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In order to support the development of new materials required for the design of next generation commercial supersonic transports, a research program is underway at NASA to assess the long term durability of advanced polymer matrix composites (PMC's). In support of this program, recent work has provided test methods and an elastic/viscoplastic constitutive model which accounted for some aspects of rate-dependent tension and compression loading behavior of IM7/5260 (graphite/bismaleimide) and IM7/8320 (graphite/thermoplastic) materials throughout a range of useful temperatures. The research effort detailed in this paper is an extension of that work and had two main objectives. The first objective was to explore the effects of elevated temperature (23°C to 200°C) on the constitutive model's material parameters. To achieve this goal, test data on the observed nonlinear, stress/strain behavior of IM7/5260 and IM7/8320 composites under tension and compression loading was collected and correlated against temperature. These tests, conducted under isothermal conditions using variable strain rates, included such phenomena as stress relaxation and short term creep. From this data, trends in the parameters at elevated temperatures were developed, differences between the two material systems were outlined, and the significance of the parameters in terms of ductility and rate dependent behavior was established. The second major goal was the verification of the model by comparison of analytical predictions and test results for offaxis and angle-ply laminates. Correlation between test and predicted behavior was performed for specimens of both material systems over a range of temperatures. Results indicated that the model provided reasonable predictions of material behavior in load or strain controlled tests. Periods of loading, unloading, short term stress relaxation, and creep were accounted for. These types of studies should be useful for making comparisons on the effect of temperature between specific laminate types and material systems.
elevated temperature, viscoplastic, creep, stress relaxation, composites
Research scientist, NASA Langley Research Center, Hampton, VA