This thesis concerns the response of two-dimensional foams to applied shear based on numerical simulations in the quasistatic limit. The effect of liquid fraction and areadisorder on the discrete topological changes (T1s) which occur as a foam flows are probed at several length scales and related to the response of the ordered hexagonal honeycomb. At the macroscopic scale, many T1s combine and the yielding of the foam can be characterized through the harmonics of the stress. Stress harmonics obtained from simulations of dry two-dimensional foams are in good agreement with experimental data for foams and other yield stress materials. At the mesoscopic scale, several T1s occur in a certain region of the foam causing the flow to localize in a region of width proportional to the square root of area-disorder. For dry two-dimensional foams I present a one-dimensional measure and a tensorial measure of foam structure which can identify the localized region from a single still image. The width of the localized region increases linearly with liquid fraction and for sufficiently high values of liquid fraction and area-disorder, the T1s fill the channel and no localization is observed. At the microscopic scale, the links between neighbouring bubbles define a pair of orientations that characterize the local bubble configurations at the instant of a T1. Macroscopic flow behaviour originates at the microscopic scale and I show that the yield stress is directly related to the orientation of the T1 events. Liquid fraction and area-disorder have, in general, the same effect at each length scale. The yield stress decreases with increasing liquid fraction and area-disorder; and the amount of flowing foam and the orientations of a T1 increase with increasing liquid fraction and area-disorder. The response of disordered foams is shown to be different to that of ordered foams in each case.

One of the most important problems facing gas turbine designers today is the ingestion of hot mainstream gases into the wheel-space between the turbine disc and its adjacent casing. A rim seal is fitted at the periphery and a superposed sealant flow is used to prevent ingress. The aim of this PhD research was to design a new rotor-stator testing facility, from which both flow physics and future heat transfer characteristics in relation to ingress could be measured and analysed, along with a detailed investigation into the sealing characteristics of turbine rim-seals there from. The rig was constructed as an engine representative model of a gas turbine wheel-space, from which data correlations could tentatively be scaled and applied to actual engine design. The novel testing facility was designed in great detail for both sealing effectiveness research and to investigate the thermal effects of hot gas ingress; insight never previously achieved. An extensive commissioning process was undertaken to ensure that the correct, albeit benign, fluid-dynamic conditions were created inside the single stage turbine rig. Effectiveness data are presented for single-clearance rim-seals in a variety of ingress conditions from which a fundamental understanding is developed for both rotationally-induced and externally-induced ingress. A newly developed orifice model is validated against the experimental data, resulting in theoretical predictions of the sealing effectiveness characteristics of various rim-seals. It is suggested that these predictions could be scaled to engine representative conditions where they could act as a future design tool for secondary air system engineers. The theory is then extended to the application of double clearance-seal configurations, whereby the beneficial aspects are shown both theoretically and experimentally, leading on to the suggestion of a possible optimisation process resulting in an ultimate double seal. It is postulated that this would be the highest performance that can ever be achieved with a double clearance configuration.

This thesis is concerned with the analysis of two-dimensional cracked specimens at load levels which induce significant plastic yielding near the crack tip. The differences between the solutions discussed here and the well known linear elasto-static ones are confined to the effects of this yielding. Small load expansions of the elastic-plastic response are constructed by the method of matched asymptotic expansions. These give the small scale yielding estimate of linear fracture mechanics as a first approximation, and provide systematic refinements which take account of the nonlinear interaction between the elastic and plastic regions. Besides providing an insight into the unknown elastic-plastic solutions, these expansions can be used directly as useful approximations in the 'medium scale yielding' range. The first half of the thesis is concerned with longitudinal shear problems in which only the out of plane displacement is non-zero. The comparative simplicity of the governing equations allows a completely analytic development. The second half deals with the technically more important problems in which both in-plane displacements are present. Their general structure is first discussed and illustrated by analysis of the simple Dugdale yielding model. A more realistic model, which admits incremental plastic flow, is then incorporated into the analysis by means of a finite element computation. Details of the elastic-plastic response of any specimen are expressed in terms of a set of standard computations that can, in principle, be performed once and for all.

No description available.

This thesis presents the work conducted under the auspices of the Brite EuRam funded project, "PROGEAR" (contract number BRPR-CT98-0703), which sought to better understand the failure modes of injection moulded plastic gears, such that their performance and reliability could be improved. In so doing, the work built upon previous research conducted at the Power Transmission Laboratory, at the University of Birmingham. This research utilised a range of materials including plain polymers and polymer composites, as well a wide variety of gear geometries. In addition, the type of testing that was carried out varied from dry running to oil-lubricated tests, using sophisticated, state-of-the-art equipment Five research areas were covered, and these were as follows: 1. The consolidation of existing knowledge in all aspects of injection moulded plastic gears, resulting in an extensive literature review. 2. The investigation of the dry-running wear resistance of plastic gears. 3. The investigation of the effects of oil-lubrication on the performance of plastic gears, includingt heir wear resistancea nd fatigues trength. 4. The investigation of the efficiency of plastic gears under dry-running, greaselubricated and oil-lubricated conditions. 5. The investigation of Pitch Line Fracture, which is a failure mode unique to dryrunning gears of specific polymeric materials. This thesis represents a significant contribution to the understanding of plastic gears, and should enable significant improvements in the power density, durability and wear resistance in future designs of plastic gears. This research also maps out important avenues for future research.

No description available.

Raman spectroscopy has been used to follow the defonnation micromechanics of a range of process---controlled fibres. The fibres were solvent spun using a N-methylmorpholine N-{)xide/ cellulose system. Single fibres are defonned in tension, and it is shown that the 1095 cm-l and 1414 cm-l Raman bands, corresponding to the C-O stretch mode and the side group (C-O--H) along the chain respectively, shift towards a lower wavenumber upon the application of external tensile defonnation. The shift profile of the 1095 cm-' band is shown to be non-linear, following the shape of the stress-strain curve, and the initial shift rate is shown to be directly related to the stiffness of the fibre. This band shift rate with respect to strain is shown to increase with increasing crystall ine orientation. The 1414 cm·1 band also shifts non-linearly and rapidly reaches a plateau with both strain and stress. This is thought to be due to the breakdown of hydrogen bonding in the structure, and a potential cause of the yield point in the mechanics of the fibres. The crystalline modulus and orientation of regenerated cellulose are measured using a wide-angle X-ray diffraction method. The crystalline modulus appears to vary with fibres having different orientations. It is shown that whilst the c-spacing of crystals increases with tensile stress, the crystalline fraction reorients to the direction of the fibre axis. An average shear modulus for these fibres is detennined by placing the data on a master curve and fitting with a model equation. Structure-property relationships are derived from the molecular and crystal defonnation characteristics of cellulose fibres using Raman spectroscopy and X-ray diffraction. This represents, for the first time, a full analysis of the relationship between structure and mechanics. at a local scale, of these regenerated cellulose fibres. Using a set of hypotheses tested on the experimental data, it is possible to distinguish unifonn stress and unifonn strain beha"iourNo portion of work referred to within this thesis has been submitted in support of an application for another degree or qualification of this or any other uni\crsity. or other institution of learning

Quasi-static and fatigue tests have been carried out to investigate mixed mode crack growth behaviour in FM73 epoxy adhesive using Double Cantilever Beam (DCB) specimens. The DCB configuration used consisted of equal thickness mild steel adherends bonded with FM73 adhesive. The joints were tested under a range of both quasi-static and fatigue mixed-mode conditions from mode I to mode II using a relatively simple, variable-mode loading fixture. Values of GI and GII for the different test conditions have been obtained using both closed-form Linear Elastic Fracture Mechanics (LEFM) methods reported previously in the literature and finite element simulations, making use of the Virtual Crack Closure Technique (VCCT). The results for the total strain energy release rates (GT) and the components (GI and GII) for the two techniques agree very well over the range of loadings applied by the jig, provided that the adhesive deformation is considered. For the quasi-static tests, the strain energy release rate components at fracture have been presented in a conventional GI/GIC (mode I) - GII/GIIC (mode II) failure envelope. It is found that the fracture energy is enhanced for mode II (In-plane shearing) dominated loading compared to mode I (opening mode) dominated loading. The ratio of GIIC/GIC was found to be 1.8. Mixed mode fracture criterion of the following form is suggested which has been found to provide a reasonably good fit to the experimental data. 1 T IIC II H IC I                   G G G G Where GIC and GIIC represent pure mode I and pure mode II fracture energies respectively and H=0.5 and T=1 are material constants obtained through experimental fit of the dataFor fatigue characterisation, the testing was carried out at a load ratio (R) of 0.1, in displacement control such that the initial maximum fatigue load was 70% of the corresponding quasi-static fracture load. The fatigue load decreased as the fatigue crack grew and this load was recorded. Crack growth was monitored and measured using a video microscope. The strain energy release rate components were determined using corresponding values of fatigue crack length and fatigue load. The results suggest that crack initiation in the test specimens is controlled by the mode I strain energy release rate component. The fatigue crack growth rates were characterised using Paris law approach from which it appears that the total strain energy release rate range, ΔGTotal, is a more dominant factor in controlling crack growth than the mode I component of strain energy release rate range, ΔGI. For a quantitative description of the mixed-mode fatigue crack growth, generalised forms of the Paris relation are developed in which the Paris constants vary with the mode-mixity in a non-monotonic way

The use of gearboxes for power transfer is widespread throughout industry. However, machines today are operating at higher speeds than ever before and gear wear and fatigue failures are serious and legitimate concerns. Incipient fault detection in gears has thus become the subject of intensive investigation and at this stage of development there are many competing condition monitoring methods based on vibration signal analysis. Vibration signals obtained from a gearbox were complex multi-component signals, generated by tooth meshing, gear shaft rotation, gearbox resonance vibration signatures and a substantial amount of noise. This thesis summarizes the research steps taken after a review of (i) current maintenance strategies, (ii) gearbox condition monitoring techniques, (iii) gear vibration fundamentals and (iv) common gearbox failure modes. A mathematical model of the gearbox was used to predict the effects of load and tooth breakage effects on induced vibrations. A test rig was built around an 11 kW two stage helical gearbox, designed and fabricated for experimental data collection. Simulation and experimental work was carried out for a healthy pair of gears under different loads for different drive speeds and a pair suffering from degrees of tooth breakage. Conventional methods using the time-domain of the vibration signal (RMS, kurtosis, skewness and the zero figure of merit) were used for detecting and diagnose the seeded faults. The total energy method was applied to the gear meshing frequency and its sidebands, as obtained from the FFT, to detect the presence of the faults, and the results compared with those obtained by the conventional techniques. The proposed method appears much more effective at detecting and diagnosing tooth breakage than statistical features extracted from the time-domain. Joint time-frequency domain techniques were then used to determine their effectiveness for diagnosing the seeded faults in the gearbox system when the gearbox operates under output loads and at different speeds. A comparison was made between empirical mode decomposition and smoothed pseudo Wigner-Ville distribution methods based on vibration signature. From the results obtained it appears that the empirical mode decompose technique offers a more effective and faster way to detect faults. To improve signal-to-noise ratio, a novel scheme based on adaptive noise cancellation technique with a least squares algorithms was used on the gearbox experimental vibration signals. It is concluded that this method offers the most effective way of all those tested to detect faults.

This study describes a new approach to the quality assessment of thermally sprayed carbide and ceramic coatings produced by High Velocity Oxy-Fuel (HVOF) and Air Plasma Spray (APS) processes. The aim of the work was to develop an experimental methodology based on Acoustic Emission (AE) monitoring of a dead-weight Vickers indentation to assess the degree of cracking and hence the toughness of the coating. AE monitoring was also applied to an industrial process as a contribution to the possibility of quality assessment during the deposition process. AE data were acquired during indentation tests on samples of coating of nominal thickness 250-325 μm at a variety of indentation loads ranging from 49 to 490 N. Measurements were carried out on six different thick-film coatings (as-sprayed HVOFJP5000/ JetKote WC-12%Co, HIPed HVOF-JetKote WC-12%Co, as-sprayed HVOFJP5000 WC-10%Co-4%Cr, conventional powder APS-Metco/9MB Al2O3 and fine powder HVOF-theta gun Al2O3) and also on soft and hard metallic samples and metals. The raw AE signals were analysed along with force and displacement history and the total surface crack length around the indent determined. Also, a selection of the indents was sectioned in order to make some observations on the sub-surface damage. The results show characteristic AE time evolutions during indentation for tough metals, hard metals, and carbide and ceramic coatings. Within each category, AE can be used as a suitable surrogate for crack length measurement for assessing coating quality. Finally, a preliminary observation on AE monitoring during HVOF (JP5000) WC- 10%Co-4%Cr thermal spraying was made. It was found that AE is sensitive to individual particle landings during thermal spraying and therefore can, in principle, be used to monitor the spray process.