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  • 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.
271

Comportement mécanique d'un alliage d'aluminium à grains ultrafins. Analyse et modélisation du rôle exacerbé des joints de grains. / Mechanical Behaviour of Ultra fine grain aluminium alloy. Analysis and modelling of the enhanced role of grain boundaries

Goyal, Anchal 29 November 2018 (has links)
Les alliages à grains ultrafins semblent prometteurs, au vu de leur forte résistance en traction et de la possibilité d'une mise en forme superplastique à basse température. Toutefois, leurs mécanismes de déformation, qui comportent une part plus ou moins forte de glissement aux joints de grains restent mal connus, et leurs performances en fatigue ont été peu étudiées. Ce travail vise à comparer et analyser le comportement viscoplastique et les mécanismes de déformation et d'endommagement en traction et en fatigue d’un alliage d’aluminium-magnésium "classique" et à grains ultrafins (600nm en moyenne) obtenu par déformation plastique sévère, selon le procédé ECAP.Des essais de relaxation, fluage et traction à diverses vitesses et températures ont permis de mesurer les évolutions des sensibilités à la vitesse en fonction de ces deux paramètres et de montrer: 1) que le raffinement microstructural accroît sensiblement la sensibilité à la vitesse 2) que ce paramètre augmente avec la vitesse de déformation 3) qu'il contrôle la ductilité du matériau à grains ultrafins, qui s'accroît donc à faible vitesse 4) que cette ductilité devient supérieure à celle du matériau classique lorsque la température s'élève. Les domaines de vitesse et température dans lesquels le raffinement microstructural accroît ou diminue la résistance en traction ont été délimités.Les mécanismes de déformation et d'endommagement des deux matériaux ont été étudiés au moyen d'essais de traction sous MEB accompagnés de mesures des champs de déformation par corrélation d'images à plusieurs échelles: méso et microscopique, grâce à des microgrilles d'or et sub-micrométrique, grâce à un mouchetis très fin obtenu par démouillage d'un film d'or. Le glissement aux joints est d'autant plus actif, dans les deux matériaux, que la température augmente et que la vitesse de déformation diminue. Dans l'alliage à grains ultrafins, il a un caractère coopératif et survient préférentiellement aux joints de forte désorientation. Les champs de déformation sont plus hétérogènes dans le matériau à grains ultrafins, où le taux de déformation dépasse 100% dans des bandes localisées.Un modèle éléments finis 2D intégrant, outre la viscoplasticité au sein des grains, un glissement visqueux des joints, a été identifié dans toute la gamme de température explorée et rend assez bien compte du comportement viscoplastique des deux matériaux et de la contribution beaucoup plus forte du glissement aux joints dans l'alliage à grains ultrafins. Il permet également de préciser comment évolue cette contribution au cours de l'écrouissage.Des essais de traction-compression à déformation plastique imposée ont permis d'étudier la plasticité cyclique et les mécanismes d'endommagement en fatigue oligocyclique et des essais à contrainte imposée, d'explorer la fatigue à grand nombre de cycles. Les essais ont été suivis d'observations des surfaces de rupture et d'une analyse statistique de l'endommagement en surface, ainsi que d'observations au MET des arrangements de dislocations. Les deux matériaux manifestent un durcissement cyclique, plus modeste dans l'alliage à grains ultrafins, qui présente, à forte amplitude, une croissance de ses grains. L'écrouissage isotrope prédomine dans l'alliage classique, où la densité de dislocations augmente fortement avec la plasticité cyclique, alors que l'écrouissage cinématique prédomine dans l'alliage à grains ultrafins, en raison de sa moindre aptitude à stocker des dislocations et de la plus grande hétérogénéité de sa déformation plastique. A même amplitude plastique, ce dernier a une durée de vie plus faible, en raison d'un amorçage bien plus rapide des fissures, à partir de particules intermétalliques. A contrainte imposée, le matériau a grain ultrafins a une durée de vie légèrement supérieure, grâce à une propagation plus lente des microfissures, dont le trajet est transgranulaire dans les plus gros grains et intergranulaire dans les plus petits. / Ultrafine grained (UFG) alloys seem promising, based on their high tensile properties and the possibility of superplastic forming at relatively low temperature. However, their deformation mechanisms are not fully understood, and their performance in fatigue has not been thoroughly investigated. This work compares the viscoplastic behavior, and the deformation and damage mechanisms in tension and fatigue of a UFG Al-Mg alloy (600 nm mean grain size) obtained by severe plastic deformation (ECAP process) with that of its coarse-grained (CG) counterpart.The strain rate sensitivity (SRS) of both materials has been measured during creep, relaxation and tensile tests run at various strain rates and temperature. Microstructural refinement is shown to increase the SRS, which rises as the strain rate decreases, and controls the ductility. The UFG material becomes softer and more ductile than the CG material at high temperature. The temperature and strain rate domain for which the UFG alloy is stronger or softer has been determined.Tensile tests run in a SEM, with DIC measurements of strain fields at meso/ micro scales (using gold microgrids printed by electron beam lithography) and at sub-micron scale (using a superfine speckle obtained by film remodelling) have shown that grain boundary sliding is more and more active in both materials as the temperature rises and as the strain rate decreases. Grain boundary sliding is cooperative and occurs mostly at high-angle grain boundaries in the UFG alloy, where the strain field is more heterogeneous, and where very high strain levels (> 100%) are often observed in localized bands.A 2D finite element model taking into account the viscoplastic behaviour inside the grains, and viscous sliding at the grain boundaries has been identified other the whole temperature range investigated. It captures well the observed behaviours and the much larger contribution of grain boundary sliding in the UFG alloy. It also provides the evolution of this contribution during strain hardening.Plastic strain-controlled push-pull tests and stress-controlled push-pull tests were run to investigate the cyclic behaviour and damage mechanisms of the two materials in low and high-cycle fatigue. The tests were followed by fractographic observations, statistical analysis of surface damage, as well as TEM observations of dislocations arrangements. Both materials exhibit cyclic hardening, although it is more modest in the UFG alloy, in which grain growth occurs at high amplitude. While isotropic hardening predominates in the CG alloy where the density of dislocation strongly increases during cyclic tests, kinematic hardening predominates in the UFG alloy, because of its limited capacity to store dislocations and its more heterogeneous plastic deformation. For a given plastic strain range, the UFG alloy has a shorter fatigue life than its CG counterpart, because of a much easier crack initiation, mostly from intermetallic particles. For a given stress range, it has a slightly higher life, due to a slower development of microcracks, which have a transgranular path in the largest grains, with some intergranular growth within the smallest grains.
272

Designing meta-organisations : an empirical study of boundary setting in large infrastructure projects

Drews, Franziska January 2018 (has links)
This study examines the organisational architecture of megaproject meta-organisations; that is project-based organisations formed to deliver one-off, capital-intensive systems. It investigates how the organisation that promotes the megaproject - the buyer organisation - divides and allocates the scope of the development work during the delivery phase across multiple suppliers. In so doing, the buyer organisation sets organisational boundaries around its own work and that of each project supplier: effectively creating the megaproject meta- organisation architecture. We use organisation design literature as the main cognitive lens to understand the architecture of megaproject organisations. This literature posits that organisational boundaries can be understood by examining the interplay of four logics: i) Transaction Cost Economics (TCE), ii) capabilities, iii) power and iv) organisational identity. The impact of these four logics on organisational boundaries has been theorised extensively in the context of the enduring firm. Here, we seek to extend our knowledge of boundaries in megaproject organisations. Unlike the enduring firm, megaproject organisations do not operate in efficient markets, are set up to have a finite-lifespan and are highly interdependent with their environment. Megaproject organisations are also a critical form of organising addressing one of the grand challenges of our time: the provision of basic infrastructure. Yet, little is known about the architecture of megaproject organisations. To address this theoretical and empirical gap, we undertook a multiple case research. We conducted an in-depth analysis of the procurement choices for four large infrastructure assets: the London Olympics 2012, Crossrail, Thames Tideway Tunnel and Heathrow's Terminal 2. The research results in the development of an original conceptual framework that illuminates how the four complementary logics are brought to bear in the organisational design choices that determine megaproject architectures. The study contends that the organisational boundaries, which demarcate the work of each supplier, are the outcome of a reconciliation of efficiency concerns and considerations about the capabilities available in-house and in the supplier market. Importantly, this reconciliation is context-sensitive. Hence, the solution space for potential organisational architectures is constrained by considerations related to power and organisational identity. Power constraints are rooted in the interdependency of the buyer with its environment, including regulatory-political and ownership relations, as well as its bargaining position vis a vis suppliers. Organisational identity constraints relate to the buyer organisation's identity, which is both pre- given and developed through self-selection over time.
273

Transborder: a cross border learning place.

January 2003 (has links)
See Siu Ching. / "Architecture Department, Chinese University of Hong Kong, Master of Architecture Programme 2002-2003, design report." / Includes bibliographical references. / Chapter 01 --- THE PHENOMENON / Chapter 02 --- THE DESIRE TO CROSS OVER / Chapter 03 --- THE DESIRE TO LEARN / Chapter 04 --- TRANSBORDER
274

Dominant singularity and finite element analyses of plane-strain stress fields in creeping alloys with sliding grain boun[d]aries

Lau, Chun Woon January 1982 (has links)
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Bibliography: leaves 156-163. / by Chun Woon Lau. / Ph.D.
275

Seleção de fronteiras para análise de ciclo de vida de sistemas que emitem poluentes tóxicos de chaminés / Boundary selection for LCA of systems with toxic stack emissions

Stelvia Vigolvino Matos 17 September 2002 (has links)
Este trabalho desenvolve um método de seleção de fronteiras para Análise de Ciclo de Vida (ACV) de sistemas que emitem componentes tóxicos. Isto envolve o desenvolvimento de um modelo de predição de concentração e dose de poluentes emitidos de chaminés, que tem a vantagem de ser simples e não requerer grande número de dados de entrada. Esse modelo e mais os dados de valoração econômica de danos ambientais disponíveis na literatura, compõem o modelo geral de estimativa de custos devido à emissão de poluentes, ou seja, a Análise de Custos em Ciclo de Vida (ACCV). O modelo geral é então usado para definir as fronteiras do sistema de ACCV. Demonstra-se a aplicação do novo método em um ciclo de vida hipotético de produção de celulose. / This work develops a method for Boundary Selection (BS) for Life Cycle Assessment (LCA) of systems with stacks emitting toxics compounds. The new approach involves the development of a model of concentrations and dosage of pollutants in the vicinity of emission source that is simple and non-intensive data is required. This model plus impact costs estimations available at environmental economic studies, comprise the general model of Life Cycle Cost Analysis (LCCA) based on exposure. This general model is then used in the Boundary Selection in LCCA. A demonstration of the application of the model is performed to a hypothetical inventory system of a pulp mill.
276

A mutabilidade das normas constitucionais e os limites de atuação do juiz. / Mutability of constitucional norms and boundaries of role of the judge

Silva, Ricson Moreira Coelho da 12 May 2014 (has links)
Este trabalho é uma análise crítica do tema da mutação constitucional. Buscouse a partir da revisitação de temas como o constitucionalismo e do conceito de Constituição, construir uma dogmática da mutabilidade das normas constitucionais e assim procedendo, verificar empiricamente, com base em precedentes da jurisprudência do Supremo Tribunal Federal, o acerto desta construção. Uma vez estabelecidas as bases para a verificação do fenômeno da mutação constitucional, foi possível parametrizar os limites possíveis a esta hipótese de atualização das normas constitucionais. Pois bem, nesses termos, fixado o conceito, verificado a sua aplicabilidade na jurisprudência, bem como estabelecidos os limites de sua utilização, foi possível ainda a partir da análise do contexto atual em que se insere o instituto, vislumbrar as hipóteses de sua aplicação futura. Em suma, trata-se de um trabalho no qual se tenta compreender o papel da jurisdição constitucional com respeito ao tema de mutabilidade das normas constitucionais, sobretudo diante da realidade pós-moderna circundante, que pela sua complexidade, influencia, sensivelmente, a normatividade dos preceitos legais e, notadamente, dos constitucionais, objeto principal do estudo. / This work is a critical analysis of the issue of constitutional change. We sought from revisiting issues such as constitutionalism and the concept of constitution building a dogmatic mutability of constitutional norms and in so doing, to verify empirically, based on the precedents jurisprudence of the Supreme Court, the correctness of this construction. Once established the basis for the verification of the phenomenon of constitutional change, it was possible to parameterize the possible limits to this hypothesis update of constitutional norms . Well , in these terms , the concept set, verifying their applicability in case law as well as established the limits of their use, it was still possible from the analysis of the current context in which it appears the institute, envision the chances of its future application. In short, it is a work in which one tries to understand the role of constitutional jurisdiction with respect to the theme of mutability of constitutional norms, particularly in the surrounding postmodern reality, which by their complexity, significantly influences the normativity of legal precepts and especially, constitutional, the main object of study.
277

Non-Schmid Effects and Criteria for Dislocation Nucleation on Different Slip Systems at Grain Boundaries

Wyman, Richard Durtschi 01 June 2016 (has links)
Criteria for grain boundary dislocation nucleation are developed. A bicrystal containing two grain boundaries is placed under varying triaxial stress states using molecular dynamics. The local resolved shear, normal, and co-slip stresses needed for grain boundary dislocation nucleation are found. A framework is developed to detect the slip system grain boundary dislocation nucleation occurs on. A survey of the different ways grain boundary dislocation nucleation occurs in the sample shows a single grain boundary can nucleate dislocations in a rich variety of ways. Using the nucleation system and resolved stress values, criteria for grain boundary dislocation nucleation on different slip systems are developed. The proposed form of nucleation criterion suggests the activation stress has a linear dependence one the resolved shear, normal, and co-slip stresses. A residual analysis largely validates the efficacy of the proposed linear model. We show that the nucleation slip system cannot be predicted by a maximum Schmid factor analysis due to the non-Schmid resolved normal and co-slip terms. We show that a system's global pressure generally fails to predict nucleation; a local stress in the grain being nucleated into should be used. Using the nucleation criteria for each slip system, a yield surface for dislocation nucleation is built for the grain boundary used in this work.
278

Geospatial boundary dynamics

White, Emily 01 December 2016 (has links)
This dissertation investigates three topics related to movement and events associated with geospatial barriers. This research makes a unique contribution to geographic information science (GIScience), by examining how boundaries influence the interactions of other geographic features and to location-based services by bringing a GIScience perspective to geofence services. Chapters 2, 3, and 4 can be considered independently as journal articles. Each of these chapters builds on the previous. Chapter 2 serves as the base by examining geospatial barriers and impedance events associated with these barriers. The relationships between barriers, the features they protect and the features that they hinder are formalized in an ontology design pattern (ODP) that also includes events associated with these features. Algorithms are written to demonstrate the ODP can be used to answer queries about the presence or absence of barrier events. Chapter 3 transitions from physical boundaries to digital boundaries by examining geofences, a location-based notification service. Interaction between users and multiple geofences are investigated when the configuration of geofences differs (i.e. disconnected, partially overlapping, and completely overlapping) and when geofences are independent or dependent (a rule states that the activation of geofence Y depends on users having visited geofence X). A geofence system is prototyped in an iOS environment and used to further evaluate user-geofence interactions. Chapter 4 expands on the topic of location- based notification regions by investigating what is required for a geofence system to handle events. How changes in conditions (e.g. the spreading of a flood or movement of a fire) interact with geofences and users and what new categories of interactions are required to handle these changes are discussed. The proposed system is applied to a scenario of flooding on multiple rivers interrupting the transportation network.
279

Perméabilité des frontières vies « personnelle et professionnelle » et usage des TIC : modèles d’articulation / Permeability of professional and personal life borders and ICT use : articulation models

El Wafi, Wafa 10 December 2016 (has links)
Plusieurs études ont montré que l’usage croissant des Technologies de l’Information et de la Communication (TIC) dans la vie professionnelle et personnelle a largement contribué au brouillage des frontières entre ces deux sphères. La perméabilité reflète le degré auquel l’individu peut être psychologiquement et/ou comportementalement engagé dans un domaine, mais physiquement et temporellement présent dans un autre. Notre objectif est d’identifier les déterminants de la perméabilité des frontières ainsi que les différents modèles d’articulation entre ces deux domaines de vie, basés sur l’usage des TIC. La méthodologie utilisée est mixte, qualitative et quantitative. Les résultats de la première étude réalisée sur la base d’un questionnaire auprès 241 personnes, confirment, d’une part, que les frontières personnelles sont les plus perméables et qu’elles dépendent de plusieurs variables organisationnelles ainsi que des caractéristiques de travail, d’autre part, que la perméabilité des frontières professionnelles ne dépend que de la flexibilité du travail. Quatre modèles d’articulation ont été identifiés: la segmentation, le débordement, l’intégration et l’hybridation. La deuxième étude conduite selon la méthode des scénarios, a permis de restituer les modèles d’articulation dans une perspective dynamique et évolutive. Les individus changent de manières de gérer leurs frontières de vie, en fonction de leur âge, des changements de leur situation familiale et professionnelle et du climat organisationnel. L’ensemble des résultats sont discutés d’un point de vue théorique, méthodologique et pratique / Several studies has shown that the increased use of information and communications technology (ICT) in professional and personal life has largely contributed to the blurring of the boundaries between these two spheres. The resultant permeability reflects the degree to which the individual can be psychologically and / or behaviorally engaged in one area, but physically and temporally present in another. Our goal is to identify the determinants of porous personal and professional borders and the different models of articulation between these two areas of life, based on the use of information and communications technology. We used a mixed, qualitative and quantitative methodology. The results of the first study made by a questionnaire survey on 241 people confirm, firstly, that personal boundaries are more permeable and that they depend on several organizational variables and work characteristics, secondly, that the permeability of professional boundaries depends only on work flexibility. Four articulation models were identified: segmentation, spillover, integration and hybridization.The second study conducted according to the scenario method, set the articulation patterns in a dynamic and evolutionary perspective. Individuals change the ways of managing their life boundaries, depending on their age, the changes in their family and work situation and the organizational climate. All the results are discussed from a theoretical, methodological and practical point of view
280

Food: A Sensuous Matter of the Everyday : A sensorial exploration of material and bounded natures of mundane food practices

Linder, Elin January 2018 (has links)
This thesis examines everyday food practices such as sensed by seven households in the city ofStockholm, Sweden. By sensuously exploring the acts of acquiring, preparing, cooking, eating,and wasting food, I analyze how food is a matter of olfactory, gustatory, auditory, tactile, andvisual significance, as much as matter per se. More specifically, I address relational andbounded aspects of food, looking at how ambient surroundings, presences of material andimmaterial factors, sensuously influence everyday experiences of food. Intrigued by the at onceinter-, extra-, and re-corporeal matters of food, I analytically position myself alongside Latour,Ingold, Douglas, and Bennett. In entertaining their theoretical lines of thoughts, using them asanalytical springboards, this thesis explores socio-material dimensions of food practices, as wellas corporeal dynamics of human-material encounters. Methodologically carried out by meansof sensuous ethnography, following Pink’s notion of participatory practice, I have during tenweeks of fieldwork—in people’s homes and in their frequented grocery stores—engaged mysenses to experientially sense the world of food, such as lived by them. In our conjoint sensorialexploration, taken-for-granted mundane understandings of what food constitutes and whatconstitutes it, have emerged as domestically diverse, bounded to sensuous perceptionsderivative of the past, carried out in the presents, and cor(po)related to the future. By surveyingsituated meanings of what is smelled when savored, tasted when flavored, seen when looked,and felt when touched, simultaneously as accounting for nonhuman matters salient to coursesof actions, the thesis remarks context-sensorial-imbued figurations of everyday food.

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