Impact of Shear History on Powder Flow Characterization Using a Ring Shear Tester.

In this study, we have investigated the impact of repeated shear displacement on powder flow properties. We show that when multiple yield loci are obtained using the same bulk solid specimen by stepping through different stress levels (i.e., stress walk [SW]), the shear deformation of the powder in a rotational shear cell, that is, Schulze Ring Shear Tester, is maximized, reducing the powder shear strength. This approach is material and time sparing; however, it imprecisely predicts better powder flowability. The magnitude of the change in the unconfined yield strength, σ, due to this prolonged shear displacement appears to be material-dependent, being less impactful for free-flowing powders. Using the SW and the individual yield loci-generated flow properties, we have demonstrated that in hopper design, the shear displacement effect impacts the computed critical arching diameter more than the critical mass flow angle. This knowledge of powder flow properties highlights limitations associated with the SW. An exponential function was found to describe the relationship between the change in σ at the highest major principal stress and the density weighted flowability, ff, with an R of 0.98. Such a model could be a valuable tool for correcting shear strength results obtained from SW, saving time, and material.