# Impact Response of Composite Sandwich Cylindrical Shells
## 3 Results
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Laminates**} & \multirow{2}{*}{**Maximum Load (N)**} & \multicolumn{3}{c}{**Displacement at Max.**} & \multirow{2}{*}{**Stiffness (N/mm)**} \\  & & & & **Load (mm)** & \\ \cline{2-2} \cline{5-6}  & **Average** & **Std.** & **Average** & **Std.** & **Average** & **Std.** \\ \hline
8C & 1801 & 212 & 3.9 & 0.7 & 812 & 29 \\
6C & 873 & 121 & 4.4 & 1.0 & 354 & 41 \\
4C & 533 & 101 & 13.2 & 2.1 & 71,7 & 8 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Effect of thickness on the compressive strength. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Laminates**} & \multirow{2}{*}{**Maximum Load (N)**} & \multicolumn{3}{c}{**Displacement at Max.**} & \multirow{2}{*}{**Stiffness (N/mm)**} \\  & & & & **Load (mm)** & \\ \cline{2-2} \cline{5-6}  & **Average** & **Std.** & **Average** & **Std.** & **Average** & **Std.** \\ \hline
8C & 1801 & 212 & 4.7 & 1.1 & 812 & 29 \\
4C + Cork + 4C & 1476 & 121 & 5.9 & 1.9 & 484 & 37 \\
4K + Cork + 4C & 1623 & 101 & 9.1 & 2.3 & 256 & 32 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Cork core effect and hybridization effect on the compressive strength. Regarding the shells with six layers of carbon fabric (Figure 5b), the damage mechanisms are similar, but due to the higher instability of the shell, the fibre breakage occur  
Figure 5: Failure mechanisms for shells with: (**a**) eight layers; (**b**) six layers: (**c**) four layers. essentially in the compression side and delaminations along the generatrix that contains the indenter/composite contact point is much longer than the previous ones (Figure 5a). Finally, for the thinner shells (Figure 5c), due to their enormous instability, it is possible to observe that the delaminations reach both edges and, in this case, some fibres break both in the compression and in the tensile side. Therefore, it is possible to conclude that the damages in thicker shells are more severe along the thickness, while in thinner ones the damages are more extensive. On the other hand, from Table 3, it is possible to observe a significant effect of the cork core and the hybrid effect on the compressive properties. For example, compared to composite cylindrical shells (8C), when a cork layer is inserted into the middle of the shell (sandwich cylindrical shells), it promotes lower maximum loads and stiffness, while the displacement at maximum load increases. Decreases of around 18% and 40.4% can be found for maximum loads and stiffness, respectively, while the displacement at maximum load increases by about 25.5%. These values are in line with others reported in the literature [31; 48; 49; 50], where the insertion of cork into composites decreases the bending strength and modulus, but the bending strain increases. In fact, cork deforms because the cell walls bend and buckle and can undergo large strain deformation. Therefore, the compression behaviour of cork is largely explained by the characteristics of its cellular structure [51]. However, other benefits are achieved, such as better impact performance [21; 30; 32; 52]. In terms of damage mechanisms, Figure 6 shows the typical failure mechanism for the composite sandwich cylindrical shells, which are in line with those observed in Figure 5a. However, because the cork layer absorbs energy, the bottom face did not evidence any visible damage. Concerning the hybridization, when a carbon skin is replaced by a Kevlar one, it is noticed that a higher maximum load and displacement are obtained, while the stiffness decreases. In this case, there were increases of about 10% and 54.2% for the maximum load and displacement, respectively, while the stiffness decreased by about 47.1%. According to [PERSON] et al. [42], regarding laminates with six layers, and comparing to cylindrical shells only with carbon fibres (6C), the maximum load is obtained for hybrid cylindrical shells involving carbon and Kevlar fibres, while the lowest value was obtained for those composed of carbon and glass fibres.