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The notched fracture strength using the double edge notched plate tension specimen of the woven C/C composite was calculated by the modified shear lag analysis method, in combination with the average stress criterion (composite fractures when the average stress over the characteristic distance in the fiber bundle at the notch tip reaches the strength of the fiber bundle). The present method could describe the following features experimentally observed in our former work. (i) The fracture strength was lower than the net strength criterion and decreased with increasing width when the debonding at the notch tip was short. (ii) The fracture strength was well estimated by the net strength criterion when the debonding was long. The results of calculation for wider specimens revealed the following; the notched strength remains nearly constant for any width given by the net strength criterion when debonding length is very long; it decreases with increasing width unless the debonding length is very long; the strength-width curve of the composite with finite debonding length is in the range of the upper bound given by the net strength criterion and the lower bound by the fracture toughness one.

The effects of interface control and matrix microstructure on the interlaminar shear strength and mode II interlaminar fracture toughness of 5H satin woven C/C composites were investigated by coating bismaleimide-triazine co-polymer (BT-resin) on the surface of carbon fiber and changing the heat-treatment temperature. Three point short beam flexure tests were carried out for the shear strength. End notched flexure specimens were used for the mode II interlaminar fracture toughness tests. Both the interlaminar strength and toughness decreased by coating BT-resin and increasing HTT from 1600°C to 2500°C. However the influence of the BT-resin coating was much larger for the mode II interlaminar fracture toughness than that for the interlaminar shear strength. This difference of the effect of interface control was discussed on the basis of microscopic observation.

Effects of fiber/matrix interface and matrix microstructure on the mode I interlaminar fracture toughness of C/C composite materials were investigated by coating bismaleimide-triazine co-polymer (BT-resin) on the surface of carbon fiber and changing the heat-treatment temperature (HTT). For the case of laminates with HTT of 1600°C (carbonized C/C composites), the initial fracture toughness, GIC, was insensitive to BT-resin coating. Moreover, the fracture toughness during crack propagation, GIR, increased by coating BT-resin. On the other hand, both GIC and GIR decreased with BT-resin coating for the laminates with HTT of 2500°C. While both GIC and GIR are insensitive to HTT for laminates without BT-resin coating, they both decreased by increasing HTT for laminates with BT-resin coating. The difference of the effects of interface control and HTT was discussed on the basis of microscopic mechanism consideration. Comparison between in-plane and interlaminar strength indicated the possibility to optimize the interface control.

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.