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.

Les travaux présentés ont consisté à développer des stratégies d'identification performantes des paramètres des lois de comportement superélastique des Alliages à Mémoire de Forme (AMF). L'objectif est de disposer d'une solution complète de caractérisation, d'identification, et de simulation de structures en AMF soumises à des sollicitations complexes. Une base de données expérimentale unifiée pour un alliage de NiTi superélastique a été établie pour une multitude de trajets de déformation multiaxiaux et à différentes températures : en traction homogène, en compression, en traction-compression et en traction-traction. Une caractérisation expérimentale a été développée sur une plate-forme multiaxiale assemblée au laboratoire durant ce travail. L'emploi de la corrélation d'images a permis d'enrichir la base de données expérimentale en déterminant pour chaque essai les champs cinématiques. Cette collection d'essais a permis de montrer l'importante différence de comportement observée entre les directions de laminage et transverse, bien que le matériau soit faiblement texturé. Des procédures d'identification du comportement thermomécanique des AMF ont été mises en place, basées sur la construction et minimisation d'une fonction objectif régularisée. La première est basée sur l'exploitation des courbes contrainte-déformation moyennes sous chargement homogène et unixial. La seconde exploite la richesse des champs de déformations mesurés en essai hétérogène. Les deux stratégies ont permis d'identifier les huit paramètres gouvernant le comportement superélastique du modèle de Chemisky et al. (Chemisky et al. 2011). Des différences entre les jeux de paramètres identifiés sont caractéristiques des effets d'anisotropie observés. Le succès de cette stratégie démontre sa pertinence et est encourageant pour l'identification de paramètres de lois de comportement anisotropes.

The unique performance offerings of NiTi based shape memory alloys (SMAs), which includes the shape memory effect (SME), pseudoelasticity (PE) and biocompatibility have led to widespread acceptance of these alloys as valuable engineering materials. Over the past several decades the complex metallurgy behind the SME and PE properties has for the most part been uncovered and the design and engineering knowhow has been demonstrated; facilitating successful application of NiTi devices in numerous industries. Specifically, more mature applications in the medical industry including medical devices such as, catheters, guide wires, orthodontic arch wires, maxillofacial reconstruction implants, minimally invasive surgical tools, and arterial and gastrointestinal stents, have become common practice in modern medicine. Recently however, there has been a drive for more demanding functionality of SMAs for example to locally modify properties creating tuneable or gradient SME and PE performance. Unique processing protocols are therefore necessary to meet these demands and allow SMAs to reach their full potential in a wider range of applications. The current thesis successfully details the application of pulsed Nd:YAG laser processing along with post-processing techniques to locally tune both the SME and PE functional properties of monolithic binary NiTi wires and strip, while maintaining confidence in the retained corrosion performance and limited release of biologically harmful Ni ions. This extensive study contains three distinct parts which include: i) application of a laser induced vaporization protocol to locally embed multiple memories in a monolithic wire actuator; ii) uncovering the process, structure, and performance relationship of combined laser, cold working, and heat treatment processes; and iii) comprehensive characterization of surface characteristics and their relationship with corrosion performance and Ni ion release from laser processed material.

Submitted by Automa??o e Estat?stica (sst@bczm.ufrn.br) on 2017-12-01T23:25:13Z No. of bitstreams: 1 ThiagoSouzaViana_DISSERT.pdf: 4828616 bytes, checksum: ee06ed8ff3676ebb8c30b440a888b38d (MD5) / Approved for entry into archive by Arlan Eloi Leite Silva (eloihistoriador@yahoo.com.br) on 2017-12-05T20:32:59Z (GMT) No. of bitstreams: 1 ThiagoSouzaViana_DISSERT.pdf: 4828616 bytes, checksum: ee06ed8ff3676ebb8c30b440a888b38d (MD5) / Made available in DSpace on 2017-12-05T20:32:59Z (GMT). No. of bitstreams: 1 ThiagoSouzaViana_DISSERT.pdf: 4828616 bytes, checksum: ee06ed8ff3676ebb8c30b440a888b38d (MD5) Previous issue date: 2017-06-30 / No presente trabalho, estudou-se a influ?ncia de tratamentos t?rmicos nas transforma??es microestruturais de uma liga NiTi com mem?ria de forma de composi??o qu?mica aproximadamente equiat?mica. Um fio de NiTi com 2,5 mm de di?metro, recebido no estado encruado, foi submetido a tratamentos t?rmicos de recozimento em temperaturas de 450 ?C e 500 ?C para observa??o das varia??es microestruturais. As microestruturas predominantes antes e ap?s o recozimento foram observadas, registrando-se um aumento na granulometria da martensita tanto com a temperatura como com a dura??o do tratamento t?rmico entre 30? e 3hs. Foram determinadas as temperaturas de transforma??o, (As, Af, Ms, Mf) de cada amostra, e as altera??es nas temperaturas de transforma??o entre o material no estado encruado e ap?s recozimento. Medidas de microdureza n?o revelaram influ?ncia significativa de tratamentos t?rmicos de recozimento e tampouco de resfriamentos a temperaturas subzero. / In the present work, the influence of heat treatments on the microstructural transformations of an approximately equiatomic NiTi memory form alloy was investigated. A NiTi wire with 2.5 mm in diameter, received in the work hardened state, was submitted to annealing heat treatments at 450 ?C and 500 ?C in order to verify the calorimetric and microstructural changes. The microstructure in the work hardened condition and after the annealing heat treatments was analyzed, which showed a coarsening of the martensite plates for higher temperatures and longer heat treatment times. The determination of the transition temperatures (As, Af, Ms, Mf) and the change in the transformation temperatures between the as received worked hardened and annealed material, were determined. Microhardness measurements did not reveal any significant influence of the annealing or subzero heat treatments on the mechanical behavior of the alloy.

Magister Scientiae Dentium - MSc(Dent) / NiTi rotary instruments have shape memory and are highly flexible and super-elastic. These properties of the metal alloy allows for ease of root canal preparation to ultimately result in a root canal preparation that has a continuous taper, while canal shape and curvature is maintained. It must be noted that the NiTi rotary instrumentation may have an effect on root canal dentine, which may manifest as dentine damage. Different NiTi rotary systems on the market vary with regards to their design features and kinematics, which may influence dentine damage. The purpose of this in-vitro study was to compare the effect of four different NiTi rotary systems, as well as stainless steel files on root canal dentine. One hundred and eighty permanent human mandibular molar mesial roots were used for the study. The total samples were randomly divided into six groups, where one group (n=30) was left unprepared to serve as the control group. The remaining five groups were randomly assigned to a nickel-titanium rotary instrumentation system and one stainless steel hand file group. Group 1: Control group; Group 2: Stainless steel files group; Group 3: Wave One (Dentsply Maillefer) rotary group; Group 4: ProTaper NEXT (Dentsply Maillefer) rotary group; Group 5: iRaCe (FKG Dentaire) rotary group; Group 6: BT-Race (FKG Dentaire) rotary group.The root canal preparations were carried out according to the manufacturers' recommendations, after decoronation of the tooth crowns. Sodium hypochlorite (5,25%) and 17% EDTA was used as a root canal irrigant and a chelating agent during canal preparation. Each root segment was sectioned at 3mm, 6mm and 9mm from the apex. The root segments were observed under a stereomicroscope at 12x magnification and digital camera at 40 x magnification for the appearance of dentine damage. The images were observed by the author and an impartial second observer. Root segments were observed for the appearance of dentine damage (microcracks, craze lines or fractures), and samples were described as having 'dentine defect' or 'no dentine defect'. Data for the different groups were collected and results were calculated and the total incidence of dentine damage was as follows: Control group = 0% Stainless steel hand files group = 0% Wave One group = 56, 67% ProTaper NEXT group = 60% iRaCe group = 60% BT-Race group = 56, 67% There are no significant differences between the groups: WaveOne, ProTaper NEXT, iRACE and BT-Race. However, there is a significant difference between the four groups and the stainless steel group (p<0.0001).

No description available.

The development of metals that can overcome the strength-ductility-weight trade-off has been an ongoing challenge in engineering for many decades. A promising option for making such materials are Metal matrix composites (MMCs). MMCs contain dispersions of reinforcement in the form of fibers, particles, or platelets that significantly improve their thermal, electrical, or mechanical performance. This dissertation focuses on reinforcement with two-dimensional (2D) materials due to their unprecedented mechanical properties. For instance, compared to steel, the most well-studied 2D material, graphene, is nearly forty times stronger (130 GPa) and five times stiffer (1 TPa). Examples of reinforcement by graphene have achieved increases in strength of 60% due to load transfer at the metal/graphene interface and dislocation blocking by the graphene. However, the superior mechanical properties of graphene are not fully transferred to the matrix in conventional MMCs, a phenomenon known as the "valley of death." In an effort to develop key insight into how the relationships between composite design, processing, structure, properties, and mechanics can be used to more effectively transfer the intrinsic mechanical properties of reinforcements to bulk composite materials, nanolayered composite systems made of Ni, Cu, and NiTi reinforced with graphene or 2D hexagonal boron nitride h-BN is studied using experimental techniques and molecular dynamics (MD) simulations. / Doctor of Philosophy / The design of new metals with concurrently improved strength and ductility has been an enduring goal in engineering for many decades. The utilization of components made with these new materials would reduce the weight of structures without sacrificing their performance. Such materials have the potential to revolutionize many industries, from electronics to aerospace. Traditional methods of improving the properties of metals by thermomechanical processing have approached a point where only minor performance improvements can be achieved. The development of Metal matrix composites (MMCs) is among the best approaches to achieving the strength-ductility goal. Metal matrix composites are a class of materials containing reinforcements of dissimilar materials that significantly improve their thermal conductivity, electrical conductivity, or mechanical performance. Reinforcements are typically in the form of dispersed fibers, particles, or platelets. The ideal reinforcement materials have superior mechanical properties compared to the metal matrix, a high surface area, and a strong interfacial bond with the matrix. Two-dimensional (2D) materials (materials made up of a single to a few layers of ordered atoms) are attractive for reinforcement in composite materials because they possess unprecedented intrinsic properties. The most well-studied 2D material, graphene, is made of a single layer of carbon atoms arranged in a hexagonal honeycomb pattern. It is nearly forty times stronger (130 GPa) and five times stiffer (1 TPa) than steel. Examples of graphene reinforcing have shown increases in strength of 60% due to load transfer at the metal/graphene interface and dislocation blocking by the graphene. Despite their exceptional mechanical properties, the superior mechanical properties of graphene are not fully transferred to the matrix when incorporated into conventional metal matrix composites. This phenomenon, known as the "valley of death," refers to the loss of mechanical performance at different length scales. One cause of this phenomenon is the difficulty of evenly dispersing the reinforcements in the matrix using traditional fabrication techniques. Another is the presence of dislocations in the metal matrix, which cause very large local lattice strains in the graphene. This atomistic-scale deformation at the interface between the metal and the graphene can significantly weaken it, leading to failure at low strains before reaching its intrinsic failure stress and strain. This dissertation aims to provide insight into how the relationships between composites' design, processing, structure, properties, and mechanics can be used to transfer intrinsic mechanical properties of reinforcements to bulk composite materials more effectively. For this, nanolayered composite systems of Ni and Cu reinforced with graphene or 2D h-BN were studied using experimental techniques and molecular dynamics (MD) simulations to elucidate the underlying mechanisms behind the composites' material structure and mechanical behavior. Additionally, we explore the incorporation of graphene in a metallic matrix that does not deform through dislocations (or shear bands), such as the shape memory alloy nickel-titanium ( Nitinol or NiTi), to avoid low strain failure of the metal/graphene interface. This theoretical strengthening mechanism is investigated by designing and fabricating NiTi/graphene composites.

The demographic change in and the higher incidence of degenerative bone disease have resulted in an increase in the number of patients with osteoporotic bone tissue causing. amongst other issues, implant loosening. Revision surgery to treat and correct the loosenings should be avoided, because of the additional patient stress and high treatment costs. Shape memory alloys (SMA) can help to increase the anchorage stability of implants due to their superelastic behavior. The present study investigates the potential of hybridizing NiTi SMA sheets with additively manufactured Ti6Al4V anchoring structures using laser powder bed fusion (LPBF) technology to functionalize a pedicle screw. Different scanning strategies are evaluated, aiming for minimized warpage of the NiTi SMA sheet. For biomechanical tests, functional samples were manufactured. A good connection between the additively manufactured Ti6Al4V anchoring structures and NiTi SMA substrate could be observed though crack formation occurring at the transition area between the two materials. These cracks do not propagate during biomechanical testing, nor do they lead to flaking structures. In summary, the hybrid manufacturing of a NiTi SMA substrate with additively manufactured Ti6Al4V structures is suitable for medical implants.

This study focuses on the laser powder bed fusion processing of NiTi alloys and the feasibility of fabricating very thin stent structures for biomedical applications. A comprehensive correlation between the process and the material’s-structure and properties is established to facilitate the fabrication of NiTi alloys with tailored properties. In the first step, the impact of LPBF processing parameters and post-treatments on evolving the microstructure, texture, superelasticity, and asymmetry is examined. Subsequently, the feasibility of manufacturing very thin mesh structured stents is scrutinized followed by in-depth investigations into differently designed stents considering properties such as surface characteristics, mechanical properties, superelasticity, and recoverability. The obtained results and the represented discussions offer imperative insights, helping to better understand the complexity of the LPBF process and the present challenging aspects. Moreover, detailed contributions are made with the goal of paving the road ahead for the production of patient-specific NiTi stents with enhanced properties. / Thesis / Doctor of Philosophy (PhD)

This thesis explores the design, implementation and performance of a new concept for a low-cost, high-resolution, lightweight, compact and highly-portable tactile display. This tactile device is intended to be used in a novel visuo-tactile sensory substitution/supplemen-tation electronic travel aid (ETA) for the blind/visually impaired.Based on the psychophysiology of touch and using Shape Memory Alloys (SMAs) as the actuation technology, a mechatronic device was designed and prototyped to stimulate the sense of touch by creating sensations of contact on the fingertips.The prototype consists of an array of 64 elements spaced 2.6 mm apart that vertically actuates SMA based miniature actuators of 1.5 mm diameter to a height range of 1.4 mm with a pull force of 300 mN up to a 1.5 Hz bandwidth. The full display weights 200 g and its compact dimensions (a cube of 8 cm side-length) make it easy for the user to carry. The display is capable of presenting a wide range of tactile binary information on its 8 x 8 matrix. Moreover, both mechanical and electronic drive designs are easily scalable to larger devices while still being price attractive.Human psychophysics experiments demonstrate the effectiveness of the tactile information transmitted by the display to sighted people and show feasibility in principle of the system as an assistive technology for the blind/visually impaired.