Fiber strength utilization in carbon/carbon composites: Part II. Extended studies with pitch- and PAN-based fibers
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J. M. Yang Department of Materials Science and Engineering, University of California at Los Angeles, Los Angeles, California 90024 (Received 29 March 1994; accepted 8 November 1994)
Measurements of fiber strength utilization (FSU) in unidirectional carbon/carbon (C/C) composites as a function of heat-treatment temperature (HTT) have been extended beyond the original group of DuPont pitch-based E-series fibers to include additional pitch- and PAN-based fibers. Fibers and composites were characterized by tensile strength, optical microscopy, SEM, fiber preferred orientation, and a single-fiber composite (SFC) fragmentation test to provide a relative measure of fiber-matrix interfacial shear strength (IFSS). Results show that fracture behavior and FSU are dominated by the degree of fiber-matrix bonding, as inferred from microscopic observations and measurements of IFSS. In the very high modulus pitch-based fibers, the behavior of the E130 is strikingly different from that of the Amoco and Nippon Oil fibers, in that it retains good bond strength and high FSU even with HT to 2400 °C, in contrast to the other very high modulus pitch-based fibers that are already weakly bonded at the lowest HTT of 1100 °C. All PAN-based fibers and lower modulus pitch fibers are characterized by strong bonding, brittle fracture, and low FSU for the 1100 °C HTT. Subsequent heat treatment of these composites to 2150 and 2400 °C, in most cases, results in significant recovery of FSU, suggesting an optimum IFSS for each composite. It is suggested that the difference in bonding between the pitch-based E-series and P-series may be related to the similarity in fine structure between the E-fibers and high-modulus PAN-based fibers.
I. INTRODUCTION In a previous study,1 we reported on the fiber utilization (FSU) in single-tow unidirectional carbon/carbon (C/C) composites of a series of DuPont mesophasederived carbon fibers E35, E75, E105, and E130 as a function of composite heat-treatment temperature (HTT). The number in the fiber designates its nominal modulus in units of Mpsi. FSU is defined as the ratio of apparent fiber strength in the C/C composite to the fiber strength in a baseline resin-matrix composite. The C/C composite matrix precursor was polyarylacetylene (PAA), which is a high-char-yield thermoset resin derived from polymerization of diethynylbenzene. Its chemistry and use as a matrix precursor in C/C's is discussed in more detail elsewhere. 23 In the earlier study,1 it was found that the 1100 °C HTT (carbonization) results in a brittle carbon matrix that bonds strongly with the three lower-modulus fibers. The result is matrix-dominated failure at failure strains well below those of the fibers, and therefore, low FSU values in the composite (24 to 35%). However, the E130 J. Mater. Res., Vol. 10, No. 3, Mar 1995
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fiber composite had a much higher FSU of 79%. Part of this higher FSU in the E130 can be attributed to the much lower-failure strain of the E130 fiber relative to the ot
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