| In the lunar exploration project,it needs to study the interaction of lunar probe and lunar regolith.Because of less true regolith samples,a similar lunar soil simulant can be used replacing the real lunar soil to meet the needs of a large number of scientific researches in lunar exploration project.Due to the constraint stress of lunar soil only(1/6g)corresponds to the same depth on earth,which leads to the true stress state of regolith is much smaller than the stress state in the study of conventional soil mechanics tests used.It is an urgent need to study the mechanical properties of lunar soil simulant under low stress levels for a better evaluation of lunar soil drilling performance and it has important theoretical and practical significance for the realization of the "lunar regolith sampling".In this thesis,using QH-E lunar soil simulant developed by Tsinghua Universityto carry out a series of triaxial compression tests under different confining pressures(low and conventional)and different relative densities.The different mechanical properties of lunar soil simulant under different stress levels and relative densities are achieved,and the microscopic parameters needed for the numerical simulation of the discrete element are given based on these triaxial compression tests.Then differentstress levels(low and conventional)and different stress paths(conventional triaxialcompression(CTC),constant principal stress triaxial compression(PTC),and hydrostatic compression(HC))are conducted by using PFC3D discrete element simulations.The strength and volumetric characteristics of QH-E lunar soil simulant under different stress levels and stress paths are analyzed and their similarirties and differences are discussed.The main work and results are as follows:(1)Using the STX geotechnical three triaxial apparatus,the experimental study on mechanical properties of QH-E lunar soil simulant under low stress level is systematically carried out,the stress-strain and volumetric change rules of QH-E lunar soil simulant under different confining pressures and relative densities are clarified.The research results indicate that QH-E lunar soil simulant has an obvious strain softening and dilatancy characteristics.Under low confining pressure,the specimens with larger relative density(83%and 92%),the volumetric strain tend to be stable after reaching the peak value,while for the specimens with relatively small density(74%),the the volumetric strain continue to decrease and do not converge.The sample of each relative density all show obvious strain softening and dilatancy.Under low confining pressure,the specimen has an obvious turning point after reaching the peak deviatoric stress,and the volumetric strain increases slowly with the increase of confining pressure;while under the conventional confining pressure,the volumetric strain gradually becomes stable with the increase of confining pressure after turning point.Under low confining pressure range,the dilatancy angle varies within the range of 10-40 degrees.Relative density increases,the dilatancy angle increases;the confining pressure increases,the dilatancy angle decreases.Under conventional confining pressure,the dilatation angle decreases slowly and shows a steady trend.(2)The micro-parameters of QH-E lunar soil simulant are obtained based on triaxial compression tests on the specimens with the relative density of 83%,a cylinder sample with the same size is established using PFC3D discrete element software,the strength and volumetric change characteristics and failure mechanisms of QH-E lunar soil simulant are investigated under conventional triaxis compression(CTC)path.The simulation results indicate that the deviatoric stress-axial strain curves obtained by PFC3D under CTC path are basically the same with the experimental curves,showing obvious strain softening and dilatancy characteristics.The deviatoric stress-axial strain curve can be divided into the hardening stage before peak deviatoric stress and the softening stage after peak stress,and the softening stage is divided into the rapid softening stage and the residual strain stage.The volumetric strain-shear strain curve is divided into linear dilatation stage and residual dilatancy stage.With the increase of confining pressure,the peak stress increases,the axial strain corresponding to the peak stress increases,and the residual stress also increases.Under the same confining pressure,the larger the initial porosity is,the smaller the peak stress and the residual stress are;the larger the particle friction coefficient is,the greater the peak stress is,and the greater the residual stress is.A small amount of shear shrinkage occurs at the initial stage,followed by a long time dilatation under CTC path.As the loading continues,the specimen generates soften and forms shear bands until it breaks down.Under low confining pressure,the "V" type double shear bands appear in the sample,while the specimen is mainly destroyed by single shear band with 52°on the shear plane under conventional confining pressure.(3)Considering the effect of stress path on mechanical properties of lunar soil,constant principal stress triaxial compression(PTC)and hydrostatic compression(HC)paths are realized to further study the mechanical properties of lunar soil simulant by using PFC3D.The results show that the QH-E lunar soil simulant still exhibits obvious strain softening and dilatancy characteristics under PTC path.The development stages of the devoatoric stress-axial strain curve and the volumetric strain-shear strain curve are coincided with the CTC path.The greater the confining pressure is,the greater the peak stress and the residual stress are.Under the same confining pressure,the larger the initial porosity is,the smaller the peak stress and the residual stress are,wheares the larger the particle friction coefficient is,the greater the peak stress and the residual stress are.Dilatancy is divided into linear and residual shear dilatancy,both the linear and residual shear dilatancies with P show a good linear relationship,but the rate of volumetric change is smaller in the residual shear dilatancy stage and than the linear shear dilatancy stage,and the greater the confining pressure P is,the volumetric strain in the residual shear stage dilatancy is more stable.(4)The simulation results under hydrostatic compression(HC)path show that:with the increase of confining pressure,volumetric strain ?v gradually increases.Under low confining pressure,?v increases fast,when the confining pressure increased gradually to the conventional level,?v increases very slow,the ?v,curve becomes more and more flat.Under the same confining pressure,the greater the initial porosity e,the larger ?v is,the greater the friction coefficient f is,the smaller ?v is.Under low confining pressure,the gap of ?v caused by e is not large,but with the confining pressure increased gradually to the conventional level,the gap of ?v caused by e become more and more large.The reduction of ?v caused by f under low confining pressure is smaller than under conventional confining pressure.(5)The failure of QH-E lunar soil simulant is dilatancy damage under CTC and PTC paths.The samples have a small amount of shear contraction at the initial stage of CTC path;whereas there is almost no shear contraction under PTC path,followed by a long period of dilation,as the loading continues,QH-E lunar soil simulant softens and forms a shear bands in the interior of the sample until it is destroyed.Under two different stress paths,the dilatancy characteristic parameters(linear dilatancy parameter aL,residual dilatancy parameter ?R and residual dilatancy point?sr)with confining pressure P show a good linear relationship,and the value of ?L is greater than aR,the volumetric strain in the resudial dilatancy stage become more stable with the increase of P.Under the same confining pressure P,the peak deviatoric stress under CTC path is lager than that of PTC path,and the greater the confining pressure P is,the bigger difference in the peak deviator stress.When the confining pressure P increases to the conventional stress level,the influence of two different paths on the strength of QH-E lunar soil simulant is greater. |
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