Visualization of Shear Motions of Cohesive Powders in the True Biaxial Shear Tester

Steady-state flow of powders is defined as a continuous deformation of the material without volume change while the stresses at the specimen's boundaries remain constant. Recent investigations have shown that this state, especially for cohesive powders, is not always as steady as it should be by definition. In this article a recent extension of the true biaxial shear tester is introduced that allows a view of the shear motion of the brick-shaped powder specimen inside the tester. By applying a dark-colored powder pattern onto a light powder sample, the movement of the powder can be captured using a CCD camera. Development of shear bands and inhomogeneities of the sample can be visualized. Experiments with a cohesive powder with purely strain-controlled, volume-preserving shear cycles, as well as mixed stress-strain controlled experiments, are presented. The recorded images as well as stress-strain data from discrete elements simulations are compared to the experimental results.     

Visualization of Shear Motions of Cohesive Powders in the True Biaxial Shear Tester

Steady-state flow of powders is defined as a continuous deformation of the material without volume change while the stresses at the specimen's boundaries remain constant. Recent investigations have shown that this state, especially for cohesive powders, is not always as steady as it should be by definition. In this article a recent extension of the true biaxial shear tester is introduced that allows a view of the shear motion of the brick-shaped powder specimen inside the tester. By applying a dark-colored powder pattern onto a light powder sample, the movement of the powder can be captured using a CCD camera. Development of shear bands and inhomogeneities of the sample can be visualized. Experiments with a cohesive powder with purely strain-controlled, volume-preserving shear cycles, as well as mixed stress-strain controlled experiments, are presented. The recorded images as well as stress-strain data from discrete elements simulations are compared to the experimental results.     

Analysis of contact force distribution in a moving granule bed subjected to shear deformation by a set of rollers

Large contact forces on granules could give rise to undesirable attrition. In a new device, referred to as the Particle Shear and Impact tester, granules are subjected to repeated and sequential impact and shearing, the latter effected by two counter-rotating rollers with differential angular speeds and an adjustable gap between the rollers. This enables the contact force distribution to be varied in order to apply representative contact forces, as experienced in manufacturing plants. Granule flow in the rollers is simulated by Distinct Element Method for several roller gap sizes. The resulting contact force distribution is compared to that from shear cell simulations for representative plant normal stresses (8 to 15 kPa), in order to calibrate the appropriate gap size. The 90th percentile of the contact forces distributions from the two simulations are matched to set the gap size. A roller gap size approximately 3.5 times the 90th percentile of the particle size (based on number distribution) gives a good match of the interparticle contact forces between the rollers and the shear cell. This enables replicating the stresses that granules experience in plants, whether during handling and transport, or during more severe stressing conditions, e.g. compaction or even grinding, thereby assessing attrition or fragmentation propensity of granules.