## 5 Conclusions  
In this paper, an urgent problem concerning the influence of the scale factor on the AE monitoring data of metallic materials was considered. * The change in primary parameters of the recorded AE signals depending on the size of the developing crack during tensile testing of steel specimens with various thicknesses was assessed. It was established that an increase in the thickness of the controlled object leads to a steady change in the primary parameters of acoustic emission data--a rise in the AE signals amplitude (\(u_{m}\)) and the AE hits rate (\(\dot{N}\)). The values of AE signals amplitude and AE hits rate recorded during the tension test of steel specimens with thickness \(t=1\) mm and \(t=6\) mm corresponded to the values (\(u_{m}=89\) dB; \(\dot{N}=27\) hits/s) and (\(u_{m}=100\) dB; \(\dot{N}=38\) hits/s) respectively. The greatest difference in the values of the primary AE parameters is observed at the initial stage of destruction--during a crack initiation in the stress concentrator (V-shape notch). We assume that this is due to the uneven crack development along the thickness in specimens of large thickness at the initial stage. In the case of thick samples, several cracks form at the initial stage of fracture, each of which is a separate AE source that increases the overall acoustic activity. * It was presented that the distribution parameters of the mean count frequency of AE signals depend on the specimen thickness. The parameter \(N_{p}/\tau_{p}\) can be used in the construction of analytical models for estimating the damage degree of the objects under study, which are invariant to the influence of the scale factor. For 30 KhGSA specimens, an empirical dependence of the quantile of the mean count frequency distribution of AE signals of level \(p=0.8\) on the specimen thickness \(t\) was obtained: \((N_{p}/\tau_{p})_{p=0.8}(t)=0.04\cdot t^{(-0.45)}+0.16\) (coefficient of determination \(R^{2}=0.98\)). * It has been shown that an increase in the specimen thickness led to a significant increment of the cumulative energy of the registered AE signals \(E_{\Sigma}(t)=-15.39\cdot t^{(-0.99)}+124.52\), (coefficient of determination \(R^{2}=0.96\)). * It has been assumed that the main factors affecting the influence of the scale effect are the overall increase in the deformable metal volume, as well as the more complex stress-strain state of the metal near the stress concentrator due to the transverse deformations present in specimens with large thicknesses. Using the DIC method, it was shown that with an increase in the specimen thickness, the strain value before failure rises, both the average strain before failure in the region near the V-shaped notch and the ultimate strain value at the fracture site. Since the plastically deformable metal is an AE source, the overall increase in its volume and the increase in the ultimate strain degree also explain the increased acoustic activity when testing thicker specimens. * The results obtained show that for a defined structural metal alloy there is an impact of the test object sizes on the AE parameters. This leads to the need to take into account the sizes of the control object, since this affects the classification of the danger degree of identified AE sources, as well as the values of the criteria for assessing the technical condition of the object. To develop a reliable method for assessing the state of the metallic object, invariant to the influence of the scale factor, the authors plan to conduct additional studies using artificial neural networks and regression analysis models. Conceptualization, A.M. and D.C.; methodology, A.M. and D.C.; experiment, A.M., D.C., E.R. and A.P. ([PERSON]); validation, A.M.; formal analysis, D.Z. and A.P. ([PERSON]); investigation, D.C.; data curation, A.P. ([PERSON]), E.K. and T.K. ; writing--original draft preparation, D.C. and A.M.; writing--review and editing, D.Z. and A.P. ([PERSON]); visualization, D.Z., A.P. ([PERSON]) and E.K. ; supervision, A.M.; project administration, A.M. and D.Z.