中文摘要：

现有的弹塑性本构模型多数是基于共轴理论建立的，且只在三轴压缩应力条件下得到验证，因而不能合理描述真三轴状态下砂土的力学特性。为此，改进三维本构模型屈服函数的π面形态，运用非共轴分又理论对真三轴状态下砂土的分又强度与分又前的应力应变关系进行分析。理论分析表明，真三轴条件下中主应力对分又前应力应变曲线及分又特性有着显著的影响，其中中主应力比超过0．2时应变局部化产生并决定了土体的峰值强度。真三轴试验结果与传统的共轴分又理论的预测结果进行对比分析，验证了非共轴分又理论预测的合理性。

英文摘要：

For numerous constitutive models are established based on the coaxial theory and validated only under triaxial compression condition, the soil behavior is inaccurately described under the true triaxial condition. A 3D non-coaxial elasto-plasticity model with a novel-surface shape was proposed to simulate stress-strain curves before bifurcation and to predict the bifurcation strength. It was shown that both the pre-bifurcation deformation and the bifurcation strength were strongly affected by the intermediate principal stress, and the peak failure stemmed from the onset of strain localization when the ratio of intermediate principal stress was beyond 0.2. It was shown by comparisons between the predicted and experimental results on dense sands that the 3D non-coaxial model was more reasonable to be used to predict the bifurcation strength than the coaxial one under the true triaxial condition.