism of 3D seven-direction braided composites. The effect of braided angles on the elastic properties and crack propagation path of the fabric was discussed. The failure modes and mechanisms of 3D seven-directional braided composites were also observed and analyzed by electron microscope and scanning electron microscope. The results show that the shear failure mode is mainly caused by matrix failure, interface damage, and fiber fracture and pull-out. The fiber cross-sections at the shear fracture surface are at a certain angle, indicating that the fibers are subjected to both transverse and axial stresses during the shear process. It also can be seen that the initial damage occurs at the interface between the matrix and the fiber bundle. As the crack propagates, tensile damage occurs inside the fiber bundle, forming cracks and eventually leading to the failure of the entire structure. The accuracy of proposed method for predicting 3D seven-directional braided composites was verified by the shear experimental results.