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The stiffness, strength, and shear properties of three polyimide resins (NR-150B2, PMR-15, and CPI-2237) combined with three different moduli graphite fibers (C-6000, F-5A, and GY-70) have been determined in the temperature range 20 to 371°C (68 to 700°F). Flexural test results from the various unidirectional composites show that stiffness retention with increasing temperature is affected only by the thermal integrity of the polyimide matrix. No loss in modulus occurs up to 316°C (600°F) for the PMR-15 and CPI-2237 based composites [Tg ≃ 377°C (710°F)] or to 260°C (500°F) for the NR-150B2 based material [Tg ≃ 349°C (660°F)], with any of the three fibers. In contrast, both flexure and shear strengths show fiber-dependent behavior, irrespective of the matrix resin, with increasing temperature. The higher modulus fiber composites (F-5A, GY-70) undergo little strength change up to 343°C (650°F). Composite strengths of the lower modulus fibers (C-6000), however, degrade by as much as 50 percent over the same temperature range. Thermal-oxidative stability of the various graphite fibers, and its effect on interfacial strength degradation, are considered primary causes for the fiber-type dominated strength behavior. In general, strength retention appears directly related to degree of graphitization (modulus) of the fibers. The accumulated mechanical property data, some previously unknown, are correlated with microstructural features such as fiber-matrix adhesion, porosity, and processing defects. This study of a family of graphite/polyimide composites provides a basis for selecting a high temperature, structural weight-saving material.
graphite/polyimide composites, thermomechanical properties, graphite fibers, thermal-oxidative stability, thermal degradation, fiber-matrix adhesion, porosity
Member technical staff, Sandia National,