中文摘要：

在平面应变状态下，由于土体在应力峰值状态出现了应变局部化现象，从而变形模式失去了原有的均匀性而呈现软化特性。为此，采用常规的弹塑性本构模型模拟土体峰值前的均匀变形，对应力峰值状态则采用非共轴的分叉理论进行预测，而土样在峰值后出现不均匀变形的宏观力学特性则通过复合体理论加以描述。理论预测表明，构建这样的软化本构模型能真实反映平面应变状态下的应力．应变特性。理论分析还表明，经典的变形分叉理论中引入非共轴弹塑性模型，才能准确地预测土体的应力峰值，这是构建平面应变状态下土体软化本构模型的关键所在。

英文摘要：

Under plane strain condition, soil always exhibits softening characteristics and prominent stress peak due to strain localization. A traditional elastoplastic constitutive model is used to model uniform deformation of pre-peak; a deformation bifurcation theory involving non-coaxial elastoplastic model is employed to predict the stress peak; and a complex theory is introduced to describe the heterogeneous deformation of post-peak. By comparing the modeling results to experimental ones, it is shown that the softening characteristics of soil under plane strain condition can be truly described by presented approaches. Moreover it is found that the most important key to predict the peak-stress is to introduce exactly a non-coaxial elastoplastic model into the classical deformation bifurcation theory.