Zhifu Shen

A novel specimen preparation method for TJ-1 lunar soil simulant in hollow cylinder apparatus

Abstract Conventional methods for hollow cylinder apparatus (HCA) specimen preparation are not applicable for TJ-1 lunar soil simulant due to its wide particle size distribution. A novel method to prepare uniform TJ-1 specimen for HCA tests is put forward. The method is a combination of the multi-layering dry-rodding method and a new under-compaction criterion in the multi-layer with under-compaction method (UCM). In the novel method, the specimen is prepared with 5 layers by dry-rodding and the UCM is used to determine the height after each layer is compacted. The density uniformity of specimen is evaluated by the freezing method to find out the best under-compaction criterion. Two HCA specimens with the same target density are prepared by the novel method and examined in the tests of pure rotation of the principal stresses. Their conformable mechanical behaviors ascertain the effectiveness of the method to produce uniform and reproducible HCA specimens. Four groups of HCA tests are carried out to investigate the anisotropic and non-coaxial behaviors of TJ-1 lunar soil simulant. The results indicate that the principal stress direction, the deviator stress ratio, the stress level and the coefficient of the intermediate principal stress significantly influence the strength and deformation properties of TJ-1 lunar soil simulant.

DEM modeling of cantilever retaining excavations: Implications for lunar constructions

Purpose – Retained excavation is important for future lunar exploratory missions and potential human colonization that requires the construction of permanent outposts. Knowledge in excavation obtained on the earth is not directly applicable to lunar excavation because of the low lunar gravity and the non-negligible adhesive van der Waals interactions between lunar regolith grains. The purpose of this paper is to reveal how the gravity level and lunar environment conditions should be considered to extend the knowledge in earth excavation response to lunar excavation. Design/methodology/approach – Two-dimensional discrete element method simulations were carried out to investigate the respective effect of gravity level and lunar environment conditions (high-vacuum and extreme temperature) on retained excavation response. A novel contact model was employed with a moment – relative rotation law to account for the angularity of lunar soil particles, and a normal attractive force to account for the van der Waals...

Preliminary experimental study on three-dimensional contact behavior of bonded granules

In order to explore the microscopic contact behaviour of structured sands, devices were developed for specimen preparation and for carrying out tests on mechanical contact behaviour of three-dimensional (3D) bonded spheres. The specimen preparation device can cement two separate aluminium hemispheres by epoxy adhesive with accurate size control. The auxiliary loading devices can carry out compression, tension, shear, bending and torsion tests and any of their combinations. The experimental results show that the peak shear force, bending moment and torque of the bonded hemispheres are normal force dependent; that is, they first increase with the normal force and then decrease after the normal force exceeds a critical value.

CPT-based estimation of bearing and deformation indexes for TJ-1 lunar soil simulant ground

The bearing capacity and deformation modulus of lunar ground are of great importance for the design of lander feet and foundations of future permanent lunar outposts. Plate Loading Test (PLT) is a reliable and the most adopted means to evaluate the two mechanical indexes on the Earth, but it is not directly applicable on lunar surface due to the difficulty in setting up the test equipment. Cone Penetration Test (CPT) is an alternative means mainly due to its easy operation and can be used to evaluate the two indexes indirectly by linking them with penetration resistance in CPT. In this paper, such relationships are established by carrying out PLT and CPT on a model ground constructed by TJ-1 lunar soil simulant in a soil chamber. Test results show that the representative bearing capacity and deformation modulus of the model ground can be estimated by penetration data through the same ground. The loading plate size-dependency of the two mechanical indexes is also accounted for in the proposed empirical equ...

Noncoaxial Behavior of a Highly Angular Granular Material Subjected to Stress Variations in Simple Vertical Excavation

AbstractExperiments were carried out using a dynamic hollow-cylinder apparatus (HCA) to investigate the effects of stress paths and principal stress rotations experienced during a simple vertical excavation on the element-scale deformation and noncoaxial behavior of a highly angular granular material. For this purpose, an improved specimen preparation technique was used to generate homogeneous HCA specimens using a multilayer undercompaction method. The stress paths used in the HCA experiments were extracted from the numerical results obtained in a discrete element simulation of a simple vertical excavation, where complex stress paths and principal stress rotations have been observed. The experimental results show that different stress paths and rotations in excavation significantly affect the element-scale deformation behavior. Noncoaxial behavior (i.e., the deviation of plastic strain increment direction from the principal stress direction) was observed in the HCA tests, which was a result of the combin...

Strength and Fabric Evolution of Unsaturated Granular Materials by 3D DEM Analyses

A three-dimensional (3D) capillary water contact model was established and implemented into the Distinct Element Method (DEM) to investigate the mechanical behavior of unsaturated granular materials. 3D assemblies of spheres were examined under triaxial compression loading with focus on the effects of suction and initial compaction density on macro-behavior (e.g. strength and deformation) and micro-structure (e.g. stress-induced fabric anisotropy). The fabric was described by the directional distribution of contact-normal and transient particle rotation axis. The DEM results show that the cohesion of the material increases with the suction and is independent of the initial compaction density while the peak friction angle only depends on the density. The loading can change the distribution of contacts in a specimen by increasing the proportion of contacts in the loading direction. Particles are statistically more prone to rotate about axes in the plane orthogonal to the loading direction when the specimen is contractive, which is followed by more prominent rotation about the loading direction when the specimen is dilative. The dense assemblies exhibit greater anisotropy than the loose ones for the two defined fabrics.

DEM Analyses of Soil Cutting Test in Lunar Ground

In future lunar exploration activity, ground excavation is required in construction of permanent lunar outpost and In-Situ Resource Utilization processes, which are important long-term goals of space program. In this paper, the Discrete Element Method (DEM) was employed to simulate horizontal lunar regolith excavation process. For this purpose, a high-efficient contact model was implemented into the DEM. The evolution of excavation resistance, slide surface and stress path are studied. Then the effects of excavation depth and rate, attack angle on the excavation resistance were analyzed under the terrestrial environment. The effects of gravity were also investigated. The simulation results show that the excavation resistance first increases rapidly to a peak and then drops to a stable value quickly. With further increase of the excavation zone, the excavation resistance again increases gradually mainly due to the accumulation of soil in front of the blade. The excavation resistance is greater under larger excavation depth, smaller attack angle, and greater excavation rate. The excavation resistance increases with the gravity level linearly. Since the excavation resistance needs to be balanced by the friction between the excavator and the ground, which is proportional to the weight of an excavator, the shallow, tilted and slow excavation is recommended based on the DEM simulation results. In this way, the mass of excavation equipment launched from the Earth can be greatly reduced in the early-stage of lunar base con

Distinct element analyses of inclined cone penetration test in granular ground

This paper is to investigate the mechanism of inclined cone penetration test (CPT) using the numerical discrete element approach. A series of penetration tests with the penetrometer inclined at different angles (i.e., 30°, 45°, 60°, 75° and 90°) were numerically performed. The velocity fields, displacements of soils adjacent to the cone tip, rotation of the principal stresses and the averaged pure rotation rate (APR) are analyzed. Special focus is placed on the penetration mechanism and the effect of inclination angle on the tip resistance. The DEM results show that soils around the cone tip experience complex displacement paths as the penetration proceeds and exhibit characteristic velocity fields corresponding to three different failure mechanisms. The principal stresses near the cone tip undergo apparent rotation, accompanied by large APR which indicates evident particle rotation adjacent to the cone. The normalized tip resistance qN( = qc/σν0) decays with penetration depth in a decreasing rate. At the same penetration depth, qN decreases with the increasing of the inclination angle of penetrometer.This paper is to investigate the mechanism of inclined cone penetration test (CPT) using the numerical discrete element approach. A series of penetration tests with the penetrometer inclined at different angles (i.e., 30°, 45°, 60°, 75° and 90°) were numerically performed. The velocity fields, displacements of soils adjacent to the cone tip, rotation of the principal stresses and the averaged pure rotation rate (APR) are analyzed. Special focus is placed on the penetration mechanism and the effect of inclination angle on the tip resistance. The DEM results show that soils around the cone tip experience complex displacement paths as the penetration proceeds and exhibit characteristic velocity fields corresponding to three different failure mechanisms. The principal stresses near the cone tip undergo apparent rotation, accompanied by large APR which indicates evident particle rotation adjacent to the cone. The normalized tip resistance qN( = qc/σν0) decays with penetration depth in a decreasing rate. At the...