Thesis (Professional Degree)--University of Missouri, School of Mines and Metallurgy, 1938. / Leaf number 26 was duplicated, constituting leaves number 26 and 26 1/2. The entire thesis text is included in file. Typescript. Title from title screen of thesis/dissertation PDF file (viewed April 27, 2010)

Thesis (M.Eng.)(Electronic)--University of Pretoria, 2005. / Title from opening screen (viewed March 20, 2006). Summaries in English and Afrikaans. Includes bibliographical references.

Thesis (Ph.D.)--Michigan Technological University, 2006. / Includes bibliographical references. Also available on the World Wide Web.

Thesis (M.S.)--Michigan Technological University, 2006. / Includes bibliographical references. Also available on the World Wide Web.

The key controlling factor in the effective energy conversion of coal to combustible gases during the UCG process is the behaviour of the pyrolysed char in the reduction zone of the UCG cavity, which has not been published in available academic literature. This study investigates the impact of the operating parameters during the reduction zone of UCG using a bespoke high pressure high temperature rig which was developed as part of this research work. This rig, operating at temperatures of up to 900 oC and at pressures up to 5.0 MPa, simulates the UCG process including each UCG zone individually for a broad range of underground conditions to a depth of 500 m. Carbon dioxide and steam were used as the primary reductants with char derived from dry steam coal and anthracite sample. Carbon dioxide and steam were injected at a variety of pressures and temperatures, plus at a range of relative H2O/CO2 proportions. The composition of the resulting product gas of both coals was measured and subsequently used to calculate carbon conversion (X), carbon conversion of combustible gases ( ), cold gas efficiency (CGE) and low heating value (LHV) of the product gas. Optimal operating conditions were determined for the dry steam coal and anthracite that produced the best gas composition both at atmospheric and elevated pressure and are unique for each UCG system. A shrinking core model was employed to describe the behaviour of the pyrolised char to determine the activation energy and pre-exponential factor at atmospheric pressure for both coals. The evolution of the volatile matter of both coals and its contribution to the overall UCG performance was also determined. An optimum H2O/CO2 ratio was determined for both coals which enhanced the gasification rate of both coal chars up to the ratio of 2:1, above this ratio the effect saturated for both coals. It was shown that pressure increases the reduction-gasification process of the chars which suggests that there is an optimum operating pressure which produces a peak in carbon conversion, CGE and LHV for the product gas over the conditions tested that differs for each coal. Therefore UCG projects aiming at reaching higher pressures will not achieve an increase in the output, unless there are some new effects occurring above 4.0 MPa. Pressure enhances the gas solid reactions and almost doubles the max carbon conversion ( of combustible gases achieved at elevated pressure compared to that at atmospheric pressure. A shrinking core model was modified to take into account the effect of total pressure to the gasification rate of dry steam coal at 900 oC and pressures ranging from 0.7 to 1.65 MPa. Reaction constants for various pressures at 900 oC were determined for both coal chars. Analysis of data shown that typical UCG operations on low rank coals provides a combustible product gas that relies heavily on releasing the volatile matter from the coal and does not depend on the carbon conversion of char to gas which justifies the high CGE and LHV of the product gas found in the field trials. It was found that carbon conversion X is not significantly affected by the type of coal and that the carbon converted during UCG is between approximately 45% for high rank coals up to 55% for low rank coals. Experimental results were used to calculate the output, size and UCG model of a potential power plant which produced realistic solutions and proves that high rank coals can be suitable for UCG projects. Anthracite can produce almost the same amount of combustible gases as the dry steam coal operating under specific conditions but with a lower CGE and LHV which suggests that anthracite may be found to be more suitable for producing hydrocarbons with UCG than energy.

In this thesis the influence of microstructure on fracture toughness is investigated for two different medium/high strength Al-alloys for aerospace application. In weldable AA6156 (Al-Mg-Si-Cu) alloy sheet, the quench sensitivity in toughness is assessed via enhanced Kahn tear tests. Toughness was seen to be reduced for both 60°C water quenched and air cooled materials cf. 20°C water quench material. Fractography via scanning electron microscopy (SEM) and synchrotron radiation computed tomography (SRCT), as well as Differential Scanning Calorimetry (DSC) and Transmission Electron Microscopy (TEM) studies, have clarified the mechanisms of the quench sensitivity with respect to toughness. Both the coverage of grain boundary decoration and precipitate free zone (PFZ) width increase with reduced quench rates. The failure morphology of the air cooled material appears consistent with classical intergranular ductile failure. Coarse voiding and shear decohesion was prevalent in 20°C water quenched material (depending on local triaxiality), whilst the 60°C water quenched material showed a mixture of transgranular and intergranular fracture modes. The experimental toughness trends are compared to models in the literature and a simple new model is suggested. Fracture toughness anisotropy of AA2139 (Al-Cu-Mg), a candidate alloy for age forming, in T351 and T8 conditions has been investigated via mechanical testing of smooth and notched specimens of different geometries, loaded in the rolling direction (L) or in the transverse direction (T). Fracture mechanisms are again investigated via SEM and SRCT. Fracture toughness is seen to be anisotropic for both heat treatment conditions tested, but is substantially reduced for the T8 condition compared to the T351. Contributions to failure behaviour have been identified with: (i) anisotropic initial void shape and growth, (ii) plastic behaviour, including isotropic/kinematic hardening and plastic anisotropy, and (iii) nucleation at a 2nd population of 2nd phase particles leading to coalescence via narrow crack regions. SRCT analysis of arrested cracks revealed alignment of voids in the crack during propagation in the rolling direction, resulting in shorter intervoid ligaments than for crack propagation in the transverse direction. Coalescence through shear decohesion in the crack initiation and propagation region was found indicating the necessity to investigate and account for this mechanism. A model based in part on the Gurson-Tvergaard- Needleman approach is constructed to describe and predict deformation behaviour, crack propagation and, in particular, toughness anisotropy. Model parameters are fitted using microstructural data and data on deformation and crack propagation for a range of small test samples. The model accounts for the material features found in the experimental study and its transferability has been shown by simulating tests of large M(T) samples showing strong fracture toughness anisotropy. A parametric study shows that nucleation of small voids at different strains for different loading directions is crucial for a correct model of toughness anisotropy; the combined effects of kinematic hardening and void growth anisotropy can not fully describe fracture toughness anisotropy.

The acoustic remote sensing of subsea gas leakage, applied to the monitoring of underwater gas discharges from anthropogenic and natural sources, is becoming increasingly important. First, as the oil and gas industry is facing increasing regulation, there is a need to put more control in the industrial process and to assess the impact on the marine environment. The applications are diverse, including: early warnings of "blow-out" from offshore installations, detection of leaks from underwater gas pipelines, gas leakage detection from Carbon and Capture and Storage facilities (a process aimed at mitigating the release of large quantities of CO2 in the atmosphere), and seabed monitoring. Second, this technology has a role to play in oceanography for a better understanding of natural occurrences of gas release from the sea floor such as methane seeps. This is of major importance for the assessment of the exchange of gas between the ocean and the atmosphere with application to global warming. All those phenomena involve the formation and release of bubbles of different sizes. These are strong sources and scatterers of sound. Within this context, this thesis draws on a two part study. The first part experimentally addresses the accuracy of a passive acoustic inversion method for the quantification of gas release. Such a technique offers the advantage of lower power requirements for long term monitoring. It is common practice for researchers to identify single bubble injection events from time histories or time frequency representations of hydrophone data, and infer bubble sizes from the centre frequency of the emission. This is well suited for gas release at a low flow rate, involving solitary bubble release. However, for larger events, with overlapping of bubble acoustic emissions, the inability to discriminate each individual bubble injection events makes this approach inappropriate. Using an inverse method based on the spectrum of the acoustic emissions allows quantification of such releases with good accuracy. The inverse scheme is tested using data collected in a large test tank and data collected at sea during the QICS (Quantifying Impacts of Carbon Storage) project. The second part of the thesis addresses the problem of quantifying gas releases using active acoustics. Single beam echosounders are commonly used instruments in fisheries acoustics. When investigating gas release from the seafloor, they are frequently employed to study the spatial distribution of the gas releases. However, few studies make use of these data to quantify the amount of gas being released. Here, using the common multi-frequency ability of these systems, an inverse method aimed at determining gas volumes is developed. This is tested against simulated data and the method shows good performances in scenarios with limited data sets (data collected at limited number of frequencies). Then, using data collected at sea from methane seeps to the west of Svalbard (from two research cruises), the method is applied and compared to independent measurements of gas fluxes.

Application prospects in the automotive industry have led to increasing studies on Al-Mg- Cu-Si alloys. In this thesis, nine Al-(1-3)Mg-(0-0.4)Cu-0.15Si-0.25Mn (in wt%) alloys with potential applications in packaging and automotive industries have been investigated. By means of mechanical testing, differential scanning calorimetry (DSC) and transmission electron microscopy (TEM), several mechanisms was identified that influence the final strength of cold rolled alloys during ageing: solid solution, work hardening, recovery and precipitation. Microstructure analyses revealed the formation of undissolved particles consuming the small Si addition, which influences age hardening behaviour of the alloys. Tensile testing was performed to evaluate the strength and work hardening. The integrated experimental results showed that for cold worked samples, b² (Mg2Si) contributes to age hardening of Cu-free alloys, whilst both b² and S (Al2CuMg) contribute to that of Cu-containing alloys. According to the experimental findings, a yield strength model has been developed to elucidate the relation between processing and the final strength. It consists of three main components: i) dissolution of intermetallic phase Mg2Si; ii) precipitation of two strengthening phases b² and S; iii) strengthening contributions from solution strengthening, dislocation strengthening and precipitation hardening due to the strengthening phases. The model was calibrated and tested using separate tensile data and was applied to predict the yield strength evolution of cold worked samples during ageing. An accuracy of 8.6 MPa (about 4% of the total range of strengths) has been achieved. Based on the analysis of the relation of work hardening with cold work, composition and ageing time, three primary findings were obtained: i) cold worked samples usually have the lowest work hardening rate (WHR); ii) WHR increases after 30-minute ageing due to recovery and iii) WHR increases with decreasing level of cold work and increasing Mg and Cu contents. Work hardening models based on the Kocks-Mecking (KM) model and the Kocks-Mecking-Estrin (KME) model have been utilized to explain the main trends. The modelling results showed that the KM model is able to predict the work hardening behaviour of cold worked samples reasonably well. However, the KME model is insufficient to fully describe that of cold-worked-and-aged samples.

Nanostructured metal films were electrodeposited through the hexagonal lyotropic liquid crystalline phase (HI). The mesoporous structure consists of porous channels (a few nm in diameter) arranged in an hexagonal array. These mesoporous metal films exhibit high surface areas supplied by the concave surface within the pores. The properties of these mesoporous materials have been investigated to gain an insight on the mesoporous structure. Cyclic voltammetry in acid of HI mesoporous Pt is similar to polycrystalline Pt made up of low index Pt facets. However CO stripping voltammetry shows differences between the HI mesoporous Pt and polished Pt electrodes. The CO stripping voltammogram for the HI mesoporous Pt electrode exhibits a CO oxidation pre-wave and CO oxidation at lower overpotentials. These differences result from the presence of trough sites corresponding to the intersection of two pore walls within the mesoporous structure. The adsorption of foreign atoms Bi and Ge on HI mesoporousPt was investigated to identify the different crystalline Pt facets. The features of the voltammetric profiles recorded in acid revealed the absence of large (111) domains and the presence of (100) terraces sites. CO stripping voltammetry for HI mesoporous Pt modified with Bi suggests the presence of a CO-Bi mixed ad layer. However, the absence of the aforementioned pre-wave was attributed to the adsorption of Bi on the trough sites thus causing inefficiency in oxygen transfer for CO oxidation. In contrast, the significant pre-wave observed for HI mesoporous Pt-Ge leads to an enhancement for CO oxidation. HI mesoporous metal films grown on microelectrodes show good stability of the measurement of hydrogen peroxide. HI mesoporous Rh with a variety of film thicknesses was extensively studied over a wide range of hydrogen peroxide concentrations. A kinetic model is proposed to describe the diffusion of hydrogen peroxide and the surface reaction in the pore. The accessibility of the ionic liquid BMIM-PF6 in the pores was investigated to assess the performance of mesoporous electrodes in supercapacitors. HI mesoporous Pt of diverse pore size and polished Pt electrodes were characterised in 1 M sulphuric acid and then tested in BMIM-PF6. The double layer capacitances were higher for the HI mesoporous Pt films thanks to their internal surface area leading to the confirmation that the ionic liquid penetrates into the pores. The analysis of electrochemical impedance spectroscopy shows that the results fit a transmission line model and provides useful parameters for the characterisation of the nanostructured Pt film in BMIM-PF6. 11.

In this EngD thesis the phenomenon of sustained macroscopic deflected crack growth (SMDCG) during fatigue in Nickel based Superalloys is discussed, also referred to as ‘teardrop’ cracking in previous work. In a corner notched bend (CNB) specimen a fatigue crack usually grows in a quarter-circular plane perpendicular to the stress axis, the SMDCG however exhibits considerable deflection from this plane at the free surfaces, so that a central planar region is enclosed by large shear terraces (thus forming the ‘teardrop’). This is difficult to characterise in terms of component lifing (both in terms of crack path and crack growth rates), and this thesis aims to further understanding of the underlying mechanisms and to develop assessment methodologies for lifing of such cracks in aeroengine components. The SMDCG effect has been observed under externally applied mode I loading in Udimet 720Li with relatively fine grain sizes of 6-18μm, which are candidate materials for aero engine turbine discs. CNB and single edge bend notch bend (SENB) fatigue tests have been carried out on cast and wrought and powder metallurgy variants of Udimet 720Li and SMDCG was observed at 300°C in air and at 300, 600 and 650°C in vacuum. A consistent measure of the onset of deflection has been defined; this is the crack tip stress intensity factor range K at which the sustained deflection from the free surface exceeds the expected shear lip area, which is estimated as the distance of the monotonic plane stress plastic zone size from the free surface. This has been used to characterise samples from other research programmes and aeroengine components from rigtests, that have shown some form of deflected crack growth, to assess whether they exhibit SMDCG or whether the apparent deflection was caused by the expected plane stress region, or other factors such as tunnelling due to creep effects or complex overall loading conditions. Fracture surfaces exhibiting SMDCG have been studied via SEM and the fractographic analysis shows a competition between local shear crack growth and mode I crack growth occurs in both the macroscopically deflected and planar regions. This indicates that the macroscopic deflection is triggered by the stress state at the free surface. Further detailed analysis of the deflected crack tip with focussed ion beam (FIB) serial sectioning combined with EBSD analysis has allowed a detailed 3D reconstruction of the crack tip interaction with local microstructure. This, together with TEM foils extracted via FIB in the same region, have confirmed that the SMDCG in these systems is not linked to any local texture effects or surface microstructural differences. Lifetime predictions for the CNB samples have been carried out based on Paris law coefficients from the SENB tests, and showed shorter lifetimes than the actual samples with deflected crack growth. Whilst this could be partially due to unaccounted for initiation effects, it could also indicate that significant extrinsic shielding caused by the deflected crack growth may actually increase fatigue lifetimes. To assess the driving force evolving during SMDCG, an Alicona Infinite Focus profilometre has been used to map the complex 3D crack shapes, from where tilt angles could be measured to define a mixed mode plane stress SENB test, with the aim of replicating the stress state that had given rise to the locally deflected shear growth and to explicitly measure crack growth behaviour in the deflected regions. However this was not achieved with this test set-up, as the achievable ΔKeff was not high enough to trigger the deflection throughout the sample. As a result the 3D map was next used to define Finite Element model to assess the local crack tip stress state of the complex deflected crack path. Crack driving forces that have been identified at the point the deflected crack path becomes self-sustaining, could be used in a lifing model to predict crack paths in aeroengine components, together with the measured onset of deflection.