Influence of heat treatment temperature on fracture mechanism of C/C composites

摘要

There are two major fracture criteria for notched C/C (carbon/carbon) composite materials. One isthat the fracture strength is determined by the fracture toughness criterion, and the other is that it issimply 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/Ccomposites were investigated by using double edge cracked plate tension specimen in order to clarifythe confusion on the fracture criterion. For the case of carbonized C/C composites (heat-treatmenttemperature 1600℃), the notched strength of wider specimens was smaller than the net sectionstress failure criterion, showing the notch effect. When the ligament length of the notch was shorterthan about 8 mm, the fracture strength of notched specimen was well correlated to the net sectionstress criterion. On the other hand, the notched strength of graphitized C/C composites (heattreatment temperature 2500 and 3000℃) was well cstimated by the net section stress criterionwithout respect to the ligament length. The strength of smooth specimen for C/C composites wasalmost the same without respect to the heat-treatment temperature, while the fiber strengthdecreased by half with increasing the heat4reatment temperature. Thus, the contribution of the fiberstrength to composites strength was much higher for the graphitized C/C composites. The change offracture mechanism in macroscopic and microscopic scale was well correlated to the change of thefracture criterion.