In this work, the constraint factor of a broad class of powders was quantified from indentation and shear cell experiments at moderate to high stresses and was found to be generally independent of the applied stress, while for four out of the twenty-five materials it exhibited fluctuations. In order to infer the unconfined yield strength from hardness measurements at low stresses, it was assumed that the constraint factor remains constant at lower stresses. Distinct Element Method (DEM) modelling was also utilised to simulate the ball indentation system, allowing the powder bed internal failure stresses to be realised in order to elucidate the behaviour of the constraint factor at low stresses. The simulations validated the assumption that the constraint factor remains constant throughout the applied stress range.
Furthermore, the applicability and reliability of both ball indentation and the FT4 shear cell were assessed in a wide range of both low and moderate to high stresses. Ball indentation gave very repeatable results throughout the whole range of stresses applied, whilst the FT4 shear cell was deemed unreliable for most materials at pre-shear normal stresses of 1 kPa and below. For all materials except the three powders that remain very cohesive throughout the stress range tested, the increase of hardness (and therefore also the unconfined yield strength inferred from ball indentation) with stress was observed to be much steeper at low stresses, as compared to higher stresses, due to a more rapid increase in packing fraction. For all model glass beads tested, except for the 0 - 20 ¼m samples, hardness was found to be independent of penetration depth in a certain depth range. In contrast, for most ?real? materials, plus the aforementioned very fine model glass beads, hardness was found to continually increase with depth, with a gradient that is independent of the applied stress and similar for all materials tested. The powders that are prone to stick-slip deviated from the above behaviour and exhibited a fluctuating force response.
The influence of a variety of particle properties on the constraint factor, and subsequently powder flowability, was also determined both experimentally and computationally. The effects of particle size, size distribution, and single particle and agglomerate shape were investigated exper