Reduction of compaction force in a confined bidisperse granular media.

Experiments and simulations of the compaction force in a confined binary granular mixture column were conducted. We measured the resistance force encountered by a piston pushing a vertical granular mixture in this confined arrangement. Granular mixtures with two different particle sizes were considered; the size ratio and the size fraction were both varied. An important decrease of the compaction force was found for volumetric fractions between 15% and 40% and size ratios larger than 3. By conducting some supplementary discrete element simulations, we found that the force chain network is fractured and redistributed when small particles are present. Hence we argue that the reduction of compaction force results from the redistribution of force within the granular column.

The micromechanical behavior of lyophilized glutaraldehyde-treated bovine pericardium under uniaxial tension.

The micromechanical behavior of lyophilized glutaraldehyde bovine pericardium undergoing uniaxial tension was studied by digital image correlation. The experiments were conducted simultaneously at macromechanical and micromechanical levels, to correlate the mechanical response of this biomaterial at different scales. From the experiments, displacement and force data were acquired; in addition, an image sequence of each sample surface was registered with a high-definition camera. With the images, it was possible to obtain the vector displacement field between pairs of images and then the in-plane strain was calculated. The secant and final moduli of this material were obtained at macromechanical and micromechanical levels. A good agreement between the micro and macro moduli was observed. This analysis is a useful alternative technique for studying this biomaterial when local properties are needed for medical applications.

Compaction force in a confined granular column.

Experiments to determine the force required to push a granular column confined within a cylinder were performed. The experimental apparatus was mounted on a material testing system machine in order to obtain force and displacement measurements simultaneously. Experiments were performed for two different sphere diameters, two different cylinder diameters and for a range of piston displacement velocities. The force necessary to displace the column increases rapidly with the column height, in accordance with Janssen's theory. More importantly, we found that this force also increases with the displacement velocity. This unexpected behavior is an indication of the transition to rate-dependent behavior in dense granular flows.