3D real-time and in situ characterisation of fibre kinematics in dilute non-Newtonian fibre suspensions during confined and lubricated compression flow

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The physical and mechanical properties of short fibre-reinforced polymer composites depend on the geometry, content, distribution and orientation of fibres within the polymer matrix. These microstructure features are mainly induced during the forming stage, i.e., when composites usually flow in moulds and behave as non-Newtonian fibre suspensions. Their flow-induced microstructures still cannot be well predicted by current rheological models. To better understand them, non-Newtonian dilute fibre suspensions were prepared and subjected to lubricated compression experiments using a micro-rheometer mounted in a synchrotron X-ray microtomograph. These experiments enabled, for the first time, fast and in situ 3D imaging of the translation and rotation of fibres in the suspending fluid. Fibre motions were compared with the prediction of the Jeffery's model. Despite the use of a non-Newtonian suspending fluid and confined flow conditions, i.e., with a gap between compression platens of the same order of magnitude than the fibre length, we showed that Jeffery's prediction was satisfactory if the fibres were sufficiently far from the compression platens (approximately at a distance of once to twice their diameter). Otherwise, the experimental average orientation rates were higher than the Jeffery's prediction.