Search

The van der Waals force is significant for the formation of microstructures of wind-blown deposits, such as loess soil. However, it is difficult to observe the effect of the van der Waals force in the natural environment and laboratory. The conventional DDA model, which simplifies contact as compressed elastic springs, has been successfully applied in simulating blocky rock behaviors, but it is incapable of simulating tension forces. In this paper, we modified the conventional DDA model by considering blocks as soil particles and by applying the van der Waals force among the particles to investigate the micro-behavior of fine soil. The modified DDA is validated by simulating two ideal experiments. Then, the falling process of fine grains (<65 μm), similar to wind-blown dust, is simulated by the modified and conventional DDA models separately. The simulated deposits of the falling grains by the two models show significant differences in microstructure. The microstructure generated by the modified DDA model is much closer to the loose structure of a real loess specimens, while that generated by the conventional DDA model is much more compact. This verifies that the van der Waals force is crucial for the formation of the loose microstructure of loess. [ABSTRACT FROM AUTHOR]

Abstract: The grains of lunar regolith are characterized with rough surfaces, angular shapes and mutual adhesions due to short-range interactions. These features control the macroscopic mechanical behavior of lunar regolith but have not been completely captured by contact models in previous Discrete Element Method (DEM) analyses. In this paper, a simplified two-dimensional microscopic contact model is proposed for high efficiency DEM analyses of lunar regolith. The model consists of three components in the normal, tangential and rolling directions respectively, plus two new parameters. A shape parameter is used to control the rolling resistance ability at the contact area between two particles to capture the features of grain shape and interlocking. The second parameter, micro-separation, which denotes the nominal minimum distance between the molecules of the two contacting particles, is introduced to account for van der Waals force as the major component of the short-range interactions that contribute to the adhesion of regolith grains in lunar environment conditions. The novel model has been implemented in a two-dimensional DEM code for numerical simulations of biaxial compression tests on lunar regolith. The effects of interparticle friction, grain shape, lunar environment conditions and void ratio on the strength of lunar regolith were numerically investigated. The results show that soils in the simulated lunar environment exhibit greater strength and more apparent strain-softening and shear dilatancy than on the Earth. The proposed model can capture the main features of the mechanical behavior of lunar regolith (apparent cohesion and high peak friction angle) and a wide range of strength indices can be obtained by the contact model. [Copyright &y& Elsevier]