## 2 Materials and Methods  
In order to study the impact of the scale factor on the results of diagnostics of metallic materials by the AE method, several experiments were carried out including tensile testing. The experience of previous research performed by the authors [22; 23; 24] on AE diagnostics of structural carbon and alloyed steels shows that a two-phase ferrite-pearlite structure can lead to a high level of \"structural noise\" associated with micro deformation of the ferrite phase, as well as the destruction of the cementite network during the material deformation. Moreover, non-metallic inclusions (sulfides, oxides, silicates, etc.) create additional noise sources. This becomes a reason for distortion of AE signals recorded during the deformation and destruction of the investigated material [25]. A high-quality alloyed steel 30 KhGSA after annealing was investigated in the current study. The chemical composition of the steel (see Table 1) was determined using the LAES Matrix atomic emission spectrometer and the Metavak-CS sulfur/carbon analyzer. The 30 KhGSA is considered to be high-quality steel due to a reduced content of harmful impurities and non-metallic inclusions. The structure of this steel in the annealed state should consist of alloyed ferrite and finely dispersed carbides located along the body of the grain and grain boundaries. Presumably, such a structure should provide a lower level of \"structure noise\", which in turn will contribute to more reliable results of the planned research tests. To analyze the actual structural and mechanical state of the metal under study (steel 30 KhGSA, sheet 6 mm), a metallographic analysis was carried out by optical microscopy and tensile testing. Flat specimens were prepared in advance to determine the mechanical properties (see Figure 1a). These specimens were made and tested in accordance with the Russian state standard GOST 1497-84 (ISO 6892-1:2019). The width was 20 mm, the gauge length was increased to 270 mm. The tension tests were performed on an Instron 5982 testing machine with a movable traverse speed of 2 mm/min using 6 specimens. The results are presented in Table 2; as an example, one of the tensile diagrams is shown in Figure 1c. Blanks for microsections were cut out from the 30 KhGSA sheet to study the microstructure. The steel structure is ferrite with finely dispersed carbides (Figure 2). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Element** & C & Cr & Mn & Si & Ni & Cu & S & P \\ \hline
**Content** & 0.29 & 1.0 & 1.1 & 0.9 & \textless{}0.1 & \textless{}0.1 & 0.003 & 0.008 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of the 30 KhGSA steel, %wt. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & **Yield Stress \(\sigma_{y}\),** & **Ultimate** & **Uniform** & **Clongation \(\delta_{u}\),** & **Total** \\  & **MPa** & **\(\sigma_{u}\), MPa** & **\%** & **\%** & **Elongation \(\delta\), \%** \\ \hline
**Value range** & 388–399 & 584–608 & 10.5–12.7 & 11.0–15.4 \\
**Average value** & 391 & 599 & 11.5 & 13.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mechanical properties of the 30 KhGSA steel (results of 6 specimens test). Figure 1: Scheme (**a**) and photo (**b**) of a specimen for tensile test for mechanical properties evaluation and stress-strain curve (**c**) of a specimen from the 30 KhGSA steel. Figure 2: 30 KhGSA steel microstructure: (**a**)—200\(\times\) magnification; (**b**)—1000\(\times\) magnification. Specimens with a V-shaped side notch (concentrator) were made to conduct experiments and study the influence of the scale factor on the AE diagnostics results. The geometry of prepared specimens is shown in Figure 3a. The presence of a side notch makes it possible to unambiguously determine the location of AE sources during the deformation and destruction of the tested material by constructing a coordinate location.