Note on the interpretation of powder shear test data

Powder shear testing has been used to assess flowability of powders for at least 48 years A.W. Jenike, Storage and Flow of Solids, Bull., Eng. Exp. Station, Univ. Utah, vol. 123, 1964]. A fundamental part of the data interpretation involves construction of Mohr’s circles such that they are tangent to the experimental locus; a set of powder shear strengths, τ_m, measured at variable applied compressive normal stresses, σ_N. Despite the customary application of this tangency criterion, we have found justifications for it to be scant.In this work, we revisit the Mohr’s circle construction and find that proper construction would require measurement of reactionary lateral powder stresses, σ_r. We further provide reasoning to support placement of the experimental locus passing through the apex (top) of Mohr’s circles for isotropic or orthotropic powders and not tangentially to them. This placement is not unexpectedly in agreement with the results of a recent numerical simulation C. Thornton, L. Zhang, Numerical simulations of the direct shear test, Chem. Eng. Technol. 26 (2) (2003) 153-156], and reveals errors in subsequently calculated constants on the order of 20%.