The strength of carbon fibres in unidirectional hybrid composites of glass and carbon fibres have been investigated. It has been shown that the strength distributions of bundles of carbon fibres impregnated with resin, both unsupported and in a hybrid, may be described by the Weibull model confirming earlier reports. The primary objective of this work has been to investigate the strength and the state of dispersion of the carbon component. Hybrid composites consisting of accurately aligned arrays of bundles in two and three dimensions have been fabricated and tested. It has been shown that the strength decreases when the bundle spacing is less than a critical distance but at very low spacings the strength begins to increase again. This observation is explained with reference to the fracture behaviour and the implications for practical composites are discussed. The hybrid composites exhibit a greater strength over impregnated bundles. An hypothesis is proposed to explain this phenomena which combines thermal effects with the constraining influence of the glass and the differences in the severity of flaws in hybrid bundles. The last two arguments result in a larger critical group of failed fibres being required in the hybrid before catastrophic failure occurs. The type of hybrid specimen tested in this work enables the fracture process in the composite to be followed closely. The investigation has in principle supported the model for composite strength proposed by Batdorf, the "critical i-plet" model. However, experimental evidence indicated that a slightly different fracture process to that proposed by Batdorf was operating. The significance of this fracture process with respect to the strength and the size effect in composites is discussed.

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The production of cellulose derived hybrid carbon nanofibre (CNF)/carbon nanotubes (CNTs) electrodes for the fabrication of supercapacitors and carbon fibre composites was investigated. The CNTs were grown via a floating catalyst chemical vapor deposition (CVD) method on the top surface of electrospun cellulose derived CNFs. These CNF and CNF/CNTs samples were then used as electrodes to produce liquid electrolyte-based supercapacitors. The growth of CNTs leads to an improvement of electrochemical performance compared to the plain CNFs. This improvement is due to the grown CNTs enlarging the reactive sites through enhanced surface area and porosity, and thereby increasing the conductivity of the system. CNTs have been also grown onto CNFs containing ferrocene and SiC particles. Composites were fabricated by combining the fibres and CNTs grown fibres with model polymers. The stress transfer properties of these materials have been estimated using an in situ Raman spectroscopic method by observing the shift of the Raman band during the tensile deformation of model polymer composites. Using this method, the elastic modulus of CNF/SiC/CNTs fibres has been estimated to be 208 ± 26 GPa. No shifts in the peak positions of bands relating to the carbon structure were obtained for in situ Raman spectroscopic studies of the CNF/CNTs fibres made from the ferrocene embedded fibres. This was thought to be due to the low yield of CNTs on the surface of the fibres. Furthermore, CNF/CNTs electrode-based structural supercapacitors, combining a solid electrolyte with the carbonized fibres, have been produced. These CNF/CNTs electrodes have a better capacitive performance than the plain CNF electrodes. There was a decrease in this performance with increased curing time of the resin, from 2 to 24 h, due to a lack of charge carrier mobility in the latter samples. A Raman spectroscopic study of the deformation of the carbon structures showed that the G-band shift towards a lower wavenumber position for the CNF and CNF/CNTs samples processed at a carbonization temperature of 2000 °C. Moduli of these fibres were estimated to be ~145 GPa and ~271 GPa, respectively, suggesting the growth of CNTs not only enhances the capacitive performance but also the mechanical properties of the structural supercapacitors. No Raman bend shift was found for the CNFs and CNF/CNTs samples processed below a carbonization temperature of 2000 °C, e.g. 900 °C and 1500 °C. This is because the graphitic structures are not well developed at carbonization temperatures below 1500 °C.

Carbon fibre reinforced plastics (CFRP's) are finding increased used as structural materials in many transport applications, particularly next generation commercial aircraft. The impact damage tolerance of these materials is relatively poor compared to conventional aircraft materials such as aluminium. As a result there is a concerted research effort to improve the damage tolerance of these materials. Understanding the microstructural mechanisms of damage can help to design improved materials. Three-dimensional X-Ray computed tomography (CT) allows these damage mechanisms to be identified and quantified non-destructively. However, a lack of published work in the field means no consistent methodologies for imaging or quantifying damage in CFRP's using X-Ray tomography exist. This thesis provides several novel methodologies for imaging and quantifying impact damage using X-Ray CT. A dual energy imaging methodology was developed to overcome the reduction in CT image quality caused by the high aspect ratio of CFRP structures. This approach resulted in a 66% increase in signal-to-noise ratio, and a 109% increase in contrast-to-noise ratio. The development of a methodology for quantifying impact damage in CFRP based on thresholding the in-plane damage area showed good agreement with ultrasonic C-scan results, and allowed correlations between impact energy, damage area and compression-after-strength to be made. Region of interest (ROI) algorithms for high magnification imaging of impact damage in CFRP plates were investigated. These algorithms were not developed by the author, but further understanding of their effectiveness and practical applications is presented in this work. Finally, a novel X-Ray tomographic imaging technique using interferometry was applied to imaging impact damage in CFRP's. This method was developed by a research group in Switzerland at the \emph{Centre Suisse d'Electronique et de Microtechnique} (CSEM) in Zurich. The work in this thesis presents the first application of the technique to image impact damage in CFRP.

This thesis investigates the behaviour and failure of simple aerospace type carbon fibre composite structures. The work focused on Acoustic Emission (AE) wave propagation in composite materials, the use of advanced AE techniques to detect, characterise and locate damage and their application to the monitoring of buckling and impact failure in large scale structures. The novelty in the work is highlighted below:

This thesis is concentrated on the long-term fracture of thick unidirectional glass and carbon fibre composites subjected to transverse stress. The objective was to develop a methodology for predicting the long term lifetime of a composite rotor used as part of a flywheel based energy storage system. The flywheel design is based on accommodating high hoop stresses induced during the high speed rotation. However, the different Poisson's ratios of the constituent materials in the rotor result in a complex stress distribution with significant stresses introduced in a direction transverse to the fibres. The possibility has been raised that the lifetime of the rotor will be limited by crack growth in this transverse direction, originating from defects (pores, cracks etc) that can be introduced into the rotor during its manufacture. The approach explored in this work has been to adopt a fracture mechanics based methodology whereby the rate of crack growth in a thick composite is measured as a function of an applied stress intensity. The basic fracture parameters for the material were measured such that the time taken for a crack to grow to a size sufficient to cause failure under an operating stress could be calculated. The materials were also examined to characterise the nature, size and extent of inherent defects. The stress distribution in the rotor under operating conditions was modelled using finite element analysis. The combination of information on inherent defects, stress directions and crack growth rates enable predictions to be made concerning the likely lifetime of the composites. Proof stress diagrams were also constructed in order to demonstrate an approach to product quality assurance testing. The end point of the work was to identify critical manufacturing defect sizes that could be tolerated under the specified operating conditions. The methodology developed for lifetime predictions was critically assessed and considered to be generally acceptable. The work did however raise some concerns regarding the applicability of a conventional fracture mechanics approach applied to heterogeneous composite systems where the size of the cracks are very small. It is recommended that future work should concentrate on studying this area with an emphasis on crack nucleation studies rather than on further crack propagation work.

The weight saving benefits of carbon fibre composites have been keenly adopted by civil aviation, with over 50% of the weight of modern designs coming from the carbon fibre components. The rapid rise in demand for this new material has led to the development of fully automated manufacturing techniques, improving rate of production and repeatability of manufacture. However, this rapid development, combined with a constant drive for increased rate of manufacture from industry can result in the formation of critical defects in the more complicated structural components. Manufacturing complex aeronautical structures from carbon fibre leads to a number of interesting mechanical problems. Forcing a multi-layered laminate to conform to a curved geometry requires individual layers to move relative to one another in order to relieve various forming-induced stresses. If the layers are constrained the dissipation of these stresses in the form of interply shear is prevented and a wide range of defects can occur, compromising the integrity of the final component. One of the most important of these is fibre wrinkling, which is effectively the buckling of one or more layers within an uncured laminate. This buckle results in a localised change in fibre orientation, which can result in a significant knockdown in part strength. A large amount of research has been conducted on carbon fibre in its cured state, when it exists as elastic fibres in an elastic matrix. Manufacturing occurs when the material is uncured however, with modern processes typically using fibres which are pre-impregnated with resin in order to reduce void content and aid fibre placement. A ply of uncured material therefore consists of stiff elastic fibres suspended in a very weak liquid viscoelastic material, whose properties are hugely influenced by temperature and rate of deformation. This thesis builds a better understanding of the mechanics involved in forming, using a series of characterisation techniques developed drawing from techniques in the literature. Part of the process involves the fitting of a one-dimensional viscoelasto-plastic model to experimental test data in order to represent the material response when shearing two plies about their interface. This model shows the material response to be dominated by the viscoelastic resin at low temperatures, before becoming frictional and fibre dominated at higher temperatures. In terms of optimum formability, a region exists in the transition from the viscous to frictional behaviour at which resistance to forming is minimised. With this data alone, optimum forming parameters such as rate of deformation, pressure and temperature can be suggested based on the material being used, along with design parameters such as stacking sequence. Another important characteristic which must be understood when considering ply wrinkling is the bending stiffness of uncured prepreg, both as a single ply and when combined to form a small laminate. A wrinkle is in effect the buckling of a single or small number of plies within a laminate, therefore by understanding the bending stiffness and process-induced loading we can begin to predict whether or not wrinkles are likely to occur for a particular manufacturing regime. In order to assess bending stiffness, a modified Dynamic Mechanical Analysis process is employed, replacing the standard Engineers Bending Theory calculations with a Timoshenko element to capture the large degree of intraply shear experienced in the bending of uncured prepreg. Finally, a small laminate scale demonstrator is considered in which a 24-ply laminate is consolidated into a female tool in such a way as to induced maximum shear strain between the plies, in order that the optimum forming parameters predicted by the characterisation tests might be validated. A simple energy minimisation model is used to predict the variation in consolidation strain around the part due to resistance to shear, using material parameters from the model describing the inter-ply shear test data. These parameters are also used to inform a novel modelling technique which has been developed parallel to this thesis, which is validated against the experimental results, and shows how the characterisation techniques can be used to advance simulation methods aimed at reducing the development time for new carbon fibre components. This work provides a set of tests and methodologies for the accurate characterisation of the behaviour of uncured carbon fibre during forming. The models developed alongside these tests allow for a detailed interrogation of the results, providing valuable insight into the mechanics behind the observed material behaviour and enabling informed decisions to be made regarding the forming process in order that the occurrence of defects might be minimised. The primary aim has been to provide a set of vital input parameters for novel, complex process modelling techniques under development, which has been achieved and validated experimentally.

This thesis contributes towards understanding of mechanical response of 3D composites and ceramics. Composite materials have widespread applications ranging from aerospace, civil sectors to sports and drones. One important application is in composite armours where composites and ceramic layers are used together. Therefore, it is important to study the mechanical response of these components to develop better armour systems. The first part of this thesis concerns with dynamic penetration response of confined ceramic targets. In the second part, mechanics of a novel 3D composite consisting of orthogonal carbon fibre tows is studied. The dynamic penetration of ceramic target by a long-rod projectile is studied using a mechanism based ceramic constitutive model. This is to capture and explain the essential physics observed during penetration of a ceramic target such as dwell and structural size effect. Dwell is captured using the constitutive model and the related physics is studied along with identification of causes of dwell. Origins of structural size effect in ceramics are identified and their influences are studied. In the second part of the thesis a novel 3D composite consisting of three mutually perpendicular orthogonal tows is studied under compression, indentation and three-point bending. Under compression along low fibre volume fraction direction (Z), the 3D composite forms stable and multiple kinks in the Z tows resulting in 10% ductility. This contrasts with traditional UD or 2D composites which fail catastrophically at 2% strain. The stability in the case of the 3D composite is due to the constraint imposed by the surrounding material. Under indentation, the 3D composite has a near isotropic and ductile response. In contrast, traditional cross-ply composites show highly anisotropic response where indentation results in brittle failure along in-plane direction. Under three-point bending, the response was ductile in Z-direction and brittle in other two directions. Overall, the 3D composite studied in this thesis shows improvement over traditional CFRPs in ductility and energy absorption capability. The 3D composite has been demonstrated to have smooth load-displacement curves reminiscent to indentation of metal in all three directions achieved at densities significantly lower than structural metals that display equivalent ductility. Thus, these 3D composites are strong candidates for applications where loading direction is unknown a-priori, and where high energy absorption is required along with reusability of the material.

The article at hand presents a novel approach to accelerating the early strength development of mineralimpregnated carbon-fibre composites (MCF) by electrical Joule heating. MCF were produced with a metakaolin-based geopolymer suspension and subsequently cured using Ohmic heating under systemically varied voltages and durations. The MCF produced were characterised in respect of their mechanical and morphological properties. Threepoint-bending and uniaxial tension tests yielded significant enhancement of MCF mechanical properties due to curing within only a few hours. Thermogravimetric analysis (TGA), mercury intrusion porosimetry (MIP), environmental scanning electron microscope (ESEM) as well as micro-computed tomography (μCT) confirmed advanced geopolymerisation by the electrical heating process and a strong sensitivity to parameter selection. After only two hours of resistance heating MCF could demonstrate tensile strength of up to 2800 MPa, showing the great potential for applying the Joule effect as a possibility to enhance the strength development of geopolymer-based MCF. Moreover, the applied method offers a huge potential to manufacture automated fast out-of-oven cured MCF with a variety of shapes and dimensions.

One major step needed for fusion to become a reliable energy source is the development of materials for the extreme conditions (high temperature, radioactivity and erosion) caused by hot plasmas. The main goal of the present study is to use and optimise ion beam methods (lateral resolution and sensitivity) to characterise the distribution of hydrogen isotopes that act as fuel. Materials from the test reactors JET (Joint European Torus), TEXTOR (Tokamak Experiment for Technology Oriented Research) and Tore Supra have been investigated. Deuterium, beryllium and carbon were measured by elastic recoil detection analysis (ERDA) and nuclear reaction analysis (NRA). To ensure high 3D spatial resolution a nuclear microbeam (spot size <10 µm) was used with 3He and 28Si beams. The release of hydrogen caused by the primary ion beam was monitored and accounted for. Large variations in surface (top 10 µm) deuterium concentrations in carbon fibre composites (CFC) from Tore Supra and TEXTOR was found, pointing out the importance of small pits and local fibre structure in understanding fuel retention. At deeper depths into the CFC limiter tiles from Tore Supra, deuterium rich bands were observed confirming the correlation between the internal material structure and fuel storage in the bulk. Sample cross sections from thick deposits on the JET divertor showed elemental distributions that were dominantly laminar although more complex structures also were observed. Depth profiles of this kind elucidate the plasma-wall interaction and material erosion/deposition processes in the reactor vessel. The information gained in this thesis will improve the knowledge of first wall material for the next generation fusion reactors, concerning the fuel retention and the lifetime of the plasma facing materials which is important for safety as well as economical reasons.