Characterization of principal diffusivities of composites by suspension method

This investigation attempts to characterize the material principal diffusivities of a unidirectional composite laminate. Diffusion of moisture in an orthotropic plate immersed in water conforms to Fick’s diffusion law. Assuming moisture absorption induces proportional hygric strain in the laminate, the hygric strain is expressed as a function of the moisture concentration that depends on the specimen geometry, material diffusivities, and diffusion duration. Immersion tests of unidirectional carbon/epoxy laminate were conducted, and suspension method was adopted to monitor continuously the hygric expansion of the specimen in water without retrieving it from water. Specimens of different dimensions were used. Data of the hygric strain verse immersion duration are used to derive the material diffusivities by least square curve fitting. The result indicates that the diffusivity in fiber’s longitudinal direction of unidirectional T700 carbon/epoxy laminate is much greater than that in the thickness direction, and also the deviations of the derived diffusivities are proportional to the square of the corresponding dimension (width or thickness).