Shape memory alloys (SMA) offer several advantages over traditional electro-mechanical devices, including: smooth, silent, clean operation; linear actuation; high power/weight ratio; scalability; and reduced part counts. These unique characteristics make them an attractive option when developing actuators, particularly at the meso- and micro-scales. However, SMAs do not typically display cyclic actuation and, therefore, require some reset force or bias mechanism in order to achieve this behaviour. Additionally, the micro-assembly of SMA material with a reset mechanism becomes increasingly difficult as the dimensions of actuators are scaled down. Therefore, actuators have been developed in which the actuation and reset mechanism are fabricated from a single piece of material. These actuators are referred to as monolithic actuators. Monolithic actuators are fabricated from a single piece of SMA material in which local annealing is used to selectively impart the shape memory effect (SME), while the remainder of the material acts as the bias mechanism. This work proposes an extension to monolithic actuators that locally varies the material composition of the monolithic component to exhibit different mechanical properties in select regions. This eliminates the need for local annealing by introducing regions of material unaffected by the annealing process. Additionally, incorporating regions of superelastic material to act as the bias mechanism greatly increases the actuator’s range of motion. These actuators are referred to as hybrid monolithic actuators. The creation of hybrid monolithic SMA actuators requires the development of both a fabrication technique and design tool. Varying the composition locally is accomplished by utilizing powder metallurgy fabrication techniques, specifically tape casting. Tapes of different compositions are cut, stacked, and sintered resulting in a monolithic component with mechanical properties that vary spatially. Tape casting NiTi from elemental powders is studied in this work, and tape recipes and sintering profiles are developed. In order to model the SMA behaviour of complex geometries, a finite element implementation of an existing lumped-element SMA model is developed. This model is used to design and simulate a prototype hybrid monolithic actuator. The prototype is fabricated and its performance used to illustrate the advantages of hybrid design over typical monolithic actuators.

Shape memory alloys (SMA) offer several advantages over traditional electro-mechanical devices, including: smooth, silent, clean operation; linear actuation; high power/weight ratio; scalability; and reduced part counts. These unique characteristics make them an attractive option when developing actuators, particularly at the meso- and micro-scales. However, SMAs do not typically display cyclic actuation and, therefore, require some reset force or bias mechanism in order to achieve this behaviour. Additionally, the micro-assembly of SMA material with a reset mechanism becomes increasingly difficult as the dimensions of actuators are scaled down. Therefore, actuators have been developed in which the actuation and reset mechanism are fabricated from a single piece of material. These actuators are referred to as monolithic actuators. Monolithic actuators are fabricated from a single piece of SMA material in which local annealing is used to selectively impart the shape memory effect (SME), while the remainder of the material acts as the bias mechanism. This work proposes an extension to monolithic actuators that locally varies the material composition of the monolithic component to exhibit different mechanical properties in select regions. This eliminates the need for local annealing by introducing regions of material unaffected by the annealing process. Additionally, incorporating regions of superelastic material to act as the bias mechanism greatly increases the actuator’s range of motion. These actuators are referred to as hybrid monolithic actuators. The creation of hybrid monolithic SMA actuators requires the development of both a fabrication technique and design tool. Varying the composition locally is accomplished by utilizing powder metallurgy fabrication techniques, specifically tape casting. Tapes of different compositions are cut, stacked, and sintered resulting in a monolithic component with mechanical properties that vary spatially. Tape casting NiTi from elemental powders is studied in this work, and tape recipes and sintering profiles are developed. In order to model the SMA behaviour of complex geometries, a finite element implementation of an existing lumped-element SMA model is developed. This model is used to design and simulate a prototype hybrid monolithic actuator. The prototype is fabricated and its performance used to illustrate the advantages of hybrid design over typical monolithic actuators.

Shape memory alloys (SMAs) are unique metallic alloys which can undergo large deformations while reverting back to their undeformed shape through either the application of heat (shape memory effect) or the removal of the load (superelastic effect). A multi-scale and multi-disciplinary approach is taken to explore the use of large diameter NiTi SMAs for applications in earthquake engineering. First, a materials characterization study is performed by studying precipitate formation, grain size and orientation, thermal transformation behavior, and strength. Cyclic tensile tests on coupon specimens and full-scale large diameter bars are then used to correlate the microstructural properties to the macroscopic behavior. Further experimental studies using NiTi wire are performed in order to optimize their properties for seismic applications. The ability of mechanical training to stabilize NiTi cyclic properties, the ability of pre-straining to increase damping levels, and the influence of different types of earthquake loadings are considered. Phenomenological mechanical models are then developed based on these results. An analytical study is then used to evaluate the performance of structural systems incorporating SMAs. One type of system evaluated includes an SMA bracing system used to modify the response of a structure during a seismic event. Overall, the results of this study have shown the ability to optimize the properties of NiTi SMAs for seismic applications through material processing. The analytical results show potential for the use of SMAs in seismic applications and provide areas for continued research.

Submitted by Emanuel Varela Cardoso (emanuel.varela@ufcg.edu.br) on 2018-06-15T19:15:40Z No. of bitstreams: 1 MAGNA SILMARA DE OLIVEIRA ARAÚJO – DISSERTAÇÃO (PPG-CEMat) 2016.pdf: 3448861 bytes, checksum: b9341836ea322c6702a8fe974682fc68 (MD5) / Made available in DSpace on 2018-06-15T19:15:40Z (GMT). No. of bitstreams: 1 MAGNA SILMARA DE OLIVEIRA ARAÚJO – DISSERTAÇÃO (PPG-CEMat) 2016.pdf: 3448861 bytes, checksum: b9341836ea322c6702a8fe974682fc68 (MD5) Previous issue date: 2016-08-18 / Capes / As Ligas com Memória de Forma (LMF) pertencem a uma classe de ligas metálicas que possuem características funcionais únicas de Efeito Memória de Forma (EMF) e Superelasticidade (SE). As LMF do sistema Ni-Ti são as mais difundidas no mercado e podem ser encontradas em diversas aplicações que abrangem, principalmente, os campos de medicina e odontologia. No entanto, muitas destas aplicações acontece sob solicitações cíclicas ou variáveis, o que torna imprescindível o estudo da vida em fadiga destes tipos de materiais. Diante disto, o presente trabalho tem como objetivo analisar o comportamento em fadiga de fios superelásticos de LMF Ni-Ti com seção transversal circular e retangular, submetidos a ensaios dinâmicos em modo de flexão simples (Single Cantilever) utilizando um equipamento de Análise DinâmicoMecânica (DMA - Dynamic Mechanical Analysis). A vida em fadiga dos fios Ni-Ti foi avaliada por meio do número de ciclos até a ruptura em função das amplitudes de deformação aplicadas durante o processo de ciclagem mecânica. Adicionalmente, a fadiga funcional foi avaliada por meio do acompanhamento da evolução da força aplicada em função do número de ciclos para diferentes amplitudes de deformação (0,7; 1,0; 1,3 e 1,6%) e níveis diferentes de frequê ncia de carregamento (0,5 e 1,0Hz). A influência simultânea da amplitude de deformação e frequência de carregamento sob a vida em fadiga dos fios foi avaliada através de um Planejamento Fatorial. Observou-se, em geral, que a força sofre um leve aumento, de aproximadamente 5%, durante os primeiros ciclos, tendendo a se estabilizar e permanecendo praticamente constante até iniciar um decaimento devido ao processo de ruptura cíclica. Constatou-se também, através das curvas de Wöhler, que o fio de seção circular possui uma vida em fadiga superior àquela do fio de seção retangular. O Planejamento fatorial utilizado permitiu a obtenção de modelos estatísticos significativos e bem ajustados. Além disso, o número de ciclos até a fratura dos fios Ni-Ti depende de forma direta da amplitude de deformação cíclica e da frequência de ensaio, situando-se na faixa de 103 a 105 ciclos, caracterizando uma fadiga de baixo ciclo. / Shape Memory Alloys (SMA) belong to a class of metallic alloys that have unique functional characteristics: Shape Memory Effect (SME) and Superelasticity (SE). The Ni-Ti SMA system are the most widespread in the market and can be found in diverse applications covering mainly medical and odontology. However, many of these applications takes place under cyclic or variables loads, which makes it necessary to study the fatigue life of these materials. Therefore, the present study aims to analyze the fatigue behavior of Ni-Ti SMA superelastic wires with circular and rectangular, cross sections subjected to dynamic tests in simple bending mode (Single Cantilever) using a Dynamic Mechanical Analysis (DMA) equipment. The fatigue life of the Ni-Ti wires was evaluated by the number of cycles until break as a function of applied strain amplitudes during the mechanical cycling process. In addition, functional fatigue was assessed by monitoring the evolution of the applied force on the number of cycles for different deformation amplitudes (0.7, 1.0, 1.3 and 1.6%) and different levels of frequency loading (0.5 and 1.0Hz). The simultaneous influence of strain amplitude and frequency on fatigue life of the wires was assessed through a factorial design. It was observed generally that the strength under goes a slight increase of approximately 5% during the first cycles, tending to stabilize and remained virtually constant until starting a cyclic decay due to rupture process. It was also observed by means of Wöhler curves, that circular section wires has a higher fatigue life to that of the rectangular wires. The factorial design used allowed to obtain significant statistical models, predictive and well adjusted. Furthermore, the number of cycles to failure of the Ni-Ti wires depends directly of the cyclic strain amplitude and frequency of testing, to stand in the range 103 -105 cycles, characterizing a low cycle fatigue.

A coupled kinetic Monte Carlo – Finite Element Analysis (kMC–FEA) method is developed with a numerical implementation in the Scalable Implementation of Finite Elements at NASA (ScIFEN). This method is presented as a mesoscale model for Shape Memory Alloy (SMA) material systems. The model is based on Transition State Theory and predicts the nonlinear mechanical behavior of the 1st order solid–solid phase transformation between Austenite and Martensite in SMAs. The kMC–FEA modeling method presented in this work builds upon the work of Chen and Schuh [1, 2]. It represents a “bottom-up” approach to materials modeling and could serve as a bridge for future studies that attempt to link ab initio methods with phenomenological findings in SMA systems. This thesis presents the derivation of the kMC–FEA model, which is then used to probe the various responses expected in SMAs and verify the influence of model parameters on simulation behavior. In a departure from the work of Chen and Schuh, the thermodynamic derivation includes an elastic transformation energy term, which is found to be a significant fraction of the total transformation energy and play an important role in the evolution of a simulation. Theoretical predictions of the model behavior can be made from this derivation, including expected transformation stresses and temperatures. A convergence study is presented as verification that the new elastic energy term proposed in this model is a reasonable approximation. A parameter sensitivity study is also presented, showing good agreement between theoretical predictions and the results of a full-factorial numerical exploration of model outputs. Model simulation demonstrates the emergence of the shape memory effect, an important SMA behavior not shown by Chen and Schuh, along with the expected superelastic effect and thermal hysteresis. Further exploration of simulated model outputs presented in this work involves comparison with experimental data and predicted output values obtained from a separate phenomenological constitutive model. This comparison shows that the kMC–FEA method is capable of reproducing qualitative, but not yet quantitative, responses of real SMA material systems. Discussion of each model parameter and its effects on the behavior of the model are presented as guidelines for future studies of SMA materials. A complete implementation of the method is contained in a new finite element software package (ScIFEN) that is available for future

The @lab is a group dedicated to the research and development of electronic textiles for architectural applications; this thesis presents the structural analyses performed by the author to improve the @lab’s projects. Also included are three investigations performed by the author that pertain to smart material applications in responsive architecture and textiles. The first investigation evaluated the feasibility of using piezoelectric materials to harvest power from human foot traffic; overall, it was determined to not be feasible. The second investigation experimentally tested how six parameters of shape memory alloy spring actuators affect their reaction times and stroke; all six parameters affected the reaction times and/or stroke. The third investigation experimentally tested how three parameters of superelastic SMA springs influence their stiffness and resonant frequencies; overall, it was found that traditional spring mechanics can be used to predict their behavior providing the internal stress does not reach the upper plateau stress.

The ability to adjust structural response to external loading and ensure structural safety and serviceability is a characteristic of Smart Systems. The key to achieving this is through the development and implementation of smart materials. An example of a smart material is a Shape Memory Alloy (SMA). Reinforced concrete structures are designed to sustain severe damage and permanent displacement during strong earthquakes, while maintaining their integrity, and safeguarding against loss of life. The design philosophy of dissipating the energy of major earthquakes leads to significant strains in the steel reinforcement and, consequently, damage in the plastic hinge zones. Most of the steel strain is permanent, thus leading to large residual deformations that can render the structure unserviceable after the earthquake. Alternative reinforcing materials such as superelastic SMAs offer strain recovery upon unloading, which may result in improved post-earthquake recovery. Shape Memory Alloys have the ability to dissipate energy through repeated cycling without significant degradation or permanent deformation. Superelastic SMAs possess stable hysteretic behavior over a certain range of temperature, where its shape is recoverable upon removal of load. Alternatively, Martensite SMAs also possess the ability to recover its shape through heating. Both types of SMA demonstrate promise in civil infrastructure applications, specifically in seismic-resistant design and retrofit of structures. The primary objective of this research is to investigate experimentally the performance of concrete beams and shear walls reinforced with superelastic SMAs in plastic hinge regions. Furthermore, this research program involves complementary numerical studies and the development of a proposed hysteretic constitutive model for superelastic SMAs applicable for nonlinear finite element analysis. The model considers the unique characteristics of the cyclic response of superelastic materials.

The ability to adjust structural response to external loading and ensure structural safety and serviceability is a characteristic of Smart Systems. The key to achieving this is through the development and implementation of smart materials. An example of a smart material is a Shape Memory Alloy (SMA). Reinforced concrete structures are designed to sustain severe damage and permanent displacement during strong earthquakes, while maintaining their integrity, and safeguarding against loss of life. The design philosophy of dissipating the energy of major earthquakes leads to significant strains in the steel reinforcement and, consequently, damage in the plastic hinge zones. Most of the steel strain is permanent, thus leading to large residual deformations that can render the structure unserviceable after the earthquake. Alternative reinforcing materials such as superelastic SMAs offer strain recovery upon unloading, which may result in improved post-earthquake recovery. Shape Memory Alloys have the ability to dissipate energy through repeated cycling without significant degradation or permanent deformation. Superelastic SMAs possess stable hysteretic behavior over a certain range of temperature, where its shape is recoverable upon removal of load. Alternatively, Martensite SMAs also possess the ability to recover its shape through heating. Both types of SMA demonstrate promise in civil infrastructure applications, specifically in seismic-resistant design and retrofit of structures. The primary objective of this research is to investigate experimentally the performance of concrete beams and shear walls reinforced with superelastic SMAs in plastic hinge regions. Furthermore, this research program involves complementary numerical studies and the development of a proposed hysteretic constitutive model for superelastic SMAs applicable for nonlinear finite element analysis. The model considers the unique characteristics of the cyclic response of superelastic materials.

The feasibility of using superelastic shape memory alloys in the retrofit of multiple frame bridges is investigated. First, three shape memory alloy constitutive models with various levels of complexity are compared in order to determine the significance of including subloops and cyclic loading effects on the structural response. The results show that the structural response is more sensitive to the shape memory alloys strength degradation and residual deformation than the sublooping behavior. Next, two parametric studies are conducted to explore the sensitivity of hinge opening to the mechanical behavior of the superelastic shape memory alloys. The first study is focused on the hysteretic properties of the alloy that could vary depending on the chemical composition or the manufacturing process of the alloy, while the second study targets the changes in the mechanical behavior of shape memory alloys resulting from the variability in the ambient temperature. The results show that the hysteretic behavior of shape memory alloys has only a slight effect on the bridge hinge opening as long as the recentering property is maintained. A detailed study on the effect of temperature shows that a reduction in the ambient temperature tends to negatively affect the hinge opening while an increase in temperature results in a slight improvement. Next, a parametric study is conducted to examine the effectiveness of shape memory alloy retrofit devices in limiting hinge openings in bridges with various properties. In addition, a comparison is made with other devices such as conventional steel restrainers, metallic dampers, and viscoelastic solid dampers. The results illustrate that superelastic shape memory alloys are superior in their effectiveness compared to other devices in the case of bridges with moderate period ratios and high level of ductility, especially when subjected to strong earthquakes.

<p></p><p>Global demand for high performance, low cost, and eco-friendly electronics is ever increasing. Ion/charge transport ability and mechanical adaptability constitute two critical performance metrics of battery and semiconductor materials, which are fundamentally correlated with their structural dynamics under various operating conditions. It is imperative to reach the mechanistic understanding of the structure-property relationships of electronic materials to develop principles of materials design. Nevertheless, the intricate atomic structure and elusive phase behaviors in the operation of devices challenge direct experimental observations. Herein, we employ a spectrum of modeling methods, including quantum chemistry, ab-initio modeling, and molecular dynamics simulation, to systematically study the phase dynamics and physico-mechanical behaviors of multiple electronic materials, ranging from transition-metal cathodes, polymer derived ceramics anodes, to organic semiconductor crystals. The multiscale atomistic modeling enriches the fundamental understanding of the electro-chemo-mechanical behaviors of battery materials, which provides insight on designing state-of-the-art energy materials with high capacity and high structural stability. By leveraging the genetic-algorithm refined molecular modeling and phase transformation theory, we unveil the molecular mechanisms of thermo-, super- and ferroelastic transition in organic semiconductor crystals, thus promoting new avenues of adaptive organic electronics by molecular design. Furthermore, the proposed computational methodologies and theoretical frameworks throughout the thesis can find use in exploring the phase dynamics in a variety of environmentally responsive electronics.</p><p></p>