Nitinol shape memory alloys have revolutionized many traditional engineering designs with the unique properties of pseudoelasticity and shape memory effect. At the present moment, primary fabrication processes for Nitinol-based devices include laser cutting and manual techniques. As the interest in incorporating Nitinol in different micro applications and devices increases, the development of effective technology for micro-welding of Nitinol becomes necessary. In general, welding processes may induce significant changes to the processed area rendering the component incompatible or unusable. Strength reduction, inclusions of intermetallic compounds, and changes in pseudoelastic and shape memory effects are all examples of how Nitinol can be affected by welding. The current study has examined the effects of two welding techniques on Nitinol: micro-resistance spot welding (MRSW) and laser micro-welding (LMW). Ni-rich Nitinol wires were welded in a crossed-wire configuration at different energy inputs by varying welding currents for MRSW and peak powers for LMW. The characterization of weld properties focused on the mechanical properties and bonding mechanisms, weld microstructure and formation, and phase transformation temperatures. Additionally, the effects of surface oxide on joint performance were also examined. During MRSW, the primary bonding mechanism was solid state, which consisted of 6 main stages: cold collapse, dynamic recrystallization, interfacial melting, squeeze out, excessive flash, and surface melting. Attempt was made to correlate the joining mechanism with the contact resistances. Joint strength and fracture mechanism were closely linked to the metallurgical properties of the welds. Finally, differential scanning calorimetry (DSC) tests showed that weld metal underwent phase transformation at lower temperatures compared to base material. The second part of this study investigated the effects of Nd:YAG laser micro welding have on Nitinol wires. The fracture strength, weld microstructure, and phase transformation temperatures resulting from the use of varying peak power inputs were studied and compared to both base metal and welds produced using the MRSW process. Results showed good retention of base metal strength and pseudoelastic properties. Moreover, the fusion zone underwent phase transformation at higher temperatures compared to base metal, which substantially altered the active functional properties of Nitinol at room temperature.

The problem of shape control of composite laminated smart structures with piezoelectric patches placed at optimal location is considered in this thesis. Laminated plate structures with piezoelectric patches for shape control applications are modeled using a shear deformable plate formulation by including the piezoelectric layers into the plate substrate. A composite plate finite element model is also developed for composite plates with self-sensing actuators. Non-linear hysteresis models for piezoelectric materials are presented and discussed. Numerical simulation of composite plate structures with piezoelectric actuators is conducted and presented. The optimization problem of finding the optimal location of actuators using a linear quadratic control algorithm is done and the results are discussed. Static shape control strategies are also discussed.

Flicker defined form (FDF) is a temporally-dependent illusion created by the counterphase flicker of randomly positioned element dots, that preferentially stimulates the magnocellular system. Previous studies have found improvement with peripheral presentation, a resistance to blur and a dependence on high temporal frequencies. (Quaid & Flanagan, 2005a; Quaid & Flanagan, 2005b). Although it is seemingly very different from most luminance defined, static stimuli, it is still unknown in what ways it differs. The current study aimed to determine how FDF varies or is similar to static, luminance defined stimuli. Current results showed that FDF could be matched to particular spatial frequencies, and improved with increasing background structure and area. Shapes could be discriminated from each other and recognized. These results suggest that although FDF is dependent on motion pathways for temporal dynamic perception, it could also benefit from the input of form perception pathways, depending on the cues present in the stimulus (e.g. background structure, area). Results also showed that FDF does not benefit from Gestalt rules of contour closure, unlike some static stimuli, although related studies have shown that FDF could still be detected in spite of blur. These studies suggest that FDF appears to rely on motion perception pathways, areas such as MT, but is easier to perceive at times due to overlap in function with shape perception pathways, areas such as IT. As such FDF shares many characteristics with other motion-defined-form stimuli, but uniquely shares aspects of form vision.

Nitinol shape memory alloys have revolutionized many traditional engineering designs with the unique properties of pseudoelasticity and shape memory effect. At the present moment, primary fabrication processes for Nitinol-based devices include laser cutting and manual techniques. As the interest in incorporating Nitinol in different micro applications and devices increases, the development of effective technology for micro-welding of Nitinol becomes necessary. In general, welding processes may induce significant changes to the processed area rendering the component incompatible or unusable. Strength reduction, inclusions of intermetallic compounds, and changes in pseudoelastic and shape memory effects are all examples of how Nitinol can be affected by welding. The current study has examined the effects of two welding techniques on Nitinol: micro-resistance spot welding (MRSW) and laser micro-welding (LMW). Ni-rich Nitinol wires were welded in a crossed-wire configuration at different energy inputs by varying welding currents for MRSW and peak powers for LMW. The characterization of weld properties focused on the mechanical properties and bonding mechanisms, weld microstructure and formation, and phase transformation temperatures. Additionally, the effects of surface oxide on joint performance were also examined. During MRSW, the primary bonding mechanism was solid state, which consisted of 6 main stages: cold collapse, dynamic recrystallization, interfacial melting, squeeze out, excessive flash, and surface melting. Attempt was made to correlate the joining mechanism with the contact resistances. Joint strength and fracture mechanism were closely linked to the metallurgical properties of the welds. Finally, differential scanning calorimetry (DSC) tests showed that weld metal underwent phase transformation at lower temperatures compared to base material. The second part of this study investigated the effects of Nd:YAG laser micro welding have on Nitinol wires. The fracture strength, weld microstructure, and phase transformation temperatures resulting from the use of varying peak power inputs were studied and compared to both base metal and welds produced using the MRSW process. Results showed good retention of base metal strength and pseudoelastic properties. Moreover, the fusion zone underwent phase transformation at higher temperatures compared to base metal, which substantially altered the active functional properties of Nitinol at room temperature.

Shape memory alloys (SMAs) are a class of alloys that display the unique ability to undergo nonlinear deformations and return to their original shape when heat is applied or the stress causing the deformation is removed. This unique shape memory characteristic is a result of a martensitic phase-change, which can be temperature induced (shape memory effect) or stress induced (superelastic effect). In this study, the cyclical behavior of NiTi, a binary shape memory alloy, is compared to the cyclical behavior of NiTiCr, a ternary SMA. The purpose of this study is to compare the behavior of a 0.085-in. diameter NiTiCr wire with the behavior of the same size NiTi wire to determine whether ternary SMAs are more viable ways to take advantage of the unique properties of SMAs for seismic applications. The experimental results showing the superelastic behavior of these alloys under cyclical tensile loading are summarized with attention to the effects of annealing temperature, strain rate, and cyclical training on the stress-strain hysteresis, maximum recoverable strain and equivalent viscous damping.

An intracranial aneurysm can be a serious, life-threatening condition which may go undetected until the aneurysm ruptures causing hemorrhaging within the brain. The typical treatment method for large aneurysms is by embolization using platinum coils. However, in about 15% of the cases treated by platinum coils, the aneurysm eventually re-opens. The solution to the problem of aneurysm recurrence may be to develop more bio-active materials, including certain polymers, to use as coil implants. In this research, a shape memory polymer (SMP) was investigated as a potential candidate for aneurysm coils. The benefit of a shape memory polymer is that a small diameter fiber can be fed through a micro-catheter and then change its shape into a three-dimensional configuration when heated to body temperature. The SMP was tested to determine its thermo-mechanical properties and the strength of the shape recovery force. In addition, composite specimens containing tantalum filler were produced and tested to determine the mechanical effect of adding this radio-opaque metal. Thermo-mechanical testing showed that the material exhibited a shape recovery force a few degrees above Tg. The effects of the metal filler were small and included depression of Tg and recovery force. SMP coils deployed inside a simulated aneurysm model demonstrated that typical hemodynamic forces would not hinder the shape recovery process. The x-ray absorption capability the tantalum-filled material was characterized using x-ray diffractometry and clinical fluoroscopy. Diffractometry revealed that x-ray absorption increased with tantalum concentration, however, not as the rule of mixtures would predict. Fluoroscopic imaging of the composite coils in a clinical setting verified the radio-opacity of the material.

Particle shape is evaluated on three scales corresponding to form, roundness and roughness. Shape at each of these scales uniquely influences material behavior. The shape of sand grains is largely formed as magma cools. Subsequent cleavage and abrasion change the roundness and roughness of particles. Published results indicate that particle shape influences several aspects of granular systems behavior including stiffness, strength, the evolution of strength anisotropy, dilation, and the development of strain localization. The crushing of granite creates a particulate material with a unique angular shape. A wide range of experimental studies implemented as part of this research permit assembling a unique database of material parameters and comparing the behavior of several crushed and natural sands. In general, the low roundness of crushed sands leads to higher maximum void ratios, lower small strain stiffnesses, and higher critical state friction angles than more rounded natural sands. It also impacts mortar strength and workability. Previous studies have emphasized size-controlled segregation. New experimental results show that differences in particle shape can also lead to segregation in a binary granular material. Round or spherical particles are more mobile than angular or flat particles. Then, the greater motion of round or spherical particles in a binary mixture subjected to horizontal or vertical vibration results in their segregation from their angular or flat neighbors. Particle shape may change significantly with stress in the case of soft particles. Therefore, the presence of shape-deformable particles decreases the stiffness of binary rigid-soft particle mixtures. However, macro-scale measurements with rigid-soft mixtures show higher stiffness than would be expected by volume averaging techniques. A subsequent microscale study shows the formation of backbone chains made of the rigid particles, partially supported by the soft particles which prevent the buckling of the load-carrying chains.

This research focuses on the characterization of the structure and mechanical behavior of metal nanowires. Molecular dynamics simulations with embedded-atom method (EAM) potentials are used. A novel shape memory effect and pseudoelastic behavior of single-crystalline FCC metal (Cu, Ni, and Au) nanowires are discovered. Specifically, upon tensile loading and unloading, these wires can recover elongations of up to 50%, well beyond the recoverable strains of 5-8% typical for most bulk shape memory alloys. This novel behavior arises from a reversible lattice reorientation driven by the high surface-stress-induced internal stresses at the nanoscale. It exists over a wide range of temperature and is associated with response times on the order of nanoseconds, making the nanowires attractive functional components for a new generation of biosensors, transducers, and interconnects in nano-electromechanical systems. It is found that this novel shape memory behavior only exists at the nanometer scale but not in bulk metals. The reason is that only at the nanoscale is the surface-stress-induced driving force large enough to initiate the transformation. The lattice reorientation process is also temperature-dependent because thermal energy facilitates the overcoming of the energy barrier for the transformation. Therefore, nanowires show either pseudoelasticity or shape memory effect depending on whether the transformation is induced by unloading or heating. It is also found that not all FCC nanowires show shape memory behavior. Only FCC metals with higher tendency for twinning (such as Cu, Au, Ni) show the shape memory because twinning leads to the reversible lattice reorientation. On the other hand, FCC metals with low likelihood of twinning (such as Al) do not show shape memory because these wires deforms via crystal slip, which leads to irreversible deformation. A micromechanical continuum model is developed to characterize the shape memory behavior observed. This model treats the lattice reorientation process as a smooth transition between a series of phase-equilibrium states superimposed with a dissipative twin boundary propagation process. This model captures the major characteristics of the unique behavior due to lattice reorientation and accounts for the size and temperature effects, yielding results in excellent agreement with the results of molecular dynamics simulations.

Geometric computations are essential in many real-world problems. One important issue in geometric computations is that the geometric models in these problems can be so large that computations on them have infeasible storage or computation time requirements. Decomposition is a technique commonly used to partition complex models into simpler components. Whereas decomposition into convex components results in pieces that are easy to process, such decompositions can be costly to construct and can result in representations with an unmanageable number of components. In this work, we have developed an approximate technique, called Approximate Convex Decomposition (ACD), which decomposes a given polygon or polyhedron into "approximately convex" pieces that may provide similar benefits as convex components, while the resulting decomposition is both significantly smaller (typically by orders of magnitude) and can be computed more efficently. Indeed, for many applications, an ACD can represent the important structural features of the model more accurately by providing a mechanism for ignoring less significant features, such as wrinkles and surface texture. Our study of a wide range of applications shows that in addition to providing computational efficiency, ACD also provides natural multi-resolution or hierarchical representations. In this dissertation, we provide some examples of ACD's many potential applications, such as particle simulation, mesh generation, motion planning, and skeleton extraction.

Most e orts on High Temperature Shape Memory Alloys (HTSMAs), have focused on improving their work characteristics by thermomechanical treatment methods. However, the in uence of transformation induced plasticity (TRIP) and viscoplasticity during actuation has not been studied. The objective of this dissertation work was to study the in uence of plasticity and viscoplasticity on the transformation characteristics that occur during two common actuation-loading paths in TiPdNi HTSMAs. Thermomechanical tests were conducted along di erent loading paths. The changes in the transformation temperature, actuation strain and irrecoverable strain during the tests were monitored. Transmission Electron Microscopy (TEM) studies were also conducted on select test specimens to understand the underlying microstructural changes. The study revealed that plasticity, which occurs during certain actuation load paths, alters the transformation temperatures and/or the actuation strain depending on the loading path chosen. The increase in the transformation temperature and the irrecoverable strain at the end of the loading path indicated that the rate independent irrecoverable strain results in the generation of localized internal stresses. The increased transformation temperatures were mapped with an equivalent stress (which corresponds to an internal stress) using the as-received material's transformation phase diagram. A trend for the equivalent internal stress as a function of the applied stress and accumulated plastic strain was established. Such a function can be implemented into thermomechanical models to more accurately capture the behavior of HTSMAs during cyclic actuation. On the contrary, although the viscoplastic strain generated during the course of constant stress thermal actuation could signi cantly reduce actuation strain depending on the heating/cooling rate. Additional thermomechanical and microstructural tests revealed no signi cant change in the transformation behavior after creep tests on HTSMAs. Comparing the thermomechanical test results and TEM micrographs from di erent cases, it was concluded that creep does not alter the transformation behavior in the HTSMAs, and any change in the transformation behavior can be attributed to the retained martensite which together with TRIP contributes to the rate independent irrecoverable strain. As a consequence, a decrease in the volume fraction of the martensite contributing towards the transformation must be considered in the modeling.