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

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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.