Shunchuan Wu

A Study of Three Intrinsic Problems of the Classic Discrete Element Method Using Flat-Joint Model

Discrete element methods have been proven to offer a new avenue for obtaining the mechanics of geo-materials. The standard bonded-particle model (BPM), a classic discrete element method, has been applied to a wide range of problems related to rock and soil. However, three intrinsic problems are associated with using the standard BPM: (1) an unrealistically low unconfined compressive strength to tensile strength (UCS/TS) ratio, (2) an excessively low internal friction angle, and (3) a linear strength envelope, i.e., a low Hoek–Brown (HB) strength parameter mi. After summarizing the underlying reasons of these problems through analyzing previous researchers’ work, flat-joint model (FJM) is used to calibrate Jinping marble and is found to closely match its macro-properties. A parametric study is carried out to systematically evaluate the micro-parameters’ effect on these three macro-properties. The results indicate that (1) the UCS/TS ratio increases with the increasing average coordination number (CN) and bond cohesion to tensile strength ratio, but it first decreases and then increases with the increasing crack density (CD); (2) the HB strength parameter mi has positive relationships to the crack density (CD), bond cohesion to tensile strength ratio, and local friction angle, but a negative relationship to the average coordination number (CN); (3) the internal friction angle increases as the crack density (CD), bond cohesion to tensile strength ratio, and local friction angle increase; (4) the residual friction angle has little effect on these three macro-properties and mainly influences post-peak behavior. Finally, a new calibration procedure is developed, which not only addresses these three problems, but also considers the post-peak behavior.

Effects of Micro-structure and Micro-parameters on Brazilian Tensile Strength Using Flat-Joint Model

Abstract It has been widely accepted that tensile strength plays a dominant role in the failure mechanism of rock or rock-like material. Tensile strength is determined mainly via two methods: the direct tension test and Brazilian test. Due to the strictness of preparing the specimen and difficulty of conducting the direct tension test, Brazilian test has been widely applied to determine the tensile strength of geo-materials. However, there is no exact standard for Brazilian test specimen. Moreover, Brazilian tensile strength (BTS) is affected by many factors, such as loading rate, loading platen width, model size. So far, most parametric studies of geo-materials have involved compression tests, but few studies have systematically focused on Brazilian test. The continuum methods have difficulty reproducing the failure process of Brazilian test, and 2D discrete element methods can not reflect the real mechanical behavior of a 3D cylindrical disk specimen. Moreover, the standard bonded-particle model has intrinsic problems in simulating geo-materials. This paper, using a 3D flat-joint model (FJM3D), investigates the effects of micro-structure and micro-parameters on BTS. The micro-structure consists of model size, model resolution, and degree of heterogeneity. The micro-parameters include the average coordination number, crack density, and bond strength. The effects on BTS are summarized, and this summary will be useful for guiding future Brazilian tests. Finally, FJM3D is used to calibrate Brisbane tuff by Brazilian test and the uniaxial compression test. The simulation results are in good agreement with those measured from experiments, and the failure process of Brazilian test is analyzed in detail at the microscale. Because of the heterogeneity of rock, cracks initiate near the loading platen instead of the center of the specimen. Even so, BTS can be an useful tensile index for geo-materials in a triaxial stress state, which is similar to the physical situations, and Brazilian test is helpful for further understanding the failure mechanism of geo-materials.

Enhanced ferroelectric and UV photocatalytic properties in a Bi4Ti3O12@ZnO core–shelled nanostructure

Composited Bi4Ti3O12@ZnO nanoparticles with a core of Bi4Ti3O12 (BIT) and a shell of ZnO have been synthesized by liquid chemical reaction method. Compared with pure BIT nanoparticles, the ferroelectricity in BIT@ZnO core–shell nanostructure was greatly enhanced. Moreover, the dielectric loss of BIT@ZnO is lower than that of BIT nanoparticles in a low frequency range. The band gap energy of BIT@ZnO core@shell nanostructure is larger than that of BIT, which formed as a type-II band alignment. Furthermore, the BIT@ZnO core–shell nanoparticles exhibit better UV photodegradation activity for organic contaminant. Such a BIT@ZnO core@shell nanostructure may have potential applications in microelectronics, photoelectronic, and photocatalytic of contamination.

Ferromagnetism studies of Cu-doped and (Cu, Al) co-doped ZnO thin films

We have studied the room temperature ferromagnetism (FM) in Cu-doped and (Cu, Al) co-doped ZnO thin films which were grown on quartz substrates by chemical method based on a sol-gel process combining with spin-coating technology. X-ray diffraction (XRD) patterns demonstrate that both the Cu-doped and (Cu, Al) co-doped ZnO films have the hexagonal wurtzite structure with c-axis orientation. Alternating Gradient Magnetometer (AGM) measurements confirm that all the doped ZnO samples are ferromagnetic at room temperature. When the doped Cu content is 1 %, the Cu-doped ZnO film has the strongest FM. The FM significantly decreases in the (Cu, Al) co-doped ZnO films. The doping of Al ions suppresses the FM induced by the doped Cu ions.

Using the Point Load Test to Analyze the Strength Anisotropy of Quartz Mica Schist Along an Exploration Adit

Xiao-Ping Zhang was formerly with Key Laboratory of Shale Gas and Geoengineering, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China.

A Study of Gravity Flow Based on the Upside-Down Drop Shape Theory and Considering Rock Shape and Breakage

The cave mining method relies on gravity to fragment the rock mass into blocks that can be extracted out of drawpoints. Several discrete element method (DEM) models on gravity flow are presented in the literature; however, only a few of those consider rock shape and secondary fragmentation. In this paper, the reliability of the particle flow code (PFC) to model gravity flow of fragmented rock is validated against known experimental results. A new method to create complex shape clusters is proposed and then used to investigate the mechanisms of gravity flow and the influence of particle bond strength on the secondary fragmentation, and the evolutions of the movement zone and extraction zone. The model results are validated against the upside-down drop shape theory for two cases: (1) constant size of fragmented rock blocks and (2) changing size due to breakage of the fragmented rock blocks. For the latter case, the results show that secondary fragmentation of weaker rocks would result in a wider movement zone and extraction zone than that of stronger rocks.

Electric-field-controlled magnetization reversal in a NiFe/BiFeO 3 /SrRuO 3 /SrTiO 3 (111) multiferroic heterostructure

Electric-Field-Control of magnetism using BiFeO3-based heterostructures is attracting a great amount of research interest. BiFeO3 (BFO) has outstanding ferroelectric properties and G-type antiferromagnetic ordering in the bulk. BFO has also a relatively high ferroelectric Curie temperature (TC~1100K), high antiferromagnetic Neel temperature (TN~640K), high remnant polarization (2Pr~110μC/cm2), and low temperature crystallization. Moreover, BFO is the only known multiferroic material which shows both ferroelectric and magnetic ordering above room temperature. Many groups have obtained encouraging results on the E-field control of BFO films. Heron et al. reported a 180 degrees anisotropic magnetoresistance (AMR) phase reversal by an in-plane electric field on BFO film [1]. This result is encouraging to further consideration of the out-of-plane electric field controlled magnetoelectric switching possibilities. However, the room temperature out-of-plane electric field control of magnetism is generally irreversible now [2]. In our study, we try to solve this problem by change the surface orientation. Several times AMR phase reversals have been observed. We try to figure out the reason of this phenomenon.