# Impact Response of Composite Sandwich Cylindrical Shells
## 3 Results
In terms of stiffness, these authors observed that the highest value was obtained for cylindrical shells only with carbon fibres, while the lowest was obtained for hybrid shells of carbon and glass fibres. Finally, the smallest displacement occurred for cylindrical shells with carbon fibres and the largest value for the system involving carbon fibres and Kevlar. This behaviour was explained by the authors as result of the intrinsic mechanical properties of fibres and respective damage mechanisms. For example, while carbon fibre composites basically fail on the compression side, the glass fibre laminates fail on the tension side [53]. Therefore, when the carbon fibres are  
Figure 6: Failure mechanisms for composite sandwich cylindrical shells. placed on the tension side and glass fibres on the compressive side, an increase in flexural strength is obtained while the flexural modulus decreases [53; 54]. However, these benefits depend on the fibre content, and according to the literature, the optimum value for hybrid carbon/glass composites is obtained for 12.5% of glass fibres placed on the compressive side [53; 55]. In terms of impact response, Figure 7 shows the force-time and energy-time curves for composite cylindrical shells with different thicknesses (Figure 7a) and for composite sandwich cylindrical shells (Figure 7b). These curves contain oscillations, which according to the literature [56; 57], result from the elastic wave and are created by the vibrations of the samples. They are typical curves, representative of the other ones, and similar to those that can be found in the bibliography [19; 20; 21]. Figure 7: Load–time and energy–time curves for: (**a**) composite cylindrical shells with different thicknesses; (**b**) composite sandwich cylindrical shells. Regardless of the system tested, composite cylindrical shells or composite sandwich cylindrical shells, it is possible to observe that all load-time curves are characterized by an increase in the load up to a maximum value (P\({}_{\text{max}}\)) followed by a drop after the peak load. However, for composite cylindrical shells with four layers (4C), after P\({}_{\text{max}}\) the load decreases and remains constant with increasing time. This atypical profile is a consequence of large damages, which can be proven through the energy-time curve where no energy is restored. Therefore, this means that the impact energy was mostly converted into damage. For the cylindrical shells with six layers (6C), the load-time curves behave similarly to the one described above. However, when it is analysed simultaneously with the energy-time curve, it is possible to conclude that despite the high damage introduced into the shell there is still a significant amount of restored energy (around 34.2% of the impact energy). Finally, for the cylindrical shells with eight layers (8C), the load-time curves present the typical behaviour reported in the literature [19; 20; 21]. Nevertheless, when the energy-time curve is compared with the others, it is possible to observe that this configuration absorbs more energy than, for example, the configuration with six layers (about 11.3% more). In this case, this higher amount of energy absorbed is reflected in larger internal damages. Figure 8, for example, shows the failure surface for the various configurations. Figure 8: _Cont._  
From Figure 8, it is possible to notice that for the thinner shells (Figure 8a), the damage starts in the impactor/composite contact zone, in the form of fibre breakage and delaminations around them. There is even full perforation of the shell along its thickness. Regarding the six-layer shells (Figure 8b), damage also starts in the impact zone with fibre breakage, but only on the compression side, and delaminations around the broken fibres that have propagated towards the edge. Although the damage is visible to the naked eye, there was no perforation of the shells due to their greater stiffness.