In-Situ Measurements of Stress Fluctuations and Kinematics in Triaxial Tests

A triaxial test on a granular material is a classical “elemental” test in geomechanics. Geomechanics researchers have increasingly acknowledged that these tests are not elemental, but in fact boundary value problems accompanied by complex grain-scale behaviors including localization and breakage. With the goal of elucidating these complex grain-scale behaviors and their connection to macroscale plasticity, we recently developed a novel triaxial compression apparatus. Our apparatus permits in-situ grain-resolved x-ray tomography for grain structure and breakage and 3D x-ray diffraction for grain stress tensors. The apparatus can uniquely be used to study sand samples as small as 1 mm diameter by 2 mm tall (containing about 1000 grains of Ottawa sand) and as large as 15 mm diameter by 30 mm tall (containing over 50000 grains of Ottawa sand). Here, we report uses of this apparatus at laboratory and synchrotron x-ray facilities to study the micromechanics of sand undergoing large strains (>30% in shear bands, 15% axially). Samples contain over 30000 sand grains, with shear bands engaging approximately 10000 grains – the largest known experiments to-date with in-situ x-ray tomography and 3D x-ray diffraction measurements. We report grain-resolved kinematics and, most importantly, stress and force fluctuations in and out of shear bands. Our results address important questions in geomechanics, such as principal stress rotations in shear bands, and important questions in granular physics, such as the nature of kinematic, stress, and force fluctuations in shear flows.