The effects of long term exposure to dry and humid environments on carbon‐epoxy and graphite‐epoxy composites have been studied. Filament wound Naval Ordnance Laboratory rings were fabricated in 1965 and were placed in dry, distilled water and sea water for 11 years. Moisture desorption tests were conducted in order to determine the water content of specimens exposed to the water environments, The effect of the history on moisture absorption characteristics was investigated by re‐exposure of partially and completely dried specimens to two different environments: distilled water at 60°C and 98% relative humidity at 60°C. The weight gain was measured and diffusion coefficients were calculated. Horizontal shear tests and flexural tests were performed on “wet” specimens (current properties) and on partially and completely dried specimens (residual properties). The shear strength of the carbon‐epoxy composites is degraded by the exposure while that of the graphite‐epoxy composite is slightly increased. The composite flexural strengths are not degraded by the exposure. These results are discussed in terms of failure modes.
Loaded transverse GRP specimens and epoxy references were exposed to different environmental conditions. Tensile strength measurements conducted under exposure and after a drying period yielded the following main conclusions: Current and residual transverse strength was not affected much by cold-water and hot dry air during a short exposure period.Under hot-water exposure, transverse strength showed a significant drop with time, especially beyond 2 days of exposure and above 25% of the preload level.Based on earlier studies, the transverse-strength degradation of GRP specimens is attributable to attack by water molecules on the fiber-glass surfaces and the fiber-matrix interfacial bonds. This process is triggered and accelerated by heat and external loading.
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