Brittle to Plastic Transition in the Dynamic Mechanical Behavior of Partially Saturated Granular Materials

The effect of liquid viscosity, surface tension and strain rate on the deformation behavior of partially saturated granular material was studied over a ten order of magnitude range of capillary number (the ratio of viscous to capillary forces). Glass spheres of average size 35 microns were used to make pellets of 35% porosity and 70% liquid saturation. As the capillary number increased, the failure mode changed from brittle cracking to ductile plastic flow. This change coincided with the transition from strain-rate independent flow stress to strain-rate dependent flow stress noted previously [Iveson, S. M., Beathe, J. A., and Page, N. W., 2002, “The Dynamic Strength of Partially Saturated Powder Compacts: The Effect of Liquid Properties,” Powder Technol., 127, pp. 149–161]. This change in failure mode is somewhat counter-intuitive, because it is the opposite of that observed for fully saturated slurries and pastes, which usually change from plastic to brittle with increasing strain rate. A model is proposed which predicts the functional dependence of flow stress on capillary number and also explains why the flow behavior changes. When capillary forces dominate, the material behaves like a dry powder: Strain occurs in localized shear planes resulting in brittle failure. However, when viscous forces dominate, the material behaves like a liquid: Shear strain becomes distributed over a finite shear zone, the size of which increases with strain rate. This results in less strain in each individual layer of material, which promotes plastic deformation without the formation of cracks. This model also explains why the power-law dependency of stress on strain rate was significantly less than the value of 1.0 that might have been expected given that the interstitial liquids used were Newtonian.