Carbon fibre reinforced material extrusion additive manufacturing: fibre orientation and mechanical properties

Material extrusion additive manufacturing (MEAM) of short-fibre-reinforced polymer composites (SFRPCs) has received increased research interest in recent years due to their potential to improve mechanical properties compared to pure polymers. Discrepancies and contradictions regarding the mechanical properties were found in the literature, due to a lack of a microscale fibre morphology and meso-scale geometric characterisation. In addition, true load-bearing areas were rarely measured due to complex specimen configuration and printing defects (inter-filament voids and non-uniform filament orientation). Single-filament-wide specimens not only overcome unnecessary complexity and common defects but also enable reliable properties along the direction of printed filaments and normal to interlayer direction (F and Z directions respectively). These are representative of the upper bound and lower bound of properties of 3D printed SFRPCs.

To fully understand the mechanics of 3D printed SFRPCs, fibre orientation and fibre reinforcing effects were studied experimentally and theoretically. Short-fibre orientation in 3D printed SFRPCs was parametrically studied in 2D and 3D experimentally. This unveiled the effects of printing parameters, non-uniform spatial distribution, and fibre length on fibre orientation. Four polymers and their short-carbon-fibre composites were 3D printed into single-filament-wide tensile-testing specimens. Tensile properties were analysed in terms of fibre reinforcement effects, mechanical anisotropy, and printing parameters. Despite widely varying properties of polymers, fibre reinforcements caused greater strength and stiffness anisotropy but lower strain-at-break anisotropy compared to pure polymers. In addition, critical effects of extrusion width on tensile strength, ductility, and stiffness were found for all materials. A brittle-to-ductile fracture transition was achieved by varying extrusion width in all materials. Compared to extrusion width, nozzle temperature and layer height showed limited and inconsistent effects on mechanical properties since they did not affect filament geometry significantly. Finally, theoretical prediction of strength and modulus was implemented via seventeen classical fibre models to test model validity and identify important factors to consider.

Effective reinforcing effects of carbon fibre were validated experimentally and theoretically (strength by up to 60% and stiffness by up to 124%). Meanwhile, short fibres did not cause great change in mechanical anisotropy, even less anisotropic for strain-at-break. By controlling extrusion width, mechanical anisotropy could be further reduced across all tested SFRPCs. This was achieved based on understanding of underlying factors for short fibre orientation and parameter inter-correlations. This reveals possibly the simplest way to control or reduce mechanical anisotropy – by increasing the extrusion width.

This thesis provides new understanding of the processing-structure-property relationship for extrusion additive manufacturing of SFRPCs, which may enlighten future research and industrial practice in process control, property control, theoretical prediction, and computational simulation of 3D printed fibre composites.