Observation des micromécanismes de déformation et d'endommagement des argilites sous sollicitation hydrique

La déformation et l'endommagement des argilites sous sollicitation hydrique sont observés au MEB environnemental. La déformation est mesurée par corrélation d'images à l'échelle micrométrique des zones argileuses entre les grains carbonatés et de quartz et révèle un couplage hydromécanique complexe. On montre l'apparition d'une déformation irréversible lors d'un cycle hydrique, ainsi qu'un endommagement sous la forme d'un réseau de microfissures localisées dans la phase argileuse ou aux interfaces grain-matrice, dont la morphologie dépend de la vitesse et du sens du chargement hydrique

Micro-scale experimental investigation of deformation of argillaceous rocks under hydric and mechanical loads

International audienc

Effects of fluid-rock interaction on rock salt strength

International audienc

De l’utilisation d’échantillons humides en porosimétrie au mercure

Les échantillons placés dans un porosimètre à mercure sont classiquement ou bien séchés ou bien lyophilisés avant expérience. Ces techniques d’analyse permettent d’avoir accès à la totalité du spectre de distribution des tailles des pores connectés. Dans le cas de roches très raides et peu poreuses, cette procédure expérimentale avec lyophilisation ou séchage risque de créer artificiellement des microfissures nouvelles altérant le réseau poreux initial, surtout si le séchage n’est pas suffisamment lent. Ce travail étudie donc la faisabilité d’essais sur échantillons humides afin d’appréhender au mieux le réseau de pore connecté « vrai »

Hydromechanical behavior of argillaceous rocks investigated by the combination of ESEM and DIC techniques

International audienc

Experimental investigation of the free swelling of crushed argillite

International audienceCallovo-Oxfordian argillite is studied in France as a possible host rock for repository of high-level radioactive wastes. A new experimental procedure is proposed to quantify the free swelling of its main constitutive clay phase, which is a key parameter to model the hydromechanical behaviour of this rock. The material was pulverised finely enough to remove any local stress that may prevent the free swelling induced by a change in water content. Powders were observed using an environmental scanning electron microscope and images recorded at various degrees of relative humidity were analysed by digital image correlation techniques to measure swelling strains. The investigation showed that the free swelling is moderate in comparison to other expansive clays (e.g. bentonite MX80). The magnitude of deformation becomes significant at high relative humidity. This swelling is not reversible and residual contraction is found after a wetting/drying cycle. These results confirm and provide a possible interpretation of some macroscopic phenomena

An advanced nonlinear signal analysis method for damage detection in geomaterials

An algorithm is described which enables us to evaluate the Volterra kernels and the corresponding transfer functions. This method is then used to detect the cracking threshold of the geomaterials under loading. The responses of a sample of sandstone under axial leading and subjected to ultrasonic excitation are analyzed. The occurrence of microcracking is characterized by the changes in the linear and nonlinear parts of the measured signal energy

Couplages hydromécaniques dans les roches argileuses : analyse par microscopie électronique environnementale et corrélation d'images

International audienc

Micromechanical experimental investigation of mudstones

International audienceThis paper reports on a micromechanical investigation of mudstones using a novel experimental method, based on a combination of environmental scanning electron microscopy (ESEM) and digital image correlation techniques. A specifically designed rig was developed, allowing in situ tests under combined hydric and mechanical loadings in the ESEM chamber. Observations were performed on the scale of the composite microstructure of the involved rocks (i.e. numerous grains of carbonate and quartz embedded in a clay matrix). Preliminary results from a uniaxial compression test on a sample with 5?4% water content are presented. Heterogeneous strain fields that correlate well with the microstructure of mudstones are illustrated, enabling different deformation modes (particularly shear bands and tensile microcracks) and their interactions to be identified

Full-Field Measurements on Low-Strained Geomaterials Using Environmental Scanning Electron Microscopy and Digital Image Correlation: Improved Imaging Conditions

International audienceFull-field strain measurement at microscale on geomaterials by means of hydromechanical in situ testing and imaging in an environmental scanning electron microscope and digital image correlation techniques is a challenging task because of both low magnitude of investigated strains (typically of the order of 10−3) and unfavourable imaging conditions. In view of improving strain measurement accuracy, three major sources of measurement errors are evaluated, and methods to minimise their effects are proposed. First, a fast and simple procedure to accurately quantify image noise is proposed and subsequently used to optimise various environmental scanning electron microscope setting parameters, such as dwell time, spot size, working distance and chamber pressure. Second, a specific procedure to limit magnification fluctuations to a sufficiently low level is described. Finally, digital image correlation systematic errors are quantified on real images, and several ways to reduce their amplitude are compared. The combination of these improvements finally allows us to reach an appropriate accuracy for overall strain measurements and characterisation of microscale heterogeneities