Experimental micromechanical analysis of a 2D granular material: relation between structure evolution and loading path

International Journal of Solids and Structures

Bagi

2004

The paper considers rotations at different scales in granular materials: the rotations of individual particles, the rolling and rigid-rotation of particle pairs, the rotational interactions of a particle within its cluster of neighbors, and the rotation of material regions. Numerical, Discrete Element Method (DEM) simulations on two-and three-dimensional (2D and 3D) assemblies show that particle rotations are diverse, that they increase with strain until the material begins to soften, and that they are expressed in spatial patterns, even at small strains. The interactions of a pair of particles are a combination of three modes: a contact deformation mode, a contact rolling mode, and a mode of rigid pair motions. Definitions are presented for each mode, including four different definitions of contact rolling. A rolling curl is also defined, which describes the cumulative rolling of neighboring particles around a central particle or sub-region. At a larger scale, material deformation and rotation are measured within small sub-regions of material, and the material deformation can be attributed to separate contributions of contact rolling, contact deformation, and the rigid-rotation of particle pairs. The diversity and extend of contact rolling were measured in 2D and 3D simulations. A dominant rolling pattern was observed, which resembles the interactions of rolling gears. This pattern can extend to distances of at least six particle diameters from a central particle.

Section: Particle Rotationsmentioning

confidence: 86%

Section: Particle Rotationsmentioning

confidence: 99%

Contact rolling and deformation in granular media

International Journal of Solids and Structures

Bagi

2004

“…the mean-field rotation). However, simulations and physical experiments have shown that although individual particle rotations can differ greatly from the mean rotation field, the mean particle rotation is very close to the meanfield rotation (e.g., Calvetti et al 1997). The current work provides an opportunity to test whether the mean rotation field differs from the mean-field rotation under conditions of non-uniform deformation.…”

Section: Particle Rotationsmentioning

confidence: 99%

Are granular materials simple? An experimental study of strain gradient effects and localization

2005

Mechanics of Materials

Experiments test the dependence of shearing stress on the first two gradients of shear strain. The tests were conducted by direct numerical simulation using the Discrete Element Method (DEM) on a large two-dimensional (2D) assembly of circular disks. The assembly was coerced into non-uniformly deformed shapes by applying body forces to the material region. The tests show that shearing stress is affected by both the first and second gradients of shear strain, and the measured responses to strain and its gradients are all incrementally non-linear. The dilation rate is unaffected by strain gradients. Particle rotations, although highly erratic, are, on average, consistent with the mean-field rotation and unaffected by strain gradients. In independent unconstrained tests, the material was sheared without body forces so that localization could freely occur. Three localization patterns were observed: microbands at very small strains; non-persistent shear bands at moderate strains; and persistent bands at large strains. The observed features of microbands and shear bands are consistent with the measured influences of shearing strain and its first two derivatives.

“…Table 1 begins with topologic characteristics, which concern the arrangement of the particles and their contacts, but without reference to their position, size, or orientation. This information can be expressed as a particle graph for both 2D and 3D assembles, which gives the topologic connectivity of the particles in a packing, as ex- [5,6,7,8,9,10,11] e Valence [12,9] b Geometric Grain size Grain shape Grain orientation [13,14,15], [16,17] b Void ratio [18], [3,19,20] b , [20] c , [21,22,23,24,25] d Void size/shape [26] b Branch vector [13,15,27,28], [29] b , orientation [6,14,30,31] e Branch vector length [4], [32] c Fabric tensor (4.2) [33,34], [8,7,11] e Loop tensor [9,35], [26] b Kinematic Particle movement [36],…”

Section: Introductionmentioning

confidence: 99%

Heterogeneity and patterning in the quasi-static behavior of granular materials

2003

Granular Matter

Heterogeneity is classified in five categoriestopologic, geometric, kinematic, static, and constitutiveand the first four categories are investigated in a numerical DEM simulation of biaxial compression. The simulation experiments show that the topology and geometric fabric become more variable during loading. The measured fluctuations in inter-particle movements are large, they increase with loading, and they extend to distances of at least eight particle diameters. Deformation and rotation heterogeneity are large and are expressed in spatial patterning. Stress heterogeneity is moderate throughout loading.