• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 3
  • 2
  • 2
  • Tagged with
  • 13
  • 13
  • 5
  • 4
  • 3
  • 3
  • 3
  • 3
  • 2
  • 2
  • 2
  • 2
  • 2
  • 2
  • 2
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Crystal plasticity modeling of structural magnesium alloys under various stress states

Stinson, Joel H 09 August 2008 (has links)
In this work, a crystal elasto-viscoplastic model was modified to account for the anisotropic mechanical response of magnesium aluminum alloys. Crystal plasticity may offer new understanding of these alloys by explicitly modeling the texture development that profoundly affects the properties of magnesium. The model is able to account for the individual slip systems of both the cubic and hexagonal phases. The constants of the model were determined from published experimental AZ31 data, and the plastic hardening response is shown to match these results well using a modification to the hardening rule to approximate the kinetics of twinning. Model aggregates were created with aluminum compositions representative of common magnesium structural alloys. This approach allows the effect of varying percentage of cubic phase on the hexagonal magnesium alloy aggregate to be studied both in terms of macroscopic response and the crystallographic changes occurring within the system.

Surfacelet-based heterogeneous materials modeling

Huang, Wei 27 August 2014 (has links)
The application of heterogeneous materials has become common in modern product design such as composites and porous media. Computational design tools for such materials, with higher complexity than the traditional homogeneous ones, will be a critical component in the realization of the heterogeneity systematically. It is foreseen that computer-aided design (CAD) systems will include computer-aided materials design modules in future so that the design of functional materials and structures can be integrated for optimal product design. The traditional CAD systems model three-dimensional (3D) geometry at macro-scales with boundary representation (B-Rep), whereas computer-aided materials design is concerned with the specification of material composition at scales ranging from nano-, meso-, to micro-. Thus, multi-scale CAD systems are desirable for the integration of product and materials information. The existing B-Rep based modeling scheme needs to be extended to incorporate heterogeneous material compositions. The new modeling scheme should also support seamless zoom-in and zoom-out operations in multi-scale CAD systems. Recently, a multi-scale model, dual-Rep, was proposed to represent geometry and material property distribution implicitly. The core part of dual-Rep is a new basis function called surfacelet. Surfacelet is able to represent boundary information more efficiently than the traditional wavelets, while keeping a unified form with wavelets so that the role exchange of boundary and internal structures during zooming operations is enabled. A surfacelet transform is able to represent microstructure distributions in 3D images with surfacelet coefficients. In this dissertation, three enabling techniques for surfacelet-based heterogeneous materials modeling are developed. First, a method of inverse surfacelet transform is developed such that the original images can be reconstructed from the surfacelet coefficients. The surface integrals of voxel (i.e., volumetric pixel) values are obtained from the surfacelet coefficients using the one-dimensional inverse wavelet transform. The images are then reconstructed by solving linear equations from discretized surface integrals. The prior knowledge of material properties and distributions is applied to solve the under-constrained problems. Second, composite surfacelets with the combinations of different types of primitive surfacelets are created to increase the flexibility of the surfacelet transform with potentially fewer surfacelets and improved reconstruction accuracy. Third, a multi-scale materials modeling method is proposed to support interactive design and visualization of material microstructures at multiple levels of details. It has the capability to support seamless zoom-in and zoom-out. This method provides a feature-based design approach based on the surfacelet basis.

Texture and Microstructure in Two-Phase Titanium Alloys

Mandal, Sudipto 01 August 2017 (has links)
This work explores the processing-microstructure-property relationships in two-phase titanium alloys such as Ti-6Al-4V and Ti-5Al-5V-5Mo-3Cr that are used for aerospace applications. For this purpose, an Integrated Computational Materials Engineering approach is used. Microstructure and texture of titanium alloys are characterized using optical microscopy, electron backscatter diffraction and x-ray diffraction. To model their properties, threedimensional synthetic digital microstructures are generated based on experimental characterization data. An open source software package, DREAM.3D, is used to create heterogeneous two-phase microstructures that are statistically representative of two-phase titanium alloys. Both mean-field and full-field crystal plasticity models are used for simulating uniaxial compression at different loading conditions. A viscoplastic self-consistent model is used to match the stress-strain response of the Ti-5553 alloy based on uniaxial compression tests. A physically-based Mechanical Threshold Stress (MTS) model is designed to cover wide ranges of deformation conditions. Uncertainties in the parameters of the MTS model are quantified using canonical correlation analysis, a multivariate global sensitivity analysis technique. An elastoviscoplastic full-field model based on the fast Fourier transform algorithm was used to used to simulate the deformation response at both microscopic and continuum level. The probability distribution of stresses and strains for both the phases in the two-phase material is examined statistically. The effect of changing HCP phase volume fraction and morphology has been explored with the intent of explaining the ow softening behavior in titanium alloys.

Modeling of Semiconductors and Correlated Oxides with Point Defects by First Principles Methods

Wang, Hao 15 June 2014 (has links)
Point defects in silicon, vanadium dioxide, and doped ceria are investigated by density functional theory. Defects involving vacancies and interstitial oxygen and carbon in silicon are after formed in outer space and significantly affect device performances. The screened hybrid functional by Heyd-Scuseria-Ernzerhof is used to calculate formation energies, binding energies, and electronic structures of the defective systems because standard density functional theory underestimates the bang gap of silicon. The results indicate for the A-center a −2 charge state. Tin is proposed to be an effective dopant to suppress the formation of A-centers. For the total energy difference between the A- and B-type carbon related G-centers we find close agreement with the experiment. The results indicate that the C-type G-center is more stable than both the A- and B-types. The electronic structures of the monoclinic and rutile phases of vanadium dioxide are also studied using the Heyd-Scuseria-Ernzerhof functional. The ground states of the pure phases obtained by calculations including spin polarization disagree with the experimental observations that the monoclinic phase should not be magnetic, the rutile phase should be metallic, and the monoclinic phase should have a lower total energy than the rutile phase. By tuning the Hartree-Fock fraction α to 10% the agreement with experiments is improved in terms of band gaps and relative energies of the phases. A calculation scheme is proposed to simulate the relationship between the transition temperature of the metal-insulator transition and the dopant concentration in tungsten doped vanadium dioxide. We achieve good agreement with the experimental situation. 18.75% and 25% yttrium, lanthanum, praseodymium, samarium, and gadolinium doped ceria supercells generated by the special quasirandom structure approach are employed to investigate the impact of doping on the O diffusion. The experimental behavior of the conductivity for the different dopants is understood in terms of the calculated lattice constants and the O migration barriers obtained from nudged elastic band calculations.

First-Principles Studies of Energetic Materials

Conroy, Michael W 26 October 2007 (has links)
First-principles density functional theory calculations were performed on a number of important energetic molecular crystals, pentaerythritol tetranitrate (PETN), cyclotetramethylene tetranitramine (HMX), cyclotrimethylene trinitramine (RDX), and nitromethane. Simulations of hydrostatic and uniaxial compressions, as well as predictions of ground-state structures at ambient conditions, were performed using the DFT codes CASTEP and VASP. The first calculations done with CASTEP using GGA-PW yielded reasonable agreement with experiment for the calculated isothermal EOS for PETN-I from hydrostatic compression data, yet the EOS for β -HMX shows substantial deviation from experiment. Interesting anisotropic behavior of the shear-stress maxima were exhibited by both crystals upon uniaxial compression. It was predicted that the <100> direction, the least sensitive direction of PETN, has significantly different values for shear stress maxima τyx and τzx, in contrast to the more sensitive directions, <110> and <001>. In addition, non-monotonic dependence of one of the shear stresses as a function of strain was observed upon compression of PETN in the <100> direction. VASP calculations were later performed, and the results yielded good qualitative agreement with available experimental data for the calculated isothermal EOS and equilibrium structures for PETN-I, β-HMX, α-RDX, and nitromethane. Using VASP, uniaxial compression simulations were performed in the <100>, <010>, <001>, <110>, <101>, <011>, and <111> directions for all crystals up to the compression ratio V/V0 = 0.70. The VASP calculations of PETN reproduced the CASTEP results of significantly different values of τyx and τzx for the insensitive <100> compression, and relatively high and equal values of τyx and τzx for the sensitive <110> and <001> compressions. A correlation between this behavior of shear stress upon uniaxial compression and sensitivity was suggested, and predictions of anisotropic sensitivity of HMX, RDX, and nitromethane were made. Further analysis of the VASP results for PETN do not indicate a correlation between sensitivity and shear stress maxima as a function of longitudinal stress, where longitudinal stress is an appropriate experimental independent variable for comparison. The validity of a correlation between shear stress maxima and sensitivity requires further investigation. Further characterization of the anisotropic constitutive relationships in PETN was performed.

Estrutura eletrônica de derivados de politieno[3,4-b]-tiofeno-co-benzoditiofeno para aplicação em camadas ativas de células solares orgânicas / Electronic structure of derivatives politieno [3,4 -b ]-thiophene-co-benzoditiofeno for application layer of active solar cells organic

Roldao, Juan Carlos 03 March 2016 (has links)
Submitted by Juan Carlos Roldão null (36780115860) on 2016-04-28T18:17:40Z No. of bitstreams: 1 Dissertação-Versão-Final-Juan_Carlos_Roldao.pdf: 3885340 bytes, checksum: 0d245c89d075ddeea3f1fc7154e9c5a9 (MD5) / Approved for entry into archive by Felipe Augusto Arakaki (arakaki@reitoria.unesp.br) on 2016-05-02T13:34:47Z (GMT) No. of bitstreams: 1 roldao_jc_me_bauru.pdf: 3885340 bytes, checksum: 0d245c89d075ddeea3f1fc7154e9c5a9 (MD5) / Made available in DSpace on 2016-05-02T13:34:47Z (GMT). No. of bitstreams: 1 roldao_jc_me_bauru.pdf: 3885340 bytes, checksum: 0d245c89d075ddeea3f1fc7154e9c5a9 (MD5) Previous issue date: 2016-03-03 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Atualmente existe uma intensa busca por novos materiais com propriedades ajustadas para utilização em células solares orgânicas de modo a obter um aumento em sua eficiência de conversão para que possam substituir os dispositivos de silício. O politieno[3,4-b]-tiofeno-co-benzoditiofeno (PTB7) é um polímero recentemente proposto na literatura e com propriedades muito interessantes em células solares orgânicas, o que o coloca como uma possível alternativa ao amplamente utilizado poli(3-hexiltiofeno) (P3HT). Tem sido relatadas modificações em diferentes posições da unidade monomérica deste copolímero, tanto na estrutura benzoditiofeno (BDT), quanto na estrutura tienotiofeno (TT), que o compõe. Estas modificações levaram a novos polímeros com propriedades diferentes e por vezes mais interessantes que aquelas do PTB7 sem substituições. O trabalho que será apresentado visou estudar as propriedades estruturais, eletrônicas e ópticas do PTB7 e possíveis alterações ocorridas devido às modificações químicas realizadas na estrutura do BDT de suas unidades monoméricas. Tal estudo utilizou ferramentas de otimização de estruturas como Mecânica Molecular, Dinâmica Molecular e o método semi-empírico Parametric Method 6 (PM6), assim como de cálculo de estrutura eletrônica de materiais, como a Teoria do Funcional da densidade (DFT) e de cálculos de propriedades ópticas como a Teoria do Funcional da Densidade Dependente do Tempo (TD-DFT). Concluímos que o PTB7 no estado sólido pode ser considerado planar. Com o nosso modelo para o PTB7, obtivemos uma diferença de energia ∆EHL entre o Último Orbital Molecular Ocupado HOMO (do inglês Highest Occupied Molecular Orbital) e o Primeiro Orbital Molecular Desocupado LUMO (do inglês Lowest Unoccupied Molecular Orbital) de aproximadamente 1,84 eV, sendo que este valor está em boa concordância com o valor experimental. Em relação às substituições químicas, estudamos teoricamente 8 derivados do PTB7 e os resultados mostraram que é possível obter compostos com uma diminuição significativa do ∆EHL e também que é possível obter compostos com valores de energia do HOMO e do LUMO mais interessantes que os do PTB7 quando na camada ativa for empregado como material doador o fenil-C61-butírico ácido metil ester (PCBM). / Currently there is an intensive search for new materials with tuned properties for use in organic solar cells to obtain an increase in its conversion efficiency and replace silicon devices. The polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7) is a polymer recently proposed in the literature and with very interesting properties in organic solar cells, which places it as a possible alternative to the widely used poli(3-hexilthiophene) (P3HT). It has been reported changes in different positions of the monomeric unit of this copolymer, both in benzodithiophene (BDT) structure, as in the thienothiophene (TT) structure that compose it. These modifications led to new polymers with different properties and sometimes more interesting than those of PTB7 without substitutions. The work to be presented aimed to study the structural, electronic and optical properties of PTB7 and possible changes due to chemical changes made in the BDT structure of its monomeric units. This study employed optimization tools like Molecular Mechanics, Molecular Dynamics and Parametric Method 6 (PM6), as well as calculations of the electronic structures with the Density Functional Theory (DFT) method, and optical properties such as the Time Dependent Density Functional Theory (TD-DFT) calculations. We conclude that the PTB7 chains in the solid state can be considered planar. With our model for PTB7, we obtained a difference ΔEHL between the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO) of approximately 1.84 eV, and this value is in good agreement with the experimental value. Regarding chemical substitutions, we studied theoretically 8 derivatives of PTB7 and the results showed that it is possible to obtain compounds with a significant decrease in ΔEHL and that it is possible to obtain compounds with HOMO and LUMO energy values more adjusted to the widely employed acceptor material phenylC61-butyric acid methyl ester (PCBM).

Characterization and Modeling of the Martensite Transformation in Advanced High-Strength Steels

Cluff, Stephen Roy 09 December 2019 (has links)
Multiple studies on the microstructures of advanced high-strength steels are presented here that seek to add to the already substantial body of knowledge on martensite in steel. These studies seek to gain additional insight into the role that the martensite transformation has on the observed mechanical properties of modern steels. Crystallographic Reconstruction of Parent Austenite Twin Boundaries in a Lath Martensitic Steel The study of post-transformation microstructures and their properties can be greatly enhanced by studying their dependence on the grain boundary content of parent microstructures. Recent work has extended the crystallographic reconstruction of parent austenite in steels to include the reconstruction of special boundaries, such as annealing twins. These reconstructions present unique challenges, as twinned austenite grains share a subset of possible daughter variant orientations. This gives rise to regions of ambiguity in a reconstruction. A technique for the reconstruction of twin boundaries is presented here that is capable of reconstructing 60 degree twins, even in the case where twin regions are comprised entirely of variants that are common between the twin and the parent. This technique is demonstrated in the reconstruction of lath martensitic steels. The reconstruction method utilizes a delayed decision-making approach, where a chosen orientation relationship is used to define all possible groupings of daughter grains into possible parents before divisive decisions are made. These overlapping, inclusive groupings (called clusters) are compared to each other individually using their calculated parent austenite orientations and the topographical nature of the overlapping region. These comparisons are used to uncover possible locations of twin boundaries present in the parent austenite. This technique can be applied to future studies on the dependence of post-transformation microstructures on the special grain boundary content of parent microstructures. Coupling Kinetic Monte Carlo and Implicit Finite Element Methods for Predicting the Strain Path Sensitivity of the Mechanically Induced Martensite Transformation The kinetic Monte Carlo method is coupled with a finite-element solver to simulate the nucleation of martensite inside the retained austenite regions of a TRIP (transformation induced plasticity) assisted steel. Nucleation kinetics are expressed as a function of load path and kinematic coupling between retained austenite regions. The model for martensite nucleation incorporates known elements of the kinetics and crystallography of martensite. The dependence of martensite transformation on load path is simulated and compared to published experimental results. The differences in transformation rates of retained austenite are shown to depend on load path through the Magee effect. The effects of average nearest neighbor distance between austenite grains is shown to affect the rate at which martensite nucleates differently depending on load path. Ductility and Strain Localization of Advanced High-Strength Steel in the Presence of a Sheared Edge The localization of strain in the microstructures of DP 980 and TBF 980 is quantified and compared. Of particular interest is the difference in final elongation observed for both materials in the presence of a sheared edge. Scanning electron micrographs of etched microstructures near the sheared edge are gathered for both materials at varying amounts of macroscopic strain. These micrographs are used to generate strain maps using digital image correlation. A two point statistical measure for strain localization is developed that utilizes strain map data to quantify the degree to which strain localizes around the hard phase of both materials. The DP steel exhibits higher strain localization around the martensite phase. Reasons for differences in strain localization and shear banding between the two materials are suggested, and the role played by the mechanically induced martensite transformation is speculated.

Development of an automated characterization-representation framework for the modeling of polycrystalline materials in 3D

Groeber, Michael Anthony 30 August 2007 (has links)
No description available.

Mesoscale Modeling of Shape Memory Alloys by Kinetic Monte Carlo–Finite Element Analysis Methods

Herron, Adam David 01 April 2019 (has links)
A coupled kinetic Monte Carlo – Finite Element Analysis (kMC–FEA) method is developed with a numerical implementation in the Scalable Implementation of Finite Elements at NASA (ScIFEN). This method is presented as a mesoscale model for Shape Memory Alloy (SMA) material systems. The model is based on Transition State Theory and predicts the nonlinear mechanical behavior of the 1st order solid–solid phase transformation between Austenite and Martensite in SMAs. The kMC–FEA modeling method presented in this work builds upon the work of Chen and Schuh [1, 2]. It represents a “bottom-up” approach to materials modeling and could serve as a bridge for future studies that attempt to link ab initio methods with phenomenological findings in SMA systems. This thesis presents the derivation of the kMC–FEA model, which is then used to probe the various responses expected in SMAs and verify the influence of model parameters on simulation behavior. In a departure from the work of Chen and Schuh, the thermodynamic derivation includes an elastic transformation energy term, which is found to be a significant fraction of the total transformation energy and play an important role in the evolution of a simulation. Theoretical predictions of the model behavior can be made from this derivation, including expected transformation stresses and temperatures. A convergence study is presented as verification that the new elastic energy term proposed in this model is a reasonable approximation. A parameter sensitivity study is also presented, showing good agreement between theoretical predictions and the results of a full-factorial numerical exploration of model outputs. Model simulation demonstrates the emergence of the shape memory effect, an important SMA behavior not shown by Chen and Schuh, along with the expected superelastic effect and thermal hysteresis. Further exploration of simulated model outputs presented in this work involves comparison with experimental data and predicted output values obtained from a separate phenomenological constitutive model. This comparison shows that the kMC–FEA method is capable of reproducing qualitative, but not yet quantitative, responses of real SMA material systems. Discussion of each model parameter and its effects on the behavior of the model are presented as guidelines for future studies of SMA materials. A complete implementation of the method is contained in a new finite element software package (ScIFEN) that is available for future

FEM Study of Metal Sheets with a Texture based, Local Description of the Yield Locus

Duchêne, Laurent 10 November 2003 (has links)
Résumé de louvrage La thèse déposée par L. Duchêne comporte cinq chapitres dont une introduction et des conclusions et perspectives et totalise 183 pages. La bibliographie compte 94 références. Louvrage a pour objet la modélisation du comportement mécanique des tôles métalliques (principalement des tôles dacier obtenues par laminage) lors des processus de mise à forme. Lanisotropie de la tôle découle de la mesure de la texture du matériau. Les processus de mise à forme étudiés sont principalement lemboutissage des tôles. Le mémoire propose le développement de deux méthodes locales dinterpolation de la surface de plasticité. Ensuite, ces modèles sont implémentés dans le code éléments finis LAGAMINE, puis validés sur quelques exemples académiques et finalement des applications industrielles sont examinées. Analyse du contenu Le premier chapitre introduit le sujet, définit le cadre dans lequel lauteur situe sa démarche et présente les originalités du travail. Le deuxième chapitre est consacré à la description du comportement microscopique des métaux et des méthodes de transition micro-macro. Le comportement dun monocristal ou plutôt le comportement dun cristal à lintérieur dun polycristal est examiné. Les modèles microscopiques de Taylor et de Bishop-Hill constituant une approche primal-dual pour ce problème sont décrits. Le comportement macroscopique du polycristal est obtenu à partir du comportement microscopique via la transition micro-macro sur base de la texture du matériau. Cette texture est généralement caractérisée par son ODF (Orientation Distribution Function). Cependant, le modèle de transition micro-macro utilisé, basé sur les hypothèses de Taylor, nécessite une description de la texture par un ensemble dorientations cristallines représentatif de la texture du matériau. Différentes méthodes dextraction de lensemble de cristaux représentatif sont décrites. Le nombre de cristaux inclus dans lensemble représentatif est un paramètre important et est discuté. Le comportement élastique et plastique du polycristal sont décrits. Outre le modèle de transition micro-macro basé sur les hypothèses de Taylor, dautres modèles plus récents et plus coûteux en temps de calcul sont décrits. Le deuxième chapitre introduit également lécrouissage du polycristal. Le troisième chapitre présente les étapes successives du développement de lapproche locale de la surface de plasticité. Etant donné que la surface de plasticité est définie dans lespace à 5 dimensions des contraintes déviatoriques, des notions de géométrie dans un espace à n dimensions sont présentées. Différentes propriétés des domaines dans lesquels la surface de plasticité est localement définie sont décrites ; ainsi que la méthode de construction des domaines et le lien entre domaines voisins. Deux méthodes dinterpolation à partir de points calculés via le modèle de Taylor de la surface de plasticité à lintérieur des domaines ont été développées et sont présentées. La méthode de hyperplans définit localement la surface de plasticité au moyen dhyperplans (des plans dans lespace à 5 dimensions des contraintes déviatoriques). La méthode dinterpolation directe contraintes-déformations permet une représentation plus précise et plus continue entre domaines voisins de la surface de plasticité. Etant donné son importance cruciale pour la convergence des simulations numériques utilisant la méthode des éléments finis, la normale à la surface de plasticité est soigneusement examinée pour les deux méthodes locales dinterpolation. Certains problèmes particuliers rencontrés lors du développement et liés à lapproche locale de la surface de plasticité sont présentés pour les deux méthodes dinterpolation. Quelques points particuliers liés à limplémentation de ces méthodes dans le code éléments finis LAGAMINE sont décrits. Le principal intérêt de lapproche locale de la surface de plasticité est son efficacité lors du calcul de lévolution de texture au cours des déformations plastiques. Un paragraphe est dès lors consacré aux détails de calcul des rotations des orientations cristallines et à limplémentation de lévolution de texture dans le code éléments finis. Le chapitre 3 se termine par un paragraphe qui analyse la précision, la robustesse et le gain en temps de calcul (par rapport à lutilisation directe du modèle de Taylor) de lapproche locale de la surface de plasticité. Le quatrième chapitre présente les validations de lapproche locale de la surface de plasticité. Trois validations académiques sont tout dabord effectuées : prédiction de lévolution de texture lors du laminage ; prédiction de leffet Swift en cours de torsion dun tube ou dun cylindre et comparaison du comportement en torsion et compression des métaux cubiques faces centrées et cubiques centrés isotropes. La première validation complexe est lemboutissage de godets circulaires à fond plat. Les résultats des simulations (distribution des déformations plastiques, évolution de la force poinçon en fonction du temps, prédiction des oreilles demboutissage et prédiction de la texture finale) sont comparés aux valeurs expérimentales. La géométrie du processus demboutissage, la mesure des valeurs expérimentales et les paramètres numériques utilisés pour les simulations sont largement détaillés. Linfluence de certains paramètres numériques sur les résultats des simulations est de plus analysée. Une seconde simulation demboutissage avec une autre géométrie et un matériau plus anisotrope est également étudiée. Les oreilles demboutissage sont particulièrement analysées ; le retour élastique dû à un retrait des outils est examiné. Le mémoire se termine par un cinquième chapitre qui présente des perspectives et des conclusions permettant à lauteur de synthétiser les apports de sa thèse et dindiquer quelles directions de recherches lui paraissent devoir être explorées dans le futur.

Page generated in 0.114 seconds