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Testing techniques were developed for the mechanical testing of metal matrix, high-modulus filament composites. The testing of boron-aluminum composites was emphasized using both unidirectional reinforcement and multidirectional reinforcement. Tension, compression, shear, fatigue, and creep testing are discussed. Because of the anisotropy of the material, certain adaptations of standard tension testing techniques were developed in the gripping, specimen design, and extensometry. The tension test at temperatures up to 500 C of materials with room-temperature tensile strengths in the reinforced direction of up to 200,000 psi and shear strengths in nonreinforced directions of 10,000 psi required specimens of modified geometry. These specimens had long grip lengths to prevent delaminating shear failures and only mildly reduced sections to prevent longitudinal shear from the shoulder into the grip. The gripping of the materials required abrasive paper or doublers to eliminate slippage in the grips without making indentations in the brittle boron fiber as occurs with serrated grips. Strain measurement was accomplished with both strain gages and clip on linear variable differential transducer extensometers. Both methods required special considerations due to anisotropy and the brittle nature of the fibers. Similarly, in compression and shear tests particular specimen designs and loading point constraints were found superior. The recommended material information input and reportable information from the tests will be discussed. The input information is more complex than is the case for isotropic materials and includes the properties of the constituent phases of the composite, the geometry of their combination, and the thermal and mechanical history of the part which lead to internal stresses between the constituents. The axial unidirectionally reinforced elastic properties of the composites can be predicted from the rule of mixtures, and the stress strain curve demonstrated three regions; elastic-elastic behavior, elastic-plastic behavior, and fiber breakage similar to that observed by McDanels et al. It was found that the rule of mixtures prediction of strength could be applied better if the statistical nature of the fiber strength and the critical reinforcing length were taken into consideration. An analysis of the state of stress of the composite structures in the tests described is presented together with a discussion of the value and limitations of the tests in determining engineering properties.
composite materials, mechanical properties, boron, aluminum, evaluation, tests
Kreider, K. G.
Program manager, United Aircraft Research Laboratories, East Hartford, Conn.