Time Dependent Properties of Injection Moulded Composites

Abstract

The uniaxial tensile creep behaviour of several thermoplastics reinforced with short glass fibres has been studied over a wide range of temperatures using specimens machined from a variety of injection moulded bars, discs and plaques. The samples cover a range of fibre volume fractions from 0,10 to 0.22 and mean fibre aspect ratios from 18.5 to 43.0. A detailed quantitative assessment of fibre orientation distribution (FOD) and fibre length distribution has been carried out for each type of moulding used with each material. This structural information, together with fibre volume fraction, fibre modulus and matrix creep data, has been used in the prediction of composite tensile modulus both parallel and transverse to the major flow direction for each type of moulding. For the theoretical predictions, the FOD was treated using the ‘laminate analogy’ approach developed by Halpin et al; the properties of a ply containing uniaxially aligned short fibres being calculated using several different theoretical approaches. At low strains, one of these approaches led to remarkably good agreement between theoretical and experimental composite moduli, over most of the very wide range of materials and temperatures studied. An empirical extension of this approach is also shown to give reasonably accurate predictions of composite creep behaviour at finite strains, where both the matrix and the composite exhibit non-linear viscoelastic behaviour.