Dynamic Mechanical Behavior of Frozen Ottawa Sand Subjected to High Strain Rate Loading
Advisor: Dr. Tugce Baser
ABSTRACT
Increasing temperatures in the Arctic region raise environmental concerns, but also bring new opportunities for civil engineers including infrastructure development, such as roads, buildings, and pipelines. Extensive research focusing on the behavior of frozen soils under quasi-static loading resulting in small strain rates exist; however, limited studies have explored their behavior under high strain rate loading, which is relevant in construction and resource extraction, etc. This research aims to investigate the dynamic mechanical behavior of frozen sands under varying thermal conditions and to characterize the impact of high strain rates on the overall stress-strain responses. Frozen Ottawa sand samples having different dry densities and initial degrees of saturation were tested at temperatures of -15, -10, and -5°C to characterize the mechanical behavior. These samples were subjected to strain rates ranging from 400 to 1600/s using an instrumented Split Hopkinson Pressure Bar (SHPB). A temperature-controlled chamber was designed and attached to the SHPB setup to maintain constant temperatures during the experiments. A high-speed infrared camera was integrated to monitor temperature variations during the impact tests for estimating the thermal energy during the tests. The stress-strain curves of frozen Ottawa sand at different temperatures were obtained, and the results indicated that the stress-strain behavior was influenced by both the strain rates and the initial degrees of saturation of the frozen sands. Specifically, the stress-strain curves exhibited peak stresses followed by pronounced strain softening when the strain rate was below 1000/s. However, at strain rates above 1000/s, relatively more brittle response was observed. The results also revealed that the strength of the frozen Ottawa sand increased as the temperature decreased, due to the enhanced bonding between the ice and soil particles. To further evaluate the behavior of frozen sands under various strain rates, numerical simulations using LS-DYNA were performed. Two numerical methods available in LS-DYNA, namely, the Finite Element Methods and the Smoothed Particle Hydrodynamics, were employed to perform the numerical simulations of the SHPB tests. Holmquist-Johnson-Cook material model was employed in the simulations. The results indicated that the numerical scheme with the Smoothed Particle Hydrodynamics produced stress-strain curve responses that were in good agreement with the experimental results as compared to those from the FE analysis. The simulations where the strain rate was beyond the values achieved during the experiments revealed the strain rate sensitivity of the frozen Ottawa sand which displayed brittle material behavior at very high rates. The results of this study can contribute to the design of infrastructure and protective structures that may face high strain rate events, impact loadings, or explosions. Future research will build on this study to develop a material model for simulating the thermo-mechanical behavior of frozen sands under extreme loading conditions.