**w/o applied filter with applied filter**

0 dB

### **Figure 15.** *Cont*.

193

10 dB

**Figure 15.** Comparison of the results for the 10% flaw depth for different excitation pulse powers. Results are shown with and without filtering.

The results obtained for a pulse with a reference power (0 dB) show a significant amount of noise. Increasing the pulse power improves the signal-to-noise ratio (SNR) and enables more accurate visualization, defect detection and identification. The use of filters allows you to significantly improve the SNR. The Table 3 below shows a comparison of the SNR values for different pulse powers.

**Table 3.** SNR (dB) values for different pulse powers with and without filtering for defect depth 10%.


The results of the experiments show that the use of the PMFES-ECT method with increased pulse power and the use of filtering significantly increases the SNR value and the possibility of detecting deeply lying defects.

In the case of a 10% defect, the gain for PMFES-ECT was as high as 5 dB compared to the MFES-ECT method.

### **6. Conclusions**

This article presents the novel pulsed multi-frequency eddy current method for nondestructive testing (PMFES-ECT). The method was initially verified by the computer simulations and then experimentally validated.

The proposed measurement method can be utilized for the inspection of conducting materials with different thicknesses. It combines the advantages of the multifrequency and pulse ECT methods. A rich frequency spectrum enables accurate and detailed identification of flaws, and the pulsed operating principle increases the range and detection of deeply located flaws. Moreover, it also reduces energy consumption, which is essential in portable systems.

The spectrogram and peak frequency enable the determination of the depth of the defect and location in examined material, while the maximum amplitude of the spectrogram is correlated with the defect size.

In future work, this method could be used together with artificial intelligence methods to create an autonomous, portable system for detecting and identifying defects.

**Author Contributions:** Idea and concept of the PMFES-ECT method, T.C.; implementation of the method, J.M.G.; software preparation, J.M.G.; resources—transducer, samples and system elements preparation, T.C.; measurement, J.M.G.; data curation, J.M.G.; writing—original draft preparation, J.M.G.; writing—correction and extensive editing, T.C. and J.M.G.; visualization, J.M.G.; supervision, T.C.; All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to a complicated structure that requires additional explanations.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
