| Vehicle mobility on unpaved surfaces is important to military, agriculture, forestry, mining, construction, and recreation industries. Because of the complicated nature of vehicle-terrain interaction, comprehensive modeling of off-road mobility is often performed using empirical algorithms. When mechanical analyses have been applied, they are generally in two dimensions, sometimes with modifications to imply the effect of the third dimension. The desire to incorporate theoretical mechanics into performance models has generated great interest in applying numerical modeling techniques to simulate the full three-dimensional interaction of the deformable tire and terrain.; To this end, a three-dimensional model simulating a tire rolling over snow or other terrain material was developed. Fresh snow and compacted sand terrain surfaces were modeled as inelastic materials using the Crushable Foam and Cap Drucker-Prager constitutive models of critical state and plasticity theory. The snow model was generated from experiments on the mechanical deformation of snow and was validated using plate sinkage test data. The soil model was constructed based on existing constitutive models from the literature for a soil similar to that used in vehicle mobility studies. Tires used during vehicle testing were simulated with (1) a rigid tire model, (2) the Shoop-Darnell tire model, which incorporates user-defined elements for the sidewalls, and (3) a modal analysis type of tire model. Comparisons of the tire models with measured deflection and contact stress indicate that both the Shoop-Darnell and modal analysis models yield accurate results, but the Shoop-Darnell model is much more computationally efficient.; The combined tire-terrain model was validated for fresh snow using force measurements collected with an instrumented vehicle and measured snow deformation under the wheel; and it was compared to snow mobility predictions made using the NATO Reference Mobility Model (NRMM). These comparisons indicate excellent agreement between the finite element model and field measurements of forces resisting motion and snow deformation under the wheel. The model also illustrates the effect of slip on sinkage. Preliminary results of the modal analysis tire model on snow show very little deformation in the tire, as expected, indicating that the rigid wheel simplification may be more applicable for soft terrain. |
