Effect of Dilatancy on Instability, Pre-Instability Strain Softening of Sand Along Proportional Strain Paths

The roles of dilatancy in granular materials are experimentally and numerically explored with regard to material instability and failure. Experimental results of sand corresponding to proportional strain paths show that either pre- or post-failure instability may take place, depending on the density and the imposed rate of volume change. Pre-instability strain softening is observed for both dense and very loose sand, however, strain softening is not a necessary condition for the occurrence of material instability. Both experimental results and constitutive modeling show that the difference between the forced rate of volume change and the inherent potential of dilation dominates the occurrence of strain softening and material instability. The dependency of inherent dilation on void ratio and stress level is a key factor in predicting the complicated stress responses of sand deforming along imposed strain paths.