## 4 Results and Discussion
### Nested Tubes  
The findings from the lateral compression examinations of composite samples, consisting of dual and triple aluminum tubing configurations, are delineated in Figure 3. The  
Figure 2: The force–displacement curves of single (**a**) aluminum and (**b**) CFRP tubes under a lateral crushing load. bi-tubular samples were fabricated by inserting tubes of small (T\({}_{\rm S}\)) and medium (T\({}_{\rm M}\)) dimensions into a larger tube, whereas the tri-tubular specimens were constructed by incorporating either two small tubes or one small and one medium tube within a large tube. An epoxy-based adhesive was employed to preserve the original positioning of the tubes during the lateral compression test. In the case of the bi-tubular specimens, the spatial separation between the outer surface of the T\({}_{\rm S}\) and T\({}_{\rm M}\) tubes and the inner surface of the large tube measured at 37 mm and 14 mm, respectively. Additionally, a 20 mm gap was observed between the T\({}_{\rm S}\) and T\({}_{\rm M}\) tubes. The dimensional selection of the tubes, allowing for the accommodation of two smaller tubes within the larger tube, was a critical consideration. Upon analysis of the force-displacement graphs presented in Figure 3a, it is observed that the force profiles initially align with those of the T\({}_{\rm B}\) tube. However, post a specific displacement threshold, the nature of the force curve alters. This suggests that deformation initially occurs solely within the T\({}_{\rm B}\) tube, and subsequently, as the T\({}_{\rm B}\) tube comes into contact with the inner tube(s), the combined deformation of these tubes results in the modification of the force curve. Utilizing the T\({}_{\rm M}\) tube internally results in a rapid escalation in the force curve at approximately 14 mm of displacement, akin to the initial response observed in the T\({}_{\rm M}\) single-tube experiment. Conversely, when a T\({}_{\rm S}\) tube is incorporated (in the NT2-T\({}_{\rm S}\) sample), this escalation is noted at a displacement of roughly 37 mm. The emergence of a stepped force curve through the integration of tubes of varying diameters is significantly noteworthy as it demonstrates the potential for deliberate manipulation of the force curve. Given that the NT2-T\({}_{\rm M}\) specimen exhibits an energy absorption capacity approximately 16% higher than that of the NT2-T\({}_{\rm S}\) specimen, it is inferred that an earlier onset of the stepped force curve phenomenon is preferable. The stepped structure of the force curve, previously mentioned, exhibits a more pronounced character in the NT3-T\({}_{\rm S}\)+T\({}_{\rm M}\) specimen, as delineated in Figure 3b. Initially, deformation occurs in the larger tube, succeeded by the T\({}_{\rm M}\) tube's deformation at approximately 14 mm, culminating in the emergence of the initial step; the subsequent step materializes with the T\({}_{\rm S}\) tube's deformation at around 34 mm. In juxtaposition, the NT2-T\({}_{\rm S}\) specimen evidences an escalation in the force curve at a displacement value of 37 mm. However, this value markedly diminishes to 14 mm in the NT3-T\({}_{\rm S}\)+T\({}_{\rm M}\) specimen, a phenomenon likely attributable to the concurrent deployment of the T\({}_{\rm S}\) and T\({}_{\rm M}\) tubes within the triply nested specimen. Conversely, the NT3-T\({}_{\rm S}\)+T\({}_{\rm S}\) specimen, employing two T\({}_{\rm S}\) tubes, does not exhibit a stepped force curve, maintaining uniformity akin to the force curves observed in single-tube specimens, as illustrated in Figure 3b. This uniformity arises from the inner tubes' interaction with the TB tube, facilitating a more consistent force curve and enabling a 12% enhancement in energy absorption at the experiment's inception. Given the dual incorporation of smaller tubes in the NT3-T\({}_{\rm S}\)+T\({}_{\rm S}\) specimen, it is anticipated that a higher specific energy value will be achieved, correlating with a reduced sample weight compared to the NT3-T\({}_{\rm S}\)+T\({}_{\rm M}\) specimen.