Analysis of operational window in Freeman Technology uniaxial powder tester

Abstract

The operational window of the recently developed Freeman Technology Uniaxial Powder Tester (UPT) in terms of specimen aspect ratio (H/D), consolidation stress, and consolidation and failure strain rates is investigated for a range of bulk solid materials. The results show that the measured uniaxial unconfined yield stress (uUYS) decreases as aspect ratio increases but this sensitivity of uUYS to H/D reduces significantly for aspect ratios of greater than 1.5. The sensitivity of uUYS to aspect ratio is attributed to the intersection of slip planes with the top and bottom platens. The critical aspect ratio (defined as a ratio beyond which uUYS becomes independent of aspect ratio) is found to be a material-specific property, but it is increased with increasing consolidation stress. A correlation between the internal angle of friction and the critical aspect ratio is found, but it does not follow previously reported work. This discrepancy could be attributed to the differences in the packing fraction of the samples in uniaxial compression and the shear cell tests which were used to measure the internal angle of friction. uUYS is found to be independent of consolidation strain rate for the range investigated here (0.005 to 0.05 s−1). A very high level of repeatability is observed for the measurement of compressibility response of bulk solids with no sensitivity to consolidation strain rate uniaxial compression; powder flow; flowability; unconfined yield stress; aspect ratio; strain rate; compressibility.