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 propagation of waves along an elastic layer has long been an area of active research since the later part of the 19th Century. Many contributions have already been made to the study of small amplitude wave propagation in a linear isotropic elastic layer and multi-layered structures, with traction-free boundary conditions on the upper and lower surface. However, the presence of pre-stress within an elastic body greatly increases the complexity of the problem, especially within multi-layer structures. The dispersion relations associated with a symmetric 4-ply (3 layer) laminate discussed in this thesis are derived and first solved numerically. The behaviour observed from the numerical investigation is then used to obtain asymptotic expansions for the phase speed.

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.

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.

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.

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.

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.

The simplest type of sandwich composite consists of two, thin, stiff strong sheets of dense material separated by a thick layer of low density material. The first major structure to incorporate sandwich panels was the Second World War "Mosquito aircraft" which consisted of a balsa wood core with plywood faces. The cellular materials used for sandwich core applications can generally be divided into two categories; honeycomb structures and polymer foams. Other cellular cores which exist are balsa wood and corrugated cores. Honeycomb materials generally have a hexagonal cellular shape, and are the core material of choice for advanced composites, In this thesis it was the aim to prepare open-cellular PolyHIPE foam core materials by the polymerisation of the continuous phase of a high internal phase emulsion (HIRE). To assess whether PolyHIPE materials are viable as core materials their flexural, compressive and shear properties were evaluated against the current commercially used core materials. It was shown that it was possible to improve the flexural, compressive and shear properties of a styrene/DVB PolyHIPE material by the addition of fibres to the material and the use of an optimised surfactant system (OSS). It was also shown that by the addition of monomers such as; butyl acrylate, 2-ethylhexyl acrylate, butyl methacrylate and methyl methacrylate to the monomer phase that the compressive properties of the S/DVB material could be varied. It was also possible to prepare a fibre-free and fibre-reinforced resorcinol- formaldehyde-based material, which had greater compressive and shear properties compared to the styrene/DVB material. Also a PolyHIPE material containing poly (ϵ-caprolactone) diacrylate showed shape-memory properties and an elongated cell structure after deformation and cooling. This elongated cell structure could lead to possible anisotropic behaviour. Finally it was concluded that the best mechanically performing PolyHIPE materials prepared competed well with the shear and compressive strength of the commercial honeycomb and foam core materials examined.

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