## 4 Results and Discussion
### Nested Tube-Reinforced EPP Foams  
This section presents the findings of testing EPP foam blocks reinforced with aluminum and CFRP composite nested tubes under in-plane crushing stresses. The EPP foam blocks employed in this investigation are consistent in terms of dimensions, each measuring 130 mm by 115 mm, with a density of 30 kg/m\({}^{3}\). Only the tubes with a larger diameter were chosen to reinforce the EPP foam. Due to the considerably greater strength of both aluminum and CFRP composite tubes relative to EPP foam, no appreciable deformation of the tubes was detected until a displacement of about 35 mm was attained from the commencement of the testing procedure. Following this displacement threshold, a progressive increase in the force curve was recorded, as illustrated in Figures 7 and 8. Figure 6: The force-displacement curves of three tube nested samples (**a**) small tubes of CFRP and Al placed in Al outer tube, (**b**) small CFRP and Al tubes placed in CFRP outer tube. Furthermore, it is noteworthy that above this displacement threshold, the force curves increase at a reduced gradient compared to those observed for both aluminum and CFRP composite tubes without EPP foam reinforcement. The deformation curves for the double and triple nested tubes, depicted in Figure 7, demonstrate the emergence of distinctive force curves for the nested specimens, similar to those shown in Figures 3 and 4, after an average displacement of 40 mm. A consistent trend is evident in the force curves of the CFRP tubes, as illustrated in Figure 8. The force values for both aluminum and CFRP composite tubes increase by an average of 1000 N with the addition of EPP foam. Analysis of the energy absorption values of the specimens revealed a significant increase in EPP foam, exceeding the total energy values of both the specimen and the foam. This result is notably significant, reflecting the interaction effect seen when individual tubes are employed together. The single T\({}_{\text{B}}\) tube and EPP foam exhibited energy absorption values of 51.1 J and 35.2 J, respectively, totaling 86.3 J, whereas the FF\({}_{\text{T}}\)B sample showed a 20% enhancement in energy absorption, as illustrated in Figures 2a and 7a. The increase was particularly significant in the CFRP composite TB tube, reaching approximately 39%, as illustrated in Figures 2b and 8a. The maximum and minimum improvements in energy absorption from interactions in the nested-tube-reinforced EPP foam specimens were observed in the FF\({}_{\text{NT2-T}}_{\text{M}}\) and FF\({}_{\text{NT3-T}}_{\text{S+T}}_{\text{S}}\) samples, with enhancements of 35% and 5% for aluminum and 25% and 52% for CFRP composite, respectively. Furthermore, the increase in energy absorption for all nested CFRP composite tube-reinforced specimens surpassed that recorded in specimens reinforced with nested aluminum tubes. Following the completion of experiments on the nested samples, the EPP foam blocks were reinforced with nested aluminum and CFRP tubes and underwent compression testing. The main aim of this reinforcing method was to restrict the lateral expansion of the aluminum and CFRP tubes, thus improving the energy absorption capacity of the samples. The compressive deformation behavior of the EPP foam blocks was meticulously examined. The findings in Table 2 demonstrate that the EPP foam blocks underwent considerable expansion when exposed to compressive stresses. This deformation was directly linked to the lateral expansion of the outer tube. The strengthening of EPP foam sought to limit lateral expansion, thus enhancing the structural integrity and energy absorption capabilities of the nested tubes. A comprehensive comparison of the force-displacement curves between the nested samples and the nested-tube-reinforced EPP foam samples demonstrated significant differences. The use of EPP foam led to an increase in the crushing displacement of the samples by roughly 30 mm.