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Graphite-fiber-reinforced polymer composite materials have become prime candidates for space structures. The high specific strengths and stiffnesses obtainable with these materials and the ability to design them with a near-zero coefficient of thermal expansion provide a flexibility that is virtually unmatched. However, microcracks resulting from thermal cycling can lead to drastic property changes of the composites. Specimens of five composite materials, each with the same epoxy-cyanate blend matrix and reinforced with different graphite fibers, were subjected to a simulated spacecraft thermal cycling environment to determine the effects of fiber physical properties on the resulting microcrack behavior. The lay-up for each material was [0/45/90/-45]s with a nominal ply thickness of 0.0125 cm (0.005 in). The specimens were cycled between -157°C (-250°F) and +121°C (+250°F) up to 500 times. They were examined at a magnification of ×400 at different thermal cycle intervals for microcracks. It was found that although the maximum crack density varied with the ply lay-up angle, it did not vary much with fiber type. However, the fiber type had a strong influence on the rate of microcrack development. This behavior was found to be best described by fitting a hyperbolic function to the microcrack density as a function of the number of thermal cycles.
epoxy-cyanate blend, graphite fibers, microcracking, polymer matrix composites, space structures, thermal cycling
Ph.D. candidate, University of Cincinnati, Cincinnati, OH
Senior researcher, NASA Langley Research Center, Hampton, VA
Professor, University of Cincinnati, Cincinnati, OH