PSI - Issue 28

L.R. Botvina et al. / Procedia Structural Integrity 28 (2020) 2118–2125 L.R. Botvina et al. / Structural Integrity Procedia 00 (2019) 000–000

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3. Results and discussion 3.1. Deformation dependences of acoustic emission characteristics

Figure 2 presents the dependences of acoustic emission parameters on relative deformation measured during the tension of specimens before (at N/N f = 0 ) and after preliminary сycling to various values of relative lifetime (at N/N f , equal to 0.3; 0.5; 0.7). An analysis of these dependencies made it possible to compare the behavior of AE parameters of steels studied and distinguish the stages of changes in AE characteristics that correlate with the stages of changes in mechanical properties and material damage. From the graphs, it follows that for both steels with an increase in the relative deformation, the accumulated number and energy of AE signals grow, but the intensity of signals decreases. The dependences of the measured characteristics on the relative strain ε*, defined as the ratio of the current strain to the fracture strain, show a certain stages, reflecting changes in strength properties during the fracture process and the development of damage. The initial I-th stage of elastic deformation before reaching the yield point is accompanied by high acoustic activity dN AE /dt and high values of the b AE -value, especially when testing the material in the initial state (Fig. 2, a, b). Its completion and transition to stages II and III lead to a decrease in acoustic activity and changes in the b AE parameter. The boundary between the second and third stages passes through the point of initial damage of material (“kink point” on deformation curve plotted in true coordinates, according to (Berezin et al., 2004) (indicated by an asterisk on the deformation curves). In the transition region from stage II and within stage III, the angular coefficient γ of the deformation dependence of the total number of AE signals also changes, which indicates the transition to the stage of localization of deformation before final fracture (Botvina et al., 2020). When stress close to the ultimate strength is reached, at the end of stage III, a kink appears on the deformation dependencies of Σ N AE , after which the values of Σ N AE remain almost constant until final fracture, and the rate of accumulation of acoustic events decreases sharply. It follows from the results that the deformation dependences of acoustic characteristics for both steels are qualitatively similar.

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