Applicability of fracture toughness concept to fracture behavior of carbon/carbon composites

Carbon/Carbon (C/C) composites have attractive mechanical properties such as superior specific strength and high elastic modulus at high temperature exceeding 2 000°C in an inert atmosphere. However, mainly due to lack of knowledge of design criteria, C/C composites have not been used in primary heat resistant structures. For example, almost no unified explanation has been given about fracture behavior of C/C composites. The objective of this paper is to examine whether the linear elastic fracture mechanics (LEFM) is able to be applied to a C/C composite. The LEFM is tried to apply to fracture behavior in tensile tests of the double-edge-notched and compact tension specimens and in the four-point bending test of the single-edge-notched specimens. It was found that the results of three kinds of fracture tests can be consistently and rationally explained by the LEFM concept with the aid of tf-curve behavior if the pre-crack length is long enough to be able to neglect the notch tip radius. From fractographic observation it was found that R-curve behavior of the C/ C composite was mainly attributed to the fiber-bridging effect near the notch tip.