Stiffness enhancement in aligned, short-fibre composites: A computational and experimental investigation

Abstract

This communication presents the results of a combined computational and experimental study with the objective of determining the extent to which the alignment of fibres improves the effective tensile modulus of short-fibre-reinforced composite materials. In the computational part, we use the boundary element method (BEM) to model the mechanical response of a fully three-dimensional, multi-particle composite rod consisting of up to 40 individual, rigid, rod-like particles randomly embedded in an elastic matrix. The BEM allows great flexibility in the geometrical placement of the reinforcing particles, thus allowing the numerical investigation of micromechanical, orientational and configurational effects without resorting to the unit cell approximation, as is almost invariably the case with previous work in this area. Besides their volume fraction and aspect ratio, the effective stiffness of the composite rod is found to depend on the relative placement of the reinforcing fibres and is predicted to be highest when the fibres are aligned in the tensile direction. In the experimental part, we apply a hydrodynamic method in order to align, in a small-scale pilot facility, short carbon fibres in an epoxy matrix. Composite samples are prepared and tested two at a time (one with randomly oriented fibres and the other with aligned fibres) and the effect of fibre alignment on mechanical properties is isolated and measured. Experimental results show that aligned fibres are approximately twice as effective than randomly oriented fibres in stiffening the composite in the alignment direction. There is a remarkable agreement between numerical predictions and experimental measurements.