## 3 Experimental Validation
### Experimental Setup and Data Acquisition  
Laminated composite plates are manufactured to obtain an imbalanced vibration dataset. Eight layers of carbon prepreg (T700 SC-12k-60E), which have dimensions of 35 cm \(\times\) 30 cm, are stacked in [0/90/0/90]\({}_{\text{s}}\) order, as shown in Figure 2. Table 1 shows the mechanical properties of the prepreg.  
To imitate various health states of the laminated composites, Teflon film (Model KSC\(-\)V1000) with a thickness of 0.03 mm and a usable heating range of 280 \({}^{\circ}\)C was inserted between the fourth and fifth layers during the stacking. The film can act like damage inside the laminated composite, so the presence of delamination could be imitated. A total of three health states of composite plates are stacked: Healthy (H), which has no delamination, and Delamination 1 (D1) and Delamination 2 (D2), which have delamination in different locations inside of the specimens.  
After stacking, the plates were cured using the hot press machine illustrated in Figure 3a. The pressure of the machine was 20 kg/cm\({}^{2}\), and the heating cycle of the plate is in Figure 3b. The cured plates that have dimensions of 35 cm \(\times\) 30 cm are then cut into five pieces to receive a beam-shape of 35 cm \(\times\) 5 cm, as shown in Figure 4. Thus, five specimens for each health state, and a total of fifteen specimens, were manufactured for all health states. The blue-colored area is used as the fixed part for the vibration experiment, while the red-colored area indicates the presence of delamination.  
Figure 5 shows the Data Acquisition system. Random vibration signals were generated through MATLAB Simulink, and they were collected in the Data Acquisition (DAQ) unit (DAQ1, model dSPACE/CLP1104). The magnitude of signals was enlarged through the amplifier (model Labworks/PA-151). These amplified signals were then received by a shaker (model Labworks/ET-126-4), which induced vibration in the specimen.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Tensile** & **Tensile** & **Elongation** & **Thermal** & **Density** & **Filament** \\
**Modulus** & **Strength** & **Elongation** & **Conductivity** & **Density** & **Diameter** \\ \hline
230 GPa & 4900 MPa & 2.1\% & 9.4 W/m\(\cdot\)K & 1.8 g/cm\({}^{3}\) & 7 \(\upmu\)m \\ \hline \end{tabular}
\end{table}
Table 1: Mechanical properties of the carbon prepreg.  
Figure 2: Laminated angles for each layer in the composite structure.  
Response signals were obtained with accelerometer (model Bruel & Kjaer/Type 4517-C) sensors that were bonded on the top surface of the specimens. Herein, for the robustness of variations in signals, signals were acquired through the same ten sensors but in different locations. Figure 6 shows the locations of the accelerometers. The figure shows that the sensors were bonded near the edges to consider the effects of twisting and bending in the specimen.  
Figure 4: Three health states of the manufactured specimens.  
Figure 5: Data acquisition system for the experiment.  
Figure 3: (**a**) Hot press machine used for manufacturing specimen. (**b**) Heating cycle for the hot press machine.  
The response signals from the sensors were then put into the amplifier (model Bruel & Kjaer/Type 2692-082) again, and after passing the DAQ (DAQ2, model NI/USB-6341), the final response vibration signals were saved in the computer.  
To simulate the imbalance problem in the experimental condition, a different number of sensors were selected for each health state. For state H, sensors from P1 to P10 were used, while for states D1 and D2, only P01 was used. For state H, 1000 signals were obtained for each sensor, and 60 signals for D1 and D2 were acquired to maximize the data imbalance problem. Because five specimens for each health state were utilized to obtain the dataset, 200 data instances for H and 12 data instances for D1 and D2 each were acquired from each specimen. Table 2 shows the number of the dataset and length of the signals for each health state.