Influence of Heat Treatment Temperature on Fracture Mechanism of C/C Composites

There are two major fracture criteria for notched C/C (carbon/carbon) composite materials. One is that the fracture strength is determined by the fracture toughness criterion, and the other is that it is simply decided by the net section stress failure criterion. In the present study, effects of crack depth, specimen size, and the heat treatment temperature on the strength and fracture mechanism of C/C composites were investigated by using double edge cracked plate tension specimen in order to clarify the confusion on the fracture criterion. For the case of carbonized C/C composites (heat-treatment temperature = 1600° C), the notched strength of wider specimens was smaller than the net section stress failure criterion, showing the notch effect. When the ligament length of the notch was shorter than about 8 mm, the fracture strength of notched specimen was well correlated to the net section stress criterion. On the other hand, the notched strength of graphitized C/C composites (heat treatment temperature = 2500 and 3000°C) was well estimated by the net section stress criterion without respect to the ligament length. The strength of smooth specimen for C/C composites was almost the same without respect to the heat-treatment temperature, while the fiber strength decreased by half with increasing the heat-treatment temperature. Thus, the contribution of the fiber strength to composites strength was much higher for the graphitized C/C composites, The change of fracture mechanism in macroscopic and microscopic scale was well correlated to the change of the fracture criterion.