A virtual experiment technique on the elementary behaviour of granular materials with discrete element method

Abstract

SUMMARYMulti-scale investigations aided by the discrete element method (DEM) play a vital role for current state-ofthe-art research on the elementary behaviour of granular materials. Similar to laboratory tests, there are three important aspects to be considered carefully, which are the proper stress/strain definition and measurement, the application of target loading paths and the designed experiment setup, to be addressed in the present paper. Considering the volume sensitive characteristics of granular materials, in the proposed technique, the deformation of the tested specimen is controlled and measured by deformation gradient tensor involving both the undeformed configuration and the current configuration. Definitions of Biot strain and Cauchy stress are adopted. The expressions of them in terms of contact forces and particle displacements, respectively, are derived. The boundary of the tested specimen consists of rigid massless planar units. It is suggested that the representative element uses a convex polyhedral (polygonal) shape to minimize possible boundary arching effects. General loading paths are described by directly specifying the changes in the stress/strain invariants or directions. Loading can be applied in the strain-controlled mode by specifying the translations and rotations of the boundary units, or in the stress-controlled mode by using a servo-control mechanism, or in the combination of the two methods to realize mixed boundary conditions. Taking the simulation results as the natural consequences originated from a complex system, virtual experiments provide particle-scale information database to conduct multi-scale investigations for better understanding in granular material behaviours and possible development of the constitutive theories provided the qualitative similarity between the simulation results from virtual experiments and observations on real material behaviour.

Macro deformation and micro structure of 3D granular assemblies subjected to rotation of principal stress axes

“…The virtual experiment procedure proposed by Li et al [33] has been implemented in the commercial software PFC3D to reproduce the behavior of three dimensional granular materials subjected to continuous rotation of principal stress axes. The samples are considered as the representative elements.…”

“…Li et al [33] proposed a numerical technique to achieve general loading path by imposing translational and rotational motion of rigid frictional boundaries. In this paper, we implement it in the commercial DEM package-Particle Flow Code in Three Dimensions (PFC3D) [34] to reproduce the material deformation characteristics to rotation of principal stress axes.…”

“…For better sample uniformity, it is suggested that the rigid massless boundary surfaces are to be generated forming a polyhedron with obtuse angles between any two neighbouring boundaries. The boundary control algorithm detailed in [33] has been used to control the displacements of boundary walls synchronously to impose the strain-controlled boundary, and to monitor the stresscontrolled boundary with a servo-controlled mechanism.…”

“…The Biot strain definition is used to describe the specimen deformation [33]. For simplicity, in this study, we impose zero rigid body rotation, and fix the position of specimen origin O throughout the simulation.…”

Contributions to the generalization of advancing front particle packing algorithms

“…Several professional and free softwares are available for this purpose. Practical applications to a wide range of problems may be found in the recent literature ; however, most of them assume spherical particles. Spheres are simple to code and easy to use, but in many cases, they cannot capture the basic dynamic mechanisms and therefore do not provide the most adequate geometric model for the particles.…”