THE SOLUTION
In order to understand these complex material systems under various stress conditions, a GDS HCA was procured. This choice was made based on the quality of the instrument, service (technical support) and prior experience of IISc in utilising GDS equipment. The hollow cylinder torsional apparatus (HCA) provides the user with the unique capability to independently control the magnitude and direction of the principal stresses by controlling the internal and external cell pressures, axial load and torque. Complex stress paths, which are encountered in various boundary value problems, can be re-examined under controlled laboratory conditions to gain a unique perspective on the response of the material. Significant advances in understanding the anisotropy of granular materials has also been possible through these tests. Additionally, the IISc group has worked to understand microstructural features, such as particle shape and angularity, and how these affect the actual yield surface of a granular material through HCA tests.
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Exploration of the state boundary loci, such as the critical state loci, not only in three dimensional principal stress space but also in the state space, has been possible through the HCA tests performed at IISc. Angular sand, spherical glass ballotini, and a wide range of particle morphologies were used as model materials for this study. The failure state is reached when the value of deviatoric stress peaks, while the critical state is defined when a constant specimen volumetric strain is observed. The critical state points are plotted in the principal stress space on an octahedral plane. These experimental results are used to calibrate some of the existing constitutive models and are a first step toward establishing models at multiple length scales.
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