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

Abstract

This paper is concerned with micromechanics of Schneebeli material specimens composed of wooden roller stacks. Several laboratory tests are carried out to analyse the material behaviour under complex loading conditions, involving loading–unloading cycles and principal axes rotations. In order to characterize micromechanical deformation features and structure evolution, a series of pictures is taken during loading. Pictures are then digitized using a stereo device, obtaining the position of each roller. Starting from these data a number of computer programs, conceived for the purpose, allow us to measure micromechanical variables and to analyse their evolution. In the following, after the description of the devices employed in this research, macromechanical results are analysed to evaluate the reliability of the laboratory model. Then, local variables are introduced and the use of continuum mechanics to describe granular materials behaviour is discussed. Finally, the evolution of local kinematic variables is described, focusing interest on the evolution of specimen anisotropy. © 1997 by John Wiley & Sons, Ltd. Mech. cohesive‐frictional mater. 2, 121–163 (1997)

Micromechanics of shear bands

“…As expected, prior to shear band formation there is compression and then within the shear band there is dilation, as revealed by the appearance of large voids within the shear band in sands (e.g. see Chapter 4 of Oda and Iwashita, 1999) and Schneebeli systems (Calvetti et al, 1997). In Fig.…”

Micromechanics of shear bands

“…As expected, prior to shear band formation there is compression and then within the shear band there is dilation, as revealed by the appearance of large voids within the shear band in sands (e.g. see Chapter 4 of Oda and Iwashita, 1999) and Schneebeli systems (Calvetti et al, 1997). In Fig.…”

Satic states of cohesive granular media

“…For the initial state, contacts are orientated with a quasihomogeneous manner for weak adhesion value, Unlike for strong adhesion and for two types of effort, two particular directions of ±45°with respect to the compression axis appear on the diagram. This "bimodal" distribution of contact directions has also been observed for non cohesive particles deposited in the gravitational field [4]. Here we observe that this effect is much more important for high adhesion values.…”

Micromechanical constitutive modelling of granular media: evolution and loss of contact in particle clusters

“…The increase in void ratio within a shear band, as shown in Figure 8, is consistent with the experimental observations of dilation and the appearance of large voids within shear bands in sands (e.g. see [2,Chapter 4]) and assemblies Schneebeli rods [12]. The total rotation in the region of the shear band, as shown in Figure 9, refers to the net accumulation of rotation ω (see Equation (3.1)) from the start of deformation up to a shear strain of 0·8.…”

Micromechanics of shear bands

“…As expected, prior to shear band formation there is compression and then within the shear band there is dilation, as revealed by the appearance of large voids within the shear band in sands (e.g. see Chapter 4 of Oda and Iwashita, 1999) and Schneebeli systems (Calvetti et al, 1997). In Fig.…”

Satic states of cohesive granular media

“…For the initial state, contacts are orientated with a quasihomogeneous manner for weak adhesion value, Unlike for strong adhesion and for two types of effort, two particular directions of ±45°with respect to the compression axis appear on the diagram. This "bimodal" distribution of contact directions has also been observed for non cohesive particles deposited in the gravitational field [4]. Here we observe that this effect is much more important for high adhesion values.…”

Micromechanical constitutive modelling of granular media: evolution and loss of contact in particle clusters

“…The increase in void ratio within a shear band, as shown in Figure 8, is consistent with the experimental observations of dilation and the appearance of large voids within shear bands in sands (e.g. see [2,Chapter 4]) and assemblies Schneebeli rods [12]. The total rotation in the region of the shear band, as shown in Figure 9, refers to the net accumulation of rotation ω (see Equation (3.1)) from the start of deformation up to a shear strain of 0·8.…”

Micromechanics of shear bands

“…As expected, prior to shear band formation there is compression and then within the shear band there is dilation, as revealed by the appearance of large voids within the shear band in sands (e.g. see Chapter 4 of Oda and Iwashita, 1999) and Schneebeli systems (Calvetti et al, 1997). In Fig.…”

Satic states of cohesive granular media

“…For the initial state, contacts are orientated with a quasihomogeneous manner for weak adhesion value, Unlike for strong adhesion and for two types of effort, two particular directions of ±45°with respect to the compression axis appear on the diagram. This "bimodal" distribution of contact directions has also been observed for non cohesive particles deposited in the gravitational field [4]. Here we observe that this effect is much more important for high adhesion values.…”

Micromechanical constitutive modelling of granular media: evolution and loss of contact in particle clusters

“…The increase in void ratio within a shear band, as shown in Figure 8, is consistent with the experimental observations of dilation and the appearance of large voids within shear bands in sands (e.g. see [2,Chapter 4]) and assemblies Schneebeli rods [12]. The total rotation in the region of the shear band, as shown in Figure 9, refers to the net accumulation of rotation ω (see Equation (3.1)) from the start of deformation up to a shear strain of 0·8.…”