## Results and Discussion
### Compression after Impact (CAI) of Structure Properties  
Figure 11 shows the variations of CAI strength performance with the displacement for the optimal banana, carbon, Kevlar, and glass sandwich structures following an impact energy of 10 J. For all sandwich panels, the structures exhibited an initial linear CAI strength before reaching a maximum value, following which the strength gradually fell, an effect that was also reported previously [47]. The figure shows that an increase in peak stress resulted in a decrease in the displacement, highlighting the presence of the stronger and stiffer core structures. The carbon/epoxy based sandwich structure was seen to offer a greater residual strength than those of other systems. Furthermore, the findings also show that the CAI strength of the natural-fiber-reinforced epoxy was still lower than its synthetic fiber counterparts. The differences in residual stress were recorded at 22%, 63%, and 136% between banana and Kevlar, glass, and carbon respectively. Figure 12 illustrates the average CAI strength versus impact energy traces of the sandwich structures, including those of unimpacted specimens. As observed, the compression strengths decreased with the increase in impact energy. The carbon fiber sandwich structure exhibited a massive residual strength under all impact energies with the optimal banana composite system providing the least. The outstanding residual strength performance of the carbon/epoxy sandwich structure was due to the high resistance to shear cracking between the carbon and epoxy of the core. The use of the surface treatment and fiber conditions of the banana pseudo-stem fiber in order to increase the surface bonding between the fiber and matrix was still insufficient and failed to offer high resistance to CAI strength compared to synthetic fibers. With respect to CAI behavior in the sandwich structures experiments, core breaking and skin buckling were identified as the major failure modes of the sandwich structures [48]. [PERSON] et al. [49] mentioned that matrix cracking and delamination between the face sheet and the core were found to be the most common damage mechanisms of the sandwich structure under residual compression strength. Figure 11: The variation of compression after impact (CAI) strength with the displacement for four different sandwich structures subjected to an energy impact of 10 J. In order to compare the properties of impact damage on the strength of the material, the ratio between the CAI strength of the specimens damaged by a given impact and the unimpacted sample was measured. Figure 13 presents the normalized residual strength as a function of the incident impact energy. The optimal banana/epoxy sandwich structure showed the greatest reduction in normalized strength; however, the lowest drop in the normalized residual strength was obtained in the carbon/epoxy core structures. In addition, the normalized CAI strength gradually decreased with the increasing incident impact energies. Similar findings were reported by [PERSON] et al. [50]. At an impact energy of 15 J, the normalized reductions of this strength were: 41% in the carbon/epoxy structure, 43% in the glass/epoxy structure, 60% in the Kevlar/epoxy structure, and 70% in the optimum banana/epoxy structure. The lowest reduction in normalized value, especially of the synthetic fiber structure, may be due to the stiffer core, making the structure more stable and with a high resistance to impact. As a result, synthetic fiber/epoxy systems were more prone to buckling during CAI testing. Figure 14 shows the CAI setup conditions of the optimum banana/epoxy sandwich structures subjected to a 15 J impact energy. The major delamination face was observed perpendicular to the loading direction. As can be seen in the figure, the failure form of the outer skin was more severe at the point of impact. In addition, the debonding phenomenon occurred between the core and the outer surface extended rapidly from the middle of the damaged area to both sides of the edge.