Thin material layers have found various applications with various roles of functions, such as in fibre reinforced laminated composite materials, in integrated electronic circuits, in thermal barrier coating material system, and etc. Interface delamination is a major failure mode due to either residual stress or applied load, or both. Over the past several decades, extensive research works have been done on this subject; however, there are still uncertainties and unsolved problems. This thesis presents the new developed analytical studies on local delamination failure of thin material layers. Firstly, the analytical theories are developed for post-local buckling-driven delamination in bilayer composite beams. The total energy release rate (ERR) is obtained more accurately by including the axial strain energy contribution from the intact part of the beam and by developing a more accurate expression for the post-buckling mode shape. The total ERR is partitioned by using partition theories based on the Euler beam, Timoshenko beam and 2D-elasticity theories. By comparing with independent test results, it has been found that for macroscopic thin material layers the analytical partitions based on the Euler beam theory predicts the propagation behaviour very well and much better than the others. Secondly, a hypothesis is made that delamination can be driven by pockets of energy concentration (PECs) in the form of pockets of tensile stress and shear stress on and around the interface between a microscopic thin film and a thick substrate. Both straight-edged and circular-edged spallation are considered. The three mechanical models are established using mixed-mode partition theories based on classical plate theory, first-order shear-deformable plate theory and full 2D elasticity theory. Experimental results show that all three of the models predict the initiation of unstable growth and the size of spallation very well; however, only the 2D elasticity-based model predicts final kinking off well. Based on PECs theory, the room temperature spallation of α-alumina oxidation film is explained very well. This solved the problem which can not be explained by conventional buckling theory. Finally, the analytical models are also developed to predict the adhesion energy between multilayer graphene membranes and thick substrates. Experimental results show that the model based on 2D elasticity partition theory gives excellent predictions. It has been found that the sliding effect in multilayered graphene membranes leads to a decrease in adhesion toughness measurements when using the circular blister test.

The objective of this study is to evaluate the bond durability of an epoxy-based polymer concrete wearing surface bonded to aluminum bridge decks. In the bridge design, an aluminum alloy bridge deck is used with a polymer concrete wearing surface. A modified mixed mode flexure fracture test was developed to assess the bond durability of specimens aged in the following environmental conditionings: 30°C [86°F], 98% RH; 45°C [113°F], 98% RH; 60°C [140°F], 98% RH; freezing and thawing; salt (NaCl) water soak; and 60°C [140°F], dry. The exposure times varied from none to twelve months. The critical strain energy release rate (Gc) of the bond was determined using a compliance technique. In spite of considerable scatter in the data, the results suggested that the interfacial bond toughness had been degraded by exposure conditions. The aging appeared to affect the polymer concrete overlay (silica aggregates/epoxy bond) as well. Fracture analysis and finite element modeling were completed for linear elastic behavior. Analytical and numerical solutions were in reasonably good agreement. Characterization of the bridge components and failure specimens were accomplished using analytical measurements including thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). Techniques employed in the surface analysis included x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). / Master of Science

La taille de la zone plastique en pointe de fissure dans un matériau est directement liée au facteur d’intensité de contrainte pour une configuration donnée. Cette étude utilise la technique d’indentation avec une très faible charge afin d’étudier la taille de la zone plastique cyclique et monotone au voisinage de la fissure. La zone plastique est caractérisée par la relation entre la dureté, le module réduit, le travail élastique, le travail plastique, l’indice de plasticité et la distance par rapport à la pointe de fissure. Les essais sont menés sur une éprouvette CT pour un alliage d’aluminium 2024T351. Dans une étude complémentaire, le contour des zones plastiques en fond de fissure en mode mixte, le rayon minimum de zone plastique (MPZR) et la direction de la fissure initiale pour l’alliage d’aluminium 2024T351 sur un “Compact Tension spécimen” est déterminé en utilisant le logiciel Matlab. Nous avons calculé la forme, la taille de la zone plastique au fond de fissure, le rayon minimum de la zone plastique par rapport à l’angle de chargement et le facteur d’intensité de contrainte en utilisant le critère de Von Mises. L’étude est effectuée pour les divers angles de chargement. Nous avons constaté que le chargement en mode mixte (13 = 600) est le plus néfaste pour le matériau. / The plastic zone size associated with a propagating crack in a material is directly related to the stress intensity factor for a given configuration. This work utilizes the ultra-low-load indentation techniques to study the cyclic and monotonic plastic zone size at the fatigue crack tip based on the relationship between the hardness, elastic work, plastic work, plasticity index and the distance from the crack tip. The study is conducted essentially on the aluminium alloy 2024T351. In a second part of this work, we study the contour of mixed mode crack-tip plastic zones, the minimum plastic zone radius (MPZR) and the direction of initial crack for the aluminium alloy 2024T351 in a Compact tension specimen by using Matlab software. We have computed the shape, size of plastic zone at crack-tip and the minimum plastic zone radius with respect to the loading angle and stress intensity factor in linear elastic fracture mechanics for plane strain condition according to Von Mises yield criteria, the study is conducted for various loading angle. We found that the mixed mode loading (F36O°) can lead to earlier material fracture earlier than any other biaxial loading.

No description available.

Pour valider expérimentalement les modèles de fissuration et d'endommagement du béton, pour identifier leurs paramètres et pour mieux caractériser le comportement du béton lors de la propagation des fissures en mode mixte, des nouveaux essais multiaxiaux sont développés. Inspiré de travaux de Nooru-Mohamed (1992) et Winkler (2001), des essais riches et discriminants sont réalisés à l'aide de techniques expérimentales modernes. Le chargement est appliqué en utilisant une machine d'essai à 6 degrés de liberté asservie par un système de pilotage 3D. Pendant tout le déroulement d’essai, l'état de fissuration est analysé par corrélation d'images numériques, ainsi que les conditions limites cinématiques. Avec le dispositif expérimental proposé plusieurs histoires de chargement sont analysées : des chargements multiaxiaux proportionnels et non-proportionnelles, avec et sans refermeture de fissure. Les résultats expérimentaux sont confrontés à des simulations numériques réalisées avec des modèles d'endommagement non-locaux (i.e., à gradient) et avec un modèle de la mécanique linéaire de la rupture couplé avec une technique X-FEM. Le travail souligne l'importance de l'utilisation de conditions aux limites précises, estimées à partir de mesures de champ pour effectuer des simulations numériques qui reproduisent au mieux les résultats expérimentaux. En outre, il est démontré que la réorientation et la bifurcation de la fissure sont essentielles afin de créer des essais de propagation de fissures vraiment discriminants. Basé sur ces résultats expérimentaux, un nouveau type d’essai est finalement proposé. Cet essai hybride consiste à résoudre un problème inverse pour définir les conditions aux limites qui imposeront un trajet de fissuration voulu. Cette nouvelle technique permet une exploration plus complète du comportement mécanique complexe du béton en un seul essai. / To experimentally validate concrete damage and fracture models, identify their parameters and better characterize the concrete behaviour during mixed-mode crack propagation, multiaxial tests are developed. Inspired by former works of Nooru-Mohamed (1992) and Winkler (2001), rich and discriminating tests are performed by using state of the art techniques, where the experimental boundary conditions are directly measured during crack propagation. The loadings are applied using a hexapod testing machine controlled by a 3D displacement system and the cracking state is analysed via digital image correlation.With the proposed experimental setup several loading histories are analysed: proportional multiaxial loading histories and non-proportional ones, with and without crack closure and friction. The experimental results are confronted with numerical simulations performed with nonlocal (i.e., gradient) damage models and with a linear elastic fracture model coupled with the X-FEM framework. The present work underlines the importance of using accurate boundary conditions, estimated from full field measurements, to perform numerical simulations that reproduce the experimental results. Moreover it is shown that crack reorientation and crack branching is vital to create discriminant crack propagation tests.Based on these experimental results a innovative hybrid test is proposed, where the boundary conditions leading to a given crack path are found by solving an inversed problem. This new discriminating concrete fracture test is able to fully investigate its complex mechanical behaviour within a single run.

This dissertation is to obtain an overall understanding of the crack path selection in adhesively bonded joints. Using Dow Chemical epoxy resin DER 331® with various levels of rubber concentration as an adhesive, and aluminum 6061-T6 alloy with different surface pretreatments as the adherends, both symmetric and asymmetric double cantilever beam (DCB) specimens are prepared and tested under mixed mode fracture conditions in this study. Post-failure analyses conducted on the failure surfaces indicate that the failure tends to be more interfacial as the mode II component in the fracture increases whereas more advanced surface preparation techniques can prevent failure at the interface. Through mechanically stretching the DCB specimens uniaxially until the adherends are plastically deformed, various levels of T-stress are achieved in the specimens. Test results of the specimens with various T-stresses demonstrate that the directional stability of cracks in adhesive bonds depends on the T-stress level. Cracks tend to be directionally stable when the T-stress is compressive whereas directionally unstable when the T-stress is tensile. However, the direction of crack propagation is mostly stabilized when more than 3% mode II fracture component is present in the loading regardless of the T-stress levels in the specimens. Since the fracture sequences in adhesive bonds are closely related to the energy balance in the system, an energy balance model is developed to predict the directional stability of cracks and the results are consistent with the experimental observations. Using the finite element method, the T-stress is shown to be closely related to the specimen geometry, indicating a specimen geometry dependence of the directional stability of cracks. This prediction is verified through testing DCB specimens with various adherend and adhesives thicknesses. By testing the specimens under both quasi-static and low-speed impact conditions, and using a high-speed camera to monitor the fracture sequence, the influences of the debond rate on the locus of failure and the directional stability of cracks are investigated. Post-failure analyses suggest that the failure tends to be more interfacial when the debond rate is low and tends to be more cohesive when the debond rate is high. However, this rate dependence of the locus of failure is greatly reduced when more advanced surface preparation techniques are used in preparing the specimens. The post-failure analyses also reveal that cracks tend to be more directionally unstable as the debond rate increases. Finally, employing interface mechanics and extending the criteria for the direction of crack propagation to adhesively bonded joints, the crack trajectories for directionally unstable cracks are predicted and the results are consistent with the overall features of the crack paths observed experimentally. / Ph. D.

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

Double cantilever beam (DCB) specimens with distinct intermetallic microstructures and different geometries were fractured under different mode ratios of loading, ψ, to obtain critical strain energy release rate, Jc. The strain energy release rate at crack initiation, Jci, increased with phase angle, ψ, but remained unaffected by the joint geometry. However, the steady-state energy release rate, Jcs, increased with the solder layer thickness. Also, both the Jci and Jcs decreased with the thickness of the intermetallic compound layer. Next, mode I and mixed-mode fracture tests were performed on discrete (l=2 mm and l=5 mm) solder joints arranged in a linear array between two copper bars to evaluate the J = Jci (ψ) failure criteria using finite element analysis. Failure loads of both the discrete joints and the joints in commercial electronic assemblies were predicted reasonably well using the Jci from the continuous DCBs. In addition, the mode-I fracture of the discrete joints was simulated with a cohesive zone model which predicted reasonably well not only the fracture loads but also the overall load-displacement behavior of the specimen. Additionally, the Jci calculated from FEA were verified estimated from measured crack opening displacements in both the continuous and discrete joints. Finally, the pad-crater fracture mode of solder joints was characterized in terms of the Jci measured at various mode ratios, ψ. Specimens were prepared from lead-free chip scale package-PCB assemblies and fractured at low and high loading rates in various bending configurations to generate a range of mode ratios. The specimens tested at low loading rates all failed by pad cratering, while the ones tested at higher loading rates fractured in the brittle intermetallic layer of the solder. The Jci of pad cratering increased with the phase angle, ψ, but was independent of surface finish and reflow profile. The generality of the J =Jci(ψ) failure criterion to predict pad cratering fracture was then demonstrated by predicting the fracture loads of single lap-shear specimens made from the same assemblies.

Double cantilever beam (DCB) specimens with distinct intermetallic microstructures and different geometries were fractured under different mode ratios of loading, ψ, to obtain critical strain energy release rate, Jc. The strain energy release rate at crack initiation, Jci, increased with phase angle, ψ, but remained unaffected by the joint geometry. However, the steady-state energy release rate, Jcs, increased with the solder layer thickness. Also, both the Jci and Jcs decreased with the thickness of the intermetallic compound layer. Next, mode I and mixed-mode fracture tests were performed on discrete (l=2 mm and l=5 mm) solder joints arranged in a linear array between two copper bars to evaluate the J = Jci (ψ) failure criteria using finite element analysis. Failure loads of both the discrete joints and the joints in commercial electronic assemblies were predicted reasonably well using the Jci from the continuous DCBs. In addition, the mode-I fracture of the discrete joints was simulated with a cohesive zone model which predicted reasonably well not only the fracture loads but also the overall load-displacement behavior of the specimen. Additionally, the Jci calculated from FEA were verified estimated from measured crack opening displacements in both the continuous and discrete joints. Finally, the pad-crater fracture mode of solder joints was characterized in terms of the Jci measured at various mode ratios, ψ. Specimens were prepared from lead-free chip scale package-PCB assemblies and fractured at low and high loading rates in various bending configurations to generate a range of mode ratios. The specimens tested at low loading rates all failed by pad cratering, while the ones tested at higher loading rates fractured in the brittle intermetallic layer of the solder. The Jci of pad cratering increased with the phase angle, ψ, but was independent of surface finish and reflow profile. The generality of the J =Jci(ψ) failure criterion to predict pad cratering fracture was then demonstrated by predicting the fracture loads of single lap-shear specimens made from the same assemblies.