Shear wave propagation in granular assemblies

Abstract

Shear wave velocity is a fundamental property of a granular assembly. It is a measure of the true elastic stiffness of a bulk specimen of discrete grains. Shear wave velocity is typically measured in the laboratory (e.g., using bender elements) or in-situ (e.g., using a seismic cone penetrometer, sCPT). In the current work, shear wave propagation is modeled numerically using the discrete element method (DEM). First, an appropriate method for measuring wave velocity is identified. The effects of particle size and elastic properties are investigated. Specimen fabric is quantified before and after wave excitation and the elasticity of the response at the scale of the particle contacts is investigated. The results show that shear wave velocity may be robustly measured for discrete numerical specimens. The ability to measure shear wave velocity using DEM simulations may provide another tool for researchers seeking to link results from physical and numerical experiments.

An efficient flexible membrane boundary condition for DEM simulation of axisymmetric element tests

“…A variety of boundary conditions have been implemented in DEM to capture the correct mechanical behavior of an assembly of particles. Rigid walls are the most common type of boundary conditions for element tests and have been used for, eg, modeling a rectangle or a hexagon in a biaxial compression test, 1,2 a rectangle in a 2D direct shear test, 3 a cube in a true triaxial test, [4][5][6][7] a parallelepiped in a direct shear test, 7,8 a cylinder in an isotropic consolidation test, 9,10 a ring in an oedometer test, 11,12 and a series of rings in a simple shear test. 13 Stresses on the walls are calculated as the sum of ball-wall contact forces divided by the wall area.…”

A discrete element method representation of an anisotropic elastic continuum

“…Mass-scaling was employed to improve calculation speed; in this approach, element sizes are increased by several orders of magnitude to increase the critical time step for numerical stability (see, e.g., (Belheine et al, 2009;Evans and Frost, 2007;Ning et al, 2015;Yun and Evans, 2011;Zhao et al, 2017)). Simulations were performed in the absence of gravity so that the increased element sizes do not generate excessive self-weight within the assembly.…”

A micromechanics-based micromorphic model for granular materials and prediction on dispersion behaviors

“…However, because of the discrete characteristics of macroscopic granular media and the complexity of contact behaviors, there still exist some problems for the study on the wave propagation: (1) The existence of wave dispersions in granular materials is still controversial [12]. (2) There are still some differences in the filtering ability and the wave velocity of granular materials predicted by different models [13][14][15]. (3) The influence of microstructures on the direction, the attenuation and the dispersion of the wave propagation should also be studied systematically.…”

“…At the micro scale, the DEM is closer to the physical essence of granular materials, and it is also convenient to simulate the wave propagation through the energy transfer among particles, which makes it convenient to investigate the influences of microscopic geometrical information such as particle sizes, particle arrangements and contact constitutive relationships on the wave propagation from the microscopic level. However, studies on the wave propagation by the DEM [12,[15][16][17][18][19][20][21][22] are still at a qualitative or initial quantification stage mostly. This is mainly because the conception of wave is under the continuum framework, and there are still problems about how to describe the wave propagation quantitatively by the DEM simulations and how to measure the influence of the discrete information through homogenization on the wave propagation.…”