Tesi sul tema "Mesoscopic mechanics"

This thesis consists of two parts. The first part explores a 2-d edge dislocation model to demonstrate characteristics of Field Dislocation Mechanics (FDM) in modeling single and collective behavior of individual dislocations. The second work explores the possibility of modelling adiabatic shear bands propagation within the timespace averaged framework of Mesoscopic Field Dislocation Mechanics (MFDM). It is demonstrated that FDM reduces the study of a significant class of problems of discrete dislocation dynamics to questions of the modern theory of continuum plasticity. The explored questions include the existence of a Peierls stress in translationally-invariant media, dislocation annihilation, dislocation dissociation, finite-speed-of-propagation effects of elastic waves vis-a-vis dynamic dislocation fields, supersonic dislocation motion, and short-slip duration in rupture dynamics. A variety of dislocation pile-up problems are studied, primarily complementary to what can be dealt by existing classical pile-up models. In addition, the model suggests the possibility that the tip of a shear band can be modelled as a localized spatial gradient of elastic distortion with the dislocation density tensor in continuum dislocation mechanics; It is demonstrated that the localization can be moved by its theoretical driving force and forms a diffuse traveling band tip, thereby extending the thin layer of the deformation band. A 3-d, parallel finite element framework of MFDM is developed in a geometrically nonlinear context for the purpose of modelling shear bands. The numerical formulations and algorithm are presented in detail. Constitutive models appropriate for single crystal plasticity response and J2 plasticity with thermal softening are implemented.

Understanding how materials deform and break is a subject of critical importance in industry. At the same time, it requires from the knowledge of the basic processes governing the phenomenon and hence, fundamental physics research is a must. The presence of power law distributions in both temporal and spatial properties and the universality of the behavior seem to suggest that fracture and plasticity could be explained as some type of critical phenomena. This means that there should be some general principles that rule the process and that are more important than a detailed description of the interactions and atomic structure of the media. Hence, simplified theoretical approaches based on fundamental concepts can help to capture the essential ingredients in the system. This Thesis is devoted to the study of the deformation and failure of materials in the presence of disorder with the help of statistical mechanics tools and models.

This PhD thesis focuses on the development of mathematical and computational models for flexoelectricity, a relatively new electromechanical coupling that is present in any dielectric at the micron and sub-micron scales. The work is framed in the context of both continuum and quantum mechanics, and explores the gap between these two disciplines. On the one hand, the focus is put on the mathematical modeling of the flexoelectric effect by means of continuum (electro-) mechanics, and the development of computational techniques required to numerically solve the associated boundary value problems. The novel computational infrastructure developed in this work is able to predict the performance of engineered devices for electromechanical transduction at sub-micron scales, where flexoelectricity is always present, without any particular restrictions in geometry, material choice, boundary conditions or nonlinearity. The numerical examples within this document show that flexoelectricity can be harnessed in multiple different ways towards the development of breakthrough applications in nanotechnology. On the other hand, the flexoelectric effect is also studied at an atomistic level by means of quantum mechanics. This work proposes a novel methodology to quantify the flexoelectric properties of dielectric materials, by means of connecting ab-initio atomistic simulations with the proposed models at a coarser, continuum scales. The developed approach sheds some light on a controversial topic within the density functional theory community, where large disagreements among different theoretical derivations are typically found. The ab-initio computations serve not only to assess the material parameters within the continuum models, but also to validate their inherent assumptions regarding the relevant physics at the nanoscale.
Aquesta tesi doctoral es centra en el desenvolupament de models matemàtics i computacionals per a la flexoelectricitat, un acoblament electromecànic relativament nou que es present en qualsevol material dielèctric a les escales microscòpica i nanoscòpica. El treball s'emmarca tant en el context de la mecànica del medi continu com de la mecànica quàntica, i explora l'espai entre aquestes dues disciplines. Per una banda, s'estudien els models matemàtics de l¿'efecte flexoelèctric mitjançant la mecànica del medi continu, i es desenvolupen tècniques computacionals necessàries per la resolució numèrica dels problemes de valor de contorn associats. La nova infraestructura computacional desenvolupada en aquest treball és capaç de predir el rendiment de dispositius funcionals per a la transducció electromecànica a la nanoescala, on la flexoelectricitat és sempre present, sense cap tipus de limitació en quant a geometria, propietats materials, condicions de contorn o no-linearitat. Els exemples numèrics en aquest document demostren que la flexoelectritat es pot aprofitar de diverses maneres per tal de desenvolupar aplicacions nanotecnològiques innovadores. Per altra banda, el efecte flexoelèctric es estudiat també a nivell atomístic mitjançant la mecànica quàntica. Aquest treball proposa una metodologia nova per quantificar les propietats flexoelèctriques de materials dielèctrics, connectant les simulacions atomístiques amb els models continus proposats. El mètode desenvolupat clarifica un tema controvertit en la comunitat de la teoria del funcional de la densitat (DFT), on els càlculs teòrics estan típicament en desacord entre ells. Les simulacions atomístiques no només serveixen per calcular els paràmetres flexoelèctrics dels materials considerats en models continus, sinó també per validar les hipòtesis en les quals es basen en relació amb les físiques rellevants a la nanoescala.

Thesis (Ph.D.)--Physics, Georgia Institute of Technology, 2008.
Committee Chair: Alexei Marchenkov; Committee Member: Bruno Frazier; Committee Member: Dragomir Davidovic; Committee Member: Markus Kindermann; Committee Member: Phillip First

Des états électroniques localisés apparaissent dans les liens faibles entre électrodes supraconductrices : les états d’Andreev. Les expériences présentées dans cette thèse explorent les propriétés de cohérence quantique de ces états, en utilisant comme liens faibles des contacts à un atome entre des électrodes d’aluminium. Les contacts atomiques sont intégrés dans une cavité microonde qui permet à la fois de les isoler et de les sonder.Dans une première série d’expériences, il est montré qu’on peut utiliser les états d’Andreev pour définir un bit quantique, le « qubit d’Andreev », qu’on contrôle à l’aide d’impulsions micro-onde.Les mesures des temps de vie de cohérence de ce qubit sont analysées en détail.Dans une deuxième série d’expérience,l’interaction entre le qubit d’Andreev et le résonateur micro-onde est utilisée pour quantifier le nombre de photons présents dans le résonateur en fonction de la puissance d’impulsions microonde à sa fréquence propre.Enfin, des sauts quantiques et des sauts de parités ont observés dans des mesures continues de l’état du qubit d’Andreev
Localized electronic states, called Andreev bound states, appear in weak-links placed between superconducting electrodes. The experiments presented in this thesis explore the coherence properties of these states. Single atom contacts between aluminum electrodes are used as weak links. In order to isolate and probe these states, the atomic contacts are integrated in amicrowave cavity.In a first series of experiments, it is shown that Andreev states can be used to define a quantumbit, “the Andreev qubit”, which is controlled using microwave pulses.Measurements of the lifetime and coherence time of this qubit are thoroughly analyzed.In a second series of experiments, the interaction between the Andreev qubit and the microwave cavity are used to determine the number of photons present in the cavity as a function of the power of microwave pulses at its eigenfrequency.Finally, quantum and parity jumps are observed in continuous measurements of the state of the Andreev dot

Les propriétés magnétiques détaillées des solides peuvent être vu comme le résultat de l'interaction de plusieurs sous-systèmes: celui des spins effectifs, portant l'aimantation, celui des électrons et celui du réseau crystallin. Différents processus permettent à ces sous-systèmes d'échanger de l'énergie. Parmis ceux-ci, les phénomènes de relaxation jouent un rôle prépondérants. Cependant, la complexité de ces processus en rend leur modélisation ardue. Afin de prendre en compte ces interactions de façon abordable aux calculs, l'approche de Langevin est depuis longtemps appliquée à la dynamique d'aimantation, qui peut être vue comme la réponse collective des spins. Elle consiste à modéliser les interactions entre les trois sous-systèmes par des interactions effectives entre le sous-système d'intérêt, les spins, et un bain thermique, dont seulement la densité de probabilité constituerait une quantité pertinente. Après avoir présenté cette approche, nous verrons en quoi elle permet de bâtir une dynamique atomique de spin. Une fois son implémentation détaillée, cette méthodologie sera appliquée à un exemple tiré de la littérature et basé sur le superparamagnétisme de nanoaimants de fer
Detailed magnetic properties of solids can be regarded as the result of the interaction between three subsystems: the effective spins, that will be our focus in this thesis, the electrons and the crystalline lattice. These three subsystems exchange energy, in many ways, in particular, through relaxation processes. The nature of these processes remains extremely hard to understand, and even harder to simulate. A practical approach, for performing such simulations, involves adapting the description of random processes by Langevin to the collective dynamics of the spins, usually called the magnetization dynamics. It consists in describing the, complicated, interactions between the subsystems, by the effective interactions of the subsystem of interest, the spins, and a thermal bath, whose probability density is only of relevance. This approach allows us to interpret the results of atomistic spin dynamics simulations in appropriate macroscopic terms. After presenting the numerical implementation of this methodology, a typical study of a magnetic device based on superparamagnetic iron monolayers is presented, as an example. The results are compared to experimental data and allow us to validate the atomistic spin dynamics simulations

Concrete is a highly non-homogeneous composite with large heterogeneities of quasi-brittle character. Failure of concrete structures is usually accompanied by cracking of concrete, which is strongly affected by the mesoscale structure and the behaviour of the interface between the aggregates and the mortar matrix, especially under complex stress conditions. Analysis of the failure mechanisms of concrete at the mesoscale is therefore crucial for a better understanding of the macroscopic behaviour of the material, which can in turn contribute to improved design of concrete structures and finding new ways to enhance the material properties. This research aims to investigate the intrinsic failure mechanisms of concrete-like materials from a mesoscale point of view. To do this, continued developments from existing work on mesoscale modelling are carried out to cater the needs of realistically simulating the damage process in concrete under complex loading conditions. The new developments focus on two key aspects. Firstly, techniques to realistically simulate the fracture process of concrete are developed and these involve the incorporation of a combined cohesive and contact mechanisms for the interface between aggregates and mortar matrix. Such interface modelling allows the crack initiation and propagation at the mesoscale to be explicitly represented. Secondly, a full 3D mesoscale finite element model for concrete-like materials with random aggregates and the possibility of high packing density is developed. Use is then made of these enhanced mesoscale models to explore the intrinsic mechanism governing the fundamental behaviour of concrete such as fracture propagation in tension and compression, the well-known size effect and the dynamic strain rate effect. The research investigation begins with an analysis of the size effect in plain concrete beams under three-point bending using a generic 2D mesoscale model. The analysis aims to provide preliminary insight into the use of a mesoscopic computational tool for examining the concrete damage mechanisms with the well-known size effect phenomenon as a benchmark scenario. The shapes and the sizes of the fracture process zone (FPZ) during the whole fracture process are captured. The role of detailed FPZ features is discussed accordingly. On the other hand, the results also point out the deficiencies of the continuum-based mesoscale framework at capturing the evolution of the local fracture process, and to resolve this problem requires explicit simulation of the initiation and propagation of the micro-cracks and thus a realistic reproduction of the fracture process zone, and this becomes the subject of research in much of the later chapters of the thesis. To cater to the needs of better representing the fracture process in concrete, a coupled cohesive-contact interface approach is proposed to model the crack initiation, crack propagation and the friction mechanism within the transition zone between the coarse aggregates and the mortar matrix. The cohesive-contact combined model is verified to perform well under simple as well as complex loading conditions. The interface approach in a mesoscale model framework provides a new platform for investigating the failure mechanisms in terms of the cohesive fracture process and the contact friction process. A more comprehensive and robust mesoscale interface modelling approach, in which the cohesive plus contact interface is inserted along all mesh grids, is developed to study the complex dynamic behaviour of concrete with the consideration that fractures can spread in a fine distributed manner within larger damage areas including the strong aggregate, particularly under high loading rate. By allowing local fractures to develop explicitly, the issues with fracture damage description with a continuum material model can be largely resolved. The effectiveness of such an approach is demonstrated and employed in an investigation into the intrinsic mechanisms governing the sensitivity of the dynamic tension resistance with the loading rate. Subsequently, a re-visit of the size effect in terms of the evolution of the fracture process zones using the mesoscale model with cohesive plus contact interface model is conducted and the results are presented. The preliminary observations from using the continuum-based mesoscale model are examined and verified. Additional insight into the fracture processes in the concrete beams with various sizes is obtained and the intrinsic mechanisms of the size effect are further discussed. On the real 3D mesoscale modelling methodology, the new development focuses on achieving a realistic representation of the actual shapes and sizes of aggregate particles and at the same time allowing for high volumetric ratios of aggregates (packing density) to be attained. In addition to specific techniques to enhance the conventional take-and-place procedure, an algorithm to generate supplementary aggregates to allow increased packing density is proposed and implemented. Example 3D mesoscale specimens so created are then verified against standard experimental tests such as uniaxial compression, uniaxial tension and compression with lateral confinements, and applied to examine the dynamic behaviour of concrete under high strain rate compression.

Nonlinear mesoscopic elastic (NME) materials present ananomalous nonlinear elastic behavior, which could not beexplained by classical theories. New physical mechanismsshould be individuated to explain NMEs response.Dislocations in damaged metals, fluids in rocks andadhesion (in composites) could be plausible. In this thesisI have searched for differences in the macroscopic elasticresponse of materials which could be ascribed to differentphysical processes. I have found that the nonlinearindicators follow a power law behavior as a function of theexcitation energy, with exponent ranging from 1 to 3 (thisis not completely new). This allowed to classify materialsinto well-defined classes, each characterized by a value ofthe exponent and specific microstructural properties. Tolink the measured power law exponent to plausiblephysical mechanisms, I have extended thePreisach-Mayergoyz formalism for hysteresis to multi-statemodels. Specific multi-state discrete models have beenderived from continuous microscopic physical processes,such as adhesion-clapping, adhesion-capillary forces,dislocations motion and hysteresis. In each model, themicroscopic behavior is described by a multistate equationof state, with parameters which are statisticallydistributed. Averaging over many microscopic elements theso-called mesoscopic equation of state is derived and, fromwave propagation simulations in a sample composed bymany mesoscopic elements, the experimental results couldbe reproduced. In the work of the thesis, I have shownthat model predictions of the exponent b ( the exponent bhas not been introduced before) are linked in a 'a priori'predictable way to the number of states and the propertiesof the statistical distribution adopted. We have classifiedmodels into classes defined by a different exponent b andcomparing with experimental results we have suggestedplausible mechanisms for the nonlinearity generation.

Nonlinear mesoscopic elastic (NME) materials present an anomalous nonlinear elastic behavior, which could not be explained by classical theories. New physical mechanisms should be individuated to explain NMEs response. Dislocations in damaged metals, fluids in rocks and adhesion (in composites) could be plausible. In this thesis I have searched for differences in the macroscopic elastic response of materials which could be ascribed to different physical processes. I have found that the nonlinear indicators follow a power law behavior as a function of the excitation energy, with exponent ranging from 1 to 3 (this is not completely new). This allowed to classify materials into well-defined classes, each characterized by a value of the exponent and specific microstructural properties. To link the measured power law exponent to plausible physical mechanisms, I have extended the Preisach-Mayergoyz formalism for hysteresis to multi-state models. Specific multi-state discrete models have been derived from continuous microscopic physical processes, such as adhesion-clapping, adhesion-capillary forces, dislocations motion and hysteresis. In each model, the microscopic behavior is described by a multistate equation of state, with parameters which are statistically distributed. Averaging over many microscopic elements the so-called mesoscopic equation of state is derived and, from wave propagation simulations in a sample composed by many mesoscopic elements, the experimental results could be reproduced. In the work of the thesis, I have shown that model predictions of the exponent b ( the exponent b has not been introduced before) are linked in a ‘a priori’ predictable way to the number of states and the properties of the statistical distribution adopted. We have classified models into classes defined by a different exponent b and comparing with experimental results we have suggested plausible mechanisms for the nonlinearity generation.

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008.
Includes bibliographical references.
Patterned pseudo spin-valve rings show great promise for device applications due to their non-volatility and variety of stable magnetic states. However, the magnetic reversal of these elements under an applied field is complex due to the magnetostatic coupling between the two ferromagnetic layers. Elliptical rings are electrically probed using highly symmetric Wheatstone bridges in conjunction with traditional four-point electrical measurements and micromagnetic simulations. New insight into domain wall nucleation and propagation events are elucidated. The resulting behavior is found to yield large signals at very low fields, making these devices ideal for device applications in data storage and computer logic. 360° domain walls are found to be extremely stable until fields as high as 10000e, but are positionally uncontrollable in elliptical rings. Rhombic rings were investigated as a geometry that can nucleate, propagate and pin domain walls more easily. Measurements and simulations confirm that the same reversal mechanisms exist and domain walls are more systematically positioned. The control over 3600 domain walls is valuable since reversals can occur without nucleation by decoupling the wall into a reverse domain. As a result, rhombic rings are useful as devices that can perform device functions at extremely low fields.
by Bryan Ng.
M.Eng.

La fatigue de contact est un des modes de défaillance prédominants des composants tels que les engrenages ou les roulements. Les mécanismes d’initiation de fissures associés à ce mode de défaillance sont fortement liés à la microstructure du matériau. Cependant, la plupart des modèles utilisés pour prédire la durée de vie se situent à l’échelle macroscopique. Un modèle basé sur une représentation de type Voronoi des grains (échelle mésoscopique) est développé afin d’analyser les mécanismes d’initiation. Le concept d’endommagement est appliqué aux joints de grain modélisés par la méthode des zones cohésives. L’objectif de ce modèle est (i) de contribuer à une meilleure compréhension de l’influence de paramètres tels que ceux liés aux conditions de contact (rugosité, lubrification) ou aux matériaux (présence d’inclusions ou gradients de propriétés et contraintes résiduelles générés par les traitements de surface…) sur les mécanismes d’initiation et (ii) de fournir une estimation de la durée de vie jusqu’à cette initiation. Un premier modèle 2D isotrope a permis de mettre en place l’approche proposée et d’analyser le comportement numérique des éléments cohésifs : influence de la valeur des raideurs cohésives et apparition de singularités aux jonctions triples. Cette singularité semble inévitable, mais l’approche consistant à considérer le joint de grain comme une unique entité, et donc à utiliser des valeurs moyennes le long du joint de grain permet de s’affranchir de cette singularité. La représentativité du modèle a ensuite été améliorée par la modélisation de l’anisotropie cristalline. Un modèle de type élasticité cubique a été utilisé pour modéliser le comportement des grains. Enfin, une analyse approfondie de l’application du concept d’endommagement aux joints de grains a permis de proposer une nouvelle formulation entraînant une influence plus réaliste de cet endommagement sur le cisaillement intergranulaire et conduisant à une durée de vie estimée (apparition des premières micro-fissures) d’un ordre de grandeur comparable à celles données par l’expérience
Contact fatigue is the predominant mode of failure of components subjected to a repeated contact pressure, like rolling element bearings or gears. This phenomenon is known as rolling contact fatigue (RCF). A large number of models have been developed to predict RCF, but there is today no complete predictive life model, and understanding RCF failure mechanism remains a significant challenge. RCF failure mechanisms are known to be very sensitive to a large number of parameters linked to contact conditions (roughness, lubrication) or materials (inclusions, gradients properties, residual stresses…). To improve knowledge about the influence of these parameters on failure mechanisms and life, a numerical model is developed to simulate the progressive damage of a component subject to rolling contact fatigue. Mechanisms associated with the initiation stage of failure process are located at a scale lower than the macroscopic scale. The proposed approach is to develop a grain level model (mesoscopic scale) in order to focus on initiation mechanisms. A Voronoi tessellation is used to represent the material microstructure. The progressive deterioration is simulated by applying the concept of damage mechanics at grain boundaries represented by cohesive elements. This approach has been first applied to a 2D isotropic model. The numerical behaviour of cohesive elements has been investigated: the influence of cohesive stiffness has been analysed and singularities at the triple junctions has been highlighted. The representativeness of the original model was improved by modelling crystal anisotropy. A cubic elasticity model was used to represent the behaviour of grains. Finally, a thorough analysis of the application of the damage concept at grain boundaries highlighted that the initial formulation results in a very low influence of the damage on the intergranular shear stress. A new formulation leading to a direct influence of the damage on the intergranular shear stress has been proposed. This new formulation has resulted in (i) a change in the distribution of micro-cracks, with coalescence between the different micro-cracks, and (ii) a large increase in the RCF life estimated by the model. The order of magnitude of the number of cycles corresponding to the first micro-cracks is comparable to that given by experiments

Le rôle des mécanismes mésoscopiques dans le traitement et la conception des aliments est encore mal compris, notamment pour la transformation orale et la perception de la texture. Malgré le développement de la physique de la matière molle, les scientifiques peinent encore à relier les comportements micro-, méso- et macroscopiques par des simulations. Cette thèse se concentre sur les simulations mécaniques, excluant les effets thermiques, chimiques et physico-chimiques, pour explorer le comportement des aliments à l'échelle mésoscopique. Nous avons développé une approche de simulation dans LAMMPS combinant des implémentations de "smoothed-particle hydrodynamics" (SPH) pour les liquides et les solides élastiques. Cette approche a été validée pour des scénarios comme l'écoulement de Couette et la déformation des granules. Les résultats démontrent l'efficacité de l'approche pour capter la dynamique des structures alimentaires et leurs interactions avec des papilles et des cils ; offrant de nouvelles perspectives sur la perception de la texture. L'étude met aussi en évidence l'impact de l'élasticité des granules et de leur fraction volumique sur les propriétés d'écoulement. Ce travail, centré sur la mécanique, reste ouvert à l'intégration future de processus thermiques, chimiques et biologiques dans les modèles alimentaires. Les recherches futures viseront à intégrer plus de physiques et à rendre les outils de simulation plus accessibles aux ingénieurs, pour favoriser les applications pratiques en science des aliments
The role of mesoscopic mechanics in food processing and design is not well understood, particularly for oral processing and texture perception. Despite the recognized importance of soft matter, the food science community has struggled to bridge the gap between micro-, meso-, and macro-scale behaviours using simulations. This thesis addresses this challenge by focusing on mechanical simulations, excluding thermal, chemical and physicochemical effects, to explore food behaviour at the mesoscopic scale. We have developed a simulation framework within the LAMMPS environment, combining smoothed-particle hydrodynamics (SPH) implementations for liquids and elastic solids. We validated the framework across scenarios such as Couette flow and deformation of granules in a flow. The results show the framework's effectiveness in capturing food structure dynamics and interactions with cilia and papillae and offer new insights into texture perception and hydrodynamics. The study also highlights how granule elasticity and volume fraction impact flow properties and their eventual role in texture perception. This work focuses on mechanics while deliberately remaining flexible enough to integrate mechanical, thermal, chemical, and biological processes in future food science models. Proposed future research includes strategies to integrate more physics and scales and efforts to improve the accessibility of simulation tools for engineers, advancing practical applications in food science

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2003.
Includes bibliographical references (p. 279-300).
Helical ribbons with pitch angles of either 11⁰ or 54⁰ self-assemble in a wide variety of quaternary surfactant-phospholipid/fatty acid-sterol-water systems. In all of the systems studied, the thermodynamically stable state for the sterol is plate like mono-hydrate crystals. However, the sterol is typically found to pass through a serious of metastable intermediates from filaments to helical ribbons to tubules before reaching the stable crystalline state. In the present work, we chose to focus on helical ribbons formed in the Chemically Defined Lipid Concentrate (CDLC) system. These helices typically have radii on the order of a few to a few tens of microns and lengths on the order of hundreds of microns. By tethering to these mesoscopic helical ribbons using Devcon 5-Minute Epoxy®, we have been able to elastically deform them, and thus examine their response to uniaxial tension. For small deformations, the low pitch helices behave like linear elastic springs with a spring constant for a typical example measured to be (4.80 +/- 0.77) x 10-6 N/m. From the observed spread in helix dimension, our theory predicts a corresponding range of spring constants for the structures of 10-7 to 10-4 N/m allowing, in principle, a great range of forces to be examined. Under larger tensions, both low and high pitch helices have been observed to reversibly separate into a straight domain with a pitch angle of 90Ê» and a helical domain with a pitch angle of (16.5 +/1 1.3)⁰ for the low pitch or (59.6 +/- 1.7)⁰ for the high pitch. Using a newly developed continuum elastic free energy model, we have shown that this phenomena can be understood as a mechanical phase transition of first order.
(cont.) From this analysis, we have also been able to determine all of the parameters within our model, and to show that it is capable of self-consistently and quantitatively explaining all of the observed properties of these self-assembled helices.
by Brice Christopher Smith.
Ph.D.

Physics
Ph.D.
With the advance in nanotechnology, we are more interested in the "smaller worlds". One of the practical applications of this is to measure a very small displacement or the mass of a nano-mechanical object. To measure such properties, one needs a very sensitive detector. A quantum point contact (QPC) is one of the most sensitive detectors. In a QPC, electrons tunnel one by one through a tunnel junction (a "hole"). The tunnel junction in a QPC consists of a narrow constriction (nm-wide) between two conductors. To measure the properties of a nano-mechanical object (which acts as a harmonic oscillator), we couple it to a QPC. This coupling effects the electrons tunneling through the QPC junction. By measuring the transport properties of the tunneling electrons, we can infer the properties of the oscillator (i.e. the nano-mechanical object). However, this coupling introduces noise, which reduces the measurement precision. Thus, it is very important to understand this source of noise and to study how it effects the measurement process. We theoretically study the transport properties of electrons through a QPC junction, weakly coupled to a vibration mode of a nano-mechanical oscillator via both the position and the momentum of the oscillator. %We study both the position and momentum based coupling. The transport properties that we study consist of the average flow of current through the junction, given by the one-time correlation of the electron tunneling event, and the current noise given by the two-time correlation of the average current, i.e, the variance. The first comprehensive experimental study of the noise spectrum of a detector coupled to a QPC was performed by the group of Stettenheim et al. Their observed spectral features had two pronounced peaks which depict the noise produced due to the coupling of the QPC with the oscillator and in turn provide evidence of the induced feedback loop (back-action). Benatov and Blencowe theoretically studied these spectral features using the Born approximation and the Markovian approximation. In this case the Born approximation refers to second order perturbation of the interaction Hamiltonian. In this approximation, the electrons tunnel independently, i.e., one by one only, and co-tunneling is disregarded. The Markovian approximation does not take into account the past behavior of the system under time evolution. These two approximations also enable one to study the system analytically, and the noise is calculated using the MacDonald formula. Our main aim for this thesis is to find a suitable theoretical model that would replicate the experimental plots from the work of Stettenheim et al. Our work does not use the Markovian approximation. However, we do use the Born approximation. This is justified as long as the coupling between the oscillator and QPC is weak. We first obtain the non-Markovian unconditional master equation for the reduced density matrix of the system. Non-Markovian dynamics enables us to study, in principle, the full memory effects of the system. From the master equation, we then derive analytical results for the current and the current noise. Due to the non-Markovian nature of our system, the electron tunneling parameters are time-dependent. Therefore, we cannot study the system analytically. We thus numerically solve the current noise expression to obtain the noise spectrum. We then compare our noise spectrum with the experimental noise spectrum. We show that our spectral noise results agree better with the experimental evidence compared to the results obtained using the Markovian approximation. We thus conclude that one needs non-Markovian dynamics to understand the experimental noise spectrum of a QPC coupled to a nano-mechanical oscillator.
Temple University--Theses

L'utilisation des composites à renforts fibreux est en continuelle croissance dans plusieurs domaines, tels que les industries aérospatiales, les transports et le génie civil. Cela est dû principalement à leur excellent ratio légèreté/performances. Les structures tressées font partie des structures textiles utilisées comme renforts pour les composites. Leur procédé de fabrication permet la réalisation de formes complexes et de géométries très variées. Cependant, les nombreux paramètres présents tant au niveau des matières utilisées qu'au niveau des procédés de mise en œuvre impliquent la nécessité de bien maitriser la technologie du tressage afin d'optimiser les paramètres de fabrication et de prédire le comportement final de ces structures de renfort. Ce projet de recherche constitue une étude des paramètres de tressage et du comportement mécanique des structures tressées. Cette étude comporte une partie expérimentale et une partie numérique. Dans la partie expérimentale, plusieurs renforts tressés tubulaires en fibre de carbone sont fabriqués à l'aide d'une machine de tressage radiale 2D couplée à un robot six axes. Des composites à base de ces renforts sont ensuite élaborés par le procédé RTM. Plusieurs essais expérimentaux sont réalisés pour caractériser le comportement des renforts secs et leurs composites afin de pouvoir évaluer l'influence des paramètres géométriques, comme l'angle de tressage, le diamètre de la tresse et le type de la tresse (biaxiale ou triaxiale), sur les propriétés mécaniques des tresses tubulaires. Dans la partie numérique de l'étude, la microtomographie aux rayons X est utilisée pour obtenir le modèle géométrique des renforts tressés. Une analyse par élément finis à l'échelle mésoscopique est réalisée en utilisant une loi de comportement hypoélastique implémentée dans Abaqus/Explicit à l'aide d'une subroutine Vumat
The use of textile composites is increasing in several areas, such as aerospace industries, transportation, civil engineering and others, due to their high strength-weight ratio. Braided structures are one of the textile reinforcements used in different industrial applications for the cost effectiveness of their manufacturing technique, its versatility and the wide range of shapes it can offer. The special structures with the special functionalities needed in each composite application make the braiding a delicate process that needs to be studied in order to fulfill the demands of each specific sector. This PhD project aims to achieve a proper understanding of the process, the structures, the various parameters and the behavior of the final products. The study is conducted using the Herzog 2D braiding machine of Ifth, which, combined with a 6 axes robot, can prototype 3D structures by over-braiding complex shaped mandrels. Multiple carbon fiber braided samples are produced by varying the process parameters (Braid angle, Braid's diameter ...) and characterized in order to assess the influence of these parameters on the braid's geometry and its mechanical properties. To reach a better understanding of the materials' behavior and to avoid the time-consuming trial and error manufacturing and testing way, a modeling procedure is necessary to support the experimental work and optimize the design phase of the braids. Different models have been developed by researchers to predict the properties of braids at different scales of the structure (microscopic - mesoscopic - macroscopic). This work will be focused on the finite element analysis at the meso-scale, i.e. the braid unit cell scale, which considers the orientation of the yarns and the braid's architecture. This analysis is conducted using a hypo-elastic constitutive law which is implemented in user subroutine Vumat in Abaqus/Explicit. In this work, the geometric model is obtained using micro-computed tomography, which is a nondestructive scanning technique that allows detailed and precise analysis of the geometry of a textile reinforcement

Les composites à matrice céramique (CMC) présentent une architecture multi-échelle complexe. Pour être utilisé en tant que composant de moteur aéronautique qui nécessitent des géométries complexes, ces matériaux doivent être tissés sous forme d'architectures textiles spécifiques. Mon travail s’est concentré sur l’étude d’une pièce de type raidisseur, et plus particulièrement sur le détail d’une jonction composite tissée. La taille caractéristique de cette pièce se situe entre les échelles méso- et macroscopique, ce qui rend impossible l’utilisation des hypothèses de séparabilité des échelles. Nous avons tout d’abord développé un montage expérimentale de flexion/cisaillement adapté à la jonction tissée. Ces essais ont non seulement permis d’identifier et de caractériser le comportement mécanique de cette pièce, mais aussi, de mettre en lumière l’interdépendance entre le chargement, l’architecture textile et les mécanismes d’endommagement, qui est particulièrement importante dans le cas de la jonction tissée. C’est pourquoi, la modélisation de ce détail de structure doit inclure une connaissance approfondie de l’architecture interne du matériau. Nous avons donc développé une approche originale de segmentation variationnelle à partir de µCT, afin de construire des modèles numériques réalistes du matériau à l’échelle mésoscopique. Cette approche repose sur une heuristique globale-locale qui améliore itérativement la ressemblance d’un modèle géométrique initial. Cette démarche a permis de construire le jumeau numérique de la jonction tissée. Le modèle final ne comportant pas d’interpénétration entre fils, un maillage tétraédrique conforme peut ensuite être généré directement à partir de l’image ainsi labellisée. Des simulations EF à l’échelle mésoscopique ont été menées en prenant en compte le comportement non-linéaire des constituants des CMC. Elles permettent de prévoir le niveau de chargement menant aux premiers endommagements. De plus, la localisation des endommagements ainsi que leurs interactions avec l’architecture méso ont également ont été reproduites de manière satisfaisante.Cependant, ces modèles incluent une description très détaillée du matériau et nécessitent donc des ressources de calcul importantes. Une description approchée de ces détails pourrait être suffisante pour obtenir une prédiction correcte des propriétés élastiques, voir de l’amorçage de l’endommagement. Nous avons donc proposé un pont méso-macro permettant de construire le comportement apparent des éléments macroscopiques à partir de l’information méso sous-jacente. Les propriétés des éléments macroscopiques sont obtenues en assimilant localement le matériau à un stratifié équivalent construit à partir des fractions volumiques et des orientations locales des constituants. Cette approche permet de réduire drastiquement la taille des problèmes EF, tout en conservant une description approchée de la méso-structure. Le modèle macroscopique enrichi permet de reproduire fidèlement les résultats obtenus à l’échelle mésoscopique, tant que la taille de filtrage reste comparable à celle des fils.Les modèles proposés ont été utilisés pour reproduire les résultats expérimentaux et approfondir leur analyse. Nous avons étudié en particulier la sensibilité aux conditions aux limites de l’essai, ainsi que l’influence des variabilités liées au procédé de fabrication des éprouvettes. Enfin, la chaine d’outils développée dans le cadre de la thèse pourra être utilisée pour étudier différentes définitions textiles de la jonction, permettant in fine de définir l’architecture optimale de la pièce
Woven ceramic matrix composites (CMC) exhibit an intricate multi-scale architecture. To be used as components of aircraft engines, the weaving of such parts could also incorporate specific features compared to « classical » woven CMC as they need to comply with complex geometries. My work focused on a stiffener-like fully woven junction that is made of a complex 3D woven fabric, and whose characteristic size lies at the frontier between the mesoscopic and the macroscopic scales, i.e. where scale separation hypothesis is not applicable.I have first developed an experimental device to perform shear/bending tests on the woven junction. These tests not only allowed to gain significant knowledge about the mechanical behavior of such part, but also to highlight the interplay between the load, material architecture and damage mechanisms that is particularly significant in the case of the woven junction. Therefore, numerical prediction of the mechanical behavior of the woven junction necessitates a sound knowledge of its inner structure.With this aim, I have developed an original segmentation method to build realistic numerical models of textile composites, using X-ray micro-computed tomography and a prior geometric model. The procedure includes a global-local heuristic to iteratively improve the resemblance of the initial model. This approach allowed to build “digital twins” of the woven junction. A conformal tetrahedral image-based mesh could then be obtained as the resulting models are free of interpenetration. Mesoscale FE simulations, including non-linear behavior laws of the yarns and matrix, allowed to predict the maximal load leading to the first damage events, and to reproduce accurately the damage localization and its interaction with the architecture.However, with such level of details incorporated in the model, the simulations necessitate significant computational resources. An approximate macro-scale description may be sufficient to evaluate the elastic properties, or even to simulate damage initiation. Therefore, we have proposed a meso-informed macroscopic modelling framework where the behaviour of the macro-elements is derived from the knowledge of the local direction and volume fraction of constituents, thanks to the digital twin. The effective behaviour of the macro-elements is obtained through an equivalent lamina. This method drastically reduces the size of the model while preserving an approximate description of the underlying local anisotropy and heterogeneities. With respect to the damage initiation, the meso-informed macroscopic model accurately reproduced the results obtained using the reference mesoscale model, as long as the filtering size remains comparable to the yarn size. This allowed to propose an optimal modelling framework with an adequate level of description of meso-details and acceptable computational requirements.Finally, I have used these models to thoroughly compare the numerical simulations with the experimental results: variabilities of experimental boundary conditions have been analyzed, as well as the influence of specific heterogeneities related to the fabrication process. We have also used this framework to explore different weaving patterns in order to obtain an optimal design of the woven junction

La simulation du procédé de fabrication de renforts fibreux secs est un enjeu majeur pour l’étude de l’élaboration de matériaux composites, dont l’utilisation dans les industries de pointe s’intensifie rapidement. Ainsi, l’influence du métier à tisser sur la qualité des renforts est primordiale dans la caractérisation de leurs propriétés mécaniques. Une campagne d’essais expérimentaux est tout d’abord réalisée, de manière à identifier les phénomènes physiques mis en jeu. Les différents modes de déformation de la mèche sont ainsi étudiés : élongation, compaction, cisaillement et distorsion. Est étudié également le comportement en flexion et en frottement, afin de mieux appréhender l’effet du procédé de tissage sur les mèches. Deux types de lois de comportement élastiques sont envisagés : une loi hypoélastique et une loi hyperélastique. Sont développées les propriétés de chacune d’entre elles, ainsi que les grandeurs caractéristiques nécessaires à leur implémentation dans le logiciel commercial ABAQUS/Standard. Les algorithmes de deux subroutines sont présentés, correspondant à l’une ou l’autre de ces lois. Le choix est fait de modéliser le comportement mécanique de la mèche à l’aide d’une loi hyperélastique isotrope transverse de type St-Venant, par l’intermédiaire de la subroutine ABAQUS/Standard UANISOHYPER_INV. Enfin, une identification des paramètres matériau à l’aide d’une méthode inverse est proposée. Sont comparés les résultats obtenus par simulation avec les résultats expérimentaux. La loi de comportement alors déterminée permet de mettre en place des simulations de procédé de tissage
Simulating the manufacturing process of woven preforms is a major stack for understanding the development of composite materials, used in high performance industries. The effect of the weaving loom on the preforms is very important to caracterize their mechanicals properties. Experimental tests are realised to identify the physical phenomenon. Different deformation modes are studied : elongation, compaction, shear and distortion. The bending and friction behavior are also important to understand the effect of weaving process. Two constitutive laws are considered : a hypoelastic law and a hyperelastic law. An analyse of their properties is presented, and their implementation in a commercial software, ABAQUS/Standard, is detailed. In this purpose, two subroutines can be used. The modelisation of the mechanical behavior of the tows is finally realised with a transversely isotropic hyperelastic St-Venant model, with the subroutine ABAQUS/Standard UANISOHYPER_INV. To conclude, an identification method is presented and the simulated results are compared to experimental tests. The obtained consitutive behavior is finally used to simulate the weaving process

L'industrie aéronautique doit faire face aux nouvelles exigences environnementales, tout particulièrement concernant la réduction de la consommation des énergies fossiles. L'utilisation de matériaux composites plus léger permet de répondre en partie à cette attente. Pour limiter les coûts lors de la fabrication et du développement des composites à renforts tissés 3D, il est nécessaire d'utiliser des outils de simulation performants. En particulier, les outils existants, qui discrétisent à une échelle mésoscopique l'architecture des tissus 3D, ne tiennent pas compte de l'influence du procédé de fabrication sur la constitution de la structure textile. Si des outils numériques dédiés à la modélisation du procédé de tressage et de tricotage sont disponibles, il n'en est rien concernant le tissage. Cette étude avait donc pour but de s'intéresser plus particulièrement à la simulation du prodécé de tissage pour pouvoir obtenir une structure de tissu sèche déformée numériquement. La production de différentes architectures de tissu en verre E dans notre laboratoire nous a permis d'observer les différents éléments en contact avec le fil ou le tissu sur la machine à tisser, par le biais de l'utilisation d'une caméra rapide par exemple. Le développement d'un modèle numérique par éléments finis reproduisant le procédé de tissage a été réalisé. Une loi de comportement isotrope transverse fut utilisée pour modéliser les fils de verre. Des premières simulations numériques encourageantes pour la fabrication d'un tissu d'armure toile et d'un tissu d'armure croisé 2-2 sont présentées et comparées avec les tissus réels produits correspondants
The aeronautical industry faces new challenges regarding the reduction of fossil fuel consumption. One way to address this issue is to use lighter composite materials. The ability to predict the geometry and the mechanical properties of the unit cell is necessary in order to develop 3D reinforcements in composite materials for these aeronautical applications. There is a difficulty to get realistic geometries for these unit cells due to the complexity of their architecture. Currently, existing tools which model 3D fabrics at a meso scale don't take into account manufacturing process influence on the shape modification of the textile structure. There is already some numerical tools that can model the braiding or knitting process, but none have been developed for weaving so far. Consequently, this study deals with the numerical simulation of the weaving process to obtain a deformed dry fabric structure. During the weaving process of E-glass fabrics, achieved in our laboratory, it has been observed that large deformations led to the modification of transverse section of meshes, or local density changes, that can modify the fabrics mechanical resistance. For this reason, a numerical tool of the weaving process, based on finite element modelling, has been developped to predict these major deformations and their influences on the final textile structure. The correlation between numerical results and fabrics produced with glass fibres has been achieved for plain weave and 2-2 twill

La simulation des procédés de mise en forme des composites à renforts tissés de type RTM est un enjeu majeur pour les industries de pointe mettant en œuvre ce type de matériaux. Au cours de ces procédés, la préforme tissée est souvent soumise à des déformations importantes. La connaissance et la simulation du comportement mécanique de la préforme à l'échelle macroscopique et à l'échelle mésoscopique s'avère souvent nécessaire pour optimiser la phase de conception de pièces composites formées par de tels procédés. Une analyse du comportement mésoscopique des préformes tissées de composites est d'abord proposée. Une loi de comportement hyperélastique isotrope transverse est développée, permettant de décrire le comportement mécanique de chacun des modes de déformation de la mèche : élongation dans la direction des fibres, compaction et distorsion dans le plan d'isotropie de la mèche, cisaillement le long des fibres. Une méthodologie est proposée pour identifier les paramètres de cette loi de comportement à l'aide d'essais sur la mèche et sur le tissu, et une validation par comparaison avec des essais expérimentaux est présentée. Une analyse du comportement macroscopique des renforts interlocks est ensuite proposée : une loi de comportement hyperélastique orthotrope est développée et implémentée. Cette loi, extension de la loi de comportement pour la mèche, est également basée sur une description phénoménologique des modes de déformation de la préforme. Une méthode d'identification des paramètres de cette loi de comportement est mise en œuvre, utilisant des essais expérimentaux classiques dans le contexte des renforts tissés (tension uniaxiale, compression, bias extension test, flexion). Cette seconde loi de comportement est validée par comparaison avec des essais de flexion et d'emboutissage hémisphérique.

Afin de pouvoir prédire le comportement des renforts de composites 3D interlock au cours d'un procédé de mise en forme, il est nécessaire de connaitre la position des mèches dans le renfort durant la phase de préformage du procédé. Les travaux présentés ici traitent de la simulation du préformage de renforts 3D épais à l'aide d'un élément fini hexaédrique semi-discret spécifique. En utilisant le principe des travaux virtuels, on distingue le travail interne virtuel dû à la tension des mèches des autres travaux virtuels. La raideur due aux tensions de mèches, qui constitue la contribution principale de la rigidité du matériau, est prise en compte à l'aide de barres incluses dans les éléments. Les rigidités dues aux autres sollicitations, comme la compression transverse, les cisaillements ou les frottements inter-mèches, sont décrites par un matériau continu additionnel. La combinaison de ce modèle discret du premier ordre et d'un matériau continu hyperélastique anisotrope dit du second ordre, pour formuler le comportement du matériau va permettre la simulation du préformage des renforts tissés épais. Conjointement aux travaux sur la simulation, des travaux expérimentaux pour l'identification des paramètres matériau de la loi de comportement ont été définis et réalisés. Ces paramètres concernent les deux parties de la formulation du comportement.

博士
國立交通大學
電子物理系所
101
Mesoscopic physics, which is in between the microscopic and the macroscopic world, contains physical features of both scales. Distinctive phenomena found in the mesosopic systems give insights into the quantum-classical correspondence which has attracted lots of attention from researchers. The related issues in mesoscopic regime have been studying and paying close attention. In the thesis we employed optical systems as analog systems to investigate the connection between quantum and classical mechanics. This statement based on the good correspondence between quantum-classical mechanics and wave-ray optics. Moreover, optical wave equation was theoretically elucidated to be in the same mathematical form as the Schrödinger equation. We provided comprehensive studies for the quantum coherent states corresponding to the optical waves. With sophisticated mathematics in quantum mechanics, we are able to understand the wonderland between wave optics and ray optics and the important roles of quantum coherent states in quantum systems. Two kinds of optical systems, light pipes and a laser resonator, were discussed in the thesis. Although it seems that the two setups are totally different, they are governed by the same theoretical foundation. Within rigorous analyses, the coherent states in corresponding quantum systems revealed intriguing patterns localized on the classical periodic orbits. The same spatial patterns could be found in the optical systems. The validation of the connection between quantum and optical coherent waves enables further studies on related research based on quantum mechanics. Another topic in the thesis is the linkage of two distinctive optical coherent states localized on the periodic orbits of Lissajous and trochoidal curves. The investigation not only visualized the insight of topology in mathematics but exhibited analog transformational relationship of particle trajectories followed by different coupling mechanisms in a two-dimensional harmonic system. Hence, the realization of the converted spatial coherent states might be an accessible method for the study of fundamental science in various branches. With theoretical analyses, the coherent waves were found to carry large orbital angular momentum and might stimulate further applications. Besides the two topics mentioned on the above, another topic has been played an important role in the mesoscopic physics—the investigation of localization for disordered wave functions in random media. In this work, we obtain the disordered wave functions from the conical second harmonic generation to explore the continuous transformation of weak localization from extended to pre-localized states. We numerically verify that the experimental density distributions with different extents of weak localization can be excellently analyzed with a reduced version of the nonlinear sigma model. This is the first time that the reduced version of the nonlinear sigma model to be applied to describe the experimental results. Moreover, we perform that the chi-square distributions with fractional degrees of freedom are practically equivalent to the density distributions of the reduced version of the nonlinear sigma model. Since the observation of the disordered wave functions is not accessible, this work might provide an approach to comprehensively study the intriguing physics behind the disordered systems. On the other hand, the present results suggest the possibility of exploiting conical second harmonic generation as a diagnostic method to understand the complex topological structure of the disordered crystals.

博士
國立清華大學
物理系
100
To realize the probability of nanodevices application using quantum phase information, dephasing processes are one of the most crucial essential issues regarding semiconducting mesoscopic systems. This thesis presents a series of experiments that study the phase coherence and dephasing mechanisms by using Aharonov–Bohm (AB) and spin-type Mach–Zehnder interferometers (SMZIs) fabricated in GaAs/AlGaAs heterostructured crystals. In chapter 1, we introduce the motivation and context for the experimental works described in the following chapters. In chapter 2, we investigate the dependence of the dephasing rate in a ballistic AB ring on the temperature, bias current, and probe configuration. First, we would like to study how the probe configuration influences the conductance and the dephasing rate. In fact, averaging of the transmission phase, in which current is carried by thermally excited or current-induced electrons, results in dephasing. We find that the appropriate energy window for dephasing is set by the drift velocity of the interfering electrons and the asymmetry of the ring path. In chapter 3, we investigate the dephasing rates in ballistic AB rings with local and nonlocal probe configurations by tuning the transmission through one arm of the ring. The dephasing rates are independent of the probe configuration, whereas the transmission through the ring paths is equal. In contrast, because AB interferometers are tuned to be strongly asymmetric, the dephasing rate of the local configuration becomes larger than that of the nonlocal configuration. We find that our observations can be explained qualitatively by voltage fluctuations from the measurement circuit, as proposed by G. Seelig, S. Pilgram, A. N. Jordan, and M. Büttiker [56]. In chapter 4, we first introduce a novel SMZI by using spin-resolved edge states in the integer quantum Hall regime. Furthermore, to investigate the phase coherence length in this interferometer, we determined the finite temperature coherence length of the spin-resolved edge states by designing interferometers of various sizes and attempted to explain the dephasing mechanism in this novel system. The phase coherence length in the present experiment, surprisingly, is noticeable larger than the charge coherence length found in an electronic MZI[75, 83]. Finally, in chapter 5, we summarize our finding from the experimental works and discuss future work based on our presented results.

碩士
國立東華大學
物理學系
104
One-dimensional growth mechanisms of mesoscopic oxides formed on heated metallic substrates have been investigated. Morphology evolution of surface oxides was observed in dependence of temperature, time, oxygen content of the ambient gas, and heating rate. Evident deviations from the description of an available nanowire growth model based thermally activated Zn migration and diffusion-limited processes have been found. Accordingly, a new model based on self-catalytic nanowire growth mechanism has been proposed, taking into account the competition between one- and two-dimensional growth modes of the surface oxides as well as the growth speeds dominated by reaction-limited processes.

碩士
國立臺北科技大學
土木與防災研究所
95
Due to the growth of microcracks leading to localization within quasi-brittle materials, the engineering structure subjected to different stress paths would be caused an unexpected damage prior to peak load. Therefore, it is more important to understand the evolution of microcracks for the stability of structural materials. This study presents a numerical simulation of displacement discontinuity behavior by using Particle Flow Code in three dimensions（PFC3D）which bases on the principle of distinct element method（DEM）. First of all, we proceed the sensitivity of parametric studies to calibrate the consistency of micro-to-macro material properties during both triaxial including unconfinement case and Brazilian test, and discuss mesoscopic behavior of quasi-brittle rock under different stress paths. Secondly, the numerical results in terms of the development of micro crack as well as the failure type of cracks between particles were compared with experimental data of acoustic emission qualitatively. It evolves the failure characteristic of quasi-brittle materials by viewing mesoscopic fracture behavior and conventional global failure criterion, and provides an option to identify the location of damage zone under certain stress level. This numerical simulation shows that, by setting proper micro-geometrical parameters, a significant influence of particle radius on some macroscopic material parameters such as E, υ, qu. In this study, 7000 balls of particle numbers were used to conduct a series of parametric studies. Furthermore, the relation between micro- and macro- material properties: micro elastic modulus versus macroscopic elastic modulus, and bond strength versus confinement strength are found the existence of linear relation. In addition, there is a fair correlation between the normal/ shear stiffness and Poisson’s ratio. Macroscopic friction angle is also controlled by micro friction coefficient in spite of the upper bound of friction angle may not be controlled well with respect to different material and stress path. According to analysis of sensitivity parametric, numerical model fitting is able to match through triaxial test including unconfing case. It shows that the micro-properties of PFC3D indeed response a good agreement with laboratory results in terms of both elastic and plastic parameters. By monitoring shear/ tensile cracks increment ratio and localization of particle cracks corresponding different load level, local shear cracking somehow dominated the damage around peak in uniaxial compression test. On the other hand, by simulating Brazilian test with same micro-properties, relatively tensile crack in element which dominates damage was found. However the estimation of tensile strength is higher than laboratory experiment value about 3 times. Finally, appearance of localization are obtained about 61% and 51% for numerical simulation and experimental AE data under uniaxial test respectively, and about 65% and 61% under Brazilian test. It shows that PFC3D could be used to verify the growth of micro cracks as well as its localization qualitively.

Tesis (Doctor en Física)--Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía, Física y Computación, 2016.
La pérdida de correlación de fase en un sistema cuántico es conocida como decoherencia y es consecuencia de las interacción con el ambiente. En esta tesis nos enfocamos en desarrollar herramientas teóricas y numéricas que permitan describir el efecto de la decoherencia en el transporte cuántico de sistemas con dependencia temporal o que presenten procesos de inversión de espín. En particular abordamos problemas de transporte a través de paredes de dominios magnéticos, magnetorresistencia gigante, dinámica cuántica decoherente y motores cuánticos. Los modelos y métodos desarrollados son los suficientemente generales como para ser aplicados en otras situaciones.
The loss of phase correlation in a quantum system is known as decoherence and it is consequence of interactions with the environment. In this thesis, we focus on developing theoretical and numerical tools to describe the effect of decoherence in quantum transport of systems with time dependence or with spin-flipping processes. In particular we address the problems of transport through magnetic domain walls, giant magnetoresistance, decoherent quantum dynamics and adiabatic quantum motors. The models and methods developed in this thesis are general enough to be applied in other situations.

During my PhD, I performed functional and structural investigations on the heart; I optimized clearing and staining protocols on cardiac tissue, and I exploited advanced imaging techniques to perform high-resolution studies working at two different projects. In the first project, the energetics of demembranated multicellular cardiac muscle strips were investigated from three Hypertrophy Cardiomyopathy (HCM) patients with the E258K mutation. Energetic measurements in multicellular preparations may suffer from artefacts, related to the density and the orientation of the contractile material. Alterations of cardiomyocytes organization or disarray, a common HCM histopathological feature at whole heart level, may decrease isometric tension while increasing the isometric ATPase of multicellular preparations, thus leading to an artificial increase in tension cost (TC). To exclude this hypothesis, a new protocol that combines a novel tissue clearing technique, previously used to clear the mouse brain 1, and an advanced optical microscopy, two-photon fluorescence microscope (TPFM), was developed. With this approach it was possible to perform a three-dimensional (3D) cytoarchitecture analysis of the myofibril orientation, with a micron-scale resolution, on a subset of human ventricular strips previously used for mechanical and energetical experiments. In the second project, action potential propagation of entire hearts was investigated from a transgenic mouse model of HCM with a mutation on the gene coding for cardiac Troponin T (cTnT). The wide field system used to perform this investigation has already been utilized to study electrical activity 2. Subsequently, on the same hearts employed for functional studies, clearing techniques were applied to make the tissue transparent and to homogenize the refractive index 3. This isnecessary to label entire samples with fluorescent proteins, used to visualize cardiac fiber structure, and to execute high resolution imaging with light-sheet microscopy for large volume acquisitions. Finally, cytoarchitecture analysis based on cardiomyocytes and myofilaments alignment was applied to correlate electro-mechanical dysfunction with structural alterations. These innovative experimental approaches will allow to dissect the morphological causes leading to alterations of electrical conduction and to electro-mechanical dysfunction, and, more generally, will represent a whole new paradigm for diagnostic and therapeutic investigations.