## 4 Results and Discussion
### Nested Tubes
The preceding text outlines the results from lateral crushing tests performed on nested CFRP composite specimens, as seen in the cited pictures (Figure 4). The experimental data primarily examine the force-displacement characteristics of both two-tube and three-tube CFRP composite specimens, with a comparison versus their aluminum counterparts. The early deformation phase is notably comparable across different specimens, due to the primary deformation of the T\({}_{\rm B}\) specimen, which reflects the behavior seen in aluminum tubes. The differences in the force curves resulting from initial contact versus non-contact with the main tube are distinctly seen in the graphs. This phenomenon is particularly pronounced in specimens featuring three tubes; nonetheless, the initial congruence of the force curves in the absence of contact (Figure 4a) is also significant as it demonstrates the efficient manufacturing of the CFRP tubes. The story clarifies the specimens' dynamic response to a load, emphasizing the cyclical adaptation of force due to gradual structural compromise and the consequent involvement of the smaller tube in the deformation process. The discussion highlights a unique energy absorption pattern between the two-tube and three-tube specimens, with the latter exhibiting a more stable force curve, despite closely comparable energy absorption levels, differing from the behavior seen in the aluminum tubes. This exposition emphasizes the intricate behavioral characteristics of CFRP composite tubes during lateral crushing and contributes to the wider discussion on the structural integrity and durability of composite materials in load-bearing applications. Figure 3: The force–displacement curves of nested aluminum (**a**) two-tube and (**b**) three-tube samples under a lateral crushing load. The progressive deformation stages of the nested aluminum samples, comprising configurations with two and three tubes, are detailed in Table 1. Initially, deformation begins in the largest aluminum tube for all samples, except for the nested sample containing two smaller tubes. The outermost tube starts to deform under the applied load until it contacts the inner tube. Once contact is made, the deformation propagates to the inner tube, causing it to deform as well. This sequential deformation pattern creates force curves characterized by distinct steps, each corresponding to the different stages of deformation in the nested structure. Similarly, the nested samples made from carbon fiber-reinforced polymer (CFRP) exhibit a progressive deformation pattern. However, due to the brittle nature of CFRP, these tubes experience cracking as deformation progresses. The cracks  
Figure 4: The force–displacement curves of nested CFRP (**a**) two-tube and (**b**) three-tube samples under a lateral crushing load. appear on the tube walls once the deformation reaches a certain threshold. Notably, the sequence of cracking is consistent across all CFRP tubes, irrespective of their diameter. Cracks first develop at the upper and lower sections of the tube and subsequently at the right and left sides. This cracking sequence significantly impacts the load-bearing capacity of the nested CFRP samples, leading to abrupt reductions in force. These sudden drops are evident in the force-displacement curves of the CFRP samples, as illustrated in Figure 4. The contrasting behavior between aluminum and CFRP tubes is further highlighted in the smooth force curves observed for the aluminum samples (Figure 3). The absence of cracking in the aluminum tubes is attributed to their ductile behavior, which allows them to undergo significant deformation without sudden failure. This ductility ensures a more gradual and continuous absorption of energy, resulting in smoother force-displacement curves compared to the CFRP samples. In conclusion, the progressive deformation behavior of nested aluminum and CFRP tubes demonstrates distinct characteristics influenced by the material properties.