Abstract:Yield and failure are two different conceptions,and they are two different steps in the deformation processes of materials. In the stress space,the failure surface of perfect plastic materials keeps unchangeable. In the strain space,however,with the constant size of failure surface,its center location moves with the increase of plastic deformation. So the traditional criteria built in the stress space cannot determine whether the material is destroyed. On the contrary,the criteria built in the strain space can properly deal with the problem. The strain criterion for ductile shear failure based on the maximum principal shear strain is established. On the basis of triaxial compression test results of normally consolidated saturated clay,the development laws of stress and volume strain(drained) or pore water pressure(undrained) with strain are analyzed;and the results of numerical simulation with ANSYS are in consistent with the test results. So it is rational to take the shear strain of initial critical state as the limit failure strain. The formulas of the maximum principal shear strain are deduced;and the calculation cases for several commonly used yield criteria are presented.