## 2 Materials and Methods
### Prototyping and Compression Tests
#### 2.3.2 Monotonic and Repeated Tests of Modified Prototypes  
This section develops Samples 3 and 4 (Figure 4e,f) to replicate the impact energy capacity of Sample 2 (Figure 7b) and ensure the reference weight of the front part of the fuselage prototype. The modification aims to improve the internal prototype structure, and the internal pattern of Sample 2 serves as the reference. Therefore, a cylindrical core with the same structure as Sample 2 (Figure 8c) served as the load-bearing core of both alternative specimens considered in this section, and the remaining internal volume had a sparse structure, ensuring the reference weight of the prototype remained consistent. This analysis employed the same 3D printing settings and LW-PLA Sample 2 (Table 3). Following the above approach, the top surface shape, extruded vertically, formed the internal stiff cylinder. The analysis of the deformed shape of the internal lattice structure  
Figure 8: Internal structure analysis of the LW-PLA prototypes: (**a**) cutting place; (**b**) Sample 1; (**c**) Sample 2. Figure 7: Load–vertical displacement diagrams of the tested prototypes: (**a**) test results; (**b**) the shaded deformation energy areas corresponding to the 300 J impact energy equivalent (refer to Table 3 for the sample notations). of Sample 1 (Figure 8b) determines the shape of the cylindrical stiffener of Sample 4. Thus, Sample 4 utilizes a 43 mm diameter cylindrical stiffener, localized at the center of the top surface of the prototype. Similar to the preliminary tests, this analysis utilizes SOLIDWORKS software (Educational Edition 2023 SP2.1, DASSAULT SYSTEMS, Velizy-Villacoublay, France) to create the fuselage front fragments. Figure 4e,f show the numerical models transformed using BAMBU STUDIO software (BAMBU LAB, Version 1.10.1.50, Shenzhen, China) for fabrication via FFF, and Table 3 summarizes the corresponding printing parameters and the resultant weights of the manufactured prototypes. The outer structure was produced with a 13% grid infill, while the inner core featured a denser 24% grid infill to enhance the load-bearing capacity. Two specimens of each prototype type were produced to compare their mechanical resistance under monotonic compression and repeated loads. Figure 9 compares the compression test results of all prototype types subjected to the monotonic compression load. Analogously to Figure 7, these diagrams demonstrate the experimentally estimated load-displacement diagrams and the same graphs, indicating the 300 J deformation-energy-equivalent areas. Both the modified prototypes (Samples 3 and 4 in Figure 9b) showed an improvement in energy absorption capacity compared with Sample 1, and the deformation required to absorb the impact energy of the modified prototypes approached the targeted results of Sample 2. To assess the reliability of the results from the modified specimens, this study employed repeated loading tests, which evaluate the rigidity changes of polymeric structures under various environmental conditions [42]. Figure 10 shows the test results of prototype Samples 3 and 4 under monotonic and repeated compression loading conditions. In this figure, the letter \"r\" in the sample notation reflects the repeated loading. All other sample parameters were identical for the same prototype type. The repeated loading followed the same protocol for both specimens. In the first stage, the specimen was loaded to achieve deformation corresponding to 300 J of energy absorption. The vertical dashed lines in Figure 10 indicate this deformation. After that, the samples were unloaded to a minor load (not zero) to prevent the sudden release of the test equipment, which is typical for such tests [42]. This release was followed by the loading repetition reaching the same compression load that was measured in the first loading stage. This loading produced a single loop. Both test specimens (Samples 3r and 4r) underwent five such loops. After that, the monotonic test was continued to estimate the residual resistance of the test specimens.