System reliability based design of aircraft wings is studied. A wing of a light commuter aircraft designed according to the FAA regulations is compared with one designed by system reliability optimization. Both the level III, and the advanced first order, second moment (AFOSM) method are employed to evaluate the probability of failure of each failure element of the system representing the wing. In the level III method the statistical correlation between failure modes is neglected. The AFOSM method allows to evaluate the sensitivity derivatives of the system safety index analytically. Furthermore, it accounts for the statistical correlation between failure modes. The results demonstrate the potential of stochastic optimization, and the importance of accounting for the statistical correlation between failure modes. Finally, it is shown that the problem associated with discontinuity of sensitivity derivatives, encountered when using second order Ditlevsen upper bounds to estimate the system failure probability, is circumvented if a penalty function method is used for optimization. / Ph. D.

An experimental investigation was conducted to find effective methods of controlling a junction vortex system around and in the wake of a NACA 0020 body with a 1.5:1 elliptical nose mounted normal to the flat surface. Control of the junction vortex system was investigated using more slender noses and leading edge fillets. The study includes surface flow visualizations, and extensive mean flow measurements. The mean flow measurements consist of velocity, total pressure, and static pressure distributions. The results of the mean flow measurements around and in the wake of a NACA 0020 body with a 1.5:1 elliptical nose show the presence of a large, dominant vortex, or a primary horseshoe vortex, in the junction between the body and the flat surface. A smaller, co-rotating vortex relative to the primary horseshoe vortex was observed at the 100 percent chord position, which is believed to be identified with the corner separation at the junction near the trailing edge. Mean flow measurements of the flow field around and in the wake of a NACA 0020 body with more slender noses were acquired. The results indicate that the more slender nose generated a weaker primary horseshoe vortex. The results also suggest that the weak primary horseshoe vortex developed by the more slender nose was dissipated alongside the body to the point where it had a very small effect on the flow field at the 100 percent chord position. The results of the mean flow measurements around and in the wake of a NACA 0020 body with a 1.5:1 elliptical nose with leading edge fillets indicate that the leading edge fillets were very effective in reducing the strength and size of the primary horseshoe vortex. The results suggest that an effective fillet should be about one boundary layer thickness high and two body thicknesses long. The vorticity identified with the corner separation vortex was observed in the wake for both leading edge shape changes and fillets. The results show that the corner separation vortex dominates the flow in the wake due to the weak effects of the primary horseshoe vortex. A trailing edge fillet was used to investigate its effects on the corner separation vortex in the wake of the NACA 0020 body with the 1.5:1 elliptical nose with a leading edge fillet. The results indicate a small decrease in the strength of the comer separation vortex at the 150 percent chord position, but the effects on the wake pattern were small. / Ph. D.

Like most agricultural production systems, effective decision making in turkey production systems requires the prediction of future status of the system and evaluation of alternative management policies. A simulation model of a turkey production system was developed to predict values of flock performance indicators of significant economic importance, namely body weight and feed consumption. Existing weather simulation models were combined and modified in order to develop a model that predicted daily dry-bulb temperature and humidity ratio outside the turkey house. The weather simulation model was validated using twenty years of daily observed weather data from Roanoke, Virginia. Thermal environment inside the turkey house was predicted from simulated outdoor weather using energy and mass balance equations. House environment prediction part of the model was validated using observed inside and outside temperature data collected at a turkey farm in Virginia. A discrete event simulation model was developed to simulate the effects of house thermal environment, feed energy, sex, and age on weight gain and feed consumption of growing turkeys. The model was validated using temperature, body weight, and feed consumption data collected at a turkey farm in Virginia. The observed average bird weights at marketing age were within 95% confidence intervals of the predicted values. However, the model underpredicted energy consumption values. The sensitivity of the model to variations in R-value, ventilation rate, and feed energy concentration was evaluated. The model was more sensitive to feed energy concentration. / Ph. D.

In this study, the objective was to develop a method of predicting the energy-absorption capability of composite subfloor beam structures. Before it is possible to develop such an analysis capability, an in-depth understanding of the crushing process of composite materials must be achieved. Many variables affect the crushing process of composite structures, such as the constituent materials’ mechanical properties, specimen geometry, and crushing speed. A comprehensive experimental evaluation of tube specimens was conducted to develop insight into how composite structural elements crush and what are the controlling mechanisms In this study, the four characteristic crushing modes, transverse shearing, brittle fracturing, lamina bending, and local buckling were identified and the mechanisms that control the crushing process defined. An in-depth understanding was developed of how material properties, affect energy-absorption capability. For example, an increase in fiber and matrix stiffness and failure strain can, depending upon the configuration of the tube, increase energy-absorption capability. An analysis to predict the energy-absorption capability of composite tube specimens was developed and verified. Good agreement between experiment and prediction was obtained. Sine-wave and integrally stiffened composite beams were evaluated. Composite energy-absorbing beams crush in modes similar to tubular specimens that are made from the same material and have similar geometry. Energy-absorption trends of the composite beams are similar to energy-absorption trends from composite tube specimens. Composite beams are equal or superior energy absorbers to comparable geometry metallic beams. Finally, a simple and accurate method of predicting the energy-absorption capability of composite beams was developed. This analysis is based upon the energy-absorption capability of the beams’ constituent elements. / Ph. D.

The biochemical pathways for the formation of the unusual amino acids found in virginiamycin M₁ and A33853 were investigated. Specifically tritiated and carbon 14 labeled serines were incorporated into virginiamycin M₁. (2S)-serine and (2S,3R)-[3-³H] serine were found to be precursors, thus giving evidence of stereochemical control in the formation of the oxazole moiety. This information allowed for postulation of a ring closure pathway. Stereochemical investigations were also carried out on the dehydroproline unit and it was shown that both (R) and (S) prolines were incorporated into the dehydroproline unit. (2S,3R)-[3-³H] proline was synthesized and upon incorporation lost the (3-³H) label as evidence of stereochemical control in the formation of the dehydroproline unit from a saturated precursor. The basic biosynthetic origins of A33853 were investigated by feeding of D-[U-¹⁴C] glucose, sodium [U-¹⁴C] acetate, (S)-[U-¹⁴C] lysine, (S)-[U-¹⁴C] aspartic acid, [carboxyl-¹⁴C] anthranilic acid, and (S)-[5-³H] tryptophan. D-[U-¹⁴C]. Glucose and (S)-[U-¹⁴C] lysine appeared to be the main precursors. ¹³C¹⁵N lysine was synthesized and used to examine the ring closure of the 3-hydroxypicolinic amide ring in virginiamycin S₁. / Ph. D.

Two experiments were conducted to evaluate the effects on applied force of structural design parameters and feedback conditions inherent in the aesthetic overlay of membrane switch touchpads. In the first experiment, which evaluated structure, 12 males and 12 females keyed 100 4-digit sequences into a computer using 6 of a total of 12 touchpads which differed in membrane ply thickness, spacer thickness, and spacer aperture diameter. The same task was completed by nine males and nine females in the second experiment, which evaluated feedback conditions inherent in flat, embossed, domed, embossed with dome, flat with escutcheon, and domed keycap aesthetic overlays. The apparatus employed for force measures was a force platform system integrating seven strain gauge force transducers. Subjects received auditory feedback for correct actuations. Results of the studies indicate that applied forces are correlated quite highly with the required actuation force (RAF) of the switch (r = 0.89, p < 0.01). However, membrane switch structure had a significant effect on the applied forces, even after the effects of RAF were controlled. Feedback conditions inherent in the aesthetic overlay also had an effect on the forces applied. A significant preference for RAF was found in the structure experiment. There were no significant differences among touchpads with respect to preference for feedback conditions inherent in different aesthetic overlays. / Ph. D.

Ground water contamination from agricultural nitrates poses potential adverse health effects to a large segment of the rural population of the United States. Contamination is especially prevalent in livestock intensive areas, which produce large quantities of animal waste with substantial nitrogen content. In this study, potential management strategies for reducing nitrate contamination of ground water from agricultural sources were examined using an economic-physical model of a representative dairy farm in Rockingham County, Virginia. A mixed integer programming model with stochastic constraints on nitrate loading to ground water and silage production was used to simulate the impacts of various nitrate loading reduction strategies on estimated farm level net returns over variable costs. A survey of all dairy operations in the county was conducted to assist in specifying the mathematical programming model, identify current nutrient management and quality issues, and gauge farmers’ attitudes toward ground water quality and agricultural chemical use. Results of the model indicate that substantial reductions in current nitrate loadings are possible with relatively minor impacts on farmers’ net returns through the use of currently practiced approaches of cost sharing for manure storage facility construction and nutrient management planning. Greater loading reductions are achievable through presently untried policies of land use restrictions, bans on purchase of commercial fertilizer, and imposition of standards on loadings to ground water. These reductions are achieved, however, at higher costs in terms of reduced net returns. Study results indicate that a wide range of policy options exist for reducing nitrate loading to ground water; these reductions, while varying in cost, do not appear to come at the expense of eliminating the economic viability of the county dairy sector. Model results indicate that reductions in nitrate loading of 40 to 70 percent (on average) could be achieved with reductions in farmers’ net returns of one to 19 percent, respectively, when cost sharing for manure storage construction was provided. Explicit consideration was given to the annual variability in nitrate loading due to weather and other factors. The result was higher policy costs than when average loadings alone were considered. / Ph. D.

This thesis is divided into two parts. The first part involves the study of molecular structure utilizing Ianthanide induced shift (LIS) and lanthanide induced relaxation (LIR) nuclear magnetic resonance (NMR). It the course of these studies, it was found that the trifluoroethoxy group is a good deactivating group towards lanthanide shift reagents (LSR) and can be used to selectively deactivate multifunctional molecules. The second part of the thesis involves the utilization of liquid-liquid intermolecular transfer (LLIT) dynamic nuclear polarization (DNP) to study the microwave power needed to achieve saturation using deuterated and nondeuterated 2,4,6-tri-tert-butylphenoxy and galvinoxyl radicals, and ¹⁴N labelled, nondeuterated and ¹⁴N labelled, deuterated 4-hydroxy-TEMPO radicals. Also, selective ¹H DNP enhancements for taxol in solid-liquid intermolecular transfer (SLIT) DNP low-to-high field transfer experiments were obtained. Finally, the syntheses of compounds with both a LSR moiety and a nitroxide radical moiety in the same molecule were examined. LLIT DNP transfer experiments for the intramolecular LSR-nitroxide radicals molecular system and the intermolecular LSR—radicals systems were studied. Conclusions based on this study provide new insight regarding approaches for new LIS and DNP studies. / Ph. D.

The development, characterization, and catalytic application of a novel zeolite with solid base properties has been completed. From isopropanol decomposition to acetone and propylene, the acid/base properties of various alkali exchanged, faujasite-type zeolites are explored. It is found that upon impregnation of CsNaY with cesium hydroxide or cesium acetate, acetone production (normally attributed to base sites) is promoted by an order of magnitude above the untreated (not impregnated) CsNaY zeolite. Furthermore, selectivity to acetone is above 97% and on a surface area basis, acetone activity is found to be comparable to MgO. Impregnation of CsNaX shows little promotion in acetone activity resulting in acetone activity comparable to the untreated CsNaY zeolite and acetone selectivity at 62%. From numerous characterization studies of the impregnated CsNaY zeolite (many in situ) it appears that a highly dispersed intracrystalline cesium oxide is formed from the decomposition of the occluded salt and is believed to be the active site for acetone formation. Evidence is provided also to indicate the presence of an isopropoxide intermediate which is responsible for the acetone and the minor amounts of propylene formed by the acetate impregnated CsNaY zeolite. The cesium acetate impregnated CsNaY catalyst was tested for activity by the base catalyzed alkylation of toluene, acetone, ethane, and methane with methanol. For toluene alkylation, it is determined that little improvement is gained over previous faujasite-type catalysts. This appears to be a result of the rapid decomposition of formaldehyde, i.e., the alkylating agent derived from the dehydrogenation of methanol. MgO is demonstrated to possess base properties similar to the acetate impregnated CsNaY zeolite, yet MgO is found to be inactive in toluene alkylation. Neither ethane or methane is alkylated at 425°C or 465°C by the acetate impregnated zeolites. Acetone is alkylated slightly to methylvinylketone and methylethylketone at 425°C but the majority of the reacted acetone appears to form aldol condensation products. / Ph. D.

Novel, high molecular weight, high glass transition temperature, photocrosslinkable polyimide and poly(imide siloxane) homo- and segmented copolymers were prepared and characterized. The polyimides were synthesized by the classical two step method of first preparing soluble poly(amic acid) prepolymers by the reaction of various aromatic dianhydrides with aromatic diamines. The siloxane modified copolymers were synthesized by reacting single or mixed components of the aromatic dianhydrides with a mixture of aromatic amine and bis(3-aminopropyl) end blocked polydimethyl siloxane oligomers in a cosolvent system of tetrahydrofuran and N-methyl-2-pyrrolidinone. These dysfunctional aminopropyl terminated siloxane oligomers were prepared through an anionic ring opening equilibration polymerization of octamethylcyclotetrasiloxane with bis(3—aminopropyl) tetramethyldisiloxane in the presence of siloxanolate catalyst. Soluble fully imidized polyimides were obtained by use of a solution imidization procedure which utilized a cosolvent system of N-methyl-2-pyrrolidinone and N-cyclohexyl-2-pyrrolidone at temperatures of approximately 170°C. The fully imidized polyimides were soluble in a variety of solvents. The homo- and copolymers have been characterized for compositional analysis by FT-IR and proton NMR spectroscopy. All polymers were characterized for their thermal properties by differential scanning calorimetry, dynamic mechanical thermal analysis and thermogravimetric analysis. All homoand copolymers possessed excellent thermal characteristics and good mechanical properties. The photosensitive properties of the polyimide and poly(imide siloxane) homo- and copolymers were investigated at the UV wavelengths of 313nm and 365nm. The photosensitivities were found to depend on both the amount of benzylic methyl substituted diamine incorporated into the polyimide backbone, and the amount of aromatic ketone concentration. High concentrations of fluorinated (6F) dianhydride were also desirable. Incorporation of the polydimethylsiloxane segments into the polyimide decreased the optical density without decreasing the photosensitivity and therefore desirably allowed thicker films to be crosslinked at lower exposure doses. The adhesion of the siloxane modified polyimides to the silicon wafers was increased with significantly increasing siloxane content and at 20 weight percent, eliminated the need for conventional coupling agents. / Ph. D.