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The strength, stiffness, fracture toughness, and crack propagation characteristics of graphite-glass intraply hybrid uniwoven and biwoven composite fabric materials are evaluated. The hybrid fabric plies were designed by modifying existing all-graphite fabrics in which were added single tows of S-glass spaced 6.35 mm (0.25 in.) apart, and 12.70-mm (0.5-in.)-wide strips of graphite were replaced with S-glass spaced 10.16 cm (4.0 in.) apart. The purpose of this hybrid design was to provide better damage tolerance and damage containment capabilities than the all-graphite system.
The test program involved unidirectional and laminated coupons for strength and fracture toughness evaluation and 30.5-cm (12.0-in.)-wide orthotropic panels for crack arrestment evaluation. The panels were designed with glass strips aligned in discrete and staggered configurations. All tests involved room temperature dry and 121°C (250°F) 1.2 percent moisture-content specimens. The test results showed that the strength and stiffness properties for the hybrid materials were within 10 percent of the comparable all-graphite materials. However, the fracture toughness was approximately 44 percent higher for the hybrid systems and was not degraded by the 121°C (250°F) wet environment. A distinct crack arrestment capability was achieved only with the discrete strip configuration, with ultimate failure strain after crack arrestment in the 0.64 to 0.76 percent strain range.
composite materials, intraply hybrid, graphite-glass hybrids, damage tolerance, fracture toughness, crack propagation, crack arrest
Engineering Specialist, Northrop Corporation, Aircraft Division, Hawthorne, Calif.