## 3 Experimental Results  
The values obtained from the experiment are directly used for further processing. The measurement results are processed in such a way that the value of the voltage phas \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Force Duration** & \(F_{1}\) **= 1.2 kN** & \(F_{2}\) **= 1.5 kN** & \(F_{3}\) **= 1.8 kN** \\ \hline \(t_{1}\) = 6 h & - & Specim 2 & Specim 3 \\ \(t_{2}\) = 12 h & - & Specim 4 & Specim 5 \\ \(t_{3}\) = 24 h & Specim 1 & Specim 6 & Specim 7 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Description of time and static force used for initiation of corrosion cracks. Figure 4: The spatial configuration of the measurement points on the specimen with the presence of corrosion cracks (**a**) and distribution of measurement points in the region of interest (**b**). of a separate probe is subtracted from the measured voltage phasor above the material. Subsequently, the absolute value of the difference and the phase shift are calculated. From these values, it is possible to calculate the frequency response of the material to the excitation signal as well as the effect of the presence of the crack. All necessary math calculations and operations are performed by Matlab software. The results were then normalized to the reference value. According to the following formula, the complex impedance of the coil is normalized by the complex impedance of the coil in the air:  
\[\Delta\mathbf{V}_{\mathrm{norm}}=\frac{\Delta\mathbf{V}_{\mathrm{sim}}}{| \mathbf{V}_{\mathrm{air}}|}=\frac{\mathbf{V}_{\mathrm{sim}-}\mathbf{V}_{ \mathrm{air}}}{|\mathbf{V}_{\mathrm{air}}|} \tag{1}\]  
where \(\Delta\mathbf{V}_{\mathrm{norm}}\) [V] is the normalized value of the voltage of the coil, \(\Delta\mathbf{V}_{\mathrm{sim}}\) [V] is the difference between the coil voltage in the air, and from the simulation, \(\mathbf{V}_{\mathrm{sim}}\) [V] is the voltage of the coil in the presence of the material and \(\mathbf{V}_{\mathrm{air}}\) [V] is the voltage of the coil in the air. The normalized coil voltage module is calculated using the following formulas:  
\[\left[\Delta\mathbf{V}_{\mathrm{norm}}\right]=\sqrt{Re\{\Delta\mathbf{V}_{ \mathrm{norm}}\}^{2}+Im\{\Delta\mathbf{V}_{\mathrm{norm}}\}^{2}} \tag{2}\]  
Figure 5 shows the waveforms of signals from eleven points of corrosion Specimen 1. This specimen is left for \(t_{3}\) = 24 h in the corrosion solution, while a force \(F_{1}\) = 1.2 kN acts on it. It can be seen from the curves that a corrosion crack has developed in the material. Based on the results, this crack probably spreads in the direction of point no. 2 since its value is the lowest of the waveforms. This assumption is also confirmed by the lower value of the signal from point No. 10, which according to the assumption, should have the same value as 11. Figure 6 represents the absolute value of the phasor voltages measured in Specimen 2. A force \(F_{2}\) = 1.5 kN acted on this specimen, and the specimen was left in the corrosion solution for \(t_{1}\) = 6 h. In this specimen, the crack extends in the direction of points No. 2 and No. 8 evenly. The signal reaches its lowest value at point No. 5. Figure 5: Graph of the absolute value of the voltage phasor of Specimen 1 corrosion crack. The curves not measured in Specim 3 are shown in Figure 7. Similarly, it can be seen from the waveforms that the crack spreads evenly in the direction of points No. 2 and No. 8. When comparing, it is possible to notice that the values of points No. 5, No. 2, and No. 8 are approximately the same except for minor deviations. However, the values of the signals from the other points already differ. This is caused by the propagation of the crack in another direction as well. But such a comparison is not plausible, even if the initial conditions of the crack production process were defined at the beginning, each crack developed and spread in the material without an external stimulus, i.e., spontaneously. Therefore, it is difficult to qualify or compare the measurement results.