Dynamic damage and fracture mechanism of three-dimensional braided carbon fiber/epoxy resin composites

Three-dimensional braided carbon fiber/epoxy resin composites are significant structural materials in the fields of astronauts and aeronautics. The effect of the process and test parameters on the mechanical properties was studied in this paper. Optical microscope and field emission gun scanning electron microscope (SEM) were used to analyze the macro- and micro-fracture morphology. The fracture morphology of the three-dimensional braided carbon fiber/epoxy resin composites was varied under different loading rates. The results indicate that the mechanical properties can be significantly affected by the parameters of braiding. By decreasing the braided angle the elastic modulo E and ultimate tensile strength showed an obvious increase. The fracture process depends on stress transfer behavior. The fracture characteristic was brittle, which originated from plastic constraint at the fiber. Fiber fracture was caused by a defect on the surface of the carbon fiber in low loading rate. As the explosive impact test was carried out in a special attachment, superior energy absorption capability and damage tolerance were found. Due to high bursting pressure a surface ablation region was formed and a notch was observed in the region. The fibers in the surface ablation region were sheared off and pulled out. The fracture of fiber was scarcely related to any fiber defects in this region. There was a damage region under the ablation layer. Fracture morphology revealed that the epoxy area between fibers showed crack growth traces, which looked like a river pattern. The river pattern is the sign of brittle fracture and the direction of the river pattern is toward the direction of crack spreading. The fracture process showed that damage formation were fiber sheared off and epoxy flaked off under the ablation region. The propagation path of the macro-crack is mainly in the braided sheaf and along the sheaf interface under the ablation region.