Localisation de la déformation dans les milieux granulaires

International audienceThis paper discusses strain localisation in granular media by presenting experimental, full-field analysis of mechanical tests on sand, both at a continuum level, as well as at the grain scale. At the continuum level, the development of structures of localised strain can be studied. Even at this scale, the characteristic size of the phenomena observed is in the order of a few grains. In the second part of this paper, therefore, the development of shear bands within specimen of different sands is studied at the level of the individual grains, measuring grains kinematics with x-ray tomography. The link between grain angularity and grain rotation within shear bands is shown, allowing a grain-scale explanation of the difference in macroscopic residual stresses for materials with different grain shapes. Finally, rarely described precursors of localisation, emerging well before the stress peak are observed and commented

The Alestle - Vol. 57 No. 30 - 01/11/2005

Vol. 57 No. 3

FEMxDEM multi-scale modelling with second gradient regularization

The multi-scale FEMxDEM approach is an innovative numerical method for geotechnical problems, using at the same time the Finite Element Method (FEM) at the engineering macro-scale and the Discrete Element Method (DEM) at the scale of the microstructure of the material. The link between scales is made via computational homogenization. In this way, the continuum numerical constitutive law and the corresponding tangent matrix are obtained directly from the discrete response of the microstructure [1,2,3]. In the proposed paper, a variety of operators, rather than the tangent consistent for the Newton- Raphson method, is tested in a challenging attempt to improve the poor convergence performance. The independence of the DEM computations between the different elements is exploited to develop a parallelized code using an OpenMP paradigm. At the macro level, a second gradient constitutive relation is implemented in order to enrich the first gradient Cauchy relation bringing meshindependency to the model. The second gradient regularization, together with the speedup provided by the parallelization allows by first time to the FEMxDEM model to be applied to real scale problems with the desired mesh refinement. Some results are given exhibiting the above findings with emphasis on aspects related to strain localisation

MECHANICAL BEHAVIOUR AND RUPTURE IN CLAYEY ROCKS STUDIED BY X-RAY MICRO-TOMOGRAPHY AND 3D-DIGITAL IMAGE CORRELATION

International audienceThe mechanical behaviour and the rupture of clayey rocks have been experimentally studied by performing in situ triaxial tests on a synchrotron beamline i.e. performing X-ray micro tomography scans under mechanical loading. The 3D images obtained at different steps of the test were then analysed by 3D-Digital Image Correlation method in order to measure incremental strain fields. The results allow to clearly detect the onset of shear strain localization and to characterize its development in a 3D complex pattern

FEMxDEM Multi-scale modelling applied to geomaterials

The multi-scale FEMxDEM approach is an innovative numerical method for geotechnical problems, using at the same time the Finite Element Method (FEM) at the engineering macro-scale and the Discrete Element Method (DEM) at the scale of the microstructure of the material. The link between scales is made via computational homogenization. In this way, the continuum numerical constitutive law and the corresponding tangent matrix are obtained directly from the discrete response of the microstructure [1,2,3]. In the proposed paper, a variety of operators, rather than the tangent consistent for the Newton-Raphson method, is tested in a challenging attempt to improve the poor convergence performance. The independence of the DEM computations is exploited to develop a parallelized code using an OpenMP paradigm. At the macro level, a second gradient constitutive relation is implemented in order to enrich the first gradient Cauchy relation bringing mesh-independency to the model. The second gradient regularization, together with the speedup provided by the parallelization allows by first time to the FEMxDEM model to be applied to real scale problems. Some results are given exhibiting the above findings with emphasis on aspects related to strain localisation

Experimental characterisation of (localised) Deformation Phenomena in Granular Geomaterials from Sample Down to Inter-and Intra-grain Scales

AbstractThis paper outlines some recent advances in the full-field experimental characterisation of the mechanics of granular geomaterials (in particular, sands) using a range of methods that provide characterisation at different scales, from the sample-scale down to the inter- and intra-grain scale. The techniques used are “full-field” approaches involving in-situ x-ray micro-tomography, 3D-volumetric digital image analysis/correlation and grain ID-tracking, in-situ 3D x-ray diffraction and in-situ, spatially-resolved neutron diffraction. These methods provide new data on the mechanics of sand at different scales, including continuum measures of strain, porosity, and fabric plus discrete measures of particle kinematics and force transmission. The results of such measurements might be used to advance higher-order continuum theories, and provide the necessary input parameters, or to calibrate discrete grain-scale simulations of sand behaviour to explore loading paths that are inaccessible in the laboratory

Caractérisation expérimentale de la déformation et de la rupture localisée dans une argilite

Nous présentons une étude expérimentale de caractérisation de la rupture par localisation de la déformation dans une roche argileuse. Des essais triaxiaux ont été effectués sur une ligne de lumière de l'ESRF pour obtenir des reconstructions microtomographiques en volume à différents stades de l'essai. Une mesure du champ de la déformation a été faite en associant aux images obtenues une méthode de corrélation d'images en volume. Ces résultats permettent d'identifier très clairement le seuil d'initiation de la localisation, et le mode de déformation dans les bandes