Powder flow characterization at low consolidation: modeling and experimental values of preshear and shear sequences

Measurements of the unconfined yield strength (fc ) of powders can be made with a variety of commercially available shear testing devices. For all of these devices, it has been observed that the reproducibility o f fc measurements is greatly reduced at low levels of major principal stress. Furthermore at low consolidation the conventional sample procedures may not be adequate since they may alter the powder consolidation condition with respect to the desired for practical application. The state of the art shear tester recently developed by Anton Paar Company is a unique instrument for the study of dry granular flows, especially in the fluidized or near fluidized state it is able to keep the powder in unconfined and not consolidated condition. Four different powders were used in this research, namely, Alumina, Glass Beads, Calcium Lactate and Sand. A specifically developed sequence of operations, including the introduction of air, allows the application of the desired stress history on the powder sample and to carry out experiments that may mimic the preshear and shear sequences operated in conventional testers. In particular, stirring the bed without air allows consolidating the material as in the preshear step, in which the shearing action is carried out till the attainment of a constant shear stress, which is assumed to correspond to the critical state. Stopping the shearing actions and using an upward gas flow means reducing the load on the shear plane by sustaining part of the bed weight by means of the gas flow. Therefore, starting a shearing action in this condition corresponds to the shear step in a rotationalshear tester. This finding suggest the possibility that, with an appropriate estimate of the normal forces applied to the shear plane, might allow the use of rheometry of aerated powder as a tool to characterize powder flow properties at low consolidation.