## 4 Discussion
The relationship between the dynamic behavior of the composite structure and the properties of the single components is very complex because of the deformation mechanisms which are involved during loading. An example is given by the simplest architecture (C3-E1-C3) where the elastic modulus of the hybrid panel with soft elastomer is lower than that of the rigid, but its loss factor is much higher, not far from the neat CFR laminate. This architecture seems to be very bad for the soft elastomer as the very low modulus is associated with a very low loss factor. The poor damping behavior in comparison with the other hybrid composites is also confirmed by the other damping properties, energy loss and acceleration damping of Table 3.  
In general, hybrid composites with the soft elastomer do not show better damping properties of those with the rigid elastomer. By comparing the same stacking sequence, apart from the discussed C3-E1-C3 case, their loss factor is higher, but energy loss and acceleration damping are lower. This strong mismatch may be justified by the effect of the displacement range, moving from 100 \(\upmu\)m of DMA tests to 10 mm of dynamic bending tests. The low rigidity of the soft rubber layer is also negative for the damping as the material is not able to consume the acquired energy. As an average on the four hybrid samples with a comparable architecture, hybrid composite laminates with the soft elastomer lose 5% of the acceleration damping. This occurrence suggests that the properties of the soft elastomer are not appropriate for a good hybrid structure as the loss in stiffness is not compensated by an improved damping. A threshold for the rubber compliance is present, over which the deforming mechanisms of the hybrid structures do not allow for further damping improvement. Moreover, the displacement scale of these deforming mechanisms is important and data from DMA tests are not sufficient to predict the behavior of complex structures under loading. It is difficult to make a comparison between the proposed architectures as it is not possible to state the acceptable reduction in stiffness for a given increase in damping. An experimental approach may be combining the measured data for stiffness and damping. In particular, the specific modulus is considered, i.e., the ratio of the elastic modulus of Table 3 with the density of Table 2. High values of the specific modulus lead to high stiffness per unit of weight, which is fundamental in aeronautics. Moreover, natural frequencies are related to the square root of the specific modulus. The specific modulus is reported in Figure 7 for all the hybrid composites, together with the data of loss energy and acceleration reduction in Table 3. The data are normalized, by the minimum and the maximum values, to obtain 1 for the best and 0 for the worst case. This is confirmed by the fact that the choice of the soft rubber is negative for all the architectures. The most complex configuration C1-E1-C2-E1-C2-E1-C1 shows the lowest values of all the parameters. Instead, the best combination of properties is found in the simplest structure C3-E1-C3 with rigid elastomer core. It is also confirmed that rubber pads on the top and the bottom of the laminate (E1-C6-E1) do not provide good dynamic performances. The use of an interlayer is suggested but the thickness of this interlayer should be designed on the basis of the properties of the elastomer. In this study, manufacturing constraints have forced us to use thick layers of elastomers, and this solution seems to affect the final properties of the hybrid laminates. In the case of low modulus rubbers, the use of complex architectures does not enable us to achieve good dynamic behaviors. Infact, Figure 7 shows that in the top of the list there are two laminates with the traditional architecture of soft core and hard skins. The proposed architectures and material combinations are quite new in the scientific database; therefore, a direct comparison with other similar studies is difficult to perform.