DECOMPOSITION OF VELOCITY GRADIENT INTO INTERPARTICLE SLIP AND ROLLING

Abstract

Some of the best existing constitutive models for granular materials are based on the microkinematic assumption that the velocity gradient is a direct result of interparticle slip. For instance in the stress-dilatancy theory (Rowe, 1962, 1971) and in the double sliding, free rotating model (de Josselin de Jong, 1971, 1976, 1977 a, b) the mechanism of interparticle slip plays a central role. Oda, Konishi and Nemat-Nasser (1982) have concluded from biaxial compression tests that also interparticle rolling appears to be a major microscopic deformation mechanism. This conclusion was based on the observed relative displacements of the interparticle contact points.In this paper a method for the determination of the relative importance of the interparticle mechanisms of slip and rolling is applied in which the measured macroscopic velocity gradient is decomposed into 2 components, namely a component due to interparticle slip and a component due to interparticle rolling and combined rotation of the rigid particles in contact. From one of the aforesaid biaxial compression tests it is found that in the post-peak interval the contribution to the velocity gradient by the mechanism of interparticle slip is still larger than the contribution by interparticle rolling but the latter is significant too.