A Comparison of Discrete Element Modeling, Finite Element Analysis, and Physical Experiment of Granular Material Systems in a Direct Shear Cell

摘要

In order to design future equipment to operate in a lunar environment, the ability to simulate such equipment using computer aided engineering (CAE) is essential. The conditions of lunar gravity cannot be reproduced on earth, thus the virtual environment is the only way to test design concepts for lunar applications. Traditional CAE for handling granular materials relies on finite element analysis (FEA), which is challenged by the lack of reliable constitutive laws. This challenge is met by using Discrete Element Modeling (DEM). However the current state-of-the-art in DEM has two major limitations which must be overcome to improve the usefulness to NASA and the commercial sector applications: the computational intensive nature of the software, and the lack of an established methodology to determine the particle properties to accurately model a given physical system. We present a three-way parallel study, physical/FEA/DEM of a direct shear test for two materials to compare the differences and similarity of the FEA and DEM results. The two materials used are 3M glass bubbles and a lunar simulant; one consists of extremely smooth particles and the other extremely rough. We test the ability of DEM for irregular particle by using simple spherical clusters to simulate such rough material. We compare the bulk friction behavior between the physical experiment and those from FEA and DEM. We also compare the internal stress field between the FEA and DEM. Both FEA and DEM can simulate the physically measured bulk friction. However, not surprisingly details of the stress field differ between different methods. To overcome the particle number limitation, DEM results are obtained using much coarser particle size. By using several different sizes we find the bulk friction depends on the particle size. Based on this study, the preliminary implications are that FEA and DEM produce different stress distributions, even in a simple geometry such as the direct shear; DEM can simulate irregular rough particle systems such as the lunar regolith; and the coarsening effect is non-trivial.