Experimental characterisations of deformation mechanisms in 2D dense assemblies subjected to shearing

Abstract

This study explored the deformation mechanisms and, in particular, the features of particle pair motions in response to shearing. A specially designed biaxial testing system with a flexible boundary equipped with an image system was set up for experimental tests. The whole testing system was modified from a CKC triaxial testing system. The particle image velocimetry (PIV) technique and close-range photogrammetry were used together to help with the investigation. The test samples TA and TB were made of randomly packed 3 D printed elliptical rods with different particle eccentricities e = 0.1, 0.2, respectively. In the examinations, the motion of a particle pair was divided into three modes, i.e. the contact deformation, the Type 4 rolling and the rigid body motion. It is found that each mode of the motion has different contributions to the sample deformation at different strain levels. The normal component of contact deformation dominates volumetric contraction at small strains and dilation at larger strains. The rigid body rotation only induces a slight dilation in sample TA and even leads to contraction in sample TB as the particles used in TB assembly become more elongated. For the sample distortion, it is mainly due to the rigid body rotation while the normal component of contact deformation makes a second contribution. The tangential component of contact deformation and the Type 4 rolling have relatively small contributions to both volumetric strain and sample distortion. The local volumetric strains arising from the contact deformation (including normal and tangential components) and Type 4 rolling mainly take place at the interfaces between moving particle clusters and vortices, also at the boundaries of micro bands or shear bands. The associated local volumetric strain due to the rigid body rotation mainly occurs inside the shear band.