DOUBLE SLIDING MODEL FOR CYCLIC DEFORMATION OF GRANULAR-MATERIALS, INCLUDING DILATANCY EFFECTS

A CONSTITUTIVE model is developed for monotonic as well as cyclic deformation of granular materials based on a double sliding mechanism. The inelastic deformation is regarded to consist of two superimposed sliding deformations, along preferred sliding planes which are obtained from a variant of Coulomb's criterion, incorporating the resistance due to the stress normal to the sliding planes and the hydrostatic pressure. The sliding is accompanied by volumetric straining which is related to the shear strain through the energy equation associated with the frictional loss. Hardening laws are proposed for the evolution of the frictional resistance which increases and saturates in continuous loading and decreases first upon unloading and then increases and saturates in reverse loading. The resistance due to hydrostatic pressure is assumed to increase with density. The model is applied to simple shear and biaxial compression tests, and is shown to have attributes of the experimentally observed results in monotonic, as well as in cyclic loadings, for both dense and loose samples. In particular, the observed initial densification and subsequent dilatancy are nicely predicted. Finally it is shown that the model predicts large increases of the confining pressure, and hence the flow stress, upon pure shearing of confined granules with very small void ratios, e.g. pulverized ceramics with inertial confinement, in shock loading of ceramics.