## Results and Discussion
### Low-Velocity Impact Response of Sandwich Structures  
Figure 5 illustrates typical load-displacement traces for the low-velocity impact response at four different impact energy levels correspond to the optimal banana, Kevlar, glass, and carbon epoxy sandwich structures. All traces initially exhibited a similar pattern with the force increasing in an \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Material**} & **Tensile Strength** & **Modulus of Elasticity** & \multirow{2}{*}{**Elongation (\%)**} \\ \cline{2-3}  & **(MPa)** & **(MPa)** & \\ \hline Neat Epoxy & 19.18 \(\pm\) 2.6 & 980 \(\pm\) 1.8 & 4.21 \(\pm\) 0.8 \\ Banana/Epoxy & 8.23 \(\pm\) 1.4 & 792 \(\pm\) 2.6 & 1.72 \(\pm\) 1.3 \\ Optimum Banana/Epoxy & 23.30 \(\pm\) 2.3 & 1628 \(\pm\) 4.8 & 2.81 \(\pm\) 2.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The average and standard deviation values of the modulus of elasticity, tensile strength, and percentage elongation of the virgin epoxy and natural fiber composites. Figure 4: Typical stress–strain curve for neat, optimal banana/epoxy composite, and banana/epoxy composite. almost linear manner before reaching a peak load. This was associated with the plastic deformation of the sandwich structures directly beneath the hemispherical impactor. Figure 5a presents typical load versus displacement traces for the impact loading of the optimal banana epoxy sandwich structures. It can be seen from the figure that increasing the incident impact energy increased the effective slope of the curve up to 3 mm of displacement, suggesting a rise in the contact stiffness of the system. Beyond this point, a non-linear trace pattern was observed, resulting in the initiation of unstable crack propagation and fiber fracturing. A very drastic fall in the contact load was apparent in the trace for the 20 J impact energy level due to sub-critical propagation and brittle failure of the core. Continued loading often resulted in microcracking and instabilities, with this being most evident in the 15 J impact energy trace where very small load jumps were apparent in the trace at displacements above 3 mm. In addition, the enclosed area within the loading and unloading curves was a measure of the energy absorbed due to the damage in the laminates. Then, the impactor was rebounding since the force and deflection values decreased in the unloading phase. At all the energy levels used in this study, the impactor did not fully perforate the sandwich structures and a portion of the impact energy was conserved as elastic energy. This energy was then transferred back to the steel impactor, causing it to rebound. [PERSON] et al. (2018) suggested that if the core structure was a highly brittle material, the rapid rebound of the skin under impact can result in debonding at the interface between the adjacent layers of the face sheet and the core. Figure 5b-d reveals the typical load displacement traces for Kevlar, glass, and carbon epoxy sandwich structures after being subjected to 5, 10, 15, and 20 J impact energies. From the figure, it is evident that the Kevlar/epoxy composite has a characteristically higher contact stiffness than that of the other composite structures; however, the carbon/epoxy structure offered less \"resting\" displacement during the unloading phase. It can be suggested that this has resulted in less of a dent depth for the carbon epoxy structures. Figure 5: _Cont._  
The variations in the peak force at different impact energy levels for composite structures are shown in Figure 6. It can be observed that the peak impact force was increased with increases in the energy level. A similar observation was also reported by [PERSON] et al. [34]. In addition, the optimal banana/epoxy composite offered a lower peak force than those of the synthetic/epoxy structures. [PERSON] et al. [35] claimed that fiber pullout and fiber compatibility were the major contributors to the toughness of the composite.