加载速率变化条件下砂土的黏塑特性及本构模型

 分析并研究饱和的、风干的砂土在平面应变压缩试验条件下的弹黏塑性特征，尤其是黏性特性。加载速率效应、蠕变以及应力松弛都是砂土材料本身黏性的反映，而与超孔隙水压力的消散无关。试验不仅实现了应变率逐步变化加载过程，同时也实现了蠕变加载和应力松弛过程。试验结果表明，在加载应变率发生突变时，对砂土应力–应变关系有明显的影响，呈现出刚性很高、近似于弹性的特性，而后随着应变的进一步增加，在明显屈服之后黏性应力逐渐衰减至基本惟一的应力–应变曲线。类似现象同样也发生在蠕变加载或应力松弛后以一恒定应变率突然重新加载的情况。基于非线性三要素模型框架，针对所观察到的砂土黏性特性，提出一种弹黏塑性本构模型。该模型可以描述砂土在任意加载历史下的黏性效应，包括加载应变率发生任意变化时的应力–应变响应、蠕变以及应力松弛。最后，利用该模型对上述砂土平面应变压缩试验结果进行模拟，所提案的三要素弹黏塑性本构模型能够很好地模拟砂土的黏性特性。

VISCOPLASTIC BEHAVIORS AND CONSTITUTIVE MODELING OF SANDS UNDER CHANGE OF LOADING RATES

Elastoviscoplastic properties，especially viscous properties of water-saturated or air-dried sands，were investigated based on the results of a series of plane strain compression(PSC) tests. Loading rate effect，creep and stress relaxation should be attributed to the viscous properties of the sand specimens，but not to the delayed dissipation of excess pore water pressure. The strain rate was changed stepwise several times；and stress relaxation tests and a set of creep tests were performed during otherwise monotonous loading at a constant strain rate. It was found when the strain rate suddenly changed，the stress also changed sharply with a very high stiffness，being close to the elastic value. Then，with an increase in the strain，the viscous stress component gradually rejoined the essentially unique stress-strain curve after having exhibited clear yielding. Similar behaviors were also found when monotonic loading was restarted at a constant stain rate following a creep loading or stress relaxation stage. An elastoviscoplastic constitutive model，described in a nonlinear three-component framework，was proposed to predict the effects of viscous property on the stress-strain relationship of sands observed in plane strain compression tests. The model is able to simulate the viscous effects of sand for arbitrary loading history，including the stress-strain behaviors during monotonic loading with step changes in the strain rate and the one after creep or stress relaxation. The results of the tested sands have been used to validate the proposed model. It is shown that the three-component elastoviscoplastic model can simulate the viscous properties of sands reasonably