Experimental And FEM Numerical Analyses Of TJ-1 Lunar Soil Simulant Excavation

There are abundant available resources on the Moon,including more than 100 kinds of minerals and nearly 225-450 million tons of rare earth elements.Especially the reserve of 3He on the Moon,a kind of clean fusion material,is 1-5 million tons which is hundreds of thousands of times more than that on the Earth.In addition,the Moon has a special environment which is quite different from that on the Earth,such as the low gravity field,high vacuum,non-magnetic field and high diurnal temperature difference.Thus the establishments of permanent lunar bases are of great significance for the research work in the fields of astronomy,physics et al.,which can also ease the burden of huge population and insufficient resources on the Earth.Therefore,various countries have explored the Moon with huge investments since the 1950 s,including the United States which is the first country to land on the moon successfully.China has also made a big progress on Chang'e lunar exploration program since 2004.In the future,the problem of lunar soil excavation will be inevitably encountered when utilizing the lunar resources and constructing the permanent outposts.However,the existing experiences and theories for excavation on the Earth may be unavailable for lunar soil excavation on the Moon because of the special environment on the Moon and the unique mechanical behavior of the lunar soil.Hence,in this paper,the lunar soil excavation was studied by conducting physical model tests on the Earth with TJ-1 lunar soil simulant,a kind of lunar soil simulant,which can capture the mechanical behavior of real lunar soil.Then the corresponding Finite Element Method(FEM)simulations were carried out.The experimental and FEM simulation results were compared and analyzed to help improve the design of the excavation scheme and excavator.The main work and conclusions are presented as follows:(1)a series of TJ-1 lunar soil simulant excavation tests was firstly conducted with various cutting depths,rake angles,blade widths,cutting speeds and blade curvature angles.The excavation resistance,soil heap in front of the blade,the soil trench behind the blade and the earth pressure in the ground were analyzed.The results show that: the excavation resistance increases to a peak value rapidly at first and then tends to be stable;The stable excavation resistance increases with the increase of the cutting depth,blade width,cutting speed and the blade curvature angle while it decreases with the increase of the rake angle.The effects of the cutting depth h and blade width w on the excavation resistance F can be described as F=ah2+bh and F = cw+d respectively,where a,b,c,d are the fitting parameters.The soil heap accumulates in front of the plate quickly and the soil trench appears behind the blade during the excavation process.Similar to the evolution trend of the cutting resistance,the sizes of the soil heap in front of the blade and the soil trench behind the blade both increase with the increase of the cutting depth,blade width,cutting speed and blade curvature angle while they decrease with the increase of the rake angle.The soil pressure below the cutting blade in the foundation gradually increases at first and reaches a peak value at a distance of nearly 100 mm.Then the earth pressure decreases and tends to be a stable value.The earth pressure placed on the soil surface can be used to monitor the affected area in front of the blade.It can be observed that the affected area increases with the cutting depth,blade width,cutting speed and the blade curvature angle while it decreases with the increase of the rake angle.(2).The triaxial compression tests and plate loading tests were firstly simulated with the Mohr-Coulomb model using the finite element software,ABAQUS,and the simulation results were compared with the experimental data to examine that if this model can be used in lunar excavation simulation.Then a series of numerical simulations of lunar excavation was conducted,where the excavation resistance and the soil ground response were analyzed.Finally the simulation results were compared with the experimental data.The results show that: the Mohr-Coulomb model can well capture the mechanical and bearing behaviors of TJ-1 lunar soil simulant,which proves that this model can be used in the simulations of excavation test.Note that the excavation displacement was applied on the soil directly to simulate the excavation process.The simulation results show that the excavation force and the affected area increase with the increase of the cutting depth,blade width,cutting speed and blade curvature angle,which are in quantitatively agreement with the experimental data.But the excavation resistance and affected area increase with the increase of the rake angle,which is totally different from the experiment.Thus,the blade cutting process,instead of directly applying the cutting displacement on the soil,was employed to carry out excavation test simulations with different rake angle.The results of numerical simulations are found to be in qualitatively agreement with the experimental data: the excavation resistance and the affected area decrease with the increase of rake angle.But the values of the excavation resistance are larger than those in the physical model tests.