Influence of evolving fibre orientation and fibre concentration distribution on polymer composite viscosity and first normal stress difference

Rheological behaviour of fibre reinforced polymer composites is largely influenced by the orientation, concentration and length distribution of fibres. There is a need to develop rheological models that incorporate simultaneous effects of fibre orientation evolution and fibre concentration migration. Further validation by comparing model predictions with experimentally measured transient rheological behaviour is also required. In this study, fibre orientation evolution was measured by X-ray Computed Tomography (X-CT) using compression moulded samples of short glass fibre filled Nylon-6 composites that were subjected to simple shear in a parallel plate rheometer. The experimental data showed a preferential alignment of fibres in the shearing direction as a function of shear strain. The measured fibre orientation evolution was fitted to the Reduced Strain Closure (RSC) orientation model via the selection of suitable fitting parameters: Fibre interaction coefficient (CI) and Strain reduction factor (κ). This component of the work established the use of X-CT to accurately measure 3-dimensional fibre orientation distributions in composite samples. The ability to measure particle concentration distribution using X-CT allowed for investigation of fibre migration during shear as well. To obtain repeatable fibre concentration distributions, injection moulded samples were used which have a distinct skin-core concentration distribution. This distribution typically has a peak concentration at the centre along the thickness direction of the sample due to the shear stress profile generated during injection moulding. As the samples were subjected to simple shear in a parallel plate rheometer, a weakening of the skin-core distribution to a uniform concentration distribution along the thickness direction was observed due to migration effects. Fibre concentration migration predictions were obtained using the Suspension Balance Model (SBM) which determines migration flux due to variance in particle stress. The novel idea of using X-CT to track fibre concentration migration and performing a comparison with model predictions is presented in this work. The transient rheological response of the fibre filled composites was measured using cone-plate and parallel-plate rheometers with a focus on viscosity and first normal stress difference results. A large truncation gap (0.8mm) and a cone angle of 0.105radians were used in the cone-plate rheometer to obtain reliable experimental data while minimising gap effects and radial shear rate variance. The measured transient viscosity and first normal stress difference results show an initial overshoot which gradually decays to a steady state value. This overshoot behaviour is attributed to the stress contributions arising from the evolution of fibre morphology, which is a transition from an initial fibre distribution to an ordered fibre distribution. The RSC model and SBM model as discussed earlier were incorporated into suitable rheology models and predictions of transient viscosity and first normal stress difference were obtained. Amongst various rheology models chosen in this study, predictions from the Evans model fitted the experimental data most accurately. The incorporation of both fibre orientation and fibre concentration evolution models in a rheology equation is a novel idea that is presented in this work. It has been determined that incorporating fibre concentration migration effects improves transient viscosity predictions from the rheological constitutive equations.