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The ability of graphite/epoxy composites to absorb moisture from the environment and the consequent degradation of resin-dependent strength properties is well understood under conditions of constant temperature and humidity. However, if the environment consists of high humidity exposure with intermittent high temperature excursions, anomalous diffusion behavior occurs accompanied by further reductions in strength. This paper reports the results obtained from a test program in which graphite/epoxy specimens were exposed to a tropical environment representative of extreme aircraft runway storage and frequent 127°C temperature excursions of long duration representative of supersonic flights.
The tests were conducted in a specially developed apparatus which held specimens in chains of five each with accompanying travelers while exposing the specimens to prescribed daily environmental conditions. The duration of the real-time simulation test was 15 months. After various intervals of exposure, weight data were gathered using accompanying travelers, and residual static tests were performed at room temperature and 127°C. The test results for the extreme environment are compared to test results obtained for specimens exposed to the same flight profile, but temperate runway storage. The results indicate that the moisture gains of the extreme environment specimens are much greater than what is thought possible to be held by the resin. Since the static compressive strength is reduced with increases in moisture content, the degradation in compressive strength is greater than what was expected.
A semiempirical transport model based on Fickian diffusion and hygrothermal history dependent material properties was developed to account for changes in absorptivity characteristics as a function of glass transition temperature (Tg) exceedances. The test data for model development were obtained from an accelerated environment test which attempted to find economic alternatives to real-time exposure tests. The moisture predictions with the new model gave reasonable correlation with real-time test observations.
composite materials, graphite/epoxy, moisture absorption, strength properties, glass transition temperature exceedance
Project engineer, Advanced Composites,,
Technical specialist, Structural Mechanics,,