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Replacing a building’s façade offers the prospect of improving the whole life performance of the building, in some instances as a favourable alternative to replacing the entire structure. This presents the opportunity to exploit the properties of advanced composite materials for maximum benefit. ‘Upwards and outwards’ retrofit, where extending floor slabs yields extra floor area, is permitted by a lightweight replacement façade, without the need to underpin foundations. For typical medium or high-rise office buildings, the extra let-able space obtained, and reduced heating and maintenance costs, can work to offset the expense of implementation. The specific materials, manufacturing processes, and façade type, most appropriate for such a scheme have been investigated. A unitised façade of sandwich panels with foam cores and pultruded GFRP skins has formed the ‘design platform’ for research conducted. It is paramount to resolve how the connections in such a façade system can meet the many requirements of an integrated building envelope. Structural integrity, enhanced environmental control, sustainability attributes, fire provisions, acoustic control, ease of manufacture, tolerance control, durability, lightness in weight, cost effectiveness and aesthetics must all be addressed simultaneously by any proposed design methodology. Investigating suitable connections through prototype development and review reveals key issues requiring targeted research. The permanent action acting on light, selfsupporting GFRP panels is small, however wind and occupancy loading impart significant imposed actions. Therefore, whilst creep deflection is often a significant consideration for structural GFRP design, quantifying fatigue performance is a higher priority for validating the ideology of polymeric facades. The unidirectional nature of pultrusion reinforcement yields a scenario of principle stresses at the panel interfaces, occurring in the weaker, secondary fibre, direction. As a consequence a fatigue-testing programme is aimed at understanding the performance and characteristics of pultruded angles compatible with ‘long-edge’ panel connections. The long-term performance of fibre-reinforced polymer (FRP) structures must be assessed if FRP is to win acceptance as a mainstream material for use in the construction industry. The environmental durability of wholly polymeric structures is often called in to question. In response, accelerated testing is usually undertaken on artificially aged FRP specimens; lack of genuine naturally aged material has previously hindered research and validation of material related design life. Case study investigation has permitted a full durability appraisal of naturally aged GFRP through laboratory testing campaign. Retrofit of existing buildings as an activity makes up 50% of all building construction in the UK. This project aims to address the shortfall in industry-required design knowledge. The tensile strength of pultruded naturally aged GFRP has been shown to reduce by only 0.65% over 17 years where natural exposure does not include UV irradiation, and by 13.1% where UV irradiation does occur as one element of exposure. The findings expose the degree of inaccuracy and fundamental flaws in existing predictive ageing models. The physical mechanisms of degradation do not match. A procedure to quantify the extent of polymer brittle hardening has been developed and applied as an analytical tool. Mechanical testing campaign has pioneered the use of the RMS (Route Mean Square) procedure to present the performance of connection specimens as a continuous function throughout programmes of fatigue testing. Testing has shown that though a threshold strain for damage accumulation does exist in complex fatigue loading of connections, and for direct tension fatigue loading.

The topic of polymer nanocomposites remains an active area of research in the dielectrics community, due to the unique electrical properties that these materials could exhibit. To explain the behaviour of these materials, the importance of clarifying the interfaces between nanoparticles and polymer matrices has been emphasised. However,understanding of the interface in nanocomposites is unsatisfactory and, consequently,many experimental results remain unexplained. This thesis reports on an investigation into a polyethylene nanocomposite system that contains varying amounts of nanosilica that differ with respect to their surface chemistry. The addition of nanosilica, even with different surface chemistries, was found to enhance the nucleation density of polyethylene and perturb the spherulitic development. While less organised lamellar structures would be expected to lead to a lower breakdown strength, this does not appearto be the case for the material systems considered here under alternating current (AC) fields. In addition, nanosilica filled polyethylene was found to absorb significantly more water than unfilled polyethylene, with the consequence that both the permittivity and the loss tangent increase with increasing duration of water immersion. However, appropriate surface treatment of nanosilica reduces the water absorption effect and modifies the dielectric response of the nanocomposites compared with those containing an equivalent amount of untreated nanosilica. Although water absorption may not be a technologically desirable characteristic, the results indicate that water molecules can act as effective dielectric probes of interfacial factors. Meanwhile, the direct current (DC) breakdown strength reduces with the inclusion of increasing amount of nanosilica in the polyethylene, but surface treatment of nanosilica improves the DC breakdown strength with respect to equivalent nanocomposites containing untreated nanosilica. Results from space charge studies reveal increased space charge accumulation in the presence of the untreated nanosilica and, upon surface treatment of the nanosilica, the charge development was suppressed in comparison with nanocomposites containing an equivalent amount of untreated nanosilica. This observation suggests that space charge accumulation and DC failure are related in these systems and it would seem that control of surface chemistry is particularly critical in connection with the use of nanocomposites in DC applications. Finally, the mechanisms underpinning the concept of filler functionalisation in nanocomposites were investigated via the use of different aliphatic chain length silane coupling agents, and the results show that long silane chains enhance the DC breakdown strength of the resulting nanocomposites. The possible further enhancement in DC breakdown strength is also highlighted. Overall, this thesis demonstrates how a nanoparticle’s interface chemistry can affect both the structure and the electrical properties of the resulting nanocomposites, and serves as an important foundation towards the engineering of nanocomposites as the reliable electricalinsulation materials of the future, through the understanding of the interface.

This research seeks to evaluate the local stress and strain distributions formed in adhesively bonded composite joints under quasi static and high strain rate loading. A literature review of current analytical, numerical and experimental studies of adhesively bonded joints is presented and identifies the lack of knowledge in the behaviour of composite bonded joints in the through-thickness direction. Detailed analysis of the stress and strain in the joint, and their impact on the development of damage prior to and during failure have been obtained using Digital Image Correlation (DIC). An experimental methodology is established to perform DIC at the mesoscopic scale using high magnification optics, enabling accurate, high spatial resolution analysis of the strains around the geometric discontinuity between adherends. It is demonstrated that the small through-thickness strains are critical in the development of damage in the joint around the discontinuity between adherends. Errors in the DIC technique are assessed using a robust morphological methodology to evaluate the quality of different speckle patterns based upon the properties of the speckles in the pattern. The strain data is manipulated to evaluate the principal stresses in the joint, which govern failure in the brittle epoxy matrix of the composite, providing a concise evaluation of the transfer of load between the adherends and damage initiation within the joint. The DIC results and methodology are validated against independent infra-red measurements using Thermoelastic Stress Analysis (TSA). Limitations in the TSA analysis approach are identified around joint discontinuity due to the varying principal stress direction. A new TSA analysis methodology is presented to overcome this. The results of the experimental analysis are used to validate a representative 2D finite element model modelling approach for adhesively bonded joints, showing good agreement to the experimental data. Finally the full-field DIC methodology is applied to analyse the response of a single lap joint during high strain rate loading, providing unprecedented full-field measurement of the strain fields up to failure.

Discontinuous fibre composites are under increasing investigation for structural and semi-structural components as they are easily automated, making it possible to remove costly hand labour based steps typically associated with advanced fibre reinforced composites. Directed fibre preforming (DFP) is one possible process which has several advantages when compared with competing techniques. Low material and process costs coupled with short cycle times means the process is suited to medium volume production (typically <10,000 ppa). Predicting mechanical performance remains a major obstacle to industrial adoption however, due to the stochastic nature of fibre distribution. This is of particular importance for structural applications where minimum property requirements and a greater certainty of performance must be achieved. This thesis employs a stochastic macroscale modelling approach to predict fibre locations during the reinforcement deposition stage. This is achieved through process characterisation studying the effects of key microstructural and process-specific parameters on fibre distribution and orientation. The proposed DFP simulation software can generate realistic fibre networks for complex three-dimensional component geometries providing feedback on preform quality. This information is used to optimise the preform structure via process input parameters such as robot trajectory and material properties with validation tests conducted to assess model accuracy. An interface between the simulation software and commercial finite element code facilitates mechanical property analysis for full-scale components using realistic load cases. The complete software package is intended to streamline the route to manufacture for DFP processes from a conceptual design stage.

Discontinuous carbon fibre composites are becoming increasingly popular in the automotive and aerospace sectors, as an alternative to textile-based fibre reinforced composites for both semi-structural and structural components. Materials are highly heterogeneous, with the random architecture leading to uncertainties when modelling and predicting mechanical performance. The microscopic characteristics are known to dominate the strength of the composite, which need to be correctly represented to improve mechanical property predictions at the macroscale. This thesis presents a multi-scale modelling approach that captures the effects of microstructural (filament level) parameters at the macro scale (component level) to predict the mechanical properties of discontinuous composite materials. In the present work, a continuum damage approach has been used to initiate and monitor failure in the models at all scales, via a user defined material (UMAT), allowing strength predictions to be made for the discontinuous material within the ABAQUS solver. Experimental testing of the material constituents (fibre bundle and matrix materials) has been performed to provide input data for the finite element analyses. Micromechanical models have been developed to calculate the properties of fibre bundles, which are used directly at the meso and macroscale. Debonding criterion has also been established and validated which has been used to demonstrate that a small interface, with a thickness of only 1% of the fibre radius, can strongly influence the stress transfer between fibre and matrix materials. Interactions between multiple fibre bundles have been considered at the mesoscale, at a range of bundle orientations and separation distances. As the separation distance between the fibre bundles decreased there was an increase in stiffness 0 f the unit cell (~1.9%) across the bundle orientations considered, however, this also coincided with greater stress concentrations (up to 9.6%) being found in the bundles aligned to the direction of loading. These stress concentrations have been used to produce a comprehensive stiffness reduction scheme at the macro scale to account for the 3D nature of the bundle interactions. A 2D macro scale model is presented for generating discontinuous random fibre architectures consisting of high filament count bundles, with interfacial debonding permitted between the bundle and matrix materials. The fibre bundles are deposited randomly in a 2D plane to provide a representative material. The model has shown that the interface between the bundle and matrix material is critical at short fibre bundle lengths (~5mm) when determining the mechanical properties of the material, with reductions in strength of up to 40% observed at low interfacial shear strengths. The results from the macro scale analysis for discontinuous materials provide predictions within ~10% for tensile stiffness and ~18% for tensile strength when compared with experimental validations.

Plant fibres, perceived as environmentally sustainable substitutes to E-glass, are increasingly being employed as reinforcements in polymer matrix composites. However, despite the promising technical properties of cellulose-based fibres and the historic use of plant fibre composites (PFRPs) in load-bearing components, the industrial uptake of PFRPs in structural applications has been limited. In developing PFRPs whose mechanical properties are well-characterised, optimised and well-predicted, this thesis addresses the question: Can PFRPs replace E-glass composites (GFRPs) in structural applications? Ensuring that the highest reinforcement potential is exploited, this research examines the mechanical properties of aligned PFRPs based on bast fibre yarns/rovings and thermoset matrices. Although aligned GFRPs are found to outperform aligned PFRPs in terms of absolute mechanical properties, PFRPs reinforced with flax rovings exhibit exceptional properties, with a back-calculated fibre tensile modulus of up to 75 GPa and fibre tensile strength of about 800 MPa. To identify the processing window which produces composites with useful properties, the minimum, critical and maximum fibre volume fraction of PFRPs have been determined, and compared to that of synthetic fibre reinforced composites. The effect of fibre volume fraction on the physical and tensile properties of aligned PFRPs has also been investigated. Furthermore, micro-mechanical models have been developed and experimentally validated, to reliably predict the effect of (mis)orientation, in the forms of yarn twist/construction and off-axis loading, on the tensile properties of aligned PFRPs. To provide a complete set of fatigue data on aligned PFRPs, the effect of various composite parameters on PFRP cyclic-loading behaviour has been illustrated through S-N lifetime diagrams. A constant-life diagram has also been generated to enable the fatigue design and life prediction of a PFRP component. At each stage, the fatigue performance of PFRPs has been compared to that of GFRPs. Finally, in directly addressing the main theme, this thesis adopts a novel comparative case study approach to investigate the manufacture and mechanical testing of full-scale 3.5-meter composite rotor blades (suitable for 11 kW turbines) built from flax/polyester and E-glass/polyester. The study claims that under current market conditions, optimised plant fibre reinforcements are a structural, but not low-cost or sustainable, alternative to conventional E-glass reinforcements.

This work addresses the characterisation of the micro-structure of fibre bundles in reinforcement textiles for composites and its influence on resin flow in liquid composite moulding (LCM) processes. Random variations in local filament spacing result in a non-uniform flow velocity field. Merging flow fronts due to differences in local flow velocities can lead to gas entrapment in the matrix phase. This results in formation of micro-scale defects, which can significantly reduce the matrix-dominated mechanical properties of the finished composite. An automated image analysis method was developed to precisely determine local filament distributions within complete fibre bundles. Based on two-dimensional micrographs, filament arrangements were characterised statistically by means of nearest neighbour distance and angle distributions. It was observed that the micro-structure becomes more uniform with increasing level of compaction. A micro-structure generator to reconstruct filament arrangements was adapted incorporating these measurement data. Transverse permeabilities derived from numerical simulations of steady-state flow on automatically discretised model domains were found to be log-normally distributed. With increasing model size, average values and widths of the distributions decrease, converging to the permeability of a complete fibre bundle. Similarly, average values and scatter decrease with increasing fibre volume fraction. The transverse permeability of random filament arrangements was found to be significantly smaller than for uniform filament arrangements. The void content in composite specimens produced by resin injection along and perpendicular to the fibre bundles was characterised with a developed image analysis process. Due to the more uniform micro-structure at increased bundle compaction, a decrease in void content was observed in the case of transverse resin injection. A first step towards void content prediction in a fibre bundle by numerical simulation of transient flow through a randomised filament arrangement was made. It was hypothesised that steady-state flow results may be used for the prediction of transient flow.

The excellent specific stiffness and strength of carbon fibre reinforced polymer composites means that the automotive sector has been investigating methods of implementing these materials into structurally demanding applications. The work detailed within this thesis supports ongoing research at the University of Nottingham into the automated manufacture of discontinuous carbon fibre reinforced polymer composite materials. Advances in the automation of composites manufacturing has meant that methods to effectively join these materials is required. This work provides a fundamental understanding of the differences that result from the adhesive bonding of a discontinuous fibre composite (DFC) compared to conventional fibre reinforced composite materials. The main objective of the project was to characterise the behaviour of adhesively bonded DFC adherends. Using a single lap shear joint geometry, an optimised fibre architecture and joint geometry was identified with a 2-part low temperature curing epoxy adhesive being characterised for industrial application. To further improve the performance of the DFC substrates, a fibre alignment technique was implemented that achieved properties comparable to those of more traditional non-crimp fabric composites. From the experimental investigations conducted, the use of discontinuous carbon fibre reinforced composites in bonded assemblies shows promise with the potential for use in structural applications.

Owing to the increasing global population and more widespread industrialisation (particularIy with respect to the developing world's aspirations to equality of standard of living), the extraction and use of materials is expanding. This ever-increasing demand for materials has meant that certain resources are under pressure. Polymer matrix composites, which are normally derived from petrochemicals with synthetic fibres requiring high amounts of energy during manufacture, are increasingly used for both structural and non-structural applications. The development of composites using bio-derived fibres or resins (or both) is an important area of research. This study considers the mechanical propelties of various bio-derived fibres, laminated with a range of standard (commercially available) and modified (tailored or with bio-derived content) thelIDosetting resins, in comparison with synthetic equivalents. The sta11ing point for the current work was to manufacture composites using commonly available natural fibres (hemp, in both chopped strand mat and unidirectional form) with commercial resin systems, primarily to investigate a potential marketing opportunity. This work has shown that it is possible to manufacture such composites, but that their mechanical properties are not particularly useful: in some cases they are in fact worse than the bulk resin. In part this is due to the compatibility of the resin with the fibres. This has been addressed to some extent by the optimisation of the composite with a coupling agent to the formulation. Of more interest however has been the identification of a processed-cellulose fibre, available in a continuous form. Composites made from this have interesting prope11ies and, due to the comparatively low density of the fibres, the specific mechanical propelties of the composites are comparable with those of glass based systems. In addition, this study has also conducted an environmental LCA of thermosetting resin manufacture; the conclusions of this have informed the corporate sponsor (Scott BadeI') of strategies for reducing the environmental impact of resin manufacture.

The performance of components, structures and vehicles are always in some way limited by the materials they are made from , often leading to compromises. This drives development to invent and discover new materials or processes to improve upon the current state of the art. In this respect the research contained in this thesis has investigated the use of plasma treatment~ as a method for modifying the properties of carbon nanotubes and carbon fibres to improve upon current composite materials technology. The initial study focused on functionalising relatively large quantities of carbon nanotubes using a unique and scalable technique suitable for industrialisation. The research began with a standard oxygen and ammonia treatment, which led to the development of an oxygen plasma treatment for increased carboxyl functionality. The important discoveries were that processing time and gas pressure had a large impact upon the agglomerate size, bulk density, surface energy, solvent stability and the quantity of carboxyl functionalisation. The treated carbon nanotube~ developed in the initial study were investigated for their use within an epoxy system. The treated carbon nanotubes were shown to disperse better, reduced resin viscosity, increase resistivity, but had little effect upon mechanical properties, degree of cure or glass transition temperature compared to the untreated carbon nanotubes. The development of the oxygen treated carbon nanotubes led the research to investigate if these treated carbon nanotubes could be used to improve the fracture toughness of a pre-preg system. The results showed that it was possible to improve initiation and propagation mode I fracture toughness significantly. Mode II results also showed increased initiation but relatively unchanged propagation toughness at lower areal densities, however in both mode I and II at the highest carbon nanotube coating density the fracture propagation resistance was reduced. The final study looked into the use of plasma treating carbon fibres as a method to modify the fibre matrix interface. The interface strength was found to improve for short oxygen and ammonia treatments, while reduce for the tetrafiuoromethane treatment on the unsized fibres . However for the commercially sized fibres each plasma treatment appeared to damage the propriety treatment in terms of interface strength. A further investigation into attaching carbon nanotubes to carbon fibres showed a dramatically reduced interface strength. The research demonstrates a variety of methods which could be used to tailor the interface for improved strength or damage tolerance.

As composite materials are now used in load conditions with increasing complexity and thickness, all the three-dimensional stress components become important and should be taken into account when predicting failures. In particular, the through-thickness stresses can play a crucial role in determining the in-plane behaviours and strength of a composite, laminate. The work presented in this PhD thesis aims to investigate failures due to complex stress fields at the root of a composite component in a dovetail assembly, where highly concentrated through-thickness stresses as well as in-plane tensile and interlaminar shear stresses are present. The problem was decoupled into two simpler multiaxial load cases which were studied separately: (1) through-thickness compression with interlaminar shear, and (2) through-thickness compression with longitudinal tension. They were investigated experimentally using new loading configuration in a biaxial test machine. This bridges the gap in reliable multiaxial experimental data which is lacking in the open literature. This was then combined with a finite element (FE) modelling approach to, develop simple failure criteria which are validated for engineering design purposes. A simple constitutive law which takes into account the effect of transverse compression and analytical tools which can be I easily utilised to predict stresses and failures in composites were also developed. The findings of this thesis were finally applied to a severely tapered dovetail composite specimen, together with some mitigation strategies, to predict its ultimate fibre failure load and the failure locations.

The recent resurgence and growing interest in high temperature structures to maximise design space and performance in motorsport applications, led to the need for greater understanding of high temperature composite materials. This study aimed to investigate suitable materials for high temperature structures with application to the motorsport environment. Composite materials were quickly identified as the most appropriate materials given the needs of motorsport, with polysialate composites championed after a thorough review of available materials. A commercially available composite reinforced with silicon carbide fibres was selected to meet the study requirements. This led to the need for thermal and mechanical characterisation in order to provide temperature dependant data suitable for accurate design of high temperature structures. The result of .this was a full engineering dataset for the most widely used polysialate composites, filling In significant gaps in the literature, whilst simultaneously producing a novel and unique high temperature tensile testing rig for this composite laminates. In turn, this new data was exploited with application to a polysialate composite case study structure; an exhaust liner used on current Formula One vehicles. This required full understanding of the thermal and mechanical load cases experienced by the liner, in order to accurately simulate the liner and environment using finite element analysis. Not only did this provide an application of the newly acquired temperature dependant material properties, but it also highlighted the need for temperature dependant properties in the design of high temperature structures. The study, its aims and approach, were validated through comparison of simulation failure predictions against component failed in service.

Defects in composite materials lead to reductions in the mechanical performance of components. The presence of fibre waviness and through-thickness ply wrinkles has been shown to reduce the Compressive strength of laminates, and are therefore subject to significant scrutiny in the aerospace industry. Whilst a large volume of industrial effort and published research has addressed the reduction of the compressive strength Due to defects, the source of such defects is currently not well understood. The work presented in this thesis aims to elucidate the source of fibre waviness and ply wrinkles. Its ultimate goal is to understand the link between material and manufacturing process variations with the formation of such defects. Once mechanisms for defect generation are understood, measures can be taken by industrial manufacturers to mitigate their formation. As a result, part acceptance rates will increase; the composites industry depends on the reliability of component manufacture. A number of themes have been investigated by proposing specific mechanisms for the formation of fibre waviness and Ply wrinkles. Such themes include fibre-matrix interaction, tool part interaction and consideration. The most significant mechanism Has been shown to be consolidation. In both prepreg and woven preform materials, poor layup has been shown to lead to severe Ply bridging at tooling radii.

The overall objective of this thesis was to develop a methodology for predicting the high strain rate properties of macro scale polymer composite materials through micro and meso scale finite element modelling. The focus in this study was on multi-layered multi-material polymer composite laminates consisting of 0/90 and ± 45 layers of either; S2 Glass/Epoxy or Carbon/Epoxy non crimp fabric. A multi-scale finite element modelling methodology was developed at the micro and meso scale for predicting the elastic response, damage initiation, damage evolution and strain rate dependence of macro scale polymer composite materials. The methodology was implemented in the Abaqus finite element package utilising Python scripting for simulation definition and Fortran code for user defined explicit material subroutines. Micro scale unit cells were tested in longitudinal, transverse and shear loading directions over a range of strain rates. The results of the micro scale testing were converted into material properties of the meso scale yarns / tows. Meso scale unit cells were tested in in-plane, through thickness and shear loading directions over of a range of strain rates. The results of the meso scale testing were converted into material properties of macro scale polymer composite materials. The modelling methodology was validated against experimental testing conducted on meso scale samples over a range of strain rates using an electromechanical universal static test machine and an instrumented falling weight drop tower. The contribution to knowledge from this study is the development of a validated modelling methodology for predicting the elastic, damage and strain rate dependent response of polymer composite materials at a micro and meso scale.

Glass matrix composites based on NextelTM alumina fibre reinforced borosilicate glass have been fabricated to improve their mechanical property and fracture toughness. In this work, a novel processing technique, which is called “sandwich” hot-pressing, has been used. It consists of arranging the reinforcing fibres in two directions with a periodic interspacing between glass slides, and submitting the material to a heat-treatment for consolidation into highly dense and transparent composites, which were proved by XRD analysis and SEM observations. These composites’ mechanical, optical and microstructural properties were studied and compared to those of the unidirectional fibre reinforced borosilicate glass composite and unreinforced glass matrix produced under the same conditions. Furthermore, a hybrid sol-gel technique has been employed for coating the fibres with a smooth and crack free ZrO2 interfacial layer to provide a weak bonding at the fibre/matrix interface to promote fibre pull-out during fracture. ZrO2 coated and uncoated fibre-reinforced borosilicate glass matrix composites were fabricated, with different sizes of optical windows including 4x4, 5x5 and 6x6 cm2. Moreover, a geometry based equation was derived to evaluate the expected light transmittance of the composites. These multi-directional fibre reinforced glass matrix composites retained at least 50% of the light transmittance and higher flexural strength compared with the unreinforced glass matrix. The highest measured flexural strength value of these composites was 56 ± 7 MPa. The composites reinforced by ZrO2 coated fibres had higher flexural strength (approx. 36%) and lower standard deviation (approx. 47%) compared with those reinforced by uncoated fibres. The introduction of a ZrO2 interfacial layer was to improve the mechanical properties and to retain the composites’ integrity, which was proved by the observations of fibre pull-out and crack deflection upon failure during mechanical tests. To investigate the microstructure of the interfacial layer in the composite, SEM, FIB-SIMS and TEM were employed. The present composites show potential for applications in architecture and special machinery requiring strong transparent windows.

The demand for oil in the world is expected to rise by 1.7% in the fourth quarter of 2012 compared to fourth quarter of 2011. In order to cater for this increasing demand, the oil and gas industry continues to explore and develop deep-sea oilfields where oil and gas risers and pipelines encounter extreme conditions. The combination of high pressure and temperature with aggressive media which contains of hydrocarbon, alkanes, acids, sour gas (H2S), and CO2, etc., requires superior material performance and durability. Conventional engineering materials, such as steel are heavy and require corrosion protection, which are currently used as risers, flowlines and choke and kill lines have reached their limits. This is because of the poor chemical resistance and damage tolerance and the high costs involved in supporting their own weight. This has motivated the industry to explore non-corroding and lighter alternative materials if deeper sea reservoirs are to be explored. One such material that has the potential to overcome such limitations thus enabling new design strategies for cost effective, weight and energy saving materials is fibre reinforced composites. The remarkable properties and the tailorability of fibre reinforcement along load paths to achieve excellent performance of the composites is an attribute not found in any other material. The aim of this research was to manufacture novel carbon fibre reinforced polyvinylidene fluoride (PVDF) composites by incorporating atmospheric plasma fluorination of the carbon fibres. Powder impregnation method was adapted for the manufacturing of continuous unidirectional (UD) carbon fibre reinforced PVDF composite prepregs. The resulting composite laminates were characterised through various macro-mechanical tests. The impact of atmospheric plasma fluorination of the carbon fibre on the tensile, flexural, short beam shear and tearing properties of the UD composites were investigated to determine whether the improvements observed in the single fibre model composite can be translated to macro-level composite laminates. Apart from this, the impact of combining both fibre and matrix modifications on the composite were studied and the preliminary results on micro-mechanical scale are presented. Finally, composite pipe structures, made by filament winding technique using unidirectional carbon fibre reinforced PVDF composite prepregs onto a pure PVDF liner were fabricated, and characterised with respect to its mechanical properties.

The work presented within this thesis concerns measurement of the fracture toughness associated with the translaminar, fibre-breaking, failure modes of composites. Loading cases of mode I tension, mode I compression and mixed mode I/II tension and shear are considered. Fracture toughness measurement for translaminar tensile failure is investigated using the compact tension specimen. A detailed analysis of data reduction schemes concludes that a modified compliance calibration technique is the most appropriate, in terms of reproducibility of results and simplicity. Investigation of specimen in-plane size, thickness and lay-up effects indicates a thickness dependence. Specimen fracture surfaces reveal that the increase in measured toughness for specimens with thicker 0° plies is due to an increased amount of fibre pull-out. Furthermore, it is found that true measurements of fracture toughness are obtained from specimens with initial notch radii less than 250 μm, for the IM7/8552 material system. Fractography reveals that the critical notch root radius is dictated by the 0° plies alone. A four-point bend specimen is used to measure the fracture toughness associated with translaminar compressive failure. This method enables the failure mode to be triggered in isolation, making it superior to other methods in the literature. Microscopy of failed specimens reveals a shear driven fibre failure at 45° to the initial notch; the cause of this failure mode is investigated using a micromechanical finite element model. Mixed-mode translaminar tensile/shear fracture is investigated using a modified compact tension configuration and a specially developed fixture. SEM of the fracture surfaces of failed specimens reveals damage mechanisms specifically caused by the introduction of a mode II component of loading which accompany a significant increase in the damage zone size. R-curves are generated which suggest that the specimen configuration used here is appropriate for characterising fracture toughness at low proportions of mode II.

This thesis describes the development of stochastic analysis frameworks for use in engineering design and optimisation. The research focuses on fibre-reinforced composites, with the stochastic analyses of an existing analytical failure model for unidirectional composites and of a unit cell numerical model of a 2D 5-Harness satin weave. Stochastic failure envelopes are generated through parallelised Monte Carlo Simulation of deterministic, analytical, physically based failure criteria for unidirectional carbon fibre/epoxy matrix composite plies. Monte Carlo integration of global variance-based Sobol sensitivity indices is performed and utilised to decompose observed variance within stochastic failure envelopes into contributions from physical input parameters. It is observed how the interaction effect can be used to identify domains of bi-modal failure, within which the predicted failure probability is governed by multiple failure modes. A reduced unit cell (rUC) model of a 5-Harness satin weave is constructed and analysed deterministically in uniaxial and biaxial loading conditions. An algorithm is developed and implemented to fully automate the rUC construction such that stochastic variations of the crimp angle can be evaluated. Monte Carlo Simulation is employed to propagate the effect of the crimp angle through the deterministic model and the probabilistic response compared with data obtained experimentally. It is observed how simulated variability compares well in uni-axial compression, but under-predicts observed experimental variability in uni-axial tension. The influence of vertical stacking sequence of plies is also demonstrated through the study of in-phase and out-of-phase periodic boundary conditions. The research highlights various, potential advantages that stochastic methodologies offer over the traditional deterministic approach, making a case for their application in engineering design and providing a springboard for further research come the day when greater computational power is available.

Failure predictions for a fibre-reinforced composite with unidirectional (UD) plies can only be relied upon provided the stress state is accurately known. This requires a prediction of the constitutive response to be made when the material is loaded. When failure does occur, matrix cracking is frequently the first mode of failure. Cracking results in a reduction of the material properties of the structure and can lead to other forms of damage. In this context, an elasto-plastic constitutive model that can accurately represent the full non-linear mechanical response of UD composites is developed, as well as the implementation of an improved model for matrix cracking. Unlike many existing constitutive models in the literature, the developed model captures some key features that are often neglected in constitutive modelling. These include the effect of hydrostatic pressure on both the elastic and non-elastic response. A novel yield function is formulated specifically for polymer-matrix fibre-reinforced composites, taking into account the presence of fibres in the material. The developed model is able to predict the non-linear response under complex loading combinations, given only the experimental response from two uniaxial tests. A non-associative flow rule is used to capture the pressure sensitivity of the material. The translation of subsequent yield surfaces under complex loading regimes is modelled by the inclusion of a non-linear kinematic hardening rule, which also allows for simulation of material unloading. The implementation of the model as a user defined material subroutine in a commercial finite element package is described. Regarding the modelling of matrix cracking, several methods are available in the literature. These models are reviewed and an existing model is combined with suitable failure criteria for the simulation of stiffness loss and crack accumulation in laminates. This model is then used to make predictions of crack accumulation and loss in stiffness of composite materials.

A literature review has been carried out relating to the damage and fracture behaviour of composite laminates based on woven fabric reinforcement containing a stress raiser in the form of either a circular hole or of a mechanically fastened joint - the tensile failure modes in these two types of problem are very similar. Closed form and finite element based approaches that enable the stress distributions in these two classes of problem are presented prior to a review of some of the existing failure models. The failure models considered are based on strength and fracture mechanics approaches, which are applied in some cases at the laminate level and in others on a ply-by-ply basis. A number of these approaches invoke the use of a characteristic distance, which is a material property, but may also be specimen geometry dependent. The aim of the present work is to develop a more unified model for damage and fracture at tensile stress concentrations within a finite element analysis. To this end a traction-separation law (based on physically meaningful material parameters) is implemented within ABAQUS CAE and used to predict the strength of woven composite plates containing open holes and mechanically fastened joints. This approach is applied first to the open hole problem within a two-dimensional framework, with reference to several data sets from the literature. Agreement is good, both with prior experimental data and other modelling approaches. The bolted joint problem is then considered and a two dimensional approach is applied to model net-tension failure data for woven GFRP bolted joints from the literature, using the same traction-separation law as applied to the open hole problem. Agreement is reasonable, but a need for a model that incorporates bolt clamp-up is apparent. Subsequently an extensive experimental data set is obtained for the failure strength of a range of woven CFRP bolted joints, both double-lap and single-lap construction with various lay-ups, plate geometries, hole sizes and bolt clamp-up. These test configurations are then modelled using a 3-D finite element framework. Good agreement between the predicted and measured bearing stress at failure was obtained for double-lap joints that failed in the net tension failure mode. Less good agreement was obtained for the single-lap joints, where the tensile fracture mechanisms were more complex and not captured fully in the model.

Previous attempts to produce carbon nanotube (CNT) ceramic composites have resulted in poorly dispersed, unaligned and non-continuous CNTs in the composites with modest improvements in properties. The research presented in this thesis pertains to the production of dense aluminoborosilicate (ABS) glass matrix composites containing aligned and continuous multi-walled carbon nanotubes (MWCNT) of millimetre lengths. This was achieved by infiltrating CVD grown MWCNT preforms using a precursor sol and sintering which achieved 80 ± 2&percnt; dense composites. Focused ion beam milling together with image analysis showed that the composites contained 20 ± 2 vol.&percnt; MWCNTs, which are aligned and continuous within the glass matrix. Indentation studies showed greater damage tolerance in the composite compared to unreinforced ABS glass. Under compression, there is no significant change in the compressive strength between the composite and the unreinforced glass. The bend strength of microcantilever beams were 1.4 to 1.3 GPa for the composite and glass respectively. Elastic modulus of 84 GPa and fracture toughness (KIC) of up to 2.4 MPa √m were obtained for the composite. The elastic modulus and fracture toughness results are an improvement of 30 &percnt; and 240 &percnt; over that of unreinforced ABS glass. Fracture surfaces showed apparent MWCNT pullout lengths of up to &Tilde; 1 μm. Analysis indicates that crack bridging by intact MWCNTs provides the majority of the improvement in fracture toughness. Interlayer sliding of the MWCNTs and "sword in sheath" failure mechanism of the MWCNTs prevented the maximum potential performance, with respect to elastic modulus and fracture toughness, from being achieved. Electrical conductivity in the alignment direction of the CNTs showed improvements by a factor of 10E6 compared to unreinforced ABS glass. Furthermore, improvement of a factor of &Tilde; 10 in the thermal conductivity was obtained for the composite over that of ABS glass.

Numerical simulations and analytical models are increasingly being sought for the design and behaviour prediction of composite materials. The use of high-performance composite materials is a growing area particularly in civilian and defence related applications. This growth demands the understanding and ability to predict how these materials will behave under their exposed environments. In this thesis, the displacement behaviour of naturally debonded composites (NDCs) from out-of-plane bending conditions is investigated. A detailed experimental programme is conducted to ascertain the phenomenological behaviours of these systems from steady-state responses for forced harmonic loading and freevibrations. The stiffness and energy dissipation behaviour are examined and a finite element (FE) model describing their behaviour is developed. Using the FE model, the experimental programme is extended through simulations and analytical models are developed to predict the displacement response behaviour. Through the exploitation of the analytical models, a linearisation is used to extract the bulk effective material properties and a constitutive model is developed to describe multi-layered 0JDCs that are reduced to a single layer describing their steady-state responses. The friction between each of the layers is included in the analytical model and is shown to have distinct behaviour for these types of composites. Acceptable agreement is observed between the analytical model predictions, the FE model and the experiments. It is well known that prevention of failure for vibratory exposed structures is a continuing challenge and that although the characterisation of damping in vibrating structures has long been an active area of research in structural dynamics, it is usually limited to experimental data. As the exploitation of lighter and more efficient structures is becoming more prevalent, the need for alternative damping materials and systems is overwhelmingly necessary. The use of NDCs poses as a viable candidate for rectifying some of these challenges, particularly since they can be designed with high levels of accuracy as opposed to a rough estimation as with many damping systems. From this work it is shown that layered NDCs have the ability to dissipate large amounts of energy from exploiting their frictional interfaces. There are many areas that NDCs could serve to be useful as damping systems.

Natural rubber (NR)/organoclay nanocomposites were prepared using organomontmorillonite (OMMT) and organo-sepiolite (OSEP). Both were found to improve modulus significantly more than equivalent amounts of conventional fillers such as carbon black for strains up to 100%. OSEP was found to increase modulus more than OMMT for a given filler content, and NR/OSEP nanocomposites also had potentially anisotropic physical properties. OMMT had more effect on vulcanisation than OSEP, although both produced considerable acceleration. The tensile stress-strain behaviour of NR/OMMT and NR/OSEP nanocomposites were studied using a number of different micromechanical models. Some models were found to give a good empirical fit with experimental data, with the best results given by the Halpin-Tsai model. Furthermore, by analysis of the vulcanisation behaviour using rheometry, and particle morphology using transmission electron microscopy (TEM), it was possible to accurately estimate the Young's modulus of a nanocomposite from knowledge of the cure onset time and the shape factor of the particles. It was discovered that unmodified montmorillonite and sepiolite clays could undergo organic modi cation in situ during mixing into NR following the addition of a suitable modifier. This resulted in vulcanisates with very similar physical properties to those found when using pre-modified OMMT or OSEP. TEM and X-ray diffraction showed that the exfoliation state of the clay modified in situ was also similar to that of pre-modified organoclay. Silane coupling agents were also used with NR/OMMT and NR/OSEP nanocomposites, producing significant increases in modulus. However, the increased modulus was only observed above 40% strain for OMMT and above 25% for OSEP. The coupling agents strengthens the rubber- ller interface preventing interfacial slippage and cavitation in the nanocomposite, and these mechanisms only begin to operate when the interfacial stress reaches a significant level. The most effective coupling agent used was bis[triethoxysilylpropyl] tetrasulfide due to its relatively high reactivity.

Due to the complex nature of fibre reinforced composite (FRC) materials, the onset of damage does not cause instantaneous failure of the entire structure. As a natural progress of the research in the area of modelling damage at multi-scales, a discrete element method (DEM) has been presented in this thesis to simulate and analyse the damage progression in FRC laminates which consists of interfacial debonding, transverse cracking, delamination, and transverse cracking and delamination under quasi-static tensile and/or thermalloadings. Regular and random packing schemes have been developed to assemble the particles to construct the DEM models of the composite materials in which two contact constitutive models, i.e. parallel bond model and contact softening model, are used to represent the linear elastic properties of the fibre and the plastic or cohesive behaviour of matrix and interface, respectively. The interlaminar delamination in composite laminates under mode I, mode II and mixed mode have been modelled and the extension of plastic zone in front of the crack tip has been predicted. The initiation and propagation of matrix cracking as well as the consequent fibre/matrix interfacial debonding process in single-fibre composite has been modelled and analysed by DEM. The effects of fibre distribution and fibre volume fraction on the transverse cracking path and the residual damage remaining in the composite lamina are studied by DEM. The progression of transverse cracking and delamination in cross-ply laminates, the cracking density as well as the stiffness degradation have been predicted by DEM and compared with those from other numerical models or experiments. A thermal expansion scheme has been developed in the DEM modelling of composite laminates, and the damage progression of matrix cracking, fibre/matrix debonding, transverse' cracking and delamination are included in the thermal-mechanical coupling DEM model with the consideration of material microstructures. The outcome of this research has validated the application of DEM in composite laminates in terms of its advantages in the modelling of damage progression and the prediction of cracking density and stiffness reduction, and also proved the potential of DE M in the future research of composite material design.

The overall purpose of this research is to detect and quantify low-velocity impact damage in structures made from composite materials. This research represents a study using simplified coupon specimens. The composite material chosen for the current research is a woven Carbon Fibre Reinforced Polymer (CFRP) prepreg with a MTM57 resin system (42%RW) with CF2900 fabric (280 g/m2, 12K and 2 x 2 twill fabric). This woven material was fabricated to produce coupon size specimens of 250 mm x 150 mm with II, 12 and 13 layers of thickness. Piezoelectric sensors of type SONOX® P5 were placed on three different locations on each of the coupon size specimens to record the responses along different directions of the ply and at different distances from the impact events. Two different approaches were used to record the acceleration response signals resulting from the impact excitation. The first approach used the LMS Testlab Impact Modal Analysis environment in order to acquire time data and produce spectra for a number of non-damaging impacts from a standard instrumented impulse hammer. The second approach used an instrumented drop-test rig to perform the potentially damaging impacts. The impact energies for this approach were set to range from 0.37 J to 41.72 J. The response signals from each test specimen were recorded using the LMS SCADAS III data acquisition system and saved for evaluation. To gather the appropriate information to make inferences regarding the extent of the damage, two different methods were used to estimate the damaged area. The first method measured the damage size using a vernier caliper directly on the impacted surface. The second method used developed X-ray films. For the latter method, the damage area was estimated as the rectangular area bounded by the width and length of the largest flaws visible parallel to the two plate axes. The correlation between the damage area in terms of the impact energy and force detected is presented and discussed. In this research, following a systematic series of experiments on the induction of impact damage in composite specimens, Scanning Electron Microscopy was used to inspect the topographies of the impacted surface at high magnifications. Two different approaches were used here to observe the type of failure modes. The first observation was on the surface defects of the impacted samples whilst the second type, usually categorised as destructive testing, visualised the cross-sectional defects to look at the internal damages. A damage model and damage pattern was developed from this work, which can provide sufficient information on the type and extent of damage. Both the damage model and pattern can be used to provide fundamental understanding of damage and failure mode progression in carbon fibre reinforced compo~ites with varying layer numbers and impact energies. Wavelet analysis is a well-known and powerful approach to feature extraction for problems in condition monitoring and damage detection. In this research, it is applied in the context of impact damage detection and quantification. The approach was based on response time signals recorded from the piezoelectric sensors. Damage indices in terms of Root Mean Square, Power Spectrum Density and Envelope Mean were presented. The results show that all three potential damage indices show a monotonic increase with impact energy and this behaviour is important when damage needs to be detected directly from the impact data. The current research was based on the idea of implementing machine-learning methods to identify and categorise (damaging and non-damaging) impacts using structural response data. To implement this idea, a novelty detection method using outlier analysis was used. This method has proved to be a successful in separating the damaged and non-damage features and classifying the types of failure modes. This method was considered an excellent approach to identify and categorise the impact events using structural response data.

Effective, non-destructive and in-situ damage detection in fibre reinforced composites (FRCs) is a field of great research interest worldwide. Currently employed optical fibre based sensors suffer from diameter mismatch between optical fibres and the reinforcing fibres, stress-transfer issues, bonding problems, temperature sensitivity and are mostly complex. Therefore the aims of this project were to investigate the use of E-glass reinforcing fibres for optical transmission and to develop a fully functional, simple and economic but effective, optical health monitoring system for epoxy matrix composites using the reinforcing fibres as the self-sensing element. E-glass fibres have been converted to optical waveguides using a variety of coatings. It has been found that reinforcing fibre could transmit light over a certain range depending on the transmitting wavelength, chemical purity of glass fibres, fibre diameter, quality and in particular the optical properties of the cladding material and fibre/matrix interface which have a strong effect on the numerical aperture. In this work, commercial grade aerospace certified epoxy, Araldite L Y IHY 5052 was employed as the matrix and to ensure that it had suitable optical properties to act as a cladding for light guiding its refracti ve index was lowered by blending with propylene carbonate. The effects of this addition on the mechanical properties of the resin were assessed. Wavelengths at which attenuation was minimised were identified and the corresponding transmitters and detectors were acquired to build a complete optical measurement setup. A variety of composite test panels were constructed including hybrid glass-carbon fibres. It is shown that the system detects damage including barely visible impact damage (BVID) and can give some indication of stress variation. Overall the system developed is simple, non- destructive, inexpensive to deploy and maintain, easy to operate, suitable for in-situ employment and gives real-time results.

Thiolated ligands are known to form self-assembled monolayers on gold nanoparticles, and the self-organisation of these ligands provides a route towards nanoparticles with programmable and complex molecular structures. This body of work investigates the structures and chemical properties of ligand monolayers based on peptides and alkanethiol derivatives. We evaluate a series of published articles reporting the peculiar self-organisation of ligands into striated domains on the surface of nanoparticles, which was shown by scanning tunnelling microscopy (STM). Image analysis of the STM micrographs shows the stripe-like domains to be scanning artefacts, and our attempts to reproduce data on nanoparticle stability and cell entry give results conflicting with those published. Self-assembled monolayers made from peptides allow the use of peptide motifs to drive self-organisation. We investigate the effects of using an amyloidogenic sequence, CFGAILSS, in a monolayer. FTIR, 2DIR and solid state NMR reveal the presence of inter-peptide hydrogen bonding consistent with a parallel β-sheet structure, which is not seen in a monolayer made from the CALNN peptide. Fluorescently labelled peptide-capped gold nanoparticles were irradiated by a femtosecond laser pulse. The timings of the reaction dynamics of this ligand release were measured by splitting the laser beam and introducing a variable delay. These measurements show that this process is a hot electron mediated process. We suggest that such laser induced release measurements can provide some insights into the intermolecular interactions within the monolayer

In the last decade, natural-fibre composites based on thermoplastics and thermosets have been embraced by car producers for use in the manufacture of door panels, seat backs and dashboards. Natural fibres have benefited from the perception that they are "green" or eco-friendly. What is proving more important is their ability to provide stiffness enhancement and sound damping at lower cost and density than glass fibres and mineral fillers. At the same time, interest has been growing rapidly in the potential offered by self-reinforced composites, such as polypropylene fibre reinforced polypropylene and polyethylene fibre reinforced polyethylene. Each of these composite systems offers its own advantages and a number of disadvantages. One way to reduce or remove many of these deficiencies would be to combine these two systems to develop a novel hybrid system that is fully recyclable. Hoo Tien N Kuan ABSTRACT Abstract The work outlined in this research project aims to develop, manufacture and fully characterise the properties of environmentally-friendly composites based on natural fibre and polymer fibre reinforcements, and their fibre metal laminates. The composites are based on natural fibres (e.g. hemp, basalt or flax) and polymer fibres (polypropylene). The composites are manufactured via a compression moulding procedure and their properties are investigated both statically and dynamically through a series of tension, flexure and low velocity impact tests. The fracture region of the laminates is also investigated using optical microscopy techniques. The mechanical response of the laminates is modelled using laminate theory and the impact response is predicted using simple impact models. A range of green composites and hybrid materials has been developed and investigated. The experimental data have shown that the composites based on basalt fibres offer an excellent all-round mechanical performance exceeding those associated with other natural fibre composites. Combining natural fibre composites with SRPP (Self- Reinforced Polypropylene) and aluminium alloy can enhance the mechanical behaviour of the composite. These environmentally-friendly composite systems offer significant potential in engineering design and could potentially help to reduce waste and pollution.

Composite parts reinforced with unidirectional or 2D woven reinforcements have been widely used for many years in numerous industries from aerospace to automotive parts. Although their use is widespread, laminated reinforcements have some significant weaknesses that limit their usefulness, including poor interlaminar strength and high cost driven by the need for lengthy hand lay-up to create complex parts. Considerable effort has been put into developing three-dimensional composite reinforcement in order to alleviate these problems. This thesis focuses on materials produced via the weaving method. "3D weaving" can produce fabrics with interlacing yams that provide genuine through thickness reinforcement. The weaving process can also produce multi-layer fabrics, reducing the amount of lay-up required to produce thick parts. A review of current literature showed that 3D woven composites have good interlaminar properties but can have poor in-plane mechanical properties, the main cause being architectural distortions such as local yam crimp and resin rich regions. It was evident that there has been relatively little study into the detail of 3D woven architecture and how it is affected by forming forces and in turn how this affects mechanical properties. In addition, [mite element modelling of 3D woven fabrics has been limited by idealised mesh generation, hampering predictive fabric analysis. The work presented here addresses the issue by developing a fundamental understanding of 3D woven composite architecture via a detailed characterisation of yam crimp and resin channels for a selection of angle-interlock weaves. The effect of compaction on these architectural features was also investigated and coupled with an innovative use of kinematic modelling that could help predict the final internal state of a compacted 3D woven fabric. Finally, compression testing of specimens reinforced using 3D woven fabrics at a range of compaction levels was used to ascertain the influence of fabric architecture on mechanical properties.

This thesis documents a programme of work undertaken from mid-2005 to mid-2009 as a part-time PhD, investigating the application of signal-processing methods to ultrasonic non-destructive evaluation (NDE) data from fibre-reinforced composite materials. The aims of the project were: •to push the boundaries of defect detectability by applying knowledge-based filtering methods to remove the response of the structure; •to produce 3D profile maps of various measured parameters to assist in differentiating between defects and structural effects; •to present the NDE information in terms of actual material properties that can be easily interpreted. These aims are linked to specific NDE problems in inhomogeneous materials: •3D characterisation of the material properties and defects in composite materials such as carbon-fibre reinforced plastic (CFRP); •3D mapping of ply wrinkling and fibre orientation in CFRP and glass-fibre reinforced plastic (GFRP). By developing and using a model, it was possible to understand the ultrasonic response of multi-layered structures when the layers themselves comprise both fibres and matrix. Various defects were inserted into the model to determine which parameters from the ultrasonic response would provide good distinction between defect types and enable quantitative 3D profiling of the required material properties. A toolset of signal-processing and image-processing algorithms was used to apply the methods to both simulated and real ultrasonic data from the above NDE problems in order to demonstrate the benefits of the new methods. At various stages through the project a validation process was undertaken to evaluate the methods for use on real composite aerostructures.

This project demonstrates the feasibility of a novel, built-in sensing system, to monitor the chemical health pf thermosetting polymer matrix composites. The system is based on the use of infrared evanescent wave spectroscopy via chalcogenide glass fibres which are embedded in the composite during production. Additionally, chalcogenide glasses were prepared in various GeS2- Sb2S3-PbS compositions, in order to produce unstructured optical fibres compatible with the curing processes for high performance epoxy-resin matrix composites.

Composite materials generally consist of different components which individually exhibit an entirely different material behaviour than within the composite. Here, two immiscible liquids are stabilised with solid particles forming liquid-liquid emulsions. Solid stabilised emulsions, also known as Pickering emulsions, have been thoroughly studied and find application in many industrial sectors. In these emulsions one liquid is generally suspended within the other in the form of droplets. Inspired by computer simulations, it should be possible to create a bicontinuous network of two immiscible liquids also stabilised with jammed particles. This will result in an attractive new material which could find possible industrial applications, for example as microreactors. This potential bicontinuous, interfacially jammed emulsion gel was dubbed bijel. Drawing together knowledge from different disciplines this thesis presents an experimental route to bijel formation. Accessing a certain type of phase separation called spinodal decomposition can be used to create a convoluted arrangement of bicontinuous interfaces. Liquid-liquid interfaces can be stabilised by exploiting the fact that solid particles can be irreversibly trapped at liquidliquid interfaces. Once trapped, the presence of the particles demands a minimum interfacial area between the two liquids. If the particles are jammed against each other this will result in the stabilisation of the liquid-liquid interfaces. To stabilise convoluted, bicontinuous interfaces in this way one type of particles must be able to concurrently stabilise two types of curvatures. Over the last three years it has been shown by several different research groups that this is possible. This thesis examines different types of temperature and pressure quenches on binary liquid systems to reach spinodal decomposition in the presence of particles. At the same time the ability of the particles to collect on the interfaces created during such phase separations is tested. It is found that temperature quenches through the critical point can lead to reproducible bijel formation resulting in the first experimental presentation of bijels. Using confocal microscopy the bijel formation process is studied in detail and properties of this new material are examined. To obtain insight into the behaviour of slowly ageing soft materials xray photon correlation spectroscopy is separately carried out on droplet emulsions.

A simple mathematical model (cylinder model) is proposed and used to evaluate various constitutive laws used in finite element codes to simulate fatigue-driven delamination in composite materials. The cylinder model has one degree of freedom and allows the evaluation of several parameters of the constitutive laws in an ffcient and accurate way without worrying about the complex interactions that may arise while using a nite element code. This cylinder model enabled a large number of analysis to be performed in a relatively short time and made possible parametric studies with tens of thousands of dfferent sets of parameter values of the constitutive laws. Different fatigue degradations strategies have been implemented into the cylinder model and investigated. A systematic comparison of the different strategies has been performed and it was found that the behaviour of the different strategies is similar but with some variations in the sensitivity to the spacing of the springs and to the size of the increment in the number of loading cycles. The results confirm that the more accurate results are found for smaller values of these two variables. A new fatigue degradation strategy is also proposed. It is based on the construction of a 3-D surface that represents the behaviour of the interface under fatigue loading. This surface has been incorporated into the cylinder model and its performance has been evaluated.

The thesis starts off with an introductory chapter on composite materials. This includes a definition of composites, a brief history of composite materials, their use in aerostructures (primarily as stiffened structures), and also optimization of composite structures. A literature review is then presented on postbuckling stiffened structures. This includes both experimental investigations on stiffened composite panels and investigations into secondary instabilities and mode jumping as well as their numerical modelling. Next, the Finite Element (FE) modelling of posthuckling stiffened structures is discussed, relating how ABAQUS models are set up in order to trace stiffened composite panels' buckling and postbuckling responses. An experimental programme conducted on an I-stiffened panel is described, where the panel was tested in compression until collapse. The buckling and postbuckling characteristics of the panel are presented, and then an FE model is described together with its predicted numerical behaviour of the panel's buckling and postbuckling characteristics. Focus then shifts to the modelling of failure in composites, in particular delamination failure. A literature review is conducted, looking at the use of both the Virtual Crack Closure Technique (VCCT) and interface elements in delamination modelling. Two stiffener runout models, representing two specimens previously tested experimentally, are then developed to illustrate how interface elements may be used to model mixed mode delamination. The previously discussed panel is revisited, and a global-local modelling approach used to model the skin-stiffener interface. FE models of a stiffened cylindrical shell are also considered, and again the postbuckling characteristics of the shell are compared with experimental results. . The thesis then moves on to optimization of composite structures. This starts off with a literature review of existing optimization methodologies. A Genetic Algorithm (GA) is devised to increase the damage resistance of the I-stiffened panel. The global-local ABAQUS model discussed earlier is used in conjunction with the GA in order to find a revised stacking sequence of both the panel flanges and skin so as to minimize skin-stiffener debonding subject to a variety of design constraints. A second optimization is then presented, this time linked to the FE model of the stiffened cylindrical shell. The objective is to increase the collapse load of the shell, again subject to specific design constraints. The thesis concludes by summarising the importance of the work conducted. FE models were created and validated against experimental work in order to model a variety of composite stiffened structures in their buckling and postbuckling regimes. These models were able to capture the failure characteristics of these structures relating to delamination at the skin-stiffener interface, a phenomenon widely observed experimentally. Various optimizations, able to account for failure mechanisms which may occur prior to overall structural collapse, were then conducted on the analysed structures in order to obtain more damage resistant designs.

The automotive and construction industries are actively involved in substituting natural fibre composites (NFCs) for other engineering materials in motor vehicles and buildings. In this work, emphasis has been placed on evaluating agave fibre composites containing well aligned, closely packed straight fibre bundles and focussing on the development of different types of practical joints for NFCs in composite structures. Two novel co-cured joints were proposed and evaluated, based on laminated and intermingled configurations.

In this thesis a long period grating (LPG) based sensor is investigated as a possible alternative to current process monitoring sensors used in the manufacture of composites to monitor cure. An LPG is demonstrated as a means of monitoring the cure of a UVcured epoxy resin. The wavelength shift of the attenuation bands were measured during the cure of the resin and compared with measurements made using a fibre optic Fresnel based refractometer. The results showed a good correlation (6 x 10 -3 rius) and illustrate the potential of the techniques for non-invasive composite material cure monitoring. Alternative fibre grating methods; a chirped LPG sensor, an in-fire Mach-Zehnder interferometer and a tilted fibre Bragg grating sensor, are also presented to demonstrate the versatility of grating based sensors for flow, high sensitivity refractive index and multi-parameter sensing, respectively. Demonstrations of LPG sensors in industrial applications are also presented and highlight the technical issues of integrating such devices in composite components and composite manufacturing processes.

Functionally graded materials (FGM) are composite materials with microstructure gradation optimized for the functioning of engineering components. For the case of fibrous composites, the fibre density is varied spatially, leading to variable material properties tailored to specific optimization requirements. There is an increasing demand for the use of such intelligent materials in space and aircraft industries. The current preferred methods to study engineering components made of FGM are mainly modelling particularly those that are finite element (FE) based as experimental methods have not yet sufficiently matured. Hence this thesis reports the development of a new Mindlin-type element and new Reissner-type element for the FE modelling of functionally graded composite (FGC) structures subjected to various loadings such as tensile loading, in-plane bending and out-of-plane bending, buckling and free vibration. The Mindlin-type element formulation is based on averaging of transverse shear distribution over plate thickness using Lagrangian interpolation. Two types of Mindlintype element were developed in this report. The properties of the first Mindlin-type element (i.e. Average Mindlin-type element) are computed by using an average fibre distribution technique which averages the macro-mechanical properties over each element. The properties of the second Mindlin-type element (i.e. Smooth Mindlin-type element) are computed by using a smooth fibre distribution technique, which directly uses the macro-mechanical properties at Gaussian quadrature points of each element. The Reissner-type element formulation is based on parabolic transverse shear distribution over plate thickness using Lagrangian and Hermitian interpolation. Two types of Reissner-type element were developed in this report, which include the Average and Smooth Reissner-type elements. There were two types of non-linearity considered in the modelling of the composite structures, which include finite strain and material degradation. The composite structures considered in this paper are functionally graded in a single direction only, but the FE code developed is capable of analysing composite structures with multidirectional functional gradation. This study was able to show that the structural integrity enhancement and strength maximisation of composite structures are achievable through functional gradation of material properties over the composite structures.

A robust method for the production of an emulsion polymer based on styrene-acrylic acid-acrylic ester was developed to give enhanced physical properties and/or reduced envhonmental impact. Replacing the methyl methacrylate content with n-butyl acrylate, tert-butyl acrylate and ethyl acrylate all gave stable polymer emulsions. Replacing methyl methacrylate with fatty acid based monomer containing no more than one polymerisable acrylate group per molecule also led to the production of a stable emulsion, with the fatty acid based monomer also acting as a self-emulsifying agent if having sufficient amphiphilic character. All stable emulsions were successfully used to produce composite materials.

This project looks at the potential for re-use of quarry wastes, in particular, Hassock, from the Lower Greensand, Hythe Beds, which in one Ragstone quarry alone, amounts to 100,000 tonnes of material annually. Quarry wastes have been solidified with a range of other waste products including Cement Kiln Dust, Ground Granulated Blast-furnace Slag and Pulverized Fuel Ash. The mixes were subjected to a CO2 atmosphere for varying lengths of time, to determine their carbonation potential and optimum CO2 exposure time, in order to produce pellets with optimum strength, measured by Aggregate Impact Value. Pellets that had been hydrated before carbonated were stronger than those carbonated immediately. However, carbonating the mixes during pelletization was found to have a beneficial effect on final pellet strength. After further hydration in air for 6 weeks, the pH of these pellets was reduced considerably, when they were further carbonated for 24 hours in the CO2 chamber. Furthermore, strength was found to be substantially increased. The manufacture of these carbonated pellets will recycle CO2 and up to 4 waste streams and modify the quarry waste by reducing the dust component to a mere trace. The Aggregate Impact Value of pellets ranges from 14 to 19 and they have a bulk density of 1030 to 1100 kg/m3, making them potentially suitable for use as Light Weight Aggregates.

In spite of their advantages, e.g. high strength, low weight, and high stiffness, composite laminates are prone to the formation of transverse cracking during both manufacture and service. It has been well recognised that the core to this damage mode is stress singularities near transverse cracks. In addition, transverse cracking may cause degradation in the thermoelastic properties of a laminate, and meanwhile new ply cracks usually form with the increase of applied loading. As a result, the stress transfer near transverse cracks, thermoelastic property degradation due to transverse cracking, and propagation of transverse cracks are all important issues in the analysis of composite laminates. The major contribution of this work is to develop a new semi-analytical method, the state space method, to evaluate the stress transfer near free edges and transverse cracks. On the basis of the generalised plane strain condition, the method overcomes the limitation of analytical methods in study nonsymmetric laminates. Moreover the method guarantees continuous fields of interlaminar stresses across interfaces between layers, which is one of the obstacles for conventional finite element method. Another contribution is to apply the stress analysis to assess the thermoelastic property degradation induced by transverse cracAking in general cross-ply and symmetric angle-ply laminates. The prediction is made by using the constitutive equation of laminates in the Classical Laminate Theory. The numerical results of stiffness degradation for nonsymmetric cross-ply laminates are revealed for the first time in the literature. The final contribution of this work is to apply the stress analysis to predict propagation of transverse cracking in general composite laminates. An energy-based cracking criterion is used to predict the crack multiplication process. The predictions for nonsymmetric laminates and the effects of shearing to transverse cracking are believed to be the first solutions in the literature.

This work draws some early baselines on the in-plane/out-of-plane properties balance in a 5HS woven carbon fabric/epoxy composite reinforced by tufting and resin injected by resin transfer moulding technique. Details of the manufacturing processes involved in the preparation of such through-the-thickness reinforced composites are presented together with analysis of the mesostructure of tufted specimens. Preforms were reinforced locally with a commercial glass or carbon fibre thread. The tufts were inserted in square arrangement with a KSL tufting tool interfaced to a 6 axis computer controlled robot arm from Kawasaki. The presence of tufts improved significantly the delamination resistance, assessed by testing double cantilever beam coupons in mode I loading configuration. In-plane tension and compression after im¬pact (CAI) tests revealed that the reinforcement resulted in a considerable increase in the post-impact residual strength value, with an accompanying drop down in static tensile modulus and strength of less than 10%. In addition to the standard coupons for the determination of the quasi-static mechanical properties, some cured miniature specimens containing a limited number of tufts were also prepared. These were tested in both uniaxial pull-out and in a mode II configuration in order to measure the bridg¬ing actions of the tufts and to determine the micromechanical failure mechanisms. The obtained crack bridging laws were used for calibrating a simple analytical model of the mechanical behaviour of a single tuft within the composite. The tufting technology was applied to an innovative concept that aims to adopt the tufting threads as a carrier for resin modifiers. For this purpose a single-filament and a multi-filament thermoplastic prototype threads were used. These threads are not intended to modify the composite fibre architecture but are expected to dissolve into and react with the host matrix upon cure. The outcome of mode I delamination and CAI tests conducted on woven preforms reinforced with such 'soluble' threads are presented and discussed.

Composite materials are usually multi-phase materials, made up from two or more phases, which are combined to provide properties that the individual constituents cannot. This technology represents an economical way to improve product performances avoiding the use of expensive materials. Composite materials can be obtained as films by means of the electrolysis of electroplating solutions in which micrometre- or submicrometre-size particles are suspended: variable amounts of these particles become incorporated in the electrochemically produced solid phase, to which they impart enhanced properties. The main aims of the present work contributing to this thesis are the study of different parameters influencing the electroco-deposition process in order to promote and improve the applicability of such a technology in the high speed electroplating industry. Following a comprehensive review on the electroco-deposition of composite coatings, the phenomena have been analysed moving from a microscopic point of view i. e. the role of the metal ions present in the electrolyte and adsorption on the inert particles and their interactions with the growing metal layer, to a macroscopic point of view i. e. the electrolyte agitation, its influence on particle motion and all the issues related to the presence of particles in an electrolyte during electroplating. In particular the inert particle influence in terms of geometry, dimension and chemical nature (spherical polystyrene particles vs. irregular alumina particles with different dimensions), the metal matrix influence (nickel, copper and zinc), the influence of electrolyte agitation (using a Rotating Cylinder Electrode cell system) and the influence of the coating thickness on particle content in the final coating, using different deposition times, have been examined. The importance of the particle shape has been highlighted showing how incorporating irregular geometries gave higher particle incorporation densities than regular geometries. The influence of the substrate finishing in terms of imperfections has been related to the particle incorporation rate showing how small surface imperfections enhanced the incorporation of particles. Different hydrodynamic regimes have been analysed resulting three different regimes being discerned: laminar, transitional and turbulent. The consequence, in terms of particle incorporation levels, has been found showing how the amount of particles in the coating changed from one regime to another. Different rate-determining steps were related to the hydrodynamics: when the regime is laminar, particles were incorporated as agglomerates and the process was under particle transfer control, whilst in the turbulent zone, the rate determining step was the velocity of reduction of the ions adsorbed on the particle surface.

Composite materials have been increasingly used in the past two decades since they offer significant potential weight reduction, part design flexibility and improved specific mechanical performance compared to traditional metals. For specific applications, braid reinforced composites offer better near net shape part and manufacturing flexibility than conventional unidirectional laminates, albeit at the expense of slightly lower in-plane stiffness and strength. Furthermore, for impact and crash applications, which is the emphasis of this thesis, their tow waviness and interlocking can offer excellent damage tolerance and energy absorption. In this work, heavy tow (24k) biaxial carbon and glass braided preforms were used to manufacture coupons and beam structures to undertake an extensive testing campaign to characterise different damage and failure mechanisms occurring in braided composites. Due to large shear deformation and surface degradation, non conventional measurement techniques based on marker tracking and Digital Image Correlation were successfully used to measure strains in the damaging material. The modelling of braided composites was conducted using the meso-scale damage approach first proposed by P. Ladevèze for unidirectional composites. The calibration of an equivalent braid unidirectional ply was achieved using the experimental results obtained for different braided coupons. Furthermore, failure mechanisms observed experimentally, such as tow stretching and fibre re-orientation occurring during loading history, were integrated into the model. A new unidirectional ply formulation was subsequently implemented into the explicit finite element code PAM-CRASHTM. Validation of the new model using single element, coupons and beams were conducted that provided a satisfying correlation between experimental tests and numerical predictions.

The behaviour of paste is complex.  It has many unusual properties that do not fit with other engineering materials.  These can be linked to the volume effects that occur as the material is sheared. A multi-phase model based on a soil mechanic approach has been developed to consider the particle, liquid and gas components.  The volume is allowed to change during shearing and links these effects to the shear stress through pore fluid pressure and surface tension.  There is also a movable reference strain to allow the model the ability to simulate both cyclic and monotonic deformations in one system. Constrained pressure tests were conducted over a fixed strain amplitude cyclic deformation.  These were conducted in a series to consider the effects of strain rate, amplitude and pressure in a paste.  They were then analysed visually and statistically to find which of the variables were significant to the stress-strain response in paste. A further series of tests carried out with an increasing amplitude cyclic test.  This test was simpler but allowed a series of different pastes to be analysed without attempting to control the pore pressure in the materials.  A number of different responses were noted and a fuller picture of paste reaction was developed. Finally, the material model was matched to physical tests to evaluate the model’s ability to fit to different paste responses.  This was completed successfully with the use of an optimisation algorithm.