100 research outputs found
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Micromechanisms of inelastic deformation in sandstones: An insight using x-ray micro-tomography
This study investigates the grain-scale mechanisms that lead to failure by strain localisation in specimens of Fontainebleau sandstone with different degrees of cementation. While the effects of inter-particle bonding on the mechanical behaviour of granular geomaterials, including soft rocks, have been largely studied, the physical micro-scale mechanisms governing the material deformation are still poorly understood. In this study, laboratory techniques have been developed to allow a non-invasive investigation of the internal deformation of sandstones during triaxial compression to failure. The material investigated was Fontainebleau sandstone, a quartzite formation from the Paris basin (France), which can be found as very hard, tightly cemented sandstone or more porous and less cemented material. Specimens with porosities of 6 and 21% were investigated. Triaxial compression tests at confining pressures of 2 and 7 MPa were conducted on dry cylindrical specimens of 11 mm diameter and 22 mm height. Three-dimensional (3D) images of the full specimens were obtained by carrying out x-ray micro-tomography scans at key points throughout the test. The high-resolution 3D tomographic images have a voxel size of 8.5 ÎŒm (0.033d50), allowing clear identification of the grains. This analysis suggests that dilatancy of the material, which depends on the degree of bonding between grains, plays a fundamental role in the failure mode of granular media. Insights into bonding rupture mechanisms and grain damage by inter- and intra-granular cracking are presented
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An experimental study of micro-scale deformation in a soft sandstone
This study investigates the grain scale mechanisms that lead to failure by strain localisation in a slightly cemented sandstone. While the effects of interparticle bonding on the mechanical behaviour of granular geomaterials, including soft rocks, have been largely studied, the physical micro-scale mechanisms governing the material deformation are still poorly understood. In this study, laboratory techniques have been developed to allow a non-invasive investigation of the internal deformation of a soft sandstone during triaxial compression to failure. The material studied is Fontainebleau sandstone, a quartzite formation from the Paris Basin, with a porosity of 21 %. Fontainebleau sandstone is a uniformly graded material with a mean grain size of 260 m. Triaxial compression tests at confining pressures of 2 MPa and 7 MPa were conducted on dry cylindrical specimens of 11 mm diameter by 22 mm height. Three-dimensional images of the full specimen were obtained by carrying out x-ray micro-tomography scans at key points throughout the test. The high-resolution tomographic images have a voxel size of 8.5 m (0.033d50) allowing a clear identification of the grains. A preliminary analysis has suggested that the failure mechanisms can be different for the samples tested at different confining pressures, which has a direct implication on the permeability of the material
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Microstructure evolution on sandstones with different degrees of cementation
This study investigates the grain scale mechanisms that lead to failure by strain localisation in specimens of Fontainebleau sandstone with different degrees of cementation. While the effects of inter-particle bonding on the mechanical behaviour of granular geomaterials, including soft rocks, have been largely studied, the physical micro-scale mechanisms governing the material deformation are still poorly understood. In this study, laboratory techniques have been developed to allow a non-invasive investigation of the internal deformation of sandstones during triaxial compression to failure. The material investigated was Fontainebleau sandstone, a quartzite formation from the Paris Basin (France) which can be found as very hard, tightly cemented sandstone or more permeable and less cemented material. Specimens with porosities 6% and 21% were investigated. Triaxial compression tests at confining pressures of 2MPa and 7MPa were conducted on dry cylindrical specimens of 11mm diameter by 22mm height. Three-dimensional images of the full specimen were obtained by carrying out x-ray micro-tomography scans at key points throughout the test. The high-resolution tomographic images have a voxel size of 8.5um (0.033d50) allowing a clear identification of the grains. The analysis suggests that dilatancy of the material which depends on the degree of bonding between grains, plays a fundamental role on the failure mode of the granular media
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An experimental investigation on the deformation of Fontainebleau sandstone
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A microstructural finite element analysis of cement damage on Fontainebleau Sandstone
This paper presents a numerical simulation that uses tomographic data to reproduce the grain-scale mechanisms taking place during deformation of Fontainebleau sandstone. Previous investigation using x-ray tomographic images acquired during triaxial compression has highlighted the role of bonding rupture mechanisms in the failure mode of the material. The model the deformation of the sandstone, images of the internal topology were used to generate an image-based finite element mesh and the grain-scale phenomena such as, the opening and propagation of the cracks associated with the debonding of the cemented grains, were reproduced using a simple constitutive model
A laboratory study of anisotropic geomaterials incorporating recent micromechanical understanding
This paper presents an experimental investigation revisiting the anisotropic stressâstrainâstrength behaviour of geomaterials in drained monotonic shear using hollow cylinder apparatus. The test programme has been designed to cover the effect of material anisotropy, preshearing, material density and intermediate principal stress on the behaviour of Leighton Buzzard sand. Experiments have also been performed on glass beads to understand the effect of particle shape. This paper explains phenomenological observations based on recently acquired understanding in micromechanics, with attention focused on strength anisotropy and deformation non-coaxiality, i.e. non-coincidence between the principal stress direction and the principal strain rate direction. The test results demonstrate that the effects of initial anisotropy produced during sample preparation are significant. The stressâstrainâstrength behaviour of the specimen shows strong dependence on the principal stress direction. Preloading history, material density and particle shape are also found to be influential. In particular, it was found that non-coaxiality is more significant in presheared specimens. The observations on the strength anisotropy and deformation non-coaxiality were explained based on the stressâforceâfabric relationship. It was observed that intermediate principal stress parameter b(b = (Ï2 â Ï3)/(Ï1 â Ï3)) has a significant effect on the non-coaxiality of sand. The lower the b-value, the higher the degree of non-coaxiality is induced. Visual inspection of shear band formed at the end of HCA testing has also been presented. The inclinations of the shear bands at different loading directions can be predicted well by taking account of the relative direction of the mobilized planes to the bedding plane
Exemple d'application de la corrélation d'image 2D à un matériau fortement confiné
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Characterization of the early strain localization in a sandstone and a clay rock
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Emergence of strain localization in porous rocks characterized by full-field measurement in plane strain condition
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