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To ensure secure geological storage of carbon dioxide it is necessary to establish the integrity of the overlying sealing rock. Seal rock fractures are key potential leakage pathways for storage systems; understanding their behaviour in the presence of CO2 under reservoir conditions is therefore of great importance. This thesis presents experimental investigations into the hydraulic behaviour of discrete fractures within low permeability seal rocks during single phase supercritical CO2 flow, under varying mechanical and thermal conditions representative of in-situ conditions. An experimental rig was designed and built to enable the controlled study of supercritical CO2 flow through 38 mm diameter samples under high pressures and temperatures. Samples are placed within a Hassler-type uniaxial pressure cell and CO2 flow is controlled via high precision syringe pumps. Flow experiments with supercritical CO2 within the pressure range 10-50 MPa were undertaken at temperatures of 38°C and 58°C with confining pressures of 35-55 MPa. The effects of stress loading and temperature change on the hydraulic properties of the fractured sample were studied; continuous differential pressure measurement enabled analysis of hydraulic response. Experiments were undertaken on a pre-existing Wissey field Zechstein Dolomite fracture and three artificial fractures (two East Brae field Kimmeridge Clay samples and one Cambrian shale quarry sample). Fracture permeabilities ranged from 8 X 10-14 m2 to 6 X 10-11 m2 with higher permeabilities observed within the harder rock samples. A broadly linear flow regime, consistent with Darcy's law, was observed in the lowest permeability sample (East Brae). A Forchheimer-type non-linear flow regime was observed in the other samples. Transmissivity variations during experiments were used to infer the mechanical impact of stress and temperature changes. An increase in effective stress resulted in transmissivity reduction, suggesting fracture aperture closure. During initial stress loading cycles, and subsequent higher temperature stress loading, a component of this transmissivity reduction was found to be inelastic, suggesting permanent modification of fracture geometry during closure. Pre- and post-experiment fracture surface characterisation provides further evidence for the occurrence of plastic deformation. Transmissivity-stress relationships were elastic during subsequent external stress-loading cycles, suggesting elastic closure and opening of fractures without additional permanent fracture geometry changes. The impact of fluid property variations on fracture hydraulic conductivity, Kfrac, was also analysed. Under constant effective stress Kfrac was found to be higher within high temperature and low fluid pressure scenarios, due to higher density/viscosity ratios. However, under constant confining pressure, fluid pressure changes are coupled both to mechanical effects (from effective stress alteration) and hydraulic effects (from viscosity variation), with opposing impacts on fracture hydraulic conductivity. At lower effective stresses mechanical effects were found to be dominant, with fluid pressure increase resulting in a notable increase to Kfrac due to aperture opening. At higher effective stresses, mechanical changes are much smaller due to increased contact area between fracture surfaces, and thus increased stiffness of fractures. Under such conditions hydraulic effects may be dominant and result in a small Kfrac reduction as fluid pressure increases, due to a reduction in the density/viscosity ratio. These results highlight that CO2 fluid property variation can have a notable influence on hydraulic conductivity under certain in-situ conditions. The single phase CO2 fracture flow experiments undertaken during this study were designed to enable a study of hydraulic and mechanical processes in isolation, without the influence of chemical processes. In-situ, the additional presence of brine and thus multiphase fluid behaviour and associated chemical processes makes the hydraulic behaviour of fractures considerably more complex. Coupled process modelling enables the relative influence of these processes to be simulated, but relies on experiments for validation. These unique experimental findings are of great value for enabling validation of such models as well as for informing analyses of geological and field studies.

La prochaine génération de moteur d'avion civil, LEAP, développé par Snecma (groupe Safran) et General Electric, intègrera de nombreuses innovations matériaux qui contribueront à la réduction de la consommation de carburant, d'émission de polluants et du bruit. Parmi ces innovations, l'utilisation d'aubes de turbine en CMC (Composites à Matrice Céramique) permettra une réduction significative de la masse du moteur. Les travaux présentés concernent à la fois la caractérisation du comportement mécanique de composites tissés 3D-SiC/Si-B-C et le développement d'une approche multi-échelle du comportement élastique adaptée aux structures CMC complexes. Un premier modèle à l'échelle du fil a été développé en prenant en compte la variabilité du matériau (porosité, architecture, usinage, etc...). Le modèle HPZ (Homogénéisation Par Zone) reposant sur la discrétisation du domaine d'homogénéisation permet de faire le lien entre l'échelle mésoscopique et l'échelle de la structure. / Due to their high thermo-mechanical properties and low densities, ceramic matrix composites (CMC) are candidate materials for hot parts in gas-turbine engines. Various applications have been identified for several types of CMC including C/SiC (nozzles), SiC/SiC (compressor blade) and all oxide composites (combustors). This work presented relates to both the characterization of the mechanical behaviour of woven composites 3D-SiC/Si-BC and the development of a multi-scale elastic behaviour suitable for complex CMC structures approach. A first model at the mesoscale has been developed taking into account the variability of the material (porosity, architecture, manufacturing, etc ...). The HPZ model ("Homogenisation par Zone" in French) based on the discretization of the homogenization field allows to link the mesoscopic scale and the scale of the structure.

This research project has investigated the mechanical behaviour of artificial turf surface systems used for sports under a range of real player movements, and the contribution of component layers to the overall system response by developing advanced measurement systems and methods. Artificial turf surface systems are comprised of a number of different materials and commonly with several layers, all of which contribute to their composite behaviour. During sports movements a player loads the surface, resulting in deformation that can change the surface behaviour, which in turn modifies the player biomechanical response. Improving the understanding of surface response to actual player loading is important for developing enhanced products for improving play performance. Likewise, by improving knowledge of surface effects on players, the understanding of injury risk can be improved. However, there is currently no published research to measure and analyse the behaviour of artificial turf system during real player locomotion. This research was undertaken to address this current lack of knowledge within the interaction between player and sports surface regarding the effects of player loading on the mechanical behaviour of artificial turf systems. In addition to support player loading regime, mechanical behaviour of hockey and third generation artificial turf surface systems and their component shockpad layers (a rubber shreds bonded shockpad and a polyurethane foam shockpad) was examined through dynamic cyclic compressive loading using an advanced material testing machine in laboratory environment. Each layer and carpet-shockpad system was subjected to controlled loading designed with previous biomechanical data at various loading frequencies (0.9 Hz, 3.3 Hz and 10 Hz) and under two different contact areas (50 mm and 125 mm diameter) to simulate aspects of player walking, running and sprinting. All layers and surface systems tested showed nonlinear stress-strain behaviour with hysteresis. Increasing the contact area resulted in reduced surface vertical deflection and more linear response. Increasing the loading frequency led to stiffer response in the lower stress range (< 400 kPa) for all surface systems. The third generation artificial turf systems showed also an increase in stiffness at higher stress range ( > 600 kPa) and a decrease in maximum strain as the loading frequency increased. Hysteresis loops obtained at different loading frequencies indicated that the amount of energy lost at the same peak load of 1900 N in each surface system decreased with an increase in loading rate. Player loading regime was performed to quantify the load/stress and the resulting surface deformation/strain under subject loading. Measurement systems including motion capture system, force plate and high speed were developed to characterise the response behaviour in a novel way. The mechanical behaviour of artificial turf surface systems under three player movement patterns (heel-toe walking, forefoot running and forefoot single leg landing) was measured. Boot-surface contact area of each movement varied during the stance. The heel-toe walking results indicated that the maximum applied stress and surface strain occurred in very early stance (first 10%) when the boot-surface contact area was small. For forefoot running and landing, the peak surface strain occurred around mid-stance concurrent with the time of peak applied stress. The maximum strain measured under running was smaller than under landing. A thin-film pressure sensing mat was used in both mechanical and player loading regimes and proved to be a useful tool for evaluating the pressure distributions and contact areas at different interfaces of the surface system. The applied stress on surface was observed to greatly reduce with depth over increasing contact area through the surface systems. Although the average pressure was reduced, pressure distribution contour showed directly under the surface load area the pressure at depth was still relatively large and that outside of this area the pressure was much lower. A comparison of the mechanical behaviour of artificial turf systems in terms of compressive strain, modulus of elasticity, stress distribution and energy loss under mechanical and player loading was evaluated. Key loading parameters in different loading regimes and their influence on surface system response were determined. The structure and material intrinsic properties of shockpad were considered to further explain the observed surface system behaviour. Two mathematical models were used to fit through the experimental data and found to be able to describe the loading behaviour. A breakthrough in understanding of the effects of real player loading on the mechanical behaviour response of artificial turf systems, and the contribution of the components to the whole system response has been achieved through the development of advanced measurement techniques.

Ce travail de thèse se situe dans le contexte du comportement à long terme des bétons dans les stockages de déchets nucléaires. L'objectif est de modéliser le comportement couplé en compression<br />avec la dégradation chimique. Dans la première partie, une campagne d'essai est effectuée où la cinétique de lixiviation chimique et les propriétés mécaniques ainsi que le comportement couplé du béton sont mis en vidence. Une méthode de lixiviation accélérée est choisie qui permet de dégrader rapidement les<br />éprouvettes. Dans la deuxime partie, le couplage chimie - mécanique est décrit. D'une part une approche simplifiée de la lixiviation du calcium est utilisée. En tenant compte de la présence des granulats, une approche par homogénéisation utilisant<br />un développement asymptotique est présentée. Elle permet de décrire la tortuosité due à la morphologie, la fraction volumique des granulats ainsi qu'à la disposition des granulats.<br />D'autre part, plusieurs modélisations mécaniques peuvent rendre compte du comportement mécanique du béton après lixiviation. Le modèle élastoplastique endommageable permet notamment de retrouver<br />les déformations permanentes observées dans les essais. La résolution du problème non linéaire est réalisée dans le contexte de la méthode des éléments finis. Les simulations numériques sont comparées avec les résultats expérimentaux et montrent un bon<br />accord. Enfin un exemple d'application au cas d'un tunnel de stockage est présenté.

The mechanical response of geomaterials is highly influenced by geometrical and material heterogeneity. To date, most modelling practices consider heterogeneity qualitatively and the choice of input parameters can be subjective. In this study, a novel approach to combine detailed micro-scale characterization with modelling of heterogeneous geomaterials is presented. The influence of micro-scale heterogeneity and microcracks on the mechanical response and brittle fracture of a crystalline rock was studied using numerical and experimental tools. An existing Combined Finite-Discrete element (FEM/DEM) code was extended to suit heterogeneous, discontinuous, brittle rocks. By conducting grid micro-indentation and micro-scratch tests, the Young's modulus and fracture toughness of the constituent phases of the rock were obtained and used as accurate input parameters for the numerical models. The models incorporated the exact phase mapping obtained from a MicroCT-scanned specimen and the existing microcrack density obtained from thin section analysis. The results illustrated that by incorporating accurate micromechanical input parameters and the intrinsic rock geometric features, the numerical simulations could more accurately predict the mechanical response of the specimen, including the fracture patterns and tensile strength. The numerical simulations illustrated that microstructural flaws such as microcracks should be included in the models to more accurately reproduce the rock strength. In addition, the differential elastic deformations caused by rock heterogeneity altered the stress distribution in the specimen, creating zones of local tensile stresses, in particular, on the boundaries between different mineral phases. As a result, heterogeneous models exhibited rougher fracture surfaces. MicroCT observations emphasized the influence of heterogeneity and, in particular, biotite grains on the fracture trajectories in the specimens. Favourably oriented biotite flakes and cleavage splitting significantly deviated the cracks. The interaction of the main crack with perpendicular cleavage planes of biotite caused strong crack deviation and termination. Considering heterogeneity and the strength degradation caused by microcracks, the simulations captured reasonably accurate mechanical responses and failure mechanisms for the rock, namely, the nonlinear stress-strain relationships. The insights presented in this study improve the understanding of the role of heterogeneity and microstructure on damage and mechanical behaviour of brittle rock.

The mechanical response of geomaterials is highly influenced by geometrical and material heterogeneity. To date, most modelling practices consider heterogeneity qualitatively and the choice of input parameters can be subjective. In this study, a novel approach to combine detailed micro-scale characterization with modelling of heterogeneous geomaterials is presented. The influence of micro-scale heterogeneity and microcracks on the mechanical response and brittle fracture of a crystalline rock was studied using numerical and experimental tools. An existing Combined Finite-Discrete element (FEM/DEM) code was extended to suit heterogeneous, discontinuous, brittle rocks. By conducting grid micro-indentation and micro-scratch tests, the Young's modulus and fracture toughness of the constituent phases of the rock were obtained and used as accurate input parameters for the numerical models. The models incorporated the exact phase mapping obtained from a MicroCT-scanned specimen and the existing microcrack density obtained from thin section analysis. The results illustrated that by incorporating accurate micromechanical input parameters and the intrinsic rock geometric features, the numerical simulations could more accurately predict the mechanical response of the specimen, including the fracture patterns and tensile strength. The numerical simulations illustrated that microstructural flaws such as microcracks should be included in the models to more accurately reproduce the rock strength. In addition, the differential elastic deformations caused by rock heterogeneity altered the stress distribution in the specimen, creating zones of local tensile stresses, in particular, on the boundaries between different mineral phases. As a result, heterogeneous models exhibited rougher fracture surfaces. MicroCT observations emphasized the influence of heterogeneity and, in particular, biotite grains on the fracture trajectories in the specimens. Favourably oriented biotite flakes and cleavage splitting significantly deviated the cracks. The interaction of the main crack with perpendicular cleavage planes of biotite caused strong crack deviation and termination. Considering heterogeneity and the strength degradation caused by microcracks, the simulations captured reasonably accurate mechanical responses and failure mechanisms for the rock, namely, the nonlinear stress-strain relationships. The insights presented in this study improve the understanding of the role of heterogeneity and microstructure on damage and mechanical behaviour of brittle rock.