Effect of strain rate on powder flow behaviour using ball indentation method

Abstract

There are a number of techniques available for characterising powder flow behaviour, most of which are carried out under quasi-static conditions. Quasi-static methods are commonly used to define the conditions for the flow initiation, but they are inadequate for characterising the shear strain rate sensitivity of powder flow. The number of devices available to capture powder behaviour under dynamic conditions is indeed very limited. The very few commercially available instruments all require a large sample, which is not desirable for early stages of product development. We have recently proposed the use of ball indentation for characterising flow resistance of cohesive powders. The technique can be applied at very low consolidation stress levels (much less than 1 kPa) and requires only a small sample quantity, typically a few mm3. Previous work showed a good correlation of flowability with established methods under quasi-static conditions. Therefore, an attempt is made to extend the method to dynamic conditions and investigate the sensitivity of the stresses on the shear strain rate for a range of powders, including glass ballotini, α-lactose monohydrate, calcium carbonate (Durcal) and limestone. The results show the existence of a threshold boundary above which the flow resistance experienced by the penetrating ball becomes dependent on the shear strain rate. This is in line with the trends reported in the literature. Therefore, the ball indentation technique has the potential to be used to assess powder flowability at high strain rates.