The hygric behavior of composites is theoretically and experimentally investigated. The theoretical function of moisture concentration distribution caused by moisture diffusion in a plate submerged in water was derived based on Fick’s diffusion law, which involves the geometry of the plate, the material diffusivity of the plate, and the duration of submergence. The hygric strain induced by the moisture absorption was derived as a function of the moisture concentration. Consequently, the hygric strain of the plate was expressed as a function of its geometry, material diffusivity, and the duration. Experiments on composite laminates submerged in water were performed to obtain data of hygric strains versus durations of submergence to calculate the material diffusivity, which dominates the hygric behavior of the material. The technique, the suspending method, was employed to measure the hygric expansion of the plates and calculate their hygric strains. The diffusivity of the material in the function of the hygric strain was determined under the condition of optimizing the fitting of the theoretically predicted hygric strains to the experimental results. The minimization of the work involved in calculating the diffusivity with acceptable precision was discussed. The diffusivity in the thickness direction and the saturated moisture concentration of carbon–epoxy composite laminate were obtained.
This study presents a new diffusion model named the gradient-dependent model. It is based on the strain-dependent model, a modification of Fick’s diffusion model, and is employed to solve penetrant diffusion in a 2D-infinite plate with finite thickness. An immersion test is performed and analyzed for studying moisture diffusing through the surfaces of a polymer matrix composite laminate into the material and the induced in-plane expansion is measured. The hygric expansion is proportional to the moisture concentration, which can be derived from the gradient-dependent model involving parameters standing for the hygric property of the material. By fitting the theoretical solution of the hygric expansion to the data, the parameters for locating the penetrant front can be obtained and they are very important in describing the hygric behavior of the material.
The dimensional change of some composite materials induced by ambient air pressure change was discovered and dubbed pneumatic strain in 2000. This pneumatic behavior closely resembles hygric behavior, and pneumatic strain is proportional to the ambient air pressure change by the coefficients of the pneumatic expansion. A method termed the suspension method was employed in this work to characterize the pneumatic behavior of unidirectional T700 carbon/epoxy composite laminate and other materials, such as Kevlar 49 fiber, aluminum plate, aluminum foil, paper, celluloid sheet and pure epoxy plate. Longitudinal and transverse pneumatic strains of T700 carbon/epoxy composite lamina induced by 1 atm air pressure change were measured. The results also indicated that Kevlar 49 fiber and aluminum have no pneumatic behavior, while paper, celluloid film and epoxy Downloaded from plate do have pneumatic behavior. Two major contributions of this work are verifying the effectiveness of suspension method for the pneumatic characterizing and discovering more materials having pneumatic behavior.
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