Validation of Powder Properties Measured by a Rotational Shear Cell

Powders will be subjected to consolidation stress during their handling, whether it is transport in a keg, storage in a hopper, or processing through an IBC on the top of a tablet press or roller-compactor. To understand the relationship between consolidation stress and the ability of a powder to transition from static to incipient flow conditions, a shear cell is commonly used. The use of rotational shear cells has become commonplace due to the advantages of easy operation, quicker testing time and unlimited shear displacement. In a rotational shear cell, a vaned head is used to induce a precisely controlled normal stress on the powder bed. While maintaining this normal consolidating stress, the head then rotates to induce a rotational stress. As the powder initially resists this movement, the shear stress increases until this resistance is overcome and the powder bed fails or shears. This is the point where flow occurs and is known as the Point of Incipient Failure (shear point). In this study, data collected from a rotational shear cell has been validated by comparing it to the certified shear stress values for the standard reference material BCR 116 limestone. To further support this investigation a uni-axial testing method was employed to compare Unconfined Yield Strength (UYS) values with those derived from a rotational shear cell, as well as to the standard values. The influence of the fitting algorithms used to derive the Flow Function (FF) values was then investigated.