Applicability of fracture toughness concept to fracture behavior of carboncarbon composites

Carbon/Carbon(C/C) composites have attractive mechanical properties such as superior specific strength and high elastic modulus at high temperature ex ceeding 2000 degrees C in an inert atmosphere. However, mainly due to lack of knowledge of design criteria, C/C composites have not been used in prima ry heat resistant structures. For example, almost no unified explanation ha s been given about the fracture behavior of C/C composites. The objective o f this paper is to examine the adequacy of the linear elastic fracture mech anics (LEFM) as the fracture criterion of notched C/C composites. Thus the LEFM was tried to be applied to fracture behavior in tensile tests of doubl e-edge-notched and compact tension specimens and in four-point bending test s of single-edge-notched specimens. It was found that the results of three kinds of fracture tests can be consistently and rationally explained in ter ms of the LEFM concept with the aid of R-curve behavior if the pre-crack le ngth is long enough to be able to neglect the notch tip radius. From fracto graphic observation it was found that R curve behavior of C/C composites wa s mainly attributed to the fiber-bridging effect near the notch tip.