Different from classical plasticity theory developed basing on stress invariants, rotation of principal stress axis causes plastic deformation of granular materials. To investigate this unique feature of granular materials, this study carries out discrete element simulations (DEM) to simulate the material response under principle stress rotation and applies homogenization to discover the paricle-scale mechanism. The simulation results show that plastic deformation is produced by rotation of principal stress axis and higher the stress ratio, the material plastic deformation gets larger and and the degree of non-coaxiality gets smaller. Continuous rotation of principal stress axis to large number of cycles brings the sample to an ultimate state with constant stress ratio and repeated strain trajectories. This ultimate state is insensitive to the initial void ratio and mainly dependent on the stress state. Larger stress ratio results in a smaller void ratio and larger strain trajectory.
To further investigate material plasticity caused by rotation of principal stress axis, numerical simulations of small loading-unloading increment have been carried out to facilitate detailed analyse of elastic deformation and material plasticity. Particle-state governing mechanism and state parameters have been proposed by detailed investigation of changes in contact forces, internal fabric and particle displacements. Together with homogenization theory, the spatial distribution of local stress increments and local strain increments and the evolution of material state variables are given, serving the development of constitutive model of granular materials considering general load paths.

rotation of principal stress axis; discrete element method (DEM); fabric evolution; non-coaxial deformation