Effects of Fiber Length on Elastic Moduli of Randomly-Oriented Chopped-Fiber Composites

Abstract

Based on a composite-cylinder model with a short cylindrical fiber embedded in the center of a cylindrical matrix, the longitudinal Young's modulus and major Poisson's ratio of a unidirectional, short-fiber composite are found in terms of the fiber volume fraction and the tip-to-tip spacing of the fibers. The expressions obtained are then modified to account for the influence of volume fraction and aspect ratio of the fibers. These results, together with Christensen and Waals' normalized expressions, are used to calculate the Young's modulus and Poisson's ratio of a randomly-oriented chopped-fiber composite in terms of the fiber volume fraction and its aspect ratio. The theory developed is then applied to examine numerically the effects of fiber length on the Young's modulus and Poisson's ratio of short glassfiber/polyester-resin composites. The results show that the Young's modulus of both unidirectional and randomly-oriented fiber composites are strongly dependent on the fiber length; so is the Poisson's ratio, though to a lesser degree.