Dynamic strength of liquid-bound granular materials: The effect of particle size and shape

The dynamic strength of liquid-bound granular pellets was measured by uniaxial compression at speeds of up to 150 mm/s. Pellets were made with a range of different powders: three fractions of spherical glass ballotini from 26 to 78 μm in size, copper powders of spherical, irregular and dendritic morphology, and also a crushed silica powder. A silicone oil with nominal viscosity of 1 Pa s was used as binder in the majority of the work. The results for the three different sized ballotini all collapsed together onto a single curve when plotted as a dimensionless peak flow stress versus the capillary number. As found previously [22] [S.M. Iveson, J. Beathe, N.W. Page, The dynamic strength of partially saturated powder compacts: the effects of liquid properties, Powder Technol. 127 (2002) 149-161], there were two distinct regions of behaviour. At low capillary numbers where viscous forces were insignificant, the flow stress was independent of strain rate (Region I), whereas at high strain rates where viscous forces dominate, the flow stress increased proportional to capillary number to the power of approximately 0.5 (Region II). The flow stresses of the non-spherical powders were higher than for the spherical powders, however, the trends were identical. There was also the same transition from brittle to plastic behaviour as observed and modeled previously for spherical particles [23] [S.M. Iveson, N.W. Page, Brittle-to-plastic transition in the dynamic mechanical behaviour of partially-saturated granular materials, J. Appl. Mech. 71 (2004) 470-475].