## 2 Materials and Methods  
An IMP503 type Epoxy matrix reinforced with T300 Twill 2 \(\times\) 2 carbon fibers (245 g/m\({}^{2}\)) was used as reference material. Several specimens are fabricated as sandwich, including a core material made of a damping rubber, whose purpose is the vibration absorption improving of the composite. Two different damping materials were used, both supplied by Gummiwerk KRAIBURG GmbH: Kraibon HHZ9578/99 (HHZ) and Kraibon SUT9609/24 (SUT). The lamination of composite and damped sandwich were made by 8 layers at 0\({}^{\circ}\) of prepreg layup and damping rubber integration between the central layers. Plates 600 \(\times\) 600 mm\({}^{2}\) were produced by vacuum bag method through autoclave for 3 h at 135 \({}^{\circ}\)C and 5 bar. The specimens were obtained from plates by water-jet cutting, avoiding the thermal degradation of the polymer matrix. The specimens were cut in square plates 100 \(\times\) 100 mm\({}^{2}\). Specimens thickness is related to the number of layer used. Then, T300/EP samples thickness was 2 mm while damping reinforce composites thickness (SUT and HHZ) was 2.8 mm. In Table 1 are reported the averaged values of Young's modulus for the composite laminate, for the pure damping layers and for the Sandwich laminates with SUT and HHZ at temperature of \(-\)20, 20 and 60 \({}^{\circ}\)C in the untreated condition. The values have been measured using the Oberst test (ASTM E756). In this way, [PERSON]'s modulus of specimens in axial direction could be obtained by a reliable alternative to tensile test. As can be noted, the Sandwich with SUT at room temperature demonstrates a lower Young's modulus than the Sandwich with HHZ. The impact tests were performed according the ASTM D7136 [27], which defines the standard for free-fall drop dart testing on composite materials. This test method is used for damage resistance evaluation. Three samples were tested for each condition. Independent variables studied in this paper were: rubber layer insertion (SUT or HHZ) on energy absorption; ageing effect on energy absorption; and dart velocity at impact on energy absorption. The damage resistance properties measured through this methodology is not an intrinsic property of the material, but it is influence by several factors. Dimensions and geometry of the specimen, sandwich layup, impactor geometry and settings, in addition to all boundary conditions, could strongly affect test results. The specimens are square plates 100 \(\times\) 100 mm\({}^{2}\) constrained by clamping fixtures on a circular hole of diameter 76 mm. Tests were conducted in the range of low-velocity impact (LVI) using 6 m/s as drop dart velocity. In order to analyze the variation in energy absorption with velocity, 1.5 m/s were subsequently tested on sandwich damping structures. The same amount of initial kinetic energy has been imposed at 90 J changing the mass of the dart from 71 kg for 1.5 m/s to 6 kg for 6 m/s. The potential energy is considered without friction loss. The acquisition rate of 1 MHz is appropriate to collect an adequate number of points for a test that normally lasts less than 100 milliseconds. Vertical impact speed is then evaluated from the \(\Delta t\) measured passing between two optical sensors. The calibration of the test velocity needs to be performed before starting with the activity specimens. Dart velocity, displacement, and energy as a function of the time are calculated by the Equations (1)-(3):  
\[v(t)=v_{o}+gt-\int_{0}^{t}\frac{F(t)}{w}\ dt\quad[\mathrm{m/s}] \tag{1}\]  
\[\delta(t)=\delta_{o}+v_{o}t+\frac{gt^{2}}{2}-\int_{0}^{t}\biggl{(}\int_{0}^{t }\frac{F(t)}{w}\ dt\biggr{)}dt\ [\mathrm{m}] \tag{2}\]  
\[E_{a}(t)=\frac{w(v_{o}^{2}-v(t)^{2})}{2}-wg\delta(t)\quad[\mathrm{J}] \tag{3}\]  
where: \(v\) is dart velocity [m/s], \(v_{o}\) is dart velocity at time = 0 s [m/s], \(g\) is the is the free fall acceleration [m/s\({}^{2}\)], \(t\) is time [s], \(F\) is load [N], \(w\) is mass of the dart [Kg], \(\delta\) is displacement [m], and \(E_{a}\) is the absorbed energy [J].