Experimental micromechanics: grain-scale observation of sand deformation

Abstract

International audienceStrain localisation plays a key role in the deformation of granular materials. Such localisation involves bands of just a few grains wide, which dominate the material's macroscopic response. This grain-scale phenomenon presents challenges for continuum modelling, which is the rationale behind models that explicitly take micro-scales into account. These in turn require micro-scale experimental analysis. In this work, X-ray tomography is used to image a small sample of oolitic sand while it deforms under triaxial compression. Grains are followed with a technique combining recent developments in image correlation and particle tracking. From these rich data, the evolution of the material in a subvolume of a thousand grains inside the sample (which contains 53 000 grains) is presented. The subvolume is chosen to lie inside the shear band that appears at the sample scale. Three-dimensional (3D) grain kinematics are analysed in three increments: the beginning of the test, the peak of the sample's macroscopic axial stress response and the residual stress state. When the sample's deformation is homogeneous (increment one) or fully localised (increment three), the kinematics of the grains in the subvolume appear to be representative of the kinematics occurring at the sample scale, allowing micro-mechanical observations to be made. In the transition from homogeneous to localised deformation (increment two), however, the scale of observation requires a zoom out of the subvolume to the sample scale in order to capture the complex mechanisms at play

Estimation of Separating Planes between Touching 3D Objects Using Power Watershed

“…In order to fully characterise the micro-mechanical evolution of the granular materials images, contact orientations are required, however the orientations measured are unusable due to strong artefacts introduced by the classical watershed used to segment particles [1]; these are probably aggravated by the fact that contact areas are relatively small in these images. A first test case will be performed on images coming from a sample (labelled ABEA02) of glass beads, which are close to perfect balls.…”

Investigation of the change in soil fabric during cone penetration in silt using 2D measurements

“…The remarkable contribution of this study is that it provides a means of relating the formation of zone with different volumetric response to the fabric of the material in particular the statistical orientation of the grains traduced by the anisotropy value, i.e., the difference between the major and minor eigenvalues of the fabric tensor. The transitional nature of silt and the coexistence of grains with distinct morphologies both in terms of grain size and grain shape, in particular the presence of needlelike shape grains originates unique extreme types of soil fabric: (1) an 'open-fabric' where the grains reorient in random orientations creating large voids between them and (2) a 'closed-fabric' type in which the grains align along a well-defined preferred orientation forming a more compacted structure with small voids within. These two distinct soil patterns can be directly linked to drainage patterns and the formation of compaction and dilation zones that leads to the partially drained conditions observed in silt.…”

“…The authors have quantified the orientation of the grains, the orientation of the contact normal and orientation of the voids and observed their changes under shearing. Andò et al [1] have measured grain rotation also during shearing of a silica sand and related it to the formation of the shear band in the specimen. Soil fabric and the mode it evolves under an imposed deformation are directly affected by grain morphology.…”

TRACKER: A particle image tracking (PIT) technique dedicated to nonsmooth motions involved in granular packings

“…It has become a powerful technique that provides reliable kinematic measurement fields, e.g., displacement, acceleration, strain, strain-rate fields. This technique is a fully non-intrusive measurement tool that can be used to follow the straining of patterns of a wide range of materials such as metals, polymers, ceramics, concretes and granular materials [6]. The latter can be treated as continuous when the followed pattern includes several grains [7].…”