堆石料加载与流变过程中塑性应变方向研究

Plastic strain directions of rockfill materials during loading and creeping

确定塑性应变增量方向是建立土体弹塑性本构模型的核心之一,弹塑性理论中通常假定塑性应变增量方向仅与应力状态有关,与应力增量无关。流变试验是一种应力状态恒定的特殊试验,应力增量为零,所有应变均为塑性变形。研究流变过程中塑性应变方向与应力状态的关系及其与加载过程中塑性应变方向的差异,对于建立土体弹塑性本构模型具有重要价值。通过对某抽水蓄能电站筑坝堆石料的大型三轴压缩试验和三轴流变试验,分别研究了加载和流变过程中剪胀比与应力比之间的关系。结果表明,三轴压缩和三轴流变过程中,堆石料的剪胀比均随着应力比的增加而减小,且相同三轴压缩应力状态下,堆石料的流变剪胀比明显大于加载剪胀比,即流变过程中堆石料剪缩性比三轴压缩过程中的剪缩性更为强烈。因此,采用相同的塑性势函数同时确定加载塑性应变方向和流变黏塑性应变方向是不恰当的,建立考虑堆石料流变的弹塑性模型时应该选用不同的应力剪胀方程或塑性势函数。

The determination of the incremental plastic strain direction（IPSD） is one of the kernels in elastoplastic constitutive models. It is generally assume that the IPSD depends only on the stress states and is not influenced by the stress increments. A creep test is a special experiment that the stress state is kept constant without increment and the strains are purely plastic. Studying the dependence of the IPSD during creeping upon the stress states and its difference from that during loading may offer valuable reference to constitutive modeling. In this study, large-scale triaxial compression experiments and triaxial creep experiments are conducted on a typical rockfill material used in a pumped storage power station. The relationships between the dilatancy ratio and the stress ratio during both loading and creeping are investigated, respectively. It is found that in both kinds of experiments the dilatancy ratio decreases nonlinearly when the stress ratio increases, and the positive dilatancy ratio during creeping is considerably higher than that during loading under the same stress state. That is to say, the shear volume contraction behavior is much more evident during creeping than that during loading. Therefore, it is irrational to use the same potential function to determine the IPSDs during both loading and creeping. Different stress dilatancy equations or different plastic potential functions are required in establishing constitutive models considering the creep behavior of rockfill materials.