A finite-element model of the ATL consolidation of thermoplastic pre-preg tape

An investigation of CF/PEEK melt behaviour under shear and compression deformation, expected during squeeze flow under an ATP roller at processing temperatures, was carried out using a combined experimental and modelling approach. The project was supported by the Irish Research Council. Rheology was employed to examine the viscosity in shear of both PEEK and CF/PEEK melts. PEEK melt was characterised using traditional rheological approaches, and was found to behave as a linear-viscoelastic, shear-thinning fluid. CF/PEEK melt was examined using a novel off-centred technique and was found to be a linear-viscoplastic, shear-thinning fluid that exhibited a yield stress. Shear banding is a phenomenon known to occur in other yield-stress fluids in the literature. Its presence in CF/PEEK would go some way towards explaining the widely varying viscosity results in the literature and the variation of viscosity with applied pressure obtained in this work. Heterogeneous, two-dimensional, finite-element modelling of carbon fibres in PEEK melt was developed in the non-linear finite-element code Abaqus®, to study CF/PEEK behaviour more deeply. Such finite-element models rely on accurate material models to describe its behaviour. PEEK melt was modelled with a PRF material model, but it was found necessary to include friction between fibres to account for the CF/PEEK yield-stress. Shear banding was observed in the finite-element model deformation, confirming the experimental results. Since shear banding cannot occur in squeeze flow such as under an ATP roller, this knowledge helped identify the most suitable homogeneous material model, also employing a PRF, to describe CF/PEEK for squeeze-flow deformation. Using this material model, squeeze flow was modelled under two different ATP rollers. It was found that squeezing under a stiff metal roller occurs at high strain rates with stresses much higher that the yield stress. In contrast, squeezing under a silicone roller occurs at low strain rates, with stresses of a similar magnitude to the yield stress. This makes it unlikely that significant squeezing will occur under this roller. Further work will be necessary to examine the additional impact of tape tension, while is likely to increase the effective yield stress of CF/PEEK when in the ATP system, making it less likely that squeezing will occur.