## 2 Numerical Simulation Procedure  
The numerical modelling approach is used mainly to reduce the necessary time for a further experimental investigation. It can be used to investigate materials with various parameters, whether geometric or electromagnetic. The results can only be taken into account if they are subsequently verified by the measurements, with an appropriate match in the results. Numerical simulations in this work are performed using the CST Studio Suite software process of modelling the interaction of the electromagnetic field with the conductive structure. The software uses the finite element method (FEM) for the calculation and analysis itself. The eddy-current probes that were used during the numerical investigation are presented, and their parameters summarized, in Table 1. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Attribute** & **Symbol** & **Value** \\ \hline Rx width & \(w_{\text{Rx}}\) & 0.7 mm \\ Rx height & \(h_{\text{Rx}}\) & 2 mm \\ Rx radius & \(r_{\text{coil}}\) & 1.4 mm \\ Rx number of turns & \(N_{\text{Rx}}\) & 140 \\ Tx width & \(w_{\text{Tx}}\) & 0.5 mm \\ Tx height & \(h_{\text{Tx}}\) & 2 mm \\ Tx radius & \(r_{\text{coil}}\) & 1.4 mm \\ Tx number of turns & \(N_{\text{Tx}}\) & 80 \\ Tx-Rx lift off & \(h_{\text{lo}}\) & 0.8 mm \\ Core height & \(h_{\text{lc}}\) & 5.7 mm \\ Core radius & \(r_{\text{c}}\) & 0.55 mm \\ Core relative permeability & \(\mu_{\text{core}}\) & 13,000 \\ Shielding width & \(w_{sh}\) & 1 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The SFECT probes simulated parameters. Figure 1: Algorithmizing of actions: the procedure used to conduct this study. Each of the new SFECT probes consists of a two-coil system. The first coil serves as a transmitter (Tx), hence as a generator of the electromagnetic field. The second coil is used to pick up the response signal and is named a receiver (Rx). The cross-section of the probes is shown in Figure 2.  
For simulation and experimental purposes, we utilized a conductive plate specimen with a thickness of \(l_{\mathrm{s}}\) = 10 mm, a width of \(w_{\mathrm{s}}\) = 150 mm, and a length of \(l_{\mathrm{s}}\) = 500 mm; the electromagnetic parameters of the stainless steel AISI 316L (chromium-nickel-molybdenum steel with a carbon content of up to 0.03%, acid and corrosion-resistant, with a little susceptibility to pitting corrosion in chloride-containing solutions) were inspected in this study. The material has the conductivity of \(\sigma\) = 1.4 MS/m and the relative permeability of \(\mu_{\mathrm{r}}\) = 1. The value of the lift-off parameter is set to lift-off = 0.5 mm. This value was also used in the implementation of the experiments and was given by the construction of the measuring probes. There were no changes in the lift-off parameter during the simulations and experiments. Each specimen contained five non-conductive defects and were rectangular in shape, as shown in Figure 3. Five artificial flaws were present in each specimen. The flaws were rectangular in shape and their dimensions varied for each specimen: specimen No. 1 contained five defects with different depths. The depths were in the range of \(d_{\mathrm{sp1}}\) = \(<\)1 mm; 9 mm\(>\), with 2 mm step. Specim No. 2 contained five flaws of varying lengths: \(l_{\mathrm{sp2}}\) = \(<\)10 mm; 30 mm\(>\) with 5 mm step. The geometry of all flaws are listed in Table 2.  
Figure 3: Spatial configuration of the specimen with presence of artificial flaws (**A**) and geometry of the flaw (**B**). Figure 2: Cross-section and geometry of SFECT probes: (**A**) air-core probe, (**B**) probe with ferrite core, (**C**) probe with ferrite core and aluminum shield. After performing each numerical simulation, the values of the magnetic induction in the whole volume of the receiving coil were extracted, then integrated and subsequently converted into the values of the voltage of the pick-up coil. These values from each simulation are used for further processing, based on the presented algorithm.