In order to investigate the stress-strain behavior and its variation with the micro-fabric for granular materials

a series of tests and corresponding micro-fabric analyses were conducted on dimensional ideal 2D granular materials with circular and elliptical shapes of cross-section

which were prepared at different maximum principal stress direction angles δ

using the biaxial testing system under three different stress paths including isotropic compression

constant lateral pressure shearing and constant mean principal stress shearing. The anisotropic property in shear strength and dilatation induced by shear were presented and analyzed. Based on the experimental results and micro-fabric analyses

the following new findings involving the failure mechanism of granular materials were confirmed

including that: (1) There exists a preferred shear direction towards which the long axis direction of particles tend to rotate as shear strain increases; and along this direction a shear band will be triggered and finally formed under large shear strain. (2) This preferred shear direction is irrelevant to the bedding plane

and the angle of the bedding plane with the preferred shear direction is about equal to 45°+ /2

where is residual internal friction angle. (3) The effect of particle rolling on the shear failure becomes more significant with the increasing angle of the bedding plane with the preferred shear direction

and such difference in the effect of particle rolling is the main reason why there is the anisotropy shear strength and different dilatation induced by shear.