Experimental study on the fabrication, high-temperature properties and failure analysis of 3D seven-directional braided composites under compression

Three-dimensional seven-directional (3D7d) braided composites had potential application in aerospace field and they were successfully fabricated. The compression mechanical properties of composites are thoroughly studied based on different load modes and high temperature influence. Then, the failure mechanism is evaluated using Macro-fracture and SEM micrographs. The results show that as temperature increases, the stress vs. strain curves decrease monotonically and present nonlinearity feature. Meanwhile, the out-of-plane compression properties among three loading direction modes is the best, following by longitudinal, and transverse is the worst. 3D7d structure significantly improves the out-of-plane properties compared to 3D6d, 3D5d and 3D4d structure. Meanwhile, it also makes the transverse properties higher than those of 3D5d and 3D4d structure. Moreover, failure mechanism of 3D7d braided composites varies with loading mode and temperature. Out-of-plane compression shows multiple 45° shear failure, and compacted gradually. Longitudinal compression behaves as local outward expansion and multiple shear features along the braiding yarn/matrix interface. Transverse compression exhibits shear fracture at 45° in-plane direction. At high temperatures, the shear fracture feature still dominated the main failure. The matrix gets soft, cracks and comes plastic, and the interfacial adhesion of fiber/matrix is significantly reduced.