## 4 Conclusions  
The main conditions that are required to be confirmed for the verification of the special SPMs according to the requirements of ISO 17025 are presented, and the main shortcomings complicating the preparation of reliable testing, inspection, and monitoring standards for WTBs certification are demonstrated.  
Lamb waves were proved to be one of the most promising tools for failure prevention and safety assurance of WTBs and an effective tool for environmental sustainability by  
Figure 8: The example of expanded uncertainty of reconstructed frequency range for phase velocity mode.  
using signals of Lamb waves. However, the limitations of Lamb waves, such as a dispersion phenomenon and an infinite number of modes, are identified as great obstacles for the application of these waves. The limitations of the Lamb waves approach are related to defining the necessary minimum number of transducers to perform SHM of the WTB structure and the minimal number of spatial measurement points for NDT as well. Therefore, future work will be dedicated to the related research.  
Moreover, two different tests that use simulated and experimental parameters must be performed to verify the accuracy of the SPM. In both cases, the results obtained must be compared with the values calculated by the reference method. When reproducing the sections of the dispersion curve, a complex analysis of the uncertainty components must be performed. For this purpose, the minimum scanning step is selected, and if necessary, the material properties of the object are identified, the average of the measurements at each point is obtained, and constant environmental conditions are ensured. This allows obtaining a sufficient amount of data for reliable estimation of the dispersion and uncertainty of the results. While analysing the metrological parameters of the method, the goal is to achieve the value of the uncertainty adequate to the required measurement accuracy. The measurement accuracy is considered an important factor in developing national and international standards for the prevention of WTB failure in the future. The general principles should expand the understanding of the measurement reliability, peculiarities of method validation, and verification. Such information facilitates the application and utilisation of the proposed methods and their choice for completing a given task  
Conceptualization, L.D.; methodology, L.D., A.M., R.R., and P.G.; software, E.Z.; validation, R.R., and L.D.; formal analysis, L.D.; investigation, L.D., R.R., and E.Z.; resources, L.D., A.M., R.R., and P.G.; data curation, R.R.; writing--original draft preparation, L.D.; writing--review and editing, L.D., A.M., R.R., P.G., and Z.S.; visualization, E.Z., R.R.; supervision, L.D. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Research Foundation of the Research Council of Lithuania under the project UNITE 'Ultrasonic tomography for the spatial reconstruction of material properties from a limited number of positions for NDT and monitoring applications', No. MIP2048.  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest.