abstract: In material science, microstructure plays a key role in determining properties, which further determine utility of the material. However, effectively measuring microstructure evolution in real time remains an challenge. To date, a wide range of advanced experimental techniques have been developed and applied to characterize material microstructure and structural evolution on different length and time scales. Most of these methods can only resolve 2D structural features within a narrow range of length scale and for a single or a series of snapshots. The currently available 3D microstructure characterization techniques are usually destructive and require slicing and polishing the samples each time a picture is taken. Simulation methods, on the other hand, are cheap, sample-free and versatile without the special necessity of taking care of the physical limitations, such as extreme temperature or pressure, which are prominent issues for experimental methods. Yet the majority of simulation methods are limited to specific circumstances, for example, first principle computation can only handle several thousands of atoms, molecular dynamics can only efficiently simulate a few seconds of evolution of a system with several millions particles, and finite element method can only be used in continuous medium, etc. Such limitations make these individual methods far from satisfaction to simulate macroscopic processes that a material sample undergoes up to experimental level accuracy. Therefore, it is highly desirable to develop a framework that integrate different simulation schemes from various scales to model complicated microstructure evolution and corresponding properties. Guided by such an objective, we have made our efforts towards incorporating a collection of simulation methods, including finite element method (FEM), cellular automata (CA), kinetic Monte Carlo (kMC), stochastic reconstruction method, Discrete Element Method (DEM), etc, to generate an integrated computational material engineering platform (ICMEP), which could enable us to effectively model microstructure evolution and use the simulated microstructure to do subsequent performance analysis. In this thesis, we will introduce some cases of building coupled modeling schemes and present the preliminary results in solid-state sintering. For example, we use coupled DEM and kinetic Monte Carlo method to simulate solid state sintering, and use coupled FEM and cellular automata method to model microstrucutre evolution during selective laser sintering of titanium alloy. Current results indicate that joining models from different length and time scales is fruitful in terms of understanding and describing microstructure evolution of a macroscopic physical process from various perspectives. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2018

A demanda por estruturas mais leves implica num ganho em economia, porém o aumento de esbeltez da estrutura pode tornar ela susceptível a instabilidade frente a tensões compressivas estáticas ou dinâmicas. A instabilidade acontece em várias escalas da estrutura analisada e pode interagir com outras formas de colapso como a propagação instável de fissuras, problema governado pela mecânica da fratura, pela plastificacão do material, ou por uma combinação dos efeitos citados. Neste contexto, no presente trabalho, se explora a capacidade do método dos elementos discretizados por barras (LDEM) na simulação de problemas de instabilidade estática e dinâmica devido as tensões de compressão. Este método permite simular o sólido como um arranjo de barras com rigidez equivalente ao contínuo que se quer representar. Leis constitutivas não lineares permitem modelar ruptura de forma simples. A equação de movimento resultante da discretização permite formular uma equação de movimento desacoplada que pode ser integrada no domínio do tempo com um método explícito (Método das Diferencias Finitas Centrais). O fato das barras serem rotuladas nos seus extremos e a solução do problema ser obtida de forma incremental permite capturar problemas com não linearidade geométrica, entre eles a instabilidade estrutural frente a tensões compressivas. Como último exemplo se realiza a análise de um painel sanduiche por flexão em três pontos, que é composto por um núcleo de poliuretano, com duas lâminas externas de material compósito, neste caso a instabilidade estrutural está associada a flambagem da camada da lâmina comprimida. Finalmente a potencialidade da metodologia de análise utilizada é discutida. / The demand for lighter structures implies a gain in economy, but the increase in slenderness of the structure may make it susceptible to instability against static or dynamic compressive stresses. Instability occurs at various scales of the analyzed structure and may interact with other forms of collapse such as unstable crack propagation, problem governed by fracture mechanics, plastification of the material, or a combination of the cited effects. In this context, in the present work, we explore the ability of the discrete elements methods by bars (LDEM) in the simulation of problems of static and dynamic instability due to the compression stresses. This method allows to simulate the solid as an arrangement of bars with rigidity equivalent to the continuum that one wants to represent. Constitutive non-linear laws allow simple modeling of rupture. The equation of motion resulting from the discretization allows us to formulate a decoupled motion equation that can be integrated in the time domain with an explicit method (Central Finite Differences Method). The fact that the bars are labeled at their ends and the solution of the problem is obtained in an incremental way allows to capture problems with geometric non-linearity, among them the structural instability against compressive tensions. The last example, the analysis of a sandwich panel by three-point bending, which is composed of a polyurethane core, with two external blades of composite material, in this case the structural instability is associated with buckling of the layer of the compressed blade . Finally, the potential of the analysis methodology is discussed.

Les travaux de cette thèse concernent le développement d’un modèle par éléments discrets du comportement thermo-mécanique d’un alliage d’aluminium et de son évolution microstructurale lorsqu’il est soumis à des conditions de grandes déformations et/ou de grandes vitesses de déformation (soudage par friction-malaxageou FSW). Le procédé de « soudage par friction malaxage » (Friction Stir Welding,FSW) est un procédé de soudage récent, inventé en 1991 par « The Welding Institute (TWI) ». Ce procédé se distingue des autres par sa capacité à souder la matière à l’état visqueux, sans atteindre le point de fusion. Le principe est relativement simple et comparable à une opération de fraisage, si ce n’est que l’outil coupant est remplacé par un outil malaxant composé d’un pion et d’un épaulement. Ce procédé revêt un fort intérêt pour l’industrie aéronautique, car il offre la possibilité de souder des alliages d’aluminium des séries 2XXX et 7XXX, réputés difficiles à souder par d’autres procédés.Le soudage par friction-malaxage étant un procédé récent, il demeure encore un sujet de recherche actif, pour mieux appréhender certains points comme l’écoulement de la matière, l’influence des paramètres du procédé (vitesse de rotation, forme de l’outil, ...)et la modification de la microstructure. Ce dernier point est particulièrement important pour les alliages à durcissement structural comme ceux des séries 2XXX et 7XXX. Pour ces alliages d’aluminium, les propriétés mécaniques dépendent étroitement de leur état de précipitation, lui-même influencé par l’apport de chaleur généré lors du soudage.L’optimisation des paramètres de soudage par rapport aux propriétés mécaniques passe obligatoirement par la prédiction de l’état de la précipitation le long du joint soudé. Le travail de la thèse vise à mettre en place des modèles à l’échelle de la microstructure permettant de prévoir la modification des propriétés de la matière liées aux paramètres du procédé. / The work of this thesis concerns the development of a model using the discrete element method, for the thermo-mechanical behavior of an aluminum alloy and its microstructural evolution when exposed to conditions of large deformations and/or high deformation rates (Friction StirWelding or FSW). Friction stir welding is a recent welding process invented in 1991 by "The welding Institute" (TWI). This process is different from the others by its ability to weld material in the viscous state, without reaching the melting point. The principle is relatively simple and similar to a milling process except that the cutting tool is replaced by a rotary tool composed of a pin and a shoulder. This process is of great interest in the aircraft industry because it offers the possibility to weld aluminum alloys 2XXX and 7XXX series, known to be difficult to weld by other processes. Since friction stir welding is a recent process, it still remains a subject of active research, to better understand certain issues such as material flow, influence of process parameters (rotation speed, tool shape, ...) and the modification of the microstructure. This last point is particularly important for structural hardening alloys such as those of the 2XXX and 7XXX series. For these aluminum alloys, the mechanical properties are highly dependent on their state of precipitation, which is itself influenced by the heat generated during thewelding. The optimization ofwelding parameters in order to obtain good mechanical properties requires the prediction of precipitation state along thewelded joint. Thework of this thesis is to develop required models at the scale of microstructure allowing to predict the modification of material properties related to the process parameters

O processo de dano em materiais quasi-frágeis pode ser caracterizado pela perda de isotropia para certos níveis de carga. A localização de deformações, o efeito cooperativo entre regiões danificadas e a avalanche de rupturas são características particulares na medição do dano neste tipo de material. As características mencionadas criam diferentes formas de dissipação de energia, que não são fáceis de representar utilizando métodos baseados na hipótese dos meios contínuos. No presente trabalho uma versão do Método dos Elementos Discretos Formado por Barras é empregado. Neste método a massa do contínuo é concentrada nos nós, os quais são interconectados por barras sem massa. Essas barras possuem uma lei constitutiva bilinear, que é usada para simular a ruptura da estrutura em estudo. A distribuição dos nós permite formar uma treliça tridimensional regular, e a partir dessa discretização espacial é possível chegar a um sistema de equações de movimento, que é resolvido com um esquema explícito de integração numérica (diferenças finitas centrais). Neste método a fratura e a fragmentação são levadas em conta de forma natural, já que as barras que rompem durante o processo são desativadas, respeitando o balanço energético. É possível introduzir heterogeneidade no modelo considerando as propriedades do material como campos espaciais aleatórios com distribuição de probabilidades de Weibull e comprimento de correlação conhecido. Nessa dissertação, é analisado o processo de dano que aparece em estruturas de geometria simples quando solicitadas até o colapso. Diferentes índices são apresentados para realizar a medição do dano. O desempenho desses índices, e a maneira com que eles ajudam na interpretação da evolução do dano, são discutidos nesse trabalho. / The process of damage in quasi-fragile materials is characterized by loss of isotropy for certain load levels. The strain localization, the cooperative effect between damaged regions and the avalanche of ruptures are particular features in measuring the damage in this kind of material. The mentioned features create different forms of energy dissipation, which are not easy to represent with a continuous approach. In the present work a version of the Lattice Discrete Element Method is employed. In this method the mass of the solid is concentrated on node points, which are interconnected by uniaxial elements. These elements have a bilinear constitutive law, which is used to simulate the rupture of the structure under study. The node distribution allows the formation of a regular three-dimensional lattice, and from this spatial discretization it is possible to arrive at a system of equations of motion, which is solved by an explicit numerical integration scheme (central difference). In this method the fracture and fragmentation are taken into account in a natural manner, since the bars that reached their limit strength during the process are disabled of the system, respecting the energy balance. It is possible to introduce heterogeneity in the model considering the material properties as random fields with spatial Weibull probability distribution and known correlation length. In this dissertation, the damage process, which appears in structures of simple geometry, when they are loaded until collapse, is analysed. Different indexes are presented to perform the measurement of the damage. The performance of those indexes, and the way they help in the interpretation of the damage evolution, are discussed in this paper.

O processo de dano em materiais quasi-frágeis pode ser caracterizado pela perda de isotropia para certos níveis de carga. A localização de deformações, o efeito cooperativo entre regiões danificadas e a avalanche de rupturas são características particulares na medição do dano neste tipo de material. As características mencionadas criam diferentes formas de dissipação de energia, que não são fáceis de representar utilizando métodos baseados na hipótese dos meios contínuos. No presente trabalho uma versão do Método dos Elementos Discretos Formado por Barras é empregado. Neste método a massa do contínuo é concentrada nos nós, os quais são interconectados por barras sem massa. Essas barras possuem uma lei constitutiva bilinear, que é usada para simular a ruptura da estrutura em estudo. A distribuição dos nós permite formar uma treliça tridimensional regular, e a partir dessa discretização espacial é possível chegar a um sistema de equações de movimento, que é resolvido com um esquema explícito de integração numérica (diferenças finitas centrais). Neste método a fratura e a fragmentação são levadas em conta de forma natural, já que as barras que rompem durante o processo são desativadas, respeitando o balanço energético. É possível introduzir heterogeneidade no modelo considerando as propriedades do material como campos espaciais aleatórios com distribuição de probabilidades de Weibull e comprimento de correlação conhecido. Nessa dissertação, é analisado o processo de dano que aparece em estruturas de geometria simples quando solicitadas até o colapso. Diferentes índices são apresentados para realizar a medição do dano. O desempenho desses índices, e a maneira com que eles ajudam na interpretação da evolução do dano, são discutidos nesse trabalho. / The process of damage in quasi-fragile materials is characterized by loss of isotropy for certain load levels. The strain localization, the cooperative effect between damaged regions and the avalanche of ruptures are particular features in measuring the damage in this kind of material. The mentioned features create different forms of energy dissipation, which are not easy to represent with a continuous approach. In the present work a version of the Lattice Discrete Element Method is employed. In this method the mass of the solid is concentrated on node points, which are interconnected by uniaxial elements. These elements have a bilinear constitutive law, which is used to simulate the rupture of the structure under study. The node distribution allows the formation of a regular three-dimensional lattice, and from this spatial discretization it is possible to arrive at a system of equations of motion, which is solved by an explicit numerical integration scheme (central difference). In this method the fracture and fragmentation are taken into account in a natural manner, since the bars that reached their limit strength during the process are disabled of the system, respecting the energy balance. It is possible to introduce heterogeneity in the model considering the material properties as random fields with spatial Weibull probability distribution and known correlation length. In this dissertation, the damage process, which appears in structures of simple geometry, when they are loaded until collapse, is analysed. Different indexes are presented to perform the measurement of the damage. The performance of those indexes, and the way they help in the interpretation of the damage evolution, are discussed in this paper.

Dans la fabrication de lentilles asphériques pour la photolithographie, l’étape delissage est critique. C’est aujourd’hui le seul procédé qui peut corriger les défauts de hautesfréquences spatiales responsables de diffusions parasites, de diminutions de transmittance etde contraste. Cette opération doit préserver la forme asphérique basse fréquence tout enlissant les défauts de hautes fréquences. Un tel comportement peut être obtenu pour des outilscombinant une couche continue flexible pour le suivi basse fréquence et une couche de poixfractionnée pour le lissage de défauts hautes fréquences. Les buts principaux de cette étudesont de prédire l’efficacité de lissage et le suivi de forme de différents outils et ensuite dedéterminer l’outil permettant le meilleur compromis. A cette fin, un modèle multi-échelles estdéveloppé. A l’échelle de l’outil entier, une étude paramétrique par éléments finis permet dedéterminer les caractéristiques de la couche flexible ainsi que la force appliquée optimale afind’obtenir l’homogénéité de la pression à l’échelle de la forme asphérique globale. A l’échelle dela couche de poix, la Méthode par Eléments Discrets est utilisée pour investiguer l’interfaceoutil-pièce. Un modèle basé sur le concept de la poutre cohésive viscoélastique est développé,prenant en compte la réponse fréquentielle de la couche de poix. La comparaison avecl’Analyse Mécanique Dynamique montre la capacité de la DEM à modéliser le comportementviscoélastique. L’opération de lissage est ensuite modélisée par DEM et analytiquement. Lesdonnées expérimentales obtenues par la méthode de la Densité Spectrale de Puissancemettent en évidence l’impact des propriétés de la poix sur l’efficacité de lissage. Lacomparaison entre les résultats numériques et expérimentaux montre que le modèle DEMdonne des résultats prometteurs pour la modélisation du lissage de défauts. / In aspherical photolithography optics manufacturing, the smoothing step iscritical. So far, it is the only process that can correct high spatial frequency defects, responsiblefor flare and transmission, contrast decrease. This operation must preserve the low frequencyaspherical shape while smoothing high frequency defects. That behavior can be obtained withtools that combine a continuous flexible layer for low frequency compliance and a fractionatepitch layer for high frequency defect polishing. The main goals of this study are predicting thesmoothing efficiency and form control of different tools, and then determining the best tool toachieve a good balance between them. To do this, a multi-scale model is developed. First, atthe whole tool scale, a finite-element parametric study yields the best characteristics for theflexible layer as well as the optimal applied force to achieve pressure homogeneity at the globalaspherical shape level. Second, at the pitch layer level, the Discrete Element Method is used toinvestigate the tool-workpiece interface. A model based on the viscoelastic cohesive beamconcept is developed, taking into account the pitch layer’s frequency response. Comparisonwith Dynamic Mechanical Analysis shows the ability of DEM to model viscoelastic behavior. Thesmoothing operation is then modeled both by DEM and analytically. Experimental data obtainedby the Power Spectral Density Method highlight the impact of pitch properties on the smoothingefficiency. Comparison between numerical and experimental data shows that the DEM modelyields promising results in defect smoothing modeling

No description available.

Doctor of Philosophy / Department of Grain Science and Industry / R. P. Kingsly Ambrose / Praveen V. Vadlani / Wheat flour processing involves gradual size reduction and size-based fractionation of milled components. The size-based separation efficiency of wheat flour particles, with minimum bran contamination, is an important flour mill operational parameter. The flour particles often behave as imperfect solids with discontinuous flow and agglomerates during the separation process due to their differences in physical and chemical characteristics. Noticeable loss in throughput has been observed during sieving of soft wheat flour compared to that of hard wheat flour due to differences in inter-particle cohesion. However, there is limited understanding on the factors that influence the inter-particulate forces. Direct and indirect methods were applied to investigate the effects of moisture content, particle size, sifter load, and chemical composition on the cohesion behavior of flours from different wheat classes. Image analysis approach was used to quantify the particle characteristics such as surface lipid content, roughness, and morphology with respect to particle size to better understand the differences between hard and soft wheat flours. Surface lipid content and roughness values showed that the soft wheat flours are more cohesive than hard wheat flours. The morphology values revealed the irregularity in flour particles, irrespective of wheat class and particle size, due to nonuniform fragmentation of endosperm particles. The chemical composition significantly contributes to the differences in cohesion and flowability of wheat flours. Based on the particle parameters, a granular bond number (GBN) model was developed to predict the dynamic flow of wheat flour. In order to further understand the wheat flour flow behavior during size-based separation, a correlation was developed using the discrete element method (DEM). The error of predictions demonstrated that this correlation can be used to estimate the sieving performance and sieve blinding phenomenon of wheat flour. The experimental results from this dissertation work and the numerical model could eventually be instrumental to improve the efficiency of size-based separation of flour from various wheat classes. In addition, the models developed in this study will contribute significantly to understand the inter-particle cohesion as influenced by chemical composition.

This study aims to deduce an appropriate shape and density for an electronic free-cell that could be placed into a silo so that position and other desired physical parameters could be recorded. To determine how density and shape affects the trajectory and displacement of free cells, the trajectory and displacement of cylindrical, cuboid and triangular prism free-cells of equivalent volume was investigated in a discharging quasi 3D silo slice. The free-cells were placed at twelve different starting positions spread evenly over one half of the 3D slice. Tests were conducted using a monosized batch of spherical particles with a diameter of approximately 5 mm. Tests were also conducted in a binary mixture consisting of particles of different sizes (5 mm/4 mm) and the same density (1.28 g/cm3) and a binary mixture consisting of particles of different size (6 mm/5 mm) and different densities (1.16 g/cm3/1.28 g/cm3).The rotation of the free cells was also briefly discussed.Computer simulations were conducted using the Discrete Element Method (DEM). The simulation employed the spring-slider-dashpot contact model to represent the normal and tangential force components and the modified Euler integration scheme was applied to calculate the particle velocities and positions at each time step. One trial of each of the metal and plastic, cylindrical, cuboid and triangular prism free cells was compared with the average of three experimental trials. The trajectory and displacement of a representative particle positioned at the same starting position as the free cell was also obtained from DEM simulation and compared with the path and displacement of each of the free cells to determine which free cell followed the particle most closely and hence to determine a suitable free cell that would move with the rest of the grains. Spherical particles are idealised particles. Therefore tests were also conducted with a small number of polyhedral particles, to deduce their flow rate and the critical orifice width at which blockages were likely to form. Simulations were also conducted to test the feasibility of the DEM in modelling the behaviour of these polyhedral particles.Results indicate that for a free cell to move along the same trajectory and have the same displacement and velocity as an equivalent particle in the batch it should have a similar density to the majority of the other particles. A cylindrical free cell of similar density to the particles was found to follow the path of the representative particle more closely than the cuboid or triangular prism. Polyhedral particles were found to have a greater flow rate than spherical particles of equivalent volume.

The goal of the project was to characterize the granular flows impact on the gas flow in a packed-bed-reactor. The study was created at Swerim as a master's thesis for Luleå University of Technology. The packed-bed-reactor geometry used in this study is a scaled down blast furnace model. The granular flow was modelled using the discrete element method (DEM) in LS-DYNA. Four models were created with different sizes and size distribution of the particles. To study the granular flows impact on gas flow, porosity is extracted from the DEM models and analyzed, since porosity has a direct impact on the gas flow. The supervisors form Swerim, Joakim Eck and Martin Flemström created computational fluid dynamics (CFD) models in Ansys Fluent using the porosity from the DEM models. The DEM results are presented as granular flow profiles. This flow profile is created by injecting particles with alternating colors to see this profile. A total of 6 images are taken over the whole process. The porosity results are presented as a porosity field plots of the extracted porosity data using MATLAB. The CFD results are presented as plots of gas velocity and absolute pressure. The results show the different characteristics of the flow in the different DEM models, and how it relates to the different porosity fields that were found. Furthermore, the CFD models show how the flow of the gas is dependent on the porosity.