The influence of an oxidation inhibitor on the elevated temperature fracture resistance of carbon/carbon composites
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This fracture study evaluates the role of a fiber/matrix interfacial glass on the toughening of two different carbon/carbon (C/C) composites. Both composites incorporate a two-dimensional layup of 8-harness satin weave continuous fiber fabric, but differ in several aspects, the most significant of which is the presence of an oxidation inhibitor in one of these. The fracture behavior of both materials was determined in three-point flexure at 20 through 1650 °C. Microstructural studies indicate that the nonhomogeneous distribution of the oxidation inhibitor within the fiber bundles controls the fracture behavior. Electron microprobe results indicate a high concentration of the glass oxidation inhibitor associated with the inter-bundle matrix, while the intra-bundle matrix is composed primarily of carbon. Accordingly, debonding along the inter-bundle interfaces characterizes the oxidation inhibited composite, whereas the nonoxidation inhibited samples debond by individual fibers. Both materials exhibit strongly rising 7?-curves throughout the test temperature range. At the higher test temperatures the oxidation inhibited C/C shows the greatest cumulative toughening component, although at a lower value of the fracture toughness. This is consistent with the observed increase in the percentage of fibers that experience individual pullout at the higher temperatures.
I. INTRODUCTION Previous room temperature fracture studies of carbon/carbon composites evaluated several weave patterns and process variables of uncoated specimens and included the determination of both fracture toughness and crack growth resistance curves.1'2 Senet et al.3'4 and Grimes and White5 have evaluated two different C/C composite architectures through test temperatures of 1650 °C. These studies found a strong wake zone contribution at all test temperatures; however, they attributed significant fracture behavior influences to the development of temperature-dependent interfacial phases. The ability to provide load-carrying capability in the region behind the primary crack tip was found to play a significant role in the slow crack growth behavior under displacement controlled conditions. Renotching procedures, where the wake region is removed from a fracture specimen during an interrupted test, were introduced as a quantitative method for separating the toughening contributions of the wake region from those of the frontal process zone. The damage zone formed near the tip of a propagating crack consists primarily of matrix subcracks that form in the vicinity of the fibers.6'7 Since the matrix of continuous fiber composites may contribute up to onehalf of the total composite stiffness,8 the development of this zone of microcracked matrix during crack growth corresponds to a decrease in the effective composite modulus, to a value significantly lower than the true J. Mater. Res., Vol. 7, No. 7, Jul 1992 http://journals.cambridge.org
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elastic constant.9 White et al.10 have shown that with the loss of the matrix strength, the bridgin
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