Stretching the limits of dynamic and quasi-static flow testing on cohesive limestone powders - 2020

Powders are a special class of granular matter due to the important role of cohesive forces. Because the flow behavior of powders depends on both their flow states and confining stresses, it is difficult to measure/quantify with only one experiment. In this study, the most commonly used characterization tests that cover a wide range of states are compared: (static, free surface) angle of repose, the (quasi-static, confined) ring shear steady state angle of internal friction, and the (dynamic, free surface) rotating drum flow angle are considered for free flowing, moderately and strongly cohesive limestone powders.

The free flowing powder gives good agreement of angles among all different situations (devices), while the moderately and strongly cohesive powders behave more interestingly. Starting from the flow angle in the rotating drum and going slower, one can extrapolate to the limit of zero rotation rate, but then observes that the angle of repose measured from the static heap is considerably larger, possibly due to its special history. When we stretch the ring shear test to its lowest confining stress limit, the steady state angle of internal friction of the cohesive powder becomes comparable with the flow angle (at free surface) in the zero rotation rate limit of the rotating drum test, by defining an appropriate effective stress.