## 3 Experimental Setup: A New Approach  
All the measurements were performed using identical measuring probes and plate specimens used in the numerical simulations. The measuring apparatus contains individual measuring components that are necessary for generating, sensing, filtering, processing, and visualizing the measured signals. The measurements were performed on both conductive specimens with EDM notches, respectively. The dimensions of all defects were precisely defined by the vendor and re-measured using (electromagnetic-acoustic-transducer-method) EMAT and RT (radiography testing) methods.  
In essence, the following instruments were required to perform the measurement using the sweep-frequency eddy-current method: the material under investigation, the  
Figure 7: Numerical simulation: current density module (Conductive current density) distribution within the conductive structure at different excitation frequencies: (**a**) \(f\) = 25 kHz, (**b**) \(f\) = 50 kHz and (**c**) \(f\) = 100 kHz, probes A, B and C.  
new SFECT probe, lock-in amplifier (Signal Recovery DSP 7280), a PC (personal computer) with the LabVIEW platform and DAQ card (National Instruments, Austin, TX, USA).  
The probe (air-core) consisted of two coils with an air-core. The measured value of the self-inductance of the coil Rx was \(L_{\mathrm{Rx1}}\) = 4.7 \(\mu\)H, and the inductance of the Tx coil was \(L_{\mathrm{Tx1}}\) = 11 \(\mu\)H. The probe (ferromagnetic-core) had the same dimensions as the previous one, it differed only in the presence of a ferromagnetic core, and the value of inductances were as follows: \(L_{\mathrm{Rx2}}\) = 25 \(\mu\)H and \(L_{\mathrm{Tx2}}\) = 64 \(\mu\)H. The third probe (ferromagnetic-core and metal shielding) was of the same dimensions as the previous ones with the additional aluminum shielding; it had the following parameters: \(L_{\mathrm{Rx3}}\) = 33 \(\mu\)H and \(L_{\mathrm{Tx3}}\) = 55 \(\mu\)H, Figure 8.  
The transmitting coil was supplied by an alternating harmonic signal (voltage) generated by the 7280 DSP Lock-in amplifier (Figure 9). The signal's magnitude was \(V_{\mathrm{PP}}\) = 0.1 V. The generated harmonic signal had a frequency range of \(f_{\mathrm{g}}\) = \(<\)1 kHz; 1400 kHz\(>\). It varied in \(f_{\mathrm{step}}\) = 5 kHz steps, with a sweeping time of \(t_{\mathrm{sweep}}\) = 2 s for each frequency. A lock-in amplifier that simultaneously served as a data-filtering device was used to measure the induced voltage of the receiving coil. Each material defect was examined for each plate by three probes. In this approach, the probe, in a static position, was used without any movement above the inspected material. Three probe locations were used in the numerical simulation as well as for whole measurements: probe in the air (case A), probe above the defect-free material as a reference (case B), and probe axially symmetrically above the material with the defect (case C).  
Data storage in the form of two separate data files was realized with the use of LabVIEW software. The data containing the real part of the induced voltage were saved separately in one file, and the data for the imaginary part were in another file. The program read \(s\) =1000 samples from the lock-in amplifier with a sampling frequency of \(f_{\mathrm{s}}\) = 10 kHz. The average value was then calculated and saved in the file using these datasets.  
Figure 8: SFECT probes used for the experiments: individual probes (**left**) and probe positioning above the inspected material (**right**).  
Figure 9: Data acquisition and signal processing procedure (**left**) and probe positioning (**right**) during the SFECT measurements.