## 3 Data Preparation  
### Experimental Setup  
Figure 2 shows the experimental setup of the electromagnetic testing system. The signal processing circuit and sensor probe were attached to an XY-stage scanner to scan entire rives on the specimen. The XY-stage scanner triggers the acquisition device with respect to the scan step of 0.5 mm at a scan speed of 32 mm/s. The lift-off was maintained at 0.3 mm during scanning using four wheels on the sensor probe. The Hall sensor array has 64 InSb elements arrayed at an interval of 0.52 mm. The magnetic source was excited with a current amplitude of 0.1 A and a frequency of 900 Hz. The skin depth was about 3.2 mm, enough to penetrate the two layers specimen (each layer has a 1.27 mm thickness). The output voltage of the Hall sensors was high-pass filtered at 284 Hz and amplified at 60 dB. The numbers of high-pass filters, amplifiers, and RMS circuits are the same as the number of Hall sensors, 64. The signal is then digitalized with a 2.441 mV/bit resolution by a NI-PCI 6071E device [21; 22]. The detailed specifications of the components are shown in Table 1.  
Figure 1: Principle of the electromagnetic testing system with the integration of machine learning techniques. \begin{table}
\begin{tabular}{c l l} \hline \hline  & **Components** & **Properties** \\ \hline \multirow{8}{*}{**Sensor probe**} & \multirow{8}{*}{_Coil_} & Thin slices: 10 \\  & & Material: Silicon steel \\  & & Size: 13 \(\times\) 20 \(\times\) 50 mm (inner dia., outer dia., height) \\  & & Turns: 1220 \\  & & Copper wire: 0.2 mm diameter \\  & & Current supply: 0.1 A \\  & & Frequency: 900 Hz \\  & & Hall elements: 64 hrs \\  & & Element interval: 0.52 mm \\  & & Length: 33.28 mm \\  & & Type: RC \\  & & High-pass filter \\  & & Cut-off frequency: 284 Hz \\  & & To remove low-frequency noise signal \\  & & Differential type: INA128 \\  & & No. of elements: 64 \\
**Signal processing** & \multirow{2}{*}{_Amplifier_} & Gain: 60 dB \\  & & ARQ436 chipset \\  & & No. of elements: 64 \\  & & Device: NI-PCI 6071E \\  & & Channels: 64 \\  & & Resolutions: 12-bit, 2.441 mV/bit \\  & & Sampling rate: 1.25 MS/s \\  & & Sampling trigger at each 0.5 mm from XY-stage scanner \\  & & Two aluminum alloy layers (Al 2024) \\  & & Size: 300 \(\times\) 300 \(\times\) 1.27 mm \\  & & No. of Rivets: 25 (AN426 AD-5-6, Air Force and Navy standard) \\  & & No. of corrosion: 25 \\
**Specimen \& Corrosion** & \multirow{2}{*}{_Artificial Corrosion_} & Diameters: 6, 8, 10, 12, 14 mm \\  & & Depths: 0.1, 0.3, 0.6, 0.9, 1.27 mm (Through) \\  & & Volumes: 2.8\(\sim\)195.4 mm\({}^{3}\) \\  & & On rivet sites of the second layer \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Properties of components in the electromagnetic testing system. Figure 2: Experimental setup of electromagnetic testing on a two-layer aluminum specimen: (**a**) sensor probe attached to XY-stage scanner with signal processing circuits, (**b**) sensor probe on a specimen, and (**c**) Hall sensor array on the sensor probe. There are two specimens with and without artificial hidden corrosion. Each specimen has two aluminum (Al 2024) layers and 25 rivets. The rivets are commercial aircraft countersunk rivets (AN 426 AD-5-6) with diameters of the head and shank of 5.85 and 3.95 mm, respectively. Artificial corrosion was fabricated on the riveting site of the upside surface of the second layer with diameters of 6.0, 8.0, 10.0, 12.0, 14.0 mm and depths of 0.1, 0.3, 0.6, 0.9, 1.27 mm. Thus, there are 25 distinguish volumes of corrosion. The sketch of the rivet with hidden corrosion on the second layer is shown in Figure 3. During the experiment, the corrosive specimen was rotated at 0\({}^{\circ}\), 90\({}^{\circ}\), 180\({}^{\circ}\), and 270\({}^{\circ}\) such that the corrosion was on the four sides of the scanning direction, which are forward, backward, left, and right side. Thus, it is needed to detect corrosion and determine the location of the corrosion. The no-corrosion specimen was rotated at 0\({}^{\circ}\) and 90\({}^{\circ}\) only.