The mechanical behaviour of cemented granular materials has been an important topic in geotechnical engineering since decades. Historically, most research on ce-mented granular materials has been performed at relatively low confining pressures. Problems relating to cemented granular materials at high-pressure are still not fully understood. However, understanding of the behaviour of cemented granular materials at high-pressure is highly important in deep foundations, particularly for offshore piling, deep mine shafts, high earth dams, and oil-bearing strata. To address the problem, artificially cemented sand specimens with varying degrees of cement contents and initial relative densities were prepared in the laboratory to simulate the natural cementation characteristics. A high-pressure triaxial compression apparatus was utilized to investigate the effect of initial relative density, cement content, and confining pressure on the mechanical behaviour of artificially cemented sand. High-pressure tests including isotropic compression, drained and undrained triaxial shearing and microscopic studies of the materials were carried out on the artificially cemented sand specimens in the Nottingham Centre for Geomechanics laboratory at the University of Nottingham. Complexities with artificial specimen preparation and with high-pressure testing were identified and tackled. The experimental results indicate that there is significant effect of cement contents and confining pressures on the mechanical behaviour of cemented materials. Particularly, these effects were notified on isotropic compression, peaks strength, strength parameters, shear banding, particle crushing, yielding, and stress-dilatancy relationships. For example, reduction in compressibility, reduction in particle crushing and shift in normal compression line by the increase in cement content of the material during isotropic compression were significant. Progressive suppression in the dilation of cemented sand by the gradual increase in confining pressure, increase in the peak strength, developing of curved failure envelope, increase in the yield strength and formation of conjugate shear banding during progressive failure during triaxial compression were worth noticeable. This concludes that the significance of high-pressure and cement content cannot be ignored in the design considerations. However, more research needs to be carried out at further high pressures in order to see the convergence of failure envelopes and the initiation of bond breakage and particle crushing to give a reasonable design framework for foundations.

This thesis concerns a study of the three-body abrasive wear behaviour of two groups of materials with different abrasive particles using the Dry Sand Rubber Wheel (DSRW) test method. This investigation can be divided into three sections: In the first section, the abrasion of a range of steels with an ash from a biomass power station was compared with that observed for abrasion with a conventional silica abrasive. It was seen that the wear rate of the steels when abraded with silica increased in proportion to the applied load and decreased with the hardness of the steel. However, the bottom-ash was more friable than the silica abrasive, and as such, significantly more abrasive crushing was observed during the tests with the bottom-ash abrasive. It is proposed that the wear is dominated by abrasion by the larger particles in the distribution, and that damage is limited by the maximum load which the particles can sustain before failing. In the second section, the motion of particles in the DSRW test with silica abrasive against a range of steels, as a function of applied load and the hardness of the steels was studied. The results showed that particle rolling through the contact is favoured by low applied loads and low testpiece hardness whereas particle sliding through the contact is favoured by high applied loads and high testpiece hardness. A model was proposed to provide an analysis of the motion of particles in the DSRW test. The effect of hardness on particle rotation is well predicted by the model, but the effect of the applied load on particle motion observed experimentally is opposite to that which is predicted by the model. The shortcomings of the model are discussed, and the model has been qualitatively modified to account for this discrepancy. In the third section, five different WC-metal cermet powders were deposited as coatings by HVOF thermal spraying. These were a WC-nickel alloy, a WC-iron alloy and three types of WC-Co powders with different carbide grain sizes. Characterisation of the coatings showed decomposition of WC during spray process for all the coatings. The results show different solubilities of W and C in the binders and different precipitation characteristics. DSRW tests were performed to assess the wear resistance of the coatings with silica and alumina abrasives. It was found that the coatings had different wear rates and mechanisms when abraded with silica compared with alumina. The differences in the wear behaviour of the coatings are due to the differences in powder characteristics, the extent of reaction and decarburisation during spraying, and the subsequent development of the microstructure in the coating during splat solidification at high cooling rates.

Ni-based superalloy weld overlays are widely used in electricity generating plants to significantly reduce high temperature corrosion problems of ferritic steel components under service conditions. Welding a nickel alloy similar to IN625 onto the outer bore of a 2.25Cr1Mo steel tube enhances its service life as a superheater tube in the highly corrosive environment of a Waste-to-Energy boiler. For the purposes of studying the effects of high temperature service on the microstructure of this laid tube with a weld overly, a series of thermal exposure tests at 650˚C was performed for different times from 1 day up to 128 days. The microstructural evolution was studied using a combination of analytical techniques along with changes in hardness profile across the interface. Changes in the microstructure were examined using OM, SEM, EPMA, EBSD and FIB-STEM. Hardness survey results indicate significant changes in the interfacial region during ageing. Formation of a soft zone ~300 μm wide and its subsequent re-hardening was observed in the steel side while the hardness of the bulk steel remained unchanged. Development of a hard band ~30 μm wide adjacent to the interface in the weld overlay region and hardening of the bulk overlay material occurs in the early stages of ageing and remained unchanged with ageing time. Thermodynamic calculations were performed using Thermo-Calc software and TCFE6 and TTNI7 databases to aid interpretation of experimental data. Microstructural evolution in the steel region is related to the carbide transformation process and carbon migration. In the bulk of the steel tube, the bimodal microstructure is stable and in the tempered martensite/bainite areas, the initial M3C transform to M23C6 through a series of metastable carbides while in the ferrite grains, M2C carbide precipitates and transforms directly to the equilibrium carbide. The main process for supply of carbon atoms is transformation of carbides and the rates of transformation are related to the as-welded microstructure. On the steel side of interfacial region where bainite was formed after welding, the stability of metastable carbides is related to the carbon content. Long term ageing causes Mo replenishment in the coexisting ferrite and fine grains have formed in this region following long term ageing. Experimental observations confirmed that a network of alloyed M23C6 carbide precipitates was formed at the interface in the steel side which are believed to interrupt the carbon migration across the interface. Moreover, there is a carbide precipitation region within ~100 μm from the interface in the weld overlay. Beyond this region intermetallic phases like Mo-rich μ and Nb-rich δ were formed in the interdendritic regions and along the grain boundaries. Hard band formation is related to precipitation of the σ phase.

Several well established techniques of soil stabilisation and soil reinforcement are available to improve properties of geotechnical materials. However, the addition of fibre into soils has its unique potential as a reinforcing agent. This is because a friction between fibre and soil particles increases the bonding between the particles of soils and this can improve the plasticity, stress-strain behaviour and failure characteristics of both cemented and uncemented soils. It also reduces the brittleness of the cemented sand. Numerous experiments on fibre-reinforced granular materials have been carried out by several researchers. However, the behaviour of fibre-reinforced cemented granular soils has not been fully understood yet. Furthermore, most experimental studies of fibre reinforced cemented materials have been carried out at relatively low confining pressures. As a result, more experiments are still needed to understand complicated behaviour of soil-cement-fibre composite materials. The main objective of this thesis is to analyse the behaviour of fibre reinforced cemented sand under wide range of confining pressures. For this GDS high pressure triaxial cell apparatus and Bishop and Wesley conventional triaxial cell apparatus have been used to carry out the tests at wide range of confining pressures from 50kPa to 20MPa. Drained and undrained tests have been carried out on polypropylene fibre reinforced sand with and without the addition of cement. Samples with varying fibre and cement content were prepared by the method of undercompaction and were cured for 28 days prior to testing. The experimental results indicate that there is a significant effect from the addition of fibre and/or cement contents and confining pressures on the mechanical behaviour of Portaway sand. Particularly, these effects were noted in drained and undrained triaxial tests, particularly peak strength, strength parameters, shear banding, particle crushing, yielding, and stress-dilatancy relationships. The addition of fibres increases the peak, yield, and ultimate strengths. Increase in confining pressure also increases the strength but the individual effect of the addition of fibres was more pronounced at low confining pressures. Progressive suppression in the dilation by the gradual increase in confining pressures as well as an increase in dilation with the addition of fibres during triaxial compression was also worth noticeable. Although, no noticeable affect was observed in isotropic compression due to the addition of fibre in both cemented and uncemented sand. An extensive series of tests were carried out but due to time constraint only one type and length of fibre was used. Therefore, more research needs to be carried out at different fibre lengths and types in order to see that whether these change the behaviour observed in this research.

GPS precise point positioning (PPP) has been used in many scientific and commercial applications due to its high computational efficiency, no need for any synchronous measurements from a nearby reference receiver and homogeneous positioning quality on a global scale. However, these merits are devalued significantly by unresolved ambiguities and slow convergences of PPP. Therefore, this thesis aims at improving PPP’s performance by resolving ambiguities for a single receiver and accelerating the convergences to ambiguity-fixed solutions in order to achieve a centimeter-level positioning accuracy with only a few seconds of measurements. In recent years, ambiguity resolution for PPP has been developed by separating fractional cycle biases (FCBs) from single-receiver ambiguity estimates. One method is to estimate FCBs by averaging the fractional parts of single-difference ambiguity estimates between satellites, and the other is to assimilate FCBs into clocks by fixing undifferenced ambiguities to integers in advance. The first method suffers from a large number of redundant satellite-pair FCBs and unnecessary 15-minute narrow-lane FCBs. Therefore, this thesis suggests undifferenced FCBs and one narrow-lane FCB per satellite-pair pass over a regional area in order to reduce the size of FCB products and achieve comparable positioning quality with that of the original method. Typical tests show that ambiguity resolution dramatically reduces the RMS of differences between hourly and daily position estimates from 3.8, 1.5 and 2.8 cm in ambiguity-float solutions to 0.5, 0.5 and 1.4 cm in ambiguity-fixed solutions for the East, North and Up components, respectively. Likewise, the RMS for real-time position estimates are reduced drastically from 13.7, 7.1 and 11.4 cm to 0.8, 0.9 and 2.5 cm. Of particular note, this improvement can be achieved even at remote receivers which are over a few thousand kilometers from the reference receivers that are used to estimate FCBs. Furthermore, this thesis improves the accuracy of narrow-lane FCB estimates with integer constraints from double-difference ambiguities. In a one-year global network analysis, the RMS of differences for the East component between the daily and IGS weekly estimates is reduced from 2.6 mm in the solutions based on original FCBs to 2.2 mm in the solutions based on improved FCBs. Although small, this improvement is significant and critical to some geophysical studies, such as tectonic motions, sea level rise, and post-glacial rebound. More importantly, for the first time, this thesis provides a theoretical proof for the equivalence between the ambiguity-fixed position estimates derived from the aforementioned two methods. This equivalence is then empirically verified by the overall minimal discrepancies of the positioning qualities between the two methods. However, these discrepancies manifest a distribution of geographical pattern, i.e. the largest discrepancies correspond to sparse networks of reference receivers. This comparison can provide valuable reference for the GPS community to choose an appropriate method for their PPP ambiguity resolution. As the foremost contribution, an innovative method is originally developed in this thesis in order to effectively re-converge to ambiguity-fixed solutions with only a few seconds of measurements. Specifically, ionospheric delays at all ambiguity-fixed epochs are estimated and then predicted precisely to succeeding epochs in the case of re-convergences. The predicted ionospheric delays are first used to correct wide-lane measurements in order to rapidly resolve wide-lane ambiguities. The resulting ionosphere-corrected and ambiguity-fixed wide-lane measurements are then used to tightly constrain narrow-lane measurements and thus speed up narrow-lane ambiguity resolution significantly. As a result, the practicability of real-time PPP is greatly improved by eliminating the unrealistic requirement of a continuous open sky view in most PPP applications. Typical tests illustrate that over 90% of re-convergences can be achieved within five epochs of 1-Hz measurements, rather than the conventional 20 minutes, even if the latency for the predicted ionospheric delays is up to 180 s. Moreover, for a van-borne receiver moving in a GPS-adverse environment where satellite number decreases significantly and cycle slips occur frequently, only when the above rapid re-convergence technique is applied can the rate of ambiguity-fixed epochs dramatically rise from 7.7% to 93.6% of all epochs. Finally, a precise positioning service for the next-generation global RTK, characterized by both global coverage and regional augmentation, is originally proposed in this thesis based on real-time PPP enhanced by rapid (re-)convergences to ambiguity-fixed solutions. It is illustrated that a globally distributed network of 38 stations can ensure that the ambiguity-fixed epochs account for over 95% of all epochs.

Using an asymptotic series approach, a thick shell theory is proposed for doubly curved sheIls with variable thickness. This theory includes the effects of transverse shear stresses and rotatory inertia. The displacement functions are designed to give non-zero transverse shear stresses internal to the shell, which satisfy the stress-free boundary conditions on the upper and lower surface. Use of the stress-free conditions makes the displacement functions, which vary through the thickness of the shell, dependent only on the middle surface displacements. This theory is applied to the twisted plate. A similar approach is applied to the cylindrical shell, but the effects of transverse normal stress are also included. The theory is applied to the problem of free vibrations of shells clamped along one edge with the other three edges free. The results obtained are compared with practical and theoretical results of other researchers, and with those obtained from thin shell theory. The twisted plate results show the answers that are expected from a thick shell theory, in that it predicts lower frequencies than thin shell theory for modes in which the wavelength/ thickness ratio is less than ten. The results for the cylindrical shell show that the inclusion of transverse normal stress to the order assumed is not warranted. The numerical techniques used for the solution of the free vibration problem are based on variational methods in which the Hamiltonian for the shell is minimised, subject to the constraints of the displacement boundary conditions.

Optimised design of textile composite structures based on computer simulation techniques requires an understanding of the deformation behaviour during forming of 3-dimensional double-curvature components. Purely predictive material models are highly desirable to facilitate an optimised design scheme and to significantly reduce time and cost at the design stage, such as experimental characterisation. In-plane shear and out-of-plane bending are usually thought to be the key forming mechanisms. Therefore, this thesis is concerned with studies of the shear and bending behaviour by experimental characterisation and theoretical modelling. Micromechanical interaction between fibre and matrix offers fundamental understanding of deformation mechanisms at the micro-scale level, leading to development of composite viscosity models, as input to shear and bending models. The composite viscosity models were developed based on rheological behaviour during movement of fibres, and validation was performed using experimental results collected from the literature. A novel characterisation method for measuring the bending behaviour, by means of a large-displacement buckling test, was attempted due to some significant advantages over other methods. Development of a bending model was also undertaken for unidirectional composites but experimental validation suggests further study may be required for woven composites. The shear behaviour was characterised using a picture frame test for viscous polymer composites. To obtain reliable experimental data, some efforts of improving the characterisation method were made. The experimental results were then used to validate a shear model, suggesting that further improvement is required, in terms of weave patterns, rate and temperature dependence.

To date, the development of an amorphous alloy has concentrated entirely on a multi-component Fe-based amorphous alloy (Fe-Cr-B based alloy systems) containing boron and/or carbon. The major driving force behind these developments lies in the desire to produce an amorphous coating which exhibits both excellent thermal stability and wear performance, whilst achieving significant cost reductions over existing materials used in wear applications. Three commercially available gas atomised Fe-Cr-B based alloy powders, namely Armacor M, Armacor C and Nanosteel SHS7170 and a locally designed powder (KC1) were employed in this study. High velocity oxy-fuel (HVOF) thermal spraying was used to deposit coatings of these multi-component Fe-based amorphous alloys, approximately 300 micrometers thick on a mild steel substrate. The microstructures of the feedstock powders and the coatings before and after heat treatment were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The as-sprayed Armacor M, Armacor C and Nanosteel SHS7170 coatings are a mixture of amorphous and crystalline phases, whereas the as-sprayed KC1 coating is composed of only an amorphous phase. The fraction of amorphous phase decreases in the order KC1 >> Nanosteel SHS7170 > Armacor C > Armacor M. The amorphous phase in these coatings crystallises on annealing at about 600-700 degree Celsius and the crystallisation temperature depends on the content of solute elements (i.e. boron and carbon). Thermal stability of the amorphous phase containing boron and carbon is higher than those phases containing boron. After heat treatment of the coatings above the crystallization temperature (~700 degree Celsius) the amorphous phase in the coatings devitrified into a multiphase structure. In the KC1 coating, formation of a very fine mixed structure of α-Fe, carbide, borocarbide and/or boride by decomposition of the amorphous phase can be observed, bringing about a hardness of (~12.4-13.2 GPa). In contrast, the hardness of the Armacor M and Armacor C coatings, containing boron decreased when annealed at temperatures of 750 degree Celsius or higher. The wear performance of the sprayed SHS7170, Armacor C and Armacor M coatings was evaluated by sliding wear (ball-on-disc) testing under unlubricated conditions. The testing was performed with ceramic and steel counterfaces as a function of load and sliding distance. The results show that the SHS7170 coating, containing larger proportion of amorphous phase exhibits higher sliding wear resistance, compared with that of the Armacor M and Armacor C coatings. Wear mechanisms were investigated and related to properties (i.e. fracture toughness) and microstructures of the coating.

The characterisation, emulation and by-emitter degradation analysis of two types of high power semiconductor laser diodes are presented in this thesis as part of an European Union (EU) project. An attempt is made using an accurate laser simulator called Speclase to learn more about the degradation of high power semiconductor laser diodes. Speclase being a single emitter simulation tool was transformed to model a bar i.e. multiple emitters, which we have named Barlase, through an external control interface written in Labview. The concept of Barlase was based on the fact that a bar is a monolithic block of multiple emitters connected in parallel with each other with a common voltage connected across them. This tool is capable of performing simulation in different modes of operation (i.e. constant current or constant power). The tool is designed to examine and emulate the degradation processes at both the laser bar and individual emitter levels of operation. It is known that, emitter degradation is faster for emitters within a bar than for identical single emitters due to a combination of packaging-induced strain and current competition between emitters amongst others. This tool shows clear evidence of the benefits of using by-emitter degradation analysis for gaining detailed understanding of individual emitters operating in a bar and for determining bar degradation mechanisms. The tool complement to the by-emitter analysis, allowing the effects of certain factors that affect the degradation of laser bars to be investigated. Various intervention measures were taken to improve upon the results of the emulation such as modifying the trap density through local heating and the use of the global thermal solver. The modification of the trap density allowed the acceptance of a spatially variable local trap density distribution that gave a more realistic and accurate simulation of the degradation behaviour. The introduction of the global thermal solver allowed the modelling of thermal cross-talk communication between the emitters, which brings about the frown shaped current/power profiles for the unaged bars (though not as pronounced as in the experiment). An attempt was made to employ this tool in the emulation of experimentally observed degradation behaviour in a 975 nm, 16 emitter infrared tapered laser bar with each group of 4 mini-array emitters. The laser bar was first calibrated to achieve a reasonable agreement between the experimental P-I curves of unaged emitters assuming identical emitters with the simulated P-I curves. The simulated P-I curve was then used to perform simulations to emulate the degradation of the laser. The simulated output power profile did not correspond well with the experimental power profile, but a good agreement was realised between the combined output powers of the bar. Better correlation was observed between the experimental and the simulated temperature profiles. This was expected since the experimental temperature was set as input for the heatsink temperature profile. This agreement therefore must not be over-emphasised. The bar emulation model was enhanced by including a global thermal solver to model the thermal crosstalk between emitters. Emulations using this model showed a clearly defined frown shaped profile in the output current and power profiles but the change was minimal. As the emulation of laser bar degradation has not been attempted before, this work is still at a very early stage. Therefore, further work is needed to achieve better agreement in the output current/power profiles and to better the model.

This research aims to study the mechanical properties of single-walled carbon nanotubes. In order to overcome the difficulties of spanning multi-scales from atomistic field to macroscopic space, the Cauchy-Born rule is applied to link the deformation of atom lattice vectors at the atomic level with the material deformation in a macro continuum level. Single-walled carbon nanotubes are modelled as Cosserat surfaces, and modified shell theory is adopted where a displacement field-independent rotation tensor is introduced, which describes the rotation of the inner structure of the surface, i.e. micro-rotation. Empirical interatomic potentials are applied so that stress fields and modulus fields can be computed by the derivations of potential forms from displacement fields and rotation fields. A finite element approach is implemented. Results of simulations for single-walled carbon nanotubes under stretching, bending, compression and torsion are presented. In addition, Young’s modulus and Poisson ratio for graphite sheet and critical buckling strains for single-walled carbon nanotubes are predicted in this research.