Particle Flow Code Numerical Study On The Mechanical Properties Of Lunar Soil And Its Simulant

The research of the mechanical properties of lunar soil is one of the major tasks in lunar exploration program.Meanwhile,lunar soil simulants are the main subject of study in the related model tests.In essence,both the lunar soil and the lunar soil simulants are granular materials,having the discrete characteristics.Thus,the properties of particle,especially the particle shape,have great influence on their mechanical properties.The discrete element method(DEM)develops numerical model based on the discrete nature of objects,and has a unique advantage in simulation study of the non-continuous particulate material such as the lunar soil.Based on the theory of discrete element method,the numerical simulation by particle flow code(PFC)is applied to analyze the correlation between the particular characteristics of lunar soil and its mechanical properties in shear strength.Also,an FEM--DEM multiscale modeling is conducted to simulation study the cone penetration test in the in-situ lunar soil.The main findings are as follows:(1)The lunar soil simulant needed for research is prepared,using volcanic ash as the raw material.Contrastively analize its basic physical properties in particulate microstructure?particle relative density?particle size distribution?bulk density?void ratio?compactness,et al.,as well as the mechanical properties of compressibility,with the lunar soil samples and other major lunar soil simulants.The comparison results show that the lunar soil simulant is similar to the lunar soil samples and other major lunar soil simulants in particulate microstructure,including the particle shape and surface roughness.And its basic physical properties and the compressibility are within the physical and mechanical property value range of the lunar soil samples and other major lunar soil simulants.(2)The PFC numerical simulation method can intuitively reflect the change of field parameters such as velocity?displacement and force,as well as it can record and extract the arrangement?contact force and displacement of particls in real time during cycling.Thus,PFC numerical simulation has its unique advantages in the research of granular material such as lunar soil and its simulant.As for the field of studying the geotechnical problems related to the lunar soil,PFC numerical simulation method is applied mainly in the following aspects:amendment of particle contact model?selection of the micro parameters?simulation of the particle shape,and the numerical study of the mechanical properties of the lunar soil and soil-wheel interaction mechanism.(3)A series of detailed tri-axial tests are carried out to get the shear strength of the lunar soil simulant.The tests results show that the peak friction angle of the lunar soil simulant lies between 43° and 51°,and the cohesion force is about 0.37?4.90 kPa.The shear strength parameter values will increase along with the relative density of sample and the confining pressure.The numerical model of tri-axial tests are established by the means of PFC 3D,and the influences of the three micro-parameters:inner-particle friction coefficient?original porosity and the stiffness of particle on the shear strength are detailedly investigated.The simulation results show that inner-particle friction coefficient affect the most on the shear strength.(4)In order to characterize the particle shape of the lunar soil simulant,their particles are scanned by stereomicroscope.The observation results show that the particles of lunar soil stimulant are of quite different shapes,and have rough surfaces.There exist apparent honeycomb-shaped pore structures within large particles,and the particle surfaces are covered with angular?serrated and grooved structures.An image processing software named Image-J is utilized to quantitatively describe the shape characteristics of particles for statistical analysis,the result shows that the lunar soil simulant has four main particle shapes:sphere,quasi-ellipsoid shape,quasi-trigonal shape and quasi-cube shape.By the means of PFC 3D,the triaxial tests of the lunar soil simulant with typical irregularly-shaped particles is numerically modeled,and the simulation results show that the particle shape has a noticeable effect on shear strength;the shear strength of particles of quasi-ellipsoid shape is the greatest of all,while the sphere particles has the minimum shear strength.With particles of the same quasi-trigonal shape,the greater ruggedness it is,the stronger shear strength the sample has.(5)Base on the test results of the particle shape characterization of the lunar soil simulant,twenty typical particle shapes of the lunar soil simulant are modelled in PFC 2D,and the biaxial test is simulated to comparatively study the effect of particle shape on the shear strength of the lunar soil simulant.The simulation results reveal that the sample with irregularly-shaped particles have more apparent pores before or after the biaxial compression,compared to the sample consisted of default round particles.Meanwhile,big pores are more likely to exist adjacent to the irregularly-shaped particles with greater size.Under the same test condition,the sample with irregularly-shaped particles have greater peak deviator stress in the loading process,and have more obvious phenomenon of the strain softening.The particle shape has a strong influence on lunar soil simulant's shear strength:internal friction angle of the sample with irregularly-shaped particles is about 5°greater than that of the sample with default round particles;as for the cohesive force,it is 0.2 kPa or so.(6)A series of cone penetration tests in the in-situ lunar soil is numerically simulated by an FEM-DEM multiscale method,and the flow situation of the particles adjacent to the cone is effectively reproduced during the penetration process.The change of gross penetration resistance?cone tip resistance and side frictional resistance of penetrometer along the penetration depth are analyzed.In addition,the effects of the low gravitational field on the Moon?the cone tip angle and the penetration rate are qualitatively studied.The simulation results show that:the gross penetration resistance and cone tip resistance increase along with cone tip angle,gravitational field and penetration velocity.Meanwhile,the side frictional resistance is mainly affected by gravitational field and penetration velocity,but it is little affected by cone tip angle.