Cohesion and Internal Friction of Fine Glass Beads as Affected by Small Intensity Vertical Vibration

Abstract

We have used a novel centrifuge powder tester to obtain the angle of internal friction and cohesion of fine glass beads as affected by previous vibration in the vertical direction. In the experimental procedure we use a small amount of mass, typically between 2 and 4 grams, contained in a rectangular cell. The bed is initialized and subjected to low intensity vertical vibrations of controlled frequency and amplitude for a fixed period of time. By means of pre-vibration the material becomes compacted. Then the cell is taken to the centrifugal powder tester, in which it is rotated around its vertical axis at increasing values of the rotation velocity. At a critical point the shear stress caused by the action of the centrifugal force is large enough to drive material avalanches. From a theoretical analysis of these avalanches based on the Coulomb’s method of wedges we derive the angle of internal friction and cohesion of the glass beads. Measurements have been performed using different masses pre-vibrated at different frequencies and amplitudes. Results from the tests are fitted to a single trend when they are plotted as a function of the effective consolidation stress imposed on the bed by means of pre-vibration. Basically, the data indicate a significant increase of cohesion and a slight decrease of the angle of internal friction as the effective consolidation on the sample is increased. The interparticle cohesion force has been estimated from the cohesion measured, and using the averaging Rumpf’s equation. For the unconsolidated samples, the value estimated agrees with the expected force due to the sum of van der Waals and capillary forces for undeformed contacts between surface asperities. However, the interparticle cohesion force increases as pre-vibration intensity is increased, being this the main reason for the increase of cohesion at the bulk level. According to theoretical estimations, the increase of the interparticle cohesion force is attributable to the plastic yield of the surface asperities at contact. The rate of increase of the interparticle cohesion force with the interparticle consolidation force is in accordance with the results predicted by a theoretical model on plastic contacts between surface asperities. It can be concluded that fine powder flowability is seriously hindered by compaction due to pre-vibration