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In the range of linear elastic fracture mechanics, the conditions for critical loading assessment of structures made of two dissimilar materials are usually based on the assumptions of prevailing normal mode of loading. The same assumption is usually applied to the stability criterion of bi-material notches. However, in engineering practice there are cases of loading and failure closed to the shear mode. The aim of the work is the assumption of the stability criterion of bi-material notches subjected to combination of normal and shear loading. The FEM model of the bi-material notch under combination of normal and shear loading mode is created and solved and evaluated as well.

The first part of this master's thesis deals with the high cycle fatigue of materials, especially on growing cracks using linear elastic fracture mechanics. Much of this work is focused on the concept of stress intensity factor. This concept is nowadays one of the most widely used concepts for describing a body with crack. The first part ends with theoretical approaches to determine the residual fatigue life of the body with a crack. The second part of this master's thesis is focused on the determination of residual fatigue life of a specified railway wheelset. An existence of crack-like defect is assumed at the railway wheelset. The goal of this master's thesis is to estimate how long it will take to grow from initial defect to a critical crack length. The last part of this master's thesis is devoted to addiction order load cycles on crack growth rate.

This Ph.D. thesis deals with modeling of compound structures containing defects such as cracks or notches. Attention was mainly focused on structures manufactured out of metals and polymers. Presented results were obtained from experimental measurements, which were compared with theoretical estimations of the ultimate states of the compound structures. Theoretical estimations were obtained by the application of the generalized fracture mechanics, whose integral part is based on numerical solution provided by finite element method. The results of the Ph.D. thesis contribute both to prediction of the limit states of compound objects, and to verification of the validity of the generalized fracture mechanics.

The presented thesis is devoted to the experimental and numerical analysis of concrete fracture under the mixed-mode I/II load. This phenomenon was analysed on various concrete grades and types which are used in the fabrication of precast concrete structural elements. Subsequently, the Brazilian disc test with central specimen was used in experimental and numerical parts. The numerical part employs both linear elastic fracture mechanics (LEFM) approach and non-linear material model to assess the concrete fracture and failure under the mixed mode I/II load. The LEFM part is dedicated to evaluation the geometry functions and higher order terms of the Williams’ expansion, while the non-linear analysis is dedicated to crack initiation and propagation throughout the specimen using the concrete damage plasticity model. The experimental part is dedicated to the analysis of the mixed mode-mode I/II fracture resistance by the generalised tangential stress (GMTS) criterion with focus set on the governing role of the critical distance rC. Furthermore, the experimental part validates the applicability of the Williams’ expansion on the concrete. For this, experimentally measured displacements by digital image correlation technique were used to calculate the Williams’s expansion terms. Lastly, the thesis deals with the influence of the aggressive environment on the material’s fracture toughness and on the fracture resistance under the mixed mode I/II has been studied.

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Hydrogen embrittlement can manifest itself as hydride formation in structures when in contact with hydrogen-rich environments, e.g. in space and nuclear power applications. To supplant experimentation, modeling of such phenomena is beneficial to make life prediction reduce cost and increase the understanding. In the present work, two different approaches based on phase field theory are employed to study the precipitation kinetics of a second phase in a metal, with a special focus on the application of hydride formation in hexagonal close-packed metals. For both presented models, a single component of the non-conserved order parameter is utilized to represent the microstructural evolution. Throughout the modelling the total free energy of the system is minimized through the time-dependent Ginzburg-Landau equation, which includes a sixth order Landau potential in the first model, whereas one of fourth order is used for the second model. The first model implicitly incorporates the stress field emanating from a sharp crack through the usage of linear elastic fracture mechanics and the governing equation is solved numerically for both isotropic and anisotropic bodies by usage of the finite volume method. The second model is applied to plate and notched cantilever geometries, and it includes an anisotropic expansion of the hydrides that is caused by the hydride precipitation. For this approach, the mechanical and phase transformation aspects are coupled and solved simultaneously for an isotropic material using the finite element method. Depending on the Landau potential coefficients and the crack-induced hydrostatic stress, for the first model the second-phase is found to form in a confined region around the crack tip or in the whole material depending on the material properties. From the pilot results obtained with the second model, it is shown that the applied stress and considered anisotropic swelling induces hydride formation in preferential directions and it is localized in high stress concentration areas. The results successfully demonstrate the ability of both approaches to model second-phase formation kinetics that is triggered by flaw-induced stresses and their capability to reproduce experimentally observed hydride characteristics such as precipitation location, shape and direction. / <p>Note: The papers are not included in the fulltext online.</p><p>Paper I and II in thesis as manuscripts.</p>

On s'intéresse plus particulièrement à la modélisation du comportement d'un matériau hétérogène méso-fissuré (béton, roche,...), soumis à une sollicitation thermo-hydro-mécanique avec prise en compte du couplage géométrique. Pour conduire cette étude, on s'appuie notamment sur les approches micro-mécaniques du comportement des milieux méso-fissurés non saturés développées depuis quelques années au Laboratoire des Matériaux et des Structures du Génie Civil - Ur Navier - Université Paris Est. Le milieu fissuré non saturé traité ici est constitué d'une matrice solide homogène élastique linéaire et de fissures connectées saturées par deux fluides immiscibles : un liquide et un gaz séparés par une surface capillaire. La fissure est traditionnellement considérée comme une cavité ellipsoïdale (cas 3D) ou elliptique (cas 2D) dont le rapport d'aspect tend vers zéro. Deux morphologies typiques de matériau sont considérés dans ce travail : la situation où les fissures sont toutes orientées dans la même direction et la situation où les fissures possèdent des orientations aléatoires. Dans une première étape, on rappelle brièvement les résultats disponibles concernant la modélisation des fissures non saturées par des cavités ellipsoïdales aplaties. A la fin de cette première partie, on complète les résultats déjà disponibles en étudiant l'influence de l'histoire de chargement sur la réponse de matériau. Dans une deuxième étape, on s'attache à valider une partie des résultats obtenus en utilisant une description des efforts capillaires dans les fissures par une précontrainte homogène en seréférant aux solutions analytiques exactes disponibles dans la littérature permettant de décrire le comportement d'une fissure isolée au sein d'une matrice élastique. Dans une troisième étape, on s'intéresse aux phénomènes de propagation des fissures en condition non saturée. Les lois de propagation sous critique et le phénomène de branchement des fissures sont également prises en compte dans cette approche. La dernière partie de la thèse concerne l'influence de la température sur le comportement des milieux poreux non saturés / The main topic of my work is the development of a micromechanical model for the behaviour of unsaturated mesocracks in media (concrete, rock...) in which the thermo-hydro-mechanical loadingsand thermo-hydro-mechanical couplings are taken into account. For this, we used the micromechanical approach model of behaviour of cracked porous media recently developed at LMSGC. My thesis is focused on the equilibrium configurations of a porous material whose pore space is saturated by a vapour and a liquid phase. The behaviour of an elastic medium containing unsaturated mesocracks is studied in the framework of a micromechanical approach. The cracks are filled by two immiscible fluids, namely a liquid and a gas, separated by a capillary interface. Furthermore, it is assumed that the set of cracks constitutes a connected network ; the capillary pressure is uniform over a representative elementary volume. The cracks are modelled as flat oblate spheroid cavities. Several geometrical configurations of cracks in porous media are considered in the framework of Eshelby-based homogenization methods (parallel cracks, randomly oriented cracks). First, a previously developed model showed that when coupling between the deformation of the cracks and the capillary forces is taken into account, there is no more a one-to-one relationship between the loading parameters and the state-variables. Thus, we describe the loading history prescribed to the material in order to compute its response. Second, we validate these results referring to the exact solutions available in the literature to describe the behaviour of a unsaturated crack within an elastic matrix. Third, the description of crack propagation in unsaturated media is considered in the framework of linear elastic fracture mechanics. The phenomenon of subcritical crack growth due to stress corrosion cracking is taken into account in this approach. Mixed mode fracture in the plane is also examined. Finally, we are interested in the influence of the temperature on the behavior of unsaturated porous media in the framework of the micromechanical approach