## 7 Materials Used in Additively Manufactured Crash Boxes
### Polymers  
Polymers are widely employed in AM due to their versatility, cost efficiency, and adaptability across various AM techniques [16]. Moreover, polymers offer notable benefits, such as low weight and processing flexibility [77]. These properties render polymers ideal for applications requiring optimized energy absorption capacity, making them a desirable option for innovative crash box manufacturing. Multiple studies have examined polymer-based crash boxes produced through AM [78; 79; 80; 81]. Recent studies have explored the influence of material selection and processing parameters on the crashworthiness of polymers used in additively manufactured crash boxes. [PERSON] et al. [82] conducted a comprehensive study on thin-walled multi-cell tubular structures, illustrated in Figure 13a, analyzing the effects of wall thickness, filament material, and printing parameters such as layer height, printing speed, nozzle temperature, and nozzle diameter. The specimens were fabricated using FDM with three types of PLA: PLA+, PLA-ST, and PLA-LW. The highest \(CFE\), reaching 0.91, was achieved using the PLA-ST filament, a wall thickness of 1.6 mm, a printing speed of 90 mm/s, a nozzle temperature of 230 \({}^{\circ}\)C, a 0.3 mm layer height, and a 0.8 mm nozzle diameter. For the maximum \(SEA\) of 19.08 J/g, the optimal setup included the PLA+ filament, a wall thickness of 1.6 mm, a printing speed of 90 mm/s, a nozzle temperature of 220 \({}^{\circ}\)C, a 0.3 mm layer height, and a 0.6 mm nozzle diameter. In a related study, [PERSON] et al. [83] investigated the performance of five polymer-based honeycomb lattice structures, shown in Figure 13b, fabricated using FDM with four polymer materials: PLA, PETG, ABS, ASA, and a fiber-reinforced material, PA12 with nano-carbon fiber. The results revealed that PETG and PLA achieve superior \(SEA\) values of 2.49 J/g and 2.6 J/g, respectively. PETG also exhibits the highest \(CFE\) at 0.83, followed by ABS at 0.78, highlighting PETG's superior performance in both \(SEA\) and \(CFE\). Some studies have focused on the cyclic compression-recovery behavior of polymers used in additively manufactured crash boxes. For instance, [PERSON] et al. [84] investigated metamaterial lattice structures, shown in Figure 13c, fabricated using the FDM technique with PLA and PETG. The study examined unit-cell layouts, including hexagonal, hybrid, and re-entrant configurations. Under one-off compression, the PETG re-entrant honeycomb exhibited the best performance. During cyclic compression, the PETG re-entrant honeycomb demonstrated superior reusability, retaining approximately 40% of its energy dissipation capacity after seventeen cycles, which is two cycles more than the hybrid counterpart and five cycles more than the hexagonal counterpart. These findings further corroborate PETG's superior performance, as noted in earlier studies. Temperature is a critical factor influencing the mechanical properties of polymers employed in additively manufactured crash boxes. [PERSON] et al. [85] investigated the effect of temperature on the performance of an origami crash box (OCB), illustrated in Figure 13d, fabricated from PLA using the FDM technique. The study evaluated the OCB at three temperatures: 30 \({}^{\circ}\)C, 40 \({}^{\circ}\)C, and 50 \({}^{\circ}\)C. The results revealed a clear temperature dependence, with \(SEA\) values of 1.88 J/g at 30 \({}^{\circ}\)C, 1.71 J/g at 40 \({}^{\circ}\)C, and 0.45 J/g at 50 \({}^{\circ}\)C, indicating a decline in \(SEA\) as temperature increases. This behavior is closely related to the glass transition temperature \(\left(T_{g}\right)\) of PLA, which lies near the tested temperature range. As the material approaches or exceeds its \(T_{g}\), it transitions from a rigid, glassy state to a more rubbery and ductile state, resulting in a significant reduction in stiffness and energy absorption capacity.