## 2 Materials and Methods
### Experimental Setup and Test Procedures  
In order to perform the measurements, each panel is fastened between two steel frames (Figure 2) by means of sixteen M10 through-screws, tightened with a controlled torque of 30 Nm. The size and mass properties of the steel frames are summarized in Table 3. The outer dimensions of the frames match the dimensions of the specimens. Hence, each specimen can vibrate as a rectangular plate characterized by the inner frame dimensions (_il_ and _is_ in Figure 2a and Table 3) and clamped at its edges. This kind of constraint aims to simulate typical boundary conditions featured by CFRP panels used in automotive applications, such as the car roof fixed to the vehicle body. Then, the steel frame is suspended by using low-stiffness elastic ropes, with the short sides vertically directed (Figure 2b). \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Density [kg/m\({}^{3}\)] & 1190 \\ Thickness (single strip, uncured) [mm] & 0.20 \\ Strip length (_sl_) [mm] & 800 \\ Strip width (_sw_) [mm] & 150 \\ Side spacing (_ss_) [mm] & 15 \\ Central spacing (_cs_) [mm] & 50 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Properties of the SMACWRAP® strips embedded in the panel. Figure 1: Schematic of the \(PY\) panel layout (top view). Experimental modal analysis (EMA) is performed by using two different excitation techniques, namely impact and shaker excitations. A modal geometry consisting of 12 nodes is adopted to represent the specimens (Figure 3). In particular, 6 nodes (referred to as _Pij_, \(i\) = 1, 2 and \(j\) = 1,..., 3) are associated with the panel; 4 nodes (_Ck_, \(k\) = 1,..., 4) are associated with the corners of the steel frame; 2 nodes (_in1_ and _in2_) are the locations on the panels excited by the shaker. It can be noticed that only six accelerometers are attached to the panels, to not significantly alter the tested system dynamics due to excessive additional masses. Such a limited spatial resolution is reasonably supposed to cause an aliased estimation of the mode shapes. A simple finite element (FE) model of the panels was thus preliminarily implemented with the aim (_i_) to let the numerical modal analysis support the experimental data processing (see Section 2.4) and (_ii_) to pinpoint adequate locations of nodes _Pij_  
Figure 2: Experimental setup: (**a**) schematic of the steel frame and of the excitation/measurement nodes; (**b**) photo of the specimen _PN_ excited by the shaker at node _in1_. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Total mass (2 frames + bolts) [kg] & 11.54 \\ Outer long side (_o_) [mm] & 800 \\ Outer short side (_os_) [mm] & 580 \\ Inner long side (_il_) [mm] & 720 \\ Inner short side (_is_) [mm] & 500 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Properties of the steel frames. for a straightforward visual identification of the main mode shapes. The vibration response of the specimens along the Z direction (perpendicular to the panel plane) is measured at all nodes _Pij_ and at node _C1_ by means of IPE piezoelectric accelerometers (PCB Piezotronics Inc., Depew, NY, USA). Node _C1_ permits to monitor the behavior of the steel frame, in order to possibly identify spurious mode shapes involving vibrations of the frame itself, which should be excluded from the analysis. Impact test is performed by exciting all the specimens' nodes except _in1_ and _in2_, perpendicularly to the panel plane, with an impulse force hammer (PCB 086C04, PCB Piezotronics Inc., Depew, NY, USA). Hence, a total amount of 7 response nodes and 10 reference nodes (i.e., the nodes impacted by the hammer) are used for the analysis (_in1_ and _in2_ are deactivated in the modal model). This setup may be seen as the combination of the so-called _roving hammer_ and _roving sensors_ approaches, thus generating a redundant dataset that may be exploited to find the most convenient subset of frequency response functions (FRFs) estimates that EMA is based on.