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It now appears generally accepted that the process of stress-induced orientation and graphitization of a thermoset-resin-derived matrix in a carbon-fiber/carbon-matrix (C/C) composite is principally a result of molecular orientation induced during the pyrolysis process as a consequence of the restraint of pyrolysis shrinkage at the fiber/matrix interface by attractive or frictional forces between fiber and matrix. We hypothesize that the critical factor for the formation of lamellar graphite (by subsequent high-temperature heat treatment), instead of fibrillar or isotropic glassy carbon, is a state of multiaxial deformation in the pyrolysis step. Finite-element stress analyses of the relative stresses in the regions of interfilament matrix, as the matrix pyrolyzes from polymer to carbon, reveal patterns of biaxial and triaxial stress consistent with experimental observations of lamellar graphite formation in the matrix by the techniques of optical microscopy, scanning electron microscopy, and transmission electron microscopy. The implications of localized matrix orientation and graphitization for C/C composite properties are discussed in terms of a “duplex” system of composite reinforcement. An example is presented showing crack deflection and blunting at the matrix/matrix-sheath interface produced as a result of such orientation and graphitization.

A test program was conducted to investigate the failure modes of undamaged and damaged graphite-epoxy cylindrical tubes subjected to internal pressure. Some tubes were intentionally damaged by either a longitudinal-line cut or a single-point impact. Both types of damage were intended to simulate fiber breakage caused by a local damage. The objectives of the program were to investigate the failure modes of graphite-epoxy cylindrical tubes subjected to internal pressure, with and without local damage, and to qualitatively determine the relative burst pressure degradation associated with preexisting local damage to the tubes. A high-speed motion analyzer was used to record images of the fractures. The images provide the failure mode information that was not available in existing literature. Testing was conducted under three conditions: hydraulic pressurization, pneumatic pressurization with solid inserts, and pneumatic pressurization with inert propellant inserts. A tube with a longitudinal cut can fail by three different modes: a local leakage mode, a bursting mode with fracture initiating from the cut, or a bursting mode with complete tube disintegration. In testing of impact-damaged tubes, the results show that the burst pressure decreases with increasing impact load. At an impact load of 1493N (335 lb), the burst pressure decreases by 33% compared to the undamaged condition.

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