In this work, beneficial effects of tensile mean strain on fatigue behavior and microstructure of superelastic NiTi (i.e. Nitinol) are studied. Most applications, such as endovascular stents made with NiTi, are subjected to a combination of constant and cyclic loading; thus, understanding the fatigue behavior of NiTi undergoing mean strain loading is necessary. Cyclic strain-controlled fatigue tests are designed to investigate the effects of tensile mean strain on fatigue of superelastic NiTi. Experimental observations show that combinations of large tensile mean strains and small strain amplitudes improve the fatigue life of superelastic NiTi. This behavior arises from reversible, stress-induced phase transformations. The phase transformations cause “stress plateaus” or strain ranges with no change in stress value. Scanning electron microscopy (SEM) of the fracture surfaces of specimens revealed generally short crack growth. Electron backscatter diffraction (EBSD) found the amount of residual martensite to be about ~8%, regardless of loading conditions

La superélasticité (SE) est la capacité d'un alliage à recouvrer sa forme initiale après une importante déformation. Cet effet, dû à une transformation martensitique réversible, est très convoité pour de nombreuses applications biomédicales. Les alliages nickel-titane sont les alliages les plus utilisés dans les applications fonctionnelles nécessitant de la SE. Cependant, l'utilisation de cet alliage dans les dispositifs biomédicaux est controversée par la présence du nickel, considéré comme élément cytotoxique et allergène. De ce fait, ces dernières années, une attention accrue a été portée aux alliages de titane [3-métastable pouvant constituer une alternative pour des applications biomédicales. En effet, ils ont l'avantage d'être élaborés à partir d'éléments biocompatibles et de présenter un comportement SE. L'objectif de cette thèse réside ainsi dans l'étude de la réponse SE des alliages de titane [3-métastable à différentes échelles dans leur forme massive et sous forme de revêtements. Deux alliages [3-métastable ont été élaborés: le Ti-27Nb {% at) et le Ti-24Nb-4Zr-4Sn (% mass). Les propriétés SE de ces alliages à l'état massif ont été caractérisées à l'échelle macroscopique et à l'échelle submicrométrique et comparées à celles du NiTi superélastique et du CP-Ti élasto-plastique. La nanoindentation a d'abord été utilisée pour sonder l'effet SE dans ces alliages massifs à l'échelle locale, et dans un deuxième temps, pour étudier l'effet de l'orientation cristallographique sur la réponse SE et mécanique à l'échelle du grain. Enfin, les propriétés mécaniques et SE de ces alliages sous forme de revêtements ont été évaluées avec ce même procédé de nanoindentation. / The superelasticity is the ability of an alloy to recover its original shape after significant deformation. This effect, due to a reversible stress-induced martensitic transformation, is highly sought after for many biomedical applications. Nickel-titanium alloys that have a very large strain recovery (in bulk and thin film state), are the alloys currently used for functional applications requiring superelasticity. However, the use of this alloy in biomedical devices is controversial by the presence of nickel, considered as allergen and presumed cytotoxic for the body. As a result, in recent years, increased attention has been given to metastable f3 titanium alloys, which may be an alternative for biomedical applications. Indeed, they have the advantage of being elaborated from biocompatible elements and exhibit a superelastic behavior. The objective of this thesis lies in the study of the superelastic response of metastable f3 titanium alloys at different scales in bulk and thin film state. Two metastable f3 titanium alloys were elaborated: Ti-27Nb (at%) alloy and Ti-24Nb-4Zr-8Sn (wt %) alloy. The superelastic properties of these two bulk alloys were characterized at macroscopic and sub-micrometric scale and compared to superelastic NiTi and elastoplastic CP-Ti. The nanoindentation was first used to evaluate the superelastic effect of various bulk titanium alloys at local scale, and in a second time, to study the effect of crystallographic orientations on the superelastic and mechanical responses at the grain scale. Finally, the mechanical and superelastic properties of metastable f3 titanium coatings were evaluated with this same nanoindentation process.

Shape memory alloys (SMAs) have recently attracted much attention as a smart material that can be used in passive protection systems such as energy dissipating devices and base isolation systems. For the purpose of investigating the potential use of SMAs in seismic engineering applications a soft computing approach, namely a neurofuzzy technique is used to model dynamic behavior of CuAlBe shape memory alloy wires. Experimental data are collected from two test programs that have been performed at the University of Chile. First, in order to evaluate the effect of temperature changes on the behavior of superelastic SMA wires, a large number of cyclic, sinusoidal, tensile tests are conducted at various temperatures. Second, to assess dynamic effects of the material, a series of laboratory experiments are conducted on a scale model of a three story model of a building that is stiffened with SMA wires and given excitation by a shake table. Two fuzzy inference systems (FISes) that can predict hysteretic behavior of CuAlBe wire have been created using these experimental data. Both fuzzy models employ a total of three input variables (strain, strain-rate, and temperature or prestress) and one output variable (predicted stress). Values of the initially assigned membership functions for each input are adjusted using a neural-fuzzy procedure to accurately predict the correct stress level in the wires. Results of the trained FISes are validated using test results from experimental records that had not been previously used in the training procedure. Finally, numerical simulations are conducted to illustrate practical use of these wires in a civil engineering application. In particular, dynamic analysis of a single story frame and a three story benchmark building that are equipped with SMA damping elements are conducted. Then, an isolated bridge that utilizes a linear rubber bearing together with SMA elements is analyzed. Next, in order to show recentering ability of SMAs, nonlinear time history analysis of a chevron like braced frame is implemented. The results reveal the applicability for structural vibration control of CuAlBe wire whose highly nonlinear behavior is modeled by a simple, accurate, and computational efficient FIS.

TiNi shape memory alloy is characterized by shape memory and superelastic effects which occur due to reversible martensite transformation. It has been recently found that TiNi alloy has superior dent and wear resistance compared to other conventional materials. The stress-induced martensite transformation exhibited by this alloy contributes to its dent and wear resistance. Much work is required to establish the fundamental principals governing the superelastic behavior of TiNi under wear and indentation conditions. Understanding the superelastic behavior helps to employ superelastic TiNi in applications where high impact loading is expected as in gears and bearings. In this study the superelastic behavior of shape memory alloys under reciprocating sliding wear and indentation loading conditions was investigated. The deformation behavior of superelastic Ti-Ni alloys was studied and compared to AISI 304 stainless steel. Dominant wear and deformation mechanisms were identified.

Nanoindentation has emerged as a leading technique for the investigation of mechanical properties on small volumes of material. Extensive progress has been made in the last 20 years in refining the nstrumentation of nanoindentation systems and in analysis of the resulting data. Recent development has enabled investigation of materials under several dynamic conditions. The palladium-hydrogen system has a large miscibility gap, where the palladium lattice rapidly expands to form a hydrogen-rich β phase upon hydrogenation. Nanoindentation was used to investigate the mechanical effects of these transformations on foils of palladium. Study of palladium foils, which had been cycled through hydrogenation and dehydrogenation, allowed the extent of the transformed region to be determined. Unstable palladium foils, which had been hydrogenated and were subject to dynamic hydrogen loss, displayed significant hardening in the regions which were not expected to have transformed. The reason for this remains unclear. Impact indentation, where the indenter encounters the sample at relatively high speeds, can be used to probe the strain rate dependence of materials. By combining impact indentation and elevated temperature indentation, the strain rate dependence of the superelasticity of nickel-titanium was probed over a range of temperatures. Similar trends in elastic energy ratios with temperature were observed with the largest elastic proportions occurring at the Austenite finish transformation temperature. Multiple impact and scratch indentation are two modes of indentation which are thought to approximate erosive and abrasive wear mechanisms, respectively. These were utilised to investigate the wear resistance of several novel coatings formed by plasma electrolytic oxidation (PEO) of Ti-6Al4-V. Multiple impact indentation results appear to subjectively rank the erosive wear performance of both ductile and brittle materials. Comparison of normalised performance of coating systems on aluminium in abrasive wear to scratch hardness showed similar degrees of resistance.

Les alliages de titane sont actuellement très utilisés comme implants orthopédiques de part leurs bonnes propriétés mécaniques, leur bonne résistance à la corrosion ainsi que leur excellente biocompatibilité. Cependant, l’alliage Ti-6Al-4V qui est le plus utilisé présente un module d'élasticité élevé (110GPa), ce qui peut provoquer le phénomène de « stress shielding » et finalement causer l’échec de l’implantation. De plus, l’utilisation à long terme de ce type d’alliage est remise en question à cause de la présence de certains éléments (Al et V) considérés comme cytotoxiques et/ou allergènes. Les alliages -métastables à base de titane peuvent être des candidats de remplacement intéressants grâce à l’addition d'éléments biocompatibles tel que Nb, Zr et Sn.L'alliage superélastique biocompatible de composition Ti-24Nb-4Zr-8Sn (% massique) a été étudié dans le cadre de cette thèse. Cet alliage montre des propriétés intéressantes telles qu’un bas module d’élasticité, une résistance mécanique élevée et une ductilité relativement importante.Dans ce travail de thèse, différents traitements thermomécaniques ont été réalisés afin d’obtenir des textures cristallographiques différentes. Les influences de changement de texture sur les propriétés mécaniques et la superélasticité ont été ainsi préalablement étudiées. La transformation martensitique a été caractérisée par des essais in situ de diffraction des rayons X sous rayonnement synchrotron (SXRD) pendant une sollicitation mécanique et par analyse mécanique dynamique (DMA) sous différentes contraintes statiques. De plus, les microstructures de déformation ont été observées par EBSD et MET pour caractériser précisément les mécanismes de déformation plastique, en particulier le maclage. / Titanium alloys have already been extensively used as orthopedic implants due to the good mechanical properties, corrosion resistance and excellent biocompability. However, the most widely used Ti-6Al-4V alloy exhibits high elastic modulus (110GPa) which would cause the stress shield effect and eventually lead to the implantation failure. Furthermore, elements of Al and V are proved to be toxic for long-term application. Low modulus metastable  titanium alloy can be a suitable candidate through proper addition of non-toxic alloying element such as Nb, Zr and Sn.The present investigated Ti-24Nb-4Zr-8Sn alloy is a new -type metastable alloy potentially interesting for biomedical applications. This alloy displays high strength, low elastic modulus, high ductility, superelastic property and good biocompatibility according to previous investigations.In this work, the as-cold rolled Ti-24Nb-4Zr-8Sn alloy was subjected to different thermo-mechanical treatments in order to introduce different crystallographic texture. Influences of texture change on mechanical properties and superelasticity have been preliminarily studied. Martensitic transformation which is responsible for the superelasticity has been characterized by both in situ synchrotron X-ray diffraction and dynamic mechanical analysis. Moreover, deformed microstructures have been observed by EBSD and TEM to characterize precisely the plastic deformation mechanisms, and particularly the twinning.

O objetivo da presente pesquisa foi avaliar o comportamento da força em função da deflexão na ativação e desativação de fios ortodônticos de níquel-titânio, de secção circular (.016"). Ensaiaram-se três marcas de fio (Forestadent, GAC e Morelli). As temperaturas de ensaio foram três (32, 37 e 42ºC). As ativações máximas foram até 1, 2, 3 e 4mm. O espécime de ensaio consistiu de uma placa de resina acrílica, sobre a qual eram fixados cilindros metálicos por meio de parafusos, simulando dentes, com distribuição semelhante aos dentes naturais. Sobre os cilindros foram cimentados os bráquetes (distância de 8mm entre eles). Sobre os bráquetes era fixado, conforme clinicamente, o arco do fio em ensaio. O “dente" correspondente ao incisivo central era liberado (desparafusado) para poder se deslocar livremente no sentido vestíbulo-lingual durante os ensaios e o fio poder sofrer a deflexão. O conjunto era levado à máquina de ensaio (Instron), com câmara de temperatura controlada. Durante o ensaio a velocidade de deslocamento para a deflexão foi de 0,5mm/min. Durante a ativação e desativação as forças foram registradas de 0,10 em 0,10mm de deflexão. Por meio de software esses valores eram impressos numericamente e em gráficos da força em função da ativação/desativação. Com 4 ativações máximas, 3 marcas de fios e 3 temperaturas obtiveram-se 36 condições experimentais e com 5 repetições (n=5) foram feitos 180 ensaios. Os gráficos obtidos mostraram uma não linearidade entre força e deflexão e com ativação de 1 e 2mm não foi detectado platô de superelasticidade, mas que foi observado na desativação, das ativações máximas de 3 e 4mm. Dos gráficos e tabelas foram extraídos valores específicos para serem analisados e comparados: 1) força máxima de ativação; 2/3) diminuição da força na desativação de 0,20mm e 0,70mm (esta só na ativação de 3 e 4mm); 4/5) extensão e forças no platô (apenas nas ativações máximas de 3 e 4mm); 6/7/8) desativação até alcançar 50g de força, a partir de 0,80 e 1,80mm (ativações de 1 e 2mm), ou no final dos platôs (ativações de 3 e 4mm), a força ao iniciar-se a desativação, afastamento da origem ao registrar 50g de força; 9) deformação permanente ao atingir a força zero. As principais conclusões foram: a força de ativação máxima aumentou com o aumento desta e da temperatura, o material Morelli apresentou a menor e o GAC a maior; a diminuição da força máxima foi tanto maior quanto maior a ativação; apenas as ativações até 3 e 4mm apresentaram platôs de superelasticidade e que foram bem mais extensos na ativação de 4mm, que por sua vez apresentaram menor força; em todos os ensaios for observada deformação permanente / The purpose of this study was to evaluate the behaviour of force as function of loading and unloading orthodontic nickel-titanium wires. Three defferent brands (Forestadent, GAC and Morelli) were tested. Testing temperature were 32, 37 and 42 o C. Loading were up to 1, 2, 3 and 4mm deflection. Specimens consisted of a acrylic base where were fixed metallic cylinders simulating teeth. Over the teeth were fixed brackets to maintain arch wire in position. Tooth corresponding to central incisor was loosened and at the correspondig bráquete was applied the force in an Instron equipment, with temperature control. Head speed was 0.5mm/min. During loading and unloading forces were read every 0.10mm deflection and registered. The pair values of forces and deflections were also plotted as graphs. With 4mm maximum loading, 3 brands and 3 temperatures were obtained 36 experimental conditions and with 5 repetitions (n=5) were performed 180 tests. Graphs showed that there is no linearity between forces and deflection and with loading up to 1 and 2mm was not obtained the formation of superelasticity plateaus. But they were observed with 3 and 4mm deflections. From the graphs and registered numerical values for each specimen were selected special values to be analysed: 1) maximum loading force; 2/3) force decrease during unloading 0.20 and 0.70mm (the last only at 3 and 4mm activation); 4/5) exte nsions and forces at plateaus (only for 3 and 4mm activations); 6/7/8) extension until 50g force, since 0.80 and 1.80mm (respectively activation up to 1 and 2mm), or ends of plateau (3 and 4mm activation). In these conditions, the force at beginning unloading. Distance of origin at register 50g force; 9) permanent deformation at zero force. The main conclusions were: maximum loading forces increase with deflection and temperature increases, and brand Morelli presented the smallest force and GAC the greatest; the decrease of forces with unloading was as larger as activation force; only the activation until 3 and 4mm presented superelasticity plateaus, and was longer with 4mm activation, but it presented lower force at plateaus; in all tests was noted permanent deformation

Les tubes à paroi mince en alliage à mémoire de forme Nickel-Titane (AMF NiTi) sont largement utilisés dans la fabrication de stents auto-expansibles. Leur fonctionnement repose sur la superélasticité (SE), comme de nombreuses autres applications des AMF NiTi dans le domaine biomédical. Le SE est un phénomène cristallographique réversible qui donne aux AMF la capacité de récupérer de grandes déformations par simple déchargement mécanique. En raison de la nature cristallographique du SE, les propriétés mécaniques liées à cet effet devraient être affectées par l'anisotropie inhérente du tube, qui émerge de son processus de fabrication. Cependant, le NiTi est encore souvent considéré comme isotrope dans la conception et l'optimisation de tels dispositifs. L'une des difficultés empêchant l'utilisation de modèles anisotropes est l'absence de caractérisations mécanique de l'anisotropie du tube NiTi. Le présent travail vise à effectuer une telle caractérisation pour un tube superélastique NiTi à paroi mince. Dans une campagne expérimentale, le comportement en traction du tube est analysé à différentes orientations et températures. La technique de corrélation d'image numérique (digital image correlation - DIC) est utilisée pour surveiller la distribution des déformations pendant les essais de traction. Les résultats montrent que toutes les propriétés analysées liées à la SE sont anisotropes. Toutes les dépendances d'orientation sont presque symétriques à 45° de l'axe du tube. Certaines propriétés dépendent également de la température, dépendance qui est également anisotrope. Une approche thermodynamique basée sur l'énergie libre de Gibbs est utilisée pour analyser ces dépendances d'orientation et de température. Avec cette analyse, il a été possible de relier l'hystérésis mécanique de la SE et les contributions irréversibles présentes dans le système. Enfin, l'influence de l'anisotropie sur la distribution des déformations est vérifiée. L'accent est mis sur l'analyse du phénomène de localisation de la déformation tout au long du chargement et du déchargement. L'inclinaison de la bande de localisation est caractérisée et évaluée avec une approche de plasticité. L'angle de la bande avant observé avec DIC est prédit en utilisant des données de vitesse de déformation globale. / Thin walled tubes of Nickel-Titanium shape memory alloys (NiTi SMA) are widely used in the fabrication of self-expandable stents. The operation of stents relies on the superelastic effect (SE), as many other applications of NiTi SMA in the biomedical field. The SE is a reversible crystallographic phenomenon that gives SMA the ability to recover large strains through simple unload. Due to the crystallographic nature of the SE, the mechanical properties related to this effect are expected to be affected by the inherent anisotropy of the tube, which emerges from its fabrication process. However, NiTi is still often treated as isotropic in the design and optimization of such devices. One of the difficulties preventing the use of anisotropic models is a lack of mechanical characterizations about the NiTi tube's anisotropy. The present work aims to perform such characterization for a thin walled NiTi superelastic tube. In an experimental campaign, the tensile behaviour of the tube is analysed at different orientations and temperatures. Digital Image Correlation (DIC) technique is used to monitor the strain distribution during tensile tests. Results show that all the analysed properties related with SE are anisotropic. All the orientation dependencies are nearly symmetrical to 45° from the tube's axis. Some properties are also dependent on temperature, a dependence that is also anisotropic. A thermodynamic approach based on the Gibbs free energy is used to analyse these orientation and temperature dependencies. With this analysis it was possible to relate the SE stress hysteresis and thermodynamic irreversible energy contributions. Finally, the influence of anisotropy on the strain distribution of tensile samples is verified. Focus is given to the analysis of the strain localization phenomenon throughout loading and unloading. The inclination of the localization front band is characterized and evaluated with a plasticity approach. The front angle observed with DIC is predicted using global strain rate data.

In the scope of this study, TiNi foams with porosities in the range of 39-64 vol% were processed from prealloyed powders by Mg space holder technique. Porous TiNi alloys displayed homogeneously distributed spherical pores with interconnections, which is suitable for bone ingrowth. Porous Ti-50.8 at%Ni alloys were processed by sintering at 1200 &deg / C for 2 h to analyze the microstructure as well as mechanical behavior. SEM, TEM and XRD studies were conducted for the characterization of microstructure and phase analyses in addition to the mechanical characterization performed by monotonic and superelasticity compression tests as well as compressive fatigue tests. It was observed that stress required to trigger martensitic transformation was decreased via increasing porosity. The monotonic compression test results also indicated that altering the porosity content of TiNi foams leads to different monotonic compression behaviors. It was observed that the foams display more bulk deformation like behavior as a composite structure composed of TiNi and macropores when the porosity content was low. As the porosity content has increased, the struts became more effective and deformation proceeds by the collapse of favorable struts. On the other hand, cyclic superelasticity tests results indicated that maximum achieved and recovered strain values at the end of fifth cycle increase while the fraction of strain recovered at the end of fifth cycle decreases with decreasing porosity content. Furthermore, the fatigue lives of the processed foams were observed to vary within a band which has a width decreasing with decreasing &sigma / max / &sigma / y yielding an endurance limit ranging in between 26-89 MPa or 0.5-0.6 &sigma / y. Fractography studies on the failed foams after fatigue testing revealed that the failure occurs by the coalescence of micro-cracks initiated from pore walls leading to macro-cracks aligned at 45o with respect to the loading axis. In addition to the mentioned characterization studies, the effects of sintering temperature and time on TiNi foams with 58 vol% porosity as well as heat treatment on the microstructure and the mechanical behavior of TiNi foams with 49 vol% porosity were analyzed with SEM and compression tests. Aging of TiNi foams with 49 vol% porosity at 450 &deg / C for 1.5 h has shown that the presence of Ti3Ni4 precipitates improve the superelastic response.

The damaging effects of strong ground motions on highway bridges have revealed the limitations of conventional design methods and emphasized the need for innovative design concepts. Although seismic isolation systems have been proven to be an effective method of improving the response of bridges during earthquakes, the performance of base-isolated structures during near-field earthquakes has been questioned in recent years. Near-field earthquakes are characterized by long period and large- velocity pulses. They amplify seismic response of the isolation system since the period of these pulses usually coincides with the period of the isolated structures. This study explores the feasibility and effectiveness of shape memory alloy (SMA)-based isolation systems in order to mitigate the response of bridge structures against near-field ground motions. SMAs have several unique properties that can be exploited in seismic control applications. In this work, uniaxial tensile tests are conducted first to evaluate the degree to which the behavior of SMAs is affected by variations in loading rate and temperature. Then, a neuro-fuzzy model is developed to simulate the superelastic behavior of SMAs. The model is capable of capturing rate- and temperature-dependent material response while it remains simple enough to carry out numerical simulations. Next, parametric studies are conducted to investigate the effectiveness of two SMA-based isolation systems, namely superelastic-friction base isolator (S-FBI) system and SMA/rubber-based (SRB) isolation system. The S-FBI system combines superelastic SMAs with a flat steel-Teflon bearing, whereas the SRB isolation system combines SMAs with a laminated rubber bearing rather than a sliding bearing. Upon evaluating the optimum design parameters for both SMA-based isolation systems, nonlinear time history analyzes with energy balance assessment are conducted to compare their performances. The results show that the S-FBI system has more favorable properties than the SRB isolation system. Next, the performance of the S-FBI systems is compared with that of traditional isolation systems used in practice. In addition, the effect of outside temperature on the seismic response of the S-FBI system is assessed. It is revealed that the S-FBI system can successfully reduce the response of bridges against near-field earthquakes and has excellent re-centering ability.