Performance of Aluminum Foam-Filled Hierarchical Thin-Walled Structures Under Axial Impact

In this study, a hierarchical aluminum foam-filled thin-walled structure is proposed and its performance under axial impact is subsequently investigated. Two primary configurations are studied, namely, a hierarchical unit-cell structure (HUCS) and hierarchical multi-cell structure (HMS), respectively. Meanwhile, based on the experimental results, models are established to further investigate the effect of geometries, foam densities and impact velocities on the impact performance of the proposed structure. Finally, an improved simplified super folding element (SSFE) theoretical model which accounts for the constraint-induced strengthening effect of the foam filler is derived and a closed-form expression for the mean crushing force (MCF) is obtained. Compared with non-hierarchical counterparts (NHUCS and NHMS), the hierarchical designs exhibited superiority in reducing deformations and enhancing specific energy absorption (SEA). Compared to non-hierarchical structures, under an impact with identical energy, the HUCS shows a 12.7% reduction in maximum deformation and a 15.4% increase in SEA at the same deformation level. Meanwhile, the HMS reduces MCF by 17.2% and initial peak force (IPF) by 20.2% compared to the NHMS. Parametric studies reveal that wall thickness has a greater influence on final deformation than foam density. Numerical results are in good agreement with the proposed SSFE model within the baseline parameter range, with typical deviations below 10%, though larger discrepancies up to 23% are observed for certain extreme combinations of wall thickness and foam density. The hierarchical multi-cell collaborative design and the MCF prediction method presented here can provide practical guidance for designing high-efficiency impact-protective structures.