Thesis (M.S.)--Ohio University, November, 1987. / Title from PDF t.p.

Thesis (M.Eng. (Mechanical and Aeronautical Engineering)) -- University of Pretoria, 2007. / Includes bibliographical references (p. 103-108)

Thesis (B.S.)--University of Missouri, School of Mines and Metallurgy, 1899. / 1899 determined to be year of publication from the "1874-1990 MSM-UMR Alumni Directory". The entire thesis text is included in file. Holograph [Handwritten and illustrated in entirety by author]. Title from title screen of thesis/dissertation PDF file (viewed )

Thesis (M.S.)--University of Nevada, Reno, 2008. / "December 2008." Includes bibliographical references (leaves 57-58). Online version available on the World Wide Web.

As coal mines get deeper, more mechanised and more productive the heat load on the ventilation system increases. In certain cases to the point where serious environmental problems may arise. To continue mining in these demanding conditions the sources of heat must be identified and evaluated so ameliorative measures may be taken. Due to the trend towards mining at greater rates and further from the shaft, mined coal on the conveyors is being recognised as a heat source of growing importance. This thesis describes its investigation. Reviews of heat sources, psychrometry,heat stress indices and heat transfer are included to provide a background framework. The evaluation of the heat released by conveyed coal itself consists of theoretical treatment and laboratory investigations of heat transfer through broken coal. A model conveyor and its instrumentation constructed in a duct are described along with underground measurements at mines. The information obtained from theoretical, laboratory and on site investigations is analysed and summarised to provide a basis for future prediction.

The principal objectives of the research reported in this thesis are: to determine the effect that sinter time has on the metallurgical behaviour of alumina-reinforced aluminium-606lmatrix composites; compare and assess the wear resistance of alumina and silicon carbide reinforced aluminium 6061-matrix composites, together with monolithic aluminium 6061 alloy; determine the effect that reinforcement particle size has on the wear resistance of aluminium 6061-matrix composites; identify the relative merits of two techniques for depositing copper coatings on to alumina reinforcements. Through investigation, a successful method of processing silicon carbide and alumina particulate-reinforced AA6061 composites, fabricated by cold uniaxial pressing with vacuum sintering, has been determined. The processing route is as follows: pressing at 400 MPa; vacuum sinter at 600°C for 30 minutes; solution heat treat for 30 minutes at 530°C then water quench; precipitation (ageing) heat treat for 7 hours at I 75°C, then air cool. Metallurgical examination of composites revealed that magnesium was found to collect at interface regions around alumina particulates, resulting in the depletion of magnesium from the aluminium 6061 matrix. The severe depletion of magnesium from the AA6061 matrix when alumina is used as a reinforcement was found to occur during long (greater than 30 minutes) sintering times using a sintering temperature of 600°C. It is postulated that the formation of spinel (MgA12O4) formed from the reaction of magnesium with alumina is a probable cause for the Mg migration. The composites containing alumina particulates were found to have lower hardness values than the monolithic alloy and composites containing silicon carbide, when sintering took place for longer than 30 minutes. Adding 5 wt% silicon to the AA6061 matrix in composites reinforced with alumina particulates was found to reduce the magnesium depletion for sinter times up to one hour at 600°C and give improved composite bulk hardness. During the research, a need for an improved wear testing machine was identified. Therefore a wear test rig, which allows samples of different materials (under different applied loads if required) to be tested simultaneously without interference between test pieces, was designed and commissioned. Two electroless methods for copper coating alumina particulates were also investigated. One method used formaldehyde as the reducing agent, while the other employed hydrazine-hydrate as the reducing agent. The latter method has proven to be quicker, and with improved results, compared to the traditional method using formaldehyde as the reducing agent.

Diamond-Like Carbon (DLC) coatings are thin protective surface coatings used to reduce friction and minimise wear in a wide range of applications. The focus of this work is the use of DLC coatings within Rolls-Royce’s pressurised water reactors. A strong understanding of material behaviour in this environment is compulsory due to the stringent safety requirements of the nuclear industry. Wear testing of a range of commercial DLC coatings against steel in water, and the dependence of the tribology on normal load, sliding distance, and environmental species, was examined. Wear depth was observed to increase with normal load, and increase non-linearly with sliding distance. Uniquely, it was suggested that the tribology of a DLC coating in water was controlled by the velocity accommodation mode (VAM) of the transfer layer. When interfacial sliding was the dominant VAM, the carbonaceous transfer layer was present at all times, and a low specific wear rate was observed. When shear and recirculation of debris was the dominant VAM, the carbonaceous transfer layer initially present was replaced by iron oxide species, and a high specific wear rate was observed as a result of a three-body mechanism involving hematite. Two individual wear models were developed to predict the wear depth of a DLC coating sliding against steel in water. Each model represents a novel extension to the current literature regarding the modelling of wear. Firstly, an analytical differential equation was derived to predict the wear depth of a ball and a flat surface, in relation to any phenomenological law for wear volume. Secondly, a unique formulation of an incremental wear model for an arbitrary geometry was developed for a DLC coating which included the growth of a transfer layer. An efficient methodology was presented to allow fast integration of the equations whilst damping numerical instabilities. A comparison between the analytic and computational wear models showed a strong agreement in the model predictions, with a comparative error of less than 5%.

Age hardening of Al-Cu-Mg type alloys occurs in two stages separated by a constant hardness plateau when the alloys are aged at 110°C to 240C after solution treatment and quenching. This work aims to develop a physically based two-stage hardening model to predict the yield strength of Al-Cu-Mg alloys with compositions in the (+S) phase region. Experiments by means of hardness and tensile tests, differential scanning calorimetry and transmission electron microscopy (TEM) have been carried out to provide the relevant information for the calibration and validation of the model. The model considers a simplified precipitation sequence which involves a pre-precipitate structure followed by S phase. This pre-precipitate structure is referred to as Cu-Mg co-clusters instead of GPB zones based on atom probe and TEM studies from collaborators and a review of the literature. The competition between the Cu-Mg co-clusters and the S phase is modelled by assuming S phase forms at the expense of Cu-Mg co-clusters. In the model, the solvi of the Cu-Mg co-clusters and the S phase are calculated, the evolution of precipitates in terms of volume fraction, average size and the solute concentration in the matrix are described and the superposition of various contributions from precipitation strengthening, solution strengthening and dislocation strengthening are modelled. Strengthening by Cu-Mg co-clusters and S phase is described by the modulus strengthening mechanism and the Orowan bypassing mechanism, respectively. The predicted contributions to the critical resolved shear stress show that strengthening in the alloys is mainly due to the Cu-Mg co-clusters in the first stage of hardening and due to the S phase in the second stage of hardening. The model takes account of the composition dependency of precipitation rate for Cu-Mg co-clusters formation as well as the amount of Cu and Mg present in undissolved intermetallic phases. With a training root mean square error of 12MPa on an artificially aged 2024 alloy, the modelling accuracy on unseen yield strength data of two other alloys is 16MPa. Using a single set of parameters, the model has been applied to predict the hardness of a 2024-T351 alloy artificially aged at low temperature followed by short term underageing at higher temperature and then room temperature ageing. Good agreement between the predictions and the experiments indicates that the hardness changes during these multi-stage heat treatments can be well interpreted by considering Cu-Mg co-cluster dissolution, S precipitation and Cu-Mg co-cluster re-formation. Application to Al-xCu-1.7Mg alloys (x=0.2, 0.5, 0.8 and 1.1at.%) has shown good predictive capabilities of the model for the first stage of hardening. It is also shown that the model is applicable to Al-Cu-Mg alloys with Si contents at levels of 0.1-0.2wt.%. Modelling results of various Al-Cu-Mg alloys during natural ageing, artificial ageing and multi-stage heat treatments indicate that the model is capable of predicting the evolution of microstructure and the yield strength as a function of composition and heat treatments, and can provide a predictive tool for predicting the strength of Al-Cu-Mg based welds.

Synchrotron radiation computed tomography (SRCT) has been used to analyse the tensile failure mechanisms in carbon fibre/epoxy composites. Two specimen types were analysed – in situ loaded coupons and filament wound samples, taken from incrementally loaded cylinders and scanned “post mortem”. The effects of fibre, matrix and interfacial properties on the initiation and accumulation of fibre breaks have been analysed. Breaks accumulated on a power law curve as a function of fibre stress; however the fibre and matrix moduli had little effect on accumulation. Initial analysis of the fibre Weibull moduli showed little correlation between Weibull modulus and break accumulation. Singlets initiated in low fibre volume fraction areas; however a full investigation into the effects of varying fibre volume fraction has not been possible. Attention was focused on the formation of interacting groups of broken fibres (clusters), as they are believed to be the strength-defining failure event. The in situ coupons had much larger maximum cluster sizes than the filament wound counterparts (14 vs. 9), and a correlation between high break density and low cluster percentage is proposed. No simple correlations were found between fibre/matrix moduli and the clustering parameters. Clusters formed in one load step, and did not grow from singlets or smaller clusters, which suggests a dynamic process. The interface is suggested to be key to damage initiation and propagation. The work provides links between experimental studies and simulation tools by informing and validating a micromechanical tensile failure model. Comparisons between experimental and modelled results found that the model accurately predicted the composite failure strain but not the complex damage accumulation processes. The model under-predicted both cluster size and the proportion of interacting breaks; this is attributed to the inaccurate modelling of the stress transfer process. Both experimentally and analytically the dominant parameter controlling clustering was the overall stress concentration factor. This has been infrequently analysed in work published in the literature, and is the recommended focus of the future work.

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