Effect of interface control on fracture behaviour of woven C/C composites

Effects of fibre/matrix interface and matrix microstructure on the strength and fracture mechanism of C/C composite materials were investigated by coating bismaleimide-triazine co-polymer (BT-resin) on the surface of the carbon fibre and changing the heat-treatment temperature. Here, the high-modulus type carbon fibre was used to avoid the change of fibre properties during heat treatment. For the case of carbonized C/C composites (heat-treatment temperature=1600°C), the notched strength of composites without BT-resin coating was smaller than the net section stress failure criterion. The notched strength of the wider specimen was smaller than the narrower specimens. Thus, the composites indicated clear notch sensitivity. On the other hand, the notched strength of C/C composites with BT resin coating was well estimated by the net section stress criterion. For the case of graphitized C/C composites (heat-treatment temperature=2500°C), the notched strength was well estimated by the net strength criterion without respect to BT-resin coating. The change of the fracture mechanism in microscopic scale was well correlated to the change of the fracture criterion.