Executive Summary

At moderate to high stresses shear cells are the preferred method of powder flow measurement, however at low stresses the determination of unconfined yield strength by this technique is often marred with inconsistencies in the measurement, or in comparison to observed behaviour. An alternative approach for measuring powder flow is ball indentation, which directly measures hardness; related to unconfined yield strength by the constraint factor. The ball indentation and shear cell methods are applied for a wide range of powders, and the constraint factor is found to increase if particle size is reduced or the particle size distribution is reduced. Constraint factor is found to be independent of stress for almost all tested powders, though varies from 1.8 – 5.5 for the different powders tested, and hence its determination is required in order to provide meaningful unconfined yield strength values from ball indentation. For many powders the variation in unconfined yield strength with major principal stress is shown to be much more pronounced at low stresses (< 1 kPa) when analysed by ball indentation. Shear cell measurements at this low stress range generally agree with this trend, though for some powders discrepancies exist between the values reported by each technique. The hardness measurement at low stress is highly reproducible, though relies on a constant constraint factor in order to determine unconfined yield strength. In contrast, the reproducibility of the yield locus generated in the shear cell weakens at lower stresses, with the minimum stress that provides reliable measurements being material dependent. For maize starch the shear cell measurement appears to be reliable even at a pre-shear normal stress of 60 Pa, and the determined unconfined yield strength agrees very well with that determined by ball inden