# Impact Response of Composite Sandwich Cylindrical Shells
## 3 Results
In addition to the reported benefits, cork is also responsible for increasing the impact threshold when inserted into polymeric composites [21; 52]. Studies developed by [PERSON] et al. [21] and [PERSON] et al. [52] showed, for example, benefits of around 9% to 12.6% in relation to laminates without cork (heat resin). However, in relation to the benefits obtained with the hybridization of composite sandwich shells, from Table 5 it is possible to observe, compared to sandwic shells with carbon alone, lower peak loads and higher maximum displacements, as well as elastic  
Figure 9: Maximum load versus areal density. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Laminates**} & \multicolumn{2}{c}{**Peak Load [N]**} & \multicolumn{2}{c}{**Max Displacement [mm]**} & \multicolumn{2}{c}{**Elastic Recuperation [J]**} \\ \cline{2-7}  & **Average** & **Std.** & **Average** & **Std.** & **Average** & **Std.** \\ \hline
8C & 1959 & 50 & 4.2 & 0.02 & 1.16 & 0.38 \\
4C + Cork + 4C & 1755 & 92 & 4.7 & 0.05 & 1.68 & 0.01 \\
4K + Cork + 4C & 1653 & 21 & 4.9 & 0.13 & 2.03 & 0.01 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Average values of the peak load, maximum displacement and elastic recuperation for composite sandwich shells. recuperation (restored energy). For example, the maximum impact load (peak load) is 5.8% lower, while the maximum displacement and restored energy are about 4.3% and 20.8% higher, respectively. In this case, the hybridization further improved the impact performance of the sandwic with the incorporation of aramid fibres (Kevlar), which agrees with studies developed by [PERSON] et al. [42]. According to these authors, composites' cylindrical shells incorporating Kevlar fibres showed the highest restored energy, while the non-hybrid ones (6C) had the lowest restored energy, in addition to lower maximum impact load and higher displacements. It was observed by the authors that, as a consequence of higher damages, the restored energy of the carbon shells (1.71 J) was the lowest, while the hybrid configuration involving carbon and Kevlar fibres (2C + 2K + 2C) showed a much higher elastic recuperation (37.4% higher). This shows that stiffer structures (see Table 3) produce higher impact forces, smaller deflections, and shorter contact duration times [36; 37; 39]. Consequently, an optimization process in design is suggested because the minimum damage zone is achieved for equal bending stiffnesses in both the axial and circumferential directions [38]. In fact, aramid fibres fail not through brittle cracks like carbon fibres, but through a series of small fibril failures, where the fibrils are molecular strands that make up each aramid fibre and are oriented in the same direction as the fibre itself. These small failures absorb a large amount of energy, leading to a very high tenacity. Finally, the failure surface observed after impact for sandwich shells is shown in Figure 10, where it is possible to observe some similarity with the surfaces obtained in the static tests. In both sandwic, the damage is very confined to the impactor/composite contact zone. For sandwic involving only carbon fibres, fibre breakage occurs on the compression side due to the low compressive strength of these fibres, and around them some delaminations occur. Regarding the hybrid sandwic, the damage mechanism is very similar but much more confined to the impactor/composite contact zone. As mentioned above, the damage appears at the top ply and propagates to the bottom layers but, due to the cork core, it is expected that it stops at the composite/cork interface and promotes large delaminations in this region. Figure 10: _Cont._