Shape synthesis (also called topological synthesis or inverse design in other disciplines) has the potential to provide antenna engineers with a useful addition to their design tools. Transmitarray antennas, which consist of a feed antenna plus a printed planar structure that emulates a lens, are able to provide high directivity antenna performance, and have been the subject of sustained research over the past ten years. The transmitarray lens consists of a lattice of cells, with each cell occupied by an element that includes conductors of specific shape. The feed field incident on each element on the input surface side of the transmitarray is transformed by each element into a field of different amplitude and phase on the output surface side of each element, providing some desired aperture distribution on the output surface. In this thesis we develop a technique, and the overall computational tool to implement it, that fundamentally allows the electromagnetics to dictate how the conducting portions of a 3-layer element must be shaped in order to obtain some specific transmission coefficient. Such shape synthesis of the elements offers the possibility of obtaining elements that have properties not obtainable using conventional elements. These techniques were applied to the shape synthesis of dual-band elements (18 GHz and 24 GHz). A transmitarray using these elements was designed and fabricated, its performance measured and compared to simulated results. An in-depth discussion of the outcome experimentally validates the shape synthesis procedure.

The Medial Axis Transform (MAT) is a powerful tool for shape analysis and manipulation. Traditional methods for working with shapes usually define shapes as boundaries between some “inside” and some “outside” region. While this definition is simple and intuitive, it does not lend itself well to the construction of algorithms for a number of seemingly simple tasks such as classification, deformation, and collision detection. The MAT is an alternative representation of shape that defines the “inside” region by its center and thickness. We present a method of constructing the MAT which overcomes a significant limitation of its use with real-world data: instability. As classically defined, the MAT is unstable with respect to the shape boundary that it represents. For data sources afflicted by noise this is a serious problem. We propose an algorithm, LSMAT, which constructs a stable least squares approximation to the MAT. / Graduate

The ability to collect valuable data concerning the stress, strains, and shape profiles of aircraft and aircraft components during flight is important to fields such as structural health monitoring, gust alleviation, and flutter control. A research interest in the form of a NASA Phase I SBIR called for possible systems that would be able to take accurate shape sensing data on a flexible wing aircraft. In a joint venture between Luna Technologies Inc. and Virginia Polytechnic Institute and State University a flexible wing wind tunnel model was designed and constructed as a test article for the Luna Technologies Inc. fiber optic shape sensing system. In order to prove the capability of a fiber optic shape sensing system in a wind tunnel environment a flexible wing test article was constructed. The wing deflections and twists of the test article were modeled using a vortex lattice method called Tornado combined with simple beam theories. The beam theories were linear beam theories and the stiffness of the composite bodies was supplied by static testing of the test articles. The code was iterative in that it ran the VLM code to estimate the forces and moments on the wing and these were applied to a linear beam which gave the wing a new geometry which in turn was run through the VLM. The wind tunnel model was constructed at Virginia Tech using 3-D printing techniques for the fuselage and foam and fiberglass for the wings. On the bottom surface of the wings the Luna Technologies Inc. fiber optic shape sensing fiber was bonded along the leading and tailing edges. The swept-wing test article was experimentally tested in the Virginia Tech 6'x6' Stability Wind Tunnel at various airspeeds and the VLM based code results were in agreement, within margins of error and uncertainty, with the experimental results. The agreement of the analytical and experimental results verified the viability of using an iterative VLM code in combination with simple beam theories as a quick and relatively accurate approximation method for preliminary design and testing. The tests also showed that a fiber optic shape sensing system can be sufficiently tested in a wind tunnel environment, and if applied carefully could perhaps in the future provide useful shape and strain measurements. / Master of Science

Indiana University-Purdue University Indianapolis (IUPUI) / The expansion of the applications of direct band gap semiconductor nanocrystals (NCs) has been a result of the control colloidal synthetic methods offer on the optoelectronic properties. These properties are readily controlled by the surface chemistry and even a small change in the surface passivating ligand can show profound effects. Furthermore, the choice of surface passivating ligand also impacts the NC shape evolution, which in turn influence the surface area, quantum yield, and charge transport properties that are critical to optimize device fabrication. In this dissertation, the unique aspects of surface chemistry that control both NC shape evolution and optoelectronic properties are investigated. We began by investigating how surface chemistry controls the shape evolution of methyl ammonium lead bromide (CH3NH3PbBr3) perovskite NCs. In addition to the surface passivating ligand, the reaction temperature and solvent system were also examined. Through a series of control experiments, the critical parameter for the formation of quantum wires (QWs) was found to be the presence of a long chain acid, while the quantum platelets (QPLs) required a long chain amine and chlorinated solvent, and quantum cube (QC) formation was kinetically driven. The higher ordered stacking of the QPLs and bundling of the QWs was also found to be controlled by surface ligand chemistry. Next we further examined how surface chemistry impacts shape evolution, but in the system of metal chalcogenide NCs. We developed a versatile, low temperature, and gram scale synthesis of QWs, QPLs, and quantum rods (QRs) using both cadmium and zinc as metal precursors and sulfur and selenium as chalcogenide precursors. Through systematic investigation of both the surface chemistry and reaction progression, the growth and formation mechanism was also determined. The 1D QW growth required a long chain amine while the QPLs required the presence of both a long and short chain amine to drive 2D growth. Finally, the QRs would found to be a kinetically-controlled process. Ultrasmall semiconductor NCs are known to possess high surface to volume ratios and therefore even a minute change in surface chemistry will have a significant impact on the optoelectronic properties. Our investigation focused on (CdSe)34 NCs, and how exchanging native amine ligands with various chalcogenol based ligands influences these properties. These NCs lie in the strong confinement regime and therefore have a higher probability of undergoing exciton delocalization, resulting in red shifts of the first excitonic peak and reduction of the optical band gap. Additionally, we examined different characteristics of the ligand (level of conjugation, electron withdrawing or donating nature of para-substitution, binding mode and head group) to examine how these parameters impact exciton delocalization. We observed the highest shift in the optical band gap (of 650 meV) after exchanging the native amine ligands with pyrene dithiocarbamate. Through this investigation it was determined that ligand characteristics (specifically conjugation and binding mode) have significant influence in the proposed hole delocalization. Finally, we continued the investigation of how surface chemistry controls optoelectronic properties of ultrasmall NCs, but expand our work to those of methyl ammonium lead halide. We developed a low temperature and colloidal synthesis of white-light emitting NCs with a diameter of 1.5 nm. Through precise manipulation of the surface halide ions, it was possible to tailor the emission to match that of nearly pure white light.

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NiTi (also known as Nitinol) is an almost equiatomic alloy of nickel and titanium and has many applications in various industries, such as biomedical, automotive, and aerospace. NiTi shape memory alloys undergo martensitic phase transformations under both thermal and mechanical loading and exhibit unique properties, such as superelasticity (SE) and shape memory effects (SME). Modeling the fatigue behavior of this alloy is very challenging due to the unique mechanical response of the material. Moreover, there are very limited studies on the fatigue behavior of this alloy under more realistic loading conditions, such as variable amplitude loading and multiaxial loading. In this study, strain-controlled cyclic experiments have been conducted in different conditions to study the variable amplitude fatigue behavior of superelastic NiTi. Nonzero mean strain/stress behavior of superelastic NiTi is investigated, and it is demonstrated that the classical fatigue models for mean strain/stress correction do not appropriately model the nonzero mean strain/stress fatigue behavior of superelastic NiTi. It is shown that, despite common metals (e.g., steel, aluminum, and titanium alloys), mean strain also affects the fatigue behavior of superelastic NiTi, as the resulting mean stress does not fully relax under cyclic load. Two energy-based fatigue models have been proposed based on the results in this study and provide acceptable correlation with experimental observations. The models proposed in this research, account for the effects of mean strain/stress and variations in cyclic deformation. The variations in the cyclic deformation can be due to several factors, such as slight changes in chemical composition, heat treatment processes, texture, etc. The predicted fatigue lives using the proposed fatigue model fall within scatter bands of 1.5 times the experimental life for constant amplitude loading. Analyses also show that the proposed total fatigue toughness parameter, ΣWt, together with the Rainflow cycle counting technique can accurately predict the fatigue life under more realistic loading condition, such as two-step (i.e. high-low and low-high) and variable amplitude load-paths.

The objective of this study was to determine the deflected wing shape and the out-of-plane loads of a large-scale carbon-composite wing of an ultralight aerial vehicle using Fiber Bragg Grating (FBG) technology. The composite wing was instrumented with an optical fiber on its top and bottom surfaces positioned over the main spar, resulting in approximately 780 strain sensors bonded to the wings. The strain data from the FBGs was compared to that obtained from four conventional strain gages, and was used to obtain the out-of-plane loads as well as the wing shape at various load levels using NASA-developed real-time load and displacement algorithms. The composite wing measured 5.5 meters and was fabricated from laminated carbon uniaxial and biaxial prepreg fabric with varying laminate ply patterns and wall thickness dimensions. A three-tier whiffletree system was used to load the wing in a manner consistent with an inlight loading condition.

This thesis presents a survey of techniques to obtain the depth component from two­-dimensional (2D) images. Two common techniques - stereo and shape from shading are examined here. Their performance is compared with an emphasis on noting the fundamental limitations of each technique. An argument is presented which suggests an adjustment of the paradigm with which stereo and shape from shading have been treated in three-dimensional vision. The theoretical development of the stereo and lighting models is followed by experiments illustrating use of these models for a variety of objects in a scene. A comparison of the results provides a motivation for combining them in a particular way. This combination is developed, and its application is examined. Using the model that is consistent for both shape and lighting, significant improvement over either stereo or lighting models alone is shown. / Thesis / Master of Engineering (ME)

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