Enhancement of Experimental Imaging Capabilities for Advanced Study of Shear Band Growth and Evolution

摘要：目前正在进行一项涉及剪切带平面应变测试和数字图像分析的实验研究，该研究涉及剪切带的平面应变测试和数字图像分析。目的是综合评价沉积孔隙比与砂体临界状态线（CSL）之间关系的性质。最近的研究结果表明，膨胀砂的局部变形（剪切带）和临界状态的实验量化表明，CSL在孔隙比-有效应力空间中的位置可能不是唯一的，而是取决于沉积孔隙比。由于临界状态概念和CSL在岩土分析和设计中广泛使用，因此这些发现的影响可能是重大的。最终目标是从结果中得出沉积孔隙比与CSL之间关系的定量描述，并提出将这些发现纳入当前液化潜力分析程序和本构模型的建议，这些分析程序和本构模型利用当前独特形式的CSL。实验是在一个先进的平面应变试验装置上进行的。该装置配置为促进剪切带的无约束形成，并允许对定位过程进行照相分析。采用数字图像相关（DIC）技术对砂土试样进行了高精度的局部位移直接测量。与之前的研究相比，采用更高分辨率的数码相机，结合图像采集与数据采集和控制的同步，将能够更准确地量化剪切带变形，包括与剪切带形成相关的位移模式。高度发达的摄影能力也将使与其他局部位移现象（如临时剪切带）相关的变形机制的定量研究成为可能。如上所述的双轴测试装置的配置提供了目前唯一可用的实验方法之一，以量化土壤中局部位移的演变。然而，不幸的是，在目前的结构中，只能看到剪切带的一个平面，这使得位移量化的有效性受到质疑，尽管它的精度很高。此外，剪切带内可能发生的面外体积交换也无法解决。本研究的目的是克服当前实验系统的局限性，通过合并修改，允许通过两个平面应变壁成像，同时保持平面外力测量能力。修改包括增加第二个数码相机，并重新设计负载墙以适应成像。这些修改将影响几个系统改进。首先，通过剪切带深度对两个平面上的局部位移进行量化，将能够确认局部导出的临界状态空隙比。其次，将剪切带两端的位移场与目前在试件深度中点可用的传感器测量结果进行实时比较，将为通过剪切带深度对变形均匀性进行更可靠的观察提供机会。测量局部变形到如此详细的水平应该有助于颗粒土的行为的理解显著。

CMS-0220309PI: Amy RechenmacherInstitution: Johns Hopkins UniversityTitle: "Enhancement of Experimental Imaging Capabilities for Advanced Study of Shear Band Growth and Evolution"Abstract:An experimental investigation is currently underway, involving plane strain testing and digital image analysis of shear bands. The objective is to comprehensively evaluate the nature of the relationship between deposition void ratio and critical state line (CSL) for sands. The study has been motivated by results of recent research in which experimental quantification of localized deformations (shear bands), and thus critical states, in dilative sands has indicated that CSL position in void ratio-effective stress space may not be unique, but rather dependent on deposition void ratio. Since critical state concepts and the CSL are widely used in geotechnical analysis and design, the impact of these findings is potentially significant. The ultimate goal is to derive from the results a quantitative description of the relationship between deposition void ratio and CSL, and to develop recommendations for incorporating the findings into current analysis procedures for liquefaction potential and constitutive models that utilize the CSL in its current unique form.Experiments are being conducted in an advanced plane strain testing apparatus. The apparatus is configured to promote unconstrained formation of shear bands and to permit the localization process to be analyzed photographically. The technique of Digital Image Correlation (DIC) is used to measure directly localized displacements in sand specimens to a high level of accuracy. The incorporation of a higher resolution digital camera than in previous studies, combined with the synchronization of image acquisition with data acquisition and control, will enable more accurate quantification of shear band deformations, including displacement patterns associated with shear band formation. The highly developed photographic capabilities also will enable quantitative study of deformation mechanisms associated with other localized displacement phenomenon, such as temporary shear bands.The configuration of the biaxial testing apparatus as described above offers one of the only currently available experimental methods to quantify the evolution of localized displacements in soils. Unfortunately, however, in its current configuration only one plane of the shear band is visualized, yielding question as to the validity of the displacement quantification, in spite of its high accuracy. In addition, the potential occurrence of out-of-plane volumetric exchange within the shear band cannot be addressed.This study aims to overcome the stated limitations of the current experimental system by incorporating modifications to allow imaging through both plane strain walls, while keeping in tact the out-of-plane force measurement capabilities. The modifications consist of the addition of a second digital camera, and load wall redesign to accommodate imaging. These modifications shall affect several system improvements. First, quantification of local displacements on two planes through the depth of the shear band will enable confirmation of locally derived critical state void ratios. Second, the real-time comparison among displacement fields at both ends of the shear band and transducer measurements currently available at midpoint of the specimen depth will provide the opportunity to make more reliable observations about deformation uniformity through the depth of the shear band. Measurements of local deformations to such a level of detail should contribute significantly to the understanding of granular soil behavior.