# Assessment of the Structural State of Dissimilar Welded Joints by the Acoustic Emission Method
## 3 Results
A cluster analysis was performed for a more detailed study of the AE data. AE signals were combined into clusters based on the waveform similarity. The cross-correlation coefficient was used as a distance measure. As a result of cluster analysis, it was found that the AE hits that were grouped into horizontal chains had a similar shape and are correlated with a cross-correlation coefficient \(r=0.65\)-\(0.8\). A representative cluster was formed with a specific signal waveform for the data obtained under cyclic loading with cycle amplitude of 300 MPa, as in Figure 6a. The quantity of AE hits in the cluster was approximately 10% of the total number of registered AE hits. The quantity of correlated signals increases with increasing load. The quantity of correlated signals at \(\sigma_{\text{max}}=400\) MPa increased to 20%. One of the typical AE signal shapes is shown in Figure 7b. AE data recorded when testing dissimilar welded joints specimens with diffusion interlayers had some peculiarities in comparison with the AE data described earlier. Figure 8 shows the data obtained during the tension test of a specimen with diffusion interlayers. An additional maximum was observed at the stress range of 250-300 MPa. It belongs to the ultimate tensile stress of the ferritic structure. Therefore, it can be assumed that the additional maximum of the AE data is an indicator of a decarbur  
Figure 8: AE data recorded under tension of a specimen of a dissimilar welded joint with diffusion interlayers. All three data arrays (stress—blue, AE hits rate—black, and AE amplitude—red) depend on the same argument (strain, %). Figure 7: Typical AE signal waveforms for defect-free specimen at \(\sigma_{\text{max}}=300\) MPa (**a**) and \(\sigma_{\text{max}}=400\) MPa (**b**). Figure 9 shows AE hit amplitude, AE hit rate and AE event location results when loading a dissimilar welded joint with diffusion interlayers with sinusoidal cyclic stress with maximum stresses \(\sigma_{\max}\) = 200, 300 and 400 MPa. The AE data obtained under cyclic loading of a specimen with carbide and decarburized interlayers differs significantly from the AE data recorded when loading a defect-free specimen (Figure 5). The difference is clearly demonstrated at low maximum stresses. Thus, at \(\sigma_{\max}\) = 200 MPa (Figure 9a), the AE hit rate sometimes exceeds 5 AE hits per cycle, and the average value is 0.95 AE hits per cycle, which is about 20 times higher than for a defect-free specimen. At the weld zone (Figure 9b), there is a cluster which suggests that the source of AE is the weld zone itself at a given loading level. At a maximum stress of 300 MPa (Figure 9c,d), the AE hit rate increases, the average value is 1.5 hits/cycle, the amplitudes of the AE hits reach 70-80 dB and the location of AE events is observed not only in the weld zone, but also in the base metal, 12 Kh18N10T steel. With a growth of the maximum stress to 400 MPa (Figure 9e,f), the level of AE hit rate decreases, and its average value is 1.1 AE hits per cycle; the location of AE events occurs mainly in the region of 09G2S steel. The difference between the AE hits parameters recorded during loading of a defect-free and a defective welded joint is clearly shown in the rise time vs. amplitude scatterplots (Figure 10). Representation of the data in the form of a scatterplot makes it possible to determine that the increase in AE hit rate occurs due to the appearance of a group of AE hits with similar rise times, which leads to a significant change in the form of the scatterplot. The scatterplot for a defect-free specimen at \(\sigma_{\text{max}}\) = 200 MPa (Figure 6a) displays a cloud of low-density, randomly scattered points. At the same time, a horizontally oriented cluster is distinguished in the lower part of the scattering diagram for a specimen with diffusion interlayers (Figure 10a). This region is formed by AE hits of different amplitudes, but with similar and low rise time values (marked with a red ellipse).