Characterizing the Hygric Behavior of Composites by Suspending Method

Abstract

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.

Characterization of principal diffusivities of composites by suspension method

“…The suspension method [10] is proposed for measuring the laminate strain of an immersed specimen. This experiment used 20 cm long, 0.5 or 1.0 cm wide, and 0.057 or 0.114 cm thick rectangular unidirectional composite specimens with both ends sealed by 0.05 mm thick aluminum foil.…”

“…where r h is the local hygric stress, Q is the stiffness of the material which is a constant for the unidirectional laminate, and e H is the laminate strain which can easily be measured by the suspension method [10]. When the diffusion causes symmetric moisture distribution with respect to the x 1 and x 3 coordinates, the x 1 -x 3 cross-section plane of the specimen in Fig.…”

“…For the same reason, r D 3 is roughly proportional to h 2 3 . The laminate of the same material and thickness with all the ends and edges sealed were tested by Tsai and his co-workers [10]. The diffusion was in thickness direction (x 3 -direction) only, and Eq.…”

“…An alternative approach based on the assumption that the hygric expansion is proportional to the moisture concentration [1], led to new techniques, namely, by measuring the hygric strain or geometry change of an immersed specimen to derive the diffusivity [6][7][8][9][10]. Among them, the suspension method is a very effective technique which directly measures the hygric strain of a specimen [10].…”

“…Among them, the suspension method is a very effective technique which directly measures the hygric strain of a specimen [10]. The data of the hygric strain verse immersion duration are used to derive the principal diffusivities of the specimen through curve fitting.…”