Alessandro Tengattini

Experimental evidence of "Granulence"

We present an experimental study of velocity fluctuations in quasistatic flow of a 2D granular material deformed in a shear apparatus named 1γ2e [1]. Radjai and Roux [2] revealed systematic similarities between velocity fluctuations observed in discrete element simulations of quasistatic flow of granular material and turbulent flows in fluids. The character of these velocity fluctuations - named granulence by [2] - manifests as a non-Gaussian broadening of the probability density function of the fluctuations as the length of the analyzed shear-window is decreased, and exhibits some space and time scaling. The experiments presented are simple shear tests on granular samples composed of about 2000 wooden rods. The kinematics of the rod centers was followed by means of 2D Particle Image Tracking (PIT) technique applied to a sequence of 24 Mpixels digital pictures acquired throughout the duration of the loading at a frequency of 0.08 image/s. This analysis confirms the existence of granulence features in a real experimental test, which is comparable to that previously observed in numerical simulations of [2]. The experimental results obtained open up a new avenue for further studies on fluctuations in granular materials. (Less)

A Micromechanics Based Model for Cemented Granular Materials

Cemented Granular Materials (CGMs) consist of a particle skeleton and a solid matrix partially filling the interstitial space. In this broad class we encounter a number of typical geotechnical materials such as sedimentary rocks (Sandstones, Conglomerates and Breccia) as well as naturally and artificially cemented sands. These materials, while showing a brittle behavior under shearing at low confining pressures, are ductile at high confinements. The micro mechanisms involved, that are cement disaggregation, grain crushing and fragment rearrangement, are known to be different in these two cases.

Double-Scale Assessment of Micro-mechanics Based Constitutive Models for Granular Materials Undergoing Mechanical Degradation

The richness across the scales of geomaterials has long been known. Yet only recently, thanks to the development of new experimental techniques, it has been possible to study the micro (grain scale) origin of some of the phenomena observed at the macro (specimen) scale. This unprecedented insight calls for new models able to build rational links between these two scales. Some recently proposed models for cemented and uncemented granular materials take advantage of this understanding to conjugate the macroscopic irreversible strains with internal variables representing a statistically averaged evolution of the micro-structure. While these models have shown their capability to reproduce the macroscopic behavior of the geomaterials they were designed for, to fully assess them and to prioritize possible enhancements, a comparison between the predicted evolution of the micro-structure and appropriate experimental data is desirable. In this contribution we study the possibility of extracting robust and statistically meaningful measurements of microstructural evolution from X-ray computed tomography images which are then compared with the micro-scale predictions of the existing micro-mechanics based models.

Development of Image Analysis Tools to Evaluate In-Situ Evolution of the Grain Size Distribution in Sand Subjected to Breakage

Grain crushing is a phenomenon of pivotal importance in the inelastic deformation of granular materials. The progressive evolution of the grain size distribution is known to play a major role in a number of geotechnical engineering problems. There is, however, a lack of experimental work tackling the quantification of the three dimensional evolution of the grain size distribution of materials undergoing grain crushing. The technological advancements in X-ray computed tomography now allow in situ, 4 dimensional (3D + time) images of geomaterials to be obtained as they evolve. While recent investigations of the kinematics of persistent grains have allowed a deeper experimental understanding of some inelastic micro-mechanisms to be obtained, a further effort is required when interpreting tomographic images in which grains are not persistent (i.e., they can break). In this contribution, a novel image-analysis technique under development is proposed to quantify the evolution of the grain size distribution as grain crushing proceeds in an experiment. This technique is applied to the analysis of 3D tomographic images of sand sheared at high confinement.