Approaching perfect energy absorption through structural hierarchy

Energy absorbing efficiency of thin-walled tubular structures is restricted by the long-wave folding elements, which makes the mean crushing force (MCF) of the thin-walled tubular structure usually much smaller than its yielding strength. Hierarchical lattice topology increases the energy absorbing ability of tubular structure notably without increasing the weight. In an effort to reveal this advantage, hierarchical rectangular tubular structures are proposed in this paper. These proposed structures have multi-cellular structure and lattice sandwich cellular walls. During crushing of these structures, three typical folding styles, i.e., macro-cell folding, micro-cell folding and hybrid folding were observed in both experiments and finite element (FE) simulations. Micro-cell folding has relative short wave length depending on the dimension of the microstructure cells. Macro-cell folding has relative long wave length determined by the dimension of the overall tubular structure. Micro-cell folding notably increases the mean crushing force (MCF) while macro-cell folding decreases the MCF. However, the maximum MCF is associated with the hybrid folding style which is a transition from micro-cell folding to macro-cell folding. When the hierarchy level of the tubular structure is gradually increased, the MCF approaches the full-plastic strength of the matrix. Through high order hierarchical topology, extraordinary energy absorption can be achieved.