On the crashworthiness of novel right-angle fractal gradient hierarchical multi-cell bi-tubular tubes under axial loading

To achieve a thin-walled structure with enhanced crashworthiness, inspired by tree fractals, a new right-angle fractal of gradient hierarchical multi-cellular square bi-tubular tube (RFGHMBT) was proposed. Its energy absorption characteristics under axial impact were systematically studied using numerical simulation methods. The results show that, for both the same wall thickness and the same mass, the high-level hierarchical multicellular bi-tubular tube has a higher energy absorption capacity and crushing force efficiency compared to the low-level hierarchical multicellular bi-tubular tube. This substantially improves crashworthiness performance. Under the condition of the same wall thickness, the specific energy absorption (SEA) and crushing force efficiency (CFE) of the third-order RFGHMBT improved by 270% and 244.13%, respectively, compared to the 0th-order. The energy absorption (EA) and CFE under the same mass condition are 1.96 times and 2.20 times higher than those of the 0th-order RFGHMBT, respectively. Further geometric parameter analysis revealed that structural parameters (inner measured square tube side length 2 l, shape scale factors γ1, γ2, wall thickness t) significantly affect the crashworthiness of the square gradient-layered multicellular bi-tubular tubes. Additionally, a comparative study with other multicellular tubes demonstrated the superiority of the proposed square gradient-layered multicellular bi-tubular tubes. The findings of this study offer an effective design concept for developing lightweight and efficient energy absorbers.