terramechanics, computational mechanics, contact problems

Traction analysis for off-road tires by FE-DEM

The performance of off-road tires depends on the contact condition between terrain and tread pattern of tires. However, this contact problem belongs to a typical non-linear problems, and it should be approximately solved by numerical analysis. In this study, we assume that the tire and subsurface soil layer, where the deformation is small, could be modelled by the finite element method (FEM) and top soil layer should be modelled by the discrete element method (DEM), where tread patterns of tire interact with terrain. We have developed a coupled FE-DEM for traction analysis of off-road tires. The contact condition of tire in rigid operating mode could successfully be analysed by this method, and the accuracy of this method was verified by an indoor traction experiments [ref 1][ref 2]. Currently, the contact condition of tire in elastic mode and the more precise analysis in 3D by FE-DEM are under study.

Lunar and planetary terramechanics

In Japan, JAXA now plans to propose a successive mission to the Moon, called SELENE-2, for a launch of vehicle in mid 2010s, where the soft-landing of a lander and the small autonomous robotic rover are two key technological highlights. In this study, based on the experiences on the collaboratory study with NASDA (currently, JAXA), we have constructed the performance analysis tool for a lugged wheel of micro lunar rover under lunar gravity condition. Demonstration:Analysis of a lugged wheel performance on sloped terrain by DEM
As our further preparation for such in situ resource utilization (ISRU) on the Moon in future, the specific shear resistance was analyzed by DEM and the verification of analysis was also done by the low gravity experiments in the parabolic flight of airplane. We showed that the specific shear resistance can be expressed by a linear relationship with relative gravity condition [ref 3].
Currently, the formation of angle of repose of sand in low gravity condition has been analysed, and running devices and motion control for a rover in ISRU have been investigated. Demonstration:Formation of angle of repose by DEM

Fundamental study on the improvement of accuracy in DEM analysis

DEM has been a popular numerical method in terramechanics these days. However, since in terms of the computational cost the model assumes in principle the virtual unit of element which does not correspond to the real particle dimension in real soil particles. Therefore, it has been an open issue how we can decide the properties used in a contact model for such virtual elements. Our approach is to use similar size of DE in both numerical analysis and experiments for cone penetration resistance, and find the important contact parameters, such as spring constants, by comparing both results. Moreover, the experience on this comparative observation will be useful for analyzing the real sand behavior in cone penetration. We believe that this approach would also be the nearest way for the improvement of numerical algorithm in DEM. Currently, the numerical analysis of bevameter test has been investigated using DEM, and the tuning of 3D DEM program is also under study.

Fundamental study on soil-machine interactions

DEM has been a popular numerical method in terramechanics these days. Soil cutting is one of fundamental problems in soil-machine system interactions. To apply DEM for various cutting conditions, an indoor experimental system was constructed with preconditioning system for soil conditions. Currently, various experimental data have been collected so that we can apply previously developed DEM program to these soil-machine systems. Moreover, we have been investigating some effective algorithms for fast and precise analysis applied to soil cutting problems by DEM.

Modelling of elongation and enlargement of root

From the viewpoint of modelling using mathematics or physics in relation to biological response to environmental conditions, we have focused on the elongation of root within the root sphere. The formation of root in elongation should be a result of contact interaction with surrounding environments. Thus, in this study, not only the soil but also the root are modelled by DEM, and the root elongation is assumed to be a mechanical insertion of virtual spring and new root DE after the end of previous elongation of virtual normal spring previously inserted with sprung condition. Fundamental interaction between soil and root could be expressed successfully. Currently, more realistic root growth with lateral enlargement stage has been under investigation[ref 4].

Improvement of turning performance of an articulated steering vehicle

Articulated steering vehicle, such as a wheel loader in construction machinery, suffers in principle the limited turning radius by the mechanism of frame steering. We have been developing a numerical methods which can be applied to the analysis of turning behavior of this vehicle. Moreover, the possibility of independent design of turning radius has been investigated numerically by the control of braking front and rear tires of inner turning circle. We have already done some experiments using a wheel loader and the results on turning radius with respect to vehicle velocity are obtained. Currently, the tire model based on terramechanics has been investigated and this model would be incorporated into the numerical analysis of turning behavior of a articulated steering vehicle.
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