GLOBAL CONSTRAINT-INSENSITIVE FRACTURE IN SIC PARTICULATE-REINFORCED AA-2009

Experimental results prove this hypothesis: The effective plastic frac ture strain (epsilon(f)-p) for a discontinuously reinforced metal matr ix composite (MMC) is insensitive to globally imposed triaxial tensile stress, because the elastic reinforcement independently produces high local matrix constraint during plastic deformation. The value e(f)-p is measured for SIC particulate-reinforced AA 2009-T6, with cylindrica l circumferentially notched (global constraint ratio, (sigma(m)/sigmaB AR, of 1.0) and smooth tensile specimens (sigma(m)/sigmaBAR of 0.3), a s a function of temperature. The MMC fractures by microvoid-based proc esses associated with the SiC at all temperatures. The ratio (r) of sm ooth to notched specimen e(f)-p equals unity from 25-degrees-C to 200- degrees-C and is less than values of 2 or higher typical of monolithic Al alloys at similar global constraint. This result establishes that global constraint does not degrade MMC fracture resistance because of the unique effect of local matrix constraint. The ratio r for the MMC increases from near 1 at 200-degrees-C to a maximum of 3.3 at 250-degr ees-C, indicating a loss of local matrix constraint, possibly due to r educed matrix-particle-load transfer or microvoid nucleation at low st rains. Uncertainties hinder precise definition of epsilon(f)-p global sigma(m)/sigmaBAR. Global constraint-insensitive fracture for the MMC suggests that plane strain fracture toughness differences between the composite and un-reinforced matrix are not well defined by smooth spec imen ductilities. Further, MMC fracture toughness should be insensitiv e to global constraint and associated cracked specimen dimensions.