Xihua Chu

Effects of Stone-Wales and vacancy defects in atomic-scale friction on defective graphite

Graphite is an excellent solid lubricant for surface coating, but its performance is significantly weakened by the vacancy or Stone-Wales (SW) defect. This study uses molecular dynamics simulations to explore the frictional behavior of a diamond tip sliding over a graphite which contains a single defect or stacked defects. Our results suggest that the friction on defective graphite shows a strong dependence on defect location and type. The 5-7-7-5 structure of SW defect results in an effectively negative slope of friction. For defective graphite containing a defect in the surface, adding a single vacancy in the interior layer will decrease the friction coefficients, while setting a SW defect in the interior layer may increase the friction coefficients. Our obtained results may provide useful information for understanding the atomic-scale friction properties of defective graphite.

Finite-Element Analysis of Failure in Transversely Isotropic Geomaterials

AbstractThe strength and failure behaviors of geomaterials, such as soils and rocks, are commonly anisotropic because of lamination and sedimentation. The main purpose of this study is to investigate the effect of anisotropy on the localization pattern and bearing capacity of geostructures by finite-element simulation. To this aim, an extended Drucker-Prager yield criterion is developed for transversely isotropic geomaterials based on the Cosserat continuum. In this criterion, the internal frictional angle is related to the material principal direction and the mixed invariant of the stress tensor and the microstructure tensor. The corresponding stress update and consistent elastoplastic tangent modulus matrix are presented. Numerical results show that the localization pattern and the bearing capacity of the geostructures are very sensitive to the material principal direction as well as the anisotropic degree.

Evolution of Anisotropy in Granular Materials: Effect of Particle Rolling and Particle Crushing

The effect of particle rolling and crushing on the evolutions of the two types of anisotropy, i.e., anisotropy of particle packing (microstructure) and anisotropy of force chains, is investigated numerically using the discrete element method. To this end, the classical fabric tensor is adopted to describe the anisotropy of microstructure, while two similar orientation tensors defined by the directions of contact forces are used to characterize the anisotropy of force chains. Numerical results show that the evolutions of anisotropy follows the same tendency as the stress–strain curve, and the anisotropy of force chains is more intense than that of the microstructure. In addition, particle rolling exerts different effect on anisotropy before and after the peak stress state, and particle crushing decreases the anisotropy of granular materials.

A Multiscale Computational Formulation for Gradient Elasticity Problems of Heterogeneous Structures

In this paper, a multiscale computational formulation is developed for modeling two- and three-dimensional gradient elasticity behaviors of heterogeneous structures. To capture the microscopic properties at the macroscopic level effectively, a numerical multiscale interpolation function of coarse element is constructed by employing the oversampling element technique based on the staggered gradient elasticity scheme. By virtue of these functions, the equivalent quantities of the coarse element could be obtained easily, resulting in that the material microscopic characteristics are reflected to the macroscopic scale. Consequently, the displacement field of the original boundary value problem could be calculated at the macroscopic level, and the corresponding microscopic gradient-enriched solutions could also be evaluated by adopting the downscaling computation on the sub-grids of each coarse element domain, which will reduce the computational cost significantly. Furthermore, several representative numerical experiments are performed to demonstrate the validity and efficiency of the proposed multiscale formulation.

A simulation based on the cosserat continuum model of the vortex structure in granular materials

Displacement fluctuation is the difference between the real displacement and the affine displacement in deforming granular materials. The discrete element method (DEM) is widely used along with experimental approaches to investigate whether the displacement fluctuation represents the vortex structure. Current research suggests that the vortex structure is caused by the cooperative motion of particle groups on meso-scales, which results in strain localization in granular materials. In this brief article, we investigate the vortex structure using the finite element method (FEM) based on the Cosserat continuum model. The numerical example focuses on the relationship between the vortex structure and the shear bands under two conditions: (a) uniform granular materials; (b) granular materials with inclusions. When compared with distributions of the effective strain and the vortex structure, we find that the vortex structure coexists with the strain localization and originates from the stiffness cooperation of different locations in granular materials at the macro level.

Anisotropy of microstructure and force chains in granular materials

Three orientation tensors are adopted to describe the anisotropy of microstructures and force chains in granular materials. Numerical investigations based on discrete element method focus on the evolution of anisotropy and the effect of particle rolling friction on anisotropy. Results show that the fabric tensor describing microstructure will be isotropic for large particle assemblies with circular grains, however, other two tensors describing force chains will be of significant anisotropy and the evolution curves of the intensity of anisotropy are similar to stress-strain curves, in addition, the particle rolling have major influences on the anisotropy of force chains.