## Results and Discussion
### Low-Velocity Impact Response of Sandwich Structures
Thus, synthetic fibers offered a significantly higher peak load, suggesting higher bonding capabilities with the matrix due to the surface smoothness and consistency of the cross-section. In the case of natural fibers, such a mechanism was not favored due to the mechanical interlocking between fibers and the matrices. In this study, to increase the compatibility between the banana fiber and the matrix, the chemical treatment was promoted; however, the peak force still remained unimpressive. The peak displacement indicates the maximum deformation of the sandwich panels, which results in a significant area of damage after subjection to different energy levels. Figure 7 shows representative maximum dent depth against energy level traces for different core composites. In general, the maximum displacement steadily increased with rising impact energy. It can be seen that carbon fiber had the highest dent depth at low energy levels and it was the lowest at higher impact energies. This was in part due to the high in-plane tensile properties of the material. Properties of the core, including rigidity and brittleness, also influenced this behavior. In a recent study by [PERSON] and [PERSON] [36], it was found that at high peak displacement, the uppermost skins of the specimens exhibited more evidence of splitting and delamination of the surface ply. Further, comparing the maximum displacement values for optimum banana and synthetic structures, it can be seen that the values vary between those structures. This helps to explain why the differences between the behavior of core structures were within the scattering; as a result, peak displacement is another independent parameter of sandwich panels. Figure 5: Typical load–displacement traces of (**a**) banana, (**b**) Kevlar, (**c**) glass, and (**d**) carbon/epoxy sandwich structures. The total impact energy is a combination of the energy absorbed and the elastic energy loss of the impacted structures [37]. If the percentage of the energy absorbed is high, the energy converted to elastic energy lost by the impactor is low. As a result, the impact energy is fully transferred to the structures at the point of maximum displacement [38]. In order to evaluate the impact response and resistance of the composite structures, the energy absorbed against impact energy traces are shown in Figure 8. The absorbed energy was measured by calculating the area under the force-displacement  
Figure 6: The variation of the maximum impact force at different impact energy levels for composite structures. Figure 7: Variation of the maximum dent depth against energy level traces for composite structures. traces. An observation of the figure suggests that the absorbed energy tended to increase with the incident impact energy, although there did appear to be scattering in the findings, particularly between the optimal banana/epoxy and carbon/epoxy composites. The lower energy absorption means that there was not much energy lost due to failure. Thus, each failure mechanism, including matrix cracks, interlayer failures, delamination, and fiber breakage, absorbed a fraction of the impact energy. The variation of energy absorbed depends on the skin thickness [39], the mechanical properties of fibers and matrices [40], the density of the core [41], and the impactor head [42]. The evolution of the damage area with impact energy for the optimum banana/epoxy sandwich structure is presented in Figure 9. The sample data was elucidated using a dye penetrant inspection beginning at the uppermost impacted face and followed by measurement of the area using ImageJ software. According to the load-displacement curves (Figure 5), the onset of damage, essentially matrix cracking, was at approximately 3000 N. As the impact energy increased, greater propagation of the damage area in the transverse direction of the specimen was obtained. As can be seen from Figure 9a, a closely peanut shape of visible marks, just like in plain laminates [43], was observed, which was dominated by matrix cracking of the skin.