ABSTRACT
Our experimental setup (Fig. 1a) consists of a biaxial test apparatus with aluminum walls resting horizontally on a sheet of Plexiglas. We use motorized linear slides to move two walls of the biaxial cell independently and precisely.The other two walls are stationary. The model granular material consists of a bidisperse mixture of polymer photoelastic disks of diameter 0.8 cm or 0.9 cm with aYoung’s modulus of 4MPa and coefficient of static friction of 0.8. The biaxial cell is placed between crossed circular polarizers and a high resolution camera captures digital images from above (Fig. 1b). The image size is 1600 ×1152 which covers approximately 250 particles around the center of the cell. For each type of loading condition, we follow the same procedure: we begin with a stress-free state and apply incremental deformations in a quasi-static manner. The imposed strains (xx = yy = |L/L|) for isotropically compressed system range from 0 to 0.016 and for the sheared system, the strains range from 0 to 0.042. Isotropic compression is applied by moving both walls inwards by the same amount. Uniaxial compression is applied by moving one of the walls inwards and pure shear is applied by moving one wall inwards and the other outwards by the same amount. In all types of loadings the speed at which the walls move is 0.024 cm/s.
