Real-time observation of damage nucleation in three-dimensionally reinforced carbon/carbon composites and role of bundle/matrix interface

Carbon/carbon composites (C/Cs) are widely used as thermal protection materials in aviation and aerospace field. Most of the fracture processes of C/Cs have been found to be profoundly affected by their interfacial properties. Specially arranged fiber bundle push-out test was utilized to determine the fiber bundle/matrix interface shear strength of one three-dimensionally reinforced C/Cs. In order to reveal the link between the interfacial properties and the failure behavior, the micromechanisms of damage initiation at the notch tip on the C/Cs was investigated in real-time during the flexural fracture tests through scanning election microscopy. Real-time fracture observation revealed that the damage was nucleated in the fiber bundle/matrix interfaces around the notch tip and the failure crack was successfully observed along the fiber bundle/matrix interface. Fiber yarns acted as an obstacle to crack propagation hence it was necessary to increase the load to propagate the crack through the next fiber yarn.