Investigation of powder flowability at low stresses: Influence of particle size and size distribution

At moderate stresses, shear cells are the preferred method of powder flow measurement. However, several industrial processes operate at low stresses, where the determination of unconfined yield strength by the shear cell technique may be inconsistent, or found not to correlate with observed behaviour. Alternatively, ball indentation can be used, which directly measures hardness; related to unconfined yield strength by the constraint factor. However, it is not known how constraint factor is influenced by particle properties. Here, ball indentation and shear cell methods are applied for glass beads of various size distributions, and the influence of particle size distribution on the constraint factor is explored. The constraint factor is shown to be independent of the pre-consolidation stress, though reduces as the d10, d50 or d90 are increased. Unconfined yield strength inferred from indentation measurements suggest that extrapolation of shear cell data to low stresses overestimates the unconfined yield strength. [Display omitted] •Extrapolating shear cell data from high to low stress overestimates yield strength.•Ball indentation measures hardness; equal to yield strength times constraint factor.•Constraint factor is independent of applied stress.•Constraint factor decreases with increasing particle size and distribution width.•Constraint factor increases with fines content, though reduces with added coarse.