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.
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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