Le comportement des alliages à mémoire de forme (AMF) et des alliages à mémoire de forme magnétiques (AMFM) est régi par les mécanismes de transformation martensitique à l'échelle de la microstructure, à l'origine de leurs propriétés remarquables (mémoire de forme, superélasticité, grandes déformations associées à la réorientation martensitique sous champ magnétique). Les mécanismes de transformation et de réorientation martensitique peuvent être induits par des sollicitations thermiques, magnétiques et / ou mécaniques et de manière couplée. La mise au point d'outils de conception fiables nécessite une meilleure prédictibilité du comportement réel des alliages à mémoire de forme sous sollicitations thermo - magnéto - mécaniques complexes.Le choix d'une modélisation multiaxiale et multi échelle est pertinent. Le modèle reporté présente une formulation unifiée, permettant de simuler aussi bien le comportement des AMF que celui des AMFM.Parallèlement au développement de ce modèle, une étude expérimentale est nécessaire afin d'une part d'identifier les propriétés intrinsèques des matériaux étudiés, et d'autre part de valider les estimations de la modélisation. A cette fin, des mesures de fractions volumiques de phase par diffraction des rayons X in situ ont été entreprises lors de sollicitations thermiques (cycles de chauffage-refroidissement), mécaniques (traction, compression, essais biaxiaux) et magnétiques (champ magnétique unidirectionnel). L'exploitation des résultats de diffractométrie permet une analyse quantitative des fractions volumiques des phases en présence. Celles-ci sont comparées aux estimations du modèle à des fins de validation. / The behavior of shape memory alloys (SMA) and magnetic shape memory alloys (MSMA) is governed by the martensitic transformation mechanisms at the scale of the microstructure. This transformation is at the origin of their remarkable properties (memory effect, superelasticity, large deformations associated with the martensitic reorientation under magnetic field). The martensitic transformation and reorientation mechanisms can be induced by thermal, magnetic and / or mechanical stresses and in a coupled manner. The development of reliable design tools requires a better predictability of the actual behavior of shape memory alloys under complex thermal-magneto-mechanical loading.The choice of multiaxial and multiscale modeling is relevant. The model proposed in this work presents a unified formulation, making possible to simulate both the behavior of SMA and MSMA.In parallel with the development of this model, an experimental study is necessary in order to identify the intrinsic properties of the materials studied and to validate the estimates of the modeling. For this purpose, measurements of phase fractions by in-situ X-ray diffraction were carried out during thermal (heating-cooling cycles), mechanical (tensile, compressive, biaxial) and magnetic (unidirectional magnetic field) loadings. The diffraction patterns allow a quantitative estimation of the volume fractions of the phases. These are compared to model estimates for validation purposes.

Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: The successful design, implementation and testing of a feedback control system for tab-deflection control of a shape memory alloy (SMA) based control surface actuator is presented. The research is performed as part of the Control Surfaces in Confined Spaces (CoSICS) research project conducted at Stellenbosch University. The research group investigates ways to provide control surface actuation in size-restricted spaces in commercial aircraft such as the Airbus A320 and A330. This is achieved by investigating the concept of trailing edge tabs to reduce the required torque load, resulting in reduced actuator requirements enabling the use of smaller actuators. This thesis contributes to the project by investigating the possibility of using SMA-based actuators in reduced hinge moment requirement applications. An SMA-based tab actuator demonstrator design is presented. Mathematical models are derived for the SMA material, thermodynamics and actuator geometry. The models are combined to formulate an SMA-based control surface actuator model. The model is utilised in four tracking feedback controller designs; two based on linear and two based on non-linear control techniques. The manufactured prototype is presented along with the incorporated hardware for controller implementation. System identification follows and validates the three mathematical models. Practical verification of the model and two of the controllers is conducted. The unimplemented controllers are implemented through a validated model simulation. Controller evaluation, based on the dynamic controller performance, is conducted. The results validate the concept of using an SMA actuator for tab-deflection control and indicate important limitations for the intended application. / AFRIKAANSE OPSOMMING: Die tesis behels die ontwerp, implementering en toetsing van ’n terugvoer beheerstelsel vir hulpvlak defleksie beheer van ’n vorm-geheue allooi (SMA) gebaseerde aktueerder. Die navorsing vorm deel van die Beheervlakke in Begrensde Ruimtes (CoSICS) navorsingsprojek by Stellenbosch Universiteit. Die CoSICS navorsing behels ’n ondersoek na beheervlak aktueering in beknopte spasies in kommersiële vliegtuie soos die Airbus A320 en A330. Die probleem word aangespreek deur ’n ondersoek na aerodinamiese hulpvlakke wat ’n vermindering in skarnier moment tot gevolg het en sodoende die aktueerder vereistes verminder. Hierdie tesis dra by tot die projek deur die moontlikheid van die gebruik van SMAgebaseerde aktueerders in verminderde skarnier moment vereiste toepassings te ondersoek. ‘n SMA gebaseerde hulpvlak demonstrasie aktueerder ontwerp word voorgelê. Wiskundige modelle vir die SMA materiaal, termodinamika en prototipe geometrie is geformuleer en gekombineer om ‘n SMA gebaseerde beheervlak aktueerder model te ontwikkel. Die model word in vier beheerder ontwerpe toegepas. Twee ontwerpe is op liniëre en twee op nie-liniëre beheer tegnieke gebaseer. Die prototipe en nodige hardeware vir beheerder implementasie is voorgedra. Stelsel identifikasie is toegepas en verifieer die drie wiskundige modelle. Praktiese verifikasie van die model en twee beheerders is gedoen. Die ongeïmplementeerde beheerders is deur die geverifieerde aktueerder model gesimuleer. ‘n Beheerder evaluasie gebaseer op die dinamiese beheerder gedrag word toegepas. Die evaluasie beklemtoon kritiese aspekte en beperkinge in verband met SMA aktueering. Die resultate regverdig die gebruik van ‘n SMA aktueerder vir hulpvlak defleksie beheer en beklemtoon belangrike beperkinge ten opsigte van die voorgestelde toepassing.

We report the fabrication of Ni2.7Mn0.9Ga0.4/InGaAs bilayers on GaAs (001)/InGaAs substrates by molecular beam epitaxy. To form freestanding microtubes the bilayers have been released from the substrate by strain engineering. Microtubes with up to three windings have been successfully realized by tailoring the size and strain of the bilayer. The structure and magnetic properties of both, the initial films and the rolled-up microtubes, are investigated by electron microscopy, X-ray techniques and magnetization measurements. A tetragonal lattice with c/a = 2.03 (film) and c/a = 2.01 (tube) is identified for the Ni2.7Mn0.9Ga0.4 alloy. Furthermore, a significant influence of the cylindrical geometry and strain relaxation induced by roll-up on the magnetic properties of the tube is found. / Dieser Beitrag ist aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.

The modeling, analysis and experimental verification of a two-degree-of-freedom typical aeroelastic section with superelastic shape memory alloy springs are presented. The focus is to investigate the effects of the phase transformation of the shape memory alloy springs on the flutter and post-flutter behaviors of the typical section. The shape memory alloy phase transformation kinetics is described by a modified version of well-known phenomenological models. The shape memory alloy spring model is based on classical spring design (with the pure shear assumption) and modified to account for the nonlinear effects of phase transformation. The cross-section of the shape memory alloy wire is represented by a linear radial distribution of shear strain and nonlinear radial distributions of shear stress and martensitic fraction. The equations of motion of a linear typical section are modified to include the shape memory alloy springs. A linear unsteady aerodynamic model is employed to determine the aerodynamic loads. The proposed model is cast into state-space representation and solved with a Runge-Kutta method. It is numerically and experimentally shown that the phase transformation of shape memory alloy springs can be effectively exploited to enhance the aeroelastic behavior of a typical section by replacing unstable flutter oscillations by stable oscillations of acceptable amplitudes over a range of airflow speeds, providing a useful method of passive aeroelastic control. Since the modified aeroelastic behavior is attractive for wind energy harvesting purposes, electromechanical coupling is also modeled in the plunge degree-of-freedom along with a resistive load in the electrical domain for electrical power estimation. The exploitation of the shape memory alloy phase transformation is more attractive for airfoil-based wind energy harvesting performance than the use of typical concentrated nonlinearities (e.g., hardening steel) in terms of enhanced electrical power output. / A modelagem, análise e verificação experimental de uma seção típica aeroelástica com dois graus de liberdade e molas com memória de forma superelásticas são apresentadas. O foco é investigar os efeitos da histerese pseudoelástica das molas com memória de forma nos comportamentos de flutter e pós-flutter da seção típica. A cinética das transformações de fase nas molas com memória de forma é descrita por uma versão modificada de modelos fenomenológicos amplamente conhecidos. O modelo de molas helicoidais com memória de forma é baseado em teoria clássica de molas (com a hipótese de cisalhamento puro) e modificado para representar os efeitos não lineares de transformação de fase. A seção transversal do fio da mola com memória de forma é representada por uma distribuição radial e linear de deformações de cisalhamento e por distribuições radiais e não lineares de tensões cisalhantes e de frações martensíticas. As equações de movimento de uma seção típica linear são modificadas para incluir as molas com memória de forma. Um modelo aerodinâmico linear não estacionário é utilizado para se determinar as cargas aerodinâmicas. O modelo proposto é representado em espaço de estados e resolvido com um método Runge-Kutta. Mostra-se, numérica e experimentalmente, que a histerese pseudoelástica de molas com memória de forma pode ser efetivamente explorada para melhorar o comportamento aeroelástico de uma seção típica ao transformar oscilações instáveis de flutter em oscilações estáveis e de amplitudes aceitáveis em uma faixa de velocidades do escoamento, provendo um método útil de controle aeroelástico passivo. Como o comportamento aeroelástico modificado (pela histerese pseudoelástica) é atrativo para a coleta de energia do escoamento, um acoplamento eletromecânico é modelado no grau de liberdade de deslocamento linear, juntamente com uma carga resistiva no domínio elétrico do problema para se estimar a potência elétrica gerada. A exploração da histerese pseudoelástica das molas com memória de forma é mais atrativa para a performance da coleta aeroelástica de energia do que o uso de não linearidades concentradas típicas (como o enrijecimento não linear do aço) em termos de melhoria na potência elétrica gerada.

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.

- / -