## 7 Materials Used in Additively Manufactured Crash Boxes
### Multi-Material  
Multi-material AM allows for the integration of geometrical complexity and material flexibility within a single structure, enabling localized material variations that can significantly enhance mechanical properties ([PERSON] et al., 2017). As a result, multi-material crash boxes can be engineered to deliver improved performance. However, due to their cost and manufacturing complexity, there are relatively few studies that explore multi-material crash boxes manufactured through AM techniques. [PERSON] et al. [PERSON] et al. (2018) proposed a novel wood-inspired, helix-reinforced cylinder design, as shown in Figure 16a. The specimens were fabricated using the MJT technique, utilizing a stiff polymer (VeroWhitePlus) for the fibers and a blend of the stiff polymer with a rubber-like transparent material (TangoPlus) for the intermediate matrix. It was found that thin reinforcement fibrils, oriented perpendicular to the primary loading direction, significantly enhanced energy absorption, which aligns with the design strategy observed in natural wood cell walls. While [PERSON] et al. [PERSON] et al. (2018) highlighted the benefits of strategic multi-material reinforcement, [PERSON] et al. [PERSON] et al. (2018) focused on understanding how different material pairings influence the energy absorption performance across various lattice structures. [PERSON] et al. [PERSON] et al. (2018) investigated multi-material combinations across three lattice geometries: one non-auxetic (hexagonal honeycomb) and two auxetic (re-entrant and anti-tetrachiral) structures, as illustrated in Figure 16c. Each geometry was evaluated using three material configurations: a single-material structure made of PLA and two dual-material structures  
Figure 15: Metals: (**a**) hexagonal thin-walled tube ([PERSON] et al., 2016); (**b**) thin-walled tubes featuring various slit dimensions ([PERSON] et al., 2017). combining PLA-TPU and PLA-nylon. The findings highlight that, in general, single-material PLA structures achieve the highest energy absorption performance, whereas PLA-nylon combinations consistently exhibit lower performance due to insufficient inter-material bonding. Furthermore, for applications requiring multiple loading cycles, the PLA-TPU combination generally offers a significant advantage. Using continuous fibers as reinforcement in thin-walled structures provides a feasible method to create strong, lightweight structures. [PERSON] et al. [42] investigated 3D-printed continuous fiber-reinforced thin-walled structures with square, circular, and hexagonal configurations, as illustrated in Figure 16b. The continuous fiber-reinforced FDM technique was employed, utilizing ramie yarn with a linear density of 24 Nm/2R as the continuous fiber and PLA as the thermoplastic matrix. Their results indicate that a combination of delamination, fiber pullout, and plastic deformation contributes to enhancing the energy absorption process. It was observed that the hexagonal structure exhibits the highest \(SEA\), whereas the square structure achieves a higher \(CFE\). Figure 16: Multi-material: (**a**) wood-inspired helix-reinforced structure [98]; (**b**) continuous fiber-reinforced thin-walled structures [42]; (**c**) multi-material lattice geometries [99]. The SEA values of different materials used in additively manufactured crash boxes, as reported in recent studies, are illustrated in Figure 17 and categorized into polymers, fiber-reinforced polymers, metals, and multi-material. The mean \(SEA\) values for polymers, fiber-reinforced polymers, and metals are 8.45 J/g, 14.23 J/g, and 28.65 J/g, respectively. These results indicate that metals generally exhibit the highest energy absorption capacity, followed by fiber-reinforced polymers, with polymers showing the lowest energy absorption capacity. The standard deviation for metals is the highest at 16.50 J/g, indicating greater variability in energy absorption performance.