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Water diffusion into composites in different directions was examined in this study with the aim of determining the best way of measuring diffusion coefficients and to provide values to compare with model predictions. Water absorption behavior of unreinforced epoxy resins and carbon fiber reinforced epoxy composite materials was investigated with long‐term exposure to different environmental conditions. Initial Fickian absorption was observed followed by a slower second stage that continues for at least 3.7 years. Fiber architecture was found to be an important aspect of controlling absorption, where water diffusion along fibers was observed to be about three times faster than across the fibers and about seven times faster than through the thickness. A three‐dimensional finite element computer model based on Fickian diffusion behavior was developed to predict the levels of moisture absorption under hot/humid environments. A multi‐scale modeling approach was used which allowed the results of simulations at the micro‐structural level to be used to predict the diffusivity in different directions. The modeled diffusion coefficients showed high dependency on the detailed micro‐structure. Experimental results provided a baseline for the validation of the model, and it was found that these data could be closely predicted using a reasonable micro‐structure characterization. POLYM. COMPOS., 39:E2305–E2315, 2018. © 2017 Society of Plastics Engineers

Defects in polymer composites associated with in-service damage induced by impact and those produced by environmental degradation due moisture absorption can have a significant influence on the mechanical properties and failure modes of the structures. This study evaluates the flexural characteristics and failure modes of 6458 carbon/ epoxy laminates under different conditions. The flexural strength and stiffness of these composites were inspected before and after subjecting to water absorption, drying, re-drying and low energy impact. It was observed that both absorbed water and low energy impact caused significant deformations and fractures which drastically reduced the load carrying ability of the 6458 carbon/epoxy laminates. Re-drying slightly improved the properties and reversed the effects of water. POLYM. COMPOS., 00:000-000, 2016.

The long-term water absorption behavior of UD and 645 carbon fiber/977-2 epoxy composite and unreinforced 977-2 epoxy resin has been studied at different environments with various temperatures and relative humidities. Fickian behavior was noticed initially followed by a slight and continuing increase in saturation with time, a behavior that continues for at least 4 years. Fickian saturation and diffusion coefficients of all materials were derived using graphical method. Initial Fickian equilibrium moisture contents were found not to be dependent on temperature but the long-term increase in solubility increased with temperature. Absorption rates showed strong dependency on temperature and relative humidity. The activation energy values for all materials in the initial stage found to be much higher (40 kJ/mole) than the long-term stage (10 kJ/mole). E a values of the neat resin were observed to be slightly lower than the composites. The slow continued weight gain in the long-term absorption experiments has been seen to be coupled with an incomplete drying and a higher diffusion rate on drying and subsequent re-conditioning. The possible reasons and mechanisms for the long-term gradual increase in moisture absorption was further investigated. It was found that molecular relaxation is the most likely dominant mechanism. POLYM. ENG. SCI., 00:000-000, 2018. V C 2018 Society of Plastics Engineers Water UptakeThe samples were dried in an oven at 708C and were weighed periodically until the sample weights stabilized. Specimens were then immersed in distilled water at 238C, 408C, 608C, 708C, and 808C. For humidity experiments, other samples were exposed to moisture uptake at different relative humidity 45%, 60%, 85%, 100% at 708C. The percentage of moisture weight gain versus time was monitored by gravimetric methods for all specimens until the specimens approached effective moisture equilibrium. Two to three specimens of each material type were conditioned to get averaged results.

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