Mechanical air entrapment, void compression and void dissolution are the main mechanisms behind void formation in liquid composite molding. Mechanical air entrapment is induced by the highly non-uniform geometry at the meso- and micro-scale and the heterogeneous properties of the reinforcing material. This results in differences in the velocity profile through the porous- and free flow domains, which can lead to air being entrapped in slow-flowing domains, by resin flowing through the faster flowing domains. Depending on the competition between the viscous flow through the free flow domains and the capillary flow through the porous domains, voids occur either in the intra-bundle domain or in the inter-bundle domain, where the competition between the two flow types can be quantified by the capillary number. Once the voids are entrapped, they can change in size due to void compression and -dissolution, all which occur on considerably different timescales. An experimental set-up was developed which uses fast radiation curing to almost instantly cure the resin during injection and thus detach the different stages of void formation from each other. In this research the relationship between the mesoscale structure of a woven fabric and mesoscale void formation was investigated. The mesoscale structure is strongly related to void formation, as viscous flow is related to the inter-bundle domain size in the through-thickness direction of the preform and capillary flow is related to the mesoscale bundle porosity. As the bundle porosity is related to both the bundle permeability, and the capillary pressure, its influences on void formation were considered to be large, and thus bundle porosity became the main research topic. A 2D semi-analytical model based on mechanical air entrapment was constructed which was capable of determining the filling times of the intra- and inter-bundle domains on the mesoscale. A parameter called the competitive number was introduced which was related to these filling times. Once the competitive number was larger than 1, spherical inter-bundle voids should be formed, once it was equal to 1 no voids should be formed and once it was lower than 1 ellipsoidal intra-bundle voids should be formed. This model was validated experimentally. Flow behavior predicted by the model was compared to video footage of the flow front propagation at the surface of the preform at the macro- and mesoscale during multiple injections. Void volumes, types and locations obtained from the model were compared to voids observed in micro-CT scanned samples and were in agreement with each other. The validated model was eventually used to investigate the effects of bundle porosity on void formation. It was found that increased bundle porosity at a constant global preform porosity, leads to increased intra-bundle flow and decreased viscous flow, thus considerably influencing void formation by mechanical air entrapment. In short injection cycles with high macroscopic flow velocities, which can lead to intra-bundle voids, it could be beneficial to switch a fabric with a higher bundle porosity, as that would effectively reduce the intra-bundle void size. For injections with low macroscopic flow velocities, the opposite would apply, thus usage of fabrics with lower bundle porosities could help to reduce the inter-bundle void volumes.

Frontal polymerization (FP) has emerged as a promising alternative to traditional bulk curing methods in recent years for manufacturing high-performance fibre-reinforced polymer (FRP) composites. The energy utilized in this self-propagating curing strategy is solely derived from the exothermic enthalpy of polymerization, making it potentially more efficient than traditional curing methods, which are extremely energy-intensive and therefore unsustainable. Research in the field of FP has been primarily focused on studying the effectiveness of the FP formulations, particularly the relationship between the monomer types and initiator concentrations on front properties. However, the research on the use of FP for manufacturing FRPs has been limited to flat rectangular plates. Therefore, this research aims to investigate the behaviour of the propagating fronts in composites with varying fibre volume fractions (Vf) within the sample and varying geometries to assess the feasibility of using FP in structures that more closely resemble real-life applications. Several test setups were explored to determine the best method for maintaining the heat balance required to sustain a propagating front. Through these trials, a stainless steel-based closed mould test setup with Teflon sheets was conceptualized and manufactured, consistently allowing for the manufacturing of composite samples with a Vf below 36%. The influence of changing Vfs on the behaviour of the front was studied, and the results were quantified using temperature data captured via thermocouples placed at strategic locations. Through a series of experiments, a critical length was established that allowed the propagation of the front from the low Vf region to the high Vf region. When the length of the low Vf region with respect to the high Vf region was smaller than the critical length, the front was seen to quench at the interface. This phenomenon was attributed to the physical reduction in the volume of the resin, which implied a reduction in the generated heat through polymerization. This led to fronts with lower peak temperatures, which upon reaching the interface would quickly fall below the threshold temperature required to sustain the front due to the higher rate of heat loss resulting from the higher fibre content. This study was followed by the manufacturing of L-shaped composite samples with uniform Vf, which showed the propagation of the front in complex geometry. Finally, the Vf was varied within the L-shaped samples, and the behaviour of the front showed similarities to the rectangular samples with varied Vf, leading to the conclusion of the presence of a critical length irrespective of geometry. These findings effectively contribute to the knowledge base necessary for implementing FP as a curing strategy for FRPs. The results obtained from this study can inform the design of manufacturing processes and setups tailored for the use of FP, potentially leading to more efficient and sustainable manufacturing practices.

In recent years, lightweight composites have become increasingly popular in several industries for the benefits they offer such as high strength to weight ratio, cost efficiency, tailorable properties and superior mechanical performance. The performance of these lightweight composites can be further optimized by using spread tows as a reinforcement solution. Tow spreading is a process of flattening fibre tows into thinner and wider tapes with a low aerial weight which increases resin impregnation of fibres, lowers void content in impregnated tows and produces lightweight composites with improved damage tolerance. The majority of tow spreading technology however is owned by companies in the form of intellectual property and patents thus limiting the scientific understanding of the process. This aspect leaves tow spreading open as a field of research for exploration and investigation. While some previous studies have investigated the process of tow spreading, the applicability of these studies has so far been limited in their experimental apparatus. These studies identified the gap that exists between the sophisticated industrial technology and the research setups hence providing an incentive to further develop an experimental facility that generates a deeper understanding of the current industrial practices in tow spreading. In this research, a mechanical tow spreader was developed that utilizes cylindrical spreader bars to facilitate the spreading of fibres. This was followed by an experimental analysis where different input parameters in the tow spreader such as tension, spreader bar orientation, heat and speed among others were investigated to understand their influence on spreading behaviour. Lastly, a microstructural investigation of spread tow was conducted where a technique to capture the spread of fibres in the tow was developed and microstructural samples were generated. These samples were then observed under a microscope to visualize the evolution of fibre distribution as the tow spreads.

This master’s thesis focuses on the design and assembly of an innovative tow spreading line for investigating carbon fiber tow spreading through the utilization of spreader bars. The primary objective was to examine the friction behavior during the spreading process by employing multiple tension sensors. Additionally, novel monitoring concepts including 4-point resistivity sensing, optical width and gap detection, and optical analysis for determining fiber orientation within the carbon tow were introduced. Through a series of experiments conducted on the newly built setup, comprehensive data was collected and analyzed. The findings revealed an intriguing observation that deviates from the established Capstan equation. It was observed that an increase in tension in the tow resulted in a reduced apparent friction coefficient during bar tow spreading. This departure from the conventional understanding of friction dynamics in this scenario contributes valuable insights to the field of carbon fiber tow spreading. Furthermore, the feasibility of utilizing resistivity measurements as a means of detecting material anomalies, such as damage or waviness, was investigated. The results demonstrated that this technique, while partially reliant on tow tension, consistently detected such anomalies. However, challenges were encountered in achieving quantitative consistency in the optical orientation analysis, making it difficult to obtain robust results in this aspect. The experimental monitoring setup also revealed an increase of approximately 20% in the width of the tow. To enhance process repeatability, recommendations are proposed to upgrade several components of the built experimental line. Moreover, it is advised to further test and develop the software analysis techniques to achieve a higher degree of repeatability, thus potentially validating the obtained results. In conclusion, this research contributes to the understanding of carbon fiber tow spreading through the design and assembly of an experimental production line, examination of friction behavior, and the introduction of novel monitoring techniques. The outcomes serve as a foundation for future investigations and advancements in this domain, with the potential to enhance the efficiency and quality of carbon fiber tow spreading processes.