The lack of understanding of plastic hardening (softening) laws, especially under anisotropic stress conditions, results in inappropriate geotechnical management. Most of the yielding envelopes do not consider the effect of intermediate principal stress and the influence of Lode’s angle. In addition, the application of plastic flow rules regarding yielding surfaces compromises the softening of rock internal friction as well as the influence of Lode’s angle on the plastic potential. Moreover, the ductility to brittleness transition in the intermediate principal stress direction still requires a theoretical foundation. In this study, based on poly-axial testing results of Yunnan sandstone, we adopted a failure criterion with the intermediate principal stress proposed by Menétrey and Willam. The proposed new failure envelope was applied to capture the plastic evolution of rock samples. A plastic hardening-softening model is constructed, based on the framework of the plastic theory. The softening envelope is modified to better present the stress drop and considers the deterioration of rock internal friction in the post-peak stage of poly-axial loading. The differential of plastic potential according to the principal stresses is also modified, considering the rotation of Lode’s angle in the poly-axial loading tests. The model results were compared with laboratory testing results, which showed great consistency across 9 different loading tests (5 under triaxial stress and 4 under poly-axial stress with 22 stress-strain curves in total). The induced brittleness by the intermediate principal stress is also well captured by the proposed model.

缺乏对塑性硬化（软化）规律的了解，特别是在各向异性应力条件下，会导致岩土工程管理不当。大多数屈服包络线没有考虑中间主应力的影响和洛德角的影响。此外，屈服面塑性流动规则的应用会影响岩石内耗的软化以及洛德角对塑性势的影响。此外，中间主应力方向的延性到脆性的转变仍然需要理论基础。本研究基于云南砂岩多轴测试结果，采用Menétrey和Willam提出的中间主应力破坏准则。所提出的新破坏包络线被应用于捕获岩石样本的塑性演化。基于塑性理论的框架，构建了塑性硬化-软化模型。修改软化包络线以更好地呈现应力降，并考虑多轴加载峰值后阶段岩石内摩擦的恶化。考虑到多轴加载试验中洛德角的旋转，根据主应力的塑性势微分也进行了修改。将模型结果与实验室试验结果进行比较，发现9次不同的加载试验（5次三轴应力试验，4次多轴应力试验，共22条应力应变曲线）具有很好的一致性。