טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentAsaf Zvi
SubjectDetermination of Discrete Element Model Parameters Required
for Simulation of Soil-Implement Interaction
DepartmentDepartment of Agricultural Engineering
Supervisors Professor Emeritus Itzhak Shmulevich
Dr. Dror Rubinshtain


Abstract

The discrete element method (DEM) seems to be a promising approach for constructing a high-fidelity model to describe soil-implement interaction. Proper prediction of this interaction using DEM depends upon the model parameters. However, there is no robust method for determining the parameters for discrete element models. The need for a robust method for calculation of the micro parameters of the model from macro properties of the martial that can be measured is the main motivation for this work.

In the first stage of the present work, the ability of the DEM method to quantitatively model soil-implement interaction was investigated. Three models were constructed: (a) a model of track-link soil interaction; (b) a rigid wheel-soil model; and (c) a model of a bulldozer blade and bucket in the digging process. The simulation results show reasonably good correlation, quantitatively and qualitatively, with previously reported results and theories.

The study focused mainly on a method for determining the discrete element model parameters for soil-implement interaction from the macro properties of the soil . The method is based on in-situ tests and an inverse solution technique. The research concentrated on quasi-static simulation in a two-dimensional and cohesionless soil model.

The in-situ tests were based on sinkage tests using two wedges with angles of 30° and 90°, respectively, and a plate. For each test, a plot of force versus displacement was drawn and the energy of each test was calculated. Using the wedge-sinkage tests and simulations results, an expression of  the energy as a function of the friction coefficient of the soil was found. Based on the slope in the elastic-plastic zone of plate-sinkage tests, an expression between the spring constant and the friction coefficient was developed. By using these expressions, the initial value of the model parameters can be determined with error of 15%. An objective function was created. This objective function was composed of the average of the sum of the energy differences between the wedges and the plate sinkage test results and the corresponding simulation results, normalized by the energy test results. An inverse solution technique that uses the Nelder-Mead algorithm of optimization was employed to find the most appropriate parameter values for this function[TB1] . The validation of the method was done by means of an independent shear grouser test. The results of this test show that the main mechanical soil parameters were reconstructed with an error of 4%.