Directional diffusion of moisture into unidirectional carbon fiber/epoxy Composites: Experiments and modeling

Abstract

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

An investigation of the long‐term water uptake behavior and mechanisms of carbon fiber/977‐2 epoxy composites

“…Using the Fickian saturation values obtained by the technique mentioned before, the initial Fickian diffusivity was calculated. To compensate for edge effect, the edge correction factor used in this study was derived by Starink, Starink, and Chamber , Eq . .…”

“…Polymer composites are often used in contact with liquids and vapors, either organic or aqueous (liquids or vapors), and such environments can affect the immediate or the long‐term performance of the material . Polymer matrix composites normally absorb small but potentially damaging amount of moisture from the surrounding environments . Typically, the moisture flow into or out of the bulk of the composite interior is slow, while the external surfaces absorb and desorb moisture almost immediately.…”

“…On the other hand, in many crosslinked and glassy polymers, when molecular relaxations or morphological effects dominate the movement of diffusing species in polymers, deviations from Fick's law can be noticed which may be associated with resin deterioration, or structural changes . The long‐term water absorption in carbon fiber/epoxy composites was studied by Arnold et al and Korkees et al . The weight gain behavior was noticed to deviate from Fickian behavior and it followed the two‐stage behavior.…”

A framework for three-dimensional finite element analysis of unidirectional and cross-ply composite layups through localized microstructures under hygromechanical conditions

“…However, from the diffusion standpoint, it is found that three-dimensional Fickian diffusion phenomenon is a more reliable method to validate the experimental data. It is because the diffusion along the fibers is higher than that of the unreinforced resin [14,22].…”

“…From the microscopic viewpoint, fiber architecture is an important aspect that can influence the moisture diffusion. Therefore, there is a need to accurately model the ingress of moisture into composites [14][15][16][17]. As the fiber diameter is fairly small in comparison with its length, most of the researches have been conducted using two-dimensional models.…”