United States

We investigate numerically the statistics of the stresses, defined for cells - the irreducible loops surrounded by particles in contact - in two-dimensional granular solids and the relation between these cell stresses and cell structures. We prepared a number of mechanically stable granular assemblies at various packing fractions and mean coordination numbers, and measured the stress statistics on each cell. We find the following. (1) The mean cell stress agrees with the bulk external pressure; (2) the probability density function of the normalised cell stress ratio collapses into a master curve; (3) the mean stress ratio increases with increasing intergranular friction; (4) the mean stress ratio per cell order decreases with increasing cell order because of their reduced stability; (5) the maximum cell stress direction correlates well with the cell’s long axis. These results are strong evidence that the cell structure and the stress field self-organise together and reveal the co-evolving correlations between the two.

Powders &amp; Grains 2021

Statistical properties of cell stresses in 2D granular solids

## Dr. Nishikant (Nish) Sonwalkar: Academician, Scientist, Musician

![](https://assets.underline.io/uploads/markdown_image/1/image/0f9b63a0b3deba4a355d98e2c87af68a.png)

Dr. Sonwalkar is regarded as a leading expert in the application of computers in education. He has extensive research and development experience in hypermedia authoring, mobile learning, adaptive learning, and brain-based synaptic learning, advanced scientific visualization, database management systems, and computer simulation.

After receiving doctoral degree from MIT in the area of Molecular Dynamics of Nano-interfaces, he joined MIT as faculty in Mechanical Engineering, but his passion for technology enabled education led him become the founding director of Hypermedia Teaching Facility (Hypermedia Lab) at MIT and later he was appointed as the Principal Educational Architect of MIT.

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NISH 2021

poster

[EPJ Web of Conferences, Volume 249 (2021)](https://www.epj-conferences.org/articles/epjconf/abs/2021/03/contents/contents.html)

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Powders & Grains 2021

Powders & Grains 2021 will be held in Buenos Aires (Argentina, VIRTUAL) July 5, 13, 21, 29 and Aug 6, 2021. The aim of the conference is to give an up-to-date picture of the broad research field of granular media, powders and grains. Contributions from experts around the world will focus on research topics that cover a wide range of open problems in the field.

We investigate failure in granular material with numerical simulations based upon DEM (Distinct Element Method). 

Failure is associated with a discontinuity in a component of the incremental strain over a plane surface through the condition of the determinant of the acoustic tensor to be zero.

DEM simulations are carried out on an aggregate of elastic frictional spheres, initially isotropically compressed and then sheared at constant pressure. 

The components of the stiffness tensor are evaluated numerically in
stressed states along the triaxial test, and employed in the condition of the acoustic tensor to predict localization.

This occurs in the pre-peak region, where the material behavior is on average incrementally frictionless, i.e., change in deviatoric stress due to the tangential forces is negligible.


Failure in granular materials based on acoustic tensor: a numerical analysis

Displacement fields in sheared particle packings often display vortex-like structures that reveal essential features about the mechanical state of the collection of particles. There are several metrics to quantify these flow field features, yet extracting such quantitative metrics from flow field or particle tracking data involves
making numerous choices on the time and length scales over which to average. Here we employ a much used experimental data set on sheared disk packings to explore how such arbitrary data mining choices affect the obtained results. We focus on calculating the strain dependent vorticity, as this metric is a differential method hence potentially sensitive to the way it is computed. We find that the total surface area with an absolute vorticity above a certain threshold approaches a plateau value as shear progresses. This plateau value exhibits a non-monotonic dependence on packing fraction. We also show which range of choices yields results that can
support an analysis method independent, physical interpretation of the flow field data.

Strain dependent vorticity in sheared granular media

We focus on localizing the source of acoustic emissions within a compressed two-dimensional granular ensemble of photoelastic disks, having as main information the arrival times of the acoustic signal to 6 sensors located in the boundaries of the system. By estimating, thanks to the photoelasticity of the grains, the wave speed at every point of the structure, we are able to compute the arrival times from every point of the system to the sensors. A comparison between the arrival time differences between every set of computed values to those from the actual measurements allows finding the source of the acoustic emissions.

Acoustic localisation in a two-dimensional granular medium

In the present work, the relaxation of several wood powders has been investigated in an annular shear cell. It is found that the slow logarithmic relaxation commonly observed for various materials including granular materials is interrupted by large events. Their frequency and amplitude are investigated with respect to particle size and stress-history by measuring stresses and strain. These large events during
the relaxation appear to be controlled by the deformation. The coarser the particles are, the bigger is the deformation of the powder bed between two fast relaxation events and during the event.

Slow and intermittent stress relaxation of biomass granular media

We analyze the dilation of the system in a cylindrical granular fault consisting of one single layer of disks submitted to both normal pressure and continuous and slow shear, which results in intermittent and sudden energy release events that reproduce the main laws of seismicity. The dilation of the system can be separated into two parts: a smooth increase of dilation, plus sudden changes both contracting and dilating the medium, which are correlated to abrupt jumps -both positive and negative- in the measured resisting torque. We explain the four possible (and existing) general scenarios combining those two variables: dilation jumps and torque jumps, thanks to the assumption of an optimal local angle in the direction of force chains, and each reorganization of the structure as a replacement of the force chain holding most of the applied stress. The average rate of increase of global dilation varies monotonically with the size of the energy release event, making dilation a plausible candidate to predict catastrophic events in such earthquake-like systems.

Dilation as a precursor in a continuous granular fault

We investigate collective motions of points in 2D systems, orchestrated by Lloyd algorithm. The algorithm iteratively updates a system by minimising the total quantizer energy of the Voronoi landscape of the system. As a result of a tradeoff between energy minimisation and geometric frustration, we find that optimised systems exhibit a defective landscape along the process, where strands of 5- and 7-coordinated dislocations are embedded in the hexatic phase. In particular, dipole defects, each of which is the simplest possible pair of a pentagon and a heptagon, come into the picture of dynamical arrest, as the system freezes down to a disordered hyperuniform state. Moreover, we explore the packing fractions of 2D disk packings associated to the obtained hyperuniform systems by considering the maximum inscribed disks in their Voronoi cells.

Dynamical arrest of topological defects in 2D hyperuniform disk packings

Granular systems are not always homogeneous and can be composed of grains with very different mechanical properties. To improve our understanding of the behavior of real granular systems, in this experimental study, we compress 2D bidisperse \md{systems} made of both soft and rigid grains. By means of a recently developed experimental set-up, \md{from the measurement of the displacement field we can} follow all the mechanical observables of this granular medium from the inside of each particle up to the whole system scale. We \md{are able to} detect the jamming transition from these observables and study their evolution deep in the jammed state for packing fractions as high as $0.915$. We show the uniqueness of the behavior of such a system, \md{in which way} it is similar to purely soft or rigid systems and how it is different from them. This study constitutes the first step toward a better understanding of the mechanical behavior of granular materials that are polydisperse in terms of grain rheology.

Highly strained mixtures of bidimensional soft and rigid grains: an experimental approach from the local scale

Earthquakes happen with frictional sliding, by releasing all the stresses accumulated in the pre-stressed surrounding medium. The geological fault gouge, coming from the wear of previous slips, acts on friction stability and plays a key role in this sudden energy release. A large part of slip mechanisms is influenced, if not controlled, by the characteristics and environment of this tribological “third body”. A 2D granular fault (mm scale) is implemented with Discrete Element Modelling (DEM). A displacement-driven model with dry contact is studied to observe kinematics and properties of the slipping zone. Changing the percentage of infill material (as matrix particles) within the granular media modifies the kinematic of the gouge and Riedel shears evolution. Low-angle Riedel shear bands are mostly observed. Their number increases with the inter-particle friction coefficient (which also influences shear bands formation in their orientation angle) and reduces with an increase in shear modulus.

Shear bands in dense fault gouge

During industrial air preparation, one of the most important issues is the proper moisture content. In vehicle brake systems, compressed air is commonly used, where condensate in the system can lead to serious problems as corrosion, frosting, etc. For avoiding malfunctions, the relative humidity of the media must decrease to 20-40%, by adsorbing its moisture content. To achieve this, air dryer units filled with desiccant granular media is commonly used in railway industry. The compressed air flows through the dryer, thus the particles can bind the water vapour. If the packing density of the desiccant can increase, the efficiency of the dryer can improve. In this paper an interesting method for container filling, the snowstorm technique was investigated and optimized by applying discrete element method. Essence of the device is a rod system placed below the filler bin, which ensures even distribution of the particles. By the numerical analysis it was revealed, the particles’ lateral velocity is increased because of the snowstorm system, but with non-optimal arrangement, the particle flow can be significantly blocked. Based on our systematic analysis, optimal geometry of the particle distribution system was found, which ensures settling more particles by 9.5% into the dryer reservoir.

Numerical investigation of snowstorm filling

The shape of the particles and local friction, separately, are known to strongly affect the macroscopic properties of an assembly of grains. But the combined effects of these two parameters still remain poorly described. By means of extensive two dimensional contact dynamics simulations, we perform a systematic analysis of the interplay between friction and shape on strength properties of granular systems.
The shape of the particles is varied from disks to triangles, while the friction is varied from 0 to 0.7. We find that the macroscopic friction first increases with angularity, but it may decline (for low friction values), saturate (for intermediates friction values), or continue to increase (for large friction values) for the most angular shapes. In other words, the effect of the particle's angularity on the shear strength depends on the level of sliding friction. In contrast, the effect of local friction on the shear strength does not depend on the specific properties of shape. The results presented here highlight the subtle coupling existing between shape and friction effects.

Statistical properties of cell stresses in 2D granular solids

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