A Study of the Influence of Surface Energy on the Mechanical Properties of Lunar Soil Using DEM
DEM was employed to investigate the mechanical behaviour of Lunar Soil, generally defined as the finest fraction of the Lunar Regolith, taking into account the main effects of the lunar environment (e.g. high vacuum, low gravity and high temperature fluctuations). The purpose is to inform the planning of future lunar exploration, both manned and robotic. The investigation has focussed on the origins of the observed cohesion of Lunar Soil, whose explanation has proved elusive until now. The effects on the stress-strain behaviour of inter-particle attractive van der Waals forces (quantified by the surface energy) were investigated by means of triaxial compression tests in a periodic cell containing 5000 spherical particles. The particle radii vary according to the typical Particle Size Distribution of the Lunar Regolith. Very dense samples, with relative density approaching 100%, were prepared under isotropic compression. Triaxial tests were then carried out at constant mean pressure for very small confining stresses in the range of 0.1 kPa to 10 kPa, corresponding to a depth from the lunar surface of approximately 0.04 m to 4 m, respectively. To assess the effect of surface energy, simulations were run with and without the presence of inter-particle adhesion, which was introduced at the contact level by implementing the JKR model. The results show that the surface energy is responsible for an apparent macroscopic cohesion at the peak of the deviatoric stress-strain curve, but does not affect the tangent friction angle. However, at the critical state, very little cohesion (if any) was observed. Furthermore, an increase of the secant friction angle is apparent only at stresses less than 5 kPa. Physical explanations of these complex phenomena are postulated, and a link between microscopic and macroscopic physical variables is proposed.