DEM simulation of soil-tool interaction under extraterrestrial environmental effects

In contrast to terrestrial environment, the harsh lunar environment conditions include lower gravity acceleration, ultra-high vacuum and high (low) temperature in the daytime (night-time). This paper focuses on the effects of those mentioned features on soil cutting tests, a simplified excavation test, to reduce the risk of lunar excavation missions. Soil behavior and blade performance were analyzed under different environmental conditions. The results show that: (1) the cutting resistance and the energy consumption increase linearly with the gravity. The bending moment has a bigger increasing rate in low gravity fields due to a decreasing moment arm; (2) the cutting resistance, energy consumption and bending moment increase significantly because of the raised soil strength on the lunar environment, especially in low gravity fields. Under the lunar environment, the proportions of cutting resistance, bending moment and energy consumption due to the effect of the van der Waals forces are significant. Thus, they should be taken into consideration when planning excavations on the Moon. Therefore, considering that the maximum frictional force between the excavator and the lunar surface is proportional to the gravity acceleration, the same excavator that works efficiently on the Earth may not be able to work properly on the Moon.Peer ReviewedPostprint (author's final draft

3D DEM analysis of soil excavation test on lunar regolith simulant

To validate the application of three-dimensional (3D) discrete element method (DEM) on modeling the excavation process, a discrete analogue of lunar regolith simulant is excavated under terrestrial conditions using DEM and the results were compared with the previous experimental data using TJ-1 lunar soil simulant. The soil failure mechanism is first described at different scales, with detailed DEM studies of excavation force, earth pressure, soil heap, void ratio changes, APR (average micro-pure rotation rate) field, particle displacements and velocities. Following these, the effects of cutting depth, cutting angle, blade width on the excavation force and size of the affected zone are analyzed and compared with the experimental data. The results illustrate that a “real-time” affected zone can be identified from the APR field and particle velocities which fluctuate significantly in a similar tendency to the excavation force at the post-peak stage. In contract, a “cumulative” affected zone can be identified from the void ratio and particle displacements, which remain increasing gradually at the post-peak stage and invariable when the excavation displacement is large enough to allow a stable soil heap to form. In addition, the simulation results can capture the effects of cutting depth, cutting angle and blade width, which reveals the validity of the numerical modeling approach and thus further studies on the effects of lunar environments on soil excavation can be carried out using DEM.Academi

Experimental Study on the Bearing Behavior and Failure Model of Digging Hold Foundation in Rock Ground

The physical model test is an effective method to study the bearing behavior of digging hold foundations due to its low cost and clear boundary conditions. Here, similar materials of rocks were configured and employed to study the bearing capacity and failure model of digging hold foundations in rock ground. Firstly, sixteen groups of material proportion schemes were employed to make similar materials of rocks, and the effects of four mix parameters were analyzed. Then, similar materials of rocks were employed to perform the uplift tests of digging hold foundations. The results show that the mass ratio of fine particles and aggregate has the greatest influence on the density and internal friction angle, while the cement moisture content has the greatest influence on the cohesion and compressive strength of similar materials of rocks. During the pull-out process of the digging hold foundation, the radial cracks radiate outward from the circumferential cracks, which is different from those in the field test because the ground is small and uniform without fissures inside. The foundation drives the surrounding similar materials to be pulled up as a whole with a certain failure angle, which increases from 35.7° to 42.5° as the internal friction angle decreases from 56° to 41°. In addition, the ratio between the equivalent shear strength in Chinese Code and uniaxial compressive strength falls in the range of 0.027–0.05

Experimental and DEM analyses on wheel-soil interaction

In this paper, the wheel-soil interaction for a future lunar exploration mission is investigated by physical model tests and numerical simulations. Firstly, a series of physical model tests was conducted using the TJ-1 lunar soil simulant with various driving conditions, wheel configurations and ground void ratios. Then the corresponding numerical simulations were performed in a terrestrial environment using the Distinct Element Method (DEM) with a new contact model for lunar soil, where the rolling resistance and van der Waals force were implemented. In addition, DEM simulations in an extraterrestrial (lunar) environment were performed. The results indicate that tractive efficiency does not depend on wheel rotational velocity, but decreases with increasing extra vertical load on the wheel and ground void ratio. Rover performance improves when wheels are equipped with lugs. The DEM simulations in the terrestrial environment can qualitatively reproduce the soil deformation pattern as observed in the physical model tests. The variations of traction efficiency against the driving condition, wheel configuration and ground void ratio attained in the DEM simulations match the experimental observations qualitatively. Moreover, the wheel track is found to be less evident and the tractive efficiency is higher in the extraterrestrial environment compared to the performance on Earth

Experimental verification on analytical models of lunar excavation

In this paper, a series of excavation tests were conducted with a carefully designed apparatus and testbed based on soil mechanics theories to obtain reliable excavation forces in Tongji-1 lunar soil simulant at first. Then the measured data were compared with the forces predicted by six typical analytical models to verify their capability of accurately capturing the effects of cutting depth, rake angle, blade width and cutting speed. The results show that for the horizontal excavation forces, the Zeng model, the Kobayashi model, the Mckyes model and the Swick and Perumpral model can capture the effects of cutting depth, and the Lockheed-Martin/Viking model could capture the effects of the cutting depth, blade width and rake angle. For the vertical excavation forces, the Swick and Perumpral model and the Mckyes model can capture the effects of the cutting depth, blade width and rake angle. The overall assessment of excavation force predictions shows that the Lockheed-Martin/Viking model, the Zeng model, the Swick and Perumpral model and the Mckyes model are recommended for predicting the horizontal excavation force, and the Swick and Perumpral model and the Mckyes model are recommended for predicting the vertical excavation force

DEM simulation of soil-tool interaction under extraterrestrial environmental effects

In contrast to terrestrial environment, the harsh lunar environment conditions include lower gravity acceleration, ultra-high vacuum and high (low) temperature in the daytime (night-time). This paper focuses on the effects of those mentioned features on soil cutting tests, a simplified excavation test, to reduce the risk of lunar excavation missions. Soil behavior and blade performance were analyzed under different environmental conditions. The results show that: (1) the cutting resistance and the energy consumption increase linearly with the gravity. The bending moment has a bigger increasing rate in low gravity fields due to a decreasing moment arm; (2) the cutting resistance, energy consumption and bending moment increase significantly because of the raised soil strength on the lunar environment, especially in low gravity fields. Under the lunar environment, the proportions of cutting resistance, bending moment and energy consumption due to the effect of the van der Waals forces are significant. Thus, they should be taken into consideration when planning excavations on the Moon. Therefore, considering that the maximum frictional force between the excavator and the lunar surface is proportional to the gravity acceleration, the same excavator that works efficiently on the Earth may not be able to work properly on the Moon.Peer Reviewe

Experimental and DEM analyses on wheel-soil interaction

In this paper, the wheel-soil interaction for a future lunar exploration mission is investigated by physical model tests and numerical simulations. Firstly, a series of physical model tests was conducted using the TJ-1 lunar soil simulant with various driving conditions, wheel configurations and ground void ratios. Then the corresponding numerical simulations were performed in a terrestrial environment using the Distinct Element Method (DEM) with a new contact model for lunar soil, where the rolling resistance and van der Waals force were implemented. In addition, DEM simulations in an extraterrestrial (lunar) environment were performed. The results indicate that tractive efficiency does not depend on wheel rotational velocity, but decreases with increasing extra vertical load on the wheel and ground void ratio. Rover performance improves when wheels are equipped with lugs. The DEM simulations in the terrestrial environment can qualitatively reproduce the soil deformation pattern as observed in the physical model tests. The variations of traction efficiency against the driving condition, wheel configuration and ground void ratio attained in the DEM simulations match the experimental observations qualitatively. Moreover, the wheel track is found to be less evident and the tractive efficiency is higher in the extraterrestrial environment compared to the performance on Earth