Research On Lunar Soil Drilling Dynamics Based On The Discrete Element Method

With the smooth development of the third moon probe phase in our country, the drilling sampling of lunar soil has been the key task in the field of space technology. It is still one of the most technically sophisticated problems in drilling field to drill lunar soil which remains with drilling problems under extreme conditions. It’s hard to perform the real environment simulation on the ground. The reason, on one hand is for extreme conditions on the moon like low-gravity, no air, no water and on the other hand the special mechanics of lunar soil with high internal friction angle and low cohesion. Additionally, space mission requires that drilling tools must meet light quality, low power consumption and large strength, therefore, it’s necessary to make a deep research on tool-soil interaction dynamics in the drilling process to provide theoretical foundation for drilling tools design, regulations design, materials design, ect. It’s hard to observe the inside situation of soil on ground experiment which also costs too much, and theoretical analysis has been confined by excessive assumptions. For that reason, discrete element method is adopted in the essay in building lunar soil constitutive model and drilling-sampling stimulation model, and also used in dynamic simulation and to analyze on low-gravity load, larger particles impact, bedding feature and drilling bit structure influence which engineering concerns about.In connection with the special mechanics features of lunar soil and based on the discrete element method, a linear stiffness with non-tension constitutive relation and a constitutive relation with apparent cohesion among discrete elements are established. Features of different tension adding ways are summarized and an optimal tension theory is put forward. According to the action form of forces and the chemical composition of lunar soil, the rationality of the spring tension is explained. An initial bonding assumption is put forward, based on which a three dimensional discrete element model of lunar soil with alterable constitutive law is finally established. The specimen preparation process under alterable compactness in traixial simulation test based on boundary vibration control is introducted, and an impact discipline of the microscopic parameters on the macro shear strength is obtained by traixial simulation. With parameter callibration a final model is confirmed. In the end, as to the model whether in shallow or in deep, the contact ration of the traixial simulation curves and experiment curves of the actual lunar soil is high, the cohesion and the internal friction angle of lunar soil in shallow are 0.90 k Pa and 42.25°, and that in deep are 2.6k Pa and 50.88°, and all meet the best estimate to ensure the precision of model and applicability under different densities.To deal with the large calculation problem in discrete element simulation, an equivalent thought is used to build the drilling models of lunar soil in shallow and deep, then under a large border drilling simulation, the particle motion law and velocity field are analyzed, and a U-shape particle moving chain around drilling bit is described. The equivalent simulation border is confirmed by comparison of stress field. Different depth environments are simulated by imposing high compacted equivalent particle group on dense lunar soil, and then equivalent depth models of drill bit and drill stem are established respectively. In order to ensure the authenticity, a pre-drilling simulation and constant-pressure control are applied. Finally, drill torque in arbitrary depth is obtained by torque composition. Drilling experiment platform of simulant lunar soil on ground is built up, and by comparison of torque curves from experiment and torque prediction curves from deep drilling simulation, the correctness of deep lunar soil drilling model is proved. By setting the gravity to lunar gravity, the drilling bit torque, drilling pipe torque and their sum torque under lunar gravity are predicted respectively to fall down to 17%, 41%, 38% of that under earth gravity. Torque without load is predicted to have significant influence.In engineering, the impact of big particle on the lunar soil drilling process is concerned very much. In this paper, the big particle whose diameter is smaller than the virtual cutting circle of drill bit is defined as relative big particle, and as super big particle if larger. Cases with relative big particle in shallow soil, super big particle in shallow soil and super big particle in deep soil are simulated. Cases with the big particle in centre or below the cutter are compared with non-particle case. The trajectory of big particle, the sampling efficiency and load curves are analyzed. The simulation results indicate that the relative big particle tends to move to terminal of the sample under the promoting of surrangding small particles, and has little influence on the drilling process. The super big particle tends to block up below the drill bit because it can not move into the drill. The impacts of super big particle on the coring property in shallow soil and deep soil are opposite. In shallow soil, the super big particle impedes coring significantly, while in deep soil, it has little impact and protects the sample particles from slide down, which improves the sampling efficiency. No matter when the super big particle is in shallow or deep soil, if it is below the centre of the drill bit, the performance of the load curves is that the axial force is improved with torque curve no change. If it is below the cutter, the axial force and torque increase both and show a large number of discrete pulse peaks. Feature of the load curves can be used to identify the presence of large particles. The case that super big particles exist in shallow lunar soil impacts the drilling process negatively and obviously. Because the force value is very small, and the characteristic is not obvious, this case is the most difficult to be identified.In engineering, two problems are concerned, one of which is the protection of different coring methods on bedding information, and the other the impact of drill bit structure on the amount of sample. In this paper, a surface slope method is proposed to describe the bedding information and a failure boundary to evaluate it. Drilling simulations with hard pipe coring and soft sack coring are performed, and the results indicate that soft sack method is superior to hard pipe method in the whole, especially in the aspect of protecting the bedding information of outboard particles. This advantage is more obvious with the increase of drilling depth. Simulation result of soft sack coring method shows that all samples come from core tube diameter range under drilling bit, sample particles bound while falling down. Combined with bedding information varying law, the circular motion of sample particles in soft bag can be described. And for the purpose of avoiding missing sample, the impact of drill bit structure is analyzed to get a result that the prism cutter with complement circle can significantly improve the efficiency of sample and the inradium expansion at the joint point between drill bit and stem has little effect on sampling.