Evaluating The Powder Testing Toolkit: Shear Testing

By Tim Freeman, Freeman Technology

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, who tackled the issue of discharge from storage vessels in the 1960s. 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. 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 and remain in use.

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.