循环动载下土体变形呈现的复杂各向异性，本质上依赖于土体微观组构特征的演化。为了揭示非比例循环动载下土体变形的微观机制，采用离散元方法模拟砂土的循环单剪行为。应用等幅剪应变的往复加载实现循环单剪应力路径，模拟得到了砂土的循环弱化、剪胀性、非共轴性及微观组构的演化规律。微观模拟表明，在循环剪切过程，土体表现为循环弱化行为，并最终趋于塑性安定状态。而试样组构主方向倾角和组构各向异性系数也会不断增大直至一个稳定值，同时主应变率、主应力轴和组构主方向旋转角度会存在一定的差异，即非共轴现象，随着单调剪切的进行，三者会逐渐趋于一致。土体的剪胀行为表现为循环压密性，而非共轴性呈现逐渐增强的演化规律。循环剪切过程中微观组构的主方向与应力主方向逐渐趋于一致，而微观组构各向异性呈现减弱的演化趋势。

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.