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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
541

Understanding the effect of surface generation rates for finish milling of Ti-5Al-5Mo-5V-3Cr aero-structural components

Cox, Adam January 2017 (has links)
A detailed investigation into the influence of cutting parameters on the surface integrity, subsurface microstructure, tool wear and fatigue performance for finish milling of Ti-5Al-5Mo-5V-3Cr landing gear components. Low cost, small scale testing methodologies have been developed to replicate complex machining strategies employed in aerospace manufacturing, to understand and develop an improved processing window. Machinability trials identifieded the influence of cutting speed (Vc), feed per tooth (fz) and surface generation rates (SGR) on surface roughness and subsurface deformation. The detrimental impact of fz can be offset somewhat by comparable increases in Vc. The subsequent reduction in imposed cutting forces, increased coolant delivery and improved chip evacuation achieves moderate responses in surface integrity and subsurface deformation. Severe plastic deformation at the near surface, exacerbated by increased cutting parameters, is accommodated through dislocation slip along basal, prismatic and pyramidal planes of primary alpha grains. Deformation modes, which are intensified by increased tool wear. A transition from abrasive to adhesive tool wear mechanisms have been identified and shown to be accelerated by increased fz. Through the application of a bespoke four point bend test rig, low cycle fatigue testing at R=0.1, has proposed some important observations for fatigue performance of machined high strength titanium allows. Cycles to failure were shown to be reduced for identical applied stress for increased SGR. Fatigue cracks has been identified within the severe plastic deformation region of the subsurface, as a result of machining. Initiation was produced through quasi-cleavage along dislocation slip planes of alpha grains and propagated in a mixed mode, significantly influenced by bulk beta grains. A competing mechanism between initiation influenced by machining damage and propagation retardation as a result of machining induced compressive residual stresses has been postulated.
542

Analysis and evaluation of multi-strand beams under static and dynamic loading

Asker, Haval January 2017 (has links)
Multi-strand systems include, but are not limited to, beam-like structure (special composites) or wire-like structure (mechanical cables and electrical wires), constructed from many long strands that are constrained rather than bonded, can provide appreciable levels of damping through the friction in the interfacial surfaces between the individual strands. These systems are generally metallic to provide dry friction damping over significantly wider working temperature ranges than is typical for common damping materials such as viscoelastic polymers. This work proposes an analytical model that describes the frictional damping behaviour of multi-strand system constructed from strands that have square and circular cross-sections. The analytical models predict the frictional system stiffness under simply supported configuration. This is followed by systematic quasi-static experiments and numerical investigations using standard finite element analyses. Simple Coulomb friction is assumed at the interfaces. Dynamic loss factors for multi-strand beams and multi-strand wires are obtained by performing forced vibration tests and are found (the loss factors) to be insensitive of the frequency. The analytical models in this study show preference in describing multi-strand systems due to some limitations in the experimental and numerical representations. This thesis provides a vital and necessary insight into the physics for stranded structures and materials that are largely prevalent in mechanical (e.g. cables) and electrical (e.g. wires) elements.
543

Gaussian process emulators for 1D vascular models

Melis, Alessandro January 2017 (has links)
One-dimensional numerical models of the arterial vasculature are capable of simulating the physics of pulse wave transmission and reflection. These models are computationally efficient and represents and ideal choice with great translational opportunities in healthcare. However, the use of these models in a patient-specific scenario is hampered by the difficulty in measuring the model inputs (parameters, boundary conditions, and initial conditions) in the clinical setting. As a result, most of the model inputs are noisy or missing, and the inputs uncertainty is transmitted to the model outputs. A fundamental step in the model development consists in performing a sensitivity and uncertainty analysis aimed at understanding how variations on the inputs affect the output variability, with the final aim of instruct the measurement process. A typical sensitivity analysis conducted by means of \break Monte Carlo sampling is computationally expensive due to the large number of runs required. A novel approach aimed at reducing the computational time consists in using a statistical emulator capable of mimicking mean and variance behaviours of the 1D deterministic model. In this study, emulators built through Gaussian process method are used to predict outcomes of a 1D finite-volume solver for networks of elastic vessels. The 1D model is discussed and validated showing good agreement with published results. The emulator approach for sensitivity analysis is validated against Monte Carlo sampling and a 99.9% reduction in computational time is obtained. This methodology is further applied in the context of cerebral vasospasm where the sensitivity analysis results are used to identify new biomechanical metrics for this pathology. The novel biomarkers are effective at detecting the cerebral vasospasm better than the currently used one. In particular, the progression of the disease is characterised from an early onset even when the vasospasm is occurring at some distance away from the measurement location.
544

Enhancing the piston effect in underground railways

Marshall Cross, Daniel January 2017 (has links)
The purpose of this study was to investigate methods of enhancing the piston effect in underground railways for the improvement of thermal conditions on platforms. In many underground railways, the piston effect is used to provide ventilation. However, in older underground railways insufficient ventilation can lead to high temperatures, largely due to heat from train braking. Additionally, the energy demand from ventilation and cooling equipment in newer underground railways can be significant. Enhancing the piston effect can provide additional ventilation for improved thermal conditions or a reduced energy demand. Two novel devices for the enhancement of the piston effect were investigated; a train fin and aerofoil. Through influencing the air flow patterns around a train, the devices alter the train air displacement and aerodynamic work. Moreover, variation of the fin size or the aerofoil angle of inclination allows the air displacement and aerodynamic work to be controlled. The influence of an enhanced piston effect on the thermal conditions on an underground platform is shown to reduce the air temperatures, through the enhanced displacement of braking heat. Two- and three-dimensional computational fluid dynamics models were developed, and verified with experimental data from the literature, to study numerically the piston effect, train fin and aerofoil and the thermal conditions on an underground platform. The results from the numerical analysis showed that a train aerofoil can increase air displacement by around 8%, with no increase in the aerodynamic work. It was found that an increase in the piston effect 10m^3s^-1 could reduce the highest air temperatures on an underground platform by between 0.16-0.29 °C. The cooling effect of enhancing the piston effect was found to be between 4.5-5.6 kW.
545

Shape Memory Alloys in the design of morphing stator vanes

Wickramasinghe, Kasun January 2016 (has links)
The thesis can be summarised as an exercise in linking the theoretical knowledge of Nickel Titanium Shape Memory Alloys to a practical space. Background work includes establishing a thorough understanding of the behaviour of Shape Memory Alloys, locating the opportunities for actuation available in gas turbine engines, and evaluating past studies that utilise SMA technologies in aerospace applications. Using this knowledge base, an experimental methodology is devised, focused on assessing criteria specifically related to SMA actuation demands. FEM techniques are employed, using data from the experimental study, to develop and analyse SMA actuator concepts. The culmination of these various streams of work effects the conceptualization of a novel solid-state flexural actuator for the gas turbine engine variable stator vane assembly.
546

The influence of stent geometry on haemodynamics and endothelialisation

Boldock, Luke January 2017 (has links)
Every year, millions of people worldwide undergo stent implantation to widen narrowed arteries or to redirect blood away from aneurysms. The rapid post-operative regrowth of a healthy endothelial layer, a key factor in stented artery repair, would reduce complications and improve quality of life for many patients. While this has long been a clinical or pharmaceutical issue, this project considers the role of local haemodynamics, specifically the effects of stent-modified wall shear stress on the endothelium. Endothelial cells have a strong mechanobiological response to wall shear stress magnitude, direction and time variance. To understand the impact of stent geometry on this response through altered fluid dynamics, a novel model vessel was developed for the deployment of a wide range of coronary and flow diverter stents in vitro. The model allowed the observation of both endothelial cell migration and, via particle tracking, disturbed flow within the stents. High-resolution micro-computed tomography scanning techniques replicated stent geometry in silico, enabling computational fluid dynamics simulations for the assessment of wall shear stress distribution. Coronary stents greatly influenced fluid flow. The orientation and distribution of tracked particle streamlines were transformed proximal to stents struts, which were also areas of reduced wall shear stress. These areas correlated with zones of reduced cell migration. Flow diverter stents had a lesser impact on observable particle flow; yet endothelial cell migration within them was completely arrested. This is likely due to their structure directing flow away from the wall and reducing shear stress to an even greater extent that coronary stents, over a more substantial area. Dissimilar cell migration between coronary and flow diverter stents is a point of possible significance as the two are treated alike with respect to post-operative care and medication. Continued analysis of various geometries may enable the efficacy of individual stent designs to be quantified or predicted. By applying this knowledge in the future, careful stent design could reduce their impact, or exert an intentional, active influence on endothelial cells, to optimise the healing process.
547

Development of an external ultrasonic sensor technique to measure interface conditions in metal rolling

Adeyemi, Gbenga Joshua January 2017 (has links)
Metal rolling is by friction which develops at metal-to-roll interfaces during the rolling process. But, friction at the metal-to-roll interface during the metal rolling process can cause roll surface damage if not controlled. Over time, friction results in downtime and repair of the mill. Therefore, lubrication is essential to control the metal-to-roll interface coefficient of friction. It is important to understand the conditions at the metal-to-roll interface to minimize energy loss and improve the strip surface finish. In this work, a new method for measurement of metal-to-roll interface conditions, based on the reflection of ultrasound, is evaluated during the cold metal rolling operation. The method is a pitch-catch sensor layout arrangement. Here, a piezoelectric element generates an ultrasonic pulse which is transmitted to the metal-to-roll contact interface. This method is non-invasive to both roll and strip during the process. The wave reflection from the metal-to-roll interface is received by a second transducer. The amplitude of the reflected waves is processed in the frequency domain. The reflection coefficient values are used to study the metal-to-roll interface conditions at different rolling parameters like, roll speeds and rolling loads. The results show that the reflection coefficient increases with increasing roll speed. This is because of reduction in the roll-bite contact area or increase in the frictional resistance of interface during the roll speed increment. However, the reflection coefficient decreases with increasing rolling load due to either increase in the roll-bite contact area or pressure. The reflection coefficient determines the oil film thickness formation at the metal-to-roll interface. In-addition, the Time-of-Flight of the reflected wave obtained from this technique is used to estimate strip thickness and roll-bite length during the rolling process. The oil film thickness in the range of 1.25µm to 3.05µm was measured during the rolling process. The film thickness increases with increasing roll speed and reduces with increasing rolling load. The roll-bite value of 5.5mm was measured during the process. The results from this study show that this ultrasonic technique can measure the metal-to-roll interface conditions (roll-bite, oil film and strip thickness) during the rolling process. This ultrasonic technique has the advantage of minor roll modification. Additionally, the experimental roll condition values obtained from the ultrasonic reflection method agree with theoretical values. The technique shows promising results as a research tool, and with further development, could be used for lubricant monitoring. Also, it can be utilized in the control system of a working mill for reduction of friction losses in the metal rolling process.
548

Dynamics of premixed flames in tube

Ebieto, Celestine January 2017 (has links)
Experimental work is reported for premixed flames propagating in tubes. The flames were ignited with a pilot flame and the flame propagation captured with high-speed cameras. Initial measurements were performed characterising the rig. These included investigations of the end configuration (open, closed, orifice plate) and whether the tube was horizontal or vertical. For horizontal tube open at both ends, the pressure signal of the propagating flame was recorded and the flame temperature distribution along the tube length was found by observation of a thin silicon carbide filament. The flame propagated steadily immediately after ignition with a curved front, then was subjected to oscillations at the middle of the tube. At the end of the tube, it regained its stability. The pressure and temperature of the propagating flame were highest at the middle point where the flame oscillated. Methane-air flames enriched with hydrogen were studied. As the hydrogen concentration was increased the acoustic pressure initially increased and then decreased this was found to be associated with the presence of Rayleigh-Taylor instabilities. For downwardly propagating flames to a closed end, methane and propane were studied. The flames initially propagated steadily, then at approximately a third of the way down the tube, the primary acoustic oscillation sets in resulting to change in the flame shape. This was then followed by a plateau of variable length before a more violent secondary acoustic oscillation. In some circumstances, flames were observed to rotate due to the primary acoustic instability. Some of the flames were subjected to Rayleigh Taylor instabilities associated with large pressure oscillations. The flame front position growth rate for both methane and propane were similar despite the differences in the fuels. There was a strong correlation between the flame oscillations and changes in CH* and C2* as well as the flame surface area.
549

Investigation into tribological performance of vegetable oils as biolubricants at severe contact conditions

Bahari, Adli January 2017 (has links)
The concern about the pollution created by the use of mineral oil based lubricants and the depletion stock of petroleum has inspired research on alternative lubricants known as biolubricants. The tribological performance of vegetable oils (palm oil and soybean oil) as biolubricants and their blends with mineral oil and anti-wear additives was evaluated in order to assess their potential use in automotive engines. The tests were performed using a reciprocating ball-on-flat test-rig at severe contact conditions with grey cast iron specimens. The performance was compared with a commercial mineral engine oil for benchmarking purposes. At severe contact conditions, the friction and wear results of vegetable oil lubricants were found to be greatly influenced by the wide hardness range of the grey cast iron specimens. The measurement of hardness on the intended wear scar region prior to testing was used in order to provide more robust tribological data. In a pure oil state, the palm oil performance was found to be competitive in friction coefficient with mineral engine oil. However, the mineral oil is far superior in wear protection over vegetable oils due to the additive package it contains its superior oxidative stability. For a vegetable oil-mineral oil blend in equal ratio, the reduction of friction and wear was not significant. However, the addition of 2% zinc dialkyl dithiophosphate in vegetable oils gave significant improvement on the friction and wear. The friction coefficient of palm oil with this additive was very close to the commercial mineral engine oil. The zinc dialkyl dithiophospate in vegetable oil was found to perform three functions; as an anti-wear agent, anti-oxidant and friction modifier. The blend of vegetable oils with boron nitride, however, did not give better results. This could be mainly due to the selection of particles size which was not suitable with the surface roughness. When putting all the results together, the downside of pure vegetable oils is found to be greater in terms of wear resistance and oxidative stability compared to friction. This effect was prominent when they were blended with mineral oils where their tribological performance dominated. However, with the commercial anti-wear additive, specifically the zinc dialkyl dithiophosphate, the vegetable oils showed positive signs as a potential candidate to be used as an alternative lubricant in automotive engine systems even though there is still much room for improvement.
550

Computational model of normal and cancer cell collective mechanics and migration

dos Santos Leite Ferreira, Ana Sofia January 2017 (has links)
Changes in the biological behaviours of cell migration and sorting are associated with cancer. Mechanistic and quantitative understanding of the mechanics of these biological processes can promote the development of anti-cancer treatments. Computational models can be used as platforms to generate this understanding and to test drugs in-silico. The heterogeneity of cancer cells constitutes one of the main drawbacks in the development of anti-cancer drugs. Cell heterogeneity must be comprehended and regarded when developing anti-cancer drugs. This heterogeneity can be accounted for using computational modelling. In addition, now that measurement technologies allow the determination of the mechanical properties of normal and cancer cells, computational models with higher mechanical fidelity are possible. In this context, a quantitative and mechanistic computational model was developed in this work to investigate the role that the mechanical properties of cancer cells play in their migration and sorting. The individual cell properties: Young’s modulus, cell-cell adhesion and local microenvironment (neighbouring cells and position within the monolayer) were found to affect intercellular stress in the first hours following cell seeding. In addition, the presence of mechanically different normal and cancer cells in co-culture results in early sorting between them and higher variation of intercellular stress when comparing to normal and cancer monocultures. Quantitative mechanical thresholds for the sorting of migrating normal and cancer cells in co-culture were defined. Sorting depended primarily on differences in the traction force of normal and cancer cells and absolute cell-cell adhesion levels, followed by the differential adhesion of normal and cancer cells. The predictions supported an integrated mechanism for the sorting of normal and cancer cells. The model also predicted that different spatial distributions of cell mechanical properties can trigger different migration modes in cancer cell populations. This suggests that the plasticity of migration of cancer cell populations is related with the heterogeneity of cell mechanics. Since the sorting of normal and cancer cells in co-culture depends on the spatial distribution of their mechanical properties, mechanical thresholds for cell sorting should additionally depend on the cell microenvironment. The effect of microtubule stabilizers on sorting was tested in-silico accounting for the changes induced in the mechanical properties of cancer cells. Microtubule stabilizers were predicted to reverse both the mechanical and migration properties of cancer cells to properties similar to the ones of normal healthy cells. The sorting of normal and cancer cells, is thereby, reduced. This study shows that individual cell mechanical properties can explain a variety of population-scale measurements and behaviours. The results emphasize the importance of investigating the changes in cell mechanics that accompany malignant transformation and their role in cancer progression.

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