# Impact Response of Composite Sandwich Cylindrical Shells
###### Abstract
[29] have also achieved significant benefits with the introduction of cork in composite laminates. For example, the latter observed a longitudinal crack and large delamination in monolithic laminates and when the cork layer was introduced, only a longitudinal crack was reported [29]. [PERSON] et al. [30] analysed the ballistic behaviour and damage tolerance of Kevlar/epoxy-reinforced composites with a matrix filled by cork powder and observed that the minimum velocity of perforation is similar, while there were fewer damaged areas when cork powder was added, in particular for velocities below the minimum perforation velocity. In terms of the residual bending strength, the authors found less sensitivity to impact velocity than samples with neat resin. In addition, cork powder reduces the amount of resin in the composite, making it more environmentally friendly. A similar benefit was observed by [PERSON] et al. [31] in terms of fatigue strength where, regardless of the lower static bending strength, the fatigue life was not affected by adding cork powder into the resin. Finally, [PERSON] et al. [32] studied the ballistic impact behaviour of monolithic laminates and composite sandwich plates incorporating a cork core. Low-velocity impact damage has previously been introduced to some laminates and sandwiches and subsequently subjected to ballistic impacts. No significant differences were observed between sandwich plates and laminates; however, the ballistic limit per area density of damaged laminates was reduced when compared to the value given for reference laminates, while in sandwich structures it was almost constant. However, all these studies were carried out on composite plates, promoting, in this context, the consolidation of knowledge rather than what happens for composites cylindrical shells. In fact, there is not a lot of literature on the subject. For example, [PERSON] et al. [33] analytically studied the contact force and central deflection of a shell for different impact conditions, shell sizes and curvature values. This analytic solution included the contact deformation and transverse shear deformation, and a good agreement was found between their results and those in the literature. The impact response of cylindrical graphite/epoxy shells was analysed by [PERSON] et al. [34; 35], and they found that higher impact energies (resulting from higher masses) promote longer contact times and more induced damage. An experimental study developed by [PERSON] [36] revealed that the impact response is very dependent on geometry. For example, stiffer structures have higher impact strength, less deflection and shorter contact time. Later, [PERSON] and [PERSON] [37] developed studies in cylindrical graphite/epoxy panels with different thicknesses, curvatures, and boundary conditions. It was noticed that the peak impact force increased for higher thickness values, while the peak centre displacement and contact duration decreased. In fact, curvature effects have become increasingly important for smaller thicknesses. On the other hand, flatter panels had higher peak loads than curved panels, as well as smaller peak displacements and contact durations. With respect to boundary conditions, they found that changing from clamped to simply supported promote lower peak loads, while the peak displacement and contact duration increase. [PERSON] and [PERSON] [38] analysed the effect of stacking sequence, radius of curvature and thickness on the impact energy threshold. The impact energy threshold is defined as the energy required to cause the initial impact damage. It was observed that the stacking sequence had a significant influence, because more interfaces between different plies can result in a higher impact velocity threshold and smaller damage size for the laminated shells. Regarding the radius of curvature, they observed that its decreasing promoted a higher impact velocity threshold and a smaller damage area.