Introducing the hierarchy into cellular materials has attracted increasing attention in the effort to pursue improved absorbed-energy abilities and impact resistance. In this paper, the dynamic crushing properties and energy absorption capacities of joint-based hierarchical honeycombs with different topologies were explored by means of explicit dynamic finite element (FE) analysis using ANSYS/LS-DYNA. Four types of joint-based hierarchical honeycombs with uniform cell-wall thickness were firstly constructed by substituting each vertex of regular honeycombs with a smaller self-similar cell (hexagon or square). The respective influences of hierarchical parameters and impact velocities on in-plane dynamic deformation modes, mechanical characteristic and energy absorption of joint-based hierarchical honeycombs were discussed. Research results showed that the hierarchy had a far greater influence on the in-plane deformation modes of honeycombs. Compared with regular honeycombs, the dynamic plateau stress and specific energy absorption of joint-based hierarchical honeycombs can be improved if the proper hierarchical parameters were chosen. Adding the joint-based hierarchy into regular honeycombs can enhance the crushing stress efficiency (CSE) of the specimens. In addition, by introducing a non-dimensional dynamic sensitivity index, the dynamic shock enhancement of hierarchical honeycombs was also investigated. These researches are useful for the multi-objective dynamic optimization design and controllable properties of cellular materials.

No abstract

The streamer discharge occurring on the surface of a conductor in rain is one of the critical problems associated with the design of high voltage direct current (HVDC) transmission lines. In this paper, the streamer discharges on the conductor surface in the presence of raindrops at atmospheric pressure are studied by use of two-dimensional particle-in-cell simulations with Monte Carlo collisions included. The influences of the drop angles, volumes, and spatial distributions of raindrops on the development of streamer discharges have been demonstrated. The formation of streamer discharges and the involved electric fields, plasma densities, propagation velocities, and discharge currents are presented. It is found that the discharge intensity decreases significantly with an increasing of the drop angle. The dividing point at the 95° angle is suggested. With an increase of the raindrop volume and the distance of raindrops, strong discharges with the advanced discharge time, faster propagation velocity, larger electric field, and higher plasma density are initiated. Moreover, the electron energy probability functions of different discharges have been studied, which are nonequilibrium and reveal kinetic behavior of the discharges. The simulation results, which are qualitatively in agreement with the previous experimental observations, bring new insight into the discharge dynamics and provide useful references for the protection of HVDC transmission lines in rainy days.

No abstract