The emerging field of nanomechanics is providing a new focus in the study of the mechanics of materials, particularly in simulating fundamental atomic mechanisms involved in the initiation and evolution of damage. Simulating fundamental material processes using first principles in physics strongly motivates the formulation of computational multiscale methods to link macroscopic failure to the underlying atomic processes from which all material behavior originates. A combined concurrent and sequential multiscale methodology is developed to analyze fracture mechanisms across length scales. Unique characterizations of grain boundary fracture mechanisms in an aluminum material system are performed at the atomic level using molecular dynamics simulation and are mapped into cohesive zone models for continuum modeling within a finite element framework. Fracture along grain boundaries typically exhibit a dependence of crack tip processes (i.e. void nucleation in brittle cleavage or dislocation emission in ductile blunting) on the direction of propagation due to slip plane orientation in adjacent grains. A new method of concurrently coupling molecular dynamics and finite element analysis frameworks is formulated to minimize the overall computational requirements in simulating atomistically large material regions. A sequential multiscale approach is advanced to model microscale polycrystal domains in which atomistically-based cohesive zone parameters are incorporated into special directional decohesion finite elements that automatically apply appropriate ductile or brittle cohesive properties depending on the direction of crack propagation. The developed multiscale analysis methodology is illustrated through a parametric study of grain boundary fracture in three-dimensional aluminum microstructures. / Ph. D.

<p> The room-temperature fracture behaviour of calcia partially stabilized zirconia (PSZ) was investigated. Fracture energy measurements were made using the standard stress intensity calibration and work to fracture techniques. The detailed nature of the PSZ microstructure was studied using scanning electron microscopy, qualitative X-ray analysis and T.E.M, surface replication. The grain structure was detenninod to be bimodal with small grains of pure zirconia dispersed along the boundaries of large grains. These large grains consist of a binary pure-zirconia/stabilized zirconia mixture. An attempt was made to relate the fracture properties to the nature of the inherent flaws present in the material. </p> <p> The strength of calcia partially stabilised zirconia was observed to depend on the size and distribution of the grain boundary precipitate of pure zirconia. It is postulated that this grain boundary precipitate causes decohesion and weakening of some of the grain boundaries due to the large internal stresses associated with its martensitic phase transformation. This phenomena of grain boundary decohesion leads to elastic nonlinearity and hysteresis. Crack propagation was always observed to proceed in a slow controlled fashion in this material. A model is proposed to explain theses observations based on the formation of a microcrack zone at the tip of a propagation crack. The occurrence of continued stable crack propagation is believed associated with increasing microcrack zone size with increasing crack length. Evidence supporting this model is presented. </p> / Thesis / Master of Science (MSc)

Ces travaux concernent l’étude numérique et expérimentale d’un panneau composite autoraidi fabriqué par le procédé d’infusion de résine (Liquid Resin Infusion LRI). Le procédé LRI permet d’intégrer des structures sur les peaux d’un panneau représentatif d’un fuselage composite. Dans l’étude numérique, des modèles éléments finis sont réalisés, pour étudier le comportement global du panneau en post-flambement. Cela permet de mettre au point un dispositif d’essai. L’approche expérimentale consiste en l’application de différentes méthodes pour contrôler la pièce et réaliser l’essai. Des essais de caractérisation sont aussi réalisés pour obtenir les propriétés mécaniques nécessaires à l’élaboration de modèles numériques locaux, permettant de décrire la décohésion des structures intégrées. / These works are related to the numerical and experimental study of a composite stiffened panel, which is manufactured by a resin infusion process (Liquid Resin Infusion LRI). This manufacturing process allows structures to be integrated onto the skins of a panel being representative of a composite fuselage. Finite element models are built along with the numerical study, in order to deal with the post-buckling global behaviour of this panel. This leads to perfect a test set-up addressed during the experimental investigation. Several experimental methods are used to check the test panel and achieve the test. Material properties are also determined through material testing intended for the development of local numerical models, describing the integrated structures decohesion.

Actuellement, l’industrie automobile est à la recherche d’une meilleure solution pour l’allégement de la structure de véhicule afin de diminuer la consommation de carburant et par conséquent diminuer les émissions nocives de CO2. Les aciers à très haute résistance (THR) mécanique permettent d’obtenir les tôles d’acier à section diminué avec les mêmes ou meilleurs propriétés fonctionnels. Les aciers Dual-Phase (DP), constitués majoritairement d’une phase ductile, la ferrite, et d’une phase dure, la martensite, occupent une place importante en tant que matériaux de structure destinés au challenge préoccupant l’industrie automobile. Une bonne résistance à l’endommagement est exigée pour leur utilisation en tant que des pièces de structures et de renfort pour l’automobile. Il a été bien établi que la résistance à l’endommagement des ces aciers Dual-Phase est contrôlée par leur microstructure. Ce travail de thèse s’est inscrit dans une logique de compréhension des mécanismes d’endommagement d’un acier Dual-Phase modèle, le DP 780, en fonction de différents paramètres microstructuraux. Deux mécanismes d’endommagement ont été identifiés pour l’acier DP 780 : la décohésion de l’interface ferrite/martensite et la formation de cavités autour des carbures, dans la martensite revenue. Un modèle qualitatif de mécanisme d’endommagement a été développé afin de pouvoir prédire l’endommagement de l’acier DP 780. Ce modèle qualitatif, développé pour l’acier DP 780, servira de base d’approfondissement de modèles plus élaborés et quantitatifs permettant la compréhension et la prédiction de l’endommagement des aciers Dual-Phase, de façon générale / In the automotive industry current environmental concerns require that the vehicle fuel consumption and CO2 emissions should be reduced as much as possible. It is therefore advantageous to reduce the weight of body in white components by replacing existing parts with higher strength, thinner gauge alternatives with equivalent or improved functional properties. Dual Phase (DP) steels are a class of high-strength low-alloy steels characterized by a microstructure consisting of martensite and ferrite. Dual Phase steels combine high strength levels with good ductility. Thus, DP steels are potentially very attractive for the automobile industry. In addition to the required high strength and ductility, DP steel has to be cold formed into complex shapes. It appears that DP steel damage behaviour is very complex and cannot be predicted using existing models based on standard mechanical properties. This work is concerned with the study of microstructural evolution and investigation of the relation between the microstructure and damage mechanisms in a reference DP 780 steel. Two damage mechanisms have been identified in this DP steel: ferrite/martensite interface decohesion and void formation at tempered carbides. A simple modeling for qualitative description of the observed damage formation mechanisms is proposed. This modeling permits a basic understanding of the experimentally observed trends and could be used as the starting point for a more detailed analysis in future