Mechanical behavior of sand subjected to impact loading
Open Access
- Author:
- Prabhu, Sudheer Sudhakaran
- Graduate Program:
- Civil Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 08, 2021
- Committee Members:
- Shimin Liu, Outside Field Member
Tong Qiu, Chair & Dissertation Advisor
Ming Xiao, Major Field Member
Todd Palmer, Outside Unit Member
Patrick Fox, Program Head/Chair - Keywords:
- Impact Loading
Sand
DEM
Particle Crushing
Equation of State
SHPB - Abstract:
- Sand is generally subjected to quasi-static loading in the field. However, for applications involving projectile impacts and surface or mine blasts, sand is subjected to high strain-rate loading. To predict the response of underground structures under these loading conditions, there is a need to accurately understand the high strain-rate response of sand overlaying these structures. Researchers noted that the split Hopkinson pressure bar (SHPB) tests provide reliable data on the high strain-rate response of sand and have been extensively used. SHPB studies on sand showed that the stress-strain response of sand is dependent on the water content of the specimen and the particle breakage of sand grains. Several numerical models have been developed through the years to capture the high strain-rate sand response accurately. However, these models were developed mainly for dry sands, completely saturated sands, and sand with high water contents. These numerical models are unable to capture the softening and subsequent stiffening stress-strain behavior observed in sands at low water contents and low stresses. In this study, SHPB tests of sands are numerically simulated and the equations of state (EOS) at different water contents are calibrated using back calculation. The back-calculated EOSs are used to calibrate a three-phase model capable of capturing the sand response at varying water contents. The response predicted by the three-phase model is validated by comparing the numerical and experimental stress-strain responses. A parametric study was carried out to understand the effect of interface friction and aspect ratio on the validity of the SHPB results by considering different aspect ratios and interface frictions for the specimens. The results iv showed that specimens with higher aspect ratio take longer time to achieve stress equilibrium while specimens with interface friction never attain perfect stress equilibrium. Another important factor that affects the high strain-rate stress-strain response of sand is the particle breakage of sand grains. Due to the difficulty of continuum-scale models in capturing particle breakage in sands, discrete element method (DEM) is used to model high strain-rate loading of sand specimens. The study tries to understand the effectiveness of particle upscaling-downscaling procedure adopted in DEM in modeling high strain-rate problems. In this study, several SHPB tests reported in literature are modeled in DEM with particle breakage introduced in these simulations. The results showed that with well-calibrated parameters for contact behavior and particle crushing, specimens with upscaled particle size distribution (PSD) provide similar dynamic stress-strain response and PSD evolution as those reported in literature. This study shows the importance of particle breakage in the stress-strain response under high stresses as the specimens without particle breakage yield a much stiffer response compared to the specimens with particle breakage. The developed DEM model will be a useful tool to model the complete SHPB test setup as the incident, reflected and transmitted stress waves can be accurately replicated.