Particle Descriptions and Grain Contact Laws for Lunar Materials: Their Implementation in a Discrete Element Model

Abstract

Lunar surface activities engage the mechanical properties of the regolith on a relatively small scale, and there is currently great interest in employing discrete element models to simulate such activities. The validity of this type of model depends critically on employing correct contact laws and particle descriptions (size, shape and surface roughness) for the materials of interest. This need has motivated us to conduct grain-scale experiments on Apollo soils, develop contact laws, and implement these laws in a discrete element model to simulate the engineering-scale behavior of the lunar regolith. The paper presents an overview of our findings with emphasis on the range of contact properties observed primarily for lunar plagioclase and pyroxene grains, with varying degrees of space weathering, and the implementation of the contact laws in a discrete element model of a standard geotechnical triaxial cell. We quantify grain size, aspect ratio, roundness, etc., and use these results to generate virtual particles with the desired population statistics. Carefully oriented SEM micrographs of each grain are used to determine the radius of curvature and surface roughness of the contact patches prior to and after mechanical testing. The paper presents normal and shear contact laws that quantify the elastic and inelastic (e.g., hysteretic) components of deformation. We observed space weathering to have a first-order effect on contact properties, reducing the effective elastic modulus of plagioclase grains by ~50% and increasing the hysteretic loss by a factor of 3-4. Implementation of the normal and shear contact models in a discrete element model is described, and the effects of the observed ranges in contact properties on the simulated triaxial behavior are demonstrated.