## 3 Results and Discussion
### Investigation of AE under Tension of Specimens with Dissimilar Welded Joints  
Figure 5 shows the data obtained under static tension of a defect-free specimen with a dissimilar welded joint. The dependence AE hits rate versus strain (Figure 5a) has two pronounced maxima corresponding to the stress of about 200 MPa and about 400 MPa. The results of the study of strains distribution on the specimen surface by the DIC method make it possible to clarify the presence of AE data peak at certain stages of the test. Figure 5b shows a diagram of a specimen with an indication of the study area with applied paint. The presence of the first AE peak (item 1 in Figure 5a) at stresses of about 200 MPa corresponds to the onset of plastic deformation of austenitic steel and (Figure 5c). With a further increase in stress to a level of about 400 MPa, the yield point of pearlitic steel is reached, as a result of which its plastic deformation occurs (Figure 5d), and a second peak is observed on the AE data pattern (item 2 in Figure 5a). After it, plastic deformation occurs in all zones of the welded joint, and this process is accompanied by a decrease in AE hits rate. In the interval when the yield point of 12 Kh18N10T austenitic steel has already been passed, and the yield point of 09G2S pearlitic steel has not yet been reached, an acoustic calm is observed, the AE hits rate significantly decreases. The period of acoustic calm is indicated in Figure 6.  
Specimens of combined welded joints with lack of penetration were tested by tension with the reinforcement bead removed. The destruction of the specimens occurred along the welded joint at a stress of 400-500 MPa. Figure 6 shows typical AE data obtained under testing of a specimen with a lack of penetration. The AE data observed during the destruction of samples with defects are generally similar to the data obtained for defect-free samples. According to the AE hits rate, two local maxima are observed, the first of which corresponds to the initiation of plastic deformation of 12 Kh18N10T steel, and the second corresponds to the destruction of a defective welded joint. To identify early predictors of fracture, the AE data were compared for defect-free and defective specimens during the period of acoustic calm before the final AE hits rate increased. The distribution function of time intervals between the adjacent hits reliably for defect-free specimen corresponds to the exponential distribution according to Pearson's goodness-of-fit criterion with \(\chi^{2}(12)=9.2\), \(p=0.06\) (Figure 7a). The character of the distribution of time intervals between adjacent AE hits, for a defective specimen, turned out to be different from exponential (\(\chi^{2}(14)=52.6\)\(p<0.001\)), due to an increase in the number of AE hits with a minimal time interval, which insignificantly affects the level of AE hits rate, but leads to disproportionality of the columns in the frequency histogram (Figure 7b). Figure 4: AE data recorded under tension of specimens made of 09G2S low-alloyed pearlitic steel. The AE data, which are recorded when testing specimens of dissimilar welded joints with diffusion interlayers (Figure 8), have some peculiarities compared to the data described earlier. As in the case of a defect-free specimen, the presence of two AE maxima (items 1 and 3 in Figure 8a) is associated with the onset of plastic deformation of the materials being welded, which is confirmed by the stress distribution shown in Figure 8b,d. Structural features associated with the presence of interlayers are manifested by the appearance of additional maximum of AE hits rate not associated with plastic deformation of the parent metals (item 2 in Figure 8a). The stress value, at which the appearance of an additional maximum is observed, approximately corresponds to 300 MPa (the ultimate tensile stress of the ferrite phase). The results of strain distributions confirm the intense deformation of a decarburized interlayer (see Figure 8c).