Fiber Strength Utilization in Carbon/Carbon Composites

The utilization of tensile strength of carbon fibers in unidirectional carboncarbon CC composites was studied for a series of four mesophase-pitch-based carbon fibers in a carbon matrix derived from a polyarylacetylene PAA resin. The fibers had moduli of 35, 75, and 130 Mpsi. Composite processing conditions ranged from the cured-resin state to various heat-treatment temperatures HTTs from 1100 to 1750 deg C for the CCs. Room- temperature tensile strength and modulus were measured for the various processing conditions, and were correlated with SEM observations of fracture surfaces, fiber and matrix microstructures, and fibermatrix interphase structures. Fiber tensile strength utilization FSU is defined as the ratio of apparent fiber strength in the CC to the fiber strength in an epoxy-resin- matrix composite. Carbonization heat treatment to 1100 deg C results in a battle carbon matrix that bonds strongly with the three lower modulus fibers, resulting in matrix-dominated failure at FSU values of 24 to 35. However, the composite with the 130-Mpsi modulus filament had an FSU of 79. It is attributed to a combination of tough fracture within the filament itself and a weaker fiber matrix interface. Both factors lead to crack deflection and blunting rather than to crack propagation. The presence of a weakened interface is inferred from observations of fiber pullout. Much of the FSU of the three lower modulus fibers is recovered by HIT to 2100 or 2400 deg C, principally as a result of interface weakening, which works to prevent matrix-dominated fracture. With HTT to 2750 deg C, there is a drop in FSU for all the composites it is apparently the result of a combination of fiber degradation and reduced matrix stress-transfer capability