Narrow Results

Due to the nonlinear nature of contacts, the real contact information is indeterminate and there is no analytical solution for discontinuous computation. In the context of two-dimensional discontinuous deformation analysis (2D DDA) for arbitrary polygon block systems, the contact indeterminacy arises from at least two aspects, namely kinematics and geometry. This study provides a specific numerical solution for 2D DDA to address the indeterminacy problem of contacts between arbitrary polygon blocks. First, local contact boundary is established by defining the solid angle of 2D blocks and the entrance solid angle between two 2D solid angles of two blocks. Second, the linearized first entrance formulas are derived to estimate the first entrance time and position. Third, an enhanced shortest exit strategy based on entrance angles is introduced to provide a unified solution of the contact points and direction of closed contacts. Some extreme and challenging examples, including oblique collision contacts, undue penetration contacts, genuine vertex–vertex contacts (including absolutely symmetric contacts), extremely abnormal block contacts (i.e., contacts between extremely short edges and/or sharp angles), multi-connected block contacts and numerous singular contacts, demonstrate the accuracy, robustness and application prospects of the proposed method in addressing difficult contact problems.

A plasmonic filter applied in visible regime is proposed. A method using discrete dipole approximation (DDA) to aid design parameters of rhombic Ag nanoparticle array is adopted for the filter design and optimization on the basis of computational numerical calculation. Influence of the particle parameters such as thickness, period and effective index of medium around the particles on the extinction spectrum is studied using the DDA-based computational analysis. The calculation results show that the thickness ranging from 35 to 45 nm and the period ranging from 350 to 560 nm are the better geometrical parameters of the rhombic Ag nanoparticle array. Considering the nano-fabrication condition, 40 nm thickness and 440 nm period were selected and fabricated using nanosphere lithography (NSL) technique. The experimental results demonstrated that the measured spectrum is basically in agreement with the theoretical spectrum derived by the DDA calculation.

Wenchuan earthquake, Ms 8.0 in magnitude and occurred on May 12, 2008 in Sichuan Province, China, triggered a lot of landslides, rock collapses, debris flow, etc. The Tangjiashan landslide, with its total volume 20.37 million m³, was the biggest and the most notable one for its effects. Based on the field geological investigation and the typical acceleration records of the main shock obtained in the period of the earthquake, numerical simulation of the whole sliding process of Tangjiashan landslide has been carried out by DDA method. It is shown that the Tangjiashan landslide was a high-speed landslide, behaved with nonlinear features in the whole sliding process. The total duration of the landslide was about 30 s while nearly all of their slipping displacements were carried out in the beginning 25 s, with the maximum sliding velocity about 30 m/s, and the average 15–17 m/s in the beginning 25 s. The crash of rock blocks induced a much higher stresses near the middle and lower parts of the landslide, with the maximum value of 6–7 MPa. The dynamic earthquake load caused an incessant deformation of the landslide, resulting in the reduction of mechanical parameters, especially the shear strength on the sliding surface and the ratio of friction coefficient on sliding surface in kinematical and static conditions are no more than 0.35. DDA simulation considering the displacement-based parameter reduction has been developed in the original DDA code, and its results primarily reflect the evolvement process of a landslide under strong seismic loads.

Drill and blast method has been widely used as an effective excavation method for underground rock caverns or tunnels. To achieve a good blast design, an understanding on the rock dynamic response and rock fragmentation process is important. In this paper, numerical simulations are performed on a typical parallel hole cut blasting based on the discontinuous deformation analysis (DDA). The blast loading is obtained from the explicit FEM code LS-DYNA and the dynamic response of the rock mass is modeled by the DDA. Different influence factors on rock fragmentation under the blast loading are investigated, including the different delay time and various rock mass properties. Such a study will have potential applications for better drill and blast designs.

The localized surface plasmon resonance (LSPR) properties of Au/Ag/graphene nanoshells are studied by discrete dipole approximation (DDA). The coupled resonance wavelengths show a remarkable dependence on the graphene thickness as well as refractive index of the surrounding medium. The resonance wavelengths of Au/Ag/graphene nanoshells red-shift as the thickness of the graphene layer is increased, when the radii of the Au core and Ag interlayer are 40nm and 45nm, respectively. Specifically, the longer wavelength red-shifts from 540nm to 740nm when the refractive index varies from 1.25 to 2.05.