Modelling the elastic and thermoelastic properties of short fibre composites with anisotropic phases

Abstract

An analytical procedure is proposed and validated for predicting the elastic anisotropy and thermal expansion behaviour of a short fibre polymer composite, where both the fibre and the polymer matrix possess anisotropic material properties. The modelling strategy is to consider the fibre composite as an aggregate of units of structure, with an averaging scheme which takes into account the state of orientation. Validation of this strategy required the accurate determination of the unit properties and a satisfactory orientation averaging procedure. First, the unit properties were determined for very highly aligned samples with either an isotropic or an anisotropic fibre (glass or carbon) and either an isotropic or an anisotropic matrix (Nylon or liquid crystalline polymer). The fibre orientation and length distribution were determined by image analysis together with measurements of their elastic and thermoelastic properties. In combination with finite element calculations developed by Gusev, these results provided the basis for validation of appropriate analytical schemes to predict the unit properties. The orientation averaging procedures were validated by measurements on a model ribbed box component manufactured from a carbon fibre filled liquid crystalline polymer (anisotropic fibre and anisotropic matrix). Measurements of Young’s modulus and the coefficient of thermal expansion were combined with the determination of fibre orientation by image analysis. The key result was that if the fibre orientation level was high, the best prediction was to assume that the matrix orientation was identical to that of the fibres. For a lower degree of alignment, a better prediction was obtained by assuming that the matrix was isotropic.