Contact evolution in granular materials with inherently anisotropic fabric

United States

This presentation shows the results of five triaxial compression tests performed on approximately 9000 oblate spheroids (lentils) with different initial orientations with respect to the loading axis (0º, 30°, 45º, 60º, and 90°). Typical stress-strain information is complemented with x-ray tomography scans every 1 % strain. Starting from an initial labelling of particles, a new technique is used to obtain unprecedented detail of tracking of all the particles through time. This rich dataset is analysed from the perspective of inter-particle contacts (building on previous metrological work on this subject) revealing that the mean contact orientation in both samples rotates towards the direction of σ1 at a rate depending on the initial orientation. Different trends are observed for the alignment of contact orientation. 


Powders &amp; Grains 2021

[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.

technical paper

Three-dimensional simulations of cone penetration tests (CPT) in cemented sandstone have been investigated using the Discrete Element Method (DEM). We generated the realistic grain size distribution of sandstone in the numerical simulations using the Ustyurt-Buzachi Sedimentary Basin samples. Numbers of numerical CPT tests at different bond strength values were performed with the penetrometer vertically moved down at a constant rate. The results of the real-world particle size distribution show that the cone penetration resistance and side friction increase with increasing depths; and with increase in bond strength the cone resistance and side friction decrease, while the friction ratio increases. The result of numerical CPT tests in cemented sandstone was found to be in good agreement with the Soil Behaviour Type (SBT) classification system from CPT data.

Numerical simulations of cone penetration in cemented sandstone

We investigate the avalanches of spherical and non-spherical granular particles inside half-filled rotating drums. The time series of the center of gravity of the particle assemblies are obtained via image analysis and their single-sided amplitude (SSA) spectra are analyzed. The spectra features of this new indicator turn out to be characteristic for the avalanches, in terms of the existence of peaks in the low-frequency range and the decay rate of high frequency components. The SSA spectrum has a peak for the packings of non-spherical particles but not for the spherical particles. The high frequency part is characterized by a power law decay { $1/f^a \ (a>0)$ }. A $1/f$-decay is found only for the spherical particles. For the packings of cornered particles, the exponents significantly deviate from $a=1$. As $1/f$ spectra are often associated with self-organized criticality and therefore a scale invariance of the dynamics, we may conclude that there is no scale-invariant structure for granular avalanches. Considering the small number of particles and the regularity of convex particle shapes being used, the spectral features revealed in this study could be utilized for validating particle simulations.

Experimental study of particle shape dependence of avalanches inside a rotating drum

The formation of shear bands is one of the most remarkable phenomena in the dynamics of granular matter. Several parameters have been so far identified to influence the behavior of the shear bands. We carried out experiments to investigate the evolution of the shear bands in the split-bottom Couette cell in the presence of confining pressure. We employed the Particle Image Velocimetry (PIV) to characterize the shear band both in the absence and presence of external pressure. Our results show that the location and width of the shear band are affected by both the confining pressure and the filling height. The shear zone evolves towards the middle of the cylinder and expands to a broader region with increasing applied pressure or filling height; also the angular velocity decreases relative to the rotation rate of the bottom disk. Our findings are consistent with prior empirical observations on the formation of wide shear bands at free surfaces.

Shearing of granular materials in a confined split-bottom Couette cell

The unknown particle scale mechanics of slow granular flows have hindered our understanding of its features, such as rate independence. The DEM simulations unveil the signatures of stick-slip pattern during flow. Furthermore, we analyzed the time evolution of contact networks and constructed a cascade failure mechanism - from the isostatic criterion - which demonstrates the emergence of stick-slip motion. This insight into micromechanics provides phenomenological reasoning for rate independence and large fluctuations in stress.

Micromechanics of slow granular flows

Granular matter transported by hydraulic conveying behaves under several transport regimes. The techncal challenge is to predict the dilute or dense regime of transport as well as the pressure drop required in order to design properly a granular hydraulic conveying system. We address this problem with a laboratory scale experimental setup able to reproduce the key features of hydraulic conveying f grains. Pressure drop measurements as well as image analysis have been used to characterize a slug transport regime and its crossover towards a moving bed transport regime. A dimensionless characteristic curves diagram has been established and a reasonably good pressure drop prediction with a generic power law is explained in details. This preliminary work has the potential to open perspectives for the design of a predictive tool for pressure drops in hydraulic conveying systems.

Hydraulic transport regimes of grains in a millifluidic confinement

Wave velocity and attenuation are two of the most important attributes of geomaterials under seismic loads. To understand how they are influenced by particle and microstructural characteristics, Discrete Elements Method simulations are carried out to simulate the propagation of linear and nonlinear waves in isotropic, frictional granular packings. The focus is on the effects of confining pressure and source characteristics (i.e., input strain and frequency) on wave velocity and attenuation. Three different approaches are used to quantify the seismic quality factor Q or damping in time and frequency domain. Our simulation results verify previous findings on the scaling of wave velocity with respect to confining pressure and coordination number. Quality factors measured both in time and frequency domains increase with confining pressure and show an interesting non-monotonic behavior with input frequency. As expected, wave velocity and quality factor for S-waves decrease with an increasing input shear strain amplitude, while for P-waves they first decrease and then increase at input strain amplitude of 6×10-4. Change of participation number shows that the inhomogeneity of the system increases with the increase of the input strain.

Elastic wave velocity and attenuation in granular material

We present the results of both experimental and numerical investigations of the silo discharge for a cohesive granular material. In our study, thanks to a cohesion-controlled granular material (CCGM) we propose to investigate the effect of the cohesive length lc, on the discharge of a silo for two different configurations, one axisymmetric, and one quasi-2D rectangular silo. In both configurations, an adjustable bottom is used to control the size of the orifice. As observed for cohesionless granular material by previous studies, the mass flow rate and the density through an orifice are mostly controlled by the diameter of the orifice D. The experimental results of the quasi-2D silo are compared with continuum numerical simulations.

The effect of cohesion on the discharge of a granular material through the orifice of a silo

Numerical models of granular materials are useful in tribology, and can be used to predict wear and friction in contacts. DEM-like simulations are used to model particles of third-body, which are partly wear debris from rubbing bodies. It has been shown that the third-body particles can have different flow regimes, depending on their mechanical properties. Among the different characteristics of flow regimes, agglomerate size seems to be crucial. A method based on vortex analysis used in fluid mechanics allows characterizing this cluster size. The results show that different vortex sizes can be observed during the simulation. In particular, it is observed that some vortexes of a characteristic size persist over time, and could be representative of agglomerates. These results pave the way for a better characterisation of the different flow regimes.

Detecting vorticity in cohesive deformable granular material

Structural characteristics of two-dimensional elliptic granular packs with various aspect ratios and intergranular friction coeffcients were studied using the Discrete Element Method (DEM). Isotropic compaction from random unjammed state leads to a jammed state with polycrystals of orientationally ordered clusters (OOC). The OOCs were identified using a cluster labelling algorithm, based on the relative angle \Delta\theta between the major axes of two contacting particles. The threshold value of \Delta\theta was optimised to give the strongest correlation between OOCs and the force chain network. We found that the resulting OOC size distribution decays algebraically with an exponent of -2, independently of grain aspect ratio and material properties.

Structural characteristics of ordered clusters in packs of ellipses

We analyse the different morphologies induced by an oscillating plate above an erodible bed. We present some data describing how the shape and the stiffness of the plate affect the main features of the generated heap. We investigate several configurations with different geometries, frequencies and the amplitudes. Some
preliminary results are available, in which the role of the flexibility of the plate is taken in account. Unlike the rigid plate in which a proper oscillation induces the formation of one heap, the morphology of the bed is now characterized by more than one heap due to a different pressure profile induced by a flexible plate.

Contact evolution in granular materials with inherently anisotropic fabric

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Contact evolution in granular materials with inherently anisotropic fabric

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Next from Powders & Grains 2021

Numerical simulations of cone penetration in cemented sandstone

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Avoiding chaos in granular dampers

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