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.

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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.

In today’s world the significance of bio-based materials are increasing rapidly because ofthe environmental concern. Material scientists are nowadays engaged in development ofsuch materials which have natural origin and degrade in its environment. Several workshave already been reported in area of thermoplastic biocomposites. However biocompositesbased on thermosets is comparatively new area of research. In this work biobasedcomposites have been developed from two different bio-based thermoset resins.The main objective of the work was room temperature curing of poly lactic acid basedresin (POLLIT™) and AESO, acrylated epoxidized soy-bean oil (TRIBEST®). These tworesin systems were impregnated with different natural fibre mats. Cure behavior wascharacterized by means of DSC (Differential Scanning Calorimeter) and results showsthat the resins have been cured at room temperature. The mechanical properties ofprepared composites were assessed by the means of flexural testing and charpy impacttesting. The viability of using these composites in structural applications are also beendiscussed. / Uppsatsnivå: D

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.

Yes / Composites have clinical application for their ability to mimic the hierarchical structure of human tissues. In tissue engineering applications the use of degradable biopolymer matrices reinforced by bioactive ceramics is seen as a viable process to increase osteoconductivity and accelerate tissue regeneration, and technologies such as additive manufacturing provide the design freedom needed to create patient-specific implants with complex shapes and controlled porous structures. In this study a medical grade poly(l-lactide) (PLLA) was used as matrix while apatite-wollastonite (AW) was used as reinforcement (5 wt% loading). Premade rods of composite were pelletized and processed to create a filament with an average diameter of 1.6 mm, using a twin-screw extruder. The resultant filament was 3D printed into three types of porous woodpile samples: PLLA, PLLA reinforced with AW particles, and PLLA with short AW fibres. None of the samples degraded in phosphate buffered solution over a period of 8 weeks, and an average effective modulus of 0.8 GPa, 1 GPa and 1.5 GPa was obtained for the polymer, particle and fibre composites, respectively. Composite samples immersed in simulated body fluid exhibited bioactivity, producing a surface apatite layer. Furthermore, cell viability and differentiation were demonstrated for human mesenchymal stromal cells for all sample types, with mineralisation detected solely for biocomposites. It is concluded that both composites have potential for use in critical size bone defects, with the AW fibre composite showing greater levels of ion release, stimulating more rapid cell proliferation and greater levels of mineralisation. / The research was funded in part by the UK EPSRC Centre for Doctoral Training in Additive Manufacturing and 3D Printing (EP/L01534X/1), the UK EPSRC Centre for Innovative Manufacture in Medical Devices (EP/K029592/1), and Glass Technology Services Ltd., Sheffield, UK.

Fibre composite materials are gaining recognition in civil engineering applications as a viable alternative to traditional materials. Their migration from customary automotive, marine, aerospace and military industries into civil engineering has continued to gain momentum over the last three decades as new civil engineering applications develop. The use of fibre composite materials in civil engineering has now evolved from non-structural applications, such as handrails and cladding, into primary structural applications such as building frames, bridge decks and concrete reinforcement. However, there are issues which are slowing the use of fibre composite materials into civil engineering. Issues include high costs, difficulties in realising potential benefits, general lack of civil engineers' familiarity with the material and relatively little standardisation in the composites industry. For composites to truly offer a viable alternative to traditional construction materials in the civil engineering marketplace, it is essential that these issues be addressed. It is proposed that this situation could be improved by demonstrating that potential benefits offered by composites can be achieved with familiar civil engineering forms. These forms must be well suited to fibre composite materials and be able to produce safe and predictable civil engineering structures with existing structural engineering methods. Of the numerous structural forms currently being investigated for civil engineering applications, the truss form appears particularly well suited to fibre composites. The truss is a familiar structural engineering form which possesses certain characteristics that make it well suited to fibre composite materials. In this research a novel monocoque fibre composite truss concept was developed into a working structure and investigated using analytical and experimental methods. To the best of the author's knowledge the research presented in this thesis represents the first doctoral research into a structure of this type. This thesis therefore presents the details of the development of the monocoque fibre composite (MFC) truss concept into a working structure. The developed MFC truss was used as the basis for a detailed investigation of the structural behaviour of the MFC truss elements and the truss as a whole. The static structural behaviour of the principal MFC truss elements (tension members, compression members and joints) was investigated experimentally and analytically. Physical testing required the design and fabrication of a number of novel test rigs. Well established engineering principles were used along with complex finite element models to predict the behaviour of the tested truss elements and trusses. Results of the theoretical analysis were compared with experimental results to determine how accurately their static structural behaviour could be predicted. It was found that the static structural behaviour of all three principal truss elements could be accurately predicted with existing engineering methods and finite element analysis. The knowledge gained from the investigation of the principal truss elements was then used in an investigation of the structural behaviour of the MFC truss. Three full-scale MFC trusses were fabricated in the form of conventional Pratt, Howe and Warren trusses and tested to destruction. The investigation included detailed finite element modelling of the full-scale trusses and the results were compared to the full-scale test results. Results of the investigation demonstrated that the familiar Pratt, Howe and Warren truss forms could be successfully manufactured with locally available fibre composite materials and existing manufacturing technology. The static structural behaviour of these fibre composite truss forms was accurately predicted with well established engineering principles and finite element analysis. A successful marriage between fibre composite materials and a civil engineering structure has been achieved. Monocoque fibre composite trusses have been developed in the familiar Pratt, Howe and Warren truss forms. These structures possess characteristics that make them well suited to applications as primary load bearing structures.

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