Abaqus选择库伦摩尔塑性模型的时候，需要选择摩擦角和剪胀角。摩擦角物理意义明确，但何为剪胀角？如何影响结果呢？（这两个问题，暂无统一解，Abaqus默认剪胀角最小是0.1°）

The angle of dilation controls an amountr of plastic volumetric strain developed during plastic shearing and is assumed constant during plastic yielding【剪胀角控制塑性体应变，一般假设在塑性变形过程中保持不变】. The value of ψ = 0 corresponds to the volume preserving deformation while in shear.【剪胀角=0，即体积不变性】

Clays (regardless of overconsolidated layers) are characterized by a very low number of dilation (ψ ≈ 0)【黏土剪胀角约为0】. As for sands, the angle of dilation depends on the angle of internal friction【砂土剪胀角约等于内摩擦角】. For non-cohesive soils (sand, gravel) with the angle of internal friction φ > 30° the value of dilation angle can be estimated as ψ = φ - 30°【对于内摩擦角>30的情况，剪胀角=内摩擦角-30】. A negative value of dilation angle is acceptable only for rather loose sands【剪缩仅针对松软砂土】. In most cases, however, the assumption of ψ = 0 can be adopted.

经验取值：紧砂剪胀角可取15°，松砂小于10°，正常固结土取0.