Mechanical Response and Energy Absorption of Bio-Inspired Auxetic Hybrid Tubular Metamaterials

Bio-inspired auxetic–conventional hybrid tubular metamaterials were investigated for lightweight energy absorption. The tubes combined 6063 aluminium alloy inner tubes with 304 stainless-steel outer tubes containing oval, circular, rotating-square, or re-entrant perforations. Experiments showed that the outer tube altered the collapse mode of the inner tube and produced topology-dependent responses. The Circle-Hybrid tube achieved the highest mean specific energy absorption (SEA) of 6.95 ± 0.04 kJ/kg; the Oval-Hybrid tube was 17.1% lower at 5.76 ± 0.25 kJ/kg but had a 24.1% lower maximum force and a CFE 6.68 percentage points higher. Its SEA was approximately 159.5% higher than that of the Oval-Single tube. A validated finite element model was used to examine oval-hole aspect ratio and inner-tube wall thickness. At fixed porosity, decreasing the aspect ratio promoted progressive folding and increased load-bearing capacity and energy absorption, while an intermediate ratio maximised CFE. Increasing the inner-tube wall thickness enhanced load-bearing capacity and energy absorption but produced more localised or asymmetric buckling. These findings demonstrate that perforation topology can tailor collapse mode, peak-force demand, crushing efficiency, and energy absorption in hybrid tubes.