Particle Scale Dynamics of Coarse Granular Material
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Developing a new wheeled or tracked vehicle for military use typically involves physical prototyping of preliminary vehicle designs and field tests to evaluate their performance in a range of conditions. Computational simulations of vehicle performance, including physics-based simulation of vehicle-terrain interaction, is an effective alternative for reducing costs of physical prototyping. Activities described in this thesis have been conducted in coordination with the Simulation Based Engineering Laboratory (SBEL) at the University of Wisconsin-Madison, which is actively engaged in physics-based simulation of vehicle-terrain interaction for military and other vehicles. The primary tool used for simulation is the Discrete Element Model (DEM) Chrono::Engine and the Chrono::Granular toolkit (Mazhar, et al., 2013). The primary objective of this research was to provide a robust physical data set to validate Chrono::Granular simulations of coarse-grained granular material behavior using relatively simple and common geotechnical tests. Tests selected included laboratory fall cone tests and direct shear tests using dry and moist preparations of 20-30 Ottawa sand and 3 mm spherical glass beads. A special direct shear device was designed, constructed, and calibrated to provide direct visual observation of particle displacements for a single plane of particles subject to shearing. Shear stress-displacement relationships measured during shearing and individual particle displacements tracked using particle image velocimetry (PIV) were made available for comparison with discrete particle displacements simulated by DEM and Chrono::Granular. Results from the fall cone testing series, including measurements of cone penetration depth as a function of time, cone apex angle, drop height, and initial particle density were also made available for direct comparison with computational simulations. The suite of fall cone and direct shear test results was analyzed to investigate relationships among the testing parameters. A case study review of similarly unconventional applications of fall cone and direct shear testing and of applications of PIV to granular material is provided. Results from the fall cone penetration test series indicate that penetration into coarse granular media is most affected over small ranges of drop height by the cone geometry (apex angles of 30° and 60°). Results from the direct shear test series indicate that shear displacement rate affects individual particle motion, with lower shear rates allowing more rotation and localized particle displacements than higher shear rates. Introducing water into the shear zone changes bulk shear strength and the character of individual particle motions. Direct shear tests with glass beads indicated that capillary bridges between particles caused displacement of particles in zones that had not moved in dry tests. Direct shear tests with sands indicated that the inclusion of water near the shear zone caused shear failure to occur outside the wetted zone along a different shear surface. Shear band development was observed to depend on shear displacement rate, applied normal force, and the presence or absence of moisture in the shear surface. Direct comparisons between the physical test results reported here and analog Chrono::Granular simulations not included as part of this thesis.