No / Paper accepted for conference.

yes / Tests results of three flanged and two rectangular cross-section concrete beams reinforced with carbon fibre reinforced polymer (CFRP) bars are reported. In addition, a companion concrete flanged beam reinforced with steel bars is tested for comparison purposes. The amount of CFRP reinforcement used and flange thickness were the main parameters investigated in the test specimens. One CFRP reinforced concrete rectangular beam exhibited concrete crushing failure mode, whereas the other four CFRP reinforced concrete beams failed due to tensile rupture of CFRP bars. The ACI 440 design guide for FRP reinforced concrete members underestimated the moment capacity of beams failed due to CFRP tensile rupture and reasonably predicted deflections of the beams tested. A simplified theoretical analysis for estimating the moment capacity of concrete flanged beams reinforced with FRP bars was developed. The experimental moment capacity of the CFRP reinforced concrete beams tested compared favourably with that predicted by the theoretical analysis developed.

Recently, there has been a significant growth in the use of composites in sectors such as automotive, aerospace and wind energy. Composites are traditionally designed for mechanical performance in terms of strength, stiffness and impact energy absorption; however multifunctionality has become the focus of researchers and designers in recent years. Multifunctional design of composites involve adding functionality such as thermal management, radiation shielding, stealth, structural health monitoring and energy storage at material level rather than adding discrete components afterwards. The aim of the current research is to incorporate multi-functionality at tow-scale both as a processing aid during manufacture and adding additional functionality during subsequent processing. Various laboratory scale machines were developed as a part of this study to identify the ideal way to spread and incorporate metallic materials into the carbon fibre tows, thereby making them multifunctional. Manufacturing processes such as co-mingling of micro-fibres, coating with metallic powder and screen printing of metallic grid lines have been developed in this work. One of the objective of this thesis is to metallise carbon tow in order to use it in conjunction with magnetic tooling, as part of the chopped fibre preforming process developed by the University of Nottingham and Bentley Motors. The performance of the metallised tow has been evaluated using characterisation tests such as magnetic pull force test, bending rigidity test etc. Finite element models have been developed to verify the experimental results of magnetic pull force and bending properties. As observed during the research, the bending properties of the carbon tow were found to influence the accuracy of the finite element modelling significantly. Study into the bending properties of the carbon fibre and Multifunctional carbon tow using two different principles such as carbon tow bending under own weight and bending due to the application of an external force were carried out. In each case the governing mathematical models were also derived.

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:

The increasing demand for polymer matrix composites (PMCs) from the airframe industry raises the issues of productivity, cost and reproducibility of manufactured PMC components. Performance reproducibility is closely related to the manufacturing technique. Resin transfer moulding (RTM) offers the advantage of flexible manufacturing of net-shape PMC components with superior repeatability starting from ready-to-impregnate dry reinforcements. An RTM apparatus was developed for manufacturing PMC plates and demonstrator components representative of actual, PMC components and PMC moulds made and used in the airframe industry. The RTM process developed in this work involved making net-shape dry carbon fibre preforms and impregnating them an epoxy resin, targeting mould applications. Thermal repeatability of different net-shape PMC components manufactured using the RTM apparatus developed in-house was investigated. Effects of bonding an outer copper plate onto the PMC material, targeting mould applications known as integrally heated copper tooling (IHCT), were explored. Heat conduction through the PMC components was studied using simulation models validated by experimental data obtained primarily by thermography. Manufactured PMC components showed good repeatability, particularly in terms of thermal behaviour. The IHCT technique was found to be well suited for mould applications. Expected advantages of thermography were materialised. Finally, the simulation models developed were in good agreement with experimental data.

There is a increasing emphasis in today’s world to use environmental friendly solutions for tribological and lubrication purposes. Use of water as a lubricant presents a cost effective and easy method of bio friendly lubrication. But, as water has low viscosity, it is necessary that the materials used in water lubricated contacts perform exceedingly well in boundary lubricated conditions. Polymer Based Materials (PBMs), are one such group of materials which have been proven to perform well in such conditions. In particular, Ultra High Molecular Weight Polyethylene (UHMWPE) has been extensively used in water lubricated contacts. But, PBMs still suffer from wear and related problems and there is room for improvement. Various methods have been tried with mixed results to improve the qualities of polymers and consequently their performance in water lubricated contacts. One such method is by inclusion of fillers. Conventionally, micron sized fillers have been used to form composites with a polymer resulting in materials with better properties. Recently, nanometer sized reinforcements have been attracting more attention due to their unique mechanical and tribological properties. Combining micrometer and nanometer sized filler in a polymer composite could help form materials with excellent properties. Such composites would be termed as a hybrid material. Therefore, the aim of this project and thesis is to experimentally investigate the influence and interaction of micro and nano carbon-based fillers on tribological behaviour of UHMWPE composites and provide further understanding of the mechanisms involved.

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.

Level set method is a boundary tracking method that uses an implicit function to define the boundary location. By using the implicit function to define the structural boundary the level set method can be used for topology optimisation. The level set method has previously been used to solve a range of structural optimisation problems. The aim of this thesis is to extend the application of the level set method to additional applications of structural optimisation. A robust method of 3D level set topology optimisation is developed and tested. The use of a hole insertion method was found to be advantageous, but not vital, for 3D level set topology optimisation. The level set method is used to optimise the internal structure of a proximal femur. Similarities between the optimal structure and real internal trabecular bone architecture suggest that the internal bone structure may be mechanically optimal. Stress constrained level set topology optimisation is performed in 2D. Stress shape sensitivities are derived and interpolated to obtain smooth boundary sensitivity, resulting in feasible stress constrained solution in numerical examples. A new generic objective hole insertion method is used to reduce dependence on the initial solution. A level set method for optimising the design of fibre angles in composite structures is also introduced. Fibre paths are implicitly defined using the level set function. Sensitivity analysis is used to update the level set function values and optimise the fibre path. The method implicitly ensures continuous fibre paths in the optimum solution, that could be manufactured using advanced fibre placement.

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.

The compressive strength of unidirectional (UD) carbon fibre reinforced plastics (CFRPs) is often only 60-70% of their tensile strength owing to premature failure associated with kink-band formation. The sudden and complex nature of kink-band formation has been hindering the progress in experimental studies on the evolution of damage in compressive failure. A better understanding of the damage mechanisms associated with kink-band formation can help to design more reliable composite structures. Therefore, the principal aim of this project is to identify, in three dimensions (3D), the key damage mechanisms underlying the initiation and propagation of kink bands in UD carbon fibre/epoxy composite. A new manufacturing method is developed to fabricate high-quality UD T700/epoxy cylindrical rods for axial compression tests and high-resolution imaging of kink bands by post mortem and in situ X-ray computed tomography (CT). The morphology of kink bands is visualised in 3D by segmenting fibre breaks at kink-band boundaries and representative longitudinal splits. The geometrical parameters of each fully developed kink band are consistent through the specimen. Radiographs obtained from ultra-fast synchrotron imaging show that a kink band initiates and propagates across the specimen in less than 1.2 ms. A scenario of kink-band failure is proposed: fibre buckling and longitudinal splitting occur prior to fibre breakage, which forms kink-band boundaries and eventually the morphology of multiple kink bands develops suddenly. 3D tomographs of the fast and unstable kink-band formation could not be captured in the axial compression experiments. Therefore, a testing method of loading notched UD carbon fibre (T800, T700 and T300)/epoxy beams using a four-point bending (FPB) fixture is developed to enable monitoring of more stable initiation and propagation of kink bands by in situ X-ray CT. Kink-band formation is significantly slowed in the FPB tests. Fibre micro-buckling accompanied by splitting, could initiate the formation of kink bands. In the T700/epoxy system, the early initiation stage of fibre micro-buckling without fracture is captured, and the critical radius of curvature of unbroken fibres prior to fracture is ～130micro metre. Unloading causes significant recovery of fibre curvature (radius of curvature ～280 micro metre) and a reduction of 10-20º in fibre rotation angle within the kink band. The results show that in situ 3D characterisation of kink bands is essential as fibre buckling is a 3D phenomenon, resulting in development of both in-plane and out-of-plane kink bands. Understanding of kink-band formation in 3D will help to establish strategies to improve the compressive strength of CFRP composites by depressing kink-band formation; in this respect lateral constraint conferred by strong interfaces is a key aspect.