Abstract: The complicated anisotropies of soil deformation under dynamic cyclic loads rely on the evolution of microscopic fabric. In order to reveal the microscopic mechanism of soil deformation under non-proportional cyclic loading, discrete element method(DEM) has been used to simulate the behavior of sand under cyclic simple shear conditions. The amplitude of shear strain is kept constant during loading to achieve cyclic simple shear stress path, the cyclic softening, dilitancy, non-coaxiality, and the evolution of fabric of sand are demonstrated. The results show that, during cyclic loading, the soil expresses a cyclic softening tendency and finally approaches a plastic stable state. The principal direction and anisotropy degree of soil fabric increase during loading and finally reach a constant value. In the meantime, there appears difference among the inclination angle of principal strain rate, principal stress axis and principal fabric, which is called noncoaxiality. The difference tends to disappear gradually during the shearing process. The dilitancy behavior of soil manifest as cyclic densification, while noncoaxiality shows an increasing tendency. The principal direction of fabric gradually agrees with the principal direction of stress during shear process, while fabric anisotropies show a softening tendency.

Key words: discrete element method(DEM), cyclic simple shear, rotation of principal stress axis, fabric