Viscous Behaviour of Fibre-filled Liquids

Doctoral thesis, 2004

Process simulation of discontinuous fibre composites is of importance in automotive industries. The performance of a fibre composite part depends to a great extent on the fibre orientation pattern. In this regard, the rheology of fibre suspensions is important and more experimental and theoretical developments are needed for suspensions in practical applications. This thesis deals with both issues and provides new insights into the rheological behaviour of fibre suspensions. The first part of the thesis focuses on the problem of measuring the viscosity of a suspension of fibres that are not short compared to the dimensions of the measurement device. Various geometry effects in shear using parallel-plate and cone-and-plate configurations are examined. A modified cone-and-plate device of unusually large diameter and truncation is developed that provides reduced free edge and wall effects. In a subsequent study using the modified geometry, a systematic study is carried out on semi-concentrated suspensions of polyamide fibres in silicone oil to understand the effect of relevant microstructural properties, i.e. volume fraction, aspect ratio and the absolute size of the fibres. In the second part, a model for short-range interactions in concentrated suspensions is introduced and combined with Shaqfeh-Fredrickson hydrodynamic model for long-range interactions. To simulate the orientation evolution, a convective discretisation technique combined with Folgar-Tucker rotary diffusion model is proposed. The model is evaluated for parallel-plate experimental results, and shows good agreement in a wide variety of conditions.