In this thesis both static and dynamic analyses of composite thin-walled structures are carried out. Most notably, the Dynamic Stiffness Method (DSM) has been extensively exploited to develop advanced formulations for plates and shells. In particular, the. Dynamic Stiffness (DS) matrices have been developed for laminated composite plates and shells using Higher-order Shear Deformation Theory (HSDT) in order to investigate their free vibration behavior and buckling characteristics. First, the Governing Differential Equations (GDEs) of motion and associated natural Boundary Conditions (BCs) (Neumann-type) for the given displacement field are derived via Hamilton's principle for both composite plate and shell structures. In the case of composite plates, the DS matrices are formulated for both out-of-plane and in-plane deformations. The GDEs for each of the two cases are solved in Levy's form separately. Next the problems for both plates and shells are reduced to a system of ordinary differential equations which are then solved by using the classical exponential solution procedure.

An investigation into the time-dependent effects of bistable morphing and deployable structures is presented, showing how the stability of these structures is influenced by viscoelastic effects and how these effects can be used as an advantage to engineer novel morphing concepts. In the context of space applications, significant relaxation effects appear in structures that have been stored for a long duration, e.g. in a space vehicle prior to launch. Here, the deployment of an ultra-thin CFRP tape spring is first studied, with the aim of understanding how long-duration stowage causes changes in its stability and how it affects its deployment behaviour. Analytical modelling and experiments show that the deployment time increases predictably with stowage time and temperature, and that analytical predictions compare well with experiments. For cases where stress relaxation become high, the structure is shown to lose its ability to deploy autonomously. Next , structures that exhibit a time-dependent effect called pseudo-bistability are considered. A pseudo-bistable structure is one that is able to gain a new stable state during relaxation and therefore becomes temporarily bist able. This is exploited to yield a novel structure which, once actuated to its second stable state, can return dynamically to its initial state after a certain recovery time without further actuation. The concept of pseudo-bistability is first demonstrated using an analytical spring-and-dashpot model, then is studied in an isotropic spherical dome through numerical modelling and experimental validation. It is shown, in particular, that a well-known geometrical parameter can be used to correlate the geometry of the dome to its recovery time. The pseudo-bistable effect is then demonstrated in a prestressed panel, and the use of composite reinforcement is considered to improve the stiffness of the structure. Results show that a limiting volume fraction exists at which the pseudo-bistable behaviour is lost. Finally, an application is proposed, whereby a pseudo-bist able structure acts as a flow controller in an air duct. Using a fluid- structure simulation, fluctuations in the outlet pressure are shown to be selectively damped by the pseudo-bistable device. In the last chapter, various other conceptual applications using time-dependent bistable devices are also explored, in the hope that this work is taken further.

When a structure is subject to cyclic loads there is a possibility of it failing due to ratchet or incremental collapse. In many engineering structures the demonstration of non-ratcheting behaviour is a fundamental requirement of the design and assessment process. Whilst it is possible to use incremental finite element analysis to simulate the cyclic response for a given load case to demonstrate shakedown or ratchet, it does not yield any information on the safety factor. In addition, there are several practical problems in using this approach to determine whether or not a component has achieved shakedown. Consequently several direct methods which find the loads at the shakedown and ratchet boundaries have been developed in the past 3 decades. In general, lower bound methods are preferred for design and assessment methodologies. However, to date, the lower bound methods which have been proposed for shakedown and ratchet analysis have not been fully reliable and accurate. In this thesis a lower bound shakedown and ratchet method which is both reliable and accurate is proposed. Previously proposed elastic plastic lower bound ratchet methods are revisited and modified to understand the limitations in current methods. From this, Melan's theorem is reinterpreted in terms of plasticity modelling and shown to have the same form as a non-smooth multi yield surface plasticity model. A new shakedown method is then proposed based on the non-smooth multi yield surface plasticity model. The new shakedown method is extended using a two stage process to determine the ratchet boundary for cyclic loads in excess of the alternating plasticity boundary. Two simplified variants of the ratchet method are also proposed to decrease the computational expense of the proposed ratchet method. Through several common benchmark problems the proposed methods are shown to give excellent agreement with the current upper bound methods which have been demonstrated to be accurate. The flexibility of the shakedown method is demonstrated by extending the method to incorporate temperature dependent yield, hardening and simplified non-linear geometric effects.

In this dissertation, the Hohmann and bi-elliptic transfers are considered with the inclusion of a plane change. The evolution of critical limits which determine the transfer offering the lowest velocity requirement, previously defined for a co-planar analysis, is shown with the inclusion of a plane change. This has not been possible in previous work as analyses have been dependent on the intermediate orbit and numerical optimisation of the plane change distribution between impulses. It is shown that the critical limits found for the co-planar analysis reduce at different rates with increasing plane change and converge on a point where both transfers offer the same velocity requirement for a given final to initial orbit ratio and plane change. Between the two limits the Area Of Uncertainty (AOU) found for the co-planar analysis is shown to reduce to the convergence point which beyond, a second AOU emerges. A detailed analysis of these critical limits, determining when each transfer should be used is performed and a simple figure is presented which would allow a mission designer to select the fuel optimal transfer dependent on the final to initial orbit ratio and plane change only. The dissertation then introduces a novel orbit transfer using both high and low-thrust propulsion systems to accommodate the current development of platforms with this technology on-board. An analytical model is created which determines when the system offers a fuel mass saving compared to a single propulsion high-thrust only transfer. In addition to this, a critical limit analysis is performed which determines the limitations of analytical models based on a quasi-circular assumption. This analysis is developed into a numerical optimisation procedure which extends the application of the transfer to allow for eccentric orbits throughout the duration of the low-thrust phase. Case studies are presented which demonstrate substantial fuel mass savings compared to the single propulsion transfer: the largest fuel mass saving is found to be 27% of the spacecraft wet mass for a transfer from a Sun-Synchronous Orbit to a highly elliptical polar orbit.

The dissertation is devoted to analysis of unstable phenomena in the centrifugal blower. These phenomena are known to be a source of serious threat to safety of the machine and the piping system connected to it. The complexity of centrifugal impellers causes that instabilities therein can have various forms and locations and have been subject of interest for over 60 years. The dissertation consists of two main parts: one of them being a result of the experimental investigations, the other one describing an outcome of the numerical simulations. In the experimental part, three methods of data analysis were applied: static, dynamic and a novel approach referred to as the quasi-dynamic one. The blower was examined in two configurations of the outlet pipe corresponding to different outlet plenum volumes. The results were presented in a form of a performance curve, amplitude plots, scalograms, phase trajectories and spectral maps. The machine was found to operate in four different regimes , namely: the stable regime, the inlet recirculation, the transient phase and the deep surge. The inlet recirculation was identifed as the earliest instability present in the inlet zone. It was characterized by strong pressure jumps and a spectral structure of broadband noise. In the transient phase, the pressure oscillation amplitude grew by one order of magnitude and appeared in all points analyzed. In the deep surge, strong pressure oscillations appeared, with one frequency component close to the Helmholtz frequency. A new factor that can be used for real-time monitoring and early detection of unstable flow phenomena was developed as a result of signal phase trajectory analysis. The second part presents results of the transient numerical simulations conducted with FLUENT and then compared to the experimental data. The full impeller geometry was simulated together with the diffuser, the volute and large volumes of the inlet and outlet pipes. Computations were run at different combinations of the boundary conditions corresponding to different operational points. Simulations with a reduced plenum volume were also conducted analogously to the experimental study. The machine was found to operate in four working regimes: stable regime, pre-surge (impeller instability), presurge (inlet recirculation) and the deep surge. An overview on the machine performance was provided together with a detailed description of particular flow structures. The results were compared to the experimental data by means of, a performance curve, phase trajectories and frequency spectra and were found to be in reasonable agreement. The numerical study introduced a possibility of a detailed analysis of unstable flow structures such as the inlet recirculation, the impeller instability, a the deep surge cycle. The study confirmed that computational methods introduce an opportunity to understand the unstable flow structures in detail and provide a missing link between real flow phenomena and mathematical surge and stall models, which is essential for effective anti-surge protection.

Aerothermodynamic investigations of hypersonic re-entry vehicles provides crucial information to other key disciplines as structures and materials, assisting the development of efficient and lightweight thermal protection systems (TPS). Under the transitional flow regime, where chemical and thermal nonequilibrium are predominant, the most successful numerical method for such studies has been the direct simulation Monte Carlo (DSMC) numerical technique. In the present work, the solver dsmcFoam has been benchmarked against experimental, numerical, and theoretical data found in the open literature for inert and chemically reactive flows. The Quantum-Kinetic (QK) chemistry model with a full set of 19 chemical reactions has been implemented into the code and it proved to be essential in the correct prediction of the shock wave structure and heating flux to the vehicle's surface during the re-entry phase. Having implemented the QK chemistry model, the dsmcFoam solver was employed to investigate thermal protection system discontinuities. These TPS discontinuities, representative of panel-to-panel joints or the impact of micro meteorites/ice droplets, were modelled as a family of cavities with different length-to-depth ratios. The results showed that the cavity length has a significant impact on the flowfield structure and aerodynamic surface quantities distribution inside and around the cavities. In addition, for L/D = 5, the flow separates at the cavity upstream lip and attaches to the cavity bottom surface, representing a potentially catastrophic feature under rarefied gas conditions. Furthermore, the same phenomena is only observed in the continuum regime when L/D > 14.

Throughout the past decade, Polypropylene (PP) has been used as a favoured matrix material with different fibres being used as reinforcement in automotive composites. One of the most popular fibres for reinforcing PP matrix is Glass fibre (GF) due to its strength and impact resistance. More recently there has been interest expressed in further reducing the weight of the vehicle and the environmental impact of the vehicle's lifecycle. One plausible way to reduce the 'environmental impact' of the vehicle is by replacing GF with lighter and more 'environmentally friendly fibres'. In order to develop environmentally friendly composites to be used in the automotive industry, it is vital to have an extensive understanding of the fibre reinforcement properties and how it contributes to the overall composite performance. In this thesis, full characterisation of three environmentally friendly fibres, Polyethylene Terephthalate (PET), Flax and Sisal, were carried out and an insight into the interaction between the fibre under investigation and the PP matrix is given. Further to this, an investigation into the accuracy of using the fibre's diameter to calculate Natural fibre (NF) properties was carried out. due to NF being non-circular in cross section. Characterisation of the fibre properties was carried out using various techniques. Single fibre tensile tests were used to investigate strength and modulus of each fibre. It was found that using the actual cross-sectional area (CSA) of the Natural fibres gave more accurate results than assuming circularity of NF. The thermoelastic properties of the investigated fibres were determined through a combination of experimental measurements and micromechanical modelling. Dynamic mechanical thermal analysis and thermal mechanical analysis techniques were employed to characterise unidirectional fibre-polyester composites over a range of off-axis loading angles. The results were input into a number of micromechanical and semi-empirical models. It was found that the investigated fibres were highly anisotropic with the fibres longitudinal modulus being greater than the fibres transverse modulus over a range of temperatures. Single fibre pull out was used to investigate the interfacial shear strength (IFSS) between the fibre under consideration and PP matrix containing various percentages of maleic anhydride Polypropylene (MAPP). It was discovered that the IFSS increased when the MAPP content increased. The IFSS was found to be low even with 10% MAPP and this was revealed to be caused by the anisotropic nature of the fibre. Furthermore, it was found that assuming circularity of NF in determining IFSS gave less accuracy than using the actual perimeter of the NF. Therefore it is highly recommended that when possible the actual perimeter and area should be used to calculate NF properties. In order to use the fibres under consideration in composites, a method to obtain the composite fibre weight fraction has to be investigated. The traditional method cannot be used as the matrix is burnt off leaving the reinforcement fibres behind however the matrix and fibres being considered have similar thermal characteristics. The DSC was found to give reasonably accurate results for obtaining the weight fraction of composite. Along with the investigation into fibre weight fraction, the fibre morphology between the PP matrix and reinforcement fibres was examined. It was found that no transcrystallisation occurred in PP at a melt flow index (MFI) of 47.

The utilisation of space provides many opportunities to deliver pioneering innovations during the 21st century. One of these opportunities is the gossamer spacecraft, an emerging technology to achieve very low mass, large area and low stowage volume. Examples include large ultra-lightweight membrane reflectors and distributed tethered formations. Gossamer spacecraft offer the potential to deliver innovative new science and applications missions to aid our growing globalised societies: high-performing communications antennae, scientific telescopes and space-based solar power collectors. However, the ability to control such large structures in space is essential for their successful operation. To this aim, this thesis investigates a novel means to control large gossamer spacecraft by exploiting modulated solar radiation pressure (SRP), thus by modifying the nominal light pressure acting on the structure in space. Various concepts have been proposed in the past to control the attitude of a gossamer spacecraft, employing complex mechanical systems or thrusters. Furthermore, methods to control the surface shape of a large membrane reflector using, for example, piezoelectric actuators, are being developed. Since on-board control systems need to be high-performance, reliable and importantly lightweight, this thesis investigates the use of thin-film reflectivity control devices across the spacecraft surface. Controlling the reflectivity modulates the Sun's light pressure acting on a thin membrane thus controlling its shape. In addition, body forces and torques become available to control the attitude of such a large structure 'optically', without using traditional mechanical systems. The concept is demonstrated first by controlling a two-mass tethered formation in a Sun-centred orbit, showing that the spacecraft attitude can be stabilised around new equilibria created by controlling the surface reflectivity of the masses. Subsequently, the concept is applied to control the attitude of a large membrane reflector, which confirms the viability of reflectivity modulation by generating variable optical torques in the membrane plane. In particular, the nominal SRP forces are modified by introducing different surface reflectivity distributions across the membrane. It is shown that through these optical torques, the reflector can be steered, for example, to a Sunpointing attitude from an arbitrary initial displacement. The analysis also considers the variation of the SRP force magnitude with changing light incidence angle towards the Sun during the manoeuvre, thereby presenting solutions to a challenging attitude control problem. Furthermore, by adopting a highly-integrated multi-functional design approach, the concept of reflectivity modulation is also employed to control the surface shape of a large membrane reflector. First, the nominal (non-parabolic) deflection shapes due to uniform SRP across the surface are presented. Subsequently, a closed-form solut ion for the reflectivity function across the membrane required to create a true parabolic deflection shape is derived. In order to improve the quite large focal lengths of the deflected shapes that can be generated for a tensioned membrane, shape control of a slack suspended surface is also considered. The achievable (shorter) focal lengths support the feasibility of exploiting modulated SRP for controlled surface deflection. In summary, this thesis demonstrates the potential of using surface reflectivity modulation to control the attitude and morphology of large gossamer spacecraft without using complex mechanical systems or thrusters. Therefore, the concept of optical control represents a major step towards highly-integrated adaptive gossamer structures and supports the development of this promising key-technology to deliver advanced space applications.

Particle-based fluid simulations can be utilised to study phenomena ranging from galaxyscale, using smoothed particle hydrodynamics, plasma physics with particle-in-cell methods, aerospace re-entry problems, using direct simulation Monte Carlo, down to chemical, biological and fluid properties at the nanoscale with molecular dynamics and dissipative particle dynamics. The information generated by particle methods, such as molecular dynamics, is converted to macroscopic observables by means of statistical averaging. A significant drawback of nano- or micro-scale modelling is the substantial noise associated with particle techniques, which disturbs the analysis of the results. The uncertainty in the mean of the ensemble is due to fluctuations caused e.g. by additional forcing terms (thermostats). Extracting the genuine information from indirect, noisy measurements is analogous to solving the ill-posed statistical inverse problem, where the object of interest is not easily accessible. The presence of noise in the data can be reduced by averaging over a large number of samples, but the computational intensity of the simulations would then be substantially increased. In order to improve the efficiency of estimating the unknown structure from the disturbed observations, a number of decomposition techniques have been applied, including: proper orthogonal decomposition, singular spectrum analysis, random QR de-noising, wavelet transform, and empirical mode decomposition. In the present work, the strengths and weaknesses of each approach, and their extensions to solving statistical inverse problems for particle simulations, are evaluated. Furthermore, we propose several novel combinations of these methods, that have the capability to improve the signal-to-noise ratio and reduce the computational cost.

In product design environments where safety is critical, industries can often be reluctant to adopt emerging technologies. This is the case for many endovascular device manufacturers, where advanced non-linear Finite Element Analysis methods are still to be incorporated into their product design functions. To encourage adoption this thesis explores the merits and limitations of newly developed constitutive models as implemented into leading numerical analysis software suites. Specific attention is given to the Holzapfel soft tissue and Auricchio Nitinol constitutive models. Limitations and merits are explored through the development of an integrated modelling framework. This framework was constructed around a case study for which Vascutek's AnacondaTM stent graft system was chosen. In its final iteration the framework took the form of a series of python scripts which could be imported into Abaqus 6.11 to generate a variety of FE studies. For the development of the python scripts, a generalised artery material model was required for which a vascular characterisation programme was initiated. This programme mechanically characterised three human abdominal aortas using uniaxial and biaxial characterisation methods. By deploying Anaconda proximal ring devices within cadaver specimens and recording deformations, the programme produced detailed out-of-plane deformation data for vascular tissue. Where possible, results obtained were combined with those found in the academic literature. From this, comprehensive data sets were created from which a generalised mechanical description for the abdominal aorta was proposed. In this study, the accuracy of such a generalised arterial description was also explored for the first time. The work herein present a range of advanced Finite Element modelling techniques. These techniques range from a bespoke three layered abdominal aorta arterial model to novel wire bundle modelling methods. A comprehensive study into Nitinol's load path dependency effects is also presented. The study correlated the effect as captured within experimental results against those predicted by Finite Element Analysis. Findings of this study highlight limitations within the Auricchio constitutive model as implemented into Abaqus 6.11 and emphasise the need to model the entire loading regime when conducting Finite Element studies. The extensive validation procedures used to determine the accuracy of the integrated modelling framework are presented. It is shown for the case study chosen, that the framework is capable of capturing a range of complex device-artery interactions. It is also shown that the model is capable of predicting deformations within 35% of that measured during device deployment and cadaver specimen pressure-inflation experiments. It is a conclusion of this thesis, that from an understanding of the limitations involved, current FE technologies could be feasibly integrated into product design methodologies through the development of robust, computationally efficient design tools.