Deformable tire on soil

cap plasticity model representing the McCormick

On harder terrain it is important to model the de-

Ranch sand (discussed earlier). The results of this

formation of the tire in addition to the deformation of

simulation are given in Figure 69, showing the maxi-

the terrain. Because of the large deformation and run

mum principal plastic strain and the principal stress

times involved, it is important that the tire model be

directions. The maximum principal stress is compres-

as efficient as possible while still maintaining an ac-

sive beneath and slightly forward of the tire. The

curate contact patch. Thus, the tire model used for

minimum principal stress is tensile and is largest just

rolling on soil was based on the formulation proposed

below and to the sides of the tire, oriented away from

by Darnell et al. (1997).

the tire. The difference between these stresses indi-

The ShoopDarnell tire model was modified to

cates the magnitude of shear stress in the soil, which

roll across deformable material, such as soil. This tire

is largest beneath and directly in front of the tire. This

was first placed on an elastic terrain material with the

corresponds to the general shear zones beneath a

properties of compacted sand. The simulation was

towed rigid wheel observed by Wong and Reece

conducted in four steps: the tire was first inflated,

(1967), shown in Figure 1.

then lowered into the soil, allowed to come to equi-

The tire-soil models are prototypes requiring re-

librium, and then rolled to the left, as seen in Figure

finement and development. They will eventually be

68. Only the tread and soil elements are shown in the

compared to field measurements of tire forces, de-

figure; the user-defined sidewall elements are not

flection, and contact area, including sinkage and

displayed. Figure 68d is an oblique view of the three-

deformation of the soil surface.

dimensional model showing the contact pressure.

51