Many lightweight, high-performance applications have accelerated the adoption of carbon fibre-reinforced thermoplastic composites with thermoplastic matrices, particularly Polyetheretherketone (PEEK) matrices. However, manufacturing high-quality thermoplastic composite tapes remains challenging because high melt viscosity limits fibre impregnation. Wet powder impregnation addresses this by coating fibres with a water-based suspension of thermoplastic particles before consolidation.

This thesis investigates slurry-based manufacturing of unidirectional carbon/PEEK tapes. It relates process parameters and slurry state to tape quality by comparing two impregnation configurations: the Kiss-roller and Nip-roller processes. The investigation is structured around systematic manufacturing campaigns where key process parameters are varied. Line speed and creel tension are investigated for both impregnation routes, and the Nip gap is also varied for the Nip-roller configuration. Tape quality is assessed using gravimetric fibre volume fraction (FVF), while lateral tape quality indicators are computed over a rectangular region of interest (ROI) spanning the full tape width at the tape’s mid-length to quantify surface quality. Cross-section microscopy is used to support and bound the interpretation of surface-based metrics.

Within this dataset, the Kiss-roller process shows readable parametric trends. In Campaign 2, increasing line speed from 3.0 to 3.6 m/min systematically decreases FVF, with a smaller drop in speed sensitivity at higher creel tension. In contrast, the Nip-roller process is dominated by strong process variability, which limits the identification of monotonic effects of line speed or creel tension when conditions are not strictly repeated. In the most comparable subset (gap 35, L = 0), FVF ranges from 64.5–67.9% at 3.6 m/min and from 64.5–68.8% at 4.2 m/min. The spread within each speed is similar to the difference between speeds. In-process qualitative observations suggest that rapid inlet-regime transitions and tow spreading or overlap quality can yield distinct deposition states under similar setpoints.

Finally, microscopy highlights a key methodological limitation of surface-based analysis: tape quality indicators quantify lateral surface quality variations in the ROI but do not provide direct evidence of through-thickness impregnation. These data provide a consistent basis for discussing the trade-offs in controllability and reproducibility between the two impregnation routes. They show that robust parametric identification requires controlling slurry rheological state for the Kiss-roller process, where surface deposition behaviour is dominant, and stabilising inlet flow regime and tow spreading conditions for the Nip-roller process. Targeted microstructural validation is required when impregnation through the tape thickness governs mechanical reliability.

The transition towards a sustainable aviation sector requires a fundamental rethinking of how structural materials are designed, manufactured, joined, and used. Lightweighting alone is insufficient if achieved through materials and processes that are energy-intensive, difficult to recycle, or incompatible with circular use. This thesis explores an alternative paradigm: sustainable structural systems that integrate bio-based materials, eco-efficient multi-material design, adhesive-free joining, and ultimately self-sensing functionality.

Firstly, the thesis develops sustainable joining strategies for wood-based structures. Ultrasonic welding is studied as an adhesive-free joining method that exploits the thermoplastic behaviour of native lignin by introducing three-dimensional printed, lignin-rich energy directors. This method has demonstrated improvements in joint strength and durability while maintaining compatibility with circular design principles and scalable manufacturing.

Secondly, the thesis advances computational and experimental methods for an eco-efficient multi-material design framework. The topology optimisation framework is extended to include environmental performance metrics alongside mechanical objectives, enabling explicit trade-offs between stiffness, mass, and environmental impact. These designs are realised using multi-material additive manufacturing, demonstrating how selective material placement can reduce waste, improve performance, and enable functional integration.

Thirdly, focusing on lignocellulosic material systems, particularly wood, lignin, and fungal mycelium, the work addresses the question of how traditionally passive bio-based materials can be transformed into functional, adaptive, and circular structures. Three complementary research directions are pursued. First, the thesis investigates fungal biology and signalling, examining how living mycelial networks embedded within composites exhibit bioelectrical activity that responds to environmental and mechanical stimuli. These findings point toward the possibility of structural materials that inherently sense and report on their own condition.

Finally, the results demonstrate that sustainable structures cannot be achieved solely through material substitution. Instead, performance, sustainability, sensing capability, and manufacturability must be addressed simultaneously. By combining living and self-sensing materials, optimised multi-material architectures, and sustainable joining and manufacturing techniques, this thesis lays the groundwork for a new class of structural systems that are lightweight, circular, and functionally active.

Sustainable aviation demands alternative propulsion system, such as Liquid Hydrogen (LH2). LH2 needs to be cryogenically stored in large, lightweight fuel tanks. Carbon-fibre Reinforced Polymer (CFRP) composite tanks offer promising weight-savings.

Liner-less composite tanks are prone to safety-critical issues such as leakage. Leakage is caused by the presence of microcrack networks in the laminate. It is known that these crack networks arise due to thermo-mechanical loads. Element level testing is required to build a foundational understanding of the damage and leakage phenomena. The bulge test rig is capable of such element level testing, by mimicing multi-axial loads experienced by the tank structure at cryogenic temperature.

A digital twin is developed to provide deeper insights into the behaviour of the test rig and specimen. The validity of this digital twin is quantified by a 'multi-sensor' experimental campaign. The validation dataset is obtained on a range of test configurations, both at room and cryogenic temperature. Specifically, strain data is acquired using strain gauges, fibre-optic sensors and digital image correlation. Qualitative damage assessment of post-mortem specimen is conducted using ultrasound and optical microscopy, to compare the inter and intra-laminar damage initiation predictions of the digital twin.

The experimental-numerical analysis of the work is envisioned to contribute to a more representative design of bulge test specimen.

Fiber-reinforced thermoplastic polymers are being considered as alternatives to thermoset composites because they are recyclable, easier to repair, less hazardous to produce, and offer better impact resistance. However, high material costs and processing challenges hinder their widespread adoption in high-performance applications. Continuous Resistance Heating Technology (CoRe HeaT) is an innovative method for the rapid processing of continuous fiber prepreg tapes, aiming to accelerate manufacturing and reduce costs. This study examines how process parameters such as moisture, maximum temperature, heating rate, and tape tension affect CoRe HeaT deconsolidated carbon fiber low-melt polyaryletherketone (CF-LMPAEK) prepreg tapes. It also investigates how consolidation parameters, such as average compaction pressure, compaction duration, tape tension, and tooling temperature, influence the quality of CF-LMPAEK pipe preforms produced using high-speed (500 mm/s) CoRe HeaT tape winding. Image processing of cross-sectional micrographs are used to quantitatively evaluate both deconsolidated tapes and wound preforms. Void content, root mean square surface roughness and waviness, and pore distribution are used as indicators for tape and laminate quality, since they can have significant impacts on mechanical performance, durability, and further stages in product manufacturing. The deconsolidation of tape is performed by heating a 70 cm long, pre-tensioned CF-LMPAEK tape clamped between two double-plate electrodes using one electrical pulse. The oven-dried tape had a 40% lower porosity as compared to the tape kept at ambient conditions. A tenfold increase in tension could also be seen to decrease porosity by 25%. Temperature and heating rate were linked in pore proliferation, while temperature alone was seen to be critical to roughness increases. A lack of tow tension was seen to cause global wrinkling of the tape, increasing waviness by 62%, while excessive tensioning was correlated with a decrease in tape width by 6.4%. The preform winding is performed using a purpose-built tape winding machine and consolidated using custom-built pressure elements, spanning rigid and conformable rollers and area compactors. The rate of porosity increase with an increase in compaction duration was seen to be negatively correlated with the applied compaction pressure. The lowest level of volumetric void content of 0.75% was achieved with the use of a silicone area compactor at 1.22 MPa within the preform tubes wound at 500 mm/s. The lowest levels of roughness and waviness, of 2.27 μm and 9.5 μm, respectively, were obtained using a a copper roller delivering 4.56 MPa at 500 mm/s winding speed. Compaction pressure was linked to a power fitting decrease in porosity. Increasing tension was also connected to a decrease in porosity, yet excessive tensioning led to gap widening, ineffective gap filling, and layer crumbling, which negatively impacted laminate quality. A marginal improvement in outer surface quality could be linked to heating the tooling above the glass transition temperature (Tg ) of LMPAEK. The impact of increasing tooling temperature on porosity and pore distribution was negligible. It was concluded that further optimization of conformable area compactors may bring significant improvements in laminate quality for winding speeds exceeding 500 mm/s, and should be further studied. Furthermore, additional studies on the impact of tape temperature at the nip-point are expected once sufficient temperature monitoring and control capabilities are achieved.

The aerospace industry has increasingly prioritized sustainability by adopting lightweight materials, shifting from metal alloys to polymers and polymer composites. Thermoplastic polymers, in particular, are gaining attention due to their recyclability and ease of reshaping. As the demand for high-performance structures grows, advanced joining techniques are becoming essential. Traditional joining methods are expected to be replaced by fusion bonding, with induction welding standing out for its non-contact nature and continuous processing capability. This technique typically relies on susceptors (e.g., metal meshes) to convert electromagnetic energy into heat. However, these materials can cause uneven heating and weaken mechanical properties. Magnetic nanoparticles present a promising alternative as minimally invasive conductive susceptors.
Originally studied for biomedical applications, magnetic nanoparticles generate heat under an alternating magnetic field through hysteresis losses. Researchers have demonstrated their ability to heat polymers above their melting points for induction welding. However, a major challenge is preventing excessive heating, which can degrade thermoplastics. To address this,it was proposed to select ferromagnetic susceptors with Curie temperatures that match thermoplastic processing temperatures. This ensures effective heating while preventing overheating, offering a self-regulating thermal solution for induction welding.

This thesis aimed to gain knowledge on thermal and magnetic capabilities of selected nanoparticles: magnetite (Fe3O4), Nickel (Ni), Nickel-Zinc ferrite (NiZnFe4O4) and Chromium Dioxide (CrO2). These materials were tested via MagneTherm equipment to identify the optimal induction heating parameters, in terms of strength and frequency of the applied magnetic field. Magnetite showed the best heating capabilities among all particle types, however due to the high detected Curie temperature it does not represent a good candidate for further research. A good alternative was found while testing Nickel and Ni-Zn ferrite nanoparticles. These materials have shown slightly lower heat generation, compensated by Curie temperatures within the range of processing temperatures of multiple thermoplastics, and consequent good potentiality for this application.
Further characterization of the particles was performed to gain better understanding on their magnetic and morphological properties and to find correlations with their heating capabilities. In this second part, more traditional techniques such as SQUID, XRD, and TGA were employed.

This study confirms that MNPs are viable candidates for non-contact induction welding in aerospace applications, presenting an efficient alternative to conventional heating methods. Induction heating experiments confirmed the correlation between alternating magnetic field parameters (amplitude, frequency) and power generation. However, further optimization of nanoparticle dispersion, morphology, composition and scalability is required for industrial implementation. Future research should explore alternative nanoparticle compositions, embedding techniques, and evaluate their performance in thermoplastic induction welding prototypes.

Unidirectional composite tapes are extensively used in automated manufacturing to produce high-performance composite parts with significant design flexibility. Their superior mechanical properties stem from the alignment of continuous fibres in a single direction, making them ideal for advanced engineering applications. However, the performance of these components is closely linked to their microstructure, and accurately predicting their behaviour requires a detailed understanding of their microstructural characteristics....

In developing Type V hydrogen tanks for energy storage and propulsion in commercial airliners, the key design criterion is maintaining tightness under cryogenic conditions. A concern is that anomalies in the laminate could cause microcracks which compromise the tightness. This study examined how resin flow, caused by an expanding mould during curing, creates a gradient in the local fibre volume fraction (FVF) across the laminate. The impact of resin system selection, autoclave cycle parameters, mandrel material, and fibre tension on the FVF gradient was investigated. Cylindrical specimens were manufactured in a process replicating automated fibre placement (AFP), with a piezoelectric sensor measuring contact pressure at the mandrel-laminate interface during the autoclave cycle as an indicator of resin flow and FVF spread. Significant correlations were found between resin flow and all parameters except fibre tension. The resin's viscosity profile and gelation characteristics had the most substantial impact on the FVF spread. Resins with high viscosity and short gelation times lowered the FVF spreads the most. Mandrel materials with lower thermal expansion coefficients (CTE) also reduced FVF gradients, though to a lesser extent. Adjusting the autoclave cycle to lower temperatures for longer periods resulted in the least significant reduction in FVF spread; however, this approach holds high potential for further improvement if more data on resin rheology and cure kinetics are available to optimise the autoclave cycle. These findings provide valuable insights for minimising FVF gradients in the design of carbon fibre-reinforced polymer (CFRP) tanks for liquid hydrogen.

Hydrogen is one of aviation's most promising fuel alternatives in the coming years. However, to store it, a large and light vessel is needed.

The solution to this problem can be the carbon fibre fuel tank. Composite structures provide significant weight savings, which is crucial in this application. However, despite its superior performance, the hydrogen molecules permeate through the CFRP tank wall. Hence, researchers must first understand the driving phenomena to design a safe and robust hydrogen tank. The only proven way to do this is through experiments, but they must also be carefully considered to deliver representative results.

In this research, the question of how to perform the permeation and leakage testing of the CFRP laminates to accurately represent the behaviour of a full-scale hydrogen tank is being answered. The numerical analysis method has been used to evaluate different testing parameters and aid the development of the new testing rig. This thesis is the collaboration between the TU Delft and the DLR.

METAL-composite laminates, also known as fibre metal laminates (FMLs), which are made by alternating thin sheets of metal alloys and layers of fibre reinforced polymers, have attracted the interest of many scholars and researchers in the field of aerospace applications, including manufacturing. These hybrid materials are widely applied owing to their significant advantages ofweight reduction, superior specific strength, higher stiffness, and more fatigue resistance than monolithic metal sheets as well as the better impact strength and damage tolerance compared with the full composites. However, the manufacturing process of such laminates is difficult as various forming and curing stages are required in combination with the complex deformation and failure mechanisms. Hence to improve the manufacturabilty of these structures, it is crucial to develop a material forming method with optimized material compositions and process parameters.

The proposed press forming process, consists of an integral forming and curing cycle, is an innovative method formanufacturing small-to-mediumsized components. The cycle involves a laminate preparing and preheating process, forming of the uncured laminate, consolidation and (partial) curing in a same mould as well as cooling and removal of the component. The most critical aspect of the cycle is a proper control of the different deformation mechanisms in different layers. For that, during the preheating stage, temperature and time needs to be carefully controlled so that the inter-ply sliding at the metal-prepreg interfaces and the intra-ply shear within the prepregs can be greatly enhanced when the resin viscosity decreases. Then, the still uncured laminate is formed and subesequently cured under pressure, avoiding a separate curing system with pressure, which is time and cost-saving...

The cellular, hierarchical nature of living materials such as wood or bone, enables their mechanical properties to vary locally. This variability grants themmulti-functionality and highmechanical performance with low resource consumption. To obtain such desirable features with man-made structures, many manufacturing restrictions need to be overcome. Currently, the most common structural anisotropic materials are fibre-reinforced composites, which allow limited control over material placement, orientation, and directional strength.

In this thesis, we explore how the additive manufacturing of Liquid Crystal Polymers (LCP) can unlock our ability to control these parameters and, therefore, expand the design space to optimize structural performance while limiting material use. To achieve this, we leverage the alignment of nematic domains in LCPs, induced by shear and elongational flow during extrusion through the 3D printer nozzle. By modulating the pressure inside the nozzle to change line width, we observed an axial stiffness range between 3 GPa and 30 GPa, associated with a failure mode shift from ductile to brittle. Demonstrating our ability to print with variable toolpathwidths and sharp curvatures,we created 3D patterns inspired by fluid dynamics. This ombination of shaping freedom and anisotropy control enabled the design of functional objects with embedded crack mitigation strategies, or improved buckling performance.

Anisotropy gradients are also present in the structure of trees. Inspired by the seamless transition of wood fibres from parallel to interlocked at the branch junctions, we manufactured sinusoidal and helical patterns with LCP. The introduction of local waviness decreased axial tensile stiffness, but significantly enhanced toughness. Tensile and interlocking performance were driven by two critical parameters: angle of maximum deviation to the axial direction, and pattern size.Moreover, resistance to normal stresses was improved, as shown by the 88% increase in load-bearing capacity compared to a nonreinforced configuration in a two-strut junction. This opens the possibility to integrate localized interlocking patterns in multi-axis loaded regions of anisotropic parts.

Manufacturing methods which utilize as littlematerial and energy as possible are needed for future space exploration missions. To assess whether the strategies mentioned previously are applicable to this context, 3D-printed LCPs were subjected to four different space environments. High-energy electron radiations created colour-centres, leading to a bright green coloration in the bulk of the specimen. However, no significant decrease in staticmechanical properties was observed, with unidirectional stiffness remaining close to 30 GPa even after intense electron irradiation and thermal cycling. These results indicate that LCPs are a promising alternative to engineering polymers like PEEK or PEI for space applications.

Future research into topology-optimization methods integrating both asymmetry between tension and compression, and mechanical property gradients, will enable to further explore the design space opened by the tunable anisotropy of LCPs.

Since the dawn of time, human beings have been always fascinated and envious of the flight of birds and human’s desire to fly has become stronger and stronger over the centuries. What child looking at a bird does not wish that they could take flight to chase it and view the at the landscape from above? The dream to fly is inside the human being since tender age, although the physical possibility does not belong to their nature. Nevertheless, humans have exploited the power of their brain and hands to get closer this dream.

Leonardo Da Vinci (1452-1519)1 was the first one that effectively studied the flight dynamics of the birds, developing several technical drawings and building flying machines between the XV and XVI century. However, for almost 400 years, no tangible developments occurred until the Wright brother’s famous flight (in 1903)2, which is the major milestone of the aeronautical world. From that far away event, enormous technological developments have been made so that a large number of apparatuses able to fly in the atmosphere and travel in Space have been developed.

One of the key technological sectors that has allowed the large growth of the aerospace industry is that of the materials. Specifically, the large use of composites materials and adhesives has allowed the design and building of modern airplanes, spacecraft and devices which are increasingly lighter, faster, stronger and tougher. Nevertheless, at the same time, the increase of aerospace performance by using new materials has raised the necessity to develop a technology which is able to provide information on the health state of the materials that composed the structures. This need is not only linked to the necessity to increase the safety but also to two other factors: to optimize the maintenance schedule of the aircrafts so that to save economical resources, and to increase our knowledge about the mechanical, damage and fracture behavior of the materials.

Although, during the decades, a number of inspection and monitoring technologies have been developed as such as piezoelectric, acoustic and ultrasound sensing, electrical gauges, fibre optic sensors, eddy current, comparative vacuum, interferometry, penetrating liquids, thermography, radiography (x-rays) and others, not one of these has proved to be applicable for the entire life cycle of the material (from the manufacturing, operating phase to the end of life) which provides in real-time enough information to effectively evaluate its health state.
Therefore, the here presented thesis aims to demonstrate the multi-sensing abilities and monitoring benefits of tilted Fibre Bragg gratings sensors (TFBGs) in order to fill a current technological gap in the previous technologies and to improve the state-of-art of the Structural Health Monitoring field. The research focused initially on the fundamentals, mathematical and numerical modelling, methodology and demodulation techniques of the TFBG sensors, successively the treatment is dedicated to the TFBG applications for simultaneous three-parameter monitoring embedded in composite material and silicone adhesive, respectively for aeronautical and space use. This selection of these materials was made from those commonly used in the aerospace industry. The application of TFBGs can be, from the very beginning, have a great impact in the scientific community and, also maybe, in industry.
Therefore, taking into account what was reported previously, the research presented in the following thesis was confronted with the aim to demonstrate that TFBG can be a promising sensor for structural health monitoring of aerospace materials, and are able to provide reliable and simultaneous multi-parameter measurements. The treatment begins with the first introduction chapter where composite materials, silicone adhesive and TFBG sensors are presented fro a historical perspective, and their current state-of-art regarding applications, issues and technological gaps in relation with the working load and environment, are reported. Then, the research questions and reasons that motivated the scientific investigation are introduced in the last part of chapter 1.
The starting point of the research can be considered chapter 2, where, first of all, the realization of a TFBG sensor customized for the desired application is confronted by providing a numerical model to simulate the spectrum based on the values of the parameters of the sensor Bragg structure. This is important to obtain TFBGs with a spectral signal usable for the simultaneous and separated measurements of different parameters before the manufacturing of the same sensor. It can be noted that, the simulation of the TFBG spectrum may bring other several benefits such as production time and costs savings. In fact, the determination, a priori, of the Bragg structure, and hence, the parameters of the TFBG manufacturing setup, allows the TFBG sensor to be obtained with the desired measuring characteristics without the waste of materials, testing and manpower time.
In chapter 3, the demodulation of the TFBG spectrum for refractometric measures was improved by developing a new technique based on the Delaunay triangulation of the datapoints that compose the spectral signal, which was demonstrated to be faster and more accurate than previous techniques. This technique is also compatible with the method to extract the strain-temperature variation from the spectrum, so that the methodology for multi-parameter measurement performed with a single TFBG is completed.
At this point, chapter 4 starts the beginning of the second part of the thesis, where the simultaneous thermomechanical measurements of the TFBG were used to determine the deformation effects of a thermal load profile on a glass-fibre/epoxy composite plate induced by heating lamps. A further step achieved in this chapter, was to compare the TFBG measurements with a classical strain sensing technique based on the use of a TFBG as standard strain gauge thermally compensated by a thermocouple. Furthermore, another comparison was performed between the experimental results and the Finite Element Model (FEM) analysis results obtained by modeling the composite sample with and without the embedded TFBG and applying a Gaussian thermal profile.
These comparisons show that the strains measured with the single TFBG are very close to the values obtained by using the classical approach and the full FEM model. The improvement regarding the single TFBG sensor can be obtained by increasing its thermal resolution which is the weak point of the simultaneous thermomechanical measurements. Nevertheless, it can be further improved by using an interrogation system with a finer wavelength scanning resolution or by designing a TFBG whose resonance peaks move in the spectrum much more with the temperature variation. The comparison of the empirical measured deformations with the strain values extracted by the FEMs analysis have highlighted the importance to model the optical fibre inside the composite to increase the accuracy of the model. In conclusion, the single TFBG sensor was able to monitor the thermomechanical trend inside the composite during the heating lamp exposure with a good accuracy as the values were close to those measured with the classical approach and full FEM simulation.
The conclusions drawn from the first and second part of the thesis are that the investigation on the multi-parameter sensing abilities of the TFBG sensors, also embedded in composite materials for a complete monitoring of their state from their manufacturing to the operating life, has given positive and promising results. Especially regarding the embedding of the sensor, each single TFBG sensor can be used in order to measure simultaneously thermomechanical and refractometric variations of the composite material where the sensor is embedded and in any step of the material life.
The second part of the thesis continues with chapter 5. Here, the TFBG sensors are embedded inside glass-fibre/epoxy resin composite material to monitor simultaneously the strain-temperature variations, the resin refractive index (RI) during the manufacturing process and the application of a thermal load profile. Regarding the TFBG monitoring performed during the composite manufacturing, several samples of different thickness were tested and sensorised. In these samples, the sensors were able to measure, simultaneously, the strain state induced in the material due to the manufacturing steps, the temperature profile, and the resin RI variations due to the crosslinking occurring during the curing. This allowed the evaluation, not only of the possible state of stress in the material during the production, but, even more interesting, the cure degree of the resin. Indeed, as anticipated already in previous research, the resin RI measured with the TFBG can indicate the curing degree matrix of the composites. The TFBGs were able to provide the entire RI profile of the resin during the curing in which three different behavior ranges were identified in combination with the temperature trend. The expected typical plateau was reached in the RI curve when the resin was considered fully cured. Furthermore, the refractometric sensing abilities of the TFBGs were also used to monitor the resin flow and speed during its infusion.
From chapter 6 on, the treatment is focused on the TFBG application study on the space qualified silicone adhesive, which regards the third part of the thesis. Silicones are strategic and widely used materials in many engineering fields, but mainly their use finds great importance in space industries, especially for electronic and structural components. Nevertheless, some parts of the spacecraft where silicone elements and adhesives are used undergo a direct exposure to the space environment, which represents harsh operating conditions and is strongly degrading for any material. In fact, perturbations as severe temperature gradients, ultra-high-vacuum, Ultra-Violet (UV) and ionizing radiations (x-rays, γ-rays, etc…), extreme thermal range and cycles, thermal shock, micro-gravity, atomic oxygen (ATOX), high accelerations, vibrations and space debris are characteristic of the space operating environment. Furthermore, it has to be also considered that these elastomers have to maintain their original mechanical and physical properties during their operational life in space, which is a challenging target. In this context, TFBG sensors embedded inside silicone adhesives, through their simultaneous thermomechanical and refractometric measuring abilities, may offer fundamental information to evaluate the internal mechanical and chemical state of the elastomers during the use in space environment. This would offer the possibility to have a technology able to provide a general view on the degradation state of the adhesive in real-time during the laboratory testing or operational life, which may improve the evaluation of its health and performance trend along the exposure time. Nevertheless, as for the composite, the monitoring technology should be always minimally intrusive, light and not affecting the material performance and properties. Hence, in order to match these requirements, each embedded TFBG sensor has to be able to perform the thermomechanical-refractometric measurements as single sensor without to be affected by the exposure to the space environment. The demonstration of a working TFBG sensor in a simulated space environment may be an interesting and promising starting point for the future development of a sensing technology able to real-time detect degradation and damages in spacecraft structures and components during the space missions.
In chapter 6, since studies in literature about the compatibility of optical fibre sensor layers in vacuum were not found, initially, the waveguide containing the TFBG was subjected to an outgassing test. Once the compatibility was verified, a TFBG was then embedded inside a space qualified silicone used as adhesive to join two micro-sheet cover glasses in order to compose a sandwich. This TFBG sensorised glass sandwich was tested inside a vacuum chamber and exposed to high-vacuum (around 10-6 mbar) and loaded with a thermal cycling profile. Hence, in this first approach, the experiment was planned to simulate the degradation on the silicone adhesive of the TFBG sensorised sample generated by outgassing (due to high-vacuum) and thermal cycling loads, which are perturbations constantly present in space environment.
The measurements of the single multi-parameter TFBG sensor, acquired during the experiment, were compared with the values obtained by using the classical sensing approach consisting in the thermal compensation of the TFBG (used as strain-gauge) through a thermocouple. The comparison highlighted the inaccuracy of the classical sensing method due to the different location of the sensors in the silicone which brings serious mistakes in the measurements. In fact, due to the absence of an atmosphere in the space environment, the transfer of heat inside a body depends only from the thermal conductivity of the material which accentuates the issue of the classical sensing approach linked to the different location of the sensors. While, the self-compensated TFBG can overcome this gap as it is thermally self-compensated. Furthermore, for a complete evaluation of the health state of an organic material such as a silicone adhesive, temperature and deformations may be not sufficient as it is not easy to obtain information on the chemical state of the material, such as the variations of the silicone RI. The embedded TFBG sensor, simultaneously sensitive to temperature and deformations, was able to provide measurements of the silicone RI that was changing due to hardening and chemical evolution induced by the thermal cycles in high vacuum environment. The chemical variations were checked also by testing a sample of pure cured silicone adhesive via the Differential Scanning Calorimetry, which showed chemical variations due to the realising of volatiles during the heating-up and cooling-down phases of the test. These results make the TFBG a promising sensor able to provide a complete evaluation of the silicone state that comprises the thermomechanical and refractometric measurements while working in space environment.
By exploiting the experiences achieved in the previous chapters, chapter 7 treats of the research focused on the detection of the UV effects induced in the silicone adhesive working in space simulated environment. This topic may be interesting for polymers used in a space working environment as long exposure to UV radiation in vacuum can cause severe damages and bring the component to failure together with the structures of a spacecraft. The reason lies in the absorption of these light wavelengths by the silicone, whose organic chemical composition is photochemically susceptible to these light wavelengths. As a consequence, the photochemical reactions cause severe degradation of the material with deterioration of the original properties and efficiency and shorter working life. Then, it is easy to understand that a sensing technology able to monitor the degradation of the silicone working in the previously described conditions, may be really useful to evaluate the state of the material during its use, but also, in the phase of laboratory testing to better detect the material behaviour and its changes. Therefore, several and different TFBG sensorised samples were tested in a high vacuum chamber provided of UV lamps which emitted a high radiation level for a certain exposure time. Hence, the acquired spectra were demodulated and used to measure the thermomechanical and refractometric variations induced by the UV exposure inside the silicone in correlation with the equivalent exposure solar hours. This allowed an analysis of the different degrees of degradation of the silicone based on the sample configuration and exposure time. In conclusion, the achievement of this research was the demonstration of a minimal intrusive sensor as the TFBG is not only compatible with operating in a space environment, but is also able to provide in-situ reliable multi-parameter sensing for the monitoring of the thermomechanical and refractometric state of the material during its operations in space environment.
The conclusions of the thesis are reported in chapter 8 where the outcomes of the conducted researches highlighted the potentiality of the TFBG as a single three-parameter sensor to monitor the state of materials for aerospace industry. As consequence, these results may induce the raising-up of the SHM concept by developing a sensing technology based on the TFBGs that are able to provide an overall view of the material state, from the manufacturing to the operational life, also working in harsh environmental conditions as those in space.

Thermoplastic composite materials are increasingly used in aircraft construction due to their high-mechanical properties, toughness, rapid processing rates and reprocessing possibilities at their end-of-life. In the production thermoplastic composite parts, 10-40% of the material remains unused, so-called production waste. Despite being a high-performance material that can be re-processed, real-world applications of the material into structures or products are lacking. This research explores how the thermoplastic composite production waste material can be utilised by designers and engineers, by demonstrating its use in a design process of new applications. This exploration resulted in the design of a structural member, that can be tailored to a specific function, and a design of a pedestrian bridge, which aims to stimulate the creative use of the material. By evaluating the design process, a framework is proposed which engineers and designers can utilise to design new applications from composite waste material.

The Nacra 17 is a foiling catamaran selected to compete in the 2024 Olympic Games. The Nacra 17 is a one-design class, which requires all equipment to be identical. The Dutch Olympic team found through testing the bending stiffness that there is a slight difference in how each of their hydrofoils performed. The question was asked: What are the manufacturing deviations in the Nacra 17 hydrofoils and how do they impact the performance? This research aims to answer the previously stated question. The 3D scanned hydrofoils validated that there is indeed a measurable difference in shape between the different hydrofoils of the Olympic team. A successful method was proposed that automatically extracts 2D profile section properties from the point-cloud data of a 3D scanned hydrofoil. This method used a rough orientation using the profile plotted in polar coordinates. The mean camber line could be extracted using the Voronoi vertex points and the offset of the profile surface. The resulting data set was fitted using non-periodic cubic B-spline curves. This produced a curve for the mean camber line and the thickness profile. Comparison of profile properties resulted in the conclusion that profile thickness (yt) could show that the two female mould halves were probably spaced differently for the different hydrofoils by a standard deviation of 0.0669 mm at a thickness of 22.6 mm. Differences are found in the maximum thickness location (xt), Leading edge radius (rle), maximum camber (yc) and its location (xc). However, the differences in lift over drag for the different hydrofoils are minimal. The size of the drag bucket did not differ more than 0.1 of the lift coefficient (cl) for the same drag coefficient. With the exception of in general one section that presented an early separation. The specific profile showing early separation changes along the span, not allowing for the generalisation for the complete hydrofoil. It shows that the curvature at the leading edge of the profile is critical for the performance of the Nacra hydrofoil. Knowing which part of the profile shape is critical. Companies can take this area into account when designing the production process for the next hydrofoil, and the Olympic team can focus on maintaining these areas of the hydrofoil. Allowing the best preparation to be competitive at the next Olympic games.

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.

Hall spars, a leading innovator in the composite mast-building industry with a long history of successful projects, provided a challenge which is the inspiration of this thesis. This thesis aims to contribute to the challenge: Joining techniques for the internal bonding of carbon fibre-reinforced polymer components into carbon fibre-reinforced polymer structures. ”Internal bonding” can also be called ”bonding.

Currently, the epoxy adhesive film is used for these types of joints. Adhesive co-bonding raises challenges due to the limited access to the mast. Challenges concerning surface preparation, low mechanical strength, reliability and labour intensity. Other techniques will be reviewed for the internally bonding limited access hollow tubes. The aim is that no new tooling is required, and the currently used materials and process parameters can be used. The thermoset resin used is an epoxy carbon fibre prepreg.

According to the literature study, potential joining techniques for this application are joining with partially cured thermosets and joining with fusion welding of thermoset composites. Differential scanning calorimetry measurements are performed to analyse the thermoset resin. Material characterisation is required to apply the joining methods with the currently used material and process parameters of Hall Spars. Process parameters for the curing model, and glass transition temperature model are derived. Using this material model allows creating the insight that partial cured joining with the thermoset resin of Hall Spars is not feasible. The structural integrity of the joined parts can not be guaranteed in the curing cycle.

Polyetherimide is used as thermoplastic material to apply the fusion welding technique on the thermoset. This thermoplastic film is co-cured with the thermoset and used as a coupling layer to join the two thermoset parts. Performing an interphase analysis with the scanning electron microscope indicates interphase formation at the PEI/epoxy interface. The higher the isothermal curing temperature, the thicker this interphase. Gelation causes the interphase formation to slow down. The interphase formation at the interface is not been observed as expected based on the literature review. This required a test if the interphase formation would have occurred at a smaller scale than the scanning electron microscopy could observe. This can be achieved by performing single-lap shear experiments. These joints are processed according to Hall Spars process parameters. Joints created with the fusion coupling technique with the PEI film show promising results. However, due to high void content, a lower ultimate lap shear strength is observed. Therefore it is advised to investigate a curing cycle with a second dwell phase. This changes the viscosity profile of the resin. Therefore allows the trapped air at the PEI/epoxy interface to leave the joint. This could potentially lead to more reliable joints.

Composite materials offer superior mechanical performance with lower weight than traditional materials. As a result, they are widely used in various industries, such as aerospace, automotive, construction, and sports. The composite industry is increasingly utilizing natural fibers and developing biodegradable composites in response to environmental sustainability concerns. Natural fibers have comparable physical and mechanical properties to glass fiber, making them suitable for use in the production of bio-composites. Despite this, natural fiber cannot fully replace glass fiber due to a variety of factors that influence the material's variable properties such as the type of fiber used, the conditions in which the fiber grows, the processing methods, and any modification of the fiber. Hence most of the research done on bio-composites has concentrated on using them in non-structural parts.

This project aims to develop and characterize a green and sustainable bio-composite system, to overcome existing application challenges. The constituent materials include a bio-resin extracted from seaweeds and Unidirectional Flax fibers as reinforcements to stiffen and strengthen for semi-structural applications. Although alginates have been studied in-depth for their biomedical applications, their potential as a bio-based matrix for biocomposites has not yet been explored.

In addition to the materials used in the composite preparation, the processing technique significantly impacts the final properties of the composite. Therefore, in the first half of the work, thorough research was conducted to study the constituent properties for their processing. The second half of the work defined a new approach to manufacturing bio-composites from natural constituents. The corresponding analysis useful for product design are thoroughly demonstrated in this thesis.

However, using natural fibers in composites and water alginate soluble matrices has three main concerns: the fiber/matrix interaction and their sensibility to moisture absorption and residual water within the composite structure. Their surfaces can be modified using physical and/or chemical methods to improve the bonding between fibers and matrix. In most studies cited in the literature, the chemical modifications employed are synthetic and toxic. It would be ideal if the chemicals used to modify natural fibers were bio-based and preserved the biodegradable nature of natural fibers.

The development of an optimized seven-step manufacturing approach is a key innovation for water removal, to enhancing fiber reinforcement and boosting overall composite performance, particularly in the use of flax fibers. This approach is notable for its novelty and challenge, as evidenced by the limited literature on the subject. It focuses on achieving sufficient impregnation by utilizing low water percentages in resin, specifically around 7\%, and applying appropriate consolidation pressure and temperatures. The most effective results were observed at 5 MPa and 95°C, which facilitated homogeneous plasticization and mouldability. Additionally, the method of wet/dry cycling, incorporating pre-soaking and heating, has proven beneficial in providing dimensional stability to flax fibers and limiting water absorption, further contributing to the technique's effectiveness.

Analytical techniques like microscopy and SEM reveal promising compatibility between components. No degradation of the bio-resin or the fibers relatable to the heat press was identified.

Finally, the research aimed to establish a meaningful relationship between the critical process variables and the properties of the bio-composite, with the ultimate goal of optimizing the production process and enhancing the quality of the final product by varying the fiber volume fraction within the range of 41-47\%.

The maximum tensile and flexural strengths achieved were 219MPa and 56Mpa, respectively. The elastic tensile and bending moduli in the composites were approximately 6.64GPa and 1.83GPa, respectively.

However, the observed properties fell below the predicted values for the biocomposite system, which were calculated using the rule-of-mixtures and Halpin-Tsai methods. Nonetheless, the experimental data confirm that these biocomposites can be used as secondary structural elements. The observed discrepancy was due to the presence of huge voids further leading to poor adhesion.

Carbon fibre reinforced plastics offer various options to improve structures made from conventional materials such as metal. Carbon fibre reinforced plastics are used increasingly in various sectors such as the aerospace, automotive or sports industry. As carbon fibre reinforced plastics are less damage tolerant as metals due to the brittle epoxy resin matrix, this project dealt with improving the damage tolerance of the CFRP-steel hybrid structures. An extensive literature review highlights the possible toughening mechanisms of epoxy resin matrices by using additives made from polymeric and inorganic materials. Furthermore, the manufacturing process resin transfer moulding, but also potential test setups were researched. Apart from toughening the matrix, the interface between the carbon fibre body and a thin steel sheet attached on top of the specimen was researched as well. For that, different interface materials such as short fibre veils, weaves and polymeric materials were reviewed. Based on the literature review a hypothesis was defined. Increasing additive concentrations, which were core-shell rubber particles and silica nanoparticles in particular, should yield an increasing impact performance. However, the influence of the manufacturing process and its limitations had to be considered as well. Furthermore, the influence of different interface materials can improve the bond strength of metal-CFRP hybrid structures. All test specimens were produced with resin transfer moulding. The matrix toughening was researched by doing an impact testing series, which was analysed with microscopy, ultrasonic testing and differential scanning calorimetry. A variety of different additive combinations were included in impact specimens. An air pressured impact cannon was used to impact the specimens. For the interface toughening, a mandrel peel test setup was developed to compare the required peel forces for different interface materials.