Strength and toughness of carbon fiber reinforced aluminum matrix composites

Abstract

Two sets of aluminum matrix composites reinforced with both uncoated and SiC-coated Pitch-55 aligned carbon fibers were prepared having a volume fraction f=0.2 of reinforcement to test certain toughening strategies proposed earlier (A.S. Argon and V. Gupta, in G.K. Haritos and O.O. Ochoa (eds.), Damage and Oxidation Protection in High Temperature Composites, Vol. 25-2, ASME, New York, 1991, p. 1). Preparatory studies showed that the Pitch-55 fibers are damaged to some degree as a result of composite sample preparation. The loss of strength in the SiC-coated fibers was much less than in the uncoated fibers. Tensile tests of both smooth and precraked composite sheets showed that while the composites reinforced with SiC-coated carbon fibers were stronger than the composites reinforced with uncoated fibers, the former had less than half the tensile toughness of the latter. The higher tensile toughness of the composites with uncoated fibers is a direct result of the more extensive debonding of fibers from the matrix during fiber fracture. This permits the formation of a thicker fracture process zone in the composites reinforced with uncoated fibers. The measured composite strength values and their trends were consistent with the predictions of theory (P.M. Scop and A.S. Argon, J. Composite Mater., 3 (1969) 30). The different tensile toughness of the composites with the coated and uncoated fibers could be accounted for by simple fracture models of fiber composites incorporating the beneficial effects of controlled debonding of fibers.