Submitted by Johnny Rodrigues (johnnyrodrigues@ufcg.edu.br) on 2018-04-26T21:22:44Z No. of bitstreams: 1 FERNANDO ANDRADE AMORIM - DISSERTAÇÃO PPGEM 2014..pdf: 6350093 bytes, checksum: 23c947f61b48ed2613bf915ed058b600 (MD5) / Made available in DSpace on 2018-04-26T21:22:44Z (GMT). No. of bitstreams: 1 FERNANDO ANDRADE AMORIM - DISSERTAÇÃO PPGEM 2014..pdf: 6350093 bytes, checksum: 23c947f61b48ed2613bf915ed058b600 (MD5) Previous issue date: 2014-08-08 / CNPq / Capes / Ligas de NiTi têm possibilitado revolucionar muitos projetos tradicionais de engenharia com suas propriedades únicas de superelasticidade (SE) e efeito de memória de forma (EMF). Com o interesse em incorporar esses materiais em diferentes aplicações e dispositivos, o desenvolvimento de tecnologia eficaz para soldagem de ligas NiTi torna-se necessário, devido ao fato de esse tipo de união proporcionar a fabricação das mais diferentes formas geométricas e combinações entre materiais similares e dissimilares. Nesse contexto, este estudo teve como objetivo geral determinar as variações das propriedades termomecânicas em juntas soldadas de fios de NiTi. Para o trabalho, fios superelásticos de uma LMF NiTi (ASTM F2063) com 0,4 e 0,9 mm de diâmetro, foram divididos em dois grupos: (a) fios sem tratamento térmico (NiTiA) e (b) fios com tratamento térmico a 400 °C durante 20 minutos (NiTi400). Em seguida estes fios foram soldados pelo processo TIG autôgeno, utilizando a soldadora Micromelt (EDG Equipamentos e Controles). A caracterização termomecânica dos fios íntegros e soldados foi realizada utilizando ensaios de calorimetria diferencial de varredura (DSC), análise dinâmico-mecânica (DMA), ensaios de tração uniaxial em diferentes temperaturas (30°C a 70°C), microscopia óptica (MO), microscopia eletrônica de varredura (MEV) e micro indentação Vickers. Os resultados obtidos demonstraram uma boa eficiência do processo TIG ao soldar os fios NiTi de ambos os grupos. Ensaios de DSC relevaram que o processo de soldagem pode promover modificações de natureza metalúrgica aos fios de NiTi, de maneira a reduzir as temperaturas de transformações de fase. Já no que diz respeito ao aspecto mecânico, os fios soldados apresentaram resistência a ruptura por tração de até 750 MPa (NiTiA ~ 70 °C) e valores de deformações de até 8,5 % (NiTiA). / NiTi alloys enabled revolutionize many traditional engineering projects with uniques properties of superelasticity (SE) and shape memory effect (SME). Interested on incorporating these materials in different applications and devices, developments of efficient welding technology for NiTi alloys becomes necessary due to the fact that this type of process can provide many differents geometric shapes and combinations of similar and dissimilar materials. In this context, this study had as main objective to determine variations in thermomechanical properties for NiTi welded wires. For this work, superelastic NiTi SMA wire (ASTM F2063) with 0.9 to 0.4 mm in diameter were divided into two groups: (a) wires without heat treatment (NiTiA) and (b) heat treated wires at 400 ° C for 20 minutes (NiTi400). Then, these wires were welded by autogenous welding process, using the Micromelt machine (EDG, Equipment and Controls). The thermomechanical characterization of as received and as welded wires was performed by using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) uniaxial tensile test at different temperatures (30°C to 70°C), optical microscopy (OM), scanning electron microscopy (SEM) and Vickers micro indentation. The results showed a good efficiency of TIG welding for every NiTi wire studied groups. DSC curves demonstrates that welding process can cause a metallurgical changes into NiTi wires, promoting changes in phase transformation temperatures. The mechanical properties of the welded wire presented a tensile rupture strength of 750 MPa (NiTi~70 °C) and strain values up to 8.5% (NiTiA).

Submitted by Johnny Rodrigues (johnnyrodrigues@ufcg.edu.br) on 2018-04-27T15:46:57Z No. of bitstreams: 1 ESTEPHANIE NOBRE DANTAS GRASSI - DISSERTAÇÃO PPGEM 2014..pdf: 4659811 bytes, checksum: 9dce2fd88b57abcccbb5be6fa913cf1b (MD5) / Made available in DSpace on 2018-04-27T15:46:57Z (GMT). No. of bitstreams: 1 ESTEPHANIE NOBRE DANTAS GRASSI - DISSERTAÇÃO PPGEM 2014..pdf: 4659811 bytes, checksum: 9dce2fd88b57abcccbb5be6fa913cf1b (MD5) Previous issue date: 2014-08-01 / CNPq / Capes / As Ligas com Memória de Forma (LMF) são um importante grupo de materiais metálicos ativos que respondem a estímulos termomecânicos por meio dos fenômenos do Efeito Memória de Forma (EMF) e da Superelasticidade (SE). Ambos os efeitos permitem recuperar grandes níveis deformações por meio de aquecimento, no primeiro caso, ou do descarregamento mecânico, no segundo. As LMF de NiTi são facilmente encontradas no mercado médico e odontológico em forma de ferramentas e acessórios para tratamentos específicos. Um destes elementos são minimolas helicoidais ortodônticas de NiTi, que alcançam deformações algumas centenas de vezes maiores que elementos unidimensionais de LMF, como fios. Por outro lado, é de amplo conhecimento que uma técnica adequada para manipular propriedades mecânicas de produtos metálicos acabados, além de variar-se a configuração geométrica, é a realização de tratamentos térmicos de recozimento. Principalmente após a realização de trabalho a frio, os recozimentos são capazes de recuperar parcial ou totalmente a mobilidade atômica no metal, o que, no caso das LMF, afeta diretamente o seu comportamento termomecânico. Neste contexto, o principal objetivo deste trabalho é estudar a influência de tratamentos térmicos de recozimento sobre a resposta termomecânica de minimolas de LMF NiTi, originalmente superelásticas. Um planejamento fatorial foi usado para avaliar a influência das variáveis temperatura e tempo de recozimento sobre algumas das principais propriedades termomecânicas das minimolas: constante de mola (rigidez), módulo de elasticidade transversal, capacidade de dissipação de energia, temperaturas de transformação, histere térmica e a entalpia de transformação. Foi demonstrado que tratamentos térmicos a temperaturas na faixa de 500 oC a 600 oC são capazes de converter as minimolas de LMF NiTi do estado superelástico para o estado de atuador, pelo aparecimento do efeito memória de forma. / Shape Memory Alloys (SMA) are an important group of metallic active materials that respond to thermomechanical stimuli through the Shape Memory Effect (SME) or the Superelasticity (SE) phenomena. Both these effects are capable of retrieving large amounts of strain by simple heating, in the former case, or simple mechanical unload, in the latest case. The SMA of the NiTi family composition exhibit superior properties when compared to other compositions, including biocompability, what brings this alloy to be widely used in medical and orthodontic fields in the form of tools and accessories to specific treatments. As an example, mini coil springs of NiTi SMA presenting superelasticity reach strain levels hundreds of times higher than one-dimensional elements, such as wires. However, a more suitable technique to manipulate mechanical properties of metallic finished products is the use of heat treatments like annealing. Mainly after experiencing cold working processes, annealing treatments are capable of partially or totally recover the atomic mobility, witch directly affects thermomechanical response of SMA. In this context, this dissertation work aims to study the influence of annealing heat treatments over thermomechanical behavior of SMA NiTi mini coil springs originally presenting the SE. A factorial design was used to evaluate the influence of temperature and time of annealing over some of the main thermomechanical springs’ properties: spring constant (stiffness), shear modulus, energy dissipation capacity, phase transformation temperatures, thermal hysteresis and transformation enthalpy availability. It was demonstrated that heat treatments between 500°C and 600°C are capable of converting the superelastic state of the mini coil springs to an actuator state, as a result of the shape memory effect appearance.

Submitted by Lucienne Costa (lucienneferreira@ufcg.edu.br) on 2018-04-30T18:33:58Z No. of bitstreams: 1 YURI JOHANN VILAR DE BRITO – DISSERTAÇÃO (PPGEM) 2017.pdf: 3327072 bytes, checksum: 64ff7fa9047331a680ddf175d1592b12 (MD5) / Made available in DSpace on 2018-04-30T18:33:58Z (GMT). No. of bitstreams: 1 YURI JOHANN VILAR DE BRITO – DISSERTAÇÃO (PPGEM) 2017.pdf: 3327072 bytes, checksum: 64ff7fa9047331a680ddf175d1592b12 (MD5) Previous issue date: 2017-05-02 / Capes / A terapia endodôntica executada em canais radiculares curvos com limas de aço inoxidável pode provocar inúmeros acidentes iatrogênicos, resultando em problemas como perfurações do canal. Na tentativa de superar as limitações desses instrumentos menos flexíveis foi desenvolvida a utilização da liga com memória de forma (LMF) de NiTi (Níquel-Titânio) para fabricação de instrumentos endodônticos. A utilização desses instrumentos endodônticos de NiTi modificou os métodos de instrumentação do sistema de canais radiculares, tornando-os mais seguros e rápidos. No entanto, ainda existem poucas informações sobre o comportamento termomecânico destes instrumentos mais avançados. Nesse contexto, no presente trabalho foram realizadas as caracterizações térmica e mecânica de três limas endodônticas de LMF NiTi: Protaper Next X2, Reciproc R25 e WaveOne Primmary. Na caracterização térmica foram realizados ensaios de calorimetria diferencial de varredura (DSC) e resistência elétrica em função da temperatura (RET) para a determinação das temperaturas de transformação de fase das limas, enquanto a caracterização mecânica consistiu de ensaios de flexão simples alternada em regime estático e dinâmico (frequência de 5 Hz) a diferentes temperaturas e deflexões máximas, usando um Analisador Dinâmico Mecânico (DMA) e finalizou com uma análise pós fagida com ensaios de calorimetria diferencial de varredura (DSC) e análise das superfícies fraturadas com um Microscópio Eletrônico de Varredura (MEV) Os resultados obtidos com as três limas revelaram que estes instrumentos endodônticos se encontram na fase conhecida como Fase R, os ensaios quase estáticos revelaram uma forte dependência da Força requerida e a Temperatura, os ensaios cíclicos em flexão alternada para impor deflexões na faixa de ±0,5 mm a ±2,0 mm as limas Reciproc R25 mostraram maior tempo de vida cíclica do que as demais, e com o aumento da temperatura de teste de 35 oC para 45 oC, verificou-se que a força para impor as deflexões foram maior e a vida em fadiga das limas reduziu consideravelmente. / Endodontic therapy performed on curved root canals with stainless steel files can cause many iatrogenic accidents, resulting in problems such punctures the canal. In an attempt to overcome the limitations of the less flexible instruments, the alloy with shape memory and NiTi (Nickel-Titanium) for manufacturing endodontic instruments were developed. The use of the NiTi instruments modified methods of instrumentation of the root canal system, making them safer and faster. However, there is still little information on the thermomechanical behavior of more advanced instruments. In this context, the present work is done the thermal and mechanical characterization of three endodontic commercial LMF NiTi files: Protaper Next X2, Reciproc R25 and WaveOne Primmary were performed. For thermal characterization were performed differential scanning calorimetry (DSC) tests and electric resistance as a function of temperature (RET) for determining phase transformation temperatures of files, while the mechanical characterization consisted of static bending tests and dynamic regime (5 Hz frequency) at different temperatures and maximum deflections using a Dynamic Mechanical Analyzer (DMA) and finalizes with post fracture analysis with differential scanning calorimetry (DSC) and fractured surface analysis with an Electron Microscope. The results obtained with the three files showed that endodontic files are in phase known as Phase R, the quasi static tests revealed a strong dependence of the Required Force and the Temperature, the cyclic tests in alternating bending to impose deflections in the Range from ± 0.5 mm to ± 2.0 mm, the Reciproc R25 showed longer cycle life than the others, and with the test temperature rise from 35 °C to 45 °C, it was found that the force to impose the deflections were higher and the fatigue life of the limes reduced considerably.

This thesis work was conducted in the Department of Mechanical Engineering at the Cape Peninsula University of Technology (CPUT) and was submitted towards the partial fulfilment of the Masters Degree in Technology: Mechanical Engineering. / Recent years have witnessed a tremendous growth and significant advances in “smart” composites and “smart” composite structures. These smart composites integrate active elements such as sensors and actuators into a host structure to create improved or new functionalities through a clever choice of the active elements and/or a proper design of the structure. Such composites are able to sense a change in the environment and make a useful response by using an external feedback control system. Depending on their applications, smart composites usually make use of either the joint properties of the structure or the properties of the individual elements within the composites. The accumulation in the understanding of materials science and the rapid developments in computational capabilities have provided an even wider framework for the implementation of multi-functionality in composites and make “smart” composites “intelligent”.

Submitted by Luciana Ferreira (lucgeral@gmail.com) on 2018-06-11T11:47:52Z No. of bitstreams: 2 Dissertação - Ana Paula Pedroso Brito - 2015.pdf: 1014799 bytes, checksum: e2dcf8a46ffc5134b0215684a6190af3 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) / Approved for entry into archive by Luciana Ferreira (lucgeral@gmail.com) on 2018-06-11T11:50:48Z (GMT) No. of bitstreams: 2 Dissertação - Ana Paula Pedroso Brito - 2015.pdf: 1014799 bytes, checksum: e2dcf8a46ffc5134b0215684a6190af3 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) / Made available in DSpace on 2018-06-11T11:50:48Z (GMT). No. of bitstreams: 2 Dissertação - Ana Paula Pedroso Brito - 2015.pdf: 1014799 bytes, checksum: e2dcf8a46ffc5134b0215684a6190af3 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Previous issue date: 2015-03-10 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / Purpose: to determine flute-to-shank ratio of rotatory nickel-titanium instruments Mtwo® #30/.05, ProTaper® #30/.09, Wizard Navigator® #30/.06 and BioRace® 35/.04. Material and methods: Instruments’ diameter were measured with a digital caliper in the first five flutes and abraded longitudinally by a diamond disk until instrument half diameter was achieved in flutes region. The samples were then fixed in stubs and SEM images were performed at 60X magnification. The flutes and shanks areas were measured, and flute-toshank ratio was achieved using software AxioVision®. Statistical analysis was performed by ANOVA and Tukey’s Test for comparison between groups. Results: There was significant statistical difference for flutes areas. Mtwo® #30/.05 had the higher shank areas and was significantly different from the other instruments, except for Wizard Navigator #30/.06 ate the second flute site. Flute-to-shank ratio showed a 3-28% variation. Conclusions: Flutes dimensions were lower than shanks’. Wizard Navigator® #30/.05 had lower flute-to-shank ratio than other files. / Objetivo: Relacionar a aresta lateral de corte e o núcleo de instrumentos endodônticos de níquel-titânio de rotação contínua Mtwo® #30/.05, ProTaper® #30/.09, Wizard Navigator® #30/.06 e BioRace® 35/.04. Material e métodos: Os instrumentos foram medidos com paquímetro digital para aferição do seu diâmetro nas cinco primeiras arestas de corte e desgastados longitudinalmente com disco diamantado flexível dupla face até atingir metade do diâmetro do instrumento nas regiões das arestas laterais de corte. A seguir, as amostras foram fixadas em stubs e obtidas as imagens por microscopia eletrônica de varredura com aumento de 60X. Foram realizadas mensurações das áreas da aresta lateral de corte e do núcleo, e a razão entre estas medidas utilizando o software AxioVision®. A análise estatística foi realizada por meio da ANOVA e do Teste de Tukey para comparação entre os grupos. Resultados: Houve diferença significativa entre as áreas das arestas para todos os instrumentos. O instrumento Mtwo® #30/.05 apresentou as maiores áreas do núcleo, com diferença significativa para todos os demais instrumentos, à exceção do Wizard Navigator® #30/.06 na região da segunda aresta. A relação entre a área da aresta de corte e do respectivo núcleo mostrou que a aresta representou áreas que variaram de 3% a 28% do núcleo. Conclusões: As dimensões da aresta lateral de corte foram inferiores às do núcleo em todos os instrumentos. O Wizard Navigator® #30/.06 apresentou uma relação entre as dimensões da aresta em relação ao núcleo inferiores aos demais

Since NiTi shape memory alloy (SMA) was discovered in the early 1960s, great progress has been made in understanding the properties and mechanisms of NiTi SMA and in developing associated products. For several decades, most of the scientific research and industrial interests on NiTi SMA has focused on its superelastic applications in the biomedical field and shape memory based “smart” devices, which involves the low strain rate (around 0.001 s^-1) response of NiTi SMA. Due to either stress-induced martensite phase transformation or stress induced martensite variant reorientation under the applied load, NiTi SMA has exhibited a high damping capacity in both austenitic and martensitic phase. Recently, there has been an increasing interest in exploitation of the high damping capacity of NiTi SMA to develop high strain rate related applications such as seismic damping elements and energy absorbing devices. However, a systematic study on the influence of strain, strain rate and temperature on the mechanical properties, phase transformation, microstructure and crystal structure is still limited, which leads to the difficulties in the design of products being subjected to high strain rate loading conditions. The four main objectives of the current research are: (1) achieve the single loading and the control of strain, constant strain rate and temperature in high strain rate compression tests of NiTi SMA specimens using Kolsky (split Hopkinson) compression bar; (2) explore the high strain rate compressive responses of NiTi SMA specimens as a function of strain (1.4%, 1.8%, 3.0%, 4.8%, and 9.6%), strain rate (400, 800 and 1200 s^-1), and temperature (room temperature (294 K) and 373 K); (3) characterize and compare the microstructure, phase transformation and crystal structure of NiTi SMAs before and after high strain rate compression; and (4) correlate high strain rate deformation with the changes of microstructure, phase transformation characteristics and crystal structure. Based on the results from this study, it was found that: (1) the compressive stress strain curves of martensitic NiTi SMAs under quasi-static loading conditions are different from those under high strain rate loading conditions, where higher strain hardening was observed; (2) the critical stress and stress plateau of martensitic NiTi SMAs are sensitive to the strain rate and temperature, especially at 373K, which results from the interplay between strain hardening and thermal softening; (3) the microstructure of martensitic NiTi SMA has changed with increasing strain rate at room temperature (294 K), resulting in the reduction in the area of ordered martensite region, while that area increases after deformation at elevated temperature (373K); (4) the phase transformation characteristic temperatures are more sensitive to deformation strain than strain rate; (5) the preferred crystal plane of martensitic NiTi SMA has changed from (11 ̅1)M before compression to (111)M after compression at room temperature (294 K), while the preferred plane remains exactly the same for martensitic NiTi SMA before and after compression at 373 K. Lastly, dynamic recovery and recrystallization are also observed after deformation of martensitic NiTi SMA at 373K.

The strain capacity of structures depends on the plastic hinge behaviour. There are different proposals in the scientific literature in order to improve this behaviour in reinforced concrete structures such as the following ones: proposals include the use of fiber reinforced concrete, very high performance concrete or replacing in the critical structure zone, the steel reinforcement with shape memory alloy and superelasticity bars (from this point forward SMA), among other solutions. However, the strain capacity of hinges is dependent on the compressed reinforcement buckling, which means a drastic diminishing in the bearing capacity and ductility. This phenomenon happens due to the cover spalling or degradation, or due to an insufficient transverse reinforcement arrangement. The design codes propose requirements related to the diameter of longitudinal bars and to the transverse reinforcement separation to assure the bearing capacity or to assure the hinge rotation without the compressed reinforcements buckle. Nevertheless, the aforementioned requirements are not valid in compressed elements made of new materials (fiber reinforced concrete, very high performance concrete or SMA bars). This doctoral thesis analyzes the compressed steel or NiTi - SMA bars behaviour in elements made of conventional, high strength or very high performance concrete, with or without fibres. Therefore, an experimental research has been carried out to study the local instability of the compressed bars (steel and NITI) in concrete elements. 32 columns subject to a bending-compression load have been analyzed. An analytic model has been proposed to analyze the buckling critical stress and length in the compressed bars in concrete elements. This model has been calibrated based on experimental tests. It has been performed an experimental and numerical research to analyze the behaviour of the NITI bars as isolated bars. It has been proposed an analytic model to calculate the relationship stress-strain of the compressed bars that includes the buckling effect. This model has been verified by both experimental and numerical results. Finally, it has been proposed an expression to calculate the maximum separation of the transverse reinforcement according to the required limited stress, for both steel and SMA bars. In order to define that stress, two criteria have been proposed: one of them is based on stresses and the other one on strains. With respect to the concrete without fibres case, the proposed expression has been compared with the current code. / La capacidad de deformación de las estructuras depende del comportamiento de las rótulas plásticas. Para mejorar dicho comportamiento en estructuras de hormigón armado, en la literatura técnica se propone utilizar hormigón con fibras de acero en su masa, hormigón de muy altas prestaciones o sustituir en la zona crítica de la estructura las armaduras de acero por barras de aleación con memoria de forma y superelasticidad (en adelante SMA) entre otras soluciones. Sin embargo, la capacidad de deformación de las rótulas está condicionada por el pandeo de la armadura comprimida, lo que supone una disminución drástica de la capacidad resistente y de la ductilidad. Este fenómeno se produce porque el recubrimiento del hormigón salta o se degrada, o por una insuficiente disposición de armadura transversal. En las normativas de diseño se proponen requisitos acerca del diámetro y de la separación de la armadura transversal para asegurar la capacidad resistente o la deformación de la rótula sin que las armaduras comprimidas pandeen. Sin embargo, dichas expresiones no son válidas en elementos comprimidos fabricados con nuevos materiales (hormigón con fibras en su masa, hormigones de muy altas prestaciones o barras de SMA). En esta tesis doctoral se analiza el comportamiento de barras comprimidas, de acero o de SMA en base NiTi, en elementos fabricados con hormigones convencionales, de alta resistencia o de muy altas prestaciones, con o sin fibras en su masa. A tal efecto, se ha ejecutado un programa experimental para estudiar la inestabilidad local de las barras comprimidas (acero y NiTi) en elementos de hormigón. Se han analizado un total de 32 soportes sometidos a una solicitación de flexo-compresión. Se ha propuesto un modelo analítico para determinar la tensión y la longitud crítica de pandeo de barras comprimidas en elementos de hormigón. Este modelo ha sido calibrado con los ensayos experimentales. Se ha realizado un estudio experimental y numérico para analizar el comportamiento de las barras de NiTi como barras aisladas. Se ha propuesto un modelo analítico para calcular la relación tensión - deformación de barras comprimidas que incluye el efecto del pandeo. Este modelo ha sido verificado tanto con los resultados experimentales como numéricos. Finalmente, se ha propuesto una expresión para el cálculo de la separación máxima de la armadura transversal en función de la tensión límite requerida, tanto para barras de acero como de SMA. Para la definición de dicha tensión se proponen dos criterios: uno basado en tensiones y otro en deformaciones. En el caso de hormigón sin fibras, la expresión propuesta se ha comparado con la normativa actual. / La capacitat de deformació de les estructures depén del comportament de les ròtules plàstiques. Per a millorar dit comportament en estructures de formigó armat, en la literatura tècnica es proposa utilitzar formigó amb fibres d'acer en la seua massa, formigó de molt altes prestacions o substituir en la zona crítica de l'estructura les armadures d'acer per barres d'aliatge amb memòria de forma i superelasticitat (d'ara endavant SMA) entre d'altres solucions. No obstant això, la capacitat de deformació de les ròtules està condicionada pel vinclament de l'armadura comprimida, la qual cosa suposa una disminució dràstica de la capacitat resistent i de la ductilitat. Este fenomen es produeix perquè el recobriment del formigó salta o es degrada, o per una insuficient disposició de l'armadura transversal. En les normatives de disseny es proposen requisits quant al diàmetre i a la separació de l'armadura transversal per assegurar la capacitat resistent o la deformació de la ròtula sense que les armadures comprimides vinclen. No obstant això, estes expressions no son vàlides en elements comprimits fabricats amb nous materiales (formigó amb fibres en la seua massa, formigons de molt altes prestacions o barres de SMA). En esta tesis doctoral s'analitza el comportament de barres comprimides, d'acer o de SMA compost per NiTi, en elements fabricats amb formigons convencionals, d'alta resistència o de molt altes prestacions, amb o sense fibres en la seua massa. A tal efecte, s'ha executat un programa experimental per estudiar la inestabilitat local de les barres comprimides (acer i NiTi) en elements de formigó. S'han analitzat un total de 32 soports somesos a una solicitació de flexo-compressió. S'ha proposat un model analític per determinar la tensió i longitud crítica de vinclament de barres comprimides en elements de formigó. Este model ha sigut calibrat amb els assajos experimentals. S'ha realitzat un estudi experimental i numéric per analitzar el comportament de les barres de NiTi com barres aïllades. S'ha proposat un model analític per calcular la relació tensió-deformació de barres comprimides que inclou l'efecte del vinclament. Este model ha sigut verificat tant amb els resultats experimentals com numérics. Finalment, s'ha proposat una expressió per al càlcul de la separació màxima de l'armadura transversal en funció de la tensió límit requerida., tant per a barres d'acer com de SMA. Per a la definició de dita tensió es proposen dos criteris: uno basat en tensions i l'altre en deformacions. En el cas del formigó amb fibres, l'expressió proposada s'ha comparat amb la normativa actual. / Pereiro Barceló, J. (2017). INESTABILIDAD DE BARRAS COMPRIMIDAS DE ACERO Y DE SMA EN ELEMENTOS DE HORMIGÓN FABRICADOS CON NUEVOS MATERIALES. RECOMENDACIONES DE DISEÑO [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/90650 / TESIS

NiTi-based shape memory alloys (SMAs) offer a good combination of high-strength, ductility, corrosion resistance, and biocompatibility that has served them well and attracted the attention of many researchers and industries. The alloys unique thermo-mechanical ability to recover their initial shape after relatively large deformations by heating or upon unloading due to a characteristic reversible phase transformation makes them useful as damping devices, solid state actuators, couplings, etc. However, there is a need to increase the temperature of the characteristic phase transformation above 150 °C, especially in the aerospace industry where high temperatures are often seen. Prior researchers have shown that adding ternary elements (Pt, Pd, Au, Hf and Zr) to NiTi can increase transformation temperatures but most of these additions are extremely expensive, creating a need to produce cost-effective high temperature shape memory alloys (HTSMAs). Thus, the main objective of this research is to examine the relatively unstudied NiTiZr system for the ability to produce a cost effective and formable HTSMA. Transformation temperatures, precipitation paths, processability, and high-temperature oxidation are examined, specifically using high energy X-ray Diffraction (XRD) measurements, in NiTi-20 at.% Zr. This is followed by an in situ XRD study of the phase growth kinetics of the favorable H-phase nano precipitates, formed in NiTiHf and NiTiZr HTSMAs, based on prior thermo-mechanical processing in a commercial NiTi-15 at.% Hf HTSMA to examine the final processing methods and aging characteristics. Through this research, knowledge of the precipitation paths in NiTiZr and NiTiHf HTSMAs is extended and methods for characterization of phases and strains using high energy XRD are elucidated for future work in the field.

Shape memory alloys (SMAs) can recover large strains (e.g., up to 8%) by undergoing a temperature-induced phase transformation. This strain recovery can occur against large forces, resulting in their use as actuators. The SMA elements in such actuators integrate both sensory and actuation functions. This is possible because SMAs can inherently sense a change in temperature and actuate by undergoing a shape change, associated with the temperature-induced phase transformation. The objective of this work is to develop an SMA based cryogenic thermal conduction switch for operation between dewars of liquid methane and liquid oxygen in a common bulk head arrangement for NASA. The design of the thermal conduction switch is based on a biased, two-way SMA actuator and utilizes a commercially available NiTi alloy as the SMA element to demonstrate the feasibility of this concept. This work describes the design from concept to implementation, addressing methodologies and issues encountered, including: a finite element based thermal analysis, various thermo-mechanical processes carried out on the NiTi SMA elements, and fabrication and testing of a prototype switch. Furthermore, recommendations for improvements and extension to NASA's requirements are presented. Such a switch has potential application in variable thermal sinks to other cryogenic tanks for liquefaction, densification, and zero boil-off systems for advanced spaceport applications. The SMA thermal conduction switch offers the following advantages over the currently used gas gap and liquid gap thermal switches in the cryogenic range: (i) integrates both sensor and actuator elements thereby reducing the overall complexity, (ii) exhibits superior thermal isolation in the open state, and (iii) possesses high heat transfer ratios between the open and closed states. This work was supported by a grant from NASA Kennedy Space Center (NAG10-323) with William U. Notardonato as Technical Officer.

Polycrystalline NiTi shape memory alloys have the ability to recover their original, pre-deformed shape in the presence of external loads when heated through a solid-solid phase transformation from a lower-symmetry B19' martensite phase to a higher-symmetry B2 austenite phase. The strain associated with a shape memory alloy in an actuator application typically has thermal, elastic and inelastic contributions. The objective of this work was to investigate the aforementioned strains by recourse to in situ neutron diffraction experiments during selected combinations of heating, cooling and/or mechanical loading. The primary studies were conducted on polycrystalline Ni49.9Ti50.1 specimens on the Spectrometer for MAterials Research at Temperature and Stress (SMARTS) at Los Alamos National Laboratory. Quantitative information on the phase-specific strain, texture and phase fraction evolution was obtained from the neutron data using Rietveld refinement and single-peak analyses, and compared with macroscopic data from extensometry. First, the lattice strain evolution during heating and cooling in an unloaded sample (i.e., free-recovery experiment) was studied. The lattice strain evolution remained linear with temperature and was not influenced by intergranular stresses, enabling the determination of a thermal expansion tensor that quantified the associated anisotropy due to the symmetry of B19' NiTi. The tensor thus determined was subsequently used to obtain an average coefficient of thermal expansion that was consistent with macroscopic dilatometric measurements and a 30,000 grain polycrystalline self-consistent model. The accommodative nature of B19' NiTi was found to account for macroscopic shape changes lagging (with temperature) the start and finish of the transformation. Second, the elastic response of B19' martensitic NiTi variants during monotonic loading was studied. Emphasis was placed on capturing and quantifying the strain anisotropy which arises from the symmetry of monoclinic martensite and internal stresses resulting from intergranular constraints between individual variants and load re-distribution among variants as the texture evolved during variant reorientation and detwinning. The methodology adopted took into account both tensile and compressive loading given the asymmetric response in the texture evolution. Plane specific elastic moduli were determined from neutron measurements and compared with those determined using a self-consistent polycrystalline deformation model and from recently reported elastic stiffness constants determined via ab initio calculations. The comparison among the three approaches further helped understand the influence of elastic anisotropy, intergranular constraint, and texture evolution on the deformation behavior of polycrystalline B19' NiTi. Connections were additionally made between the assessed elastic properties of martensitic NiTi single crystals (i.e., the single crystal stiffness tensor) and the overall macroscopic response in bulk polycrystalline form. Lastly, the role of upper-cycle temperature, i.e., the maximum temperature reached during thermal cycling, was investigated during load-biased thermal cycling of NiTi shape memory alloys at selected combinations of stress and temperature. Results showed that the upper-cycle temperature, under isobaric conditions, significantly affected the amount of transformation strain and thus the work output available for actuation. With the objective of investigating the underlying microstructural and micromechanical changes due to the influence of the upper-cycle temperature, the texture evolution was systematically analyzed. While the changes in transformation strain were closely related to the evolution in texture of the room temperature martensite, retained martensite in the austenite state could additionally affect the transformation strain. Additionally, multiple thermal cycles were performed under load-biased conditions in both NiTi and NiTiPd alloys, to further assess and understand the role of retained martensite. Dimensional and thermal stabilities of these alloys were correlated with the volume fraction and texture of retained martensite, and the internal strain evolution in these alloys. The role of symmetry, i.e., B19' monoclinic martensite vs. B19 orthorhombic martensite in these alloys was also assessed. This work not only established a methodology to study the thermal and elastic properties of the low symmetry B19' monoclinic martensite, but also provided valuable insight into quantitative micromechanical and microstructural changes responsible for the thermomechanical response of NiTi shape memory alloys. It has immediate implications for optimizing shape memory behavior in the alloys investigated, with extension to high temperature shape memory alloys with ternary and quaternary elemental additions, such as Pd, Pt and Hf. This work was supported by funding from NASAÂ s Fundamental Aeronautics Program, Supersonics Project (NNX08AB51A) and NSF (CAREER DMR-0239512). It benefited additionally from the use of the Lujan Neutron Scattering Center at Los Alamos National Laboratory, which is funded by the Office of Basic Energy Sciences (Department of Energy) and is operated by Los Alamos National Security LLC under DOE Contract DE-AC52-06NA25396.