Experimental investigation of cohesive powder rheology

An annular shear cell-type cohesive powder rheometer has been fabricated, w hich measures the shear and normal stress states over a range of shear rate and powder concentration. The rheological behavior of a 30-mu m spherical silica powder and a 40-mu m industrial polymer powder have been experimenta lly investigated for powder flow in the frictional regime. Although the sil ica powder appears to be cohesionless, based on Jenike shear cell yield loc us measurements, and the industrial powder has strong cohesion, they both d emonstrate similar rheological behavior in the frictional flow regime. At l ow shear rates, the shear stress increases with increasing shear rate until a critical shear rate is reached, at which point the shear stress decrease s with increasing shear rate. The shear-to-normal stress relation with shea r rate, for frictional flow, is apparently independent of powder concentrat ion, and the data approximately collapse into a single curve for a given po wder. The requirements for a constitutive model relating the stress tensor to the rate of deformation for the powder assembly are discussed. (C) 2000 Elsevier Science S.A. All rights reserved.