Lunar Soil Simulants’ Constitutive Relations Based On PFC3D

Engineering application and foundation design on lunar surface requiresaccurate geotechnical behaviors of lunar soil simulants, resulting in manyinvestigations on development and mechanical properties of lunar soil simulant, butlittle studies on its constitutive relations. Here mechanical properties of lunar soilsimulants are explored via geotechnical tests, providing valid parameters for lunarsurface engineering. PFC3D code is used to numerically simulate the lunar soilsimulant, calibrating its microscopic parameters which influence the simulants’strength. In sight of the difference between lunar and terrestrial environment as wellas the particle property of lunar soil simulants, a proper constitutive model isestablished based on the theory of granular material mechanics and contactmechanics.Grain-size distribution tests are conducted to determine the grain-sizedistribution of lunar soil simulants, the compression coefficient of lunar soilsimulants is calculated through consolidation tests, and direct shear tests and triaxialcompression tests are conducted to study the shear strength of the simulants. Intriaxial test, a new sample preparation mold is designed for lunar soil simulants,making the test feasible and efficient. Different ranges of confining pressures areacted on the samples in triaxial tests, describing the shear strengths in higher andlower pressures respectively.Base on the mechanical parameters resulted from laboratory tests, PFC3D codeis used to study the microscopic mechanical behaviors of lunar soil simulants. Acylinder-shaped wall and two loading walls are modeled in the first step, and thenFISH language is programmed to form the particle assembly whose particledistribution corresponds to that of lunar soil simulants. The micro parameters aredebugged repeatedly referring to the known macro physical and mechanicalparameters until the simulated stress-strain relations conform to the practicalconditions, based on which the contact mechanics model is established for the numerical simulation.As a result of the specialty of the mechanical behavior of lunar soil (orsimulants), the paper considers the possible influence of surface energy on the shearstrength of lunar soil simulants. A granular material model which takes particleinteractions of lunar soil simulants into account is developed to reproduce themechanical behavior of lunar soil simulants under earth conditions. The constitutivemodel herein is extended from a model for granular materials, including surfaceenergy forces between particles to describe the possible behavior of lunar soil underextremely low atmospheric pressure conditions. In such case, the model showsrelatively increased shear strength due to the effect of surface-energy forces, whichis in appropriate agreement with the observations of lunar soil made on the Moon’ssurface.