# Impact Response of Composite Sandwich Cylindrical Shells
## 4 Conclusions  
This work studied the effect of thickness on a carbon composite sandwich cylindrical shell and the benefits of a carbon composite sandwich cylindrical shell incorporating a cork core, compared to a conventional carbon composite cylindrical shell. Static bending tests and low-velocity impact tests were performed on specimens that were freely supported on the curved edges, while the straight edges were bi-supported.  
In terms of static bending properties, it was possible to find a significant effect of the thickness. For example, compared to thinner shells (4C), thicker shells (8C) have higher peak load and stiffness values of about 237.9% and 2149.3%, respectively, while the displacement at maximum load decreased 70.5%. Regarding the carbon composite sandwich cylindrical shells incorporating cork core, decreases of around 18% and 40.4% were found for the maximum load and stiffness, respectively, while the displacement at maximum load increases about 25.5%. These benefits were explained by the intrinsic properties of the cork. However, when the sandwich shell was hybridized with Kevlar fibres, higher maximum loads (10%) and displacements (54.2%) were obtained, while the stiffness decreased (47.1%) compared to the conventional carbon composite cylindrical shell.  
Finally, in terms of impact performance, it was observed that larger thicknesses promote significant improvements in the restored energy because, for example, the perforation was observed for the thinner shells (4C). For composite sandwich shells, improvements in impact strength were obtained when a cork layer was introduced, but they were further increased when a carbon skin was replaced by a Kevlar one (hybridization effect). For example, compared to the conventional carbon shell, when a cork layer was inserted into the carbon shell, it allowed the maximum load to be decreased, but increased the maximum displacement and the restored energy. This trend was further evidenced with the replacement of a carbon skin by Kevlar fibres. Comparing the elastic recuperation, for example, improvements of around 44.8% and 20.8% were found, respectively.  
Therefore, this study allows us to conclude that numerous industrial applications that use cylindrical sandwic can benefit from the use of a cork core and the face that is eventually subject to impact loads should incorporate Kevlar fibres to increase the sandwich's impact strength. This optimized lay-up is suggested because these fibres fail through a series of small fibril failures, while carbon fibres exhibit a brittle collapse.  
Figure 10: Failure mechanisms after impact for: (**a**) carbon sandwich shells; (**b**) hybrid sandwich shells.  
**Author Contributions:** C.A.C.P.C. analysed the results and helped to write the manuscript; F.V.P.N. performed the impact test and produced the laminates; P.N.B.R. analysed the results and helped to write the manuscript. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Acknowledgments:** This research is sponsored by national funds through FCT--Fundacao para a Ciencia e a Tecnologia, under the project UIDB/00285/2020.  
**Conflicts of Interest:** The authors declare no conflict of interest.