Widespread use of solid oxide fuel cells is hindered by a lack of long-term durability of seals between metallic and ceramic components caused by thermal expansion mismatch induced cracking. A novel gas seal design incorporating an engineered thermal expansion gradient in a SrO-La₂O₃-A₂O₃-B₂O₃-SiO₂ glass matrix with a TiNiHf shape memory alloy mesh for active stress relief and crack healing is being developed. Coefficient of thermal expansion (CTE) measurements of the seal and fuel cell components shows the possibility for a thermal expansion gradient. Differential scanning calorimetry and microscopy have shown that the TiNiHf alloy has a shape memory transition in the desired range of 200-250ºC. The oxide glass partially crystallizes during thermal cycling which has been observed through X-ray diffraction and dilatometry. The CTE decreases from 9.3Ã 10-6/°C to 6.6Ã 10-6/°C after thermal cycling. Neutron diffraction data from TiNiHf /glass composite samples reveals that the TiNiHf alloy has the ability of absorbing residual stresses from a glass matrix during martensitic phase transition. There is evidence from microscopy that the glass composition is important in determining if reaction will occur with the TiNiHf alloy. The TiNiHf alloy mesh structures can be created using the 3D printing process. This process has been adapted to allow for printing of very thin wire mesh structures of Ni and NiTi powders with a more suitable binder solution. A bi-layer test fixture has been developed which will be useful for assessing leak rate through seal materials. / Master of Science

The work herein is, in part, motivated by the idea of creating optimized, actuating structures using additive manufacturing processes (AM). By developing a consistent, repeatable method for designing and manufacturing multi-material compliant mechanisms, significant performance improvements can be seen in application, such as increased mechanism deflection. There are three distinct categories of research that contribute to this overall motivating idea: 1) investigation of an appropriate multi-material topology optimization process for multi-material jetting, 2) understanding the role that manufacturing constraints play in the fabrication of complex, optimized structures, and 3) investigation of an appropriate process for embedding actuating elements within material jetted parts. PolyJet material jetting is the focus of this dissertation research as it is one of the only AM processes capable of utilizing multiple material phases (e.g., stiff and flexible) within a single build, making it uniquely qualified for manufacturing complex, multi-material compliant mechanisms. However, there are two limitations with the PolyJet process within this context: 1) there is currently a dearth of understanding regarding both single and multi-material manufacturing constraints in the PolyJet process and 2) there is no robust embedding methodology for the in-situ embedding of foreign actuating elements within the PolyJet process. These two gaps (and how they relate to the field of compliant mechanism design) will be discussed in detail in this dissertation. Specific manufacturing constraints investigated include 1) "design for embedding" considerations, 2) removal of support material from printed parts, 3) self-supporting angle of surfaces, 4) post-process survivability of fine features, 5) minimum manufacturable feature size, and 6) material properties of digital materials with relation to feature size. The key manufacturing process and geometric design factors that influence each of these constraints are experimentally determined, as well as the quantitative limitations that each constraint imposes on design. / Ph. D.

During the past decade, Shape Memory Alloys (SMAs), particularly Nickel-Titanium (NiTi) alloys, have received increasing attention mainly because of their promising role to be integrated into multifunctional systems for actuation, morphing, and sensory capabilities in a broad variety of applications including biomedical, aerospace and seismological engineering. The unique performance of all the novel devices developed by SMAs relies on either the shape memory effect or pseudoelasticity, the two distinctive properties of SMAs. Both these unique properties are based on the inherent capability of SMAs to have two stable lattice structures at different stress or temperature conditions, and the ability of changing their crystallographic structure by a displacive phase transformation between a high-symmetry austenite phase and a low-symmetry martensite phase, in response to either mechanical or thermal loading. These properties make them a superior candidate for using as damping materials under high-strain-rate loading conditions in different engineering fields. SMA materials used in the most applications are polycrystalline in nature. In polycrystalline SMAs at the bulk-level, in addition to the phase transformation at the lattice-level, the thermomechanical response is also highly sensitive to the microstructural properties. In this work, the microstructure, as well as defects, such as dislocations and the stacking faults, are studied in the NiTi crystalline structure. In addition, the performance of NiTi under shock wave loading and vibrations, and their energy dissipation capabilities are examined using computational modeling, globally and locally. The effect of graphitic and metal structures, as reinforcements, on the performance of NiTi matrix composites under static and shock stress wave loading conditions is also investigated at the atomistic scale. / PHD / During the past decade, Shape Memory Alloys (SMAs), particularly Nickel-Titanium (NiTi) alloys, have received increasing attention mainly because of their promising role to be integrated into multifunctional systems for actuation, morphing, and sensory capabilities in a broad variety of applications including biomedical, aerospace and seismological engineering. The unique performance of all the novel devices developed by SMAs relies on their ability of changing their crystallographic structure by a displacive phase transformation between a high-symmetry austenite phase and a low-symmetry martensite phase, in response to either mechanical or thermal loading. These properties make them a superior candidate for using as damping materials in different engineering fields. In this work, the microstructure, as well as defects are studied in the NiTi crystalline structure. In addition, the performance of NiTi under shock wave loading and vibrations, and their energy dissipation capabilities are examined using computational modeling, globally and locally. The effect of graphitic and metal structures, as reinforcements, on the performance of NiTi matrix composites under static and shock stress wave loading conditions is also investigated at the atomistic scale.

The unique thermo-mechanical behavior of Shape Memory Alloys (SMAs), such as their ability to recover the original shape upon heating or being able to tolerate large deformations without undergoing plastic transformations, makes them a good choice for actuators. This work studies their application in the aerospace and defense industries where SMA components can serve as release mechanisms for gates of enclosures that have to be deployed remotely. This work provides a novel approach in evaluating the stress and heat induced change of phase in a SMA, in terms of the transformation strain tensor. In particular, the FEA tool ANSYS has been used to perform a 2-D analysis of a Cu-Al-Zn-Mn SMA specimen undergoing a nontraditional loading path in two steps with stress and heating loads. In the first load step, tensile displacement is applied, followed by the second load step in which the specimen is heated while the end displacements are held constant. A number of geometric configurations are examined under the two step loading path. Strain results are used to calculate transformation strain which provides a quantitative measure of phase at a material point; when transformation strain is zero, the material point is either twinned martensite, or austenite depending on the temperature. Transformation strain value of unity corresponds to detwinned martensite. A value between zero and one indicates mixed phase. In this study, through two step loading in conjunction with transformation strain calculations, a method for mapping transient non-uniform distribution of phases in an SMA is introduced. Ability to obtain drastically different phase distributions under same loading path by modifying the geometry is demonstrated. The failure behavior of SMAs can be designed such that the load level the crack initiates and the path it propagates can be customized.

Les Alliages à Mémoire de Forme (AMF) sont des matériaux actifs ayant des propriétés mécaniques spectaculaires comparées aux autres métaux : effets mémoire simple et double sens, pseudo-élasticité et amortissement. Les propriétés des AMF ont pour origine physique une transformation austénite – martensite pilotée par la température et le niveau de contrainte dans le matériau. Les phases austénite (A) et martensite (M) sont présentes respectivement à haute température et à base température. L’effet mémoire, quant à lui, réside dans la capacité du matériau à retrouver la forme austénitique initiale par élévation de température, après avoir été déformé de manière permanente à l’état martensitique à basse température. Le comportement mécanique des structures en béton est gouverné par le processus d’endommagement du matériau. Ce processus peut être retardé en appliquant un chargement uni ou multi-axial de compression, dans le but de contrer les contraintes locales de traction auxquelles le béton est peu résistant. Cette thèse porte sur l’utilisation d’alliages à mémoire de forme (AMF) pour la création d’états de précontrainte dans des composants en béton. Le travail repose sur deux approches : expérimentation et modélisation. Dans la première partie, des essais préliminaires concernent l’étude du comportement thermomécanique de l’AMF en Ni-Ti. Cette réponse complexe est étudiée de manière séparée à l’aide d’une machine de traction – compression uni-axiale couplée à des moyens de chauffage et de refroidissement. Ensuite, des fils d’AMF sont utilisés pour la création de précontraintes dans des poutrelles et de confinements dans des cylindres en béton. Les fils sont étirés à l’état martensitique avant d’être fixés à leurs extrémités sur des éprouvettes en béton. L’activation thermique de l’effet mémoire provoque la mise en contrainte du béton. Et puis, des essais d’écrasement des cylindres sont réalisés pour estimer l’amélioration des performances du béton confiné à l’aide de fils d’AMF. Les résultats montrent que l’effet de confinement permet d’améliorer fortement la performance mécanique en compression du béton. Dans la deuxième partie, un modèle thermomécanique est élaboré pour l’analyse du comportement de fils d’AMF sollicités en traction-compression alternée uni-axiale. Une procédure de calcul numérique pas-à-pas est développé pour la simulation du comportement de fils en AMF pour l’ensemble de la procédure de création d’effet de précontrainte. Cette simulation donne une description fine des mécanismes au sein du fil au cours des essais sur des composants en béton-AMF. L’interaction complexe entre le béton et l’AMF est précisément analysée grâce à l’utilisation du modèle thermomécanique de l’AMF. Enfin, les études de cette thèse confirment une possibilité du champ d’application des AMF dans la thématique du renforcement préventif des structures en béton. / Shape memory alloys (SMAs) are active materials that exhibit special properties such as pseudoelasticity and memory effect. These properties are resulting from austenite vs. martensite reversible transformations governed by temperature and mechanical stress states. The austenite phase (A) and the martensite phase (M) are present respectively at high and low temperature. The shape memory effect is the ability of the material to retain a deformation gained in the martensite phase, i.e. at low temperature, and then to recover its initial shape when it returns to the austenite phase upon temperature increase. The mechanical behaviour of structural concrete is governed by a process of damage. The damaging process can be delayed by applying a uni- or multiaxial compression in order to counterbalance local tensile stresses in the material. The present thesis deals with the use of shape memory alloys (SMAs) to create prestress states in concrete components. The work is based on two approaches: experimental and modelling. In the first part, preliminary tests concern the studies of the thermomechanical behaviour of Ni-Ti SMA. This complex response is studied singly by means of a MTS uniaxial testing machine and heating-cooling systems. Then, SMA wires are used to create prestress states in small-scale concrete beams as well as confinement states in concrete cylinders. They were given a prestrain in a martensitic state before being firmly fixed on concrete components. Thermal activation of the memory effect in the SMA wires caused their tensioning, which resulted in reaction in the creation of stresses in the concrete. Moreover, crush tests of concrete cylinders are performed in order to estimate the improvement of the mechanical performance of concrete confined by means of SMA wires. The test results show that the confinement effect can improve strongly the mechanical performance of concrete. In the second part, a thermomechanical model is performed to analyze the behaviour of SMA with an extension to allow for uniaxial traction-compression. A steps by steps process of the numerical simulation is developed for SMAs during the process of prestress creation. This simulation gives a detailed description of the mechanisms of SMA wires which lead to the process of experimental studies on the SMAs-concrete components. A complex interaction between the concrete and the SMAs is evidenced by means of the thermomechanical model of SMAs. Finally, studies presented in the present thesis confirm the possibility to use SMA as preventive reinforcement for application to civil engineering structures.

Les propriétés thermomécaniques étonnantes des alliages à mémoire de forme (AMF) sont mises à profit dans de nombreux domaines. Ce matériau est capable de mettre en mouvement une structure suite à un changement de température. Or les façades de bâtiments contemporains, pour s’adapter à des conditions climatiques variables, doivent réguler le passage de la lumière et de l’énergie thermique, par exemple au moyen de systèmes motorisés. Le potentiel de fils AMF pour l’actionnement de protections solaires en façade est exploré ici. La modélisation du couplage mécanique induit par l’introduction de tels matériaux dans une structure a été peu étudiée : l’AMF agit sur la structure qui en retour modifie le comportement de l’AMF. La première étape de ce travail a consisté en une contribution à la modélisation du comportement thermomécanique de ce matériau reposant sur le choix d’une énergie libre, d’un potentiel de dissipation et de plusieurs variables internes. Deux modèles unidimensionnels ont été proposés : un premier modèle monocristallin reproduit de façon simplifiée le comportement du matériau, et un second modèle polycristallin propose une description plus fidèle. En parallèle un dispositif d’essai original à température contrôlée a été développé, il a permis une caractérisation fiable de fils Nickel-Titane et l’identification des paramètres des modèles. Dans un second temps ces modèles ont permis de résoudre des cas de couplage élémentaires (fil AMF + ressort, lame élastique + fil AMF noyé) pour des chargements thermomécaniques simples, et des solutions analytiques ont été établies. Les modèles ont été implémentés numériquement via un script matériau utilisateur (UMAT) pour le logiciel éléments finis ABAQUS et au moyen d’un algorithme d’optimisation sous contraintes. Ceci permet de simuler la réponse couplée de systèmes structuraux a priori quelconques intégrant des AMF, connectés à ou noyés dans, une structure. Dans un troisième temps, divers actionneurs ont été conçus, réalisés et testés dans le cadre de l’occultation solaire des façades. Le principe est d’utiliser un cycle de température permettant à l’AMF de déformer la structure, puis à l’énergie élastique de déformation de la structure d’assurer le retour à la forme originale. Le comportement réel de ces actionneurs a été comparé aux calculs analytiques et éléments finis. Des tests cycliques ont également été réalisés / The surprising thermomechanical properties of shape memory alloys (SMA) are harnessed in many engineering fields. This material is able to set a structure in motion upon a temperature change. Today, contemporary building facades must adapt to variable climate conditions as well as to evolving building use and occupancy. In particular, they must regulate light and thermal energy passing through the facade, with motorized systems, for example. We explore the potential of SMA wires for putting in motion solar shading devices in facades. The modelling of the mechanical coupling induced by the introduction of such materials in a structure has received little attention as of now. The SMA acts on the structure which in return modifies the SMA behavior. The first step of this work is a contribution to modelling the thermomechanical behavior of this material through the choice of a free energy, a dissipation potential and internal variables. We propose two one-dimensional models: a first monocrystalline model reproduces the material behavior in a simplified way, and a second polycrystalline model offers a more accurate description of it. An original temperature-controlled testing apparatus was developed in parallel. This led to a reliable characterization of Nickel-Titanium wires and the identification of the model parameters. In a second stage, these models allowed to solve elementary coupling cases (SMA wire + Spring, Elastic plate + Embedded SMA wire) for simple thermomechanical loadings and we established analytical solutions. The models were then numerically implemented via a user-material script (UMAT) for the finite elements software ABAQUS, by using a constrained optimization algorithm. This enables the simulation of the coupled response of, in principle, any structural system including SMA wires, connected or embedded in the structure. Finally, we designed, fabricated and tested different actuators in the context of sunlight control in facades. The working principle lies in using a temperature cycle which allows the SMA to deform the structure, and then allows the elastic strain energy in the structure to ensure the return to the original shape. The real behavior of these actuators have been compared to analytical and finite element calculations. We also performed cyclic tests

[pt] O estudo de materiais inteligentes tem instigado várias aplicações nas mais diversas áreas do conhecimento (da área médica à industria aeroespacial). Os materiais mais utilizados em estruturas inteligentes são as ligas com memória de forma, as cerâmicas piezoelétricas, os materiais magneto-estrictivos e os fluidos eletro- reológicos. Nas últimas décadas, as ligas com memória de forma vêm recebendo atenção especial, sendo utilizadas principalmente como sensores ou atuadores. Existe uma gama de fenômenos associados a estas ligas que podem ser explorados. Visando uma análise mais precisa do comportamento destes materiais, tem se tornado cada vez maior o interesse no desenvolvimento de modelos matemáticos capazes de descrevê-los de maneira adequada, permitindo explorar todo o seu potencial. O objetivo deste trabalho é propor um modelo constitutivo unidimensional que considera quatro variantes de microconstituintes (austenita, martensita induzida por temperatura, martensita induzida por tensão trativa e martensita induzida por tensão compressiva) e diferentes propriedades para cada fase. O efeito das deformações induzidas por temperatura é incluído na formulação. O modelo contempla ainda o efeito das deformações plásticas e o acoplamento entre os fenômenos de plasticidade e transformação de fase. Além disso, são introduzidas modificações na formulação que permitem o alargamento do laço de histerese da curva tensão-deformação, fornecendo resultados mais coerentes com dados experimentais. Por fim, incorpora-se a assimetria no comportamento tração-compressão. A validação do modelo é obtida comparando os resultados numéricos obtidos através do modelo com resultados experimentais encontrados na literatura para ensaios de tração a diferentes temperaturas e para a assimetria no comportamento tração- compressão. / [en] The study of intelligent materials has instigated many applications within the various knowledge areas (from medical field to aerospace industry). The most used materials in intelligent structures are the shape memory alloys (SMA), the piezoelectric ceramics, the magnetostrictive materials and the electrorheological fluids. In the last decades, SMAs have received special attention, being mainly used as sensors or actuators. There is a number of phenomena related to these alloys that can be explored. Aiming a more precise analysis of SMA behavior, the interest on the development of mathematical models capable of describing these phenomena properly has grown, allowing to explore all their potential. The aim of this work is to propose a unidimensional constitutive model which considers four microconstituent variants (austenite, martensite induced by temperature, martensite induced by tensile loading and martensite induced by compressive loading) and different material properties for each phase. The effect of thermal strains is included in the formulation. The model considers the effect of plastic strains and the plastic-phase transformation coupling. Besides, some changes are introduced in the formulation in order to enlarge the stress-strain hysteresis loop, resulting in better agreements with experimental data. Eventually, the tensioncompression asymmetry is incorporated. The model validation is obtained through the comparison between the numerical results given by the model and experimental results found in the literature for tensile tests at different temperatures and for tension- compression asymmetry.

Submitted by MARCELO FRANCISCO MAESTÁ null (mfmaesta@gmail.com) on 2017-02-22T00:41:17Z No. of bitstreams: 1 Tese MArcelo Francisco Maesta.pdf: 11803249 bytes, checksum: c99c02808167e733ec292281a2b8c183 (MD5) / Approved for entry into archive by LUIZA DE MENEZES ROMANETTO (luizamenezes@reitoria.unesp.br) on 2017-02-24T18:06:13Z (GMT) No. of bitstreams: 1 maestra_mf_dr_ilha.pdf: 11803249 bytes, checksum: c99c02808167e733ec292281a2b8c183 (MD5) / Made available in DSpace on 2017-02-24T18:06:13Z (GMT). No. of bitstreams: 1 maestra_mf_dr_ilha.pdf: 11803249 bytes, checksum: c99c02808167e733ec292281a2b8c183 (MD5) Previous issue date: 2016-12-21 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / A busca por aeronaves capazes de modificar sua geometria melhorando suas características aerodinâmicas incentivou diversos autores a sugerirem modelos de asas adaptativas. Tais modelos utilizam atuadores leves de modo a substituir os atuadores clássicos convencionais sem, no entanto, comprometer a e ciência de voo da aeronave. Dentre os materiais utilizados para isto se destacam as ligas de memória de forma (Ni-Ti), que são capazes de converter energia térmica em energia mecânica e, uma vez deformadas, podem retornar a sua condição original de forma através de seu aquecimento. Neste contexto, o presente trabalho objetiva controlar a posição angular de um aerofólio utilizando para isto um par de os de liga de memória de forma. No modelo de asa proposto, deseja-se estabelecer uma forma para o per l aerodinâmico a partir da determinação de um ângulo entre duas seções da asa. Este ângulo é atingido pelo efeito de memória de forma da liga através da passagem de uma corrente elétrica. A função da corrente elétrica é alterar a temperatura dos atuadores através do efeito Joule, modificando a forma da liga. Devido à presença de efeitos não-lineares, principalmente no modelo matemático da liga, propõe-se a aplicação de controladores não-lineares do tipo liga-desliga. / The search for aircraft capable of modifying its geometry improving its aerodynamic characteristics, encouraged several authors to suggest models of adaptive wings. These models use lightweight actuators to replace conventional classic actuators without, however, compromise aircraft flight efficiency. Shape Memory Alloys (SMA) can be used efficiently for this application. These materials are capable of converting thermal energy into mechanical energy and a deformed time, can return to its original condition so through its heating. The current work aims to control the angular position of an airfoil using a couple of alloy wires of shape memory. In the proposed wing model, it is desired to establish a way for the aerodynamic pro le of the determination of an angle between two sections of the wing. This angle is attained by the alloy shape memory effect by passing an electric current. The function of the electric current is to change the temperature of the actuators through the Joule effect, modifying the shape of the shape memory alloy. Due to the presence of non-linear effects, especially in the mathematical model of the alloy, it proposes the application of nonlinear controllers type on-of

Shape memory alloys (SMAs) provide exciting opportunities for miniature actuation systems. As SMA actuators are scaled down in size, cooling increases and bandwidth, improves. However, the inclusion of a bias element with which to cycle the SMA actuator becomes difficult at very small scales. One technique used to avoid the necessity of having to include a separate bias element is the use of local annealing to fabricate a monolithic device out of nickel titanium (NiTi). The actuator geometry is machined out of a single piece of non-annealed NiTi. After locally annealing a portion of the complete device, that section exhibits the shape memory effect while the remainder acts as structural support and provides the bias force required for cycling. This work proposes one such locally-annealed monolithic SMA actuator for future incorporation in a device that navigates the digestive tract. After detailing the derivation of lumped electro-mechanical models for the actuator, a description of the prototyping procedure, including fabrication and local annealing of the actuator, is provided. This thesis presents the experimental prototype actuator behaviour and compares it with simulations generated using the developed models.

Shape memory alloys (SMAs) provide exciting opportunities for miniature actuation systems. As SMA actuators are scaled down in size, cooling increases and bandwidth, improves. However, the inclusion of a bias element with which to cycle the SMA actuator becomes difficult at very small scales. One technique used to avoid the necessity of having to include a separate bias element is the use of local annealing to fabricate a monolithic device out of nickel titanium (NiTi). The actuator geometry is machined out of a single piece of non-annealed NiTi. After locally annealing a portion of the complete device, that section exhibits the shape memory effect while the remainder acts as structural support and provides the bias force required for cycling. This work proposes one such locally-annealed monolithic SMA actuator for future incorporation in a device that navigates the digestive tract. After detailing the derivation of lumped electro-mechanical models for the actuator, a description of the prototyping procedure, including fabrication and local annealing of the actuator, is provided. This thesis presents the experimental prototype actuator behaviour and compares it with simulations generated using the developed models.