Ball Indentation on Cohesive Powder Beds

13 In the shear deformation of powder beds beyond the quasi-static regime the shear stress is dependent 14 on the strain rate. Extensive work has been reported on the rapid chute flow of large granules but the 15 intermediate regime has not been widely addressed particularly in the case of cohesive powders. 16 However in industrial powder processes the powder flow is often in the intermediate regime. In the 17 present work an attempt is made to investigate the sensitivity of the stresses in an assembly of 18 cohesive spherical particles to the strain rate in ball indentation using the Distinct Element Method. 19 This technique has recently been proposed as a quick and easy way to assess the flowability of 20 cohesive powders. It is shown that the hardness, deviatoric and hydrostatic stresses within a bed, 21 subjected to ball indentation on its free surface, are dependent on the indentation strain rate. These 22 stresses are almost constant up to a dimensionless strain rate of unity, consistent with trends from 23 traditional methods of shear cell testing, though fluctuations begin to increase from a dimensionless 24 strain rate of 0.5. For dimensionless strain rates greater than unity, these stresses increase, with the 25 increase in hardness being the most substantial. These trends correlate well with those established in 26 the literature for the Couette device. However quantitative value of the strain rate boundaries of the 27 regimes differ, due to differences in the geometry of shear deformation band. Nevertheless, this 28 shows the capability of the indentation technique in capturing the dynamics of cohesive powder flow. 29 30