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Notre nouvelle approche expérimentale consiste à étudier la fissuration de matériaux mous, principalement des gels polymériques et colloidaux, qui ont des tailles microstructurales micrométriques. Cette augmentation de la taille microscopique va avoir pour conséquence d’augmenter la taille de la zone de process et va rendre son observation plus facile avec des moyens standard de microscopie (à transmission et confocale).Pour se faire, nous avons mis au point un nouveau dispositif expérimental pour étudier la propagation de fissures dans des matériaux mous. Cette expérience permet de faire croître une fissure de manière contrôlée dans un échantillon mou et d’inspecter la pointe de fissure à haute résolution pour des fissures se propageant entre 1 µm/s and 1cm/s. En travaillant avec des gels de polymère physiques, nous avons analyse la forme de fissure ainsi que les champs de déplacement proches pointe (en utilisant des techniques de corrélation d’image) à petites et grandes échelles et à différentes vitesses. Nous avons montré qu’il existait une séparation d’échelles spatiales entre les échelles où l’élasticité linéaire s’applique, les échelles auxquelles les non linéarités émergent et les échelles auxquelles la dissipation se produit. Cette dernière échelle n’a pas pu être investigué dans le cas de gels polymériques. De récentes expériences sur des gels colloïdaux, ayant une longueur micro-structurale plus grande que celle des gels polymers, montre que nous sommes capables de sonder en temps réel les échelles d’endommagement lors de la fissuration. / Our novel experimental approach consists in studying fracture mechanics of soft materials, mainly polymer and colloidal gels, which have microstructures with large typical length scales. This increase in the microscopic length scale will consequently increase the typical size of the process zone and make its observation easier with standard microscopy techniques (optical or confocal).To do so, we designed a novel experimental device to study crack propagation in such soft materials. This experiment enables us to grow a unique crack in a controlled way in a soft specimen and to look at the crack tip at high magnification for crack velocities between 1 µm/s and 1cm/s. Working on physical polymer gels, we analysed the crack shape and crack displacement fields (using Digital Image Correlation) at large and intermediate scales for various velocities. We realized there was a separation of scales between the scale at which LEFM applies, the scale at which elastic nonlinearities emerge and the scale at which dissipation occurs. This last scale could not be investigated with the polymer gel. Recent experiments on colloidal gels, which have a microscopic length scale bigger than the one of polymer gels, show that we are able to probe damage at the microstructural scale.

In packaging industries, the demand for usage of Low Density Poly Ethylene foil is of profound interest. In the past, research was carried out on finite and infinite plates with varying crack lengths but having constant crack width. In this thesis, a detailed analysis on crack initiation is carried out on finite plates by varying width of the hole. The hole shapes for stress analysis include circle, ellipse and rectangular notch. Initially, maximum stress is found out using Linear Elastic Fracture Mechanics (LEFM) theory and compared with Finite element method (FEM) results. Secondly using Elastic Plastic Fracture Mechanics theory (EPFM), critical stress and geometric function are evaluated theoretically by Modified Strip Yield Model (MSYM) and numerically by ABAQUS. Finally, a tensile test is conducted to validate the theoretical and numerical results.  By varying the width of the hole, a study on the parameters like critical stress, geometric function is presented. A conclusion is drawn that the effect of hole width should be considered when calculating fracture parameters.

In this thesis, we have developed a Subtructured Finite Element / eXtended Finite Element (S-FE/XFE) method. The S-FE/XFE method consists in decomposing the geometry into safe FE-domains and cracked XFE-domains, and solving the interface problem with the Finite Element Tearing and Interconnecting method (FETI).This method allows for handling complex crack configurations in 3D structures with common commercial FE software that do not feature the XFEM. The method is also extended to a mixed dimensional formulation, where the FE-domain is discretised with shell elements while the XFE-domain is modelled with three-dimensional solid elements. This is the so-called S-FE Shell/XFE 3D method. The mixed dimensional formulation is more convenient than a full XFE-3D formulation because it significantly reduces the computational cost and it is more accurate compared to a full shell model because it includes three-dimensional local features such as three-dimensional crack. The compatibility of the displacements through the interface is ensured using the Reissner-Mindlin equation. The method has been extensively validated towards both academic problems and semi-industrial benchmarks in order to demonstrate the benefits of this approach. Among them, the S-FE/XFE method is applied to a crack analysis in a section of a compressor drum of a turbofan engine. The results obtained with the S-FE/XFE method are compared with those obtained with a standard FE computation. Furthermore, two applications of the S-FE shell/XFE 3D approach are proposed. First the load carrying capacity of a section of stiffened panel containing a through-the-thickness crack is investigated (this is the one-bay crack configuration). Second, the ability of the method for handling small surface cracks in large finite element models is addressed by looking at a generic 'large pressure panel' presenting realistic crack configurations.

The rise of the Environmentalism in every sector of the Industry has lead the aircraft and engine manufacturing companies to develop new generations of more environmentally friendly engines. The companies, encouraged to this purpose, are in a constant research for new manufacturing and production techniques, in order to improve their products, from the environmental point of view, by gaining efficiency in the manufacturing techniques and reduce the fuel consumption and emissions in-flight. Having in mind this scenario, the sponsor of this Project is interested in understanding how changing the materials of the blades, titanium alloys currently, for other lighter materials, such as composites, is going to have an effect in the overall gas turbine efficiency. In the particular case of this Project, it will be studied the influence of the Stiffness and coating Thickness of those non-metallic materials suitable to be employed as coatings on gas turbine fan blades, from the icing point of view. The work procedure will be based on a study of Linear Elastic Fracture Mechanics of bi-material junctions and will extrapolate the general problem to the ice-coatings case, by getting experimental data from tests carried out in an Icing Tunnel. It will be observed that the coating Stiffness has an influence on the Adhesion Level of ice to less stiff materials, if compared with the Adhesion Level of ice to metals. Besides, it will be described how a 0.5 millimetres thin polymeric coating placed over a metallic substrate is enough to reduce the Adhesion Level of ice, hiding any effect that the underneath materials might have on the Adhesion Level.

The rise of the Environmentalism in every sector of the Industry has lead the aircraft and engine manufacturing companies to develop new generations of more environmentally friendly engines. The companies, encouraged to this purpose, are in a constant research for new manufacturing and production techniques, in order to improve their products, from the environmental point of view, by gaining efficiency in the manufacturing techniques and reduce the fuel consumption and emissions in-flight. Having in mind this scenario, the sponsor of this Project is interested in understanding how changing the materials of the blades, titanium alloys currently, for other lighter materials, such as composites, is going to have an effect in the overall gas turbine efficiency. In the particular case of this Project, it will be studied the influence of the Stiffness and coating Thickness of those non-metallic materials suitable to be employed as coatings on gas turbine fan blades, from the icing point of view. The work procedure will be based on a study of Linear Elastic Fracture Mechanics of bi-material junctions and will extrapolate the general problem to the ice-coatings case, by getting experimental data from tests carried out in an Icing Tunnel. It will be observed that the coating Stiffness has an influence on the Adhesion Level of ice to less stiff materials, if compared with the Adhesion Level of ice to metals. Besides, it will be described how a 0.5 millimetres thin polymeric coating placed over a metallic substrate is enough to reduce the Adhesion Level of ice, hiding any effect that the underneath materials might have on the Adhesion Level.

This thesis is on fatigue crack growth assessments of a thermomechanical loaded Marine Boiler- Sunrod CPDB12. The installation position of the marine boiler in the ship in relation to its fatigue life under mode 1 loading is investigated. Thermomechanical loading embodies pressures, temperatures, RAO, subjected to the rigid body dynamic of ship in the marine environment.   Linear elastic fracture mechanics (LEFM) method was used is predicting the growth rates of the welding flaws at the joint based on stress range of the Paris law relationship. FEA Numerical simulation delivered better crack growth rate assessments and life predictions of the smallest detectable flaws in the boiler.   The identified smallest detestable flaws at the welding joint diminishing the designed safe life of the boiler significantly.  Also, installation position within the ship do affect the fatigue life of the boiler.

La rupture macroscopique d’un matériau intervient généralement lorsque des micro-défauts coalescent, plutôt que par la propagation catastrophique d’une seule fissure. Il est donc souhaitable d’étudier des configurations de rupture où de multiples fissures interagissent. Les paires de fissures en-passant (EP), où deux fissures parallèles croissent l’une vers l’autre, sont particulièrement intéressantes d’un point de vue applicatif. Cette configuration de rupture se retrouve aussi bien dans des situations naturelles (os, dorsales océaniques,…) qu’industrielles (génie civil, pièces métalliques,…). Malgré la diversité de tailles et de matériaux dans lesquels ces fissures existent, leurs trajectoires ont une forme typique en crochet quasi-universelle dont l’origine, résultant de l’interaction fissure-fissure répulsive puis attractive, est mal comprise. En particulier, le comportement répulsif initial semble mettre à mal la mécanique élastique linéaire de la rupture (MELR). Dans cette thèse, nous avons d’abord étudié les fissures EP dans le cadre de la MELR. L’étude de l’angle initial de déviation et la simulation de trajectoires a montré contre toute attente que la MELR permet de reproduire qualitativement la forme en crochet. Prédire précisément certaines caractéristiques, comme l’intensité de la phase répulsive, nécessite plus de finesse au niveau de la représentation du comportement matériau. Nous avons ensuite utilisé un modèle par champ de phase pour enrichir le modèle matériau. Les nouvelles trajectoires simulées étant fortement influencées par la longueur caractéristique du champ de phase, il est possible d’obtenir un modèle plus juste quantitativement. Une perspective intéressante reste de relier cette longueur à la microstructure du matériau / Macroscopic failure of a material happens generally through the coalescence of micro-defects rather than the catastrophic propagation of a single crack. It is therefore advisable to study fracture problems in which many cracks interact. The case of en-passant crack pairs (EP-cracks), two parallel and offset cracks approaching each other by propagating through their inner tips, presents a marked interest as these cracks can be found in various natural (bones, oceanic rifts,..) or industrial (civil engineering,…) situations. Despite the large variety of scales and materials in which these cracks are observed, their trajectories present a remarkably self-similar hook-shape. This shape result from the crack-crack interaction, first repulsive before becoming attractive, and its origin is poorly understood. In particular, the initial repulsive behaviour seems to question the validity of linear elastic fracture mechanics (LEFM). In this thesis, we first studied EP-cracks in the LEFM framework. The study of the initial kink angle and the simulation of crack paths showed against all expectations that LEFM is able to reproduce qualitatively the hook-shaped paths. Precise predictions of specific characteristics, such as the magnitude of repulsion, requires a more refined model of the material behaviour. We then used a phase-field model to augment the material representation. As they are strongly influenced by the characteristic length scale of the phase-field, the new simulated trajectories indicate that it is possible to develop a more quantitatively correct model. An attractive prospect is to link this characteristic length to the material microstructure