Mémoire (M.A.) - Université du Québec à Trois-Rivières, 2008. / Comprend des réf. bibliogr. (f. 191-197).

Les réfractaires électrofondus, qui constituent l’objet de ce travail, appartiennent au système alumine-zircone-silice. Ils sont obtenus par coulée dans des moules à des températures supérieures à 2000°C, rendant très difficile toute instrumentation. De nombreux phénomènes intrinsèques au matériau interviennent lors du refroidissement qui suit la coulée. Parmi ces derniers, cette recherche a essentiellement porté sur la transformation de phase (de tétragonale à monoclinique) de la zircone et aux phénomènes associés (gonflement, plasticité,…).A partir d’essais mécaniques à haute température réalisés en laboratoire, les lois de comportements thermiques et mécaniques ont été caractérisées et modélisées en cours de transformation de la zircone. La plasticité à très bas seuil de contrainte observée a, en particulier, été décrite par une vitesse de déformation dérivée du modèle de Leblond, une fonction de rendement de type Cam-clay sans consolidation et une fonction de rendement définissant l’avancement de la transformation en fonction de la température. Après implémentation dans un code de calcul par éléments finis et validation par confrontation avec des résultats d’essais sous contraintes multiaxiales, ce modèle a été assemblé aux autres composantes du comportement mécanique (fluage, élasticité,…), pour décrire l’ensemble des phénomènes thermomécaniques observés lors du refroidissement.Parallèlement, des coulées de blocs en laboratoire, instrumentées par des thermocouples et des capteurs d’émission acoustique, ont permis de reconstruire par simulation numérique l’évolution du champ de température à l’intérieur de la dalle au cours du refroidissement. L’enthalpie de solidification et celle associée à la transformation de phase, préalablement quantifiée par ATD, ont été prises en compte. L’application du modèle mécanique complet, associant toutes les composantes du comportement, a permis de calculer l’évolution du champ de contraintes généré par les gradients thermiques en fonction du temps et, en particulier, de mettre en évidence le rôle essentiel joué par la transformation de phase sur la relaxation des contraintes. / Fused-cast refractories, which are concerned by this work, belong to the alumina-zirconia-silica system. They are obtained by casting in molds at temperatures higher than 2000°C, that make very difficult any instrumentation. Many phenomena intrinsic to the material occur during cooling-down after casting. Among these latter, this research essentially focused on the phase transformation (from tetragonal to monoclinic) of zirconia and the associated phenomena (swelling, plasticity,...).From high temperature mechanical tests performed in laboratory, the thermal and mechanical behavior laws were characterized and modeled during the zirconia transformation. Plasticity at very low stress threshold was observed. A Leblond type model has been extended by introducing a Cam-clay yield function without consolidation. In this model, the progress of the transformation is controlled by the evolution of the temperature. This model was complemented by other components of the mechanical behavior (creep, elasticity, ...). It has been validated by experimental tests under multiaxial loadings that replicate the main thermomechanical phenomena observed during cooling.In parallel, blocks casted in laboratory conditions, instrumented with thermocouples and acoustic emission sensors, allowed a numerical simulation of the change in temperature field within the block during cooling-down. This simulation took into account the solidification enthalpy and the enthalpy associated to the phase transformation, previously quantified by DTA. The implementation of the complete mechanical model integrating all the behavior components led to a calculation of the stress field changes generated by thermal gradients as a function of time and, in particular, to highlight the essential role played by the phase transformation on stress relaxation.

This thesis examines the relation between classical and quantum mechanics from philosophical, mathematical and physical standpoints. It first presents arguments in support of "conjectural realism" in scientific theories distinguished by explicit contextual structure and empirical testability; and it analyses intertheoretic reduction in terms of weakly equivalent theories over a domain of applicability. Familiar formulations of classical and quantum mechanics are shown to follow from a general theory of mechanics based on pure states with an intrinsic probability structure. This theory is developed to the stage where theorems from quantum logic enable expression of the state geometry in Hilbert space. Quantum and classical mechanics are then elaborated and applied to subsystems and the measurement process. Consideration is also given to spacetime geometry and the constraints this places on the dynamics. Physics and Mathematics, it is argued, are growing apart; the inadequate treatment of approximations in general and localization in quantum mechanics in particular are seen as contributing factors. In the description of systems, the link between localization and lack of knowledge shows that quantum mechanics should reflect the domain of applicability. Restricting the class of states provides a means of achieving this goal. Localisation is then shown to have a mathematical expression in terms of compactness, which in tum is applied to yield a topological theory of bound and scattering states: Finally, the thesis questions the validity of "classical limits" and "quantisations" in intertheoretic reduction, and demonstrates that a widely accepted classical limit does not constitute a proof of reduction. It proposes a procedure for determining whether classical and quantum mechanics are weakly equivalent over a domain of applicability, and concludes that, in this restricted sense, classical mechanics reduces to quantum mechanics.