Insights into wide variations in carbon fibre/polyetheretherketone rheology data under automated tape placement processing conditions

Although carbon fibre/polyetheretherketone has been extensively characterised, literature reveals large variations in reported values of melt viscosity. Partly due to this lack of clarity, process models of carbon fibre/polyetheretherketone during automated tape placement tend to be overly simplistic, often assuming Newtonian behaviour, even though this is completely at odds with experimental data. This paper seeks to provide insight into why these wide variations exist, via rheological characterisation, utilising a novel single-ply test method to eliminate inter-ply slip. Several previously unreported and non-intuitive trends are found, e.g. shear viscosity increases with temperature, depends significantly on applied pressure, and increases substantially with time, even in an inert atmosphere. The results here, and in the literature, are explainable if carbon fibre/polyetheretherketone melt is regarded as a yield-stress fluid in which boundary-lubricated, fibre-to-fibre friction determines the viscosity at low strain rates. Additionally, shear banding can occur at low strain rates, if pressure and strain magnitude are low, significantly affecting the results obtained.