基于应变响应包络的颗粒材料增量力学行为研究

砂土的力学响应具有显著的增量非线性特征,这与离散颗粒的微观结构,即组构特性密不可分。采用应变响应包络可以较好地获得材料在不同加载方向上的增量力学响应。在物理试验中无法同时获得同一试样在不同加载方向下的宏、微观响应,故采用离散元方法,对具有不同应力历史、不同应力状态和不同级配的试样在Rendulic平面上的增量力学响应进行了系统研究和分析。离散元模拟结果表明,传统的塑性理论不能很好地描述具有复杂应力历史试样的增量力学响应,而基于颗粒间接触法向的组构增量与剪应变之间在多种不同的工况条件下均具有较强的线性相关性。当砂土的相对密实度相同时,该线性系数主要与围压的大小有关,对颗粒级配、应力历史以及应力比的变化均不敏感。由于组构的大小可以较好地量化砂土的内结构各向异性程度,进而表征应力历史的作用。上述模拟结果为在本构建模中引入组构演化机制,综合反映外荷载和内结构各向异性对砂土增量本构关系的影响提供了较好的微观物理依据。

Incremental behavior of granular soils: a strain response envelope perspective

The mechanical behavior of granular materials is characterized by strong nonlinearity, which is known to be directly linked to the details of the underlying microstructure of contacts, or fabric. The incremental behavior in different loading directions can be obtained by performing a series of axisymmetric stress probing on the specimens. Since the macro- and micro-scale response in different loading directions cannot be obtained experimentally on the very same specimen, discrete element method is employed in the present study, and the stress probing tests on the Rendulic plane are carried out on the specimens of different stress histories, stress states and particle size distributions (PSD). Simulation results indicate that the classic plasticity theories are unable to describe all the observed features when the specimens have different stress histories. However, a linear correlation between the deviatoric strain and the deviatoric part of the contact-normal-based fabric can be obtained. For the specimens with the same relative density, only confining pressure is found to have a significant effect on the linear factor while the linear factor seems to be insensitive to the variation of the stress ratio, stress history and PSD. Since the anisotropy of sand and therefore the impact of stress histories can be characterized by the deviatoric part of the fabric tensor, these observations would provide the necessary micro-scale physical basis for the development of elasto-plastic incremental constitutive relations of granular soils.