Influence of uniaxial and biaxial tension on meso-scale geometry and strain fields in a woven composite

Representative volume element (RVE) of a woven composite is often idealised to have identical geometry in the warp and weft directions. While this may be true in the case of consolidating a completely relaxed preform with equal warp and weft densities, majority of woven composites have non-ideal RVE geometry. The present paper investigates the influence of membrane stresses – applied during the moulding process – on the mechanical properties of the finished composite. Crimp interchange due to uniaxial stress and tow flattening due to biaxial stress have been computed by a fabric compliance model developed by the authors. Compliance model is based on the principle of stationary potential energy by taking into consideration tow bending, compression and extensional energies, and the external work done by the tensile forces. It has been shown that a uniaxial tensile stress applied to the preform results in an increase in tensile modulus and a corresponding reduction in the transverse modulus, as a result of reduced crimp in the loading direction with a corresponding increase in the transverse direction. Tensile strength and stiffness increase in the loading direction, and the mode of failure initiation changes from ‘knee phenomena’ to tow failure. A biaxial stress applied during the forming stage results in crimp reduction in both axial and transverse directions; this leads to an improved tensile modulus and strength of a composite laminate.RVE of the deformed geometry has been modelled using ABAQUS CAE pre-processor. The elastic moduli were computed using the concept of equivalent strain energy proposed by Zhang and Harding. The resulting strain fields were compared for various cases of uniaxial and biaxial extension.