This thesis reports on the aerodynamic and structural study carried out on flapping wings and flapping vehicles. Theoretical and experimental investigation of aerodynamic forces acting on flapping wings in simple harmonic oscillations is undertaken in order to help conduct and optimize the aerodynamic and structural design of flapping wing vehicles. The research is focused on the large scale ornithopter design of similar size and configuration to a hang glider. By means of Theodorsen’s theory the aerodynamic forces on a thin aerofoil subject to heaving, pitching, and combined heaving and pitching motions are carefully studied. The analytical method is then employed to calculate the lift acting on the rigid flat plate undergoing small simple harmonic oscillations at different airspeeds and frequencies. The theoretical calculations are compared with experimental results which show reasonably good agreement. However experimental study shows that the wing frame deformation induces extra aerodynamic forces which can change the overall wing performance. Hence an experimental investigation focusing on wing flexibility effect on aerodynamic forces is also carried out. Three wings of similar planform geometry but slightly different degree of flexibility are manufactured for wind tunnel testing. Test results show that the wing deformation not only affects the aerodynamic forces but also the required power for various wing flapping motions. By understanding the aerodynamic performance of flapping wings from both theoretical and experimental studies the preliminary design of large scale ornithopter is carried out based on a hang glider prototype. Theoretical and experimental studies are carried out to validate aerodynamically the loading acting on the wing and finite element analysis is carried out to evaluate the structural strength. In addition, DeLaurier’s method is employed to calculate the aerodynamic forces of flapping wings by taking into account of the wing aspect ratio. The test results show good agreement with the theoretical calculation by DeLaurier’s method. However the FEA results indicate structural failure based on the original calculation by assuming the wing is completely rigid. Modification of aerodynamic modelling is carried out to reassess the structural strength by taking into account of the wing deformation and possible effect due to large angle of attack which shows a much more reasonable stress distribution on the entire wing structure without failure. Furthermore three wing planform and structural modifications are carried out to improve the aerodynamic performance of the flapping wing. Finally the folding wing design case is selected as the optimal design which produces the highest overall positive lift and a variable geometric system is employed to control the folding motion of the wing.

This thesis reports the development of a method for predicting the fretting fatigue life of a system which takes into consideration the material removed as a result of fretting wear. The first implementation is based on a critical plane, multiaxial fatigue model and a damage accumulation framework. The model is applied to both ‘cylinder on flat’ and ‘rounded edge punch on flat’ geometries, for which experimental data from the literature is used for comparison. The method is able to predict a number of key experimentally observed phenomena, which existing approaches are unable to do. The dependence of fretting fatigue life on slip amplitude is captured demonstrating a critical range of slip amplitudes, relating to the partial slip regime, for which a minimum in life is predicted. The method is also shown to predict the occurrence of cracking at specific locations in the slip region. The method indicates that these phenomena are dependent on the relative rates of wear and fatigue damage occurring across the contact. The second implementation treats the nucleation and propagation fatigue phases separately. The fatigue model adopted above is reformulated to serve as a nucleation model, whilst the crack propagation phase is based on a fracture mechanics perspective. The method is used to study the effect of wear on both the propagation and nucleation aspects of fatigue. The method is also employed to investigate the role of fretting wear in fretting fatigue crack arrest.

Creep failure of the weld structure in P91 steel components in high temperature power plant applications is often a key factor limiting the lifetime of the components. Whilst creep failure in weld heat-affected zone (HAZ) regions has been studied widely, the creep properties of the weld metal itself have been less well documented. In this work, the creep response of P91 weld metal in isolation was investigated in terms of microstructural evolution and mechanical properties. The microstructural examination of P91 multi-pass weld metal revealed a typical weld metal structure including columnar regions and refined regions. The columnar region exhibited high hardness whilst the refined region exhibited lower hardness. The anisotropic creep behaviour of P91 weld metal was observed in creep tests of both longitudinal and transverse specimens at 650ºC and various stress levels. This behaviour can be correlated with the microstructural anisotropy observed, where longitudinal specimens with banded columnar regions and refined regions parallel to the stress axis had longer creep life than transverse specimens with overlapped typical-shape beads. Longitudinal weld specimens showed higher strain to failure than transverse specimens. The microstructural investigation of creep tested P91 weld metal revealed two primary modes of creep fractures. In addition to creep fractures along columnar grain boundaries (typical of weld metal creep failure), creep fractures were also found along creep-weak white-bands which had formed at the inter-bead boundaries. The white-band regions consisted of material where the M23C6 carbides had dissolved during creep testing; the loss of carbides had allowed recrystallisation of the martensitic structure to ferrite and consequently this material was much softer than the bulk weld metal. The element mapping over the weld metal by laser-induced breakdown spectroscopy (LIBS) demonstrated that there was significant inhomogeneity in the distribution of certain elements, most significantly, chromium, manganese and molybdenum. This inhomogeneity resulted in strong activity gradients in carbon (even though the carbon concentration was homogeneous following welding) resulting in carbon loss from the alloy-depleted regions, the associated dissolution of carbides and the recrystallisation that accompanied this, and thus the poor mechanical properties which resulted in creep failure. The inhomogeneity in the distribution of certain alloying elements can be partially attributed to the solute partition of alloying elements during weld solidification which has been confirmed with examination of simulation P91 TIG welds. However, the homogeneity of weld metal in this case required mixing of a base steel (the core rod in the weld consumable) and particles of various ferro-alloys (delivered into the weld pool from the flux). It is argued that poor mixing in the stagnant layer (unmixed zone) at the solid-liquid interface during weld solidification also makes a significant contribution to the formation of alloy-depleted regions. The formation of white-bands has been modelled using Thermo-Calc based on the understanding of the formation mechanism involving solute partition and subsequent carbon diffusion out of the alloy-depleted region. A good correlation to experimental results has been shown in the prediction of limiting carbon concentration and M23C6 carbide content in white-bands. In addition, it was also suggested that depletion of carbides and carbon are strongly linked and that depletion of alloying elements only above a critical value will result in total carbide loss and thus recrystallisation into a white-band.

Rowing is an intensive, all-body sport, where bad technique can lead to injury. Crew cohesion, particularly timing, is vital to the performance of the boat. The coaching process, and injury prevention, could be enhanced if data relating to the movement of the oarsmen could be collected, without hindrance to the oarsmen, during on-water training. Literature until recently has concentrated upon boat-centric measurement. Advances in wireless technology have made feasible the collection of data from multiple physically separate sites, including on-body. After analysis of candidate radio standards, a Zigbee wireless Body Sensor Network (BSN) was designed and developed to synchronously collect data from several sensors across the wireless BSN. By synchronising sensor nodes via scheduled synchronising messages from the central coordinating node, synchronisation within 0.79msec ±0.39ms was achieved. Minimisation of the on-time of the sensor node radios currently extends the battery life by a factor of 5. Acceleration and muscle activity data collected using the wireless BSN was compared to data synchronously collected using proven motion analysis techniques to validate the system. Synchronous muscle activity data was collected via the wireless BSN from several muscles during both land-based and on-water rowing and the results compared. The system was proven to facilitate the identification of bad rowing technique, as well as differences in muscle recruitment between land- and water-based rowing. Data collection from a rowing crew was also demonstrated, and their muscle activity and inter-crew timing analysed. With an additional sensor node upon the boat, it is possible to correlate acceleration and muscle activity from the oarsman with acceleration of the boat itself. A novel, power-optimised wireless sensor network has been designed and demonstrated to facilitate on-water rowing monitoring that can be extended beyond single oarsman measurements to analyse the interaction and cohesion of a crew and their impact upon boat performance.

The thesis examines the behaviour and design of unbraced steel frames with rigid and semi-rigid connections. An approximate hand method has been developed for the calculation of second-order elasto-plastic failure loads for single storey frames. Studies were carried out to propose limiting values of frame parameters, so that the first-order plastic theory can be used as a safe design method for single storey frames. A second-order e1asto-plastic computer analysis program has been developed. The program takes into account the main non-linear phenomena that occur in real frame structures. These include geometric non-linearity, material non-linearity and, the most important of all, the non-linear connection behaviour. The program can deal with any non-linear moment-rotation characteristic resulting from test data or analytical curves. The analysis program has then been used to check the adequacy of the wind connection design method. The program for static load collapse was further developed to investigate the response of the structure to cyclic loading. The program was used to investigate the incremental collapse behaviour, including alternating plasticity and shakedown of multistorey frames with rigid and semi-rigid connections.

The world population regardless of location and development stage needs energy and water. According to the United Nations, the present world population stands at about 6.7 billion with an average annual growth rate of 1.3%. Population increase calls for increased allocation of water for domestic use, agriculture and industrial use. The increased water allocation among different sectors has always resulted in conflicts among users, and stress on freshwater environment. Therefore it is essential that water resources be developed in a sustainable manner to accommodate future generations to meet their water needs. In recent years studies regarding stress on water resources due to population increase have always been done without considering the effect of climate change while studies on the effect of climate change on river flows have always ignored the effect of population increase. An assessment of the sustainability of the water resources primarily of the Central and Northern highland river basins of Malawi is presented in this thesis based on basin hydrology, human health, environment and climate change. A complete hydrological data set is not readily available in developing countries like Malawi. That being the case, a method of estimating missing data in hydrological data records has been presented. Climate change predictions have been done based on United Kingdom (UK) Meteorological Office Hadley Centre HadCM3 experiments. All the river basins in the Central and Northern part of Malawi drain into Lake Malawi. Lake Malawi plays a major role in the provision of energy for the country and water supply to the southern part of Malawi through its outlet Shire river. Planning of alternative water resources schemes on river basins in the northern part of Malawi needs an assessment of the hydrological behaviour of the lake. In view of this, the report further explores the sustainability of water levels of Lake Malawi based on generated climate scenarios. A method of extending river flow records based on climate scenario is presented. In the proposed method a simple rainfall runoff model of Linear Perturbation Model has been used to extend flow records with inputs from HadCM3 experiments. The results showed a good correlation between predicted and observed series. Climate change prediction downscaled from HadCM3 general circulation models using statistical techniques were used to create 25 year river flow scenarios from 2001 to 2100. The thesis further reports a method of extending evaporation data based on climate change prediction since evaporation plays a major role in the development of irrigation. The future evaporation scenarios have been incorporated in the water balance model of Lake Malawi for the assessment of sustainability and future water levels of the lake. A method of formulating water resources sustainability index through the integration of knowledge from hydrology, human health and environment is presented. The Water Sustainability Index has been developed as a tool for assessment of multipurpose water resources development comparing one river basin with another in a sustainable manner. In conclusion the method proposed in this thesis can be used as a tool for assessing the strategic sustainability of water resources development as planned under the Malawi National Water Development Programme (MNWDP) phase II. The goal of MNWDP II is to develop a water resources investment strategy for Malawi by looking at the current available water resources and the impact of developing these water resources. Although it is hoped that the methods can be of benefit under MNWDP II, investigations into other techniques would be beneficial.

The major part of the work presented in this thesis is an investigation of the strut and tie method for designing 2-D in-plane, reinforced concrete structures. Two important issues relating to this method are addressed. Firstly, the issue of visualising an appropriate strut and tie model is dealt with. In many situations it may be difficult to visualise an appropriate model for a given structural system. Here, a convenient method of visualising strut and tie models is presented. Using elastic finite element analysis, low stressed parts of a structure are removed in a step by step process until the main stress paths, which represent the ties and struts, are defined. The second important issue to be addressed is that of serviceability of the designed structure because the strut and tie model naturally represents a great departure from the elastic stress distribution. Since the strut-tie model is used to design for the ultimate load situation, it is necessary to assess the suitability of the same model in relation to serviceability characteristics of the resulting design. It is important that ductility of the structure should be maintained at ultimate loads while avoiding excessive deflections and cracking at service loads. A wide variety of structures were designed, and to assess the performance of each design, non-linear finite element analysis was used. Verification of some of the numerical results was carried out through physical testing in the laboratory which also allowed the serviceability behaviour of the structures to be assessed. The test program comprised of three corbel joints and two frame corner joints. It was concluded that design from the strut and tie method can produce adequate performance both at service and ultimate loads. In terms of ultimate load prediction, the strut-tie method can produce results of comparable accuracy to non-linear finite element analysis.

Historically the inter-flood period has been disregarded from investigations as it was deemed that the stability of non cohesive beds could only be altered by above threshold flows capable of sediment transport. However, this is at odds with more recent ‘stress history’ data which provides unequivocal evidence that entrainment thresholds can be delayed to higher shear stresses after being subjected to longer periods of sub threshold flows. The magnitude of this effect appears related to the surface grain size distribution and relative grain size effects, whilst the specific mechanics associated to generating a more resistant bed under sub-threshold flows are merely speculated upon. The aim of the present thesis is therefore to provide a comprehensive and quantitative data set on stress history that specifically address comparative grade effects and provides a detailed mechanistic understanding of the processes responsible for generating a more resistant bed configuration under sub threshold flows. Using a range of grain size distributions, a series of flume based experiments assess two main aspects of the stress history process. Firstly the effects of grain size distribution on the relationship between stress history duration and entrainment threshold is quantified. This is split into two sets of experiments based on the duration of the applied sub threshold antecedent flow, prescribed as 50% of the critical shear stress ( ) of the median grain size (D50). The antecedent durations of first set of experiment ranged from 0 to 60 minutes, whilst the antecedent duration of the second set of experiments ranged from 0 to 960 minutes. To ascertain the effect of the antecedent period on critical entrainment threshold and transported bedload, each experiment is concluded with a stability test composed of incrementally increased flow discharges until critical threshold conditions were reached. Secondly, aspect of stress history investigated uses high resolution laser scanning to assess bed topography and particle repositioning in order to ascertain the granular mechanics underpinning the stability process. The bed is scanned before and after the application of the applied antecedent flow with changes to bed surface structure described using Digital Elevation Models (DEM’s), statistical analysis and 1D and 2D semi-variograms to analyse scaling behaviour. In all experiments, increasing the antecedent flow duration significantly increases river bed stability in that the critical shear stress increases by up to 25% where uniform beds are more responsive to antecedency than bimodal beds. Laser based analysis reveals that vertical settlement, localised changes to bed roughness, pockets of more pronounced development of hiding effects, and particle repositioning are all mechanisms by which the bed reorganises under an applied sub threshold flow. However, the different bed grain size distributions cause significant differences in the importance of each mechanism in determining the magnitude of stress history induced bed stability.

Lumbar back pain has considerable socio-economical impacts, motivating a recently increasing interest from the research community. Yet, mechanisms triggering pain are not fully understood and this considerably hinders the development of efficient treatments and therapies. The objective of this thesis is to participate to the general understanding of the biomechanics of the spine through the development of computational strategies for the intervertebral disc. The intervertebral disc is a complex structure mainly comprised of the nucleus pulposus and the annulus fibrosus. The nucleus pulposus is the gelatinous core of the disc, which consists of a charged and hydrated extra-cellular matrix and an ionised interstitial fluid. It is enclosed in the annulus fibrosus which is formed by concentric layers of aligned collagen fibre sheets, oriented in an alternating fashion. A biphasic swelling model has been derived using mixture theory for soft, hydrated and charged tissues in order to capture the salient characteristics of the disc's behaviour. The model fully couples the solid matrix under finite deformations with the ionised interstitial fluid. The nucleus is assumed to behave isotropically while the effects of the collagen fibres in the annulus fibrosus are accounted for with a transversely isotropic model. The fixed negative charges of the proteoglycans, which induce an osmotic pressure responsible for the swelling capabilities of the disc, are constitutively modelled under the simplifying Lanir hypothesis. A Newton-Raphson solver was specifically built to solve the resulting nonlinear system of equations, together with a verification procedure to ensure successful implementation of the code. This was first reduced to the one dimensional case in order to demonstrate the appropriateness of the biphasic swelling model. The three dimensional model exhibited numerical instabilities, manifesting in the form of non-physical oscillations in the pressure field near boundaries, when loads and free-draining boundary conditions are simultaneously applied. As an alternative to considerable mesh refinement, these spurious instabilities have been addressed using a Galerkin Least-Square formulation, which has been extended for finite deformations. The performance and limitations of the GLS framework, which drastically reduces the pressure discrepancies and prevents the oscillations from propagating through the continuum, are demonstrated on numerical examples. Finally, the current state of the model's development is assessed, and recommendations for further improvements are proposed.

There has always been an ambition from Structural Engineers to design structures which are as efficient as possible, yet meet current design requirements. In the modern era, this process has relied heavily on the use of computational packages to carry out detailed analyses. However, for the design requirements to be met, the materials being used must be accurately understood and their behaviour must be accurately captured. In materials such as concrete, this understanding can be obtained by carrying out detailed analyses of the material at the mesoscale. At this scale, capturing the behaviour of concrete can be accomplished by recognising three distinct phases. First, the heterogeneities themselves, which are stiff particles usually made of crushed rock; second, we have the cement matrix which surrounds the heterogeneities and third, we have the weak interfacial transition zone (ITZ) between the two. By accurately describing the behaviour and interaction of these three phases, the complex behaviour of concrete can be captured. This thesis brings together several novel contributions in order to create a computational modelling framework for modelling fracture in concrete at the mesoscale in three dimensions. All cracks are discrete in nature and restricted to element interfaces. As fracture can generally be considered to be a stress driven problem, an accurate description of the stress state in the model is essential. To achieve this, hybrid-Trefftz stress elements are used for the bulk elements in the mesh. The hybrid-Trefftz stress elements are characterised by their separate approximations of the stresses over the element domain and the displacements over the element boundary, allowing for a much higher order of approximation for the stresses to be utilised. The discrete cracks are modelled using continuous interface elements to allow the non-linear behaviour of concrete to be captured. The initiation and evolution of cracks was modelled using a plasticity model for the interface elements. This model followed the work of Winnicki and its implementation is presented in this thesis. Unlike the conventional method of using interface elements, where they are present in the mesh from the start of the analysis, in this work, interface elements are inserted dynamically as and when the material yield criterion is violated. A crack insertion methodology is presented for the inclusion of discrete cracks along element boundaries in 3D. The procedure takes advantage of properties of the hybrid-Trefftz stress elements to aid in its implementation. In particular, due to displacement continuity being enforced in the weak sense, faces of the same tetrahedron can be considered to be independent from each other but still produce statically admissible results. Results are presented showing that this methodology works well for different topological scenarios. To generate an accurate geometric representation of the mesostructure, a method called the Maximum Level Set method was developed. For a given geometry, this method generated the mesostructure by placing aggregates within the domain at a maximum distance from both the boundary of the specimen and existing particles in a sequential manner. A Level Set function is used to calculate these distances and is updated as each sequential particle was added. This procedure is compared to the standard Random Sequential Addition method, used throughout the literature, and shows favourable results both in terms of computational cost and the suitability of the geometry for use in the creation of finite element meshes. Finally, the application of the overall framework to realistic problems is presented. The procedure for carrying out an entire load step using a standard Newton-Rhapson procedure is outlined and the determination of the parameters used in the constitutive law is presented. Preliminary results are that demonstrated the performance of the overall framework. In these results, the softening of concrete due to fracture was captured.