Teeth trauma has become one of the most serious physical problems that people are suffering from in the past years. Early diagnosis and management are desperately needed to improve tooth survival, functionality and avoid the tooth loss. However, the diagnosis of cracked tooth could be challenging due to the small size of the crack. Therefore, a mechanics understanding of the tooth fracture is demanding from the perspective of developing a framework for failure prediction in clinical research and bio-mimetic restorative materials. This study focuses on characterising the anisotropic fracture behaviour and the crack shielding mechanisms in elephant dentin. This is often used as a structural analogue for human dentin due to the similarities in microstructure and chemical composition, in order to avoid the test-piece size restrictions, given the larger size of ivory than human teeth. Compact tension test-pieces were extracted from different locations on the ivory tusk so as to have different crack growth directions relative to the microstructure to inspect the fracture anisotropies. The fracture toughness as a function of the crack extension was assessed in terms of fracture resistance curves (R-curve). The accumulative crack-tip strain fields were also measured for the first time in dentin using digital image correlation technique (DIC) to investigate the capability of crack-tip elastic/plastic deformation before material failure. Investigation of crack morphologies, the interaction between crack and the microstructures, the fracture surfaces using both 2-D and 3-D techniques could provide with insights into extrinsic shielding mechanisms. Surface and volume crack opening displacement (COD) were measured for the first time optically and by X-ray computed tomography to investigate the effect of extrinsic crack-tip shielding. The displacement fields around the crack-tip obtained by DIC were fitted using Westergaard’s analytical solution to extract the effective stress intensity factor, by comparing this to the applied load, the efficiency of the crack-tip shielding could be evaluated. A novel cohesive element model (traction-separation law) was then established based on the COD results to simulate the physical process of crack-tip shielding. It is the first time the cohesive model has been adapted to studying the direct crack behaviour measured by in-situ experiment to predict the crack growth. This model was then validated using the crack-tip strain field and R-curve obtained from the experiment measurement.

L'objectif de ce travail est la mise en oeuvre de la mécanique de la rupture et d'un modèle d'endommagement afin de vérifier l'intégrité du réservoir cryotechnique d'ARIANE 5 en présence de défauts. Ces derniers sont généralement assimilés à des fissures surfaciques semi-elliptiques. L'approche globale basée sur l'analyse FAD fournit une prédiction trop conservative lorsque cette méthode utilise le facteur d'intensité K et la ténacité KIc. Ce conservatisme peut toutefois être partiellement levé à condition d'introduire la notion d'intégrale J et d'utiliser une ténacité dérivée de la courbe JR - ∆a afin d'autoriser une légère avancée de la fissure dans le cas d'un matériau ductile. Cette courbe JR - ∆a s'obtient à partir d'essais sur des éprouvettes fissurées de différentes géométries (CT, SENT et SCT). L'approche locale de la rupture constitue cependant la seule alternative possible afin de statuer sur l'acceptabilité d'un défaut suffisamment important pour générer une plasticité étendue. Cette approche nécessite alors de décrire précisément les champs de contrainte et déformation en pointe de fissure. Par conséquent il est nécessaire de tenir compte du caractère anisotrope de la plasticité, lié au procédé de laminage, lors de la modélisation du comportement élasto-plastique del'alliage d'aluminium 2219 T87. Dans cette étude un critère phénoménologique est utilisé pour modéliser la plasticité dont l'anisotropie est décrite par différents coefficients. L'optimisation de ces paramètres repose sur la comparaison entre les résultats issus de simulations numériques et ceux obtenus par des essais sur des éprouvettes lisses et entaillées. L'approche local de la rupture repose également sur la compréhension des micromécanismes de rupture. L'alliage d'aluminium 2219 T87 étudié dans le cadre de cette thèse présenteune rupture ductile dont la phase de germination s'effectue sur les inclusions Al2Cu et constitue le mécanisme prépondérant de la rupture. L'endommagement est modélisé par le modèle de Gurson modifié par Needleman et Tvergaard (GTN). L'anisotopie de rupture est simulée par deux lois de germination différentes selon le sens de laminage et le sens transverse. / The purpose of this work is to use fracture mechanics and damage model to assess the structural integrity of the ARIANE 5 tank containing defects. These flaws are usually assumed to be semi-elliptical surface cracks.This situation can be analysed using the FAD (Failure Assessment Diagram) approach. This method is based on the stress intensity factor K and a material fracture toughness (Kmat) which can be the plane strain fracture toughness (KIC) for a conservative prediction. A more realistic prediction is achieved by employing the integral J and a material parameter derived from crack resistance curve JR - ∆a . This curve is obtained by using cracked test samples with different geometries (CT, SENT and SCT). The local approach to fractureis an alternative method to assess failure in particular in the case of large scale plasticity. In this case, the previous method is not able to give accurate predictions. Analyses based on local approach concepts requirean accurate evaluation of strain and stress fields near the crack tip. It is therefore important to take plastic anisotropy into account to model the plastic behavior of the rolled sheets. In this work this is done using amacroscopic phenomenological model. The parameters of this model are determined by minimizing of the difference between the simulation results and the test measurements for smooth and notched tensile test samples. The local approach to fracture is also based on the understanding of the failure micromechanismsof materials. Ductile fracture of metallic materials can be described as a three stage process. The first is void initiation at inclusions, the second is void growth and the third is void coalescence. Nucleation of defectsat Al2Cu inclusions is the main damage mechanism. In this work, the proposed model for ductile fracture is based on the Gurson model modified by Needleman and Tvergaard (GTN). This model is extended to account for fracture anisotropic by considering two nucleation gaussian laws respectively for rolling andtransverse directions.