Behavior of Hybrid Metal Matrix CompositesReinforced with Continuous andDiscontinuous Fibers

The appeal of hybrid composites is the ability to create materials with properties which normally do not coexist such as high specific strength, stiffness, and toughness. One possible application for hybrid composites is as backplate materials in layered armor. Layered armor systems employing an energy absorbent backplate can defeat projectiles at lower weights than traditional steel armor, increasing the fuel efficiency and mobility of armored vehicles. Fiber reinforced composites have been used as backplate materials due to their potential to absorb more energy than monolithic materials through microfragmentation of the fiber, matrix, and fiber-matrix interface at similar to lower weights. Composite backplates are traditionally constructed from graphite or glass fiber reinforced epoxy composites. However, continuous alumina fiber reinforced aluminum metal matrix composites (MMC) have superior specific transverse and shear properties compared to epoxy composites. Unlike the epoxy composites, they have the ability to absorb additional energy through plastic deformation of the metal matrix. Although, these enhanced properties may make continuous alumina reinforced MMCs advantageous for use as backplate materials, they still exhibit low strains to failure and therefore have low toughness. One possible solution to improve their energy absorption capabilities while maintaining the high specific stiffness and strength properties of continuous reinforced MMCs is through hybridization. In the past, the strain to failure of multiple low elongation composites has been increased through hybridization with a high elongation component. Two examples include the addition of whiskers to particulate composites and using both high and low elongation fibers in continuously reinforced composites. In an attempt to increase the strain to failure and energy absorption capability of a continuous alumina reinforced Nextel™ MMC, it was laminated with a high elongation discontinuous alumina Saffil® layer. Uniaxial tensile testing of hybrid composites with varying Nextel™ to Saffil® reinforcement ratios resulted in composites with non-catastrophic tensile failures and an increased strain to failure than the single reinforcement Nextel™ MMC. The tensile behavior of two hybrid continuous and discontinuous alumina fiber reinforced MMCs will be reported, as well as a description of the mechanics behind their unique behavior.