Mode-I fracture failure mechanism of 3-D braided composites under low-velocity wedge-loaded impact

Mode-I low-velocity impact fracture very often happens during the lifetime service of fiber-reinforced composites. Three-dimensional braided carbon fiber/epoxy composites (3DBC) have higher fracture toughness than laminate, while the Mode-I fracture is also an important behavior. Here we report the Mode-I fracture of 3DBC under low-velocity impact. A single cleavage triangle (SCT) specimen was prepared for the Mode-I impact test. A high-speed camera was used to capture images of the fracture initiation and growth. The inner damages were observed using X-ray microcomputed tomography (Micro-CT). The fracture behaviors were also compared among three braided angles and three impact energies. We found that the crack propagation follows the path of the crimped yarn. The propagation direction changes upon reaching interweaved points. The energy absorption at rupture increases as the braiding angle. A finite element analysis (FEA) model was developed to analyze the internal crack propagation behaviors and failure mechanisms. The fracture mechanisms of 3DBC have been compared between the tests and the FEA results. It was found the braided angle of 45°has a higher impact fracture toughness than the can 10°, 20° and 30° samples.