# Assessment of the Structural State of Dissimilar Welded Joints by the Acoustic Emission Method
## 3 Results  
It is advisable to start with considering patterns of AE data recorded under static loading (described in detail in [22]) to gain deep knowledge and understanding of AE data parameters under cyclic loading. Figure 4 shows the AE data obtained under static tension of a specimen without interlayers. There are two pronounced maxima: the first approximately corresponds to the yield strength of 12 Kh18N10T steel (stress is about 250-300 MPa); the second one is achieved at the stress level which corresponds to the yield stress of 09G2S steel (about 400 MPa). Figure 3: The design of the test specimen (**a**) and the experimental installation photo (**b**). Based on this information, tests with maximum cycle stresses \(\sigma_{\max}\) = 200, 300 and 400 MPa are of greatest interest. The stress of 200 MPa is close to the maximum allowable stress (only elastic deformation), and 300 and 400 MPa stress levels cause the start of plastic deformation. These values correspond to the yield strength of 12 Kh18N10T and 09G2S steels, respectively. Figure 5a,b show AE data obtained under cyclic loading of a dissimilar welded joint without diffusion interlayers at \(\sigma_{\max}\) = 200 MPa. Figure 5a shows AE hit amplitudes and AE hit rate, and Figure 5b reveals the result of AE event location. Since at \(\sigma_{\max}\) = 200 MPa, the deformation has been predominantly elastic, AE generation was not very active. No more than 1-2 AE hits were recorded per one loading cycle. The average activity was 0.05 AE hits per cycle, and the AE hit amplitudes did not exceed 65 dB. The location of AE events occurred non-intensively and almost evenly over the surface of the specimen. At the maximum stress \(\sigma_{\max}\) = 300 MPa (Figure 5c,d), AE hits were generated more actively, the average activity value was 0.2 AE hits per cycle, the overall amplitude level increased and high-amplitude AE hits with an amplitude of 70-75 dB appeared. AE event location was determined outside the welded joint in the base metal, 12 Kh18N10T steel. At the maximum stress \(\sigma_{\max}\) = 400 MPa (Figure 5e,f), the generation of AE hits was quite active, and the AE hit rate reached 6-8 hits per cycle, with AE events located mainly outside the welded joint in the base metal, 09G2S. To identify some failure predictors of a dissimilar welded joint, it is informative to represent AE data using a scatterplot. One axis plots the rise time of the AE hit, and the other axis shows its amplitude (Figure 6). The scatter with the value of \(\sigma_{\max}\) = 200 MPa in the absence of possible AE sources is a cloud of randomly scattered points; at \(\sigma_{\max}\) = 300 MPa corresponding to the plastic deformation of 12 Kh18H10T steel, an ordered arrangement of indications on the scatterplot is observed, and AE hits are grouped into horizontal chains (selected regions in Figure 6b). Groups of AE hits have different amplitudes, but their rise time values are close to each other. At \(\sigma_{\max}\) = 400 MPa, when plastic deformation of 09G2S steel occurred, many points are displayed on the scatterplot, and many horizontal oriented zones are formed. Figure 4: AE data recorded under tension of a specimen with a dissimilar welded joint of 12 Kh18N10T to 09G2S steels without diffusion interlayers. All three data arrays (stress—blue, AE hits rate—black, and AE amplitude—red) depend on the same argument (strain, %). Figure 5: AE data obtained during cyclic loading of a welded joint without diffusion interlayers. Location of AE events at \(\sigma\)max = 200 MPa (**a**), \(\sigma\)max = 300 MPa (**c**), \(\sigma\)max = 400 MPa (**e**); AE hit count rate values at \(\sigma\)max = 200 MPa (**b**), \(\sigma\)max = 300 MPa (**d**) and \(\sigma\)max = 400 MPa (**f**). Figure 6: Rise time vs. amplitude scatterplots for AE hits registered under cyclic loading of a defect-free welded joint at \(\sigma_{\text{max}}\) = 200 MPa (**a**), \(\sigma_{\text{max}}\) = 300 MPa (**b**) and \(\sigma_{\text{max}}\) = 400 MPa (**c**).