Experimental Study of Stress Rotation Effects in Cross-Anisotropic Sand

本课题的研究目标是研究主应力旋转过程中交叉各向异性砂土的行为，建立考虑应力旋转影响的本构模型。为了实现这一目标，将进行一系列系统的扭转剪切试验，以确定在？a1-directions, ?Ñ，并在中间主应力值范围内，如b = (？<s:1> 2 - ? <e:1> 3)/(？<s:1> 1 - ? <e:1> 3)。这些实验结果将有助于指导和形成土壤本构模型的发展，更真实地模拟土壤的真实行为。仅根据应力不变量建立的工程材料弹塑性硬化本构模型可以很好地适用于各向同性材料。它们的衍生物，旋转运动学硬化模型，也可以捕捉到大应力逆转的影响，只要没有应力旋转发生，它们就可以用来模拟各向异性行为。在塑性公式中只使用应力不变量的含义是主应力方向（不是应力增量）与主应变增量方向（同轴度）一致。然而，对交叉各向异性土的力学行为观察表明，除非主应力方向与主材料方向一致，否则主应力方向与主应力方向旋转时的主应变增量方向可能不一致。为了模拟跨各向异性材料在主应力旋转期间的行为，有必要涉及所谓的联合（或混合）不变量。它们用来表示相对于材料主方向的应力主方向。因此，可以预测非同轴性，并与所提出的扭转剪切实验的实验结果进行比较。将注意在扭剪试验中主应力旋转过程中观察到的主应力和应变率的非同轴性，以及它们的弹塑性本构模型的建模。该研究将有助于建立更真实的原位土壤行为模型。真正的土壤，当它们在地下被发现时，明显表现出跨各向异性的行为，具有旋转对称的垂直轴。这种真实的行为通常被认为是各向同性的，因此许多观察到的行为模式不能被正确预测。这项研究对社会的好处将是改进岩土工程结构和土-结构相互作用的行为预测，以及随之而来的更大的安全和经济建设。

The research objectives of this project are to study the behavior of cross-anisotropic sand during rotation of principal stresses and to develop a constitutive model that incorporates effects of stress rotation. To achieve this goal a systematic series of torsion shear tests will be performed to determine the drained behavior of sand deposited with cross-anisotropic fabric over a range of ?ã1-directions, ?Ñ, and over a range of intermediate principal stress values as indicated by b = (?ã2 - ?ã3)/(?ã1 - ?ã3). These experimental results will help guide and form the basis for the development of a constitutive model for soils that more realistically models the real behavior of soils in-situ. Elasto-plastic hardening constitutive models for engineering materials formulated in terms of stress invariants alone may work well for isotropic materials. Their derivatives, the rotational kinematic hardening models, may also capture the effects of large stress reversals and they may be used to model anisotropic behavior as long as no stress rotation occurs. The implication of using only stress invariants in the plasticity formulation is that the directions of principal stresses (not stress increments) coincide with the directions of principal strain increments (coaxiality).However, observation of the behavior of cross-anisotropic soils have shown that unless the principal stress directions line up with the principal material directions, then the principal stress directions may not coincide with the directions of principal strain increments during rotation of principal stress directions. To model the behavior of cross-anisotropic materials during principal stress rotation, it is necessary to involve socalled joint (or mixed) invariants. They are used to indicate the principal directions of stress relative to the principal directions of the material. Thus, non-coaxiality may be predicted and compared with the experimental observations from the proposed torsion shear experiments. Attention will be paid to the non-coaxiality of principal stresses and strain rates observed during principal stress rotation in the torsion shear tests as well as their modeling by the elasto-plastic constitutive model.The research will contribute to establishment of more realistic modeling of soil behavior as it occurs in-situ. Real soils, as they are found in the ground, clearly behave cross-anisotropically with a vertical axis of rotational symmetry. This real behavior is most often assumed to be isotropic, and a number of observed behavior patterns are therefore not predicted correctly. The benefits of the research to society will be improved predictions of the behavior of geotechnical engineering structures and soil-structure interaction as well as consequent greater safety and economy of construction.