On the out-of-plane crashworthiness of incorporating hierarchy and gradient into hexagonal honeycomb

Hierarchical design and gradient design have proven to be effective in improving the crashworthiness of honeycombs. In this study, a novel graded hierarchical hexagonal honeycomb (GHHH) is proposed by introducing wall thickness variation into the vertex-based hierarchical hexagonal honeycomb (VHHH). The VHHH is obtained by replacing every vertex of the regular hexagonal honeycomb (RHH) with a smaller hexagon. Numerical simulations and theoretical analysis are performed to study the crashworthiness performance of GHHH under the out-of-plane impact. The numerical results show that the specific energy absorption (SEA) of GHHH can be 146.09%, 39.01%, and 50.23% higher than that of RHH, VHHH, and graded hexagonal honeycomb (GHH), respectively, while their peak stresses are nearly the same. In addition, a theoretical model for the plateau stress of GHHH is developed, and the theoretical values show good consistency with numerical results of GHHH with in-extensional mode. The findings of this study provide an effective guideline for the design of honeycombs with enhanced crashworthiness.