Papers by Author: Xiu Xi Wang

Abstract: Three armchair single-walled carbon nanotubes (SWCNTs) (7, 7), (12, 12), (17, 17) and three zigzag SWCNTs (12, 0), (16, 0), (20, 0) are investigated in this paper, using the molecular dynamic (MD) method with the second-generation Tersoff-Brenner (TB) potential. The Poisson’s ratio of these nanotubes under tensile and compressive loading is obtained. The effect of the strain and size on the Poisson’s ratio of nanotubes is analyzed systematically, from the viewpoints of the structure and the averaged atomic potential energy of nanotubes. The results show that the Poisson’s ratio of nanotubes decreases as the strain increases. The Poisson’s ratios of nanotubes of larger chiral angle decrease more quickly. For nanotubes of the same chiral angle, the larger the diameters of nanotubes are, the larger their Poisson’s ratios become. Moreover, the Poisson’s ratios of nanotubes of larger diameter are more approaching.

Abstract: The compressive deformation of metallic glass Cu was studied under uniformly distributed strains with different rates at 1 K temperature using molecular dynamics simulations. The interaction between atoms in the system adopts the embedded atom method (EAM) reported by Mishin. We found that MG Cu is an elastic/perfect plastic material and the Young modulus is about 50 Gpa with strain rates from 0.1ns-1 to 10 ns-1. At low strain rates the sample deforms inhomogeneously and the amorphous phase transforms continuously to a crystalline phase. It was observed that the nucleation, growth and mergence processes of crystalline are induced by stress. At high strain rates the system passes through plastic deformations homogeneously and keeps the amorphous structure. The higher flow stress occurs at higher strain with the strain rate increasing. The stress effect is an important factor that induces MG crystallization just like temperature effect. The relationship between characteristic nucleation rate and strain rate determines the different deformation mechanism of MG.

Abstract: Configurations of single-walled carbon nanotubes (SWCNTs) with randomly distributed vacancies were generated by numerical method. Molecular dynamics (MD) method was used to investigate the compressive mechanics properties of SWCNTs with vacancies. The simulation results show that the SWCNTs with vacancies have more complicated deformation procedures and the Young’s modulus is lower than the corresponding perfect SWCNTs. The Young’s modulus of the SWCNTs with no more than 20 vacancies ranges from 940Gpa to 620Gpa, and its value is approximately linearly proportional to the number of vacancies. It is found that local buckling first appears in the surface region having high density of vacancies of (10, 10) SWCNTs under axial compression. As the loading increases, SWCNTs with more vacancies have more complicated buckling configuration and sluggish energy variation. Under a case of the same displacement load the more vacancies the SWCNT has, the more complicated the mechanical behavior is. SWCNTs with 20 vacancies can still maintain self structure stabilization, this validates that SWCNTs have good spacial stabilization.

Abstract: Numerical simulation of hydraulic fracturing propagations in the permeable reservoirs was carried out with the finite element analysis software (ABAQUS). A model of coupling the stress equilibrium and fluid continuity equations was proposed and implemented. The nonuniform of sink pore pressure on the fracture surfaces which changes associated with the propagation of fracture was described by a self-developed subroutine through the FLOW in ABAQUS. Samples under different conditions were conducted for studying the rules of the propagation of hydraulic fracturing. The results show that the permeability at the fracture tip is more serious than any other places of the fracture face. The model also illustrates that the fracture geometry is mainly determined by the minimal in-situ stress. The model can be used to simulate the effects of hydraulic fracturing pressures and injection rates on fracture propagation. The results are of much significance for the design of hydraulic fracturing treatments.

Abstract: Three-dimensional finite element simulations were carried out to investigate the hydraulic progressive damage and associated flow behavior in rock. In this study cohesive elements were used to simulate the damage of rock. A three-dimensional coupled pore fluid flow and stress model was proposed. The commercial engineering software ABAQUS is employed to simulate the damage process in rock along several predefined paths. A user-subroutine named FLOW was developed to enhance the capability of ABAQUS to deal the moving loadings. With the proposed coupling model, we studied the stress distribution, the pore pressure, the fluid loss, the geometry of the progressive damage. The results show that the length and the width of the path of the progressive damage are strongly influenced by both the hydraulic pressure and the injection time. The results provide good interpretation and understanding of the mechanism of hydraulic progressive damage in rock. This study is very useful and important to the oil engineering and some other rock engineering fields.

Abstract: Numerical simulations have been carried out to determine the mechanical property of single crystal copper nanowire subjected to tension using the molecular dynamics method. The mechanism of deformation, strength and fracture are elucidated based on these numerical simulations. No strengthening is found after yielding of the single crystal nanowire. The simulation results show that the strength of copper nanowire is far greater than that of realistic polycrystalline bulk copper. By decreasing the size of the nanowire's cross-section, which leads to an increase of the ratio of surface atoms, the yield stress is increased. The strain rate has an influence on strength, and mechanism of deformation and fracture. When the strain rate is comparatively low, plastic deformation arises from dislocation slips and twins. However, when the strain rate is sufficiently high, amorphization is a dominant factor of plastic deformation and super-plasticity is found. The fracture process is demonstrated using the atomic images.