This study proposes a nitinol-based thin-walled arch wedge support (AWS) and validated its performance in shock absorbing by using finite element analysis (FEA) method. Five human movements, two boundary conditions, and three thicknesses were implemented in FEA models to systematically reveal how those parameters and factors affect the response of the AWS. Due to the lack of data, the FEA models were meshed with elements of different sizes and used for simulations until the results converged. The simulation results showed that the thin-walled nitinol AWS with the selected thicknesses can withstand different human movements under both boundary conditions. In another word, the AWS will retain its original shape give the force conditions with no permanent deformation. Based on the initial numerical results, the AWS design can be further optimized before experimentation and testing. The potential of replacing the plastic AWS with additive manufactured nitinol AWS is verified from this study.

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.

En este trabajo de Tesis se estudió la electrosíntesis de distintos recubrimientos utilizando Nitinol (NiTi) como sustrato. Se electroformaron películas de polipirrol (PPy) a partir de diferentes soluciones electrolíticas. La superficie de la aleación se modificó también mediante la técnica de anodizado. Para caracterizar los distintos recubrimientos, se utilizaron técnicas electroquímicas y de superficie, con el objetivo principal de determinar las propiedades anticorrosivas. Se emplearon soluciones electrolíticas que contenían diferentes inhibidores de la corrosión, tales como molibdato, nitrato y citrato para realizar la electrosíntesis de PPy. Se determinó que la presencia de iones molibdato en la matriz polimérica confiere al sustrato una mayor resistencia a la corrosión. El polímero formado potenciostaticamente en una solución de pH 12 que contiene nitrato y molibdato es el más eficaz en términos de adhesión y protección contra la corrosión. La electrosíntesis de PPy también pudo realizarse a partir de soluciones de sulfosuccinato. El polímero fue formado potenciostaticamente en soluciones de pH 1 y 7. Las películas obtenidas fueron muy adherentes y presentaron una conductividad eléctrica aceptable. Si bien ambos recubrimientos mejoraron la protección del sustrato, las películas formadas en la solución de menor pH ofrecieron los mejores resultados. También se realizó la electrosíntesis de películas de PPy sobre NiTi a partir de soluciones de salicilato de sodio (NaSa) de diferentes viii concentraciones. El empleo de una alta concentración de NaSa condujo a la formación de microtubos rectangulares huecos de PPy, con buenas propiedades anticorrosivas. Se estudió la inmovilización de especies de plata y de cobre en el polímero con morfología microtubular. En el caso de la plata, el depósito se realizó en condiciones de circuito abierto y la cantidad de especies incorporadas dependió del grado de oxidación del polímero. Los depósitos de cobre se obtuvieron potenciostaticamente. Finalmente, se evaluó la actividad antibacterial de los recubrimientos ante las bacterias Gram positivas Staphylococcus aureus y Staphylococcus epidermidis. Ambas cepas resultaron sensibles a los recubrimientos modificados con plata mientras que no se obtuvieron resultados positivos para los recubrimientos modificados con cobre. Se inmovilizaron también especies de galio, zinc y cobalto en los microtubos de PPy electrosintetizados sobre NiTi. En este caso, la actividad antibacteriana de los recubrimientos se evaluó frente a Escherichia coli. Los mejores resultados se obtuvieron con las películas que contenían galio. Estas películas también otorgaron una buena protección anticorrosiva al sustrato. Por otra parte, y con el fin de mejorar la resistencia a la corrosión del NiTi, se realizó un proceso de anodizado tanto bajo control potenciostático como galvanostático en soluciones ácidas y alcalinas de molibdato. El proceso de anodizado redujo considerablemente el contenido de Ni y aumentó el de Ti en el óxido superficial. Este enriquecimiento de titanio en la superficie está asociado con el incremento de la resistencia a la corrosión. El óxido formado presentó una superficie lisa, sin grietas o poros. Se obtuvieron resultados similares al anodizar el NiTi en presencia de bis(2-etilhexil) sulfosuccinato de sodio. En este caso, el óxido formado consistió en TiO2, no detectándose Ni en su superficie. / In this Thesis work, the electrosynthesis of different coatings on NiTi as a substrate was studied. Polypyrrole (PPy) films were electrosynthesized using different electrolytes. In addition, the surface of the alloy was modified by means of the anodization technique. To characterize the coatings, different electrochemical and surface techniques were used, with the main objective of determining the anticorrosive properties. The electrosynthesis of PPy films from solutions containing corrosion inhibitors such as molybdate, and citrate was performed. The presence of molybdate ions in the polymer matrix conferred a greater resistance to corrosion. The polymer formed potentiostatically in a pH 12 solution, containing nitrate and molybdate was the most efficient in terms of adhesion and corrosion protection. Electrosynthesis of PPy was also possible using solutions of sulfosuccinate. The PPy was formed potentiostatically in solutions of pH 1 and 7. The films were very adherent and had an acceptable electrical conductivity. While both coatings improved substrate protection, the films formed at a lower pH ((in a pH 1 solution)) offered better results. The electrosynthesis of PPy films onto NiTi from sodium salicylate (NaSa) solutions of different concentrations was also performed. A high NaSa concentration leads to the formation of hollow rectangular microtubes, with good corrosion protection properties. The immobilization of silver and copper species on PPy rectangular microtubes was studied. In the case of silver, the deposition was performed at open circuit conditions and the amount of incorporated species depended on the oxidation degree of the polymer. Copper deposition was performed potentiostatically. Finally, the antibacterial activity of the coatings against the Gram-positive bacteria Staphylococcus aureus and Staphylococcus epidermidis was evaluated. Both strains resulted sensitive to the coatings modified with silver while no positive results were obtained in the case of copper. Gallium, zinc and cobalt species were also immobilized in PPy microtubes electrosynthesized on NiTi. In this case, the antibacterial activity of the coatings was evaluated against Escherichia coli. The best results were obtained for the coatings containing gallium species. This film also exhibited a good corrosion protection performance. In order to improve NiTi corrosion resistance, protective films were formed on its surface by means of anodization under potentiostatic or galvanostatic control in the presence of molybdate, in alkaline and acidic solutions. The anodization process reduced considerably the Ni content and increased the amount of Ti in the oxide film. This titanium enrichment on the outermost surface is associated with the enhanced anticorrosion performance. The oxide grown presented a smooth surface, without cracks or pores. Similar results were obtained when NiTi was anodized in the presence of sodium bis 2-ethylhexyl sulfosuccinate. In this case, the oxide formed consists of TiO2, while no Ni was found in the outermost surface.

This work presents a novel approach to arranging shape memory alloy (SMA) wires into a functional heat engine. Significant contributions include the design itself, a preliminary analytical model and the realization of a research prototype; thereby, laying a foundation from which to base refinements and seek practical applications. Shape memory alloys are metallic materials that, if deformed when cold, can forcefully recover their original, "memorized" shapes, when heated. The proposed engine consists of a set of SMA wires stretched between two crankshafts, synchronized to rotate in the same direction. Cranks on the first crankshaft are slightly longer than cranks on the second. During operation, the engine is positioned between two distinct thermal reservoirs such that half of its wires are heated while the other half are cooled. Wires on the hot side attempt to contract, driving the engine in the direction that relieves the heat-induced stress. Wires on the cold side soften and stretch as the engine rotates. Because the force generated during heated recovery exceeds that required for cooled deformation, the engine is capable of generating shaft power. Limited experimental measurements of shaft speed were performed. An analytical model of the engine predicts that the maximum output power for the prototype, under test conditions, should be 0.75 W. Thermal efficiency, though not measured or calculated in this work, is expected to be low. Potential applications may include the conversion of waste heat into shaft power. / Master of Science

Recent advances in materials engineering have given rise to a new class of materials known as active materials. These materials when used appropriately can aid in development of smart structural systems. Smart structural systems are adaptive in nature and can be utilized in applications that are subject to time varying loads such as aircraft wings, structures exposed to earthquakes, electrical interconnections, biomedical applications, and many more. Materials such as piezoelectric crystals, electrorheological fluids, and shape memory alloys (SMAs) constitute some of the active materials that have the innate ability to response to a load by either changing phase (e.g., liquid to solid), and recovering deformation. Active materials when combined with conventional materials (passive materials) such as polymers, stainless steel, and aluminum, can result in the development of smart structural systems (SSS). This Dissertation focuses on characterization of SMAs and structures that incorporate SMAs. This characterization is based on a hybrid analytical, computational, and experimental solutions (ACES) methodology. SMAs have a unique ability to recover extensive amounts of deformation (up to 8% strain). NiTiNOL (NOL: Naval Ordinance Lab) is the most commonly used commercially available SMA and is used in this Dissertation. NiTiNOL undergoes a solid-solid phase transformation from a low temperature phase (Martensite) to a high temperature phase (Austenite). This phase transformation is complete at a critical temperature known as the transformation temperature (TT). The low temperature phase is softer than the high temperature phase (Martensite is four times softer than Austenite). In this Dissertation, use of NiTiNOL in representative engineering applications is investigated. Today, the NiTiNOL is either in ribbon form (rectangular in cross-section) or thin sheets. In this Dissertation, NiTiNOL is embedded in parent materials, and the effect of incorporating the SMA on the dynamic behavior of the composite are studied. In addition, dynamics of thin sheet SMA is also investigated. The characterization is conducted using state-of-the- art (SOTA) ACES methodology. The ACES methodology facilitates obtaining an optimal solution that may otherwise be difficult, or even impossible, to obtain using only either an analytical, or a computational, or an experimental solution alone. For analytical solutions energy based methods are used. For computational solutions finite element method (FEM) are used. For experimental solutions time-average optoelectronic holography (OEH) and stroboscopic interferometry (SI) are used. The major contributions of this Dissertation are: 1. Temperature dependent material properties (e.g., modulus of elasticity) of NiTiNOL based on OEH measurements. 2. Thermomechanical response of representative composite materials that incorporate NiTiNOL“fibers". The Dissertation focuses on thermomechanical characterization of NiTiNOL and representative structures based on NiTiNOL; this type of an evaluation is essential in gainfully employing these materials in engineering designs.

As ligas Ni-Ti, também conhecidas como Nitinol, são baseadas na fase martensítica NiTi (estrutura monoclínica B'19), exibem propriedades únicas, como efeito de memória de forma e super elasticidade. A sua temperatura de transformação martensítica depende da composição e processamento do Nitinol. Seu valor típico é uma faixa de temperatura entre -80 a 100° C. A utilização de elemento de liga pode contribuir para evitar a formação indesejável de Ni3Ti e NiTi2. De acordo com a seção isotérmica a 900°C do sistema Ti-Ni-Fe, existe uma ampla região monofásica estendida do NiTi ao FeNi. A moagem de alta energia pode produzir nanomateriais e estruturas metaestáveis. O presente trabalho relata a caracterização microestrutural das ligas Ni50Ti48Fe2 e Ni50Ti45Fe5 preparadas por trituração de bolas de alta energia e subsequente sinterização. As misturas de pó elementar foram processadas em diferentes tempos (1, 3 e 5 horas) num moinho de esferas planetário Fritsch P-5 sob atmosfera de argônio utilizando frascos de aço endurecido (225 mL) e esferas (12 mm de diâmetro), velocidade rotativa de 200 rpm, E uma proporção em peso bola-pó de 10:1. Corpos verdes com 10 mm de diâmetro foram compactados por prensagem axial e sinterizados a 900°C, por 16h sob atmosfera de argônio. As informações sobre as transformações de fase nos pós moídos e ligas sinterizadas foram obtidas por difração de raios X, microscopia eletrônica de varredura, espectrometria de dispersão de energia e análise térmica (DSC / TG). Apenas os picos Ni e Ti foram observados em padrões DRX dos pós Ni50Ti48Fe2 e Ni50Ti45Fe5 moídos durante 5 h. Além disso, os maiores picos de Ni e Ti foram alargados e movidos de direção, indicando que os átomos de ferro foram preferencialmente dissolvidos na estrutura de Ni para formar soluções sólidas estendidas. As microestruturas das ligas Ni50Ti48Fe2 e Ni50Ti45Fe5 sinterizadas foram formadas principalmente pela fase NiTi (estrutura monoclinica B'19), e suas transformações martensíticas foram próximas a 50°C. / Ni-Ti alloys, also known as Nitinol, are based on the martensitic NiTi phase (monoclinic B’19 structure) exibit unique properties such as shape memory effect and super elasticity, and its martensitic transformation temperature depends on the nitinol composition and processing. Its typical value is a temperature range between -80-100 °C. The use of alloying can contribute to avoid the undesirable Ni3Ti and NiTi2 formation. According to the isothermal section at 900oC of the Ti-Ni-Fe system, it exists a wide single-phase region extended from the NiTi to FeNi. High-energy ball milling can produce nanomaterials and metastable structures. The present work reports on the microstructural characterization of the Ni50Ti48Fe2 and Ni50Ti45Fe5 alloys prepared by high-energy ball milling and subsequent sintering. The elemental powder mixtures were processed for different times (1, 3 and 5 hours) in a planetary Fritsch P-5 ball mill under argon atmosphere using hardened steel vials (225 mL) and balls (12mm diameter), rotary speed of 200 rpm, and a ball-to-powder weight ratio of 10:1. Green bodies with 10 mm diameter were compacted by axial pressing and sintered at 900oC for 16h under argon atmosphere. Information on the phase transformations in the milled powders and sintered alloys were obtained by X-ray diffraction, scanning electron microscopy, energy dispersive spectrometry, and thermal analyses (DSC/TG). Only the Ni and Ti peaks were noted in XRD patterns of the Ni50Ti48Fe2 and Ni50Ti45Fe5 powders milled for 5 h. In addition, the major Ni and Ti peaks were broadened and moved toward the indicating that the iron atoms were preferentially dissolved into the Ni lattice to form extended solid solutions. The microstructures of the sintered Ni50Ti48Fe2 and Ni50Ti45Fe5 alloys were mainly formed by the NiTi phase (monoclinic B’19 structure), and their martensitic transformations were close to 50oC.

Many applications require deployable structures to meet operational objectives such as satellites that unfurl antenna arrays. Typically, most deployment efforts involve the use of explosive and non-explosive actuators (EAs and NEAs respectively) that have implementation drawbacks such as the expense associated with special handling and the bulk encountered with mounting the devices. To mitigate EA and NEA drawbacks, the integration of shape memory alloys (SMA) as a deployment actuator was investigated. SMA specimens were heated and pulled to failure developing an environmental and structural operating envelope for application as deployment mechanisms. A Finite Element Model (FEM) was also created to model the response behavior induced during specimen testing so that modeled performance could be used in lieu of testing when integrating SMA actuators into deployment systems. Experimental results verified that SMAs can be implemented as deployment actuators. Recorded data showed that SMA fracture is possible over a wide range of temperatures and strains, filling a material performance gap not found in the literature. The obtained information allows design engineers to appropriately size SMAs given design requirements achieving the desired deployment effects. The Finite Element Model was partially successful, capable of emulating strained ambient material behavior up to approximately 6.1%. The limited response is due to lack of experimentally derived large stress and strain available for model emulation.

<p>Covering the metal surface of blood-contacting cardiovascular implants (stents, ventricular assist devices) with functional endothelium may reduce the incidence of clotting and restenosis complications and also reduce the need for risky anticoagulation therapy following implantation of such devices. We developed a novel cell therapy for seeding autologous endothelium onto blood-contacting vascular stents at the point of care to reduce thrombosis and stent restenosis. The proposed research tested the following hypotheses: (1) autologous endothelial cells (ECs) can spread on titanium (Ti) tubes and reduce thrombosis on the Ti surface <italic>in vivo</italic>; (2) shear stresses on the surfaces of an implanted carotid artery stent will be conducive to EC retention and function under arterial flow; and (3) nitinol stents seeded with ECs at the point of care will remain adherent and functional after stent deployment and arterial fluid shear stress conditions <italic>in vitro</italic> and <italic>in vivo</italic>. Based on the experiments reported herein, the primary conclusions of the dissertation are as follows: (1) autologous ECs significantly reduce thrombosis on Ti surfaces implanted into the bloodstream <italic>in vivo</italic>; (2) shear stresses on stent surfaces under carotid artery flow conditions are sufficiently low to be compatible with EC retention and function; (3) ECs seeded onto nitinol stents by infusion at the point of care are retained and spread to form a functional layer following deployment and arterial flow conditions both <italic>in vitro</italic> and <italic>in vivo</italic>.</p> / Dissertation

O desempenho das endopróteses vasculares está relacionado diretamente à sua força radial. Esta é uma importante propriedade mecânica destes dispositivos médicos, influenciada por características da matéria-prima, pelos parâmetros de processamento empregados e pela morfologia do produto. Em um ensaio de força radial, determina-se a força necessária para comprimir e para expandir uma endoprótese, ou seja, em função da variação do seu diâmetro é medida a força envolvida. Uma característica desejada nestes ensaios é que a endoprótese se comprima e se expanda de forma homogênea, mantendo sempre sua seção na forma circular, condição esta que a maioria dos métodos hoje utilizados não atende. O presente trabalho buscou desenvolver um método para ser utilizado na determinação da força radial de endopróteses aórticas. Para tanto, um dispositivo composto por uma cinta polimérica acoplada a uma máquina universal de ensaios foi criado. Para verificar a manutenção da forma circular da endoprótese, imagens foram obtidas no decorrer do ensaio. Os resultados mostraram que o dispositivo desenvolvido é capaz de manter a forma circular das endopróteses durante todo o ensaio. Para examinar a capacidade do dispositivo em detectar diferenças nas forças radiais apresentadas por endopróteses em diferentes condições, foram realizados dois estudos, um variando o diâmetro dos fios (0,2, 0,3 e 0,4 mm) utilizados na fabricação das endopróteses e outro variando o tempo de tratamento térmico (30, 45 e 60 min) das endopróteses. Sabendo-se também que as propriedades elásticas da matéria-prima (Nitinol) têm relação direta com a força radial que as endopróteses apresentam, os resultados dos testes de força radial das amostras foram confrontados com ensaios mecânicos de carregamento e descarregamento nos fios da matéria-prima. Os resultados indicaram que o dispositivo foi capaz de detectar diferenças entre valores de força radial com amostras em diferentes condições (tanto de diâmetro de fio como de tratamento térmico) e que os resultados obtidos foram coerentes com testes de carregamento e descarregamento realizados nas amostras de matéria-prima. / The performance of vascular endoprosthesis is directly related to their radial force that is an important mechanical property of these medical devices. It is influenced by characteristics of the raw materials, the processing parameters and the morphology of the final product. The load necessary to compress and expand an endoprostheses is determined by a radial force test, which the load involved is measured by variation of its diameter. A characteristic required during these tests is homogenous compression and expansion of the endoprostheses maintaining its diameter value, in which most methods do not attend it. The aim of this study was develop a method to determine the radial force of aortic endoprostheses. Thus, a device composed with polymer film-type connected a universal test machine was created. Results showed that device developed is able to maintain the circular section (diameter) of the endoprostheses after test. Two studies were performed in order to check the devices’s ability and detect differences in radial forces of the endoprosthesis produced by different conditions. The first condition presented the varying of the wire diameter (0.2, 0.3 and 0.4mm), that was used in the production of the endoprosthesis, and the second one showed different times of heat treatment (30, 45 and 60 min). It is also well known that the elastic properties of the raw material (Nitinol) are directly related to the radial force presented by endoprosthesis. So, samples were compared with wire loading and unloading mechanical tests. Results indicated that the device was able to detect difference among values of radial force in samples at different conditions (both wire diameter and heat treatment time) and they showed consistence of loading and unloading tests performed on samples of raw material.

O desempenho das endopróteses vasculares está relacionado diretamente à sua força radial. Esta é uma importante propriedade mecânica destes dispositivos médicos, influenciada por características da matéria-prima, pelos parâmetros de processamento empregados e pela morfologia do produto. Em um ensaio de força radial, determina-se a força necessária para comprimir e para expandir uma endoprótese, ou seja, em função da variação do seu diâmetro é medida a força envolvida. Uma característica desejada nestes ensaios é que a endoprótese se comprima e se expanda de forma homogênea, mantendo sempre sua seção na forma circular, condição esta que a maioria dos métodos hoje utilizados não atende. O presente trabalho buscou desenvolver um método para ser utilizado na determinação da força radial de endopróteses aórticas. Para tanto, um dispositivo composto por uma cinta polimérica acoplada a uma máquina universal de ensaios foi criado. Para verificar a manutenção da forma circular da endoprótese, imagens foram obtidas no decorrer do ensaio. Os resultados mostraram que o dispositivo desenvolvido é capaz de manter a forma circular das endopróteses durante todo o ensaio. Para examinar a capacidade do dispositivo em detectar diferenças nas forças radiais apresentadas por endopróteses em diferentes condições, foram realizados dois estudos, um variando o diâmetro dos fios (0,2, 0,3 e 0,4 mm) utilizados na fabricação das endopróteses e outro variando o tempo de tratamento térmico (30, 45 e 60 min) das endopróteses. Sabendo-se também que as propriedades elásticas da matéria-prima (Nitinol) têm relação direta com a força radial que as endopróteses apresentam, os resultados dos testes de força radial das amostras foram confrontados com ensaios mecânicos de carregamento e descarregamento nos fios da matéria-prima. Os resultados indicaram que o dispositivo foi capaz de detectar diferenças entre valores de força radial com amostras em diferentes condições (tanto de diâmetro de fio como de tratamento térmico) e que os resultados obtidos foram coerentes com testes de carregamento e descarregamento realizados nas amostras de matéria-prima. / The performance of vascular endoprosthesis is directly related to their radial force that is an important mechanical property of these medical devices. It is influenced by characteristics of the raw materials, the processing parameters and the morphology of the final product. The load necessary to compress and expand an endoprostheses is determined by a radial force test, which the load involved is measured by variation of its diameter. A characteristic required during these tests is homogenous compression and expansion of the endoprostheses maintaining its diameter value, in which most methods do not attend it. The aim of this study was develop a method to determine the radial force of aortic endoprostheses. Thus, a device composed with polymer film-type connected a universal test machine was created. Results showed that device developed is able to maintain the circular section (diameter) of the endoprostheses after test. Two studies were performed in order to check the devices’s ability and detect differences in radial forces of the endoprosthesis produced by different conditions. The first condition presented the varying of the wire diameter (0.2, 0.3 and 0.4mm), that was used in the production of the endoprosthesis, and the second one showed different times of heat treatment (30, 45 and 60 min). It is also well known that the elastic properties of the raw material (Nitinol) are directly related to the radial force presented by endoprosthesis. So, samples were compared with wire loading and unloading mechanical tests. Results indicated that the device was able to detect difference among values of radial force in samples at different conditions (both wire diameter and heat treatment time) and they showed consistence of loading and unloading tests performed on samples of raw material.