The in-plane dynamic crushing behavior and energy absorption capacity of self-similar hierarchical honeycombs under different impact velocities are numerically studied using ANSYS/LS-DYNA. First, the hierarchical honeycomb models with uniform cell-wall thickness are constructed by replacing every three-edge structure nodes of a regular honeycomb with smaller self-similar hexagons of the same orientation. The respective influences of hierarchical parameters, bulk materials, and impact velocities on the macro-/micro-deformation behaviors, the dynamic strength, and the specific absorbed energy of hierarchical honeycombs are explored in detail. The results show that the crushing strengths and energy-absorbing capacities of honeycombs significantly improve when adding the hierarchy into conventional cellular structures. The variation of hierarchical parameter changes the local dynamic evolution of stress waves, which further results in different macro-/micro-deformation properties. Through the proper choice of hierarchical parameters and bulk materials, the optimal crushing strength and the maximum absorbing energy could be obtained.

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SMetal hollow sphere (MHS) structures with a density gradient have attracted increasing attention in the effort to pursue improved energy absorption properties. In this paper, dynamic crushing of MHS structures are discussed. MHS of different gradients are modelled by stacks of hollow spheres of the same external diameter but different wall thicknesses in the crushing direction. Then based on the dynamic performance of MHS structures with uniform density, a crude semi-empirical model is developed, showing the effect of density gradient in energy absorption and protection against dynamic crush. The model provides some guidance in the design of MHS structures. Following this, dynamic responses of density graded MHS foams are analyzed using explicit finite element simulation and the proposed formula. Results show that the simple semi-empirical model can predict the response of density gradient MHS foams, thus be used for the gradient design of MHS structures.

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