Crashworthiness of vertex based hierarchical honeycombs in out-of-plane impact

Abstract

As a class of widely observed materials in nature, hierarchical micro structures maybe of superior mechanical properties. In this study, we incorporate the concept of hierarchy into honeycomb structures for enhancing their crashworthiness performance. Hierarchical honeycombs are constructed by replacing every vertex of a regular hexagonal network with a smaller hexagon topology and repeating this process for constructing fractal-appearing honeycombs with higher order of structural hierarchy. To examine the crashing characteristics the hierarchical honeycombs with the first-order and second-order structures were investigated under an out of the cross-sectional plane loading. A parametric study on structural variables (γ1, γ2) which are defined by γi=Li/L0 (L0 is the edge length of regular honeycomb cell, Li is the edge length of i-th order hexagon in hierarchical honeycombs) and oblique-wall angles θ which is defined by the angle of oblique-wall edge to the vertical direction are undertaken with three different densities. A comparison between regular honeycombs and hierarchical honeycombs was conducted. The results showed that the out-of-plane energy absorption of the first-order hierarchical honeycombs with 0.04≤γ1≤0.2 and the second-order hierarchical honeycombs with 0.15≤γ1≤0.2 and 0.04≤γ2≤0.08, as well as the hierarchical honeycombs with oblique-wall angle θ from 30° to 50° have better overall performance. Further, it was found that the specific energy absorptions (SEA) of the first-order hierarchy with γ1=0.08 and the second-order hierarchy with γ1=0.20, γ2=0.06 were improved about 81.3% and 185.7%, respectively. Moreover, their corresponding peak forces (Pmax) do not increase much compared with the regular honeycomb under the same density, indicating that hierarchical honeycombs can be an ideal lightweight structure for designing crashworthy structures.