The effect of fibre-bundling on the mechanical properties of a short-fibre composite.
It has been suggested that the use of fibre bundles rather than individual fibres can
improve the toughness properties of a short-fibre composite. Previous experimental
work on this topic employed materials in which bundles were impregnated prior to
manufacture or materials with poorly defined fibre-bundling. This study is the first to
consider the mechanical properties of a series of materials where the bundles have been
impregnated during manufacture of the material, and the materials tested contained a
well-defined proportion of fibres within bundles of a known size.
A novel manufacturing technique has been developed that can be used to produce short
carbon fibre reinforced polypropylene materials with a controlled proportion of fibres
in bundles. Materials manufactured in this work contained 0 %, 25 %, 50 %, 75 % and
100 % of the fibres in bundles. The fibres had a length of 5 mm or 10 mm and the
bundles contained either 1000 or 6000 fibres.
An increase in the proportion of fibres within bundles results in a decrease in the tensile
modulus of the short-fibre composites. This decrease was less severe for materials
containing bundles with a greater aspect ratio or laminates with a greater thickness. A
model for the modulus of the materials has been developed which illustrates some of
the effects of fibre-bundling on the structure of a short-fibre composite. For the
materials studied, tensile strength of materials containing bundles was one quarter of
the tensile strength of the filamentised material.
Only one combination of fibre length and bundle size resulted in a clear increase in
toughness, as measured by JJ, compared to the filamentised material and this increase
appears to be due to areas of unreinforced matrix in the material. Materials containing
both filamentised fibres and fibre bundles had relatively low values of J, The fracture
surfaces were imaged and three distinct ways in which a bundle may fail have been
identified. Discussion of the fracture mechanisms active in these materials concludes
that the use of fibre-bundling to improve toughness is unlikely to be effective due to the
mechanism that has been proposed