Measurements of unconfined yield stress at low stresses are often inconsistent, or do not correlate with observed process behaviour. Over the last decade or so a number flowability measurement techniques operable at low stresses have been introduced, or become more prominent. A few of these devices are also capable of flowability measurements at strain rates beyond the quasi-static regime. One such technique is ball indentation, which directly measures hardness; the resistance of the bed to plastic deformation. The unconfined yield stress is directly related to the hardness by the constraint factor, which is dependent on particle properties, although the constraint factor cannot yet be determined a priori. The flowability of titania is measured here using ball indentation and a Schulze ring shear tester. In contradiction to previous research on ball indentation, the bed hardness is found to be approximately constant at dimensionless penetration depths of 0.1 – 0.3, yet increases beyond this range. This could suggest that the suitable penetration depth range is not only dependent on indenter size, but particle size also. The bed hardness is found to increase approximately linearly with consolidation stress, and correlates with unconfined yield stress measurements from a shear cell at normal stresses of 3 – 15 kPa. The constraint factor is found to be approximately constant at higher stresses, but increases slightly at lower stresses. Inferred yield stress values at low stresses are greater than those extrapolated from shear cell measurements, again in contradiction to previous findings on other powders. Future work will utilize DEM to explore the variation of constraint factor at lower stresses and for varying particle properties. The flowability of silanised glass beads of a range of surface energy values will be characterised by ball indentation at quasi-static and dynamic conditions, along with non-spherical particles, such as calcium carbonate and limestone.
This technique further benefits from the requirement of only a small powder quantity.