Theses

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  • Twinning and texture development in an extruded AM30 magnesium alloy during compressive deformation
    Twinning and texture development in an extruded AM30 magnesium alloy during compressive deformation
    This study was aimed at evaluating the microstructure, mechanical behavior and texture response of extruded AM30 magnesium alloy in uniaxial compression with special attention to the effect of compressive strain amount, sample orientation, loading direction, compressive prestrain, and annealing. Compressive deformation along the extrusion direction (ED) resulted in sigmoidal true stress-true strain behavior together with three distinct stages of strain hardening, due to the presence of two sets of basal textures {0001}<2110> and {0001}<1010>, with caxes aligned nearly parallel to the normal direction (ND) of the extruded plate which facilitated the occurrence of {1012} extension twinning. The effect of in-plane loading direction, i.e., 0°,15°, 30°, 45° from the ED on the compressive flow behavior was investigated, which revealed nearly constant yield and ultimate compressive strengths but with a remarkable increase of fracture strain compared to the ED sample. The effect of pre-strain along the ED on the formation of twinning and texture was investigated during re-compression along the ED, transverse direction (TD) and ND. In the two-step ED-ED compression, the disappearance of twin boundaries or the coalescence of twins via twin growth was observed. After pre-strain along the ED, re-compression along the TD showed two seemingly opposite phenomena, i.e., the formation of new twins and de-twinning to be coexistent due to the presence of multiple sets of textures after the first-step pre-straining. The de-twinning activity decreased and the texture weakening was achieved with increasing pre-strain in the ED while doing recompression along the ND at a constant strain amount. Texture measurements revealed that the c-axes of hcp unit cells were always rotated towards the compression direction, regardless of compression in the ED, TD or ND. The annealing temperature and time also had a pronounced effect on microstructure and texture evolution. With increasing annealing time, the twins in the pre-compressed samples were observed to disappear gradually, as demonstrated by a decreased volume fraction of twins and weakened texture which became more randomly distributed. As a result, during re-compression along the ED, fewer twin formation and less intense texture were observed, resulting in a significant increase of ductility.
    Two 19th Century Japanese Souvenir Travel Albums At The Art Gallery Of Ontario
    Two 19th Century Japanese Souvenir Travel Albums At The Art Gallery Of Ontario
    This paper analyzes two nineteenth-century Japanese souvenir travel albums from the Art Gallery of Ontario's collection of photography. The project includes: a literature survey discussing sources and researchers of early photography in Japan; contextual research on the introduction of photographic technology in nineteenth-century Japan and the influence that traditional woodblock prints may have had on the genre of commercial souvenir photographs of Japan; and a detailed description of both album with a potential attribution. Furthermore, the applied component of the project, which entailed documentation of both albums in the form of a catalogue of their 100 tinted albumen prints, is included as an appendix. The paper also provides recommendations for the optimal storage and preservation of both albums, as well as a housing solution. The cataloguing and housing of the two albums will enhance accessibility and facilitate future research of these albums.
    Two-photon polymerization using femtosecond laser
    Two-photon polymerization using femtosecond laser
    The demand for microfabrication by laser technology has been the foundation of the new age of three-dimensional micro-structuring processes. One of these new processes that has been established in recent years has been the two-photon polymerization (2PP) technique by femtosecond lasers. Unlike the well known stereo-lithography (SL), this newly developing technology provides simple and rapid fabrication procedures and demonstrates much better quality and structural resolution. In addition, by using computer generated 3D models, this technique can produce any kind of 3D structures that can be then fabricated and integrated into complicated devices. Currently used strategies and methods to produce the smallest possible feature by using 2PP process are acceptable but the[sic] strive for enhanced results is necessary in order to continue the progression of such technique.This thesis proposes a broader knowledge on laser parameters that affect 2PP process and the implementation of such parameters in order to produce the smallest feature size and the highest aspect ratio structure possible. A high repetition rate, high power femtosecond laser is investigated for exceeding the quality demand established by the current market. An experimental study of the proposed laser system and the analysis of control parameters, such as laser power and repetition rate, are presented. The influence of pulse width, laser power, substrate material, and scanning speed on aspect ratio is analyzed in detail. The produced microstructure with the measured aspect ratio indicates the capability of meeting the quality and requirements that have been established by the current market.
    Two-price policy for a newsvendor product supply chain with time and price sensitive demand
    Two-price policy for a newsvendor product supply chain with time and price sensitive demand
    In this study, a dominant manufacturer wholesales a technological product to a retailer. In technology-related industries, the obsolescence of an existing product and/or the appearance of a new product decrease the attractiveness of the existing product. This study also assumes that the market demand is stochastic and price-sensitive, where this price-sensitivity increases by time. Hence, retailers need to decline the retail price during the product life cycle to alleviate the effect of time on the demand. Here, two models/cases are considered. In the first model, the retailer decreases the retail price at midlife without any compensation from the manufacturer. In the second model, the manufacturer gives rebate to the retailer when the retailer declines the retail price at midlife. In addition, the performance of the proposed models is numerically compared with wholesale-price-only and the buyback policies.
    Typha Latifolia response to Oliotrophic and Eutrophic nitrogen and phosphorus loading rates under laboratory conditions
    Typha Latifolia response to Oliotrophic and Eutrophic nitrogen and phosphorus loading rates under laboratory conditions
    Typha latifolia is an aggressive rhizomatous emergent wetland plant that can invade wetlands resulting in near monotypic Typha stands. T. latifolia is also one of the most commonly used macrophyte species in constructed wetlands. The hypothesis that elevated nitrogen and phosphorus concentrations observed in nonpoint source runoff increases T. latifolia fitness and potentially T. latifolia invasiveness was tested under semi-controlled laboratory conditions. A protocol was developed to propagate T. latifolia from seed in low P sediment to simulate an oligotrophic pre-impact reference treatment. Microcosms provided with hypereutrophic levels of P combined with oligotrophic or eutrophic levels of N had significantly greater shoot biomass and maximum leaf height compared to oligotrophic N and P treatment microcosms. These results indicated that high P often found in runoff may contribute to T. latifolia invasion. We recommend that noninvasive species of macrophytes be used in constructed wetlands to prevent impact to ecologically sensitive areas.
    U.S. immigration and media bias surrounding the reporting of the Deferred Action for Childhood Arrivals (DACA) and Deferred Action for Parents of Americans and Lawful Permanent Residents (DAPA) immigration policies
    U.S. immigration and media bias surrounding the reporting of the Deferred Action for Childhood Arrivals (DACA) and Deferred Action for Parents of Americans and Lawful Permanent Residents (DAPA) immigration policies
    In 2012, President Barack Obama used his executive power to bypass Congress and unilaterally pass a controversial immigration policy called the Deferred Action for Childhood Arrivals (DACA) program and two years later its successor, the Deferred Action for Parents of Americans and Lawful Permanent Residents immigration policy. This MRP explores whether a media slant is salient in the editorial reporting surrounding these policies from two major U.S. political networks‐‐ The FOX News Channel (FOX) and the Cable News Network (CNN). Previous academic research (Iyengar & Hahn, 2009; Stroud, 2007) has indicated that CNN’s audience tends to be left-leaning favoring the Democratic Party, while rightleaning conservative Republicans tune into FOX for their political information (Gil de Zúñiga, Correa and Valenzuela, 2012). Keeping this in consideration, would the political networks tailor its digital editorial content to mimic its audiences’ political preference? Borrowing from Benson and Wood’s (2015) media frames surrounding undocumented immigration, a framing analysis and a textual content analysis were employed on the digital editorial content published by FOX and CNN from July 2014 and February 2015. The findings revealed that both networks published messaging aligned with its audiences’ political affiliation. The FOX News Channel emphasized how undocumented immigrants were a problem for society and authorities and published content which contained anti‐Democrat rhetoric and was acutely critical of President Obama. Conversely, the framing analysis revealed the Cable News Network was more likely to accentuate the problems for immigrants and defend President Obama and his unilateral exercises of constitutional powers.
    Uber vs. Pubic transit friend or foe? : an empirical study of ridership trends across Uber cities in the USA
    Uber vs. Pubic transit friend or foe? : an empirical study of ridership trends across Uber cities in the USA
    Uber is a disruptive transportation network company (TNC) that is affecting the way people move in cities. While its effects on the taxi industry are increasingly clear, little research has been conducted to study its effect on public transit ridership. This study uses descriptive statistics to establish demographic and socio-economic profiles of cities that Uber has targeted, and a quasi difference-in-difference analysis to explore Uber's effect on transit ridership levels in order to determine whether the service is acting as a complement or substitute to public transit. The results from this high-level study suggest that Uber my be more of a complement to transit over time, and in large dense regions with highly-educated, affluent, productive labour markets that already have very high transit ridership. Finally, recommendations informed by these findings are offered for transit agencies, policy makers, and municipal governments as they continue to determine how to regulate Uber and similar ride sourcing services
    Ultra-high frequency acoustic impedance imaging of cancer cells
    Ultra-high frequency acoustic impedance imaging of cancer cells
    Acoustic impedance maps of cells can be used to gain insight into its microstructures and physiological state. Information about the cell’s microstructures can be acquired from the acoustic impedance map fluctuations. The maps can also help identify the dominant scattering source in cells. Furthermore, the cell’s physiological state can be inferred from the average acoustic impedance values as many physiological changes in the cell are linked to the alteration in the mechanical properties. A method called acoustic impedance imaging has been used to measure the impedance of biological tissues. We used an acoustic microscope attached to a transducer with a center frequency of 375MHz to acquire acoustic impedance images of breast cancer cells. The generated images suggest that the nucleus has an acoustic impedance similar to the surrounding cytoplasm. Fluorescence and confocal microscopy were used to correlate acoustic impedance images with the cell microstructure (the nucleus). Simulation results demonstrate the system’s capability in detecting cell microstructures close to the substrate. The average acoustic impedance were used to differentiate between single-live, clustered-live and clustered-fixed cancer cells with a measured values of 1.60±0.01 MRayl, 1.61±0.02 MRayl and 1.55±0.02 MRayl respectively.
    Ultrasonic Spot Welding of Similar and Dissimilar Alloys for Automotive Applications
    Ultrasonic Spot Welding of Similar and Dissimilar Alloys for Automotive Applications
    Lightweighting has been regarded as a key strategy in the automotive industry to improve fuel efficiency and reduce anthropogenic environment-damaging, climate-changing, and costly emissions. Magnesium (Mg) alloys and Aluminum (Al) alloys are progressively more used in the transportation industries to reduce the weight of vehicles due to their high strength-to-weight ratio. Similarly, high strength low alloy (HSLA) steel is widely used to reduce gauge thickness and still maintain the same strength, and thereby reduce vehicle weight as well. A multi-material design of automotive structures and parts inevitably involve similar Mg-to-Mg and dissimilar Mg-to-Al, Al-to-steel, and Mg-to-Cu joints. Ultrasonic spot welding (USW) – a solid-state joining technique has recently received significant attention due to its higher efficiency in comparison with conventional fusion welding techniques. In this study, USW was used to generate similar joints of low rare-earth containing ZEK100 Mg alloy sheets and dissimilar ZEK100-to-Al5754, Al6111-to-HSLA steel, and Mg-to-Cu joints at different levels of welding energy or welding time. To optimize welding process and identify key factors affecting the weld strength, microstructural evolution, microhardness test, tensile lap shear test, fatigue test, and fracture analysis were performed on similar and dissimilar ultrasonic spot welded (USWed) joints. Dynamic recrystallization and grain coarsening were observed during Mg-to-Mg similar welding while rapid formation and growth of interface diffusion layer were observed in all dissimilar joints in the present study. It was due to significantly high strain rate (~103 s-1) and high temperature generated via frictional heating during USW. The interface diffusion layer was analyzed by SEM, EDS and XRD phase identification techniques which showed the presence of eutectic structure containing intermetallic compounds (IMCs). As a result, brittleness at the interface increased. The Zn coating in dissimilar USWed Al-to-steel joints eliminated the formation of brittle IMCs of Al-F, which were replaced by relatively ductile AlZn eutectic. The optimum welding energy or welding time during similar and dissimilar USW of lightweight alloys with a sheet thickness of 1-2 mm was in the range of ~500 J to 2000 J (~0.25 s to 1 s).
    Ultrasonic spot welding of lightweight alloys
    Ultrasonic spot welding of lightweight alloys
    Automotive and aerospace sectors have a pressing need for structural components that are lighter and stronger, aiming to improve energy efficiencies and reduce anthropogenic environment. Steel has already a wide variety of structural applications in the transportation industry due to its excellent properties. To further reduce CO2 emissions, lightweight magnesium (Mg) and aluminum (Al) alloys have increasingly been used in the vehicle fabrication due to their lower density, higher specific strength and stiffness, excellent size stability and process ability. The structural application of these alloys inevitably involves welding and joining of similar Mg-to-Mg and Al-to-Al, and dissimilar Mg-to-Al, Mg-to-steel and Al-to-steel. Resistance spot welding produces coarse grains, large defects and thick brittle intermetallic compounds (IMCs) in the weld metal. Alternative solid-state welding processes are being considered such as ultrasonic spot welding (USW), which produces coalescence through the simultaneous application of localized high-frequency vibratory energy and moderate clamping forces. In this study, USW was successfully carried out on similar Mg alloy and dissimilar Mg-to-Al, Mg-to-steel and Al-to-steel alloys. The overall objective of this work is to gain a better understanding of the dominant factors determining the joint performance, with particular emphasis on the microstructural evolution, crystallographic texture, micro-hardness, lap shear strength, fatigue resistance, fatigue life prediction model and fracture analysis of similar and dissimilar USWed joints. Overall, USWed Mg-to-Mg is stronger and more consistent in terms of weldability than the dissimilar USWed Mg-to-Al, Mg-to-steel and Al-to-steel. This was attributed to the large volume of thick brittle IMCs and significantly higher welds center hardness in dissimilar metals welding, which is the main cause of joint failure. The IMCs were confirmed by XRD, EDS and micro-hardness measurement tests.. Therefore, another objective of this study is to minimize the presence of brittle IMCs and engineer an acceptable intermetallic layer to produce sound joints between Mg-to-Al, Mg-to-steel and Al-to-steel. A third material (tin foil or zinc coating) was placed in-between the work pieces. With this procedure, the lap shear strength of the welded samples was increased. The detailed microstructural characterization and mechanical properties of welded joints with an interlayer are presented.