# Impact Response of Composite Sandwich Cylindrical Shells
## 3 Results
Finally, for thicker shells (Figure 8c), the damage is more localized due to a higher thickness, and the indications expressed in Figure 8c show that it develops more along the thickness in form of delaminations. In this case, thickness is an important parameter both in static and impact strength. In fact, according to [PERSON] and [PERSON] [38], higher thicknesses promote smaller areas of damage because displacement is less in thicker shells, and the damage size is related to total dynamic deformation. Compared to the flat plates, the damage size becomes smaller and its position changes from the lower interface to the upper interface in composite shells [38]. Therefore, the damage appears first at the top ply and then propagates to the bottom layers; however, the maximum damage area occurs at the top surface [38]. In addition, it should also be taken into account that the severity of the damage is higher because carbon fibres fail mainly on the compression side in composites exposed to bending mode [53]. On the other hand, studies developed by [PERSON] [36] revealed that stiffer structures have a higher impact strength, less deflection and shorter contact times. According to the literature [21], the value of P\({}_{\text{max}}\) is very important because it depends on the impact energy and represents the peak load value that the composite can tolerate, under a particular impact level, before undergoing major damage. Therefore, Table 4 presents the average values, and respective standard deviation, of the peak load and other parameters (maximum displacement and rebounded energy (elastic recuperation)) obtained from the impact tests. The elastic recuperation is the difference between the absorbed energy and the energy at peak load [19]. From the results, it is possible to observe that all properties are very dependent on thickness, reproducing the same effect found for static properties. Figure 8: Failure mechanisms after impact for shells with: (**a**) four layers; (**b**) six layers; (**c**) eight layers. In terms of maximum load (P\({}_{\text{max}}\)), this value increases with increasing thickness, reaching an increase of 232.6% for the analysed shells (between 4C and 8C). On the other hand, the displacement decreases by around 46.2% between shells with six and eight layers, while for shells with four layers of carbon fabric the collapse was observed (no restored energy). This behaviour agrees with the studies developed by [PERSON] and [PERSON] (2017) and [PERSON] et al. (2019); [PERSON] et al. (2020), where it was observed that the peak impact load increased with increasing thickness, while the peak centre displacement and contact duration decreased. [PERSON] et al. (2019) observed that the maximum load increased with the thickness of the plate, while the displacement decreased, due to delaminations that proved to be the main mode of damage. They found that higher thicknesses were responsible for larger delaminations. Regarding the elastic recuperation, a decrease of around 32.2% was found between shells with six and eight layers, while for composite shells with four layers of carbon fabric no restored energy was observed. As described above, although visible damage is more severe in shells with six layers of carbon (6C), the energy absorbed is less than in thicker shells (8C). In fact, the maximum damage area occurs at the top surface and propagates to the bottom ([PERSON] et al., 2019), but smaller areas are expected with increasing thickness due to smaller displacements. However, although the damage at the top surface is less, it propagates along the thickness, promoting, in this case, delaminations capable of absorbing impact energy. According to the literature ([PERSON] et al., 2019), during an impact event, the energy is absorbed by matrix cracking, delamination and fibre breakage, but most of it is absorbed essentially by deflection and delaminations.