# Impact Response of Composite Sandwich Cylindrical Shells
## 3 Results  
Initially, the static properties were analysed and, for this purpose, bending tests were carried out for each configuration. Figure 3 shows the effect of thickness while Figure 4 shows the cork core effect and the hybridization effect in composite sandwich cylindrical shells. Typical load-displacement curves are shown, but they reflect the behaviour observed for each configuration. From the static curves related to the effect of thickness (Figure 3), it is possible to note for all materials a linear regime up to a value from which there is a non-linear regime that  
Figure 4: Cork core effect and hybridization effect on the compressive curves. Figure 3: Effect of thickness on the compressive curves. contains the maximum load. However, increasing the thickness promotes longer linear regimes. For example, compared to cylindrical shells with four layers of carbon, where the linear regime goes up to a load around 120 N, this value is about 269% and 750% higher for cylindrical shells with 6 and 8 layers of carbon, respectively. The zigzag aspect of the curves results from fibre breakage and the subsequent drop in load is a consequence of the propagation of delaminations initiated at the regions with broken fibres, which is reported in the literature [5; 6]. A similar behaviour is found when the cork core is introduced or for hybrid sandwich cylindrical shells but with different values. All properties that can be obtained from these curves are shown in Tables 2 and 3 in terms of average results and respective standard deviation. Stiffness was defined as the slope in the linear region of the load-displacement response and the displacement is the value obtained for the maximum load. The displacement, as reported above, was obtained directly from the crosshead displacement that is automatically measured by the testing machine. From Table 2, it is possible to observe a significant effect of the thickness on the bending properties, which agrees with the literature [44; 45; 46]. According to [PERSON] and [PERSON] [46], for example, an increase in the thickness of the shell reduces the instability region. From a quantitative point of view, and compared to thinner shells (4C), the load increases by 63.8% and 237.9% when the cylindrical shells are composed of 6 and 8 layers (6C and 8C), respectively. A similar trend was observed for stiffness, but with values of 393.7% and 1042.1%, respectively. On the other hand, in terms of displacement, as expected, this parameter decreases when the thickness increases. In this case, and compared to thicker cylindrical shells (8C), displacement at maximum load increases by around 12.8% and 238.5% for shells composed of 6 and 4 layers, respectively. In terms of damage mechanisms (see Figure 5), the failure analysis shows fibre fractures, which started in the fibres under compression, and some delaminations around the broken fibres. All these failure mechanisms arose in the indenter/composite contact region and agree with the literature [42; 47], where the compressive stress concentration at the loading nose contact region associated with the low compressive strength of the fibres promotes the failure damage described above. For example, Figure 5a shows the failure mechanisms of the thicker shell, where the broken fibres are observed in the contact region of the indenter/composite and the consequent delaminations that result from this collapse. Subsequently, their propagation is visible along the generatrix that contains the contact point and towards the edge of the plate. However, because the stiffness is high, they do not reach the edge of the shell and fibre breakage occurs along the thickness (from the compression to the tensile side).