## 6 Geometries Used in Additively Manufactured Crash Boxes
### Bio-Inspired Structures  
Animals and plants have adapted to harsh environments by optimizing their biological architecture throughout millions of years of evolution. These structures possess remarkable features, including lightweight construction and exceptional energy absorption capabilities. Bio-inspired structures are engineered designs that mimic the efficient characteristics of biological structures [20]. Bio-inspired structures often involve intricate designs that present substantial manufacturing challenges when using traditional methods. AM offers a suitable alternative, enabling the precise fabrication of these complex geometries. Several studies have focused on bio-inspired structures produced through AM [67; 68; 69; 70]. Several studies have explored bio-inspired tubular structures, leveraging the capabilities of AM to realize complex designs that offer enhanced energy absorption performance. [PERSON] [71] proposed a thin-walled multi-cell tube with DNA-like helical ribs, fabricated from PLA using the FDM technique, as shown in Figure 11a. The study investigated various design parameters, such as spiral turns, inner diameter, and the number of spirals. The results demonstrated a significant increase in energy absorption, with \(SEA\) improving by 166.62% and \(CFE\) by 169.70% compared to a single-tube design. [PERSON] et al. [72] drew inspiration from bamboo nodes to design Bionic Tubes with Bamboo Cross-section (BTBCs) and Bionic Tubes with Rib (BTRs), as shown in Figure 11b, manufactured from 316L stainless steel using the SLM technique. The study examined design parameters such as the number of inner tubes, tube diameter, number of ribs, rib location, and rib size. The results showed that BTBC-1 achieved the best \(SEA\) improvement with a 108.97% increase, while the best \(CFE\) improvement was observed in BTBC-2, which showed a 46.3% increase. [PERSON] et al. [73] proposed a bio-inspired thin-walled corrugated tapered tube modeled on the barnacle structure, as shown in Figure 11c. The specimens are produced using SLM with stainless steel 316L. The results indicate that the number of corrugations and the corrugation amplitude significantly affect the energy absorption capacity. The \(SEA\) of the corrugated tapered tube shows a 15.75% improvement over the traditional tapered tube. Building on their earlier work, [PERSON] et al. [74] further advanced the barnacle-inspired design by introducing hierarchical tapered tubes. Three design configurations are compared: the conventional tapered tube (CTT), the hierarchical tapered tube without corrugation (HTT), and the hierarchical corrugated tapered tube (HCTT), as shown in Figure 11d. The specimens are fabricated using SLM with 316L stainless steel. The results demonstrate that increasing the number of substructures significantly enhances the energy absorption capacity. The \(SEA\) of the HTT structure with four substructures achieves an 85.8% increase compared to the CTT structure. Honeycomb lattice structures are recognized for their efficient energy absorption capacity, making them ideal for crashworthiness applications. [PERSON] et al. [75] proposed a bio-inspired hierarchical circular honeycomb (BHCH) structure inspired by the natural architecture of wood and compared its performance to that of a conventional circular honeycomb (CH), as depicted in Figure 11e. The specimens, fabricated from nylon using FDM technology, revealed that the BHCH structure achieves a 45.3% improvement in \(SEA\) compared to the corresponding CH with an identical wall thickness. Extending the exploration of bio-inspired lattice structures, [PERSON] et al. [76] proposed five bio-inspired lattice structures, shown in Figure 11f, drawing inspiration from the arrangement of femur bones.