Test study on mechanical properties of lunar soil simulant under high compactness condition

Abstract: In order to analyze the mechanical properties of lunar soil simulant at different densities，3 types of high density lunar soil simulant (JLU5-1, JLU5-2 and JLU5-3, which has been adopted in the Chinese lunar exploration program) was prepared by a self-designed vibrating device, the shearing and penetrating tests of 4 different soil relative density (0.85, 0.9, 0.95 and 0.99) were performed, the influence of relative density on shear strength, cohesion, internal friction angle, cone index and cone index gradient were analyzed. The result showed that the shear strength of 3 lunar soil simulant ranged from 36.6 to 158.4 kPa. The shear strength increased with the increase of relative density, the average change rate was 11.1%, which indicated that the shear capacity was enhanced with the increase of soil relative density. JLU5-3 had the largest shear strength, while the shear strength of JLU5-2 was smallest. The cohesion ranged from 14.9 to 68.0 kPa, which was about 3 to 10 times higher than that of lunar soil simulant JSC-1A and TJ-1. The cohesion increased by 35.8% with the increase of relative density, the average change rate for different lunar soil simulant was 24.7% (JLU5-1), 22.8% (JLU5-2) and 37.5% (JLU5-3), respectively. When the relative density reached 0.9, the cohesion of JLU5-3 was larger than that of JLU5-1 and JLU5-2, and the difference between them increased apparently, which may due to the effect of interface energy and viscidity became more obvious with the decreasing of soil grain size. The internal friction angle ranged from 53.3o to 67.7o, which had no obvious variation trend with the increase of relative density. The internal friction angle of JLU5-2 was always smaller than that of the JLU5-1 and JLU5-3 under different soil relative density conditions. The internal friction angle of JLU5 under high compactness condition was apparently larger than other series lunar soil simulant regolith under natural condition. When penetration depth reached 40 mm in the tests, the cone index of 3 types high-density simulant lunar soil were 129.8 MPa (JLU5-1), 142.9 MPa (JLU5-2) and 175.2 MPa (JLU5-3), respectively. Cone index presented the fluctuations but generally increased with the increase of penetration depth, smaller grain size lead to the increasing of cone index under same testing condition, which was similar to variation trend of cohesion. The cone index of JLU5-3 increased by 13.4% on average compared with that of JLU5-2, and the cone index of JLU5-2 increased on average by 20.5% compared with that of JLU5-1. Cone index gradient was defined as the curve gradient of the cone index versus with penetration depth. When the relative density changed from 0.85 to 0.9 and 0.95 to 0.99, the cone index gradient obviously increased, while there was no significant increase for cone index gradient when relative density changed from 0.9 to 0.95. The average increasing ratio of cone index gradient was 50.6% with the increasing of relative density, greater than that of the cohesion, which indicated that cone index gradient was more sensitive to soil relative density. For different types of lunar soil simulant, JLU5-3 had larger shear strength, cohesion and cone index and cone index gradient than that of JLU5-1 and JLU5-2 under same testing conditions, which may due to JLU5-3 had smaller particle size. Numerical model for penetration characteristic test had been conducted by using discrete element method software (EDEM), simulation results showed that, the simulative value of cone index was always smaller than that of the testing value, however, their variation trend versus with the relative density was consistent. A linear relation model between simulative and testing value was established, the determination coefficient value of the proposed model was 0.87. The simulation method could provide technique method for cone index prediction of lunar soil simulant under high compactness condition. The results of this paper were expected to provide references for drilling sampling mission of lunar soil, optimization design of drilling mechanism, and establishment of mechanical interaction model between drilling component and lunar soil.