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Applications like military, agriculture, and extra-planetary explorations require the successful navigation of vehicles across different types of terrain like soil, mud, and snow. As the properties of the terrain heavily influence the interaction with the tire, it is necessary to characterize the terrain from a tire performance and vehicle mobility perspective. Failure to properly understand the tire-terrain interaction can lead to undesirable conditions like loss of vehicle mobility due to excessive sinkage. As a result, it is essential to understand the tire terrain interaction between an off-road tire and a sandy terrain. This study was done to assess the performance of tires in both pure slip (only traction and braking) and combined slip conditions (steering and acceleration). A single-wheel indoor test rig was used to conduct tests under different conditions and a force transducer was used to capture the forces and moments generated in the tire hub. In addition to this, the tire footprint was captured with the help of a light-based 3-D scanner. Key parameters were defined in the 3D scan, and these parameters were correlated to the input test conditions. Additionally, a grid of force sensors was made, and measurements of the normal force acting at a depth below the undisturbed terrain were taken. Inferences were made about the linear speed of the wheel and the length of the pressure bulb under the tire. / Master of Science / Several applications like military, extra-terrestrial exploration, and motor racing require vehicles to navigate off-road terrains like soil, snow, and ice. The tire interacts with these off-road terrains very differently from the way it interacts with the road. It is important to understand this interaction correctly as this interaction generates all the forces needed by a vehicle to perform various maneuvers like acceleration, braking, and turning. If not accounted for properly, there can be undesirable conditions like loss of vehicle mobility due to excessive sinkage in sand. Tests were performed where an off-road tire ran on a non-cohesive, loose soil under different slip ratios, slip angles, and camber angles in an indoor test rig. The forces and moments acting on the tire during the tests were measured and its variation with input conditions was studied. A light-based 3D scanner was used to capture the tire rut profile on the soil after each test. The important parameters of the tire rut were defined and the variation of these parameters with input parameters were studied. Additionally, the stresses developed below the soil surface were measured with the help of a sensor grid, which was also used to verify the linear speed of the tire and infer the length of the zone inside the soil that is affected by the tire.

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The emerging field of nanomechanics is providing a new focus in the study of the mechanics of materials, particularly in simulating fundamental atomic mechanisms involved in the initiation and evolution of damage. Simulating fundamental material processes using first principles in physics strongly motivates the formulation of computational multiscale methods to link macroscopic failure to the underlying atomic processes from which all material behavior originates. A combined concurrent and sequential multiscale methodology is developed to analyze fracture mechanisms across length scales. Unique characterizations of grain boundary fracture mechanisms in an aluminum material system are performed at the atomic level using molecular dynamics simulation and are mapped into cohesive zone models for continuum modeling within a finite element framework. Fracture along grain boundaries typically exhibit a dependence of crack tip processes (i.e. void nucleation in brittle cleavage or dislocation emission in ductile blunting) on the direction of propagation due to slip plane orientation in adjacent grains. A new method of concurrently coupling molecular dynamics and finite element analysis frameworks is formulated to minimize the overall computational requirements in simulating atomistically large material regions. A sequential multiscale approach is advanced to model microscale polycrystal domains in which atomistically-based cohesive zone parameters are incorporated into special directional decohesion finite elements that automatically apply appropriate ductile or brittle cohesive properties depending on the direction of crack propagation. The developed multiscale analysis methodology is illustrated through a parametric study of grain boundary fracture in three-dimensional aluminum microstructures. / Ph. D.