Terramechanics of Saturated Clays: Assessing Tire Performance Through Experimental and Numerical Approaches

Abstract

As autonomy begins to be integrated into combat vehicles, the necessity for high-fidelity virtual proving grounds becomes imperative. Performance prediction tools capable of accurately predicting drawbar performance and sinkage are useful for path prediction and navigation without the loss of mobility. This thesis focuses on formulating the physics underlying such predictive tools, particularly in the challenging terrain of saturated clays. Modeling cohesive soils with high water content is challenging due to their low shear strength and highly deformable nature, necessitating systematic consideration in both material modeling and numerical techniques. Saturated clay is marked by high plasticity, considerable deformability, and history-dependent effects, leading to non-linear behavior under various loading scenarios. Notably, under the substantial shear loads exerted by the tire, estimating the yielding mechanism and residual strength of the clay is crucial for multipass scenarios. From a fundamental soil mechanics perspective, the pore water pressure that accumulates in the soil significantly influences this performance, yet it remains underexplored. The rapid shear loading experienced by the soil can result in changes in pore water pressure of the soil, which may take extended periods to recover. To address this, an investigation of the time-scale effect with tire passes is considered. This study will differentiate between the short-term effects of a single pass and immediate multipass scenarios, compared to the multipass effect observed after a substantial duration, allowing for pore water dissipation. To illustrate this theory, this thesis emphasizes experimental and modeling techniques aimed at capturing the influence of pore water. The soil is characterized through geotechnical tests, and different material models are developed based on these findings. The impact of the soil constitutive law on the overall tire-soil simulations is examined, considering both total and effective stress frameworks. Additionally, different numerical techniques and their corresponding solutions are investigated. A user-defined material model, grounded in critical state soil mechanics with post-yielding softening effects, is proposed to represent the residual conditions of the soil fabric in tire-soil interactions. Complementarily, an advanced FE tire model is developed for tire mobility studies. Full-scale testing is conducted on a terramechanics tire-soil bin rig at Virginia Tech to validate both the FE tire model and the tire-clay interaction model. Cone Penetration Test (CPT) results are simulated to verify the in situ conditions of the soil. This tire-soil testing is carried out over a two-week period, facilitating the evaluation of time-scale effects on drawbar pull, sinkage, and rut formation in the soil. Ultimately, the insights gained from this research aim to enhance the physics and predictive capabilities of simulation tools, thereby supporting the development of virtual proving grounds and test beds for challenging terrains