title: Advanced Carbon Thermoplastic Composite Post-Forming Process-Property Optimisation
Creators: name: Li, Mengyuan
Publisher: name: The Australian National University
Year: 2024

description: Carbon fibre reinforced polymer (CFRP) materials are becoming more popular in high-performance industries such as aerospace, automotive, energy and marine for their high stiffness and strength to weight ratios and excellent corrosion resistance. When applied as tubular structures, CFRP tubes combined the design versatility of the tubular geometry with its desirable material properties, opening up the possibility of a wider range of structural applications from traditional panelling and tub components to spaceframes and piping. CFRP tubes can currently be produced with both thermoset and thermoplastic matrices via a variety of manual and automated techniques. Curved tubes can be produced directly using techniques such as wet layup and resin transfer moulding, which are often costly and inefficient due to design-specific tooling. Techniques such as pultrusion, filament winding, braiding, and automated tape placement (ATP) can be utilised to rapidly produce straight tubes at lower costs. Tight or continuous bends are not possible with these techniques due to geometrical restrictions of the tools, restricting the designs to be manufactured. Therefore, a post-forming process is required to produce curvatures from straight tubes produced to enable rapid manufacture of curved carbon fibre reinforced thermoplastic (CFRT) tubular components. This Thesis presents a novel CFRT tube post-forming setup design technique utilising localised induction heating, the corresponding forming models, forming results of carbon fibre reinforced polyetheretherketone (CF/PEEK) tubes, as well as the mechanical behaviours of post-formed tubes. For which, the post-forming technique and property change of CFRT tubes based on the rotary draw bending (RDB) process were investigated. In this process, tubes undergo RDB at elevated temperature, where their matrix is softened to allow fibre reorientation and relocation to form curvatures when the bending load is applied. Post-forming experiments were conducted to optimise the post-forming process as well as to evaluate experimental results. Process optimisation focused on the important aspects of tooling and parameters. Heating experiments were conducted to assess localised heating and uniform heating and forming experiments were conducted to derive the optimum internal mandrel design and the corresponding RDB parameters. Key findings from the experiments include: 1. Localised induction heating can reduce heating time from approximately five hours using an oven (for uniform heating) to less than five minutes while resulting in more voids in tube bending zones; 2. A steel spring bending mandrel is the optimum design which provides sufficient internal support to CFRT tubes during bending to maintain tube geometrical integrity. It also catalyses the induction heating process to reduce heating time and can sustain high temperatures to enable forming of tubes with high temperature matrices; and 3. The optimised post-forming technique using an induction heater incorporated RDB setup is capable of producing consistent and high-quality bends of varying angles of the same bending ratio using the same set of dies as validated using both CF/PA6 and CF/PEEK tubes. Property optimisation focused on the geometrical and mechanical property change of the post-formed tubes. From the experimental results, optical characterisation were performed to measure tube axial and circumferential parameters and fibre orientations before and after forming. Three-point bending tests were conducted using CF/PA6 tubes which were straight and post-formed to 90 dgrees bends with a bending ratio of 2 using either induction or oven heating to analyse tube stiffness change and failure modes. Using micro-computed tomography (_muCT), tube wall thickness and void contents were measured and failure mechanisms were identified. Key findings summarising the process optimisation are: ...