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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.

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A technique was developed for determining the longitudinal shear modulus of uniform diameter single fibers. A length of single fiber suspended from one end and with a weight attached at the other end was used as a torsional pendulum. Its longitudinal shear modulus can be expressed as a function of the frequency of oscillation of the torsional pendulum. Tests were conducted on five different types of fibers: carbon, Kevlar 49, silicon carbide, K-glass fibers and G-glass fibers.

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