The understanding of the strengthening mechanisms is crucialboth in the development of new materials with improvedmechanical properties and in the development of better materialmodels in the simulation of industrial processes. The aim ofthis work has been to study different strengthening mechanismsfrom a fundamental point of view that enables the developmentof a general model for the flow stress. Two differentmechanisms namely, solid solution strengthening and grain sizestrengthening have been examined in detail. Analytical modelsproposed in the literature have been critically evaluated withrespect to experimental data from the literature. Two differentexperimental surface techniques, atomic force microscopy (AFM)and electron backscattered diffraction (EBSD) were used tocharacterize the evolving deformation structure at grainboundaries, in an ultra low-carbon (ULC) steel. A numericalmodel was also developed to describe experimental featuresobserved locally at grain boundaries. For the case of solid solution strengthening, it is shownthat existing models for solid solution strengthening cannotexplain the observed experimental features in a satisfactoryway. In the case of grain size strengthening it is shown that asimple model seems to give a relatively good description of theexperimental data. Further, the strain hardening in materialsshowing a homogenous yielding, is controlled by grainboundaries at relatively small strains. The experimentalresults from AFM and EBSD, indicate more inhomogenousdeformation behaviour, when the grain size is larger. Bothtechniques, AFM and EBSD, correlate well with each other andcan be used to describe the deformation behaviour both on alocal and global scale. The results from the numerical modelshowed a good qualitative agreement with experimentalresults. Another part of this project was directed towards thedevelopment of continuum models that include relevantmicrostructural features. One of the results was the inclusionof the pearlite lamellae spacing in a micromechanically basedFEM-model for the flow stress of ferriticperlitic steels.Moreover a good agreement was achieved between experimentalresults from AFM and FEM calculations using a non-local crystalplasticity theory that incorporates strain gradients in thehardening moduli. The main philosophy behind this research has been to combinean evaluation of existing strengthening models, with newexperiments focused on studying the fundamental behaviour ofthe evolving dislocation structure. This combination can thenbe used to draw general conclusions on modelling thestrengthening mechanisms in metals. <b>Keywords:</b>strengthening mechanisms, flow stress, solidsolution strengthening, grain size strengthening,micromechanical modelling, AFM, EBSD

Powder compaction is a production method commonly used in the manufacturing industry today. In order to minimize costly experiments and to optimize serial production of details several methods to analyze the powder compaction process are developed and used. One method is to use micromechanical analysis where the local description of contact between two individual particles is of great importance. In this dissertation a visco-plastic contact law has been used and further developed in order to understand the powder compaction process at packing, low relative density compaction up to high relative density compaction. In order to relax some assumptions from previous theoretical studies simulation with the discrete element method (DEM) was performed. Up to 10.000 spherical particles were used in packing and early compaction simulation. It was found that rearrangement of particles is one of the major densification mechanisms in the early phases of compaction. At die compaction this effect of rearrangement was shown to be more pronounced than predicted from theoretical analyses. It was also found that the size ratio of particles is of importance when the number fraction of small particles in the compound is high. The finite element method has been used for numerical analyses to investigate the local contact problem between two particles when self-similarity no longer prevail. Based on the numerical results a suggestion for an approximate compliance relation was made. With this approximate formula the local compliance behaviour between two dissimilar particles was analysed. These findings are directly applicable to simulations with the discrete element method. Finally, an investigation using the finite element method to evaluate the range of the accuracy for theoretical and approximate compliance formula has been done with compounds of different regular lattices. It was found that the range of accuracy is much dependent on the number of contacts within the lattices, specially new forming contacts during the compaction. / QC 20101007

<p>The understanding of the strengthening mechanisms is crucialboth in the development of new materials with improvedmechanical properties and in the development of better materialmodels in the simulation of industrial processes. The aim ofthis work has been to study different strengthening mechanismsfrom a fundamental point of view that enables the developmentof a general model for the flow stress. Two differentmechanisms namely, solid solution strengthening and grain sizestrengthening have been examined in detail. Analytical modelsproposed in the literature have been critically evaluated withrespect to experimental data from the literature. Two differentexperimental surface techniques, atomic force microscopy (AFM)and electron backscattered diffraction (EBSD) were used tocharacterize the evolving deformation structure at grainboundaries, in an ultra low-carbon (ULC) steel. A numericalmodel was also developed to describe experimental featuresobserved locally at grain boundaries.</p><p>For the case of solid solution strengthening, it is shownthat existing models for solid solution strengthening cannotexplain the observed experimental features in a satisfactoryway. In the case of grain size strengthening it is shown that asimple model seems to give a relatively good description of theexperimental data. Further, the strain hardening in materialsshowing a homogenous yielding, is controlled by grainboundaries at relatively small strains. The experimentalresults from AFM and EBSD, indicate more inhomogenousdeformation behaviour, when the grain size is larger. Bothtechniques, AFM and EBSD, correlate well with each other andcan be used to describe the deformation behaviour both on alocal and global scale. The results from the numerical modelshowed a good qualitative agreement with experimentalresults.</p><p>Another part of this project was directed towards thedevelopment of continuum models that include relevantmicrostructural features. One of the results was the inclusionof the pearlite lamellae spacing in a micromechanically basedFEM-model for the flow stress of ferriticperlitic steels.Moreover a good agreement was achieved between experimentalresults from AFM and FEM calculations using a non-local crystalplasticity theory that incorporates strain gradients in thehardening moduli.</p><p>The main philosophy behind this research has been to combinean evaluation of existing strengthening models, with newexperiments focused on studying the fundamental behaviour ofthe evolving dislocation structure. This combination can thenbe used to draw general conclusions on modelling thestrengthening mechanisms in metals.</p><p><b>Keywords:</b>strengthening mechanisms, flow stress, solidsolution strengthening, grain size strengthening,micromechanical modelling, AFM, EBSD</p>

<p>Powder compaction is a production method commonly used in the manufacturing industry today. In order to minimize costly experiments and to optimize serial production of details several methods to analyze the powder compaction process are developed and used. One method is to use micromechanical analysis where the local description of contact between two individual particles is of great importance. In this dissertation a visco-plastic contact law has been used and further developed in order to understand the powder compaction process at packing, low relative density compaction up to high relative density compaction.</p><p>In order to relax some assumptions from previous theoretical studies simulation with the discrete element method (DEM) was performed. Up to 10.000 spherical particles were used in packing and early compaction simulation. It was found that rearrangement of particles is one of the major densification mechanisms in the early phases of compaction. At die compaction this effect of rearrangement was shown to be more pronounced than predicted from theoretical analyses. It was also found that the size ratio of particles is of importance when the number fraction of small particles in the compound is high.</p><p>The finite element method has been used for numerical analyses to investigate the local contact problem between two particles when self-similarity no longer prevail. Based on the numerical results a suggestion for an approximate compliance relation was made. With this approximate formula the local compliance behaviour between two dissimilar particles was analysed. These findings are directly applicable to simulations with the discrete element method. Finally, an investigation using the finite element method to evaluate the range of the accuracy for theoretical and approximate compliance formula has been done with compounds of different regular lattices. It was found that the range of accuracy is much dependent on the number of contacts within the lattices, specially new forming contacts during the compaction.</p>

Morphotrope PZT-Keramiken sind Festkörperlösungen aus Bleizirkonat und Bleititanat mit chemischen Zusammensetzungen um die 47% Ti-Anteil. Sie weisen im gepolten Zustand die größten piezoelektrischen Koppelkonstanten auf und sind daher von speziellem Interesse. Zur Vorhersage des Polungszustandes und der Bauteilfestigkeit in komplexen Bauteilen werden elektromechanisch gekoppelte Materialmodelle benötigt. In dieser Arbeit wird ein mikromechanischer Modellansatz aus der Literatur aufgegriffen. Ausgangspunkt ist ein dreidimensionales tetragonales Modell, welches ein repräsentatives Volumenelement des Kornverbundes und ein mikroskopisches Kornmodell vereint. Damit gelingt die Beschreibung der Korninteraktionen infolge unterschiedlicher Polungszustände der Körner. Die Domänenstruktur der Körner wird mittels der Volumenanteile der kristallographischen Varianten dargestellt. Ein vereinfachter Satz an mikroskopischen Materialkonstanten wird anhand experimenteller Daten und theoretischer Betrachtungen aus der Literatur abgeleitet. Die für zwei Lastfälle berechneten makroskopischen Materialantworten zeigen explizit, dass das tetragonale Modell nicht in der Lage ist, das Verhalten morphotroper PZT-Keramiken nachzubilden. Aus diesem Grund wird das Modell im Hinblick auf die besondere kristallographische Struktur morphotroper PZT-Keramiken um eine rhomboedrische Phase in veränderlichen Anteilen erweitert. Die somit berechneten makroskopischen Antworten stimmen sowohl quantitativ als auch qualitativ gut mit experimentellen Ergebnissen überein. Der Einfluss der im Modell berücksichtigten Kristallstruktur auf die makroskopische Materialantwort wird in der Arbeit ausführlich analysiert. / Morphotropic PZT ceramics are solid solutions made of lead zirconate and lead titanate with chemical composition around 47% Ti-content. When poled they possess the greatest piezoelectric coupling constants for which they are of special interest. Predicting the poling condition and the strength in complex devices requires electromechanically coupled material models. Within this work, a micromechanical modelling approach is utilised. Starting point is a three-dimensional tetragonal model, which combines a representative volume element of the grain compound together with a microscopic grain model. This allows the consideration of grain interaction due to different poling conditions of the grains. The domain structure of the grains is captured by volume fractions of the crystallographic variants. A simplified set of microscopic material constants is derived from experimental and theoretical data given in the literature. The macroscopic material response, which is computed for two load cases, shows explicitly that the tetragonal model is not capable of reproducing the behaviour of morphotropic PZT ceramics. Therefore, the model is extended by the rhombohedral phase in varying quantity with view of the specific crystallographic structure of morphotropic PZT ceramics. The so computed macroscopic response shows a quantitatively as well as qualitatively good agreement with experimental results. The effect of the crystallographic structure which is considered within the model on the macroscopic material response is extensively analysed.

Morphotrope PZT-Keramiken sind Festkörperlösungen aus Bleizirkonat und Bleititanat mit chemischen Zusammensetzungen um die 47% Ti-Anteil. Sie weisen im gepolten Zustand die größten piezoelektrischen Koppelkonstanten auf und sind daher von speziellem Interesse. Zur Vorhersage des Polungszustandes und der Bauteilfestigkeit in komplexen Bauteilen werden elektromechanisch gekoppelte Materialmodelle benötigt. In dieser Arbeit wird ein mikromechanischer Modellansatz aus der Literatur aufgegriffen. Ausgangspunkt ist ein dreidimensionales tetragonales Modell, welches ein repräsentatives Volumenelement des Kornverbundes und ein mikroskopisches Kornmodell vereint. Damit gelingt die Beschreibung der Korninteraktionen infolge unterschiedlicher Polungszustände der Körner. Die Domänenstruktur der Körner wird mittels der Volumenanteile der kristallographischen Varianten dargestellt. Ein vereinfachter Satz an mikroskopischen Materialkonstanten wird anhand experimenteller Daten und theoretischer Betrachtungen aus der Literatur abgeleitet. Die für zwei Lastfälle berechneten makroskopischen Materialantworten zeigen explizit, dass das tetragonale Modell nicht in der Lage ist, das Verhalten morphotroper PZT-Keramiken nachzubilden. Aus diesem Grund wird das Modell im Hinblick auf die besondere kristallographische Struktur morphotroper PZT-Keramiken um eine rhomboedrische Phase in veränderlichen Anteilen erweitert. Die somit berechneten makroskopischen Antworten stimmen sowohl quantitativ als auch qualitativ gut mit experimentellen Ergebnissen überein. Der Einfluss der im Modell berücksichtigten Kristallstruktur auf die makroskopische Materialantwort wird in der Arbeit ausführlich analysiert. / Morphotropic PZT ceramics are solid solutions made of lead zirconate and lead titanate with chemical composition around 47% Ti-content. When poled they possess the greatest piezoelectric coupling constants for which they are of special interest. Predicting the poling condition and the strength in complex devices requires electromechanically coupled material models. Within this work, a micromechanical modelling approach is utilised. Starting point is a three-dimensional tetragonal model, which combines a representative volume element of the grain compound together with a microscopic grain model. This allows the consideration of grain interaction due to different poling conditions of the grains. The domain structure of the grains is captured by volume fractions of the crystallographic variants. A simplified set of microscopic material constants is derived from experimental and theoretical data given in the literature. The macroscopic material response, which is computed for two load cases, shows explicitly that the tetragonal model is not capable of reproducing the behaviour of morphotropic PZT ceramics. Therefore, the model is extended by the rhombohedral phase in varying quantity with view of the specific crystallographic structure of morphotropic PZT ceramics. The so computed macroscopic response shows a quantitatively as well as qualitatively good agreement with experimental results. The effect of the crystallographic structure which is considered within the model on the macroscopic material response is extensively analysed.

Ce travail porte sur l'incorporation de nanorenforts biosourcés, c'est-à-dire des nanofibrilles de cellulose (NFC), dans les matériaux composites à matrice polymère et les mousses. Ces nouveaux matériaux biosourcés peuvent par exemple être utilisés pour la conception de structures sandwich. L'étude à caractère expérimental, théorique et numérique s'articule autour de trois axes visant à optimiser tant les procédés d'élaboration que les propriétés en service de ces matériaux.Dans un premier temps, la rhéologie des suspensions concentrées de NFC, fluides à seuil thixotropes, a été étudiée aux échelles macro- et mésoscopiques en utilisant un dispositif original de rhéométrie couplé à des mesures de champs cinématiques par vélocimétrie ultra-sonore. Nous montrons ainsi que l'écoulement des suspensions de NFC est fortement hétéro-gène et présente des glissements aux parois, de multiples bandes de cisaillement couplés avec des écoulements de type « bouchon ». Sur la base de cette étude, un modèle rhéolo-gique multi-échelles est proposé. Ce modèle tient compte d'une part de l'architecture aniso-trope des réseaux connectés de NFC dans ces suspensions, et d'autre part des interactions mécaniques et physico-chimiques aux échelles nanométriques. Il permet de montrer que les interactions colloïdales et hydrodynamiques, ainsi que la tortuosité et l'orientation des NFC jouent un rôle majeur sur la contrainte seuil et sur le comportement rhéofluidifiant de ces suspensions.Dans un deuxième temps, des nanocomposites à matrice polymère ont été élaborés sous forme de films en faisant varier sur une très grande plage la fraction volumique de NFC. En utilisant d'une part des techniques de microscopie (AFM, MEB) et de diffraction aux rayons X, et d'autre part des essais mécaniques (traction, DMA) nous montrons (i) que les NFC ont une orientation plane et s'organisent en réseaux connectés par des liaisons hydro-gènes, (ii) que ces réseaux jouent un rôle majeur sur le comportement mécanique des nano-composites et (iii) que le comportement élastique des nanocomposites est bien en deçà des prévisions données par les modèles micromécaniques de la littérature. De là, nous proposons un modèle multi-échelles alternatif où les principaux nano-mécanismes de déformation sont ceux se produisant dans les parties amorphes des NFC et au niveau des très nombreuses interfaces entre NFC.Enfin, nous avons étudié l'influence des conditions d'élaboration, de la nature et de la con-centration des NFC sur les microstructures (microtomographie synchrotron à rayons X), les propriétés mécaniques (essais de compression) et les micro-mécanismes de déformation (essai in situ en microtomographie) de mousses préparées par cryodessiccation de suspensions aqueuses de NFC. / This study focuses on the use of cellulose nanofibrils (NFCs) as bio-based nano-reinforcement in polymer composites and foams. These renewable materials can be used in place of traditional materials such as for instance to produce sandwich panels. This experi-mental, theoretical and numerical work aims at optimizing the processing of these NFC-based materials as well as their use properties.In the first part of this work, the rheology of concentrated NFC suspensions, that behave as thixotropic yield stress fluids, is investigated at macro- and mesoscales using an original rheo-ultrasonic velocimetry (rheo-USV) setup allowing the local flow kinematic to be obtai-ned. We show that the flow of NFC suspensions is highly heterogeneous and exhibits com-plex situations with the coexistence of wall slippage, multiple shear bands and plug-like flow bands. Using this experimental database, we develop an original multiscale rheological model for the prediction of the rheology of NFC suspensions. The model takes into account the anisotropic fibrous nature of NFC networks as well as colloidal and mechanical interaction forces occurring at the nanoscale. The model predictions prove that colloidal and hydrody-namic interaction forces together with the orientation and the wavy nature of NFCs play a major role on the yield stress and shear thinning behaviour of the suspensions.In the second part of this work, NFC-reinforced polymer nanocomposite films are processed for a wide range of NFC contents. Using advanced microscopy techniques (AFM, SEM), X-ray diffraction and mechanical tests (tensile and DMA tests), we show (i) that NFCs form highly connected nanofibrous structures with in-plane random orientation, (ii) that these connected NFC networks play a leading role on the mechanical behaviour of the nanocompo-sites and (iii) that the elastic properties of nanocomposite films are much lower than those predicted from the micromechanical models of the literature. In light of these observations, we propose an alternative multiscale model in which the main involved deformation nano-mechanisms are those occurring both in the amorphous segments of the nanofibers and in the numerous nanofiber-nanofiber contact zones.Finally, in a third part we focus on the influence of the processing conditions, the suspension type and the NFC concentration on the microstructure (using X-ray synchrotron microto-mography), the mechanical properties (using compression tests) and the deformation micro-mechanisms (using in situ compression test with X-ray microtomography) of various foams prepared from NFC suspensions by freeze-drying.

Ce travail porte sur l'endommagement des matériaux composites stratifiés utilisés notamment pour la réalisation de pièces structurales minces. La dégradation de ces matériaux induite par la création et le développement de surfaces de décohésion internes est abordée sous deux angles. Une campagne expérimentale a tout d'abord été menée sur des stratifiés en carbone-époxy réalisés par infusion de résine liquide et sollicités en traction uniaxiale. Cette étude propose une analyse originale à l'aide de trois techniques optiques permettant une caractérisation de l'endommagement par mesures de champs : cinématiques (par stéréo-corrélation d'images), thermiques (par thermographie infrarouge) et densimétriques (par tomographie à rayons X). Le second volet du travail concerne la modélisation de la microfissuration dans le contexte d'une anisotropie initiale. A cette fin, une homogénéisation bidimensionnelle de milieux orthotropes fissurés permet la prise en compte de défauts d'orientation arbitraire et des effets unilatéraux (ouverture-fermeture des microfissures) au sein d'une formulation énergétique en déformation. Sur cette base, un modèle de comportement est proposé dans le cadre de la thermodynamique des processus irréversibles avec variables internes. Des simulations numériques permettent de démontrer les capacités prédictives de la formulation, en particulier la représentation du comportement non linéaire de ces matériaux, l'interaction entre les anisotropies initiale et induite et la restitution des propriétés élastiques lors de la fermeture de défauts. / This work deals with the damage of laminated composite materials used in particular for the production of thin structural parts. The degradation of these materials induced by the creation and growth of internal microcracks is considered from two angles. An experimental campaign was first conducted on carbon-epoxy laminates made by liquid resin infusion and loaded in uniaxial tension. This study proposes an original analysis using three advanced optical techniques that allow the damage characterization through full-field measurements : kinematic (with stereo-image correlation), thermal (with infrared thermography) and density (with X-ray tomography). The second part of the work concerns the modeling of microcracking in the context of initial anisotropy. To this end, a two-dimensional homogenization of orthotropic cracked media allows consideration of arbitrary orientation of defects and unilateral effects (opening and closing of microcracks) within a strain energetic formulation. On this basis, a constitutive model is proposed in the framework of thermodynamics of irreversible processes with internal variables. Numerical simulations demonstrate the predictive capabilities of the formulation, in particular the representation of the nonlinear behavior of these materials, the interaction between initial and induced anisotropies and the recovery of elastic properties at the closure of microdefects.

Durant ces dernières décennies les matériaux composites organiques ont subi un très grand essor dans le domaine des structures aéronautiques. Leur principal avantage est d’alléger les structures tout en gardant de bonnes propriétés mécaniques. De plus, leur microstructure leur permet d’avoir un caractère multi-fonctionnel, ce qui facilite leur intégration pour remplacer les technologies existantes. Dans l’industrie aéronautique, il existe un besoin croissant de grande quantité de petite et moyenne pièces (clips, éléments de jonctions). Cependant, il est aujourd’hui difficile de fabriquer en série des pièces ayant des formes tridimensionnelles complexes par des procédés conventionnels (autoclave). Ainsi, l’orientation envisagée est d’utiliser les procédés de la « famille » automobile pour des applications aéronautiques « semi-structurales », comme le moulage par injection de composites thermoplastiques renforcés de fibres courtes. Cette application nécessite une maîtrise et une fiabilisation du procédé ainsi que des propriétés induites. Ceci a été réalisé par l’identification et la quantification des effets des paramètres qui influent significativement sur la microstructure et les propriétés macroscopiques, par un plan d’expériences. De plus, le dimensionnement de telles pièces requiert une modélisation robuste du comportement mécanique pour prédire au mieux leur capacité d’utilisation. Les données sur la microstructure ont permis d’alimenter un modèle micromécanique comportant un critère d’endommagement de l’interface fibre/matrice. Développé sur un code éléments finis industriel, il a permis de prédire les résultats expérimentaux d’une pièce industrielle. / During the last decades, organic composite materials have undergone great development in the field of aeronautical structures. Their main avantage is to reduce the structures weight while maintaining good mechanical properties. In addition, their microstructure allows them to have a multi-fuctional nature, which facilitates their integration to replace existing technologies. In the aviation industry, there is a growing need for large amount of small and medium parts (clips, connecting elements). However, nowadays it is difficult to produce parts with complex by conventional methods dimensional shapes (autoclave). Thus, the considered path is tu use methods of the automotive “family” for “semi-structual” aerospace applications such as injection-moulding of thermoplastic composites reinforced by short fibres. This application requires a mastery and reliability of the process and the induced properties. This was achieved by the identification and quantification of the parameters effects that significantly influence the microstructure and macroscopic properties, by a design of experiments. Moreover, the dimensioning of such parts requires a robust mechanical behabior modelling to predict the best use of their capacity. The data on the microstructure enable to feed a micromechanical model featuring damage criteria of the fiber/matrics interface. Developed on a industrial finite element code, it was used to predict the experimental results of an industrial part.

Wood is a complex organic orthotropic viscoelastic material with acellular structure. When stressed, wood will deform over timethrough a process called creep. Creep affects all wooden structureand can be difficult, time-consuming and expensive to measure. For this thesis, a simple computer model of the woodenmicrostructure was developed. The hypothesis was that the modelledmicrostructure would display similar elastic and viscoelasticproperties as the macroscopic material. The model was designed by finding research with cell geometries ofconiferous trees measured. The model considered late- and earlywoodgeometries as well as growth rings. Rays were ignored as they onlycomposed 5-10% of the material. By applying a finite element method, the heterogeneous late- andearlywood cells could be homogenized by sequentially loading thestrain vector and calculating the average stress. The computer model produced stiff but acceptable values for theelastic properties. Using the standard linear solid method to modelviscoelasticity, the computer model assembled creep curvescomparable to experimental results. With the model sufficiently validated, parametric studies on thecell geometry showed that the elastic and viscoelastic propertieschanged greatly with cell shape. An unconventional RVE was alsotested and shown to give identical result to the standard RVE. Although not perfect, the model can to a certain degree predict theelastic and viscoelastic characteristics for wood given itscellular geometry. Inaccuracies were thought to be caused byassumptions and approximations when building the model.