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hort-fibre reinforced composites (SFC) are used extensively in consumer goods, high-end industrial products, as well as many applications where strength, directional stiffness and light weight are desirable. During the
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injection moulding process of the SFC, the polymer matrix is melted and the system becomes a fibre suspension in a viscoelastic fluid. Flow causes the fibres to become oriented and the orientation pattern ultimately controls the mechanical properties of the moulded part. In the Department of Mechanical Engineering, two different microstructural approaches have been used to model and study the fibre orientation process in injection moulding of SFC -the dissipative particle dynamics (DPD) approach and the double layer boundary integral method (DLBIM).
The DPD approach is a meso-scale simulation technique that allows us to model the micro-structural behaviour of complex fluids, such as those containing short fibres and large molecules, from which the macroscopic properties of the fluid are deduced via a statistical sampling process. In our study, the fibres are modelled as osculating multi-bead rods suspended in either a Newtonian fluid (modelled by free particles) or a viscoelastic fluid (modelled by linear chains). The alignments of the fibres in a shear flow of such fluids are followed as a function of time in the model (Figure 1). The effects of fibre concentration, fibre geometry, and flow conditions on the extent of fibre alignment have been investigated in detail. |
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The second approach, the DLBIM, enables one to study the mobility of fibre suspensions and their orientation processes in a molten polymer during moulding. The DLBIM approach was formulated for Stokes flow with viscoelastic fluids. In simple viscous shear flow, a prolate spheroid undergoes a close periodic motion known as Jeffery’s orbit. Figure 2 (a) shows the comparison of Jeffery’s orbit to the DLBIM simulation with viscoelastic fluids (the latter reflects closely the viscoelastic nature of injection moulding flow). Figure 2(b) further shows the prediction of a single fibre motion by the DLBIM model, which concurs well with previous experiments.
Our works show that both the DPD approach and the DLBIM technique are powerful tools in modelling the physics of rod-like particle suspensions, which can be found in many important and diverse applications like short DNA separations, pulp suspensions, and carbon nanotubes, to name a few. |
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