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Rengaraj, Kishen
Languages: English
Types: Unknown
Increased pressure on the transport industry to reduce greenhouse gas emissions has hastened the adoption of high performance composites, particularly in the aerospace industry where the value of weight saving is very high. However, the current method of choice for manufacturing high performance composites (autoclave processing) is not cost effective for processing large (greater than 5m2) structural composite components. Developments in Out-of-Autoclave (OoA) prepreg systems have facilitated the use of vacuum only consolidation pressure to process laminates with autoclave level mechanical properties. However, owing to the low consolidation pressure, the process is heavily dependent on de-bulk quality and low cure temperatures; leading to reduced margin for error as well as long cycle times. In parallel, developments in high heating rate OoA processes have been shown to enable short cure cycle times and autoclave-level mechanical properties; albeit with a high tendency towards porosity. To date, studies on high heating rate OoA processing have been limited and the processes are not well understood. \ud \ud The main objectives of this self-funded study were to understand the mechanism of void growth mitigation in high heating rate OoA processes and to study the feasibility of achieving further reduction in cycle time and cost, whilst maintaining high mechanical properties. \ud \ud The primary mechanism of void growth was identified and an analytical model was used to predict the propensity for void growth during a given cure cycle. The model outcome highlighted a window within the cure cycle during which void growth takes place. It was hypothesised that a reduced time to resin gelation in high heating rate processes can reduce the window for void growth, leading to lower laminate porosity. A novel high heating rate pressurised tooling system (the Pressure Tool) was developed to process laminates at 15oC/min combined with the application of up to 7 Bar hydrostatic pressure. The Pressure Tool was used to verify the hypothesis that reduction in size of the window for void growth, facilitated by high heating rate, can lead to lower laminate porosity. Good agreement was observed between the model outcome and the experimental results. \ud \ud Studies have claimed that the reduction in resin minimise viscosity due to high heating rate can lead to gains in mechanical properties; sometimes even higher than that of autoclave cured laminates. OoA prepregs cured using up to 15oC/min heating combined with up to 3 Bar hydrostatic pressure did not result in the claimed additional gain in mechanical properties. The study confirmed earlier suggestions that additional factors such as void geometry and location within the laminate have to be taken into consideration.\ud \ud The final part of this thesis addresses the physical limitations to high heating rate processes; such as, the effect of tooling material, process ancillaries, laminate thickness and resin kinematics on reducing cure cycle time. The poor thermal characteristics of commonly used process ancillaries limit the dissipation of energy released by the laminate during cure. Due to which, laminate core temperature can exceed by up to 5oC, even if the laminate is processed on a highly conductive tooling material. The optimum tooling material to achieve reductions in cure cycle time whilst minimising laminate core thermal overshoot was found to have a combination of high thermal conductivity and low thermal mass. However, currently used tooling systems are not optimum for achieving further reductions in cycle time, due to unfavourable combination of thermal mass and thermal conductivity. Furthermore, the high reactivity of current resin systems and the inherently poor thermal conductivity of the polymer matrix limits the gains in cure cycle times that can be achieved.
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