In this series of editorials I’m looking at how well the traditional powder testing toolkit meets the requirements of today’s powder processors. In the last editorial I put tapped density methods under the spotlight; and now it’s time to consider shear testing.
Shear testing has its roots in the pioneering work carried out by Jenike in the 1960’s to tackle the issue of discharge from storage vessels. The technique was developed to support design methodologies that brought a numerical approach to the specification of powder handling equipment for the very first time. The task of developing a design algorithm for hoppers and silos to deliver controlled powder discharge was a major challenge, and the work has stood the test of time remarkably well. While discharge and hopper design remain an imperfect science, the strategies developed fifty years ago have not been substantially improved upon and remain in use.
Figure 1 – Traditional Shear Cell
In simple terms shear testing involves measuring the forces required to shear one consolidated powder plane relative to another. Similar methods are applied to measure wall friction, the friction between a construction material surface and a powder, to provide most of the data required for hopper design. At the base of the hopper, powder consolidated by the weight of material above it, is subject to this type of stress as it moves within the powder bed, or relative to the vessel wall, so the relevance of these test methods is clear.
Over the decades since shear testing was conceived the need for process-relevant powder characterisation has encouraged the application of shear testing well beyond this original intent. This is understandable but has brought into focus some of the technique’s limitations.
One issue is that shear testing is most suitable and accurate for cohesive powders. With a cohesive material the shear forces measured are relatively large but with less cohesive samples the forces become diminishingly small. The ‘free-flowing’ classification of Flow Function, a primary shear parameter, in fact covers a broad range of flow behaviours, and shear testing is simply not as differentiating as, for example, dynamic characterization (see below).
A second, arguably more important issue is that the conditions applied during shear testing are not representative of those that prevail in, for example, a fluidized bed or during gravitationally induced filling. For certain processes, the response of the powder to air is of crucial importance and this is something that cannot be directly investigated via shear analysis. More broadly, trying to infer from shear data how a powder will behave under any conditions that are markedly dissimilar from the test environment can be both inaccurate and inefficient.
There is no intention here to dispute the value of the shear testing for the purpose for which it was designed. And I would also argue that beyond this shear testing has a role to play in providing insight into the nature of powders for more general study. I would however suggest that other techniques, most especially dynamic testing, are more appropriate for powder characterization as shear analysis starts to reach its limits. Recognizing these limits and bringing the most appropriate technique to bear are crucial as processors push towards greater manufacturing efficiency.
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