A study of strain localization in triaxial tests on sand, using computed tomography is reported. The inception and the development of the localization are detected and described, both qualitatively and quantitatively. The complex geometrical structures involved in the localization patterns are described and the local void ratio evolution in the localization zone(s) is determined. Dense and loose Hostun RF sand specimens were tested under a confining pressure of 60 kPa. It is shown that strain localization can occur in different localization patterns depending on test conditions; a mechanism involving, simultaneously, a central cone and a set of planes in pairs was found for many of the specimens tested. The comparison of the local void ratio evolution in the shear zones with the global void ratio measurement supports the important conclusion that a limit void ratio is reached in the shear zones. This ratio is significantly different from the final void ratio defined from the global measurements; the lack of physical relevance of the latter is established. Cet article présente les résultats dune étude consacrée à la localisation de la deformation dans Pessai triaxial sur sable, utilisant la tomographie numérisée. On pent ainsi détecter et décrire la naissance de la localisation, depuis ses premières manifestations jusquà la rupture. La description obtenue est à la fois qualitative et quantitative: les structures géométriques complexes qui apparaissent sont caractérisées, et évolution locale de Pindice des vides pent ôtre mesurée. Des échantillons denses et l&cric;ches de sable dHostun RF ont é testés sous une contrainte de confinement de 60 kPa. On montre que la localisation pent sorganiser de fagon assez varée du point de vue géométrique, suivant les conditions dessai; cependant on a observé dans plusieurs cas un mécanisme similaire, qui comprend un c&cric;ne central associé à des paires de plans se coupant suivant une arête située dans le plan dune des extrémités de échantillon. éode de la variation de Pindice des vides dans les zones de cisaillement met en évidence une limite atteinte dans ces zones; par comparaison avec Pindice des vides moyen dans échantillon, on constate que cette limite est sensiblement différente de celle quon pourrait déduire des mesures globales, si Pon ignorait Pexistence de déformations localisées dans échantillon. Ainsi on doit conclure que les mesures globales sont généralement insuffisantes pour caractériser Pindice des vides en grandes déformation dans les échantillons triaxiaux sur les mat6riaux denses.

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This paper describes an original imaging technique, named Locadiff, that benefits from the diffuse effect of ultrasound waves in concrete to detect and locate mechanical changes associated with the opening of pre-existing cracks, and/or to the development of diffuse damage at the tip of the crack. After giving a brief overview of the theoretical model to describe the decorrelation of diffuse waveforms induced by a local change, the article introduces the inversion procedure that produces the three dimensional maps of density of changes. These maps are interpreted in terms of mechanical changes, fracture opening, and damage development. In addition, each fracture is characterized by its effective scattering cross section.

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The porosity and transfer properties of a very low porosity material (granite) are measured. A new procedure is defined using a capillary test and X-ray computed tomography (CT) scanning. Injected volumes are very low, i.e. a few cm 3 for a sample volume of 1 dm 3, using a fluid/rock ratio lower than 0.1%. This technique allows monitoring of the anisotropy of fluid flow during the test. Flow along the injection direction is higher than along the perpendicular direction. Saturation depends on the specific saturation of each mineral zone. Multiscale analysis allows defining the flow conditions as being controlled at both the mineral and the sample scale. Results indicate the specific role for various constituting parts of the material. High speed flow occurs in the crack network of K-feldspar, while the storage function is localized in the reaction zone forms by quartz and muscovite.

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Matrix investigations (X-Ray tomography, porosimetry by mercury injection, SEM analysis) around stylolites revealed major zones that represent different states in the propagation of the pressure solution structure.Near the stylolite termination, a significant increase of porosity relative to the far-field host rock porosity and variations in the shape of matrix particles are associated with the lateral propagation of the dissolution zone in the plane of the seam. Close to the sides of the seam, this porosity enhancement zone is found again and may be responsible for vertical development of the stylolite style. Above and below the stylolite seam, the rock matrix is less porous than the reference state and this region appears to have been a site of precipitation of diffused solute. These observations imply that the enhanced porosity state around the stylolite tip is a transient one. This zone becomes a site of deposition as the stylolite tip propagates through it.

Until now, observations of mechanically and thermally induced microcracks in rocks could only be carried out by indirect measurements or destructive observations on samples brought back to atmospheric pressure conditions. A special triaxial test cell was designed in order to perform direct observations during loading. The use of a cell in tomography apparatus involves new devices: (1) a movable horizontal load frame around a scanner; and (2) a test cell transparent to X‐rays, able to withstand up to 28 MPa maximum confining pressure and temperatures of up to 180 °C. Volumetric strains are compared with radiological density measurements. The first processed X‐ray images locating microcracks during propagation are also presented. Mineralogical effects on the crack location can be demonstrated. Strain inferred from CT density measurement is clearly correlated with the strain usually measured by a strain gauge. Different phases of mechanical behaviour are described: contracted phase and failure by macrocrack formation. The principal results obtained with this tool are the description of the porosity formation and macrocracking. Results show two principal factors localizing the porosity. First, the diffused porosity volume is controlled by mineralogical parameters, quartz and plagioclase grains, and boundaries of biotite grains during the thermal and mechanical loading. Second, macrocracking begins at the perimeter of the central section of core and grows towards the sample/piston interface. It seems that the first macrocracking is not located in the high‐porosity zone formed during the loading phase, but in a relatively low‐porosity zone.