Study On Mechanical Behavior Of Lunar Soil Disturbance On Surface Sampling

The moon,as the closest body to the earth,is rich in minerals,energy and special space environment,so it has always been an important object of space research in the world.China has become the third country to successfully sample the moon after the United States and the former Soviet Union,with the successful completion of the orbiting,landing and return lunar exploration missions of the “Chang ’E Project”.In terms of sampling methods,except for the earliest Apollo program,which was sampled by astronauts,the subsequent studies of various countries were mainly unmanned and autonomous sampling.The experience of Luna program of the former Soviet Union shows that the failure rate of unattended deep drilling is higher due to the complexity of technology and equipment.However,the shallow surface sampling method has simple technology,simple machine and high sampling success rate,which can be used as a powerful supplement to deep drilling.In addition,the study of the surface lunar soil can provide detailed information on the early evolution history of the solar system,solar wind properties and radiation characteristics of the lunar soil exposure,meteorite impact records,composition and distribution characteristics of the lunar crust,and the evolution history,etc.The surface samples are as important as the deep samples.Therefore,the method of lunar soil surface sampling needs to be further studied.However,the lunar soil surface sampling method is flexible,which undoubtedly increases the difficulty of theoretical analysis and optimization design.Therefore,lunar surface sampling design needs a supporting theory based on the physical and mechanical properties of the lunar soil itself,which can be flexibly applied to different sampling methods.It is relatively easy to determine the physical and mechanical properties of the lunar soil in the static state.However,when the sampling tool enters the lunar soil,it will inevitably disturb the lunar soil in a certain range.It is difficult to determine whether the physical and mechanical properties of the lunar soil in the disturbed range are changed or whether the change affects the further development of the sampling tool.In this paper,a series of related studies were carried out on the common sampling methods of lunar soil surface.(1)The research status of the property of the natural lunar soil and the sampling method of the lunar surface were investigated,and an experimental device was developed to test the disturbance of the lunar soil,the core of which was how to evaluate the disturbance of the lunar soil.The lunar soil is a kind of easily disturbed granular material,if the traditional geotechnical test method is used to test its properties,it will inevitably cause secondary disturbance.In this paper,a standard probe was inserted into the simulated lunar soil before and after the disturbance and the disturbance was judged by the change of penetration resistance of the probe at the same position.Theoretical analysis of probe penetration resistance shows that it is related to the gravity of lunar soil,internal friction Angle and cohesion.After the soil is disturbed,its physical and mechanical properties change,which will inevitably lead to the change of probe penetration resistance.Therefore,probe penetration resistance can be used as a comprehensive index to evaluate lunar soil disturbance.(2)Because the amount of real lunar soil is too precious to carry out experiments,the simulated lunar soil with similar physical and mechanical properties was selected instead.In this paper,the disturbance experiment of simulated lunar soil was carried out.According to the concept of soil disturbance degree,the concept and calculation method of lunar soil disturbance degree were proposed by taking probe penetration resistance as a comprehensive parameter.The degree of disturbance was calculated based on the results of the experiment,and the degree and range of disturbance were judged according to the degree of disturbance.(3)From the macroscopic point of view,the lunar soil disturbance will cause the change of probe penetration resistance,and from the microscopic point of view,the lunar soil disturbance will cause the change of its density(mainly density)and shear strength(mainly internal friction Angle).Therefore,the lunar soil density and internal friction Angle are regarded as the characteristic parameters of the disturbance mechanics.In order to quantitatively describe the relationship among probe penetration resistance,density and internal friction Angle,the overburden experiment was carried out on simulated lunar soil,and the regression analysis was performed on the experimental results.The results show that there is a strong linear positive correlation between density and internal friction Angle.There is a correlation between density,internal friction Angle and the slope of probe penetration resistance.The formulas for simulating lunar soil density and internal friction Angle from probe penetration resistance were obtained by regression analysis.Then the relationship between disturbance conditions and mechanical properties(density,internal friction Angle)of lunar soil can be obtained.(4)Neural network was used to predict the probe penetration resistance based on the lunar soil disturbance conditions,and then the density and internal friction angle were calculated.Finally,a prediction model of Characteristic parameters of disturbance mechanics based on the lunar soil disturbance conditions was obtained.Different numbers of neurons and different algorithms will affect the prediction accuracy.Therefore,the training performance of 3-12 neurons,BR algorithm,LM algorithm and SCG algorithm are compared respectively.Finally,LM algorithm is selected to use 10 neurons to predict φ and ρ values of scoop experiment and dig experiment.The prediction performance shows that the correlation coefficient R between the predicted value and the real value is greater than 0.86,indicating a high correlation.The correlation coefficient R between the predicted value and the real value is much higher than 0.9,indicating a high correlation.The prediction results can meet the needs of subsequent theoretical calculation.Therefore,BP neural network LM algorithm is used to obtain the prediction model of disturbance mechanical characteristic parameters based on disturbance conditions.(5)Based on modularization,a mechanical model of lunar soil sampling was established,which was composed of three models: motion,tool and mechanics.The motion model group has two basic modules,three-dimensional uniform linear motion and three-dimensional uniform circular motion.The combination of these two basic modules defines all types of sampled motion.The tool model group has two basic modules: flat plate and curved plate.Each module is described by its section parameters,extension parameters and cutting parameters.The combination of these two basic modules defines the sampling tools of all shapes.The mechanical model group was established based on the assumption that the external forces borne by the tool were synthesized from all the contact points of the lunar soil during the sampling process.Each contact point could be simplified into a hexahedron,and the forces on six surfaces could be calculated according to the classical soil mechanics theory.The mechanical models of lunar soil sampling for different conditions can be established by combining different basic modules in three model groups.The lunar soil sampling model with disturbance correction can be obtained by introducing the prediction model of disturbance mechanical characteristic parameters predicted by disturbance conditions.The mechanical model of lunar soil sampling before and after modification was verified by the penetration resistance data of shovel experiment and torque data obtained of scoop experiment.It is found that the mechanical model of lunar soil sampling after introducing disturbance correction is closer to the real value.(6)Using the revised mechanical model of lunar soil sampling to optimize sampling method,the results showed that: in order to achieve the maximum sampling with the fixed power,we could increase the width of the sampling tools to increase the sampling depth of sampling volume,reduce the tool movement speed to reduce power consumption,for shovel sampling,could also decrease the angle of entry to increase sampling amount;in order to achieve the goal of the lowest energy consumption per unit sampling amount,a larger sampling depth,a lower movement speed and a smaller angle of entry can be selected as far as possible,and shovel is more energy efficiency than the scoop.