Investigation of the Shear Stress Dynamics on Silty Loam Soil and Measurement of Traction-Wheel Slip Relationship of a Tractor Tire

The Hohenheim Tire Model (HTM) was developed and validated at the University of Hohenheim. It simulates the specific behavior of large-volume agricultural tires and is used in conjunction with multi-body models of tractors and other agricultural machines. Special focus is placed on driving dynamics and comfort. Currently the HTM is being expanded for implementation on soft soils. Therefor the soil's shear response using Bevameter tests and the relationship traction-wheel slip using a single wheel tester are investigated. In field tests, the effect of shear rate on soil mechanical parameters was examined. The shear device of the Bevameter was used to conduct experiments in a soil bin with silty loam at eight shear rates and five vertical loads. The Mohr-Coulomb failure criterion was calculated and the measured values were fitted to the Wong-Preston and the Janosi-Hanamoto approaches. The shear stress-shear displacement curves show two different behaviors with respect to shear rate. At high shear rates, the shear stress reaches a peak and then drops to a residual value, which is typical for cohesive soils. At low shear rates, the trend of the curves is exponential, which is typical for granular soils. The cohesion and angle of internal friction values are comparable to those from literature. The angle of internal friction has no correlation with shear rate while the cohesion shows a low correlation. Traction-wheel slip curves for the tractor tire 480/70 R24 with two tire loads (17 and 22 kN) and at 2 km∙h-1 were measured under identical conditions as the shear experiments. For the higher tire load, there is a tendency toward a larger tractive force. The curves of both wheel loads have equal gross traction coefficients. For the prediction of the traction force, the parameters obtained in the shear tests and the method of Wong and Preston-Thomas are used.