单剪条件下主应力大小及主应力轴方向

对单剪条件下的应力莫尔圆变化过程进行详细分析，将莫尔圆从初始状态到破坏状态的大、小主应力变化过程看做由若干个等增量步组成，从而推导出任意剪应力状态与初始状态对应的小主应力差值与两者大主应力差值的比为一常数，该常数可以根据初始状态莫尔圆和破坏时对应的莫尔圆确定。推导出单剪条件下任意剪应力对应的大、小主应力及主应力轴方向的表达式。讨论了土体材料参数及法向应力对主应力轴偏转过程的影响以及大、小主应力与剪应力的关系。结果表明：在土体材料参数一定的条件下，主应力轴方向由应力比唯一确定；法向应力一定，土体的摩擦角越小，相同剪应力对应的主应力偏转角越大；剪切起始阶段，主应力变化缓慢，随着剪应力增大，主应力急剧变化至土样破坏时保持不变。

Size and axis direction of principal stress in simple shear

The changing process of stress Mohr circle in simple shear is analyzed in detail. In the analysis, the process is regard as a components of some incremental step in equal; and two variables are difined: one is the difference between two minor principal stresses corresponding to a random shear stress state and the initial state, another is the difference between two major principal stresses corresponding to a random shear stress state and the initial state; Then the ratio value of the two variables is proved to be constant, which can be got by the Mohr circle in initial state and failure state. Furthermore, expressions of the principal stresses and the corresponding axis directions related to a random shear stress in simple shear are deduced. The influence of soil character parameters and normal stress on the principal stress axis directions is discussed. The relationship between principal stress and shear stress is also evaluated. Studying shows that under the same soil character parameters, the direction of principal stress axis are only decided by the stress ratio; for constant normal stress and shear stress, the rotation angle of principal stress axis increases with the decrement of internal friction angle of the soil, the principal stresses change slowly at the beginning of shearing, change sharply with the increament of shear stress, and keep constant in the failure state.