###### Abstract
Subsequently, using theoretical and experimental values, they designed a model of the problem. The results of the experiment showed a short measurement time and a measurement error of less than 8% or 18%. The SFECT method is not only used for material thickness measurement, conductivity measurement, and crack detection but also for steel age investigation, as stated in Ref. [13] by [PERSON] et al. In the study, they did not use a classic surface coil as a detection coil, but a coil wound on a U-shaped core. In the work they were based on, they found that the advantage of the U-shaped coil is a constant magnetic induction flux between the poles of the core, which is more sensitive to changes in the microstructure, which is related to the age of the steel. They used coils from different windings for the measurement. The result of the work was that the coil with 100 turns is the most sensitive in the required frequency range. The age of the steel can be distinguished using the impedance measured at an excitation frequency of 3200 Hz. Carbide precipitates were the main factor in the change of electrical resistance in the aging stages. Using the SFECT technique, a good correlation between hardness and normalized impedance at the maximum frequency response was observed. In 2018, [PERSON] and [PERSON] published the design of a programmable SFECT module Ref. [14]. The module combined the function of an excitation signal generator as well as a recording device. The last area where the SFECT method is used is the detection of cracks in the material. In 2020, [PERSON] et al. published an article Ref. [15] in which they describe crack characterization by analyzing the probe signal using a fast algorithm. In the study, the dependence of the impedance of the coil on the depth of the crack was solved by numerical simulation. Then, using a deterministic algorithm and an inverse technique, they calculated the depth of the crack from the simulations. Our previous work focused on SFECT Refs. [16] and [17] and dealt with the possibility of refinement of the crack-response signal's resolution by a fixed probe. We used a new eddy-current probe of our design, containing galvanically isolated excitation and receiving electromagnetic system with different configurations investigated. As part of the design of the probe and verification of its properties, we first carried out numerical simulations to determine the perspective of three structural arrangements of the probe. Numerical simulations were performed for different static positions of the probes in the space around the defect. Defects used in numerical simulations were modeled from real laboratory samples of defects. These are samples with artificial electric discharge machine (EDM) notches. Their detailed description and description of the procedure can be found in the literature Ref. [16] and [17]. Both computational and experimental methods were used to investigate two specimens of conductive austenitic steel. The obtained results demonstrated that the geometry of the crack significantly influences differential response through its amplitude and phase, and it was also found that the geometry of the crack and the signal frequency response are correlated. Additionally, it was found that deeper cracks may still be discriminated from one another based on individual signals, even for frequency ranges in the hundreds of kHz range. The described SFECT method for inspecting conductive structures can be implemented in two different ways under real-world circumstances: in the first, it can be used after a prior study using the usual ECT method, and in the second, it can be used without a prior investigation. The measuring probe needs to be positioned precisely where problems are anticipated in this situation. This crucial knowledge provides a fresh perspective on how the SFECT with fixed probes might be used for the electromagnetic non-destructive evaluation of cracks. When using the SHM approach, there is often a situation where the network of sensors is placed over the investigated area.