Continuous carbon fibre reinforced plastic composites – additive manufacturing, characterisation and modelling

Mechanical Engineering not elsewhere classified;Stereolithography (SLA), a pioneering additive manufacturing (AM) process, has been widely utilised in various industries. However, the limited mechanical properties of printed parts have prevented SLA from further development in high-value applications. This can be resolved by adding continuous fibres during the SLA process, i.e., directly printing continuous fibre reinforced plastic (CFRP) composites with significantly improved mechanical properties. Also, using AM allows for the fabrication of CFRP composites with customised designs, which is impossible for traditional production methods. However, research on CFRP composites produced by SLA is severely lacking. Therefore, this thesis aims to establish a novel notion for fabricating CFRP composites utilising SLA and investigate the mechanical properties of produced samples.

In this thesis, AM process for producing CFRP composites was investigated through a bottomup SLA machine and a top-down digital light processing (DLP) machine with compromised printing parameters. A series of experiments were carried out to examine the effects of postcuring time, printing angle, fibre layout, and fibre volume fraction on the tensile behaviour of printed samples. Fibre pull-out experiments were conducted to determine the interface strength of printed composites in terms of embedded fibre length/area and interface resin. In addition, compact-tension experiments were carried out to study the effects of sample thickness, postcuring time, fibre volume fraction and fibre positioning on fracture toughness. Furthermore, a series of finite element models were developed to simulate and predict the impacts of fibre volume and position on composite mechanical properties.

This study revealed that the mechanical properties of printed samples were not affected by the printing direction, and the SLA process can provide sufficient interfacial strength. The printed sample's elastic modulus and tensile strength increased as the fibre volume percentage increased. This study also proved that incorporating a fibre bundle could significantly increase the fracture toughness, especially when the bundle was positioned near the crack tip. Employing a top-down machine enabled more complex fibre configurations and greater interfacial strength than bottom-up machines. For the first time, the dark area problems, a result of light blockage by fibre bundles, were reported and could be overcome by applying the postcuring process. The simulation results were generally in good agreement with the experimental data, demonstrating the capability of the models in forecasting the mechanical performance of composites prior to the actual SLA process.

To summarise, the SLA method overcame the limitations of traditional composite manufacturing methods and significantly improved printed parts' mechanical properties. It broadens the material range available for AM composites and represents a significant step forward in developing commercial SLA machines for printing CFRP composites with complex 3D structures.