## 2 Materials and Methods
### Strengthening Technologies and Stress Device  
As the test object, welded joints were chosen, since they are the most vulnerable elements of structures operating under long-term cyclic loads. S-n curves of welded joints show differences in the impact that the welded joint zone has on the resistance to long-term cyclic loads and short-term failure loads. The service life of objects operating under low tension is limited by their welded joints. Therefore, it is necessary to monitor only the fracturing of those structural elements of the welded joint that affect service life.  
The proposed approach was tested on steel plate samples with welded joints. The plates were made of the ST3 PS steel (similar to steel A 414 Grade A) with the following dimensions: 150 \(\times\) 25 \(\times\) 4 mm. By means of semi-automatic welding, welded joints were made on both sides in the middle of the sample. The samples were divided into four groups and strengthened using three technologies: heat treatment [23; 24; 25; 26; 27], ultrasonic treatment [28; 29; 30], and structural hardening by means of beveling [31; 32].  
The first group of samples included plates that were not treated.  
In the second group, heat treatment (tempering) was applied to relieve residual stresses in welded joints. The operation was carried out in a SNOL 7.2/1100 muffle furnace by heating the samples at a rate of 10 \({}^{\circ}\)C per minute until the temperature reached 600 \({}^{\circ}\)C. This temperature was maintained for two hours, after which the samples were left to cool in the furnace. During high tempering, hardness parameters decrease and become homogeneous, while plasticity and impact strength increase, with residual stresses decreasing by 70% to 80%.  
In the third group, the welded joints on the samples were subjected to ultrasonic impact treatment (UIT) at a frequency of 20.1 kHz and a power of 200 watts.  
In the fourth group, beveling was performed on both sides of the plates. Beveling promotes stronger welds and reduces the welding time.  
\begin{table}
\begin{tabular}{c c c c} \hline
**AE Indicator** & **Micromodel** & **Nanomodel** & **Macromodel** \\ \hline \(X_{AE}\) (s\({}^{-1}\)) & dln\(\xi\)/d\(t\) & \(\gamma\dot{\varphi}/KT\) & - \\ \(Y_{AE}\) (Pa\({}^{-1}\)) & dln\(\xi\)/d\(\sigma\) & \(\gamma/KT\) & dln\(Nc/\)d\(\sigma\) * \\ \(W_{AE}\) & dln\(\xi\)/d\(Ks\)** & \(\omega=\gamma\sigma/KT\) & ln\(N_{B}\)-ln\(N_{cv}\)** \\ \hline \end{tabular}  
* \(N_{c}\), \(N_{B}\), and \(N_{cv}\) are s-n curve parameters of the material; **\(K_{s}\) is the stress factor (ratio of test stress to working stress).  
\end{table}
Table 1: Concentration and kinetic parameters of acoustic emission for assessing the quality of strengthening technologies [16].  
The samples were stressed using a Zwick/Roell Z100 universal testing machine, which creates uniform tension. Figure 1 shows the samples and the equipment that were used in the experiment. The results were processed using the approach described above.  
For registration of AE signals, a dual-channels acoustic emission system SDAE with 2 TAE P113c type was used. The extended frequency range of the registered signals was 20-1000 kHz. The lower level of the system discrimination threshold was determined by the need to eliminate electromagnetic interference and was 47 dB on each channel; the blocking time was 48,000 \(\upmu\)s, which made it possible to register microcracks larger than 100 \(\upmu\)m. A range of digital parameters characterizing the signals coming from the preamplifier was transmitted via the CAN2.OB interface in the computer. The scheme of running the experiment is shown in Figure 2.  
Figure 1: The process of conducting the experiment. (**a,b**) Equipment and acoustic emission transducers located on the sample and the process of acoustic emission recording. (**c**) A typical sample of the ST3 PS steel.