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Directional Dependence of Fracture Toughness in Hot-Pressed SiC-Whisker Reinforced Alumina at Room and Elevated Temperatures

High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN 3 7831-6069

Mattison K. Ferber

High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN 3 7831-6069

Azar Parvizi-Majidi

Department of Mechanical Engineering and The Center for Composite Materials, University of Delaware, Newark, DE 19716

The directional dependence of fracture toughness was investigated in commercially available hot-pressed SiC-whisker reinforced alumina composites at room and elevated temperatures. Whisker orientation was transversely isotropic with respect to the pressing axis in these composites. Composite bend bars were chevron-notched and loaded in four-point bending to induce stable crack propagation in two different crack systems: defined here as a combination of crack plane and direction with reference to the pressing axis. The average fracture toughness, K_Ic, was found to be approximately 30% less for a crack system with its crack plane parallel and crack direction perpendicular to the pressing axis compared to a crack system with both its crack plane and direction parallel to the pressing axis through 1200°C. The crack systems with the lower fracture toughness exhibited a characteristic high concentration of whiskers aligned parallel to the crack direction within the crack plane; this appeared to he associated with decreased or unpromoted crack deflection. This fracture mechanism evidently dominated the fracture process through 1200°C because measured fracture toughnesses were independent of temperature; crack-wake toughening mechanisms, whose efficiency is a function of the residual thermal stresses, are expected to be temperature-dependent.

Key Words: fracture toughness • whisker • crack deflection • ceramic composite • elevated temperatures

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