Nonwoven materials are engineered fabrics, produced by bonding constituent fibres together by mechanical, thermal or chemical means. Such a technology has a great potential to produce material for specific purposes. It is therefore crucial to develop right products with requested properties. This requires a good understanding of the macro and micro behaviours of nonwoven products. In last 40 years, many efforts have been made by researchers to understand the performance of nonwoven materials. One of the main research challenges on the way to this understanding is to link the properties of fibres and the fabric's random fibrous microstructure to the mechanisms of overall material's deformation. The purpose of this research is to study experimentally and numerically the deformation mechanisms of a low-density thermally bonded nonwoven fabric (fibre: Polypropylene; density: 20 gsm). The study started with tensile experiments for the nonwoven material. Specimens with varying dimensions and shapes were tested to investigate the size-dependent deformation mechanisms of the material. Based on obtained results, representative dimensions for the material are determined and used in other experimental and numerical studies. Then standard tensile tests were performed coupled with image analysis. Analysis of the obtained results, allowed the tensile behaviour of the nonwoven material to be determined, the initial study of the effects of material's nonuniform microstructure was also implemented. Based on the experimental results obtained from tensile tests, continuous finite-element models were developed to simulate the material properties of the nonwoven material for its two principle directions: machine direction (MD) and cross direction (CD). Due to the continuous nature of the models, they were only used to establish the mechanical behaviour of the material by treating it as a two-component composite. The effects of bond points, which are a stiffer component within the material, were analysed. Due to the limitations of the continuous FE models, experimental studies were performed focused on the material s microstructure. The latter was detected using an x-ray Micro CT system and an ARAMIS optical strain analysis system. According to the obtained images, the nonwoven fabric is a three-component material. The effects of material's microstructure on stress/strain distributions in the deformed material were studied using advanced image analysis techniques. Based on the experimental results, a new stress calculation method was suggested to substitute the traditional approach, which is not suitable for the analysis of the low density nonwoven material. Then, the fibres orientation distribution and material properties of single fibres were measured due to their significant effects on overall mechanical properties. Finally, discontinuous finite-element models were developed accounting for on the material's three-component structure. The models emphasised the effects of the nonuniform and discontinuous microstructure of the material. Mechanical properties of fibres, the density of fibrous network, the fibres orientation distribution and the arrangement of bond points were used as input parameters for the models, representing features of the material's microstructure. With the use of the developed discontinuous models, the effects of material's microstructure on deformation mechanisms of the low-density nonwoven material were analysed.

The thermal and mechanical properties of the Mg-based bulk metallic glasses are reported in this thesis. The original ingots were prepared by arc melting and induction melting. The thermal and mechanical properties of the Mg-based bulk metallic glasses are reported in this thesis. The original ingots were prepared by arc melting and induction melting. The Mg65Cu25Gd10 and Mg65Cu15Ag10Gd10 bulk metallic glasses with different diameters from 3 to 6 mm were successfully fabricated by conventional copper mold casting in an inert atmosphere. The Mg65Cu25Gd10 bulk metallic glass shows the high glass forming ability and good thermal stability. However, the addition of Ag in the Mg65Cu15Ag10Gd10 alloy degrades the thermal stability. Based on the DSC results, the supercooled liquid region

Zr-2.5Nb is currently used for pressure tubes in the CANDU (CANada Deuterium Uranium) reactor. A complete understanding of the deformation mechanism of Zr-2.5Nb is important if we are to accurately predict the in-reactor performance of pressure tubes and guarantee normal operation of the reactors. This thesis is a first step in gaining such an understanding; the deformation mechanism of ZrNb alloys at room temperature has been evaluated through studying the effect of texture and microstructure on deformation. In-situ neutron diffraction was used to monitor the evolution of the lattice strain of individual grain families along both the loading and Poisson’s directions and to track the development of interphase and intergranular strains during deformation. The following experiments were carried out with data interpreted using elasto-plastic modeling techniques: 1) Compression tests of a 100%Zr material at room temperature. 2) Tension and compression tests of hot rolled Zr-2.5Nb plate material. 3) Compression of annealed Zr-2.5Nb. 4) Cyclic loading of the hot rolled Zr-2.5Nb. 5) Compression tests of ZrNb alloys with different Nb and oxygen contents. The experimental results were interpreted using a combination of finite element (FE) and elasto-plastic self-consistent (EPSC) models. The phase properties and phase interactions well represented by the FE model, the EPSC model successfully captured the evolution of intergranular constraint during deformation and provided reasonable estimates of the critical resolved shear stress and hardening parameters of different slip systems under different conditions. The consistency of the material parameters obtained by the EPSC model allows the deformation mechanism at room temperature and the effect of textures and microstructures of ZrNb alloys to be understood. This work provides useful information towards manufacturing of Zr-2.5Nb components and helps in producing ideal microstructures and material properties for pressure tubes. Also it is helpful in guiding the development of new materials for the next generation of nuclear reactors. Furthermore, the large data set obtained from this study can be used in evaluation and improving current and future polycrystalline deformation models. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2008-09-05 13:51:30.42

No presente trabalho é abordada uma metodologia analítica para o cálculo de deformabilidades de ligações típicas de concreto pré-moldado, levando-se em conta os mecanismos básicos de deformação na ligação. Foram estudas duas ligações típicas viga-pilar de concreto pré-moldado. A primeira ligação é uma ligação com almofada de elastômero e chumbador. É apresentado um desenvolvimento analítico para o cálculo da deformabilidade ao cisalhamento da ligação. Também, é abordado um procedimento para a determinação da resistência ao cisalhamento. Foram realizados ensaios de cisalhamento, de flexão e torção nessa ligação, onde foram variados a geometria da almofada e o diâmetro do chumbador. A segunda ligação é uma ligação resistente à flexão com chapas soldadas. É apresentado um desenvolvimento para o cálculo da deformabilidade à flexão da ligação. Foram realizados dois ensaios de flexão alternada em um protótipo com ligações soldadas e um protótipo monolítico, o qual serviu de referência para avaliar a rigidez da ligação. Os valores calculados da deformabilidade ao cisalhamento, para os protótipos com almofada de elastômero e chumbador, foram, em média, 23% superiores aos valores experimentais. Todavia, este equacionamento não se aplica ao caso de uma ligação com almofada com um fator de forma maior do que 5. As resistências ao cisalhamento calculadas corresponderam a valores entre 96 a 100% em relação aos valores experimentais. A ligação com chapas soldadas apresentou uma rigidez à flexão da ordem 83% da rigidez da ligação monolítica. O valor calculado para a rigidez à flexão secante foi 5% superior à rigidez apresentada pela ligação ensaiada. O momento máximo de cálculo da ligação, estimado com base na rigidez calculada, foi 6% superior ao momento de ruptura. A partir dos resultados experimentais, demonstrou-se que, através da utilização de procedimentos analíticos para a determinação das deformabilidades, pode-se obter uma boa estimativa para os valores experimentais, constituindo-se assim em uma alternativa viável e em uma ferramenta de grande potencial a ser explorado para o projeto das ligações de concreto pré-moldado. / This thesis discusses an analytical methodology to determine the deformability of beam-column connections for precast concrete structures, considering the basic mechanisms of deformation in these connections. Two typical beam-column connections it was studied. For the first one, which consists of a connection with bearing pad and dowel bars, is presented an analytical development to determine its shear deformability. In addition, it is discussed a procedure to determine its shear resistance and some tests, which include shear, bending moment and sprain about beam axis, are also reported, where it were used different parameters to the bearing pad geometry and to the dowel bar diameter in the connections. For the second connection, which consists of a moment resistant connection with welded plates, is presented an analytical development to determine the connection flexibility. In order to evaluate the flexibility behaviour of the connection, two tests are reported. In the first one was used a rigid connection, which was used as a stiffness reference. As a result, the theoretical shear deformability, calculated to the prototypes with bearing pad and dowel bars, presented 23% more than the experimental results. Therefore, this calculus was not applied for pads with shape factor bigger than 5. The predicted shear resistance suited to values between 96 and 100% in relation to experimental values. The stiffness response of the resistant moment connection suited about 83% of the rigid connection stiffness. The theoretical secant stiffness of the connection it was 5% bigger than the experimental response. The predicted ultimate bending moment it was 6% bigger than moment crack of connection. Based on the analysis of the results, it was demonstrated that, the employ of analytical developments to determine deformabilities in the precast concrete connections is viable and provides good estimated values. In other way, the use of those analytical developments together with available procedures consist on important design tools to be applied for the precast concrete connections.

No presente trabalho é abordada uma metodologia analítica para o cálculo de deformabilidades de ligações típicas de concreto pré-moldado, levando-se em conta os mecanismos básicos de deformação na ligação. Foram estudas duas ligações típicas viga-pilar de concreto pré-moldado. A primeira ligação é uma ligação com almofada de elastômero e chumbador. É apresentado um desenvolvimento analítico para o cálculo da deformabilidade ao cisalhamento da ligação. Também, é abordado um procedimento para a determinação da resistência ao cisalhamento. Foram realizados ensaios de cisalhamento, de flexão e torção nessa ligação, onde foram variados a geometria da almofada e o diâmetro do chumbador. A segunda ligação é uma ligação resistente à flexão com chapas soldadas. É apresentado um desenvolvimento para o cálculo da deformabilidade à flexão da ligação. Foram realizados dois ensaios de flexão alternada em um protótipo com ligações soldadas e um protótipo monolítico, o qual serviu de referência para avaliar a rigidez da ligação. Os valores calculados da deformabilidade ao cisalhamento, para os protótipos com almofada de elastômero e chumbador, foram, em média, 23% superiores aos valores experimentais. Todavia, este equacionamento não se aplica ao caso de uma ligação com almofada com um fator de forma maior do que 5. As resistências ao cisalhamento calculadas corresponderam a valores entre 96 a 100% em relação aos valores experimentais. A ligação com chapas soldadas apresentou uma rigidez à flexão da ordem 83% da rigidez da ligação monolítica. O valor calculado para a rigidez à flexão secante foi 5% superior à rigidez apresentada pela ligação ensaiada. O momento máximo de cálculo da ligação, estimado com base na rigidez calculada, foi 6% superior ao momento de ruptura. A partir dos resultados experimentais, demonstrou-se que, através da utilização de procedimentos analíticos para a determinação das deformabilidades, pode-se obter uma boa estimativa para os valores experimentais, constituindo-se assim em uma alternativa viável e em uma ferramenta de grande potencial a ser explorado para o projeto das ligações de concreto pré-moldado. / This thesis discusses an analytical methodology to determine the deformability of beam-column connections for precast concrete structures, considering the basic mechanisms of deformation in these connections. Two typical beam-column connections it was studied. For the first one, which consists of a connection with bearing pad and dowel bars, is presented an analytical development to determine its shear deformability. In addition, it is discussed a procedure to determine its shear resistance and some tests, which include shear, bending moment and sprain about beam axis, are also reported, where it were used different parameters to the bearing pad geometry and to the dowel bar diameter in the connections. For the second connection, which consists of a moment resistant connection with welded plates, is presented an analytical development to determine the connection flexibility. In order to evaluate the flexibility behaviour of the connection, two tests are reported. In the first one was used a rigid connection, which was used as a stiffness reference. As a result, the theoretical shear deformability, calculated to the prototypes with bearing pad and dowel bars, presented 23% more than the experimental results. Therefore, this calculus was not applied for pads with shape factor bigger than 5. The predicted shear resistance suited to values between 96 and 100% in relation to experimental values. The stiffness response of the resistant moment connection suited about 83% of the rigid connection stiffness. The theoretical secant stiffness of the connection it was 5% bigger than the experimental response. The predicted ultimate bending moment it was 6% bigger than moment crack of connection. Based on the analysis of the results, it was demonstrated that, the employ of analytical developments to determine deformabilities in the precast concrete connections is viable and provides good estimated values. In other way, the use of those analytical developments together with available procedures consist on important design tools to be applied for the precast concrete connections.

Les durées de vie classiques des pièces en aéronautique sont de plusieurs dizaines d’années. Cependant, certaines applications en marge impliquent des durées de vie bien plus courtes, sans réparation ou récupération des pièces. Les modèles de conception classiques doivent être adaptés et la démarche du choix matériau se faire « au juste besoin », autorisant l’utilisation des matériaux aux conditions limites de leur intégrité. Afin d’estimer ces limites, la caractérisation à plus hautes températures d’alliages existants est entreprise. C’est dans cette optique que se placent les travaux de thèse présentés dans ce manuscrit. L’alliage étudié est le Ti-6Al-4V (TA6V) forgé qui possède à l’issu du traitement thermomécanique une microstructure duplex. Il est actuellement l’alliage de titane le plus couramment utilisé en aéronautique et son utilisation est généralement limitée aux environs de 350°C pour des durées de vie classiques. Dans le but d’utiliser cet alliage pendant une dizaine d’heure, l’étude menée consiste à caractériser le TA6V de 20°C à 600°C. La caractérisation se centre, dans un premier temps, sur l’état métallurgique de la matière initiale issue du galet forgé et sur sa stabilité en température. Ensuite, le comportement mécanique du TA6V est étudié de 20°C à 600°C en traction, mettant en évidence une sensibilité de la contrainte d’écoulement à la vitesse de déformation dépendant de la température. Ce comportement est mis en lien avec le phénomène de vieillissement dynamique. La caractérisation du comportement mécanique est poursuivie par une campagne étendue de fluage de 20°C à 600°C pour différents niveaux de contraintes (de 0,3 à 1 fois la limite d’élasticité en traction). Ces essais montrent différents comportements en fonction de la température. La matière déformée en traction et en fluage est analysée en microscopie électronique en transmission afin d’apporter des informations sur les mécanismes de déformation gouvernant les différents comportements de l’alliage. Les campagnes de caractérisation en traction et en fluage ont permis d’établir un modèle de comportement viscoplastique du TA6V de 20°C à 600°C validé par l’ajustement des résultats obtenus à l’issue d’essais thermomécaniques complexes avec la simulation de ces essais par éléments finis. La corrélation des résultats en traction et en fluage et la détermination des mécanismes de déformation conduit à une discussion sur le comportement viscoplastique du TA6V, pour finalement aboutir à une proposition de modélisation du fluage du TA6V de 20°C à 600°C. Le modèle permet de reproduire qualitativement des courbes de fluage à partir de la sensibilité à la vitesse de déformation mesurée au cours d’essais de traction. / Classical life time of aeronautic parts lasts several decades. However, for some special applications with short life time and without repairs or recovery of parts, material design is tailored “close to real needs”. This justifies characterization at higher temperatures of well-known alloys and not developing new alloys. The study presented in this manuscript is included within this frame of short life applications. Forged Ti-6Al-4V (Ti-64) alloy with a bimodal microstructure is the most common titanium alloy in aeronautic and is usually limited below 350°C applications during classical life time. In order to use this alloy during a ten hour application, this thesis consists in characterizing Ti-64 from 20°C to 600°C. In a first time, characterization is focused on initial metallurgical state coming from a forged billet and on its thermal stability. Then, mechanical behavior of Ti-64 is studied by tensile testing from 20°C to 600°C, highlighting strain rate sensitivity (SRS) of flow stress. SRS is depending on temperature. This dependency is usually due to dynamic strain ageing phenomenon. Mechanical behavior characterization continues with creep testing from 20°C to 600°C for several stress levels (from 0.3 to 1 time yield stress values). Different behaviors versus temperature are revealed. Deformed samples by tensile testing and creep testing are analyzed by transmission electronic microscopy to bring information about deformation mechanisms controlling the different behaviors of the alloy. Thanks to tensile and creep testing, a viscoplastic modeling of Ti-64 from 20°C to 600°C has been performed and validated by fitting results from complex thermo mechanical tests with finite elements simulations. Comparison of mechanical behavior with deformation mechanisms leads to a discussion about viscoplasticity of Ti-64, and finally results in a proposal modeling creep behavior of Ti-64 from 20°C to 600°C. The model is able to estimate qualitatively creep curves using strain rate sensitivity measured during tensile tests.

A presente tese tem por objetivo contribuir para o conhecimento do comportamento dependente do tempo das rochas de anidrita e gipso. O maior interesse é fornecer argumentos convincentes sobre os mecanismos físicos responsáveis pelas deformações lentas observadas em algumas escavações subterrâneas realizadas em maciços dessas rochas na Europa, aspecto este que hoje está bastante confuso e escassamente estudado. A hipótese central desta pesquisa é que o comportamento dependente do tempo das rochas de anidrita e gipso deve-se parcial e/ou totalmente às propriedades reológicas (fluência) e não exclusivamente a expansão decorrente da transformação anidrita – gipso como a maioria dos autores sustenta. Este problema será analisado a partir de uma perspectiva experimental através de ensaios específicos. Os ensaios de expansão axial livre em rochas de anidrita permitiram concluir que quando submersas em água destilada, experimentam diminuição de volume por dissolução. Já as amostras de anidrita em contato com solução saturada em 'CA'SO IND.4' mostram uma relação não linear entre deformação axial e o tempo indicando uma taxa de expansão que decresce com o tempo. As taxas de expansão axial variam entre 0,3 e 2,4%/ano. A novidade é que esta expansão se deve à deposição de uma camada contínua formada por neocristais de gipso e não a expansão por hidratação da anidrita como acontece com os argilominerais. O mecanismo de transformação consiste na dissolução da anidrita e posterior precipitação do gipso em condições de sobre-saturação da solução. A transformação mineralógica acontece, na natureza, em condições propícias de temperatura e saturação da solução com íons 'CA POT.2+' e 'SO IND.4'POT.2-'. Propõe-se uma teoria alternativa sobre deformações lentas de maciços sulfáticos baseada na força de cristalização decorrente da deposição de cristais de gipso nas juntas do maciço rochoso. Propõe-se utilizar as formulações termodinâmicas propostas para expansão de concretos por crescimento de etringita e gipso. Os ensaios de fluência uniaxial sob compressão axial constante mostram que ambos os tipos de rochas sulfáticas apresentam comportamentos elasto-visco-plásticos e taxas de deformação axial e lateral da ordem de '10 POT.-12' a '10 POT.-10' 'S POT.-1'. A deflagração da fluência secundária acontece sob tensões de 4 – 6 MPa para gipsitas e de 25 – 40 MPa para anidritas. As análises sobre os possíveis mecanismos de deformação por fluência foram inconsistentes e nenhuma conclusão definitiva foi atingida. Aparentemente a baixas tensões atuariam mecanismos de difusão e dissolução por pressão. A tensões intermediárias dominariam mecanismos de deslocamentos intra e intercristalinos e a altas tensões as rochas deformariam por propagação de microfissuramento. Ensaios de fluência acoplados a sensores de emissão acústica mostram a manifestação de eventos microssísmicos que usualmente se atribuem à propagação de microfraturas mas que podem dever-se a outros mecanismos. Tanto as taxas de expansão axial como de fluência são compatíveis com as taxas de convergência medidas em algumas obras subterrâneas escavadas em maciços sulfáticos, verificando a hipótese central da pesquisa / The purpose of this thesis is to contribute for the knowledge of time-dependent behavior of sulfatic rocks of anhydrite and gypsum. Emphasis is given to provide straightforward arguments about physical mechanisms responsible for creep observed in some underground excavations in those rocks in Europe. Explanation for this phenomena is not clear yet and studies about them are still scarce. The backbone idea behind this research is that the time-dependent behavior of anhydrite and gypsum is totally or at least partially due to rheological properties, and not exclusively due to swelling resulting from the anhydritegypsum transformation. This problem was investigated experimentally with specific tests. Free swelling tests on anhydrite led to the conclusion that when immersed into distilled water, volume decrease due to dissolution is observed. Anhydrite samples in contact with a saturated solution of 'CA'SO IND.4' present a nonlinear relationship between axial strain and time, indicating swelling rate decreasing with time. Axial swelling rates vary between 0,3% and 2,4%/year. The new concept is that this swelling is due to the deposition of a continuous layer of gypsum composed by gypsum neo-crystals, and not due to hydration swelling of anhydrite, a usual with clay minerals. The transformation mechanism consists of anhydrite dissolution and later precipitation of gypsum under over-saturation condition of the solution. The mineralogical transformation takes place in nature under favorable conditions of temperature and solution saturation with 'CA POT.2+' and 'SO IND.4'POT.2-' ions. An alternative theory is proposed about creep of sulfatic rock masses based on crystallization forces resulting from the deposition of gypsum crystals in the rock masses joints. The use of thermodynamic formulations is proposed for concrete swelling due to the growth of ettringite and gypsum. Uniaxial compression creep tests show that both types of sulfatic rocks present elasto-visco-plastic behavior and axial and lateral strain rates of the order of '10 POT.-12' to '10 POT.-10' 'S POT.-1'. Triggering of secondary creep takes places under stresses of the order of 4 – 6 MPa for gypsum and 25 – 40 MPa for anhydrite. The analyses about the possible creep mechanisms were not consistent and no definite conclusion has been reached get. Apparently under low stresses diffusion and pressure dissolution mechanism dominate. Under intermediate stresses, intra- and inter crystalline displacement mechanisms seem to dominate, and under high stresses rock deform due to microcrack propagation. Creep tests monitored with acoustic emission devices show the occurrence of microseismic events attributed to microcrack propagation. Both axial swelling and creep rock are compatible with convergence rocks measured in some underground works excavated in sulfatic rock masses, following the central idea of this thesis

Les polymères semi-cristallins sont des matériaux très répandus dans notre vie quotidienne et sont utilisés dans une large gamme d'applications, généralement sous des sollicitations viscoélastiques. Par conséquent, nombreux sont les travaux de recherche qui ont été menés ces dernières années afin d’étudier leurs propriétés élastiques et leurs micro-mécanismes de plasticité ou d'endommagement apparaissant en leur sein à l'échelle locale . Cependant, l'observation in situ de l'amorçage de ces mécanismes demeure problématique et requiert l’emploi d’équipements complexes. Dès lors, nous proposons d’utiliser des techniques d'analyse non destructives fondées sur la détection et la propagation d'ondes ultrasonores (US) afin d’obtenir de nouvelles informations sur l'initiation de la plasticité et de l'endommagement de polymères semi-cristallins. Plus précisément, nous avons utilisé les techniques de contrôle par ondes US et émission acoustique (EA) afin de caractériser la plasticité et l'endommagement de plusieurs PE , d’un PP et d’un PVDF lors d'essais de traction uniaxiale. La technique de contrôle US a permis de montrer que l'atténuation US de différents types d'ondes est élevée et augmente lorsque le taux de cristallinité du matériau diminue. Pour les ondes guidées, nous avons montré l'influence de la géométrie des éprouvettes ainsi que celle de la fréquence des ondes sur l'atténuation. Lors d’un essai de traction, une importante modification des paramètres US est observée lors du passage dans le domaine plastique, traduisant l'évolution de l'état de la microstructure, en particulier celui du réseau cristallin. La formation de micro-cavités a un impact significatif sur l'atténuation des ondes. L'effet de l'orientation des chaînes macromoléculaires a également été mis en évidence. L'activité acoustique des matériaux étudiés est faible mais il a été possible de vérifier que la majorité des signaux d'EA détectés proviennent bien des micro-mécanismes de plasticité et d'endommagement. L'effet de la vitesse de déformation est significatif et nous avons montré que la localisation de certains signaux est possible lorsque cette vitesse de déformation est élevée. L'activité acoustique présente trois phases au cours des essais de traction, ce qui nous a permis de proposer en conséquence un modèle de répartition des sources d'EA sur les éprouvettes. L'activité acoustique démarre toujours avant le seuil de plasticité montrant ainsi que des micro-mécanismes de plasticité et d'endommagement s'initient aux faibles déformations. La détection de signaux d'EA avant le seuil de plasticité dépend aussi du taux de cristallinité. Le nombre de signaux d'EA détectés ainsi que leur énergie augmentent avec le taux de cristallinité du matériau. Un critère de plasticité a donc été proposé. / Semi-crystalline polymers are widely used materials in our everyday life and in a large range of applications, generally under visco-elastic solicitations. Consequently, many of the recent years researches study their elastic properties and their plasticity or damage micro-mechanisms occurring at a local scale (nano and micrometer). However, in situ observations of the initiation of these mechanisms (e.g. shear crystallites, cavitation or martensitic transformation) remain problematic and require the use of complex devices. Therefore, we propose to use non-destructive evaluation techniques based on the detection and the propagation of ultrasonic (US) waves in order to obtain new information about the initiation of plastic deformation and damage of semi-crystalline polymers. More specifically, we have used US and acoustic emission (AE) techniques to characterize the plasticity and damage of several PE, a PP and a PVDF during tensile tests. The US monitoring technique showed that the US attenuation of several waves is high and increases when the degree of crystallinity of the material decreases. For guided waves, we showed the effect of the specimens’ geometry and the waves frequency on the US attenuation. A significant change of US parameters is observed at the elastic-plastic transition, reflecting changes in the microstructure’s state, in particular in the crystal network. The formation of micro-cavities has a significant impact on the attenuation. The effect of the orientation of macromolecular chains has also been highlighted. The acoustic activity of studied materials is weak but the majority of detected AE signals have been shown to actually originate from plasticity and damage micro-mechanisms. The effect of the strain rate is significant and we have shown that the localization of few signals is possible when the strain rate is high. The acoustic activity presents three phases during tensile tests, which allowed us to propose a model based on the distribution of AE sources on the specimens. The acoustic activity always starts before the yield point showing that plasticity and damage micro-mechanisms are initiated at small strains. The detection of AE signals before the yield point also depends on the crystallinity of the material. The number of AE signals and their energy increase with the degree of crystallinity. A plastic criterion has been proposed. The correlation between the acoustic signals and the different mechanisms is complex, however it seems that the cavitation, the breakage of crystalline lamellae and the martensitic transformation are responsible for the release of acoustic energy.

Dans cette étude, nous proposons d’utiliser la compression in situ dans le MET afin de caractériser les propriétés mécaniques de nanoparticules céramiques dont la taille caractéristique est de l’ordre de quelques dizaines de nanomètres. Nous appliquerons cette méthode à des nanocubes monocristallins de MgO, une céramique modèle dont la plasticité est bien connue dans le matériau massif. Les essais de nanocompression montrent que les nanocubes de MgO se déforment de façon homogène jusqu’à de grandes déformations (>50%) sans fissure apparente. L’analyse des résultats est assistée par des méthodes de corrélation d’images numériques et simulations de type dynamique moléculaire. Le mécanisme de déformation et l'effet de taille sur la limite élastique sont identifiés. Dans une deuxième partie de la thèse, nous présentons une étude sur des nanoparticules d’alumine de transition compactée en CED (Cellule à Enclumes en Diamant) à température ambiante, sous plusieurs pressions (5 GPa, 15 GPa et 20 GPa). Des lames minces préparées par FIB ont été étudiées en MET. Des images HRTEM montrent une texture cristallographique qui devient plus importante à des pressions plus élevées. Une orientation cristallographique préférentielle est observée, avec les plans {220} de la phase gamma de l’alumine la plupart du temps parallèles à la surface de contact avec une particule voisine. Ce comportement mécanique est cohérent avec un système de glissement, connu pour les structures spinelles. Une corrélation de ce comportement avec les tests in situ MET réalisés sur des nanoparticules similaires, par Emilie Calvié lors de sa thèse, est présentée. Enfin, des clichés de diffraction de type Debye Scherrer sont réalisés sur ces lames minces de nanoparticules d’alumine de transition compactées en CED à différentes pressions. L’analyse quantitative de ces clichés montre une transformation de phase de ces nanoparticules d’alumine de phase gamma en phase delta, de manière croissante avec la pression. / In this study, we propose an innovative mechanical observation protocol of ceramics nanoparticles in the 100nm size range. This Protocol consists of in situ TEM nanocompression tests of isolated nanoparticles. Load–real displacements curves, obtained by Digital Image Correlation, are analyzed and these analyses are correlated with Molecular Dynamics simulations. By this protocol a constitutive law with its mechanical parameters (Young modulus, Yield stress...) of the studied material at the nano-scale can be obtained. In situ TEM nano-compression tests on magnesium oxide nanocubes are performed. Magnesium oxide is a model material and its plasticity is very well known at bulk. The MgO nanocubes show large plastic deformation, more than 50% of plastic strain without any fracture. The TEM results are correlated to MD simulations and the deformation mechanism can be identified.The size effect and the electron beam effect on the yield strength are investigated. In a second part of the dissertation, we present a study on transition alumina nanoparticles compacted in a Diamond Anvil Cell at different uniaxial pressures. Thin Foils of these compacted nanoparticles are prepared by FIB for HRTEM Observations. Their analysis reveals the plastic deformation of the nanoparticles. The crystallographic texture observed inthese compacted nanoparticles in DAC shows a preferred orientation of the {110} lattice planes, orientated perpendicular to the compression direction. This is compatible with the slip system. This argument was reinforced with a preferred orientation of slip bands observed during in situ TEM nano-compression tests. Moreover, electron diffraction patterns (Debye Scherrer) analysis on these compacted transition alumina nanoparticles reveals the decrease of the presence of gamma-alumina and the increase of delta-alumina with increasing pressure. This reveals the phase transformation with increasing pressure from gamma to delta* alumina.

A presente tese tem por objetivo contribuir para o conhecimento do comportamento dependente do tempo das rochas de anidrita e gipso. O maior interesse é fornecer argumentos convincentes sobre os mecanismos físicos responsáveis pelas deformações lentas observadas em algumas escavações subterrâneas realizadas em maciços dessas rochas na Europa, aspecto este que hoje está bastante confuso e escassamente estudado. A hipótese central desta pesquisa é que o comportamento dependente do tempo das rochas de anidrita e gipso deve-se parcial e/ou totalmente às propriedades reológicas (fluência) e não exclusivamente a expansão decorrente da transformação anidrita – gipso como a maioria dos autores sustenta. Este problema será analisado a partir de uma perspectiva experimental através de ensaios específicos. Os ensaios de expansão axial livre em rochas de anidrita permitiram concluir que quando submersas em água destilada, experimentam diminuição de volume por dissolução. Já as amostras de anidrita em contato com solução saturada em 'CA'SO IND.4' mostram uma relação não linear entre deformação axial e o tempo indicando uma taxa de expansão que decresce com o tempo. As taxas de expansão axial variam entre 0,3 e 2,4%/ano. A novidade é que esta expansão se deve à deposição de uma camada contínua formada por neocristais de gipso e não a expansão por hidratação da anidrita como acontece com os argilominerais. O mecanismo de transformação consiste na dissolução da anidrita e posterior precipitação do gipso em condições de sobre-saturação da solução. A transformação mineralógica acontece, na natureza, em condições propícias de temperatura e saturação da solução com íons 'CA POT.2+' e 'SO IND.4'POT.2-'. Propõe-se uma teoria alternativa sobre deformações lentas de maciços sulfáticos baseada na força de cristalização decorrente da deposição de cristais de gipso nas juntas do maciço rochoso. Propõe-se utilizar as formulações termodinâmicas propostas para expansão de concretos por crescimento de etringita e gipso. Os ensaios de fluência uniaxial sob compressão axial constante mostram que ambos os tipos de rochas sulfáticas apresentam comportamentos elasto-visco-plásticos e taxas de deformação axial e lateral da ordem de '10 POT.-12' a '10 POT.-10' 'S POT.-1'. A deflagração da fluência secundária acontece sob tensões de 4 – 6 MPa para gipsitas e de 25 – 40 MPa para anidritas. As análises sobre os possíveis mecanismos de deformação por fluência foram inconsistentes e nenhuma conclusão definitiva foi atingida. Aparentemente a baixas tensões atuariam mecanismos de difusão e dissolução por pressão. A tensões intermediárias dominariam mecanismos de deslocamentos intra e intercristalinos e a altas tensões as rochas deformariam por propagação de microfissuramento. Ensaios de fluência acoplados a sensores de emissão acústica mostram a manifestação de eventos microssísmicos que usualmente se atribuem à propagação de microfraturas mas que podem dever-se a outros mecanismos. Tanto as taxas de expansão axial como de fluência são compatíveis com as taxas de convergência medidas em algumas obras subterrâneas escavadas em maciços sulfáticos, verificando a hipótese central da pesquisa / The purpose of this thesis is to contribute for the knowledge of time-dependent behavior of sulfatic rocks of anhydrite and gypsum. Emphasis is given to provide straightforward arguments about physical mechanisms responsible for creep observed in some underground excavations in those rocks in Europe. Explanation for this phenomena is not clear yet and studies about them are still scarce. The backbone idea behind this research is that the time-dependent behavior of anhydrite and gypsum is totally or at least partially due to rheological properties, and not exclusively due to swelling resulting from the anhydritegypsum transformation. This problem was investigated experimentally with specific tests. Free swelling tests on anhydrite led to the conclusion that when immersed into distilled water, volume decrease due to dissolution is observed. Anhydrite samples in contact with a saturated solution of 'CA'SO IND.4' present a nonlinear relationship between axial strain and time, indicating swelling rate decreasing with time. Axial swelling rates vary between 0,3% and 2,4%/year. The new concept is that this swelling is due to the deposition of a continuous layer of gypsum composed by gypsum neo-crystals, and not due to hydration swelling of anhydrite, a usual with clay minerals. The transformation mechanism consists of anhydrite dissolution and later precipitation of gypsum under over-saturation condition of the solution. The mineralogical transformation takes place in nature under favorable conditions of temperature and solution saturation with 'CA POT.2+' and 'SO IND.4'POT.2-' ions. An alternative theory is proposed about creep of sulfatic rock masses based on crystallization forces resulting from the deposition of gypsum crystals in the rock masses joints. The use of thermodynamic formulations is proposed for concrete swelling due to the growth of ettringite and gypsum. Uniaxial compression creep tests show that both types of sulfatic rocks present elasto-visco-plastic behavior and axial and lateral strain rates of the order of '10 POT.-12' to '10 POT.-10' 'S POT.-1'. Triggering of secondary creep takes places under stresses of the order of 4 – 6 MPa for gypsum and 25 – 40 MPa for anhydrite. The analyses about the possible creep mechanisms were not consistent and no definite conclusion has been reached get. Apparently under low stresses diffusion and pressure dissolution mechanism dominate. Under intermediate stresses, intra- and inter crystalline displacement mechanisms seem to dominate, and under high stresses rock deform due to microcrack propagation. Creep tests monitored with acoustic emission devices show the occurrence of microseismic events attributed to microcrack propagation. Both axial swelling and creep rock are compatible with convergence rocks measured in some underground works excavated in sulfatic rock masses, following the central idea of this thesis