The work in this thesis is inspired by the fabrication of Zn₄₅Au₃₀Cu₂₅. This is the first alloy undergoing ultra-reversible martensitic transformations and closely satisfying the cofactor conditions, particular conditions of geometric compatibility between phases, which were conjectured to influence reversibility. With the aim of better understanding reversibility, in this thesis we study the martensitic microstructures arising during thermal cycling in Zn₄₅Au₃₀Cu₂₅, which are complex and different in every phase transformation cycle. Our study is developed in the context of continuum mechanics and nonlinear elasticity, and we use tools from nonlinear analysis. The first aim of this thesis is to advance our understanding of conditions of geometric compatibility between phases. To this end, first, we further investigate cofactor conditions and introduce a physically-based metric to measure how closely these are satisfied in real materials. Secondly, we introduce further conditions of compatibility and show that these are nearly satisfied by some twins in Zn₄₅Au₃₀Cu₂₅. These might influence reversibility as they improve compatibility between high and low temperature phases. Martensitic phase transitions in Zn₄₅Au₃₀Cu₂₅ are a complex phenomenon, especially because the crystalline structure of the material changes from a cubic to a monoclinic symmetry, and hence the energy of the system has twelve wells. There exist infinitely many energy-minimising microstructures, limiting our understanding of the phenomenon as well as our ability to predict it. Therefore, the second aim of this thesis is to find criteria to select physically-relevant energy minimisers. We introduce two criteria or selection mechanisms. The first involves a moving mask approximation, which allows one to describe some experimental observations on the dynamics, while the second is based on using vanishing interface energy. The moving mask approximation reflects the idea of a moving curtain covering and uncovering microstructures during the phase transition, as appears to be the case for Zn₄₅Au₃₀Cu₂₅, and many other materials during thermally induced transformations. We show that the moving mask approximation can be framed in the context of a model for the dynamics of nonlinear elastic bodies. We prove that every macroscopic deformation gradient satisfying the moving mask approximation must be of the form 1 + a(x) ⊗ n(x), for a.e. x. With regards to vanishing interface energy, we consider a one-dimensional energy functional with three wells, which simplifies the physically relevant model for martensitic transformations, but at the same time highlights some key issues. Our energy functional admits infinitely many minimising gradient Young measures, representing energy-minimising microstructures. In order to select the physically relevant ones, we show that minimisers of a regularised energy, where the second derivatives are penalised, generate a unique minimising gradient Young measure as the perturbation vanishes. The results developed in this thesis are motivated by the study of Zn₄₅Au₃₀Cu₂₅, but their relevance is not limited to this material. The results on the cofactor conditions developed here can help for the understanding of new alloys undergoing ultra-reversible transformations, and as a guideline for the fabrication of future materials. Furthermore, the selection mechanisms studied in this work can be useful in selecting physically relevant microstructures not only in Zn₄₅Au₃₀Cu₂₅, but also in other materials undergoing martensitic transformations, and other phenomena where pattern formation is observed.

O seguinte trabalho visa o estudo, desenvolvimento e teste de um controlador de impedância genérico, que seja adequado para controle de exoesqueletos e outros sistemas robóticos, e que permita a utilização de um atuador de liga de memória de forma (SMA), entre outros tipos de atuador. Um segundo objetivo é avaliar a viabilidade da utilização de atuadores de SMA em aplicações de exoesqueletos para humanos, partindo dos resultados obtidos com o controlador e atuador propostos. Para atingir estas metas, foi projetado e construído um protótipo de exoesqueleto de membro inferior com um grau de liberdade, sendo que um atuador baseado em fios de liga de memória de forma foi utilizado. O algoritmo de controle de impedância foi desenvolvido e testes foram realizados primeiro por meio de simulações e posteriormente em ensaios práticos com o protótipo. Os resultados experimentais confirmaram a expectativa indicada pelos resultados numéricos. Embora o controlador de impedância haja funcionado como se pretendia, o atuador deixou a desejar devido, sobretudo, à sua lentidão de resposta. O texto apresenta uma breve revisão teórica sobre controle de impedância, exoesqueletos biomecatrônicos e ligas de memória de forma. Ademais, detalha o modelo matemático do problema e o aparato experimental projetado para realizar os ensaios que serviram de base para a análise do problema. Após apresentação e discussão dos resultados, é feita uma análise da viabilidade de aplicação destes conceitos a exoesqueletos para uso em humanos, concluindo que ainda há desafios tecnológicos importantes a serem vencidos antes da implementação prática de exoesqueletos com atuadores de SMA; contudo, esta implementação não pode ser descartada. Detalhes construtivos e programas desenvolvidos para simulações e controle do protótipo são apresentados nos apêndices. / This work proposes the study, development and test of a generic impedance controller, which must be adequate to control exoskeletons as well as other robotic systems, and must allow the use of a shape memory alloy (SMA) actuator, among others kinds of actuation. A second objective is to evaluate if it is possible to use SMA actuators in exoskeletons for humans, taking into account the results obtained with the proposed controller and actuator. To achieve such goals, a one degree of freedom, lower limb exoskeleton prototype was designed and built, while an actuator based on SMA wires was used. The impedance control algorithm was developed and tests were made, first by means of simulations and later by tests on the prototype. Experimental data confirmed the expected results obtained by simulations. Although the impedance controller has worked as desired, the actuator did not meet the expectations, especially because of its slow response. The text brings a brief theoretical review about impedance control, biomechatronic exoskeletons and shape memory alloys. In addition, it details the mathematical model of the problem and the experimental apparatus, designed to the execution of tests which would serve as a foundation to the problem analysis. After the results are presented and discussed, the viability of use of the proposed concepts to SMA-actuated-exoskeletons is analyzed, concluding that there are still important technological challenges to be overcome before the practical implementation of exoskeletons with SMA actuators; however, this implementation cannot be discarded. Constructive details of the prototype and the programs developed to simulate and control it are presented in the appendixes.

We study the plasticity mechanisms of diffusionless martensite phase transformation in Nickel-Titanium, one of the most widely used shape memory alloys. The research here involves four thrusts focusing on different length and time scales: (I) Molecular statics and dynamics simulations are applied to study the nanotwin structures and temperature-driven B2 → B19′ phase transitions. (II) Molecular dynamics simulations are performed to explore the stress-driven martensitic phase transformation governing the pseudoelasticity and shape memory effects in NiTi nanopillars. (III) Monte Carlo simulations are conducted to characterize the temperature- driven B2 → B19 phase transition and the patterning of martensitic nanotwins in NiTi thin films. (IV) Phase field simulations are performed to predict the formation and evolution of complex martensitic microstructures, including the detailed analysis of twin compatibility under complex loading conditions. We also study the nucleation-controlled plasticity mechanisms in different metals of Cu, Al and Ni. Our work focuses on understanding how dislocations nucleate in single crystals. Interatomic potential finite element method is applied to determine when, where and how dislocations nucleate during nanoindentation in metals such as Cu, Al and Ni.

The exciting thermomechanical properties of nickel-titanium shape memory alloys have sparked significant research efforts seeking to exploit their exotic capabilities. Until recently, the performance capabilities of nickel-titanium devices have been inhibited by the retention of only one thermomechanical characteristic. However, laser processing technology promises to deliver enhanced material offerings which are capable of multiple functional responses. Presented in this thesis, is an investigation of the effects of laser processing on the thermomechanical behaviour of nickel-titanium shape memory alloys. In the context of this work, laser processing refers to removal of alloy constituents, as in the case of laser ablation, or alternatively, addition of elements through laser alloying. The effects of laser ablation on the composition, crystallography and phase transformation temperatures of a nickel-titanium strip have been studied. Application of laser energy was shown to ablate nickel constituents, induce an austenite-martensite phase change and cause an increase in phase transformation onset temperatures, which correlated well with reported findings. Laser processing of a nickel-titanium wire was shown to locally embed an additional thermomechanical response which manifested as unique shape memory and pseudoelastic properties. Localized alloying of ternary species via laser processing of nickel-titanium strip was investigated. Synthesis of a ternary shape memory intermetallic within the laser processing region was achieved through melting of copper foils. Results from thermoanalytical testing indicated that the ternary compound possessed a higher phase transformation temperature and reduced transformation hysteresis in comparison to the reference alloy. Indentation testing was used to demonstrate the augmented thermomechanical characteristics of the laser processed shape memory alloy. In order to demonstrate the enhanced functionality of laser processed nickel-titanium shape memory alloys, a self-positioning nickel-titanium microgripper was fabricated. The microgripper was designed to actuate through four different positions, corresponding to activation of three embedded shape memory characteristics. Thermoanalytical and tensile testing instrumentations were used to characterize the thermomechanical performance of the laser processed nickel-titanium microgripper. Results indicated that each of the laser processed microgripper components possessed unique mechanical and shape memory recovery properties.

Ο θερμομηχανικός χαρακτηρισμός του κράματος NiTi και η ενσωμάτωση του σε πολυμερική μήτρα με στόχο τη γέννηση εσωτερικών μηχανικών τάσεων όταν υπάρξει θερμική διέγερση, είναι επιγραμματικά ο σκοπός της παρούσας εργασίας. Το ‘ευφυές’ σύστημα που μελετάται στην εργασία αυτή αποτελείται από εποξειδική ρητίνη, ενισχυμένη με ίνες Kevlar 29 και ενσωματωμένα σύρματα Νικελίου-Τιτανίου. Στο πρώτο πειραματικό μέρος, περιλαμβάνεται η μελέτη και ο θερμομηχανικός χαρακτηρισμός του υλικού. Χρησιμοποιήθηκαν σύρματα NiTi διαμέτρου 0.3mm, αλλά και ράβδοι για τη διεξαγωγή κάποιων συγκεκριμένων πειραμάτων. Οι πειραματικές τεχνικές περιλαμβάνουν μηχανικά πειράματα εφελκυσμού σε σερβοϋδραυλικό πλαίσιο δοκιμών, ηλεκτρονιακή μικροσκοπία σάρωσης (SEM), οπτική μικροσκοπία, χρήση θερμοκάμερας υπερύθρου ακτινοβολίας, διαφορική θερμιδομετρία σάρωσης (DSC), δυναμική μηχανική ανάλυση (DMA), μέτρηση ηλεκτρικών ιδιοτήτων καθώς και ένα πρωτοποριακό σύστημα χαρακτηρισμού υλικών (σύστημα THERMIS), που αναπτύχθηκε στο εργαστήριο. Το δεύτερο τμήμα, περιλαμβάνει τη μελέτη του υβριδικού σύνθετου υλικού. Η παρασκευή του πραγματοποιείται σε αυτόκλειστο φούρνο (autoclave) και για την ενεργοποίηση του και την καταγραφή των παραμέτρων κατά τη λειτουργία του, χρησιμοποιείται το σύστημα THERMIS. Για να επιτευχθεί η σύγκριση μεταξύ της συμπεριφοράς του κράματος όταν αυτό ενεργοποιείται με και χωρίς την πολυμερική μήτρα να το περιβάλει, έχουν επιλεγεί δύο τύποι πειραμάτων: συνεχής ενεργοποίηση για μεγάλο χρονικό διάστημα (χαλάρωση τάσης ενεργοποίησης - activation stress relaxation) και κυκλική ενεργοποίηση-απενεργοποίηση για μεγάλο αριθμό επαναλήψεων (λειτουργική κόπωση – transformation fatigue). Τα πειράματα της λειτουργικής κόπωσης στα σύρματα, έδειξαν ότι η αρχική αναπτυσσόμενη τάση, μειώνεται εκθετικά, συναρτήσει των κύκλων ενεργοποίησης. Σύμφωνα με τα αποτελέσματα, η κοπωτική συμπεριφορά του σύρματος, δεν εξαρτάται από τη διάρκεια του χρόνου θέρμανσης ανά κύκλο, αλλά από τον αριθμό των κρυσταλλογραφικών μετασχηματισμών μεταξύ οστενιτικής - μαρτενσιτικής φάσης και αντίστροφα. Ο ρυθμός υποβάθμισης της ικανότητας του κράματος να ασκεί μηχανική τάση, είναι πολύ έντονος κατά τη διάρκεια των πρώτων εκατοντάδων κύκλων και μειώνεται όσο το φαινόμενο εξελίσσεται. Στα σύνθετα υλικά, όπως και στην περίπτωση των συρμάτων SMA, η υποβάθμιση της λειτουργικής ικανότητας των υβριδικών συνθέτων, φαίνεται ότι δεν εξαρτάται από το χρόνο της θέρμανσης ανά κύκλο, αλλά μόνο από το πλήθος των κρυσταλλογραφικών εναλλαγών. Στα πειράματα χαλάρωσης τάσης, η αρχική τάση των συρμάτων ήταν γύρω στα 500MPa και σύμφωνα με τα πειραματικά αποτελέσματα, με την πάροδο του χρόνου και υπό την επίδραση της θερμοκρασίας, λαμβάνει χώρα εκθετική μείωση του μεγέθους της. Το επίπεδο της θερμοκρασίας λειτουργίας, επηρεάζει δραματικά την υποβάθμιση της μηχανικής αναπτυσσόμενης τάσης, της οποίας ο ρυθμός είναι ιδιαίτερα αυξημένος κατά τις πρώτες ώρες λειτουργιάς του υλικού. Ποιοτικά, το ίδιο φαινόμενο συμβαίνει και στην περίπτωση της χαλάρωσης τάσης ενεργοποίησης των υβριδικών συνθέτων, με τη διαφορά ότι η υποβάθμιση είναι σαφώς πιο έντονη. Προτείνεται τέλος, η μελέτη της υποβάθμισης της ικανότητας των ενεργοποιητών, με βάση στατιστικά εργαλεία και μεθόδους, κατά αναλογία με άλλα κοπωτικά φαινόμενα στη φύση (κυρίως στη μηχανική), εφόσον ουσιαστικά πρόκειται για ακολουθία δράσεων που τελικά οδηγούν στην απώλεια της ικανότητας των υλικών μας να ασκούν τάση. Συνηγορεί εξάλλου σε αυτό και η μορφή των πειραματικών καμπυλών, που παρουσιάζουν μεγάλη ομοιότητα με τις αντίστοιχες καμπύλες S/N, στη μηχανική κόπωση των υλικών. / The present work, aims to the thermo-mechanical characterization of the NiTi Shape Memory Alloy and the characterization of ‘smart’ hybrid composites with embedded SMAs, under thermal activation. The composite structure that is being investigated consists of an epoxy resin matrix, Kevlar 29 fibers and NiTi SMA wires. The first experimental section deals with the thermo mechanical characterization of the Shape Memory Alloy. 0.3 mm in diameter wires were used. The experimental techniques, include mechanical tests using a servo-hydraulic testing apparatus, scanning electron microscopy (SEM), optical microscopy, thermal IR camera imaging, differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), electrical measurements and a novel thermo mechanical characterization system (Thermis), which was tailor made according to the needs of this project. The second experimental section deals with the characterization of the composite material, which was developed using a special purpose furnace (autoclave) and was tested on the Thermis system. In order to compare the functionality of the SMA either in free condition or embedded in a polymer matrix, two experiments were chosen: continuous activation for a long time (activation stress relaxation) and recurrent thermal activation (transformation fatigue). The transformation fatigue experiments showed that the initially developed mechanical stress, reduces exponentially, according to the number of the crystalline transformations. The functional fatigue life of the alloy, does not depend on the heating time per cycle, but is strongly correlated to the number of the recurrent transformation between martensite and austenite. The stress reduction rate is increased during the first cycles and reduces as the phenomenon develops. As it concerns the composite materials, like the SMA wire case, the reduction of the stress generation capability, is not affected by the heating time per cycle, and is only related to the number of the crystalline transformations. During the stress relaxation experiments, the initial developed stress of the wires was about 500 MPa and according to the experimental results, as time passes and under the influence of the thermal field, the stress reduces exponentially. The temperature level strongly affects the reduction phenomenon and the reduction rate is very high during the first hours of the experiment. The same behavior is observed not only at the SMA wires but also at the composites, as well, noting that reduction in the later case is much more intense. At the end, the study of the fatigue and relaxation phenomena, using a statistical approach, is suggested, like many other fatigue cases in nature (especially in mechanics). The fatigue curves presented here resemble to the S/N curves that can derived from the case of mechanical fatigue of other structural materials, like steel or CFR composites.

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M. Tech. Metallurgical Engineering. / StudIes the tribocorrosion behaviour of copper-nickel-titanium shape memory composite reinforced by zirconia,synthesized through powder metallurgy process. The research aims to achieve the following objectives: 1. Study the tribocorrosion mechanisms of the composites in NaCl solution (typical human body fluid). 2. Investigate the tribocorrosion mechanisms of the composites in other environments typical of some engineering applications.The proposed study on incorporating zirconia into the matrix NiTiCu through powder metallurgical process and investigations of the phenomenon of joint wear-corrosion synergism occurring in sodium chloride considered typical of human body system and sulphuric acid environment typical of wide range engineering applications is therefore very novel. It is therefore aimed that information on the tribocorrosion behaviour of NiTiCu as well as with zirconia incorporation will form basis for typical compositional formulation approaches for improved bio-tribocorrosion improvement in biomedical applications and actuators used in other engineering applications.

Previous experimental observations have shown that the pseudoelastic response of NiTi shape memory alloys (SMA) is localized in nature and proceeds through nucleation and propagation of localized deformation bands. It has also been observed that the mechanical response of SMAs is strongly affected by loading rate and cyclic degradation. These behaviors significantly limit the accurate modeling of SMA elements used in various devices and applications. The aim of this work is to provide engineers with a constitutive model that can accurately describe the dynamic, unstable pseudoelastic response of SMAs, including their cyclic response, and facilitate the reliable design of SMA elements. A 1-D phenomenological model is developed to simulate the localized phase transformations in NiTi wires during both loading and unloading. In this model, it is assumed that the untransformed particles located close to the transformed regions are less stable than those further away from the transformed regions. By consideration of the thermomechanical coupling among the stress, temperature, and latent heat of transformation, the analysis can account for strain-rate effects. Inspired by the deformation theory of plasticity, the 1-D model is extended to a 3-D macromechanical model of localized unstable pseudoelasticity. An important feature of this model is the reorientation of the transformation strain tensor with changes in stress tensor. Unlike previous modeling efforts, the present model can also capture the propagation of localized deformation during unloading. The constitutive model is implemented within a 2-D finite element framework to allow numerical investigation of the effect of strain rate and boundary conditions on the overall mechanical response and evolution of localized transformation bands in NiTi strips. The model successfully captures the features of the transformation front morphology, and pseudoelastic response of NiTi strip samples observed in previous experiments. The 1-D and 3-D constitutive models are further extended to include the plastic deformation and degradation of material properties as a result of cyclic loading.

Concrete-filled buckling restrained braces (BRBs) was first developed in 1988 in Tokyo, Japan, to prevent the steel plates in the core portion from buckling, leading the steel core to exhibiting a more stable and fully hysteretic loop than conventional steel braces. However, past studies have shown that buckling restrained braced frames (BRBFs) have a large residual deformation after a median or high seismic event due to steel’s residual strain. In order to address this issue, innovative self-centering SMA braces are proposed and installed in the originally unbraced bays in existing BRBFs to become a hybrid frame system where the existing steel BRBs dissipate energy induced by external forces and the newly added self-centering SMA braces restore the building configuration after the steel BRBs yield. A case study of conventional three-story BRBF retrofitted by the proposed self-centering SMA braces is carried out to develop systematic retrofit strategies, to investigate the structural behavior, and to probabilistically assess their seismic performance in terms of interstory drifts, residual drifts, and brace deformation, as compared to the original steel BRB frames. Finally, the developed brace component fragility curves and system fragility curves will be further used for the assessment of downtime and repair cost.

Over the last few decades, Shape Memory Alloys (SMAs) have been increasingly explored in order to take advantage of their unique properties (i.e., pseudoelasticity and shape memory effect), in various actuation, sensing and absorption applications. In order to achieve an effective design of SMA-based devices a thorough investigation of their behavior in the presence of cracks is needed. In particular, it is important to understand the effect of phase transformation on their fracture response. The aim of the present work is to study the effect of stress-induced as well as thermo-mechanically-induced phase transformation on several characteristics of the fracture response of SMAs. The SMA thermomechanical response is modeled through an existing constitutive phenomenological model, developed within the framework of continuum thermodynamics, which has been implemented in a finite element frame-work. The effect of stress-induced phase transformation on the mechanical fields in the vicinity of a stationary crack and on the toughness enhancement associated with crack advance in an SMA subjected to in-plane mode I loading conditions is examined. The small scale transformation assumption is employed in the analysis according to which the size of the region occupied by the transformed material forming close to the crack tip is small compared to any characteristic length of the problem (i.e. the size of the transformation zone is thirty times smaller than the size of the cracked ligament). Given this assumption, displacement boundary conditions, corresponding to the Irwin’s solution for linear elastic fracture mechanics, are applied on a circular region in the austenitic phase that encloses the stress-induced phase transformation zone. The quasi-static stable crack growth is studied by assuming that the crackpropagates at a certain critical level of the crack-tip energy release rate. The Virtual Crack Closure Technique (VCCT) is employed to calculate the energy release rate. Fracture toughness enhancement associated with transformation dissipation is observed and its sensitivity on the variation of key characteristic non-dimensional parameters related to the constitutive response is investigated. Moreover, the effect of the dissipation due plastic deformation on the fracture resistance is analyzed by using a Cohesive Zone Model (CZM). The effect of thermo-mechanically-induced transformation on the driving force for crack growth is analyzed in an infinite center-cracked SMA plate subjected to thermal actuation under isobaric mode I loading. The crack-tip energy release rate is identified as the driving force for crack growth and is measured over the entire thermal cycle by means of the VCCT. A substantial increase of the crack-tip energy release rate – an order of magnitude for some material systems – is observed during actuation as a result of phase transformation, i.e., martensitic transformation occurring during actuation causes anti-shielding that might cause the energy release rate to reach the critical value for crack growth. A strong dependence of the crack-tip energy release rate on the variation of the thermomechanical parameters characterizing the material response is examined. Therefore, it is implied that the actual shape of the strain- temperature curve is important for the quantitative determination of the change of the crack-tip energy release rate during actuation.