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1
Underhill, Daniel Martin Lennard. "Ferromagnetic shape memory alloys." Thesis, University of Cambridge, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.607746.
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Kelly, Brian L. "Beam shape control using shape memory alloys." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 1998. http://handle.dtic.mil/100.2/ADA358806.
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Thesis (M.S. in Astronautical Engineering) Naval Postgraduate School, December 1998."December 1998." Thesis advisor(s): Brij N. Agrawal, Gangbing Song. Includes bibliographical references (p. 55). Also available online.
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Dai, Liyang. "Elasticity in ferromagnetic shape memory alloys." College Park, Md. : University of Maryland, 2004. http://hdl.handle.net/1903/2047.
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Thesis (Ph.D.) -- University of Maryland, College Park, 2004.Thesis research directed by: Material Science and Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.
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Seaton, Alexander B. "Thermomechanical deformation of shape memory alloys." Thesis, Loughborough University, 2006. https://dspace.lboro.ac.uk/2134/20317.
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NiTi is a shape memory alloy and can undergo crystallographically reversible martensitic transformation under applied loads resulting in recoverable of strains of the order of 5 %. The single crystal properties of shape memory alloys have been studied extensively in the past and a good understanding of the mechanical properties of the material in this form has been acquired. However, when used in practical applications shape memory alloys are used in their polycrystalline form. In a polycrystalline form the deformation behaviour may be quite different to that of a single crystal due to the constraints of surrounding grains and anisotropy of material properties. In the case of shape memory alloys these are anisotropic elastic and transformation properties. The main focus of the work in this thesis is the deformation behaviour of commercial rod samples of NiTi while under thermomechanical loads. The grain-orientation-specific internal strain development and phase faction evolution within particular grain orientations is evaluated during deformation by the in-situ neutron diffraction technique. The experimental results presented include stress-induced martensitic transformation, cooling through the martensitic transformation under a fixed stress, the generation of recovery stresses while heated under constraint, and studies of the detwinning of the B 19' martensite phase under compressive and tensile loading. In addition, the effect of ageing on mechanical properties of NiTi is investigated via the method. Changes in the load partitioning behaviour is noted for NiTi cooled under a fixed tensile stress of 200 MPa which compare well with modelling predictions in the literature. Large changes in the mechanical properties of NiTi as a results of ageing are ascribed to the presence of the R-phase due to the formation of precipitates during ageing. Evidence of detwinning of B 19' martensite in both tension and compression is found, in contrast to other work in the literature.
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Vieira, Luís Manuel Alberty. "Laser welding of shape memory alloys." Master's thesis, Faculdade de Ciências e Tecnologia, 2010. http://hdl.handle.net/10362/4760.
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Dissertação apresentada na Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa para obtenção do grau de Mestre em Engenharia MecânicaA necessidade de desenvolver técnicas avançadas de união para ligas com memória de forma tem-se revelado um assunto da maior importância, uma vez que as suas propriedades funcionais,nomeadamente o efeito de memória de forma e a superelasticidade, se revestem de enorme valor para aplicações actuais ou emergentes. De entre as ligas com memória de forma, o NiTi é a mais aplicada em campos tecnológicos tão diversos como a indústria biomédica, aerospacial e automóvel,o que se deve às suas características, como sejam: as elevadas biocompatibilidade e resistência à corrosão. Por estas razões, tem sido investigadas técnicas de ligação para estas ligas. No entanto, a sua ligação a outros materiais constitui um desafio cada vez maior permitindo explorar novos domínios de aplicação. O principal objectivo deste estudo é compreender o efeito da soldadura laser em aspectos estruturais, mecânicos e funcionais, tanto em ligações similares envolvendo NiTi, como dissimilares. Foram produzidas juntas similares topo a topo utilizando um laser de Nd:YAG em modo contínuo e estudados os efeitos da direcção de laminagem na configuração de junta e dos parâmetros do processo nas caraterísticas das juntas. A soldadura dissimilar de NiTi com Ti-6Al-4V foi realizada com um laser de fibras operando em modo contínuo. Adicionalmente, soldaram-se arames de NiTi com aço inoxidável austenítico utilizando uma fonte laser de Nd:YAG operando em modo pulsado. Foram projectados e produzidos sistemas de fixação e de protecção gasosa específicos para estas aplicações. Foram desenvolvidos e/ou adaptados métodos de ensaio para a avaliação da macro e microestructura, do comportamento mecânico cíclico e da capacidade de memória de forma. Utilizaram-se técnicas de análise como a Calorimetria Diferencial de Varrimento (DSC), a Microscopia Electrónica de Varrimento (SEM), EDS para identificação de espécies químicas e microdureza para avaliar as juntas soldadas. Foram produzidas juntas soldadas sem defeitos de soldadura utilizando parâmetros de processo optimizados, as quais apresentaram elevada tensão de rotura (acima de 400 MPa), patamares superelásticos até níveis de deformação próximos de 8%, comportamento cíclico superior ao material base e fractura dúctil. Foi observada baixa tensão de rotura nas juntas dissimilares sobrepostas com aço inoxidável AISI 316LN, devido à fractura prematura pela zona afectada pelo calor, no lado do NiTi. Nas juntas topo a topo de NiTi com Ti-6Al-4V a zona revela uma estrutura de solidificação rápida do tipo dendrítica na qual se propagaram fissuras com origem em defeitos de soldadura, tais como falta de penetração.
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Kockar, Benat. "Shape memory behavior of ultrafine grained NiTi and TiNiPd shape memory alloys." Thesis, [College Station, Tex. : Texas A&M University, 2007. http://hdl.handle.net/1969.1/ETD-TAMU-2543.
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Prothero, Lori Michelle Gross Robert Steven. "Shape memory alloy robotic truss." Auburn, Ala, 2008. http://repo.lib.auburn.edu/EtdRoot/2008/SUMMER/Aerospace_Engineering/Thesis/Prothero_Lori_16.pdf.
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Mirzaeifar, Reza. "A multiscale study of NiTi shape memory alloys." Diss., Georgia Institute of Technology, 2013. http://hdl.handle.net/1853/49071.
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Shape memory alloys (SMAs) are widely used in a broad variety of applications in multiscale devices ranging from nano-actuators used in nano-electrical-mechanical systems (NEMS) to large energy absorbing elements in civil engineering applications. This research introduces a multiscale analysis for SMAs, particularly Nickel-Titanium alloys (NiTi). SMAs are studied in a variety of length scales ranging from macroscale to nanoscale. In macroscale, a phenomenological constitutive framework is adopted and developed by adding the effect of phase transformation latent heat. Analytical closed-form solutions are obtained for modeling the coupled thermomechanical behavior of various large polycrystalline SMA devices subjected to different loadings, including uniaxial loads, torsion, and bending. Thermomechanical responses of several SMA devices are analyzed using the introduced solutions and the results are validated by performing various experiments on some large SMA elements. In order to study some important properties of polycrystalline SMAs that the macroscopic phenomenological frameworks cannot capture, including the texture and intergranular effects in polycrystalline SMAs, a micromechanical framework with a realistic modeling of the grains based on Voronoi tessellations is used. The local form of the first law of thermodynamics is used and the energy balance relations for the polycrystalline SMAs are obtained. Generalized coupled thermomechanical governing equations considering the phase transformation latent heat are derived for polycrystalline SMAs. A three-dimensional finite element framework is used and different polycrystalline samples are modeled. By considering appropriate distributions of crystallographic orientations in the grains obtained from experimental texture measurements of NiTi samples the effects of texture and the tension-compression asymmetry on the thermomechanical response of polycrystalline SMAs are studied. The interaction between the stress state (tensile or compressive), number of grains, and the texture on the thermomechanical response of polycrystalline SMAs is also studied. For studying some aspects of the thermomechanical properties of SMAs that cannot be studied neither by the phenomenological constitutive models nor by the micromechanical models, molecular dynamics simulations are used to explore the martensitic phase transformation in NiTi alloys at the atomistic level. The martensite reorientation, austenite to martensite phase transformation, and twinning mechanisms in NiTi nanostructures are analyzed and the effect of various parameters including the temperature and size on the phase transformation at the atomistic level is studied. Results of this research provide insight into studying pseudoelasticity and shape memory response of NiTi alloys at different length scales and are useful for better understanding the solid-to-solid phase transformation at the atomistic level, and the effects of this transformation on the microstructure of polycrystal SMAs and the macroscopic response of these alloys.
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Brewer, Andrew Lee. "Shape memory response of ni2mnga and nimncoin magnetic shape memory alloys under compression." [College Station, Tex. : Texas A&M University, 2007. http://hdl.handle.net/1969.1/ETD-TAMU-1341.
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Toker, Guher P. "CHARACTERIZATION OF THE SHAPE MEMORY BEHAVIOR OF HIGH STRENGTH NiTiHfPd SHAPE MEMORY ALLOYS." UKnowledge, 2018. https://uknowledge.uky.edu/me_etds/114.
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NiTiHf alloys have emerged as potential materials for applications requiring high transformation temperatures (> 100 °C) with high strength and work output. Although they have high transformation temperatures, their low damping capacity, brittleness and poor superelastic responses (of Ti-rich NiTiHf) impedes their wider usage in many industrial applications. In this study, the quaternary alloying element of Pd has been added to NiTiHf alloys to improve and tailor their shape memory behavior,. NiTiHfPd alloys were systematically examined regarding the composition and heat treatments effects.
Effects of substituting Hf with Ti on the shape memory behavior of NiTHfPd alloys were investigated. There compositions were selected as Ni40.3Ti34Hf20Pd5 Ni40.3Ti39.7Hf15Pd5 and Ni40.3Ti44.7Hf10Pd5 (at.%). Their transformation temperatures, microstructure and shape memory properties were revealed and compared with conventional shape memory alloys. It was revealed that their transformation temperatures increases but transformation strain decreases with the increment of Hf content.
Additionally, superelastic responses of Ni45.3Ti29.7Hf20Pd5 andNi45.3Ti39.7Hf10Pd5 alloys were investigated. Transformation temperatures of polycrystalline Ni45.3Ti29.7Hf20Pd5are highly dependent on aging temperatures and they can be altered widely from room temperature to 250 oC.
Finally, the damping capacity of the Ni45.3Ti39.7Hf10Pd5 polycrystal and [111]-oriented Ni45.3Ti29.7Hf20Pd5 single crystal were investigated. The damping capacities were found to be 16-25 J.cm-3, and 10-23 J.cm-3 for the Ni45.3Ti39.7Hf10Pd5 and [111]-oriented Ni45.3Ti29.7Hf20Pd5 alloys, respectively.
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Liang, Chen. "The constitutive modeling of shape memory alloys." Diss., This resource online, 1990. http://scholar.lib.vt.edu/theses/available/etd-08232007-113153/.
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Page, David Gordon. "Electrodeposition of thin film shape memory alloys." Thesis, University of Newcastle Upon Tyne, 2001. http://hdl.handle.net/10443/470.
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There is considerable potential for the use of thin film shape memory alloys in the field of microtechnology due to their high power to volume ratio. The main obstacles for fabrication arise mainly due to the narrow regime over which shape memory behaviour is observed and the paucity of process techniques. Shape memory transition in brass only occurs in the alloy composition range 38.5 - 41.6 wt %% zinc. This study used a pyrophosphate electrolyte containing Cu2P2O7, Zn2P2O7 salts and an excess of K4P207 and KNO3, for brass deposition as a replacement for cyanide electrolytes because it is non-toxic and noncorrosive. A rotating disc electrode was employed to systematically examine polarisation data and a rotating cylinder electrode was employed to produce thin brass films and deduce the current efficiencies of copper, zinc and brass deposition with respect to deposition potential. Thin films were plated between 5- 301im, they all displayed a smooth, uniform homogenous deposit with no precipitates or oxide inclusions. The current efficiencies were found to be < 45% for copper, < 15% for zinc and between 10 - 30% for brass. The microstructural characterisation of the Cu-Zn thin alloys was undertaken by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray fluorescence (EDAX), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). XRD showed all the electrodeposited Cu-Zn alloys to have same phase composition as those predicted by the equilibrium phase diagram for Cu-Zn. This confirmed the existence of the parent p-phase within the shape memory composition range, which undergoes the martensitic transformation. TEM showed these foils to be composed of a matrix of a, p and martensite nano sized grains (< 40nm) co-existing with a sparse distribution of larger grains (200-300nm). The larger grains were always martensite in nature, recognisable by their twinning planes. Differential scanning calorimetry analysis shows evidence of a martensitic transformation change for the thin brass films.
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Hartwell, Ashley (Ashley Jessica). "Shape Memory Alloys for small scale actuation." Thesis, Massachusetts Institute of Technology, 2018. http://hdl.handle.net/1721.1/118714.
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Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2018.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 51-52).
Shape Memory Alloys (SMAs), materials that can undergo a fully recoverable strain change due to a thermal cycle, and which can be produced in a form that is superelastic are only utilized limitedly. In this thesis, I investigated the relationship between the material properties of shape memory alloy micro-wires and their mechanical performance. This study was conducted with two main types of SMAs, the first a commercially available NiTi wire, and the second an emerging Cu-based alternative. This comparison allows an understanding of the current state of the art for small scale actuation with SMA wires, and to evaluate the Cu-based alternative SMA, which has a reduced cost and improved thermal properties. This thesis evaluates Cu-based SMAs as substitutes for NiTi in terms of activation strain of wire during a shape memory cycle, power consumption during actuation, heating and cooling times during actuation, and cost. Furthermore this thesis includes studies on the processing of Cu-based alternatives to enhance shape memory properties of interests, such as transformation temperature and fatigue, and suggests future work to improve Cu-based SMA wires..
by Ashley Hartwell.
S.M.
14
Guo, Weimin M. Eng Massachusetts Institute of Technology. "Orthopaedic applications of ferromagnetic shape memory alloys." Thesis, Massachusetts Institute of Technology, 2008. http://hdl.handle.net/1721.1/45957.
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Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008.Includes bibliographical references (leaves 36-40).
Ferromagnetic shape memory alloys (FSMAs) are a new class of magnetic field-actuated active materials with no current commercial applications. By applying a magnetic field of around 0.4 T, they can exert a stress of approximately 1.5 MPa, exhibiting a strain of up to 6%. This thesis evaluates their technical and commercial feasibility in orthopaedic applications. Remote actuation is a key advantage FSMAs have over current implant materials. Also, the human body temperature is constant, providing a stable environment for FSMAs to operate. A number of potential orthopaedic applications are proposed and evaluated. Out of these, the most prominent application is the spinal traction device. It is a temporary implantable device, intended to perform internal spinal traction. A design has been proposed, with suggestions of suitable materials for its various components and appropriate device dimensions. Preliminary market and cost analyses have been conducted. This orthopaedic technology is currently in its infant stage. To commercialize this device, more trials are needed.
by Weimin Guo.
M.Eng.
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Kelly, Alex Bhattacharya Kaushik Murray RIchard M. "A constitutive relation for shape-memory alloys /." Diss., Pasadena, Calif. : California Institute of Technology, 2009. http://resolver.caltech.edu/CaltechETD:etd-09292008-204618.
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Zhang, Yahui. "Low cycle fatigue of shape memory alloys." Thesis, Université Paris-Saclay (ComUE), 2018. http://www.theses.fr/2018SACLY004/document.
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Dans cette thèse, nous proposons une analyse globale multi-échelles de la fatigue à faible nombre de cycles des matériaux à mémoire de forme (MMF). Dans un premier temps, une large campagne d’essais a été menée pour différents chargements thermomécaniques comprenant des tests de fatigue sous contrainte et déformation imposée et pour différentes fréquences de chargement. A partir des résultats des essais, un critère de fatigue, basé sur l’énergie de déformation, a été développé ; on montre que l’énergie de déformation est un paramètre pertinent pour prédire la fatigue des MMF en tenant compte du couplage thermomécanique et du type de chargement : contrainte ou déformation imposée. Ensuite, en prenant appui sur la répartition de l’énergie de l’hystérésis en dissipation et énergie stockée, on avance une interprétation physique du mécanisme de la fatigue des MMF. Dans la troisième partie, on propose une modélisation multi-échelles de l’initiation des fissures de fatigue dans les MMF à partir de la notion de plasticité de transformation (PlTr). Dans ce cadre, on montre que la fatigue de MMF est contrôlée par la (PlTr) et que la température maximale lors de la transformation de phase est le paramètre à retenir pour prédire la rupture par fatigue des MMF. Le modèle permet également de prédire le lieu d’initiation des premières fissures de fatigue. Enfin, un procédé – fondé sur l’«éducation» des MMF – permettant d’améliorer la résistance à la fatigue est proposéThe thesis proposes a multi-scale comprehensive analysis of low cycle fatigue of shape memory alloys (SMAs). First, low cycle fatigue of SMAs is experimentally investigated; comprehensive tensile-tensile fatigue tests under both stress and strain controlled loadings at different frequencies are carried out and results are discussed. Second, a new strain energy-based fatigue criterion is developed; it is shown that the use of total strain energy is a relevant parameter to predict fatigue lifetime of SMAs for different thermomechanical conditions and under different types (strain-control or stress-control) loadings. A physical interpretation of the mechanism related to the low-cycle fatigue of SMAs is then provided based on the conversion of hysteresis work into dissipation and stored energy. Third, fatigue crack initiation during cyclic stress-induced phase transformation is modeled based on transformation induced plasticity (TRIP); it is shown that the maximum temperature during the cyclic loading is a relevant indicator of the fatigue of SMA. Furthermore, the effect of the macroscopic mechanical load on the the fatigue lifetime is addressed as well as the spatial location of crack initiation. Finally, a mechanical training process that allows enhancing resistance to low cycle fatigue of SMAs is proposed
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Zhang, Yahui. "Low cycle fatigue of shape memory alloys." Electronic Thesis or Diss., Université Paris-Saclay (ComUE), 2018. http://www.theses.fr/2018SACLY004.
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Dans cette thèse, nous proposons une analyse globale multi-échelles de la fatigue à faible nombre de cycles des matériaux à mémoire de forme (MMF). Dans un premier temps, une large campagne d’essais a été menée pour différents chargements thermomécaniques comprenant des tests de fatigue sous contrainte et déformation imposée et pour différentes fréquences de chargement. A partir des résultats des essais, un critère de fatigue, basé sur l’énergie de déformation, a été développé ; on montre que l’énergie de déformation est un paramètre pertinent pour prédire la fatigue des MMF en tenant compte du couplage thermomécanique et du type de chargement : contrainte ou déformation imposée. Ensuite, en prenant appui sur la répartition de l’énergie de l’hystérésis en dissipation et énergie stockée, on avance une interprétation physique du mécanisme de la fatigue des MMF. Dans la troisième partie, on propose une modélisation multi-échelles de l’initiation des fissures de fatigue dans les MMF à partir de la notion de plasticité de transformation (PlTr). Dans ce cadre, on montre que la fatigue de MMF est contrôlée par la (PlTr) et que la température maximale lors de la transformation de phase est le paramètre à retenir pour prédire la rupture par fatigue des MMF. Le modèle permet également de prédire le lieu d’initiation des premières fissures de fatigue. Enfin, un procédé – fondé sur l’«éducation» des MMF – permettant d’améliorer la résistance à la fatigue est proposéThe thesis proposes a multi-scale comprehensive analysis of low cycle fatigue of shape memory alloys (SMAs). First, low cycle fatigue of SMAs is experimentally investigated; comprehensive tensile-tensile fatigue tests under both stress and strain controlled loadings at different frequencies are carried out and results are discussed. Second, a new strain energy-based fatigue criterion is developed; it is shown that the use of total strain energy is a relevant parameter to predict fatigue lifetime of SMAs for different thermomechanical conditions and under different types (strain-control or stress-control) loadings. A physical interpretation of the mechanism related to the low-cycle fatigue of SMAs is then provided based on the conversion of hysteresis work into dissipation and stored energy. Third, fatigue crack initiation during cyclic stress-induced phase transformation is modeled based on transformation induced plasticity (TRIP); it is shown that the maximum temperature during the cyclic loading is a relevant indicator of the fatigue of SMA. Furthermore, the effect of the macroscopic mechanical load on the the fatigue lifetime is addressed as well as the spatial location of crack initiation. Finally, a mechanical training process that allows enhancing resistance to low cycle fatigue of SMAs is proposed
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Friend, C. M. "Factors affecting reversible shape-memory." Thesis, University of Surrey, 1985. http://epubs.surrey.ac.uk/847449/.
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In the last twenty years Reversible Shape-Memory (RSM) alloys have become the source of considerable technological interest as a result of their ability to generate spontaneous and reversible changes of shape on thermal cycling. This has led to the development of a range of reversible shape-memory devices for thermostatic sensing applications. In these devices the alloy is subjected to several thousand shape-memory cycles and the stability of the reversible shape-memory is therefore an important alloy property. Data on the effect of shape-memory cycling on the long-term stability of the reversible shape-memory, however, is extremely limited. The present work, conducted to fill this gap, has shown that there is an inherent instability in the reversible shape-memory, with changes in the operating temperatures and cumulative reductions in the maximum shape-strain output of actuators on long-term thermal cycling under conditions simulating real devices. Extensive investigation has shown that these instabilities result from a number of sources, ageing of the shape-memory martensites and most importantly from morphological disruptions in the "trained" martensites caused by two-stage stress-induced transformation and due to the build-up of transformation-induced dislocation debris. This shape-strain degradation has also been successfully modelled by means of a simple two-stage stress-induced martensitic transformation model.
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Young, Avery W. "A Study on NiTiSn Low-Temperature Shape Memory Alloys and the Processing of NiTiHf High-Temperature Shape Memory Alloys." Thesis, University of North Texas, 2018. https://digital.library.unt.edu/ark:/67531/metadc1157642/.
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Shape memory alloys (SMAs) operating as solid-state actuators pose economic and environmental benefits to the aerospace industry due to their lightweight, compact design, which provides potential for reducing fuel emissions and overall operating cost in aeronautical equipment. Despite wide applicability, the implementation of SMA technology into aerospace-related actuator applications is hindered by harsh environmental conditions, which necessitate extremely high or low transformation temperatures. The versatility of the NiTi-based SMA system shows potential for meeting these demanding material constraints, since transformation temperatures in NiTi can be significantly raised or lowered with ternary alloying elements and/or Ni:Ti ratio adjustments. In this thesis, the expansive transformation capabilities of the NiTi-based SMA system are demonstrated with a low and high-temperature NiTi-based SMA; each encompassing different stages of the SMA development process. First, exploratory work on the NiTiSn SMA system is presented. The viability of NiTiSn alloys as low-temperature SMAs (LTSMAs) was investigated over the course of five alloy heats. The site preference of Sn in near-equiatomic NiTi was examined along with the effects of solution annealing, Ni:Ti ratio adjustments, and precipitation strengthening on the thermomechanical properties of NiTiSn LTSMAs. Second, the thermomechanical processability of NiTiHf high-temperature SMA (HTSMA) wires is presented. The evolution of various microstructural features (grain size reduction, oxide growth, and nano-precipitation) were observed at incremental stages of the hot rolling process and linked to the thermal and mechanical responses of respective HTSMA rods/wires. This work was carried out in an effort to optimize the rolling/drawing process for NiTiHf HTSMAs.
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Fung, Cheung Kwan. "Thermal mechanical behaviour of NiTi shape memory alloy." access abstract and table of contents access full-text, 2004. http://libweb.cityu.edu.hk/cgi-bin/ezdb/dissert.pl?msc-ap-b21174076a.pdf.
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Thesis (M.Sc.)--City University of Hong Kong, 2004.At head of title: City University of Hong Kong, Department of Physics and Materials Science, Master of Science in materials engineering & nanotechnology dissertation. Title from title screen (viewed on Aug. 31, 2006) Includes bibliographical references.
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Penar, Bradley W. "Recentering Beam-Column Connections Using Shape Memory Alloys." Thesis, Georgia Institute of Technology, 2005. http://hdl.handle.net/1853/7185.
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Shape memory alloys are a class of alloys that display the unique ability to
undergo large plastic deformations and return to their original shape either
through the application of heat (shape memory effect) or by relieving the
stress causing the deformation (superelastic effect). This research takes
advantage of the unique characteristics of shape memory alloys in order to
provide a moment resisting connection with recentering capabilities.
In this study, superelastic Nitinol, a nickel-titanium form of shape memory
alloy that exhibits a flag-shaped stress versus strain curve, is used as the
moment transfer elements within a partially restrained steel beam-column
connection. Experimental testing consists of a one-half scale interior
connection where the loading is applied at the column tip. A pseudo-static
cyclic loading history is used which is intended to simulate earthquake
loadings. The energy dissipation characteristics, moment-rotation
characteristics, and deformation capacity of the connection are quantified.
Results are then compared to tests where A36 steel tendons are used as the
moment transfer elements. The superelastic Nitinol tendon connection showed
superior performance to the A36 steel tendon connection, including the ability
to recenter without residual deformation.
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Tarhan, Elif. "Ageing Characteristics Of Copper Based Shape Memory Alloys." Phd thesis, METU, 2004. http://etd.lib.metu.edu.tr/upload/3/593541/index.pdf.
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Martensite-to-Beta transformation temperatures of CuAlNiMn and CuAlNi shape memory alloys has been determined by differential scanning calorimetry (DSC). In CuAlNiMn alloys, each new betatizing treatment has resulted in randomly varying transformation temperatures on the same specimen and an anomalously diffuse and serrated Martensite-to-Beta transformation peaks in the first cycle. Therefore, as quenched alloy samples were thermally cycled for three times in DSC prior to ageing to obtain thermally stable and reproducible transformation temperatures and to eliminate the anomalous effect of betatizing on the transformation temperatures.
CuAlNiMn alloys were aged in martensitic condition at temperatures in the range 80
C to 150&
#61616
C for 24 hours to 312 hours ageing periods. Both A_s and A_f temperatures have increased with ageing temperature and time while M_s and M_f temperatures have not changed during martensite ageing. Transformation temperatures of CuAlNi alloys, on the other hand, have not changed during martensite ageing. In this respect, CuAlNiMn alloys were found to be more prone to martensite stabilization than the CuAlNi alloys. Through Transmission Electron Microscope investigation in the Cu-12.6wt%Al-5.9wt%Ni-1.8wt%Mn alloy aged at 150&
#61616
C for 312 hours has revealed no sign of precipitate formation and it has been concluded that the &
#65533
precipitates pinning martensite boundaries&
#65533
mechanism could not be responsible of martensite stabilization. Beta phase ageing of CuAlNiMn alloys at temperatures 200&
#61616
C, 230&
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C, 250&
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C and 270&
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C, have drastically shortened the periods for stabilization to the extent that &
#946
-to-M transformation completely ceases. With regard to the Manganese content, highest Manganese bearing alloy was the one stabilized first and the lowest manganese containing one was the longest lasting alloy during beta phase ageing. Beta stabilization was not observed in any of the four CuAlNi alloys at the end of 96 hours ageing at 200&
#61616
C while beta stabilization was realized after 26, 38 and 11 hours ageing at the same temperature in the three Mn containing alloys studied. In conclusion, on the basis of ageing studies at 200&
#61616
C, with regard to beta stabilization, CuAlNi alloys were found to be more resistant to high temperature ageing than CuAlNiMn alloys. Equilibrium &
#947
_2 and &
#945
phases were observed with coupled-grown lamellar morphologies in Cu-13.6%Al-3.0%Ni alloy aged above 400&
#61616
C.
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Penrod, Luke Edward. "Fabrication and characterization of porous shape memory alloys." Texas A&M University, 2003. http://hdl.handle.net/1969.1/145.
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This work details an investigation into the production of porous shape memory alloys (SMAs) via hot isostatic press (HIP) from prealloyed powders. HIPing is one of three main methods for producing porous SMAs, the other two are conventional sintering and selfpropagating hightemperature synthesis (SHS). Conventional sintering is characterized by its long processing time at near atmospheric pressure and samples made this way are limited in porosity range. The SHS method consists of preloading a chamber with elemental powders and then initiating an explosion at one end, which then propagates through the material in a very short time. HIPing provides a compromise between the two methods, requiring approximately 5 hours per cycle while operating in a very controlled environment. The HIPing method gives fine control of both temperature and pressure during the run which allows for the production of samples with varying porosity as well as for finetuning of the process for other characteristics. By starting with prealloyed powder, this study seeks to avoid the drawbacks while retaining the benefits of HIPing with elemental powders.
In an extension of previous work with elemental powders, this study will apply the HIP method to a compact of prealloyed powders. It is hoped that the use of these powders will limit the formation of alternate phases as well as reducing oxidation formed during preparation. In addition, the nearspherical shape of the powders will encourage an even pore distribution. Processing techniques will be presented as well as a detailed investigation of the thermal and mechanical properties of the resulting material.
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Chatziathanasiou, Dimitrios. "Cyclic multiaxial behavior modeling of Shape Memory Alloys." Thesis, Paris, ENSAM, 2016. http://www.theses.fr/2016ENAM0015/document.
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De nouvelles approches phénoménologiques sur la modélisation du comportement des AMFs sont nécessaires pour tenir en compte leur réponse complexe sous chargement multiaxial. L’effet de l’anisotropie induit une dépendance de leur comportement inélastique de la direction du chargement pour des cas superélastiques. La réorientation martensitique affecte drastiquement la réponse du matériau sous chargement non-proportionnel. La charge répétitive modifie aussi certaines propriétés du matériau. L’objectif de cette étude est de proposer un nouveau modèle constitutif thermodynamique robuste pour les AMFs, focalisé surtout sur des compositions NiTi équiatomiques pour capter la transformation martensitique anisotrope et la réorientation des variantes martensitiques. Une nouvelle approche mathématique est introduite pour permettre la prise en compte de l’anisotropie de contraintes et l’évolution des déformations inélastiques lors de la transformation directe, causée par les conditions de mise en forme de structures en AMFs. Cette méthode est évaluée en employant des courbes contraintes-déformations résultant de chargements proportionnels simulés par un modèle micromécanique. Un modèle phénoménologique considérant surtout la réorientation martensitique et mettant en évidence le fort couplage thermomécanique est développé. Il est implémenté dans une plate-forme numérique en C++, SMART+, et évalué en exécutant des simulations des expériences non-proportionnelles existantes. Des structures complexes sont également simulées en employant la Méthode des Élements Finis. La dernière partie de ce travail concerne l’étude expérimentale des effets du chargement cyclique sur l’évolution des déformations résiduelles et le seuil de transformation des alliages NiTi sous sollicitation uniaxiale et biaxialeNew phenomenological approaches in modeling the behavior of SMAs are needed to account for their complex response under multiaxial loading. The effect of anisotropy induces a dependence of their inelastic behavior to the direction of the loading for superelastic cases. Martensitic reorientation affects drastically material response under non-proportional loading. Repeated loading also alters certain material properties. The goal of this study is to propose a new robust thermodynamic constitutive model for SMAs with focus on equiatomic NiTi compositions to capture anisotropic martensitic transformation and reorientation of martensitic variants, always taking in mind the strong thermomechanical coupling. A new mathematical approach is introduced to account for the anisotropy of stresses and the evolution of inelastic strains during forward transformation caused by the forming conditions of SMA structures. This method is evaluated by utilizing stress-strain curves resulting from proportional loading simulated with a micromechanical model. A phenomenological thermodynamic model considering especially martensitic reorientation and exhibiting the strong thermomechanical coupling is developed. It is implemented on a numerical platform in C++, SMART, and evaluated by simulating existing non-proportional experiments. Complex structures are also simulated using Finite Element Analysis. The last part of this work concerns the experimental study of the effects of cyclic loading to the evolution of residual strain and transformation threshold of NiTi under uniaxial and biaxial testing
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Muite, Benson K. "Analysis, modelling and simulation of shape memory alloys." Thesis, University of Oxford, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.543534.
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Delemont, Michael A. "Seismic retrofit of bridges using shape memory alloys." Thesis, Georgia Institute of Technology, 2001. http://hdl.handle.net/1853/32787.
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TIYYAGURA, MADHAVI. "TRANSMISSION ELECTRON MICROSCOPY STUDIES IN SHAPE MEMORY ALLOYS." Master's thesis, University of Central Florida, 2005. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/3913.
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In NiTi, a reversible thermoelastic martensitic transformation can be induced by temperature or stress between a cubic (B2) austenite phase and a monoclinic (B19') martensite phase. Ni-rich binary compositions are cubic at room temperature (requiring stress or cooling to transform to the monoclinic phase), while Ti-rich binary compositions are monoclinic at room temperature (requiring heating to transform to the cubic phase). The stress induced transformation results in the superelastic effect, while the thermally induced transformation is associated with strain recovery that results in the shape memory effect. Ternary elemental additions such as Fe can additionally introduce an intermediate rhombohedral (R) phase between the cubic and monoclinic phase transformation. This work was initiated with the broad objective of connecting the macroscopic behavior in shape memory alloys with microstructural observations from transmission electron microscopy (TEM). Specifically, the goals were to examine (i) the effect of mechanical cycling and plastic deformation in superelastic NiTi; (ii) the effect of thermal cycling during loading in shape memory NiTi; (iii) the distribution of twins in martensitic NiTi-TiC composites; and (iv) the R-phase in NiTiFe. Both in situ and ex situ lift out focused ion beam (FIB) and electropolishing techniques were employed to fabricate shape memory alloy samples for TEM characterization. The Ni rich NiTi samples were fully austenitic in the undeformed state. The introduction of plastic deformation (8% and 14% in the samples investigated) resulted in the stabilization of martensite in the unloaded state. An interlaying morphology of the austenite and martensite was observed and the martensite needles tended to orient themselves in preferred orientations. The aforementioned observations were more noticeable in mechanically cycled samples. The observed dislocations in mechanically cycled samples appear to be shielded from the external applied stress via mismatch accommodation since they are not associated with unrecoverable strain after a load-unload cycle. On application of stress, the austenite transforms to martensite and is expected to accommodate the stress and strain mismatch through preferential transformation, variant selection, reorientation and coalescence. The stabilized martensite (i.e., martensite that exists in the unloaded state) is expected to accommodate the mismatch through variant reorientation and coalescence. On thermally cycling a martensitic NiTi sample under load through the phase transformation, significant variant coalescence, variant reorientation and preferred variant selection was observed. This was attributed to the internal stresses generated as a result of the thermal cycling. A martensitic NiTi-TiC composite was also characterized and the interface between the matrix and the inclusion was free of twins while significant twins were observed at a distance away from the matrix-inclusion interface. Incorporating a cold stage, diffraction patterns from NiTiFe samples were obtained at temperatures as low as -160ºC. Overall, this work provided insight in to deformation phenomena in shape memory materials that have implications for engineering applications (e.g., cyclic performance of actuators, engineering life of superelastic components, stiffer shape memory composites and low-hysteresis R-phase based actuators). This work was supported in part by an NSF CAREER award (DMR 0239512).M.S.M.E.
Department of Mechanical, Materials and Aerospace Engineering;
Engineering and Computer Science
Materials Science and Engineering
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PAIVA, ALBERTO. "MODELING OF THERMOMECHANICAL BEHAVIOR OF SHAPE MEMORY ALLOYS." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2004. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=4942@1.
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CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOO 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.
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.
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Koumatos, Konstantinos. "The formation of microstructure in shape-memory alloys." Thesis, University of Oxford, 2012. http://ora.ox.ac.uk/objects/uuid:1089932b-d36e-4414-b128-6f7bcfe9cdf3.
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The application of techniques from nonlinear analysis to materials science has seen great developments in the recent years and it has really been a driving force for substantial mathematical research in the area of partial differential equations and the multi-dimensional calculus of variations. This thesis has been motivated by two recent and remarkable experimental observations of H. Seiner in shape-memory alloys which we attempt to interpret mathematically. Much of the work is original and has given rise to deep problems in the calculus of variations. Firstly, we study the formation of non-classical austenite-martensite interfaces. Ball & Carstensen (1997, 1999) theoretically investigated the possibility of the occurrence of such interfaces and studied the cubic-to-tetragonal case extensively. In this thesis, we present an analysis of non-classical austenite-martensite interfaces recently observed by Seiner et al.~in a single crystal of a CuAlNi shape-memory alloy, undergoing a cubic-to-orthorhombic transition. We show that these can be described by the general nonlinear elasticity model and we make some predictions regarding the admissible volume fractions of the martensitic variants involved, as well as the habit plane normals. Interestingly, in the above experimental observations, the interface between the austenite and the martensitic configuration is never exactly planar, but rather slightly curved, resulting from the pattern of martensite not being exactly homogeneous. However, it is not clear how one can reconstruct the inhomogeneous configuration as a stress-free microstructure and, instead, a theoretical approach is followed. In this approach, a general method is provided for the construction of a compatible curved austenite-martensite interface and, by exploiting the structure of quasiconvex hulls, the existence of curved interfaces is shown in two and three dimensions. As far as the author is aware of, this is the first construction of such a curved austenite-martensite interface. Secondly, we study the nucleation of austenite in a single crystal of a CuAlNi shape-memory alloy consisting of a single variant of stabilized 2H martensite. The nucleation process is induced by localized heating and it is observed that, regardless of where the localized heating is applied, the nucleation points are always located at one of the corners of the sample - a rectangular parallelepiped in the austenite. Using a simplified nonlinear elasticity model, we propose an explanation for the location of the nucleation points by showing that the martensite is a local minimizer of the energy with respect to localized variations in the interior, on faces and edges of the sample, but not at some corners, where a localized microstructure can lower the energy. The result for the interior, faces and edges is established by showing that the free-energy function satisfies a set of quasiconvexity conditions at the stabilized variant throughout the specimen, provided this is suitably cut. The proofs of quasiconvexity are based on a rigidity argument and are specific to the change of symmetry in the phase transformation. To the best of the author's knowledge, quasiconvexity conditions at edges and corners have not been considered before.
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Dabbaghi, Hediyeh. "Oxidation Analysis of Additive Manufacturing Shape Memory Alloys." University of Toledo / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1596450323778946.
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Chen, Xue. "Magneto-mechanical behaviors of ferromagnetic shape memory alloys." Palaiseau, Ecole polytechnique, 2013. http://pastel.archives-ouvertes.fr/docs/00/84/86/30/PDF/Thesis_xue-Chen_ENSTA.pdf.
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Ferromagnetic Shape Memory Alloys (FSMA) are promising candidates for sensors and actuators for their high-frequency response and large reversible strain. The aim of this dissertation is the analysis of the magneto-mechanical behaviors of FSMA. In this aim, we study, both experimentally and theoretically, the martensite reorientation in FSMA. Firstly, a 2D/3D magneto-mechanical energy analysis is proposed and incorporated into phase diagrams for a graphic study of path-dependent martensite reorientation in FSMA under 3D loadings. Criteria and material requirements for obtaining reversible strain in cyclic loadings are derived. The energy analysis predicts that FSMA in 2D/3D configurations (multi-axial stresses) has much more advantages than in 1D configuration, e. G. , higher output stress and more application flexibility. Secondly, to validate the predictions of the energy analysis, 2D experiments are performed on FSMA and results reveal that the intrinsic dissipation and the transformation strain due to martensite reorientation are constant in all tested 2D stress states. Moreover, preliminary results validate that the output stress of FSMA in 2D configuration (magnetic field with biaxial stresses) is larger than in 1D configuration, and the output stress can be increased by increasing the auxiliary stress. Finally, to predict the magneto-mechanical behaviors of FSMA in general multi-axial loadings, a 3D constitutive model is developed within the framework of thermodynamics of irreversible processes. All the martensite variants are considered and the temperature effect is also taken into account. Model simulations agree well with all the existing 1D/2D experiments. The model is further incorporated into finite element analysis for studying the non-linear bending behaviors of FSMA beams. The sample-geometry effect and the material anisotropic effect are systematically investigated.
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Azeem, Mohammed Abdul. "Diffraction investigations of high temperature shape memory alloys." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/11138.
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Shape memory alloys are intermetallic materials with a unique ability to revert to a predefined physical shape by virtue of diffusionless transformations. Recent interest by aerospace and automotive industries to exploit the functionalities of these materials in future energy efficient designs has renewed scientific research in this field. However, the current understanding of transformation hysteresis is inhibited by experimental difficulties associated with viewing the transformations and therefore most of our knowledge is confined to symptomatic bulk properties such as those accessible from calorimetry and dilatometry. In the current study, in situ synchrotron X-ray diffraction (SXRD) was used to accurately document the adaptability of unit cells of participating phases during transformation in a series of high temperature shape memory alloys (HTSMAs). Selected alloys based on NiTi, ZrCu and NiMnGa systems were prepared in vacuum arc melter, homogenized and rolled to grain size adequate for SXRD experiments. The resulting diffraction patterns were Rietveld refined and the evolution of unit cell parameters of participating phases were recorded as a function of temperature. It was observed that the lattices of participating phases undergo a significant overall dilation during transformation. The lower symmetry martensite unit cell was observed to undergo unprecedented anisotropic strains, reaching as high as 1.2% in certain alloys. The high temperature higher symmetry austenite was observed to complement the changes in martensite lattice during heating and vice versa was observed during cooling. These changes were mostly observed in final stages of transformation. Surprisingly, a negative coefficient of thermal expansion was observed in the b lattice parameter of the monoclinic martensite in NiTi based alloys. The implications of such strains on the current phenomenological martensitic transformation models that takes into consideration lattice parameters away from the transformation regime for prediction of orientation between participating phases has been discussed.
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Saghaian, Sayed M. "SHAPE MEMORY BEHAVIOR OF SINGLE CRYSTAL AND POLYCRYSTALLINE Ni-RICH NiTiHf HIGH TEMPERATURE SHAPE MEMORY ALLOYS." UKnowledge, 2015. http://uknowledge.uky.edu/me_etds/65.
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NiTiHf shape memory alloys have been receiving considerable attention for high temperature and high strength applications since they could have transformation temperatures above 100 °C, shape memory effect under high stress (above 500 MPa) and superelasticity at high temperatures. Moreover, their shape memory properties can be tailored by microstructural engineering. However, NiTiHf alloys have some drawbacks such as low ductility and high work hardening in stress induced martensite transformation region. In order to overcome these limitations, studies have been focused on microstructural engineering by aging, alloying and processing.
Shape memory properties and microstructure of four Ni-rich NiTiHf alloys (Ni50.3Ti29.7Hf20, Ni50.7Ti29.3Hf20, Ni51.2Ti28.8Hf20, and Ni52Ti28Hf20 (at. %)) were systematically characterized in the furnace cooled condition. H-phase precipitates were formed during furnace cooling in compositions with greater than 50.3Ni and the driving force for nucleation increased with Ni content. Alloy strength increased while recoverable strain decreased with increasing Ni content due to changes in precipitate characteristics.
The effects of the heat treatments on the transformation characteristics and microstructure of the Ni-rich NiTiHf shape memory alloys have been investigated. Transformation temperatures are found to be highly annealing temperature dependent. Generation of nanosize precipitates (~20 nm in size) after three hours aging at 450 °C and 550 °C improved the strength of the material, resulting in a near perfect dimensional stability under high stress levels (> 1500 MPa) with a work output of 20–30 J cm– 3. Superelastic behavior with 4% recoverable strain was demonstrated at low and high temperatures where stress could reach to a maximum value of more than 2 GPa after three hours aging at 450 and 550 °C for alloys with Ni great than 50.3 at. %.
Shape memory properties of polycrystalline Ni50.3Ti29.7Hf20 alloys were studied via thermal cycling under stress and isothermal stress cycling experiments in tension. Recoverable strain of ~5% was observed for the as-extruded samples while it was decreased to ~4% after aging due to the formation of precipitates. The aged alloys demonstrated near perfect shape memory effect under high tensile stress level of 700 MPa and perfect superelasticity at high temperatures up to 230 °C. Finally, the tensioncompression asymmetry observed in NiTiHf where recoverable tensile strain was higher than compressive strain.
The shape memory properties of solutionized and aged Ni-rich Ni50.3Ti29.7Hf20 single crystals were investigated along the [001], [011], and [111] orientations in compression. [001]-oriented single crystals showed high dimensional stability under stress levels as high as 1500 MPa in both the solutionized and aged conditions, but with transformation strains of less than 2%. Perfect superelasticity with recoverable strain of more than 4% was observed for solutionized and 550 °C-3h aged single crystals along the [011] and [111] orientations, and general superelastic behavior was observed over a wide temperature range. The calculated transformation strains were higher than the experimentally observed strains since the calculated strains could not capture the formation of martensite plates with (001) compound twins.
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Kotamala, Sreenath. "PRESTRESSING OF SIMPLY SUPPORTED CONCRETE BEAM WITH NITINOL SHAPE MEMORY ALLOY." Text at OhioLINK ETD Center (Requires Adobe Acrobat Reader for viewing), 2004. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=toledo1091806884.
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Thesis (M.S.V.)--University of Toledo, 2004.Typescript. "A thesis [submitted] as partial fulfillment of the requirements of the Master of Science degree in Civil Engineering." Includes bibliographical references (leaves 63-64).
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Wasylyszyn, Jonathan Allen. "Active control of underwater propulsor using shape memory alloys." Thesis, Texas A&M University, 2005. http://hdl.handle.net/1969.1/4926.
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The development of a leading edge propeller blade reconfiguration system using Shape Memory Allow (SMA) muscles is presented. This work describes the design and testing of a leading edge flap, which is used to alter the local camber of a propeller blade. The leading edge flap is deflected by SMA wires housed in the blade and maintained in a fixed position with a shaft locking and releasing mechanism. A locking and releasing mechanism is utilized so that constant actuation of the SMAs is not required to maintain leading edge deflection. The profile at 70% span of the propeller blade was used to create a two-dimensional blade for leading edge flap design implementation and load testing. Deflection of up to five degrees was obtained with the final design of the leading edge flap and locking and releasing mechanism. The SMA muscles used to deflect the leading edge were actuated electronically through resistive heating and were controlled by a proportional/integral gain control algorithm with closed-loop feedback from a linear displacement sensor within the blade. With the final design of the leading edge flap and locking and releasing mechanism, a preliminary design for a three-dimensional propeller was created.
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Kiefer, Bjoern. "A phenomenological constitutive model for magnetic shape memory alloys." Texas A&M University, 2006. http://hdl.handle.net/1969.1/4712.
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A thermodynamics-based constitutive model is derived which predicts the nonlinear
strain and magnetization response that magnetic shape memory alloys (MSMAs) exhibit
when subjected to mechanical and magnetic loads. The model development
is conducted on the basis of an extended thermo-magneto-mechanical framework.
A novel free energy function for MSMAs is proposed, from which the constitutive
equations are derived in a thermodynamically-consistent manner. The nonlinear and
hysteretic nature of the macroscopic material behavior is captured through the evolution
of internal state variables which are motivated by the crystallographic and
magnetic microstructures of MSMAs. Model predictions are presented for different
relevant loading cases and analyzed in detail. Finally, magnetostatic boundary
value problems for MSMAs are considered and numerically solved using the finite
element method. For these computations the developed constitutive model provides
the nonlinear magnetic properties of the MSMA. The knowledge of the magnetic field
distribution in the computational domain as a function of the applied field, which
results from this magnetostatic analysis, is useful for the proper interpretation of
experimental results as well as the design of experiments and applications.
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Subramaniam, Ameendraraj. "Fatigue behavior of copper zinc aluminum shape memory alloys." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape17/PQDD_0013/MQ32256.pdf.
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Rajagopalan, Sudhir. "INSTRUMENTED NANOINDENTATION STUDIES OF DEFORMATION IN SHAPE MEMORY ALLOYS." Doctoral diss., University of Central Florida, 2005. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/3283.
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Near equi-atomic nickel titanium (NiTi) shape memory alloys (SMAs) are a class of materials characterized by their unique deformation behavior. In these alloys, deformation mechanisms such as mechanical twinning and stress induced phase transformation between a high symmetry phase (austenite) and a low symmetry phase (martensite) additionally occur and influence mechanical behavior and thus their functionality. Consequently, applications of SMAs usually call for precise phase transformation temperatures, which depend on the thermomechanical history and the composition of the alloy. Instrumented indentation, inherently a mechanical characterization technique for small sampling volumes, offers a cost effective means of empirically testing SMAs in the form of centimeter scaled buttons prior to large-scale production. Additionally, it is an effective probe for intricate SMA geometries (e.g., in medical stents, valves etc.), not immediately amenable to conventional mechanical testing. The objective of this work was to study the deformation behavior of NiTi SMAs using instrumented indentation. This involved devising compliance calibration techniques to account for instrument deformation and designing spherical diamond indenters. Substantial quantitative information related to the deformation behavior of the shape memory and superelastic NiTi was obtained for the first time, as opposed to existing qualitative indentation studies. For the case of shape memory NiTi, the elastic modulus of the B19' martensite prior to twinning was determined using spherical indentation to be about 101 GPa, which was comparable to the value from neutron diffraction and was substantially higher than typical values reported from extensometry (68 GPa in this case). Twinning at low stresses was observed from neutron diffraction measurements and was attributed to reducing the elastic modulus estimated by extensometry. The onset of predominantly elastic deformation of the twinned martensite was identified from the nanoindentation response and the elastic modulus of the twinned martensite was estimated to be about 17 GPa. Finite element modeling was used to validate the measurements. For the case of the superelastic NiTi, the elastic modulus of the parent austenite was estimated to be about 62 GPa. The onset of large-scale stress induced martensite transformation and its subsequent elastic deformation were identified from the nanoindentation response. The effect of cycling on the mechanical behavior of the NiTi specimen was studied by repeatedly indenting at the same location. An increase in the elastic modulus value for the austenite and a decrease in the associated hysteresis and residual depth after the initial few cycles followed by stabilization were observed. As for the case of shape memory NiTi, finite element modeling was used to validate the measurements. This work has initiated a methodology for the quantitative evaluation of shape memory and superelastic NiTi alloys with instrumented spherical indentation. The aforementioned results have immediate implications for optimizing thermomechanical processing parameters in prototype button melts and for the mechanical characterization of intricate SMA geometries (e.g., in medical stents, valves etc.) This work was made possible by grants from NASA (NAG3-2751) and NSF (CAREER DMR-0239512) to UCF.Ph.D.
Department of Mechanical, Materials and Aerospace Engineering;
Engineering and Computer Science
Materials Science and Engineering
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Green, S. M. "The surface performance of Ni-Ti shape memory alloys." Thesis, University of Nottingham, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.294709.
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Scarsbrook, G. A. "Martensite stabilisation in Cu-Zn-Al shape memory alloys." Thesis, University of Cambridge, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.373697.
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Kottenstette, Nicholas E. (Nicholas Eugene). "Designing mechanisms with shape memory alloys and permanent magnets." Thesis, Massachusetts Institute of Technology, 1997. http://hdl.handle.net/1721.1/43923.
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Vedantam, Srikanth 1972. "Constitutive modeling of Cu-Al-Ni shape memory alloys." Thesis, Massachusetts Institute of Technology, 2000. http://hdl.handle.net/1721.1/34342.
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Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2000.Includes bibliographical references (leaves 103-110).
Certain alloys can exist in multiple phases, which, in the context of solids, essentially mean multiple crystallographic structures. For example, at certain compositions, a Cu-Al-Ni alloy can exist as a cubic lattice (austenite), an orthorhombic lattice ([beta]'1-martensite) or a monoclinic lattice (([beta]'1-martensite). The material changes from one phase to another under various conditions of thermal and/or mechanical loading. Under certain loads, multiple phases can coexist and when this happens, a sharp interface separates any two phases. As the stress or temperature changes, the interface propagates through the material and particles transform from one phase to the other as they cross the moving phase boundary. (A martensitic phase can exist in the form of many "variants", and an interface between co-existing variants is a twin boundary.) The constitutive modeling of such materials is made difficult by the inherent anisotropic nature of such materials and by the non monotonicity of the stress-strain curves. We develop a systematic method by which we can calculate the free-energy of such a material based on its symmetry. The velocity with which interfaces propagates controls the rate of phase transformations (i.e. the "kinetics"). It is well known that classical balance laws are insufficient for a complete description of the behavior of materials undergoing phase transformations. The classical continuum theory describes the bulk regions (regions away from the interfaces) in a satisfactory manner but leaves a gap in the information concerning the interface. This lacuna has been filled by either including nucleation and kinetic criteria that are consistent with the second law of thermodynamics, or by regularizing the continuum theory in some consistent manner. The above treatments seek to provide information on the boundaries between the phases. However, they suffer from the drawback that even though they are meant to be continuum scale descriptions of microscale phenomena which take place on the transformation front they do not model the physics of the transformations. A more natural way of obtaining the relevant information would be to directly study the transformation process at a microscale and then perform an appropriate homogenization so that the resulting law is applicable at a continuum scale. Such an approach would facilitate a deeper understanding of the transformation process as well as enable the continuum theory to reflect the micromechanical processes that govern the transformation. We develop a lattice model of twin and phase boundaries that accounts for microstructural effects. The model incorporates the effect of ledges in the interface. A quasi continuum model is obtained by approximating the resulting difference-differential equation of motion of the ledge, but retaining leading discreteness effects. The quasicontinuum model now models the interface at a continuum scale but incorporates lattice effects. The kinetic relation obtained from such a model explains the experimentally observed difference in the stress required for moving boundaries between different variants of martensite. The kinetic relation obtained for phase boundaries has the feature that the hysteresis loops do not decrease in size to zero for vanishing loading rates.
by Srikanth Vedantam.
Sc.D.
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Chen, Ta-Tung. "Electrochemical micromachining of microdevices from NiTi shape memory alloys." Thesis, Cranfield University, 1999. http://dspace.lib.cranfield.ac.uk/handle/1826/10697.
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This thesis aimed to develop a reproducible process for batch-fabricating microdevices
required for MEMS and medical applications, such as micro actuators and stents, from
heat-sensitive NiTi shape memory materials. Electrochemical micromachining was
chosen to carry out this work. This is a non-traditional machining process involving
photoresist processing and electrolytic etching which has received much attention
recently for the processing of thin films. The electrolyte used was a non-aqueous solution
of 5% sulphuric acid in methanol.
The optimum parameters for the photoresist processing were obtained by evaluation of
the thickness and exposure time of the KTFR photoresist coating. A quantitative
investigation of the electrolytic etching of NiTi was carried out to study the influence of
applied voltage, etch time and line width of the test pattern on the etching behaviour, e.g.
etch rate, undercut, depth of etch and etch factor. The anodic polarisation behaviour of
NiTi in 5% sulphuric acid in methanol was investigated under a potentiostatic control
system to establish the optimum etching parameters.
The materials used for the fabrication of micro actuators (required by Forschungszentrum
Karlsruhe, Germany to make a prototype microvalve) were NiTi alloy thin film materials
(sputtered or cold-rolled) with thicknesses ranging from 5 to 46J...lm displaying a one-way
or two-way shape m:emory effect. A variety of optimised designs of micro actuator were
successfully etched electrolytically at 8V. The etch rate was found to depend directly on
the anodic current density. The addition of a third alloying element such as Pd or eu
reduced the anodic current density and maintained a similar etch rate. However it
resulted in the breaking of the films during etching due to the reduction in the ductility of
the material.
The materials for the micro fabrication of stents were 100J...lm thick NiTi sheets. The
problem of non-uniform metal dissolution was observed. However, by adding a
sacrificial etch band as a current 'robber', periodic rotation of the anode and properly
adjusting the electrochemical and geometric parameters, the stents were etched
successfully with improved yield and dimensional accuracy.
44
Palanki, Zoltan. "Effect of the applied stress on shape memory alloys." Besançon, 2009. http://www.theses.fr/2009BESA2002.
Full textAbstract:
Dans cette thèse, deux modèles ont été utilisés. Celui développé à Debrecen (Hongrie) a permis de calcul les termes d'énergie non-chimique au début et à la fin de transformation martensitique. Nonobstant, non seulement ces termes sont indispensables mais la connaissance de la dépendance des termes dissipatifs élastiques à la fraction de martensite transformée est, elle aussi, importante. En conclusion, une extension de modèle s'est avérée nécessaire. Ce nouveau modèle a été utilisé afin de rationaliser les mesures effectuée sur des alliages à mémoire de forme CuAINi poly et monocristallins. Les résultats de mesures sur l'échantillon monocristallin n'ont pu être interprétés que si le concept de deux types de phase martensitique est introduit. Le modèle de Besançon (France), développé pour expliquer la transformation martensitique, introduit ce deux martensites différentes (la martensite auto-accommodante et la martensite induite par la contrainte). 0n verra que cette partition n'a aucun sens crystallographique mais est simplement utile dans une approche phénoménologique. Après qu'un lien ait été établie entre ces deux modèles et que deux phase martensitiques aient été prises en compte dans le modèle de Debrecen, la détermination des paramètre nécessaires aux simulations devient possible, et la comparaison a pu être réalisée entre les boucles hystérésis mesurées et calculéesIn this thesis two models will be concerned. The one developed earlier in Debrecen enabled to calculate the non-chemical energy terms at the start and at the end of martensitic transformation. Nevertheless not only theses points are relevant but the knowledge of the dependence of the dissipative and elastic terms on the transformed martensitic fraction in the whole transformation range is important too. Consequently an extension of the model was needed. This new model was used to evaluate the data measured on polycrystalline an single crystalline CuAINi shape memory alloys. The results of the measurements on single crystalline samples could be explained only if two types of martensitic phases were taken into account. Indeed the Besançon model developed for simulation of the martensitic transformation counts with two different martensitic phases too, namely temperature and stress induced ones. After finding the connection between these two models and took them also into account in the Debrecen-model the determination of the input parameters for the simulations became possible and comparison between the measured and calculated hysteresis loops had been made
45
Adiguzel, O. "Phase Transition and Functional Characteristics of Shape Memory Alloys." Thesis, Sumy State University, 2012. http://essuir.sumdu.edu.ua/handle/123456789/35059.
Full textAbstract:
Shape-memory alloys are a new class of functional materials with a peculiar property known as shape
memory effect. These alloys have an ability to recover a particular shape. The origin of this phenomenon
lies in the fact that the material changes its internal crystalline structure with changing temperature.
Copper based ternary alloys exhibit shape memory effect in metastable - phase field. These alloys undergo
two ordered transitions on cooling, and bcc structures turn into B2(CsCl) or DO3(Fe3Al) -type ordered
structures. The ordered structures martensitically undergo the non-conventional layered structures on further
cooling. These structures are called as 3R, 9R or 18R martensites depending on the stacking sequences
on the close-packed planes of the matrix.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/35059
46
Yang, Fan. "Precipitate Phases in Several High Temperature Shape Memory Alloys." The Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1354541472.
Full text
47
Daghia, Federica <1980>. "Active fibre-reinforced composites with embedded shape memory alloys." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2008. http://amsdottorato.unibo.it/962/1/Tesi_Daghia_Federica.pdf.
Full textAbstract:
This dissertation concerns active fibre-reinforced composites with embedded
shape memory alloy wires. The structural application of active materials allows to develop adaptive structures which actively respond to changes in the
environment, such as morphing structures, self-healing structures and power
harvesting devices. In particular, shape memory alloy actuators integrated
within a composite actively control the structural shape or stiffness, thus influencing the composite static and dynamic properties. Envisaged applications
include, among others, the prevention of thermal buckling of the outer skin of
air vehicles, shape changes in panels for improved aerodynamic characteristics
and the deployment of large space structures.
The study and design of active composites is a complex and multidisciplinary topic, requiring in-depth understanding of both the coupled behaviour of
active materials and the interaction between the different composite constituents. Both fibre-reinforced composites and shape memory alloys are extremely
active research topics, whose modelling and experimental characterisation still
present a number of open problems. Thus, while this dissertation focuses on
active composites, some of the research results presented here can be usefully
applied to traditional fibre-reinforced composites or other shape memory alloy
applications.
The dissertation is composed of four chapters.
In the first chapter, active fibre-reinforced composites are introduced by
giving an overview of the most common choices available for the reinforcement, matrix and production process, together with a brief introduction and
classification of active materials.
The second chapter presents a number of original contributions regarding
the modelling of fibre-reinforced composites. Different two-dimensional laminate theories are derived from a parent three-dimensional theory, introducing
a procedure for the a posteriori reconstruction of transverse stresses along the
laminate thickness. Accurate through the thickness stresses are crucial for the
composite modelling as they are responsible for some common failure mechanisms. A new finite element based on the First-order Shear Deformation Theory and a hybrid stress approach is proposed for the numerical solution of the
two-dimensional laminate problem. The element is simple and computationally
efficient. The transverse stresses through the laminate thickness are reconstructed starting from a general finite element solution. A two stages procedure is
devised, based on Recovery by Compatibility in Patches and three-dimensional
equilibrium. Finally, the determination of the elastic parameters of laminated
structures via numerical-experimental Bayesian techniques is investigated. Two
different estimators are analysed and compared, leading to the definition of an
alternative procedure to improve convergence of the estimation process.
The third chapter focuses on shape memory alloys, describing their properties and applications. A number of constitutive models proposed in the literature, both one-dimensional and three-dimensional, are critically discussed and
compared, underlining their potential and limitations, which are mainly related
to the definition of the phase diagram and the choice of internal variables. Some
new experimental results on shape memory alloy material characterisation are
also presented. These experimental observations display some features of the
shape memory alloy behaviour which are generally not included in the current
models, thus some ideas are proposed for the development of a new constitutive
model.
The fourth chapter, finally, focuses on active composite plates with embedded shape memory alloy wires. A number of di®erent approaches can be used
to predict the behaviour of such structures, each model presenting different advantages and drawbacks related to complexity and versatility. A simple model
able to describe both shape and stiffness control configurations within the same
context is proposed and implemented. The model is then validated considering
the shape control configuration, which is the most sensitive to model parameters. The experimental work is divided in two parts. In the first part, an active
composite is built by gluing prestrained shape memory alloy wires on a carbon
fibre laminate strip. This structure is relatively simple to build, however it
is useful in order to experimentally demonstrate the feasibility of the concept
proposed in the first part of the chapter. In the second part, the making of
a fibre-reinforced composite with embedded shape memory alloy wires is investigated, considering different possible choices of materials and manufacturing
processes. Although a number of technological issues still need to be faced, the
experimental results allow to demonstrate the mechanism of shape control via
embedded shape memory alloy wires, while showing a good agreement with the
proposed model predictions.
48
Daghia, Federica <1980>. "Active fibre-reinforced composites with embedded shape memory alloys." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2008. http://amsdottorato.unibo.it/962/.
Full textAbstract:
This dissertation concerns active fibre-reinforced composites with embedded
shape memory alloy wires. The structural application of active materials allows to develop adaptive structures which actively respond to changes in the
environment, such as morphing structures, self-healing structures and power
harvesting devices. In particular, shape memory alloy actuators integrated
within a composite actively control the structural shape or stiffness, thus influencing the composite static and dynamic properties. Envisaged applications
include, among others, the prevention of thermal buckling of the outer skin of
air vehicles, shape changes in panels for improved aerodynamic characteristics
and the deployment of large space structures.
The study and design of active composites is a complex and multidisciplinary topic, requiring in-depth understanding of both the coupled behaviour of
active materials and the interaction between the different composite constituents. Both fibre-reinforced composites and shape memory alloys are extremely
active research topics, whose modelling and experimental characterisation still
present a number of open problems. Thus, while this dissertation focuses on
active composites, some of the research results presented here can be usefully
applied to traditional fibre-reinforced composites or other shape memory alloy
applications.
The dissertation is composed of four chapters.
In the first chapter, active fibre-reinforced composites are introduced by
giving an overview of the most common choices available for the reinforcement, matrix and production process, together with a brief introduction and
classification of active materials.
The second chapter presents a number of original contributions regarding
the modelling of fibre-reinforced composites. Different two-dimensional laminate theories are derived from a parent three-dimensional theory, introducing
a procedure for the a posteriori reconstruction of transverse stresses along the
laminate thickness. Accurate through the thickness stresses are crucial for the
composite modelling as they are responsible for some common failure mechanisms. A new finite element based on the First-order Shear Deformation Theory and a hybrid stress approach is proposed for the numerical solution of the
two-dimensional laminate problem. The element is simple and computationally
efficient. The transverse stresses through the laminate thickness are reconstructed starting from a general finite element solution. A two stages procedure is
devised, based on Recovery by Compatibility in Patches and three-dimensional
equilibrium. Finally, the determination of the elastic parameters of laminated
structures via numerical-experimental Bayesian techniques is investigated. Two
different estimators are analysed and compared, leading to the definition of an
alternative procedure to improve convergence of the estimation process.
The third chapter focuses on shape memory alloys, describing their properties and applications. A number of constitutive models proposed in the literature, both one-dimensional and three-dimensional, are critically discussed and
compared, underlining their potential and limitations, which are mainly related
to the definition of the phase diagram and the choice of internal variables. Some
new experimental results on shape memory alloy material characterisation are
also presented. These experimental observations display some features of the
shape memory alloy behaviour which are generally not included in the current
models, thus some ideas are proposed for the development of a new constitutive
model.
The fourth chapter, finally, focuses on active composite plates with embedded shape memory alloy wires. A number of di®erent approaches can be used
to predict the behaviour of such structures, each model presenting different advantages and drawbacks related to complexity and versatility. A simple model
able to describe both shape and stiffness control configurations within the same
context is proposed and implemented. The model is then validated considering
the shape control configuration, which is the most sensitive to model parameters. The experimental work is divided in two parts. In the first part, an active
composite is built by gluing prestrained shape memory alloy wires on a carbon
fibre laminate strip. This structure is relatively simple to build, however it
is useful in order to experimentally demonstrate the feasibility of the concept
proposed in the first part of the chapter. In the second part, the making of
a fibre-reinforced composite with embedded shape memory alloy wires is investigated, considering different possible choices of materials and manufacturing
processes. Although a number of technological issues still need to be faced, the
experimental results allow to demonstrate the mechanism of shape control via
embedded shape memory alloy wires, while showing a good agreement with the
proposed model predictions.
49
Turabi, Ali S. "EFFECTS OF MAGNETIC FIELD ON THE SHAPE MEMORY BEHAVIOR OF SINGLE AND POLYCRYSTALLINE MAGNETIC SHAPE MEMORY ALLOYS." UKnowledge, 2015. http://uknowledge.uky.edu/me_etds/58.
Full textAbstract:
Magnetic Shape Memory Alloys (MSMAs) have the unique ability to change their shape within a magnetic field, or in the presence of stress and a change in temperature. MSMAs have been widely investigated in the past decade due to their ability to demonstrate large magnetic field induced strain and higher frequency response than conventional shape memory alloys (SMAs). NiMn-based alloys are the workhorse of metamagnetic shape memory alloys since they are able to exhibit magnetic field induced phase transformation. In these alloys, martensite and austenite phases have different magnetization behavior, such as the parent phase can be ferromagnetic and martensite phase can be weakly magnetic. The magnetization difference between the transforming phases creates Zeeman energy, which is the main source for magnetic field induced phase transformation, is unlimited with applied field and orientation independent. Thus, metamagnetic shape memory alloys can be employed in polycrystalline form and provide higher actuation stress than conventional MSMAs. High actuation stress levels and frequencies in metamagnetic shape memory alloys are promising for magnetic actuation applications.
Effects of heat treatments and cooling rates on the transformation temperatures, magnetization response and shape memory behavior under compressive stress were explored in Ni45Mn36.5Co5In13.5 [100] oriented single crystalline alloys to obtain high transformation temperatures, large magnetization difference, and low hysteresis behavior. It was found that transformation temperatures increase with higher heat treatment temperatures and decrease drastically at lower cooling rates. Temperature hysteresis decreased with increasing heat treatment temperatures. It was revealed that transformation temperatures, hysteresis, and magnetization response can be tailored by heat treatments via modifying interatomic order.
Magnetic and mechanical results of NiMn-based metamagnetic alloys in single and polycrystalline forms as functions of composition, stress, temperature and magnetic field (up to 9 Tesla) were revealed through thermal-cycling under stress and magnetic field; stress-cycling as functions of temperature and magnetic field; and magnetic-field-cycling under stress at several temperatures experiments. Single crystalline samples of NiMnCoIn showed recoverable strain of 1.5 % due to magnetic field induced reversible phase transformation under constant stress and strain of 3.7 % by magnetic field induced recovery after variant reorientation of martensite. The magnetic field effect on the superelasticity and shape memory effects were also explored in selected orientations of [100], [110] and [111].
Fe-based ferromagnetic shape memory alloys have received considerable attention due to their better workability, strength, and lower cost compared with commercial NiTi based SMAs. The shape memory properties of a ferrous single crystalline alloy, FeNiCoAlNb, were investigated along the [100] orientation by thermal cycling under constant stress and superelasticity tests in both tension and compression. Aging was used to form nano-size precipitates to demonstrate shape memory behavior and tailor the shape memory properties. It was found that after proper heat treatments, [001] oriented FeNiCoAlNb showed a compressive strain of 15%, low temperature dependent superelastic behavior, high compression-tension asymmetry, and high compressive strength (~3GPa). The orientation dependence of the mechanical properties of FeNiCoAlNb single crystals were investigated along the [100], [110], [012] and [113] orientations. In addition, martensite phase showed higher magnetization than austenite phase as opposed to NiMn-based metamagnetic shape memory alloys. This magnetization difference is promising because it can allow magnetic field induced forward transformation. Ferrous alloys have great potential for high strength, temperature independent, and large scale actuator applications.
50
Urbina, Pons Cristina. "Improvement of the one-way and two-way shape memory effects in ti-ni shape memory alloys by thermomechanical treatments." Doctoral thesis, Universitat Rovira i Virgili, 2011. http://hdl.handle.net/10803/37358.
Full textAbstract:
Ti-Ni phase transformation behaviour is very sensitive to the thermal and mechanical history of the alloy. Thermomechanical cycling through the full transformation range may degrade the Ti-Ni functional properties (functional fatigue). These repeated transformation cycles cause changes in the SMA phase transformation behaviour due to the formation and accumulation of defects in the alloy microstructure.
The main objective of this thesis is to establish the relationships between the changes in Ti-Ni phase transformation behaviour caused by thermomechanical processes, especially in the R-phase range, and the functional properties of the Ti-Ni shape memory alloys (SMAs). Establishing these relationships should allow us to find appropriate thermomechanical processes to substantially improve the Ti-Ni one-way and two-way shape memory effects.
To achieve this objective, several experimental techniques are used including measuring variations of the electrical resistivity with temperature, X-ray diffraction, isothermal tension testing, thermal cycling under constant stress, and thermal cycling under zero stress.
This study of the phase transformation changes caused by thermomechanical processes has led to a new way of interpreting resistivity curves for calculating the transformation temperatures. Moreover, we have determined how the R-phase influences the functional properties of SMA and, finally, we have substantially improved the properties of one-way and two-way shape memory effects by using thermal processes that avoid permanent deformation of the alloy.Las transformaciones de fase en aleaciones de NiTi son altamente dependientes de la composición de la aleación, así como de la historia térmica y mecánica previa al uso de la SMA. El objetivo principal de esta tesis es establecer los vínculos existentes entre los cambios producidos en las transformaciones de fase por procesos termomecánicos y las propiedades funcionales en aleaciones con memoria de forma de Ti-Ni, tal que nos permita hallar los procesos termomecánicos más adecuados que proporcionen una mejora substancial en las propiedades funcionales de estas aleaciones. La determinación de estas relaciones, prestando especial atención a la fase-R, nos debe proporcionar las claves para incrementar las propiedades de memoria de forma y doble memoria de forma. Para la consecución de este objetivo se han usado diferentes técnicas de caracterización experimental: variación de la resistividad eléctrica con la temperatura (ER), difractometría de rayos X (DRX), ensayos de tracción isotérmicos, ciclados térmicos a tensión constante y ciclados térmicos a tensión nula. A través del estudio exhaustivo de las transformaciones de fase, se ha aportado una nueva interpretación de las curvas de resistividad para el cálculo de las temperaturas de transformación, se ha determinado la influencia de la fase-R en las propiedades funcionales de las SMA y se han mejorado substancialmente las propiedades de memoria simple y doble memoria de forma mediante procesos térmicos que evitan deformaciones permanentes antes del uso de la aleación.
Les transformacions de fase en aliatges de NiTi són altament dependents de la composició de l'aliatge, així com de la història tèrmica i mecànica prèvia a l'ús de la SMA. L'objectiu principal d'aquesta tesi és establir els vincles existents entre els canvis produïts en les transformacions de fase per processos termomecànics i les propietats funcionals en aliatges amb memòria de forma de Ti-Ni, tal que ens permeti trobar els processos termomecànics més adequats que proporcionin una millora substancial en les propietats funcionals d'aquests aliatges. La determinació d'aquestes relacions, prestant especial atenció a la fase-R, ens ha de proporcionar les claus per incrementar les propietats de memòria de forma i doble memòria de forma. Per a la consecució d'aquest objectiu s'han usat diferents tècniques de caracterització experimental: variació de la resistivitat elèctrica amb la temperatura (ER), difractometria de raigs X (DRX), assaigs de tracció isotèrmics, ciclats tèrmics a tensió constant i ciclats tèrmics a tensió nula. A través de l'estudi exhaustiu de les transformacions de fase, s'ha aportat una nova interpretació de les corbes de resistivitat per al càlcul de les temperatures de transformació, s'ha determinat la influència de la fase-R en les propietats funcionals de les SMA i s'han millorat substancialment les propietats de memòria simple i doble memòria de forma mitjançant processos tèrmics que eviten deformacions permanents abans de l'ús de l'aliatge.