Stress inhomogeneity in powder specimens tested in the Jenike shear cell: Myth or fact?

In the mechanical characterization of powders using the direct shear testers such as the Jenike shear cell, the existence of a uniform or well-defined stress field in a powder specimen is assumed. This assumption has not been subjected to any serious scrutiny in the literature. In this study, the normal stress variation in a silica powder was locally determined by locating a pressure sensitive TekScan pad at the bottom section of a Jenike shear cell. A computer simulation of the consolidation and pre-shearing stages of the Jenike test procedure was performed using the Discrete Element Method (DEM). The paper presents both experimental and computational evidence for the existence of a complex stress field in the powder specimen, thus clearly invalidating the long-standing stress homogeneity assumption in the direct shear testing of powders. The implications of the stress inhomogeneity in terms of the accuracy of the material properties extracted from the Jenike test are also presented.