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.

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.

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.

Martensite-to-Beta transformation temperatures of CuAlNiMn and CuAlNi shape memory alloys has been determined by differential scanning calorimetry (DSC). In CuAlNiMn alloys, each new betatizing treatment has resulted in randomly varying transformation temperatures on the same specimen and an anomalously diffuse and serrated Martensite-to-Beta transformation peaks in the first cycle. Therefore, as quenched alloy samples were thermally cycled for three times in DSC prior to ageing to obtain thermally stable and reproducible transformation temperatures and to eliminate the anomalous effect of betatizing on the transformation temperatures. CuAlNiMn alloys were aged in martensitic condition at temperatures in the range 80&amp / #61616 / C to 150&amp / #61616 / C for 24 hours to 312 hours ageing periods. Both A_s and A_f temperatures have increased with ageing temperature and time while M_s and M_f temperatures have not changed during martensite ageing. Transformation temperatures of CuAlNi alloys, on the other hand, have not changed during martensite ageing. In this respect, CuAlNiMn alloys were found to be more prone to martensite stabilization than the CuAlNi alloys. Through Transmission Electron Microscope investigation in the Cu-12.6wt%Al-5.9wt%Ni-1.8wt%Mn alloy aged at 150&amp / #61616 / C for 312 hours has revealed no sign of precipitate formation and it has been concluded that the &amp / #65533 / precipitates pinning martensite boundaries&amp / #65533 / mechanism could not be responsible of martensite stabilization. Beta phase ageing of CuAlNiMn alloys at temperatures 200&amp / #61616 / C, 230&amp / #61616 / C, 250&amp / #61616 / C and 270&amp / #61616 / C, have drastically shortened the periods for stabilization to the extent that &amp / #946 / -to-M transformation completely ceases. With regard to the Manganese content, highest Manganese bearing alloy was the one stabilized first and the lowest manganese containing one was the longest lasting alloy during beta phase ageing. Beta stabilization was not observed in any of the four CuAlNi alloys at the end of 96 hours ageing at 200&amp / #61616 / C while beta stabilization was realized after 26, 38 and 11 hours ageing at the same temperature in the three Mn containing alloys studied. In conclusion, on the basis of ageing studies at 200&amp / #61616 / C, with regard to beta stabilization, CuAlNi alloys were found to be more resistant to high temperature ageing than CuAlNiMn alloys. Equilibrium &amp / #947 / _2 and &amp / #945 / phases were observed with coupled-grown lamellar morphologies in Cu-13.6%Al-3.0%Ni alloy aged above 400&amp / #61616 / C.

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.

Submitted by Johnny Rodrigues (johnnyrodrigues@ufcg.edu.br) on 2018-03-14T21:54:07Z No. of bitstreams: 1 NATHÁLIA CRISTINA MORAIS LIA FOOK - DISSERTAÇÃO PPGEQ 2015..pdf: 2182368 bytes, checksum: 5948d7190b44ee52e0cbfdbf788ee8dd (MD5) / Made available in DSpace on 2018-03-14T21:54:07Z (GMT). No. of bitstreams: 1 NATHÁLIA CRISTINA MORAIS LIA FOOK - DISSERTAÇÃO PPGEQ 2015..pdf: 2182368 bytes, checksum: 5948d7190b44ee52e0cbfdbf788ee8dd (MD5) Previous issue date: 2015-10-05 / Capes / As ligas de Ni-Ti têm possibilitado uma melhoria em muitos projetos tradicionais de engenharia e das áreas médicas e ortodônticas, com suas propriedades únicas de superelasticidade e efeito de memória de forma. Nas últimas décadas, as pesquisas e análises para aplicações destas ligas se tornaram cada vez mais específicas, para estudos sobre micro e nano atuadores visando aplicações em dispositivos médicos e ortodônticos e em microssistemas eletromecânicos. Assim, os processos de soldagem se tornaram importantes aliados, promovendo a união entre atuadores de liga de memória de forma (LMF) com semelhantes e dissemelhantes, uma forma de aumentar as aplicações úteis de materiais disponíveis, principalmente como biomateriais. Nesse contexto, este estudo teve como objetivo geral a avaliação da resistência à corrosão em uma região de solda obtida por microssoldagem TIG autógena em fios de Ni-Ti superelásticos com fios de Ni-Ti termoativados em solução que simula a saliva humana. Os fios soldados também passaram por um tratamento térmico feito com base em um planejamento fatorial 3². A caracterização dos fios íntegros e soldados (com e sem tratamento térmico) foi realizada utilizando ensaios de calorimetria diferencial de varredura (DSC), microscopia eletrônica de varredura (MEV) e também quanto à resistência à corrosão. Os resultados obtidos revelaram que o processo de soldagem TIG utilizado neste trabalho gerou juntas soldadas de excelente qualidade, apresentando uma boa resistência à corrosão no eletrólito que simula a saliva humana. O planejamento experimental utilizado para verificar a influência da temperatura e do tempo do tratamento térmico nos resultados de corrosão gerou um experimento ótimo com temperatura de 350°C e um tempo no intervalo de 20 a 40 minutos. Estes resultados foram confirmados através dos ensaios de espectroscopia de impedância eletroquímica. / The Ni-Ti alloys have enabled the improvement in many projects from the traditional engineering and from the medical and orthodontic areas with their unique properties of superelasticity and shape memory effect. In recent decades, researches and analysis for applying these alloys have become increasingly more specific, tending to studies on micro and nano actuators targeting applications in medical and orthodontic devices and in electromechanical microsystems. Thus, the welding processes have become important allies by promoting the union between shape memory alloy actuators (SMA) with similar and dissimilar ones, which is one way of increasing the useful applications of available materials, especially biomaterials. In this context, this study aimed at analyzing the corrosion resistance in the weld region obtained by autogenous TIG micro welding in Ni-Ti superelastic wires with thermo-active Ni-Ti wires in a solution that simulates human saliva. The welded wires also Soldiers wires also undergone heat treatment made based on a factorial design 3. The characterization of the original and welded wires (with and without heat treatment) was performed using differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) tests and also regarding its resistance to corrosion. The results revealed that the TIG welding process used in this work generated welds of excellent quality, showing a good resistance to corrosion in the electrolyte that simulates human saliva. The experimental design used to investigate the influence of temperature and time of heat treatment in corrosion results generated a great experiment with temperature of 350 °C and time in the range of 20 to 40 minutes. These results were confirmed by means of electrochemical impedance spectroscopy tests.

Ce travail de thèse est consacré à l’étude de la dissipation mécanique produite par des alliages à mémoire de forme Cu-Zn-Al lors d'un chargement mécanique cyclique, ainsi qu'à l’influence de la composition chimique sur cette grandeur. Divers alliages ont été élaborés dans ce but, chacun présentant une faible variation de composition par rapport à l’autre. Une procédure expérimentale originale a été mise au point pour mesurer cette dissipation mécanique car elle se traduit par une source de chaleur très inférieure à celles dues à d’autres phénomènes comme la chaleur latente de changement de phase ou le couplage thermoélastique. Les éprouvettes ont ainsi été soumises à divers essais cycliques à température ambiante constante alors qu’une caméra infrarouge filmait les champs thermiques sur leur surface. Ces films thermiques ont ensuite été traités pour en extraire cette dissipation mécanique. Divers niveaux de dissipation correspondant à divers niveaux d’irréversibilité mécanique ont ainsi été mis en évidence. / The study deals with the mechanical dissipation in Cu-Zn-Al shape memory alloys subjected to cyclic mechanical loading, as well as with the influence of the chemical composition on this dissipation. Various alloys were prepared for this purpose, each of them featuring a slight change with respect to the others. An original procedure has been proposed to measure mechanical dissipation because it is very low compared to other heat sources such as latent heat or thermoelastic coupling. The specimens have been subjected to cyclic tests at constant ambient temperature while an infrared camera grabbed the thermal images. These thermal maps have been then processed to extract mechanical dissipation. Various levels have been found, corresponding to various levels of mechanical irreversibilities.

Submitted by Maria Medeiros (maria.dilva1@ufcg.edu.br) on 2018-07-16T13:34:47Z No. of bitstreams: 1 ECLYS DE OLIVEIRA SOARES MONTENEGRO - DISSERTAÇÃO (PPGEM) 2016.pdf: 5996362 bytes, checksum: 84827496654dcae4004aea6958439570 (MD5) / Made available in DSpace on 2018-07-16T13:34:47Z (GMT). No. of bitstreams: 1 ECLYS DE OLIVEIRA SOARES MONTENEGRO - DISSERTAÇÃO (PPGEM) 2016.pdf: 5996362 bytes, checksum: 84827496654dcae4004aea6958439570 (MD5) Previous issue date: 2016-09-01 / Estudos recentes têm mostrado que as telas de titânio estão sendo utilizadas para auxiliar na recuperação de fraturas ósseas em diversas partes do corpo humano, como face, mandíbula, crânio e joelho. Esses componentes apresentam como vantagens uma elevada resistência mecânica somada a uma baixa espessura, necessária para a ancoragem de partes fraturadas e importante para prevenir a irritação no processo pós-operatório, buscando ainda reduzir a taxa de re-operação. Assim, vislumbrando uma melhor eficiência futura dessa aplicação, surge o interesse em analisar o comportamento termomecânico desse tipo de implante, porém fabricado a partir das ligas com memória de forma (LMF), que são materiais que apresentam propriedades funcionais como o efeito memória de forma (EMF) e a superelasticidade (SE). Estas peculiaridades, aliadas a biocompatibilidade das LMF Ni-Ti tem levado à sua utilização no desenvolvimento de dispositivos médicos implantáveis. Nesse contexto, aplicar telas de LMF Ni-Ti, com boa resistência mecânica e deformações reversíveis, para potencializar aplicações biomédicas em substituição a telas de titânio, é um desafio tecnológico atual. Sendo assim, o presente trabalho teve por objetivo realizar a caracterização termomecânica de telas de LMF Ni-Ti e Ni-Ti-Cu produzidas por fundição de precisão com três geometrias celulares distintas (circular, hexagonal e quadrada) e em três estados (brutas de fundição, tratadas termicamente e laminadas). Os resultados obtidos mostraram que as telas produzidas apresentaram a transformação de fase característica dos fenômenos de EMF e SE, além de deformações reversíveis em tração da ordem de até 5%. O tipo de geometria celular foi o fator de maior influência nos valores de resistência mecânica e os melhores resultados foram verificados nas telas de geometria circular. Nos ensaios termomecânicos de flexão, além do tipo de célula, os resultados foram bastante influenciados pela espessura das telas e tratamentos térmicos utilizados. Dessa forma, as telas produzidas apresentam características funcionais adequadas para potencializar aplicações biomédicas a partir de LMF Ni-Ti em substituição as telas de titânio puro, que não se beneficiam de propriedades funcionais de EMF e SE. / Recent studies have shown that titanium meshes are being used to assist in the recovery of bone fractures in various parts of the human body such as the face, jaw, skull and knee. These components have advantages as a high strength coupled with a low thickness required for anchoring of fractured parts important to prevent irritation postoperatively process still looking to reduce the rate of re-operation. Thus, by anticipating a future better efficiency of this application, arises interest in analyzing the thermomechanical behavior of this type of implant, but manufactured from alloys with shape memory (SMA), which are materials that exhibit functional properties such as shape memory effect (SME) and superelastic (SE). These peculiarities, coupled with biocompatibility of LMF NiTi has led to their use in the development of implantable medical devices. In this context, apply SMA meshes, with good mechanical strength and reversible deformation to enhance biomedical applications replacing titanium screens, it is a current technological challenge. Therefore, this study aimed to carry out the thermomechanical characterization of Ni-Ti and Ni-Ti-Cu SMA meshes produced by precision casting with three different cell geometries (circular, hexagonal and square) and three states (as foundry, thermally treated and laminated). The results showed that the screens produced showed the phase transformation phenomena characteristic of EMF and SE, and reversible deformation in order draw up to 5%. The type of cell geometry was the most influential factor in the strength values and the best results were obtained in the circular geometry screens. In the thermomechanical bending tests, and the type of cell, results were greatly influenced by the thickness of the screens and thermal treatments. Thus, the meshes produced had enough features to enhance biomedical applications from SMA to replace the titanium meshes, which do not benefit from functional properties.

Le SMA Nitinol est largement utilisé dans de nombreux domaines de recherche récemment ( astronautes, biomédical ) et la combinaison de leurs propriétés dans la structure de conception désirée, en particulier les matériaux de l'architecture, est développé dans la dernière décennie. Des études récentes fabriqués avec succès la structure cellulaire, en particulier nid d'abeil, par processus thermomécanique à partir de tubes ou de barres mais les caractères de la liaison entre les éléments constitutifs ne sont pas clarifiées.Ce travail est consacré à l'étude de la liaison entre NiTi alliage quasi- équatomic qui est créé par le processus frittage des tubes ou des fils et par le procédé de soudage de tubes. Les liaisons obtenues des deux méthodes semblent forts et ils sont analysés par les essais de métallurgique et mécanique. Les résultats ont conduit aux conclusions suivantes :Pour les processus de frittage, les liaisons pour les des fils et des tubes semblent être forte quand ils sont chauffés à 1200oC pendant 1 heure avec les fils et 2 heures avec les tubes, respectivement. Cependant, le traitement entraîne le changement de la composition dans le matière. Le traitement à 900oC peut aider l'homogénéisation de la liaison, mais la phase inattendu TiNi2O est existé. En outre, la diffusion entre le Nitinol et Al2O3 est commencé quand ils sont chauffés à 1200oC.Pour le procédé de soudage, les liaisons entre tubes soudés sont forts avec seulement la phase TiNi mais la microstructure a changé. Il ya 2 nouvelles zones existantes à l'intérieur de la liaison de soudure: la zone de soudage, la zone affectée thermique. Les résistances de traction de la liaison de soudge une fois sont 12N/mm et 50N/mm pour tubes ayant la paroi 0,12 mm et la paroi 0,3 mm, respectivement. L'optimisation des paramètres de soudage montre que l'énergie de soudure a un effet fortement sur la création et la résistance de la liaison. La liaison est amilioré si l'énergie augmente. Charge de soudage a un rôle important pour améliorer la résistance de la liaison, et la charge de soudage est adapté pour le tube est 100N. Le deuxième fois de soudage peuvent améliorer la résistance de la liaison de la paroi du tube de 0,3 mm, mais il diminue après le troixième fois de soudage. En plus, l'addition de fois de soudage sous pression plus que de 100N conduit à la réduction de la résistance de la liaison. Ainsi, les paramètres de soudage doivent être tout d'abord examiné base de l'épaisseur de paroi du tube, puis l' énergie de soudage, la pression de soudure et la soudure fois. / Nickel-Titane shape memory alloys are widely used in many fields (aerospace, biomedical) and the combination of their outstanding properties in designed structures, namely architecture materials, has been considered in last decade. Recent studies successfully fabricated cellular structures, in particular honey combs, by thermo-mechanical processing from tubes or bars but the properties of the bonds created between the components need to be carefully investigated.This work is dedicated to the study of the bonds between NiTi near-equatomic alloy (Nitinol) elements, which are created by sintering together tubes or wires and by welding tubes under load. These bonds are characterized from metallurgical, microstructural and mechanical points of view. The obtained results led to the following conclusions.To provide a reasonably strong bond between wires and between tubes, sintering should be operated at least at 1200°C during one hour under the maximal load allowed by the experimental device, 3.5 N. However, this treatment causes intense compositional changes inside the material. A subsequent aging treatment at 900oC can help in homogenizing the material but prejudicial TiNi2O phase still exists. Besides, the interdiffusion between Nitinol elements and alumina tools at 1200°C perturbs sintering experiments. The sintering route has thus found to be inadequate unless the used device allows applying a higher load, so that the temperature can be set down.Tube welding has been more successful in terms on bond strength and NiTi phase conservation, although important microstructure changes have been observed. Three zones have been identified after welding, the weld zone, with large and long grains, the heat-affected zone, with smaller, spherical grains, and the non-affected zone. The extent of these zones is estimated from local hardness measurement. The tensile resistance of the bonds is about 12 and 50 N/mm for tubes having 0.12 and 0.3 mm thickness, respectively. A standard aging treatment does not significantly change these values although it allows material homogenization. The parameters that mainly influence the resistance and the microstructure of the bond are the weld energy, the rate of release of this energy and the load. Achieving successive welding steps is not clearly beneficial.

Les travaux de cette thèse portent sur la caractérisation du polymères piézoélectriques de PVDF et celles de ses composites avec un alliage à mémoire de forme, pour des applications de récupération l'énergie thermique. Tout d'abord, une discussion est donnée sur les avancées actuelles des technologies de récupération d'énergie ainsi que leurs intérêts économiques. Des valeurs typiques de l'énergie pouvant être générée sont estimées, ainsi que des énergies nécessaires pour certaines applications.Une attention particulière est accordée aux principes de fonctionnement des matériaux pyroélectriques et piézoélectriques. Le PVDF et l'alliage à mémoire de forme NiTiCu sont également introduits.Des techniques de caractérisation adaptées sont introduites pour par voie direct caractériser le PVDF en tant que générateur de charges électriques, et son aptitude à la récolte de l'énergie thermique. Puisque le PVDF est un matériau très souple, la flexion à quatre points, la flexion sur tube, et la machine de traction sont utilisés pour étudier sa réponse piézoélectriques directe en mode quasi-statique, ainsi que les changements de propriétés piézoélectriques sous contrainte. Des mesures d'auto-décharge sous différents champs électriques appliqués, températures et contraintes sont effectuées pour étudier la stabilité du matériau.Un concept de récupération d'énergie utilisant des composites de matériaux fonctionnels de familles différentes est introduit. Ici, le couplage entre un matériau piézo-/pyroélectrique et un alliage à mémoire de forme est proposé. Le voltage pyroélectrique simple est combiné avec un voltage piézoélectrique induit par la transformation de phase de l'alliage à mémoire de forme, pour augmenter l'énergie totale générée par le système en chauffant. Une preuve de concept est présentée d'abord pour un matériau semi-flexible basé sur une céramique PZT, et ensuite pour le PVDF qui est entièrement flexible.Enfin, un circuit de gestion d'énergie a été conçu et intégré au récupérateur d'énergie en PVDF. Les hauts pics de tension générés lors du chauffage or refroidissement sont abaissés par un convertisseur de type buck à deux étages jusqu'au une tension de sortie utile stable. L'énergie de sortie est utilisée pour alimenter une carte d'émission sans fil. Ainsi, une chaîne complète de génération d'énergie, exploitant des variations de température et allant jusqu'au l'émission de données représentatives de l'événement thermique survenu est présentée.Les résultats de ces travaux concernent un large spectre d'applications potentiels, particulièrement les capteurs autonomes sans fil, et des objets de l'Internet of Things, avec une flexibilité mécanique élevée, une épaisseur réduite et de faible coût de maintenance. / This work deals with the characterization of piezoelectric polymers PVDF and its composites with shape memory alloys, for thermal energy harvesting applications. First, we discuss current advancements on energy harvesting technologies as well as their economical interests. Typical values of energy that can be generated are given together with energies typically needed for applications.Particular attention is given to the functioning principles of pyroelectric and piezoelectric materials. PVDF and shape memory alloy NiTiCu are also introduced.Custom characterization techniques are introduced to characterize PVDF piezoelectric properties relevant to generator applications and to evaluate its suitability for thermal energy harvesting. Since PVDF is a very flexible material, four-point bending, tube bending and a tensile machine experiments are used to study its piezoelectric response in quasi-static mode, as well as changes in piezoelectric properties with increased strain. Self-discharge measurements under various applied electric fields, temperatures and strains are performed to study the stability of material.A concept of composite energy harvesting, utilizing two materials of different families, is introduced. Here, we propose the coupling of piezo-/pyroelectric material and shape memory alloy. The pure pyroelectric voltage is combined with generated piezoelectric voltage, induced by shape memory alloy transformation, to increase the total energy generated by the system during heating. The proof of concept is shown first for ceramic PZT-based semi-flexible material and then for fully flexible PVDF.Finally, a power management circuit was designed and integrated with the PVDF energy harvester. High generated voltage peaks at heating are lowered by a two-step buck converter to a useful stable output voltage. Output energy are used to power a wireless emission card. Thus, a complete power generation chain from temperature variations to data emission is presented.The results of this work concern a wide range of applications, especially modern autonomous wireless sensors and Internet of Things objects, with low profile, high mechanical flexibility and low maintenance costs.