## 4 Discussion  
The loss factor is representative of the intrinsic damping behavior of a material. In a rough approximation, it is half of the damping coefficient. Materials with high loss factors generally show a loss of storage modulus. Therefore, it is always necessary to find the optimal compromise between the stiffness and damping of structures, depending on the design requirements. In complex structures, such as the hybrid composite sandwiches, the damping and elastic properties of the elements are averaged. In Figure 6b, the storage modulus is reported as a function of the loss factor for all the samples. As a general trend, E\({}^{\prime}\) decreases by increasing tg \(\delta\), but some exceptions are present. Hybrid composites, with rigid rubber interlayers, have a higher storage modulus than those with soft interlayers, independently from the architecture. Moreover, all the hybrid composites have a storage modulus lower than the next CFR laminate, but higher than the neat elastomers. Conversely, their loss factor is always higher than that of the neat CFR laminate, but lower than that of neat elastomers. As a consequence, hybrid laminates have dynamic properties in the middle of the neat samples, and the type of the elastomer interlayer is recognizable as well. Elastomers have a very low storage modulus and very high loss factor in comparison with the neat CFR laminate: E\({}^{\prime}\) is four orders of magnitude higher, tg \(\delta\) is one order of magnitude lower. In particular, the loss factor of the soft elastomer is 0.32, and 0.5 is considered as a reference for optimal damping. This large mismatch between the sandwich components produces low rigidity panels. For both elastomers, moving from the structure with one internal rubber layer (C3-E1-C3) to two (C3-E2-C3), the stiffness reduces: 63% for the rigid elastomer and 86% for the soft elastomer. The stiffening effect of the higher distance of the composite skin is already lost by doubling the rubber core. Other data \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Hybrid Laminate** & **E\({}^{\prime}\), MPa** & **tg \(\delta\)** & **Stiffness, N/mm** & **E, MPa** & **Loss Energy, J/cm\({}^{3}\)** & **Acceleration** \\ \hline _Rigid elastomer_ & & & & & & \\ C3-E1-C3 & 4546 & 0.145 & 24.0 & 1919 & 63.3 & 90.9\% \\ C3-E2-C3 & 1662 & 0.120 & 34.2 & 659 & 72.3 & 92.5\% \\ C1-E1-C2-E1-C2-E1-C1 & 649 & 0.167 & 18.3 & 175 & 52.3 & 91.7\% \\ C1-E1-C4-E1-C1 & 1830 & 0.129 & 32.6 & 734 & 61.4 & 89.1\% \\ C2-E1-C2-E1-C2 & 1213 & 0.164 & 19.2 & 453 & 53.9 & 88.9\% \\ E1-C6-E1 & 1498 & 0.097 & 39.1 & 956 & 12.8 & 93.9\% \\ _Soft elastomer_ & & & & & & \\ C3-E1-C3 & 574 & 0.070 & 16.1 & 428 & 7.2 & 90.2\% \\ C3-E2-C3 & 83 & 0.145 & 11.1 & 48 & 5.8 & 88.5\% \\ C1-E1-C2-E1-C2-E1-C1 & 17 & 0.242 & 3.7 & 8 & 4.1 & 80.0\% \\ C1-E1-C4-E1-C1 & 98 & 0.171 & 8.5 & 51 & 7.2 & 88.9\% \\ \hline
**Neat material** & & & & & & \\ \hline
3-ply CFR laminate & 31,625 & 0.029 & & & & \\ Rigid elastomer & 6 & 0.307 & & & \\ Soft elastomer & 1 & 0.323 & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Data from DMA and quasi-static bending tests on small samples. also confirm that all the hybrid composites are dominated by the elastomer compliance. Lower elastomer thickness would lead to improvement in the composite stiffness, but this goal requires different raw materials which are still not available. The hybrid panel with all the six CFR plies together (E1-C6-E1) shows the highest stiffness, 63% higher than C3-E1-C3, but the loss factor is very close to the neat CFR laminate. Therefore, in absence of an elastomer interlayer, the composite panel has almost no damping. An evident result is that all the hybrid composite laminates have very good agglomeration despite the different mechanical behaviors of CFR plies and elastomers. The feasibility of manufacturing this new typology of hybrid composite has been shown by using the state-of-the-art technologies of aeronautics, even if the optimal thickness of the elastomer layer has yet to be found.