The dynamic strength of partially saturated powder compacts: the effect of liquid properties

The flow stress of partially saturated pellets was measured at deformation velocities varying from 0.01 to 150 mm/s. The pellets, 20 mm in diameter and 25 mm high, were made with glass ballotini of a surface mean particle size, dp, of 35 μm. Water, glycerol and a series of silicone oils were used as the liquid binder, covering viscosities, μ, and surface tensions, γ, ranging from 0.001 to 60 Pa·s and 0.025 to 0.072 N/m, respectively. It was found that there was a critical strain rate (which was binder dependent) below which the peak flow stress, σpk, was independent of the strain rate, ε. Above this critical strain rate, the flow stress increased with increasing strain rate. When plotted in terms of two dimensionless groups, the results collapsed onto one curve of the form:Str*=k1+k2Canwhere Str*=σpkdp/γcosθ is the dimensionless peak flow stress and Ca=μεdp/γcosθ is the dimensionless capillary number, the ratio of viscous to capillary forces. θ is the contact angle (assumed zero in this work). The best fit values of the parameters were: k1=5.3±0.4, k2=280±40 and n=0.58±0.04. This result suggests that viscous forces are negligible at low strain rates, but become dominant at high strain rates. k1 is related to the static peak compressive strength of the pellets, k2 determines the critical Ca at which viscous effects become significant and n gives the power law dependence of viscous forces on the strain rate. This work is significant for granulation research since it highlights the fact that strengths measured under pseudo-static conditions may not be representative, even qualitatively, of how materials behave at the higher strain rates encountered in commercial granulators.