STRENGTH AND TOUGHNESS OF CARBON-FIBER-REINFORCED ALUMINUM MATRIX COMPOSITES

Two sets of aluminum matrix composites reinforced with both uncoated a nd SiC-coated Pitch-55 aligned carbon fibers were prepared having a vo lume fraction f = 0.2 of reinforcement to test certain toughening stra tegies proposed earlier (A. S. Argon and V. Gupta, in G. K. Haritos an d 0. 0. Ochoa (eds.), Damage and Oxidation Protection in High Temperat ure Composites, Vol. 25-2, ASME, New York, 1991, p. 1). Preparatory st udies showed that the Pitch-55 fibers are damaged to some degree as a result of composite sample preparation. The loss of strength in the Si C-coated fibers was much less than in the uncoated fibers. Tensile tes ts of both smooth and precracked composite sheets showed that while th e composites reinforced with SiC-coated carbon fibers were stronger th an the composites reinforced with uncoated fibers, the former had less than half the tensile toughness of the latter. The higher tensile tou ghness of the composites with uncoated fibers is a direct result of th e more extensive debonding of fibers from the matrix during fiber frac ture. This permits the formation of a thicker fracture process zone in the composites reinforced with uncoated fibers. The measured composit e strength values and their trends were consistent with the prediction s of theory (P. M. Scop and A. S. Argon, J. Composite Mater., 3 (1969) 30). The different tensile toughness of the composites with the coate d and uncoated fibers could be accounted for by simple fracture models of fiber composites incorporating the beneficial effects of controlle d debonding of fibers.