No description available.

<p>With the poor room temperature formability of magnesium, rare earth (RE) additions have proven a promising avenue for wrought magnesium products. However, not much is known regarding the effect of these elements on strength and ductility. Stanford et al. (2010b) summarized it best: “Although the addition of rare earth elements offers the possibility of greatly improved mechanical properties, we still lack fairly basic knowledge about the behaviour of these alloying elements”[p.6773]. Through a systematic study across three Mg-RE binary systems, the effect of gadolinium, samarium and scandium on solution strengthening, work hardening behaviour and strain rate sensitivity of Mg-RE binary alloys have been characterized.</p> <p>The results suggest that samarium offers the greatest solid solution strengthening in both tension and compression relative to the other two binary systems. For the binary alloys explored, only gadolinium and samarium in compression followed the Labusch theory of solution strengthening; in which the yield strength scales with c^2/3 . Gadolinium additions provide the largest strength and ductility in tension and compression. Increasing solute content in the binary systems leads to a decrease of the strain rate sensitivity of the alloys. At the highest level of solute, both Mg-Gd and Mg-Sm exhibit negative strain rate sensitivity under tension. In compression, Mg-Gd also exhibits a negative strain rate sensitivity at the highest level solute, whereas Mg-Sm has a low, but positive value. The asymmetry in work hardening behaviour and the form of the flow curves between tension and compression is the result of the difference in the dominating modes of deformation at a given stage of the plastic flow. The extent of the work performed established the hardening levels and thermodynamic deformation parameters which control the flow stress and work hardening behaviour, that can be applied in future work.</p> / Master of Applied Science (MASc)

The objective of this research was to investigate the hydrophilic and mechanical properties of kenaf fiber treated with vinyl tris(2-ethoxymethoxy) silane (VTEMS) and the resulting fiber/resin composite. The hypothesis was that silane-modified kenaf fibers, when used to make Sheet Molding Compounds (SMCs), would improve the water resistance and mechanical properties compared to unmodified kenaf fibers, and these potentially improved SMCs may be a useful material for the automotive and other industries. Fourier transform infrared (FTIR) spectroscopy was used to analyze the modified fibers and the resulting SMCs, and some silica-lignocellulosic bonding was observed. The water absorption of composites decreased as the loading of the silane reagent increased. The MOR and MOE mechanical properties of the SMCs also initially improved as the silane reagent levels increased from 0 (untreated) to 10% silane, but at levels above 10% VTEMS no further enhancement in the mechanical properties was obtained.

Intermetallic composites of molybdenum disilicide reinforced with various amounts of silicon carbide particulate were produced by combustion synthesis from their elemental powders. Elemental powders were mixed stoichiometrically then ball-milled. The cold- pressed mixture was then chemically ignited at one end under vacuum at approximately 700°C. The combustion temperature of the process was approximately 1600°C which was lower than the melting point of molybdenum disilicide. This processing technique allowed the fabrication of the composites at 700°C within a few seconds, instead of sintering at temperatures greater than 1200°C for many hours. The end product was a porous composite, which was densified to >97% of the theoretical density by hot pressing. The grains of the matrix were 8-14 μm in size surrounded by SiC reinforcement of 1-5 μm. The morphology and structure of the products were studied by x-ray diffraction and scanning electron microscopy (SEM). Samples were prepared for hardness, fracture strength, and toughness testing at room temperature. There were improvements in the mechanical properties of the composites with increasing SiC reinforcement. The hardness of the materials increased from 10.1 ± 0.1 GPa (959 ± 13 kg/mm2) to 11.7 ± 0.6 GPa (1102 ± 52 kg/mm2) to 12.7 ± 0.4 GPa (1199 ± 36 kg/mm2) with the 10 vol% and 20 vol% SiC reinforcement, respectively. The strength increased from 195±39 MPa to 237±39 MPa with 10 vol% and to 299 ± 43.2 MPa with a 20 vol% SiC reinforcement. The fracture toughness increased from 2.79 ± 0.36 MPa.m1/2 to 3.31± 0.41 MPa.m1/2 with 10 vol% SiC and to 4.08± 0.30 MPa.m1/2 with 20 vol% SiC. The increase in hardness and flexural strength is due to the effective load transfer across the strong interface in the composites. The main toughening mechanism is crack deflection by the residual stress in the materials, induced by the differences in the thermal expansion coefficients and the elastic moduli of the matrix and reinforcement. / Thesis / Master of Engineering (ME)

Yes / This study's objective was to conduct a comparative analysis and characterization of the microstructural evolution within the weld nugget for joints of AA5083-H112 and AA6061-T6 produced by friction stir spot welding (FSSW) and modified friction stir clinching (MFSC) processes. The mechanical performance of the welded joints was assessed in shear using a single lap joint. The microstructural study identified significant variation in joint microstructure and material flow due to the differences in the tool geometry and methodology used for the welding processes. The results show that the use of modified friction clinching process improves the welded joint by eliminating keyholes/hook defects leading to the formation of high-strength joints. Mechanical characterization of the welded joints indicated that the shear strength increased significantly from 78.69 MPa for the conventional FSSW to 131 MPa for the MFSC process.

Japanese quail were used as animal models in four experiments to evaluate the effects of supplementing diets with different lipids on bone chemical, mechanical, and histological properties. In Exp. 1, laying hens were fed a basal diet containing either 5% soybean oil (SBO), hydrogenated soybean oil (HSBO), chicken fat (CF), or menhaden fish oil (FO). The addition of SBO in the maternal diet increased the levels of total n-6 fatty acids and arachidonic acid (AA, 20:4n-6) in yolk and tibial bones of newly hatched progeny (P<0.01), whereas the maternal FO diet elevated the concentrations of total n-3 fatty acids, eicosapentaenoic acid (EPA, 22:5n-3), docosahexaenoic acid (DHA, 22:6n-3) and total saturated acid, but greatly decreased the amount of AA in both egg and progeny tibiae (P<0.01). The maternal HSBO diet resulted in the accumulation of trans-18:1 fatty acid in egg yolks and tibiae at hatch. The addition of FO or HSBO to the maternal diet significantly lowered the ex vivo PGE2 production of tibiae in newly hatched quail compared to those from hens given the SBO or CF diets (P<0.01). In Exp. 2, the addition of different lipids in the maternal diets did not affect growth, tibial length, diameter or collagen content of the progeny. However, supplementing the maternal diet with 5 % FO or HSBO increased the percent bone ash , increased bone pyridinium crosslinks of collagen, enlarged the cartilaginous proliferative and hypertrophied zones, increased diaphyseal cortical thickness of the tibiae in embryos (P<0.05), and subsequently increased tibial shear force, stiffness (P<0.05) and improved cortical thickness, density and trabecular density in early growth and development of progeny compared to those from hens consuming the SBO or CF diets (P<0.05). In Exp. 3, male quail at one month of age were fed a basal diet containing either 5% SBO, HSBO, CF or FO for seven months. Long-term supplementation in the diets of different lipids did not affect body weight, food intake, tibial length or diameter, but the FO group had the highest tibial percent ash, and both FO and HSBO increased tibial mineral content in aged quail compared to those fed the SBO or CF diets (P<0.05). At 8 months of age, quail fed FO had the highest concentrations of (n-3) fatty acids (20:5n-3, 22:5n-3, 22:6n-3) but the lowest amounts of 20:4n-6 in lipids from tibial cortical bone, whereas the SBO and CF diets greatly elevated (n-6) fatty acids and 20:4n-6 levels. The HSBO diet which contains t18:1 fatty acid resulted in t18:1 accumulation in bone. Long-term supplementation with FO or HSBO increased tibial shear force, stiffness and shear stress, as well as improved cortical thickness and density compared with the SBO or CF diets ( P<.05). In Exp. 4, the addition of SBO or CF to the diet for seven months decreased tibial mineral content compared to the FO diet (P<0.05). Quail fed SBO increased collagen concentration in the tibiae (P<0.05), but the level of collagen crosslinks was higher in quail fed FO or HSBO compared to those given the SBO or CF diets (P<0.05). The PGE2 production in bone organ culture and marrow was greatly increased in quail maintained on the SBO or CF diets (P<0.05). PGE2 production in the bone microenvironment was negatively correlated with the tibial percent ash and collagen crosslinks but had a positive correlation with tibial collagen concentration. The results of these studies demonstrate that either supplementing the maternal diets with or long-term exposure to different lipids alters the chemical composition and metabolism of skeletal tissue in both embryos and aged quail. Maternal dietary SBO or CF had an adverse effect on bone growth and development in embryos. Likewise, long-term exposure to SBO or CF diet impaired bone metabolism and remodeling. In contrast, the FO or HSBO diet had beneficial effects on bone modeling in embryos and remodeling in adult quail. / Ph. D.

The purpose of this study was to characterize the craze growth behavior of polycarbonate (PC) as a function of stress level, model the residual mechanical properties of PC at various craze levels and strain rates, and determine if the total surface area of crazing is the sole factor in residual properties or if the crazing stress plays a role. To obtain these goals, a new in-situ reflective imaging technique was developed to quantify the craze severity in transparent polymers. To accomplish the goal of craze growth rate characterization, polycarbonate samples were placed under a creep load in a constant temperature, constant humidity environment. Using the new technique, the relative craze density was measured as a function of time under load at stresses of 40, 45, and 50 MPa. The craze growth rates were found to increase exponentially with stress level, and the times to 1% relative craze density were found to decrease exponentially with stress level. One exception to this behavior was found at a crazing stress of 50 MPa at which over half of the samples tested experienced delayed necking, indicating competitive mechanisms of crazing and shear yielding. The draw stress was found to be a lower bound below which delayed necking will not occur in a reasonable time frame. The yield stress, elastic modulus, failure stress, and ductility were correlated to crazing stress, relative craze density, and strain rate using a Design of Experiments (DOE) approach. The yield stress was found to correlate only to the strain rate, appearing to be unaffected by the presence of crazes. No correlation was found between the elastic modulus and the experimental factors. The failure stress was found to decrease with an increase in relative craze density from 0 to 1%, increase with an increase in crazing stress from 40 to 45 MPa, and correlate to the interaction between the crazing stress and the strain rate. The ductility of polycarbonate was found to decrease significantly with an increase in relative craze density, a decrease in crazing stress, and an increase in strain rate. The craze microstructure was correlated to the magnitude of stress during craze formation. The area of a typical craze formed at 40 MPa was measured to be more than 2.5 times larger than the area of a typical craze formed at 45 MPa. The fewer, but larger, crazes formed at the lower stress level were found to decrease the failure strength and ductility of polycarbonate more severely than the large number of smaller crazes formed at the higher stress level. / Ph. D.

This dissertation explores the properties of sulfonated poly(arylene ether sulfone)s for desalination membrane applications. A multi-scale approach is used to understand the relationships between the chemical structure of the polymer, the equilibrium water content, and the bulk properties. The polysulfones investigated here are aromatic polymers with relatively high ion contenremain in the glassy state at room temperature even when fully hydrated. In order to better understand the effects of water on these ionic polysulfones molecular dynamics (MD) simulation is used to investigate ion aggregation and hydration at the atomic scale. MD simulations show that the sulfonate and sodium ions are not simply paired. Instead, they form an ionic network. The molecular nature of melting water within sulfonated polysulfones is also examined by combining differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and MD simulation. Experimental evidence shows that at high ion contents, the spacing between the ionic groups impacts the amount of melting water present in the polymer. We conclude that the amount of melting water in the polymer is more closely related to geometric clustering effects than electrostatic effects. Finally, molecular-scale insight is used to understand the trends in hydrated tensile modulus and hydrated glass transition (Tg) temperatures in sulfonated polysulfones. Polymers with a more rigid backbone show different trends compared to those with a more flexible backbone. The modulus and Tg trends for the more flexible backbone are qualitatively consistent with the increase in intra-chain ionic associations (loops) predicted by the sticky Rouse model. / Doctor of Philosophy / This dissertation investigates new materials that could be used to make better membranes that can remove ions (salt) from water. Existing materials are too soft or too brittle when they are fully immersed in water. Consequently, they must be combined with more durable materials in order to make useful membranes. We would like to design durable ionic polymers (large chain-like molecules with ions attached) that interact with water and other ions in a very specific way in order to make membranes that can remove salt efficiently. The goal of this research is to create tools that can describe how changes to the chemical structure of the polymer impact how the polymer, water, and ions interact with each other so that we can improve membrane properties. We find that the ions on the polymer chain interact with each other to form threads, which can form a network inside of the polymer under the right conditions. When the ions are located far apart on the polymer chain, the ion threads link one polymer chain to another polymer chain. These ionic links strengthen the polymer network. However, when the ions are located closer together on the polymer chain, the chain starts to form loops between neighboring ions. As the number of loops increases, the polymer quickly becomes softer and more gel-like. We also find that water molecules are distributed within the polymer and are not always located next to the ions. When there is more water inside the polymer, the water molecules begin to group together to form clusters. At low temperatures, water molecules that have fewer than four neighboring molecules cannot freeze. However, water in a cluster of five molecules or more can freeze into an ice crystal. The insights gained from this research will help the community to design better polymers for desalination membrane applications.

The effect of specimen size on the uniaxial tensile stress/strain response of sublaminatelevel scaled composite specimens, and the four point flexure load/deflection response of ply- and sublaminate-level scaled composite specimens was investigated. Three laminates were studied in the tensile program, namely [+30/-30/90₂]_ns, [+45/-45/0/90}_ns and [90/0/90/0|_ns, where n = 1, 2, 3, 4. Two material systems were used, namely AS4/3502 graphite/epoxy and APC-2 graphite/PEEK, to investigate the relative effect of resin toughness. Three laminates were also studied in the flexure program, The baseline lay-ups were (±45/0/90}_2ns, [0/90/0/90J_2ns and [±45/±45J_2ns, where n = 1, 2, 4. Ply- and sublaminate-level scaling were used to increase specimen thickness. All flexure specimens were of AS4/3502 graphite/epoxy. Enhanced X-ray radiography and edge photomicroscopy were used to examine damage development in specimens loaded to various fractions of their ultimate stress. This nondestructive examination was coupled with observations of critical events in the stress/strain response to try to correlate scaling effects with the damage development in the specimens. Analytical and numerical methods were employed in order to understand the stresses driving certain damage modes observed. 2-D and 3-D finite element models were used to find delamination stresses in an undamaged laminate, and an approximate clasticity approach was used to find stresses duc to cracks in the 90° plies. It was found that the tensile strength of the [+30/-30/90₂]_ns and [+45/-45/0/90}_ns laminates gencrally increased as n increased. This effect was more pronounced for the matrixdominated [+30/-30/90₂]_ns. Both the [+30/-30/90₂]_ns and the quasi-isotropic [+45/-45/0/90}_ns laminates seemed to be approaching a maximum strength, beyond which the strength scaling either stops, or is reversed. As # increased from 1 to 4, these two laminates exhibited a delay in the onset of certain damage mechanisms, such as delamination and transverse matrix cracking. The [90/0/90/0|_ns laminates showed no tensile strcss/strain response scaling effects, although the stress at which first ply failure occurred was found to increase as 7 increased. (±45/0/90}_2ns and [±45/±45J_2ns flexure specimens showed no strength scaling cffects when sublaminate-level scaling was uscd, but significant decreases in s{rength were found when specimen size was increased using ply-level scaling. [0/90/0/90J_2ns specimens showed no global load/deflection scaling effects. / Ph. D.

In order for future space transportation vehicles to be considered economically viable, the extensive use of lightweight materials is critical. For spacecraft with liquid fueled rocket engines, one area identified as a potential source for significant weight reduction is the replacement of traditional metallic cryogenic fuel tanks with newer designs based on polymer matrix composites. For long-term applications such as those dictated by manned, reusable launch vehicles, an efficient cryo-tank design must ensure a safe and reliable operating environment. To execute this design, extensive experimental data must be collected on the lifetime durability of PMC's subjected to realistic thermal and mechanical environments. However, since polymer matrix composites (PMC's) have seen limited use as structural materials in the extreme environment of cryogenic tanks, the available literature provides few sources of experimental data on the strength, stiffness, and durability of PMC's operating at cryogenic temperatures. It is recognized that a broad spectrum of factors influence the mechanical properties of PMC's including material selection, composite fabrication and handling, aging or preconditioning, specimen preparation, laminate ply lay-up, and test procedures. It is the intent of this thesis to investigate and report performance of PMC's in cryogenic environments by providing analysis of results from experimental data developed from a series of thermal/mechanical tests. The selected test conditions represented a range of exposure times, loads and temperatures similar to those experienced during the lifetime of a cryogenic, hydrogen fuel tank. Fundamental, lamina-level material properties along with properties of typical design laminates were measured, analyzed, and correlated against test environments. Material stiffness, strength, and damage, will be given as a function of both cryogenic test temperatures and pre-test cryogenic aging conditions. This study focused on test temperature, preconditioning methods, and laminate configuration as the primary test variables. The material used in the study, (IM7/PETI-5), is an advanced carbon fiber, thermoplastic polyimide composite. / Master of Science