## 2 Materials and Methods
The study of the scale factor influence on the AE monitoring results was carried out by tensile testing to failure using an Instron 5982 testing machine (see Figure 3b) with an electromechanical drive to ensure the minimum level of acoustic noise. A pair of VS150-RIC quasi-resonant AE transducers (AET #1 and AET #2 in Figure 3a) with a built-in preamplifier with a gain of 34 dB and maximum sensitivity at a frequency of 150 kHz was used. The AETs used are highly sensitive in the frequency range from 120 to 450 kHz, which makes it possible to reduce the effect of mechanical and electromagnetic noises recorded at frequencies below 50 kHz and above 500 kHz, respectively. The transducers were installed on the specimen at a distance of about 130 mm of the lateral V-shaped notch and fastened with clamps, providing the necessary acoustic contact. Thus, a location zone with a size of \(B=260\) mm was formed, and in the subsequent analysis of the AE, sources coordinates the distance \(X\) was measured from the location of the upper acoustic emission transducer (AET #1). The thickness of the specimens was chosen as a variable dimensional (scale) parameter. Specimens with a nominal thickness of 1 to 6 mm (1 mm increment) were made. Other dimensions were the same for all specimens. To ensure that the properties and structure of the material under test were as close as possible to each other and did not change from specimen to specimen, all specimens were cut from the same 6 mm thick metal sheet. The required thickness of the specimen  
Figure 3: Scheme of a specimen from 30 KhGSA steel with a side notch for testing to assess the scale factor impact on the AE signal parameters (**a**) and photo of experiments using an AE system (**b**): 1—a specimen with a V-shaped notch; 2—quasi-resonant transducers VS150-RIC; 3—clamps. was provided by grinding. A total of 18 specimens were prepared--three specimens of each thickness. At the initial stage of experimental studies, the optimal parameters of the Vallen AMSY-6 measuring system for collecting and processing acoustic signals were determined. The AE signals threshold \(u_{th}\) was defined based on the condition \(u_{th}\geq u_{n}+6\) dB (\(u_{n}\) is the maximum noise signal amplitude) and equaled to \(u_{th}=36\) dB. The bandwidth of the digital filters was chosen to be \(\Delta f=95\)-\(850\) kHz. To construct a linear location of acoustic sources arising in the process of tensile testing before failure, the value of the propagation velocity of AE signals \(V_{g}\) was obtained. The calculation of the \(V_{g}\) parameter was carried out according to the results of preliminary tests, which consisted in simulating acoustic signals using a Hsu-Nielson source. The velocity value was \(V_{g}=3300\) m/s with the size of the location zone \(B=260\) mm. A series of experimental studies consisted of the registration of AE signals during tensile testing to failure were performed. The movable traverse speed was 2 mm/min during the tests. Synchronization of the AE diagnostics results with the size of a developing crack was done by the digital image correlation (DIC) method using the LaVision StrainMaster optical control system. The imaging system consisted of two Imager SX video cameras, a LaVision PTU synchronization device for simultaneous acquisition of images, and a personal computer with DaVis 8.4 software (LaVision GmbH, Gottingen, Germany). To increase the contrast of the image, a random pattern (speckle) was created on the surface of the specimens using white and black aerosol paints. During the test, images of the specimen surface in the notch area were recorded at a frequency of 2 fps. As an example, in Figure 4 photos of the specimen surface and strain fields during one of the experiments are shown. For an accurate assessment, this parameter was determined from the vector fields of displacements calculated when processing experimental images using the algorithm developed by the authors of the paper.