Modeling Moisture Diffusion Behaviors in Polymers and Polymer Composites
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Date
2018-07-30
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Abstract
This study presents a transient analysis of fluid diffusion behavior in polymers, fiber reinforced polymer (FRP) composites, and polymeric sandwich composites. The studied polymers and polymer composites consist of pure resins and polymeric foams, which are considered to have isotropic diffusion behavior; FRP composites, which typically have anisotropic diffusion behavior because of the presence of the fibers; and sandwich composites, which have different constitutes with different diffusion behaviors at various geometries. Diffusivity coefficients are first determined by assuming the diffusion to follow the classical Fickian diffusion. Different constitutive models are considered in order to understand the diffusion behavior of fluid through polymers and polymer composites. In some cases, fluid sorption led to quite significant changes of volume, and the diffusion process cannot be well described by the Fickian diffusion. In such situations, the coupled deformation-diffusion model for linear elastic isotropic materials is adopted, as a first approximation. This coupled deformation-diffusion model reduces to a Fickian diffusion model when the coupling parameters are absent and the volume changes in the solid polymers during diffusion are negligible. A finite difference method is used in order to solve for the coupled deformation-diffusion model. The coupled deformation-diffusion model is used to predict the one-dimensional moisture diffusion in thin polymer plates and the multi-axial three-dimensional moisture diffusion in dogbone specimens and polymeric foams. FRP composites typically have anisotropic diffusion behavior, where diffusivity in the fiber direction is faster than in the transverse direction. A micromechanical model that includes fiber volume contents and diffusivities of the constituents (fiber and matrix) is considered in order to gain fundamental insight into the effects of microstructural morphologies and constituents’ diffusivities on the diffusion process in the FRP specimens. The matrix and the fibers are modeled separately. And the micromechanical model is implemented in a continuum three-dimensional FE. The micromechanical model uses anisotropic diffusion constant from calibration and the simulation results are compared with the experimental results of thin laminates with different fiber arrangements. Finally, a multi-scale analysis of fluid diffusion in sandwich composites is performed. The simulation results are compared with the experimental data.
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diffusion, polymer, polymer composite, deformation, micromechanics, modeling, finite difference, finite element