Influences of critical fragment migration characteristics of lunar soil on drilling resistance

The critical fragment refers to the lunar soil particles with average diameter greater than or equal to the diameter of coring bit hole, which is widely distributed in lunar subsurface soil. On one hand, due to the drilling effect in the drilling process, some of the critical fragments are placed from the bottom of the hole and embedded into the borehole wall to produce the hole wall insertion phenomenon, thus increasing consumption of the cutting load power. On the other hand, the others cannot be collected by the coring hole following the rotation of the drilling tool, thus increasing the risk of drilling failure. Firstly, the critical fragment cutting model for lunar soil is established, the particle size of the critical fragment and its interaction position with cutting edge are considered, the sensitivity of cutting resistance is analyzed, and it is obtained that the change of position has the most obvious influences on cutting resistance. Then, the concept of normal overlap ratio is put forward to study the influences of position change on the migration characteristics and cutting resistance of the critical fragment. The rotary motion of cutting edge is equivalent to the linear one, and the influences of normal overlap ratio on the cutting resistance are verified by the discrete element method, so as to obtain the relationship between the normal overlap ratio and the phenomenon of being placed from the bottom of the hole and embedded into the borehole wall to produce the hole wall insertion, as well as the characteristics of the corresponding time-domain change curve of cutting resistance. Finally, through the simulation tests on the migration characteristics of the critical fragment, it is found that the normal overlap ratio and cutting resistance have a linear relationship in the same direction. When the critical fragment produces the process of being placed from the bottom of the hole and embedded into the wall, the time-domain curve features of the corresponding cutting resistances are continuous peak points and only one peak point respectively. The above results may provide the theoretical basis for the parameter identification of drilling conditions in lunar sampling mission.