Improving energy absorption capability of the hexagonal honeycomb and re-entrant structures using a combined hierarchical design.

Hexagonal honeycombs and re-entrant lattices have been widely used in energy-absorption applications. The hexagonal honeycomb structure, known for its positive Poisson's ratio and high specific stiffness and strength, has long been considered an effective energy absorber. However, with the increasing use of auxetic structures, their efficiency in energy absorption applications is being explored. Auxetic lattices exhibit a negative Poisson's ratio, which significantly enhances their energy absorption capacity. In this study, a combined hierarchical lattice structure is proposed by integrating the advantageous features of both hexagonal honeycomb and re-entrant lattices. Using additive manufacturing, the proposed lattice was fabricated, and its energy absorption behavior was experimentally investigated through quasi-static compression testing. Furthermore, the repeatability of the experimental results was verified using the finite element method. This numerical framework was then employed to simulate the energy absorption behavior of the base honeycomb and re-entrant lattices. To judge the effectiveness of the lattice structures in energy absorption, four parameters, including peak force, initial stiffness, specific energy absorption, and crush force efficiency, were selected as evaluation criteria. Accordingly, it was depicted that the re-entrant lattice in axial compression cannot be considered as an efficient energy absorber because of a very high peak force and lower crush force efficiency in comparison with the new structures. In addition, the hierarchical design of the new lattice enhanced its deformation and failure modes, delaying wall collapse and improving structural integrity under compression.