PSI - Issue 23

M.R. Tyutin et al. / Procedia Structural Integrity 23 (2019) 559–564 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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According to the measurement results, dependenci es of the estimated characteristics on the relative deformation ε* were plotted. The relative defor mation ε* was defined as the ratio of the current strain to the strain at specimen failure. The damage analysis was performed on the basis of the Kachanov-Rabotnov kinetic concept (Kachanov 1974; Rabotnov 1966): = В ( (1− ) ) , (1) where ω – is the damage parameter, as which we took S *, B – constant, m – exponent. Figures 1a, c shows the stress-strain diagrams of studied steels, the dependences of the acoustic emission activity ( Ṅ AE ), the b AE -value and the intensity of the self-magnetic field ( H r ) on the relative strain ε *. Figures 1b, d shows the stress-strain diagrams, the magnetic field strength H EC assessed by eddy current method, and the coercive force H C on ε* . The dependence of the percentage of the martensitic phase (fig. 1d) resulting from deformation-induced martensitic transformation in unstable austenitic steel is also presented on the relative deformation ε* . The achievement of yield stress and ultimate strength with an increase in the load cause corresponding changes in the magnetic and acoustic parameters. As can be seen from Fig. 1a, c, the dependencies of the AE activity of the steels under study are different both in the shape and in the values of the measured parameter. At testing the low-carbon steel specimens (fig. 1a), the AE activity maximum occurs at the beginning of the plastic flow, which is connected, as is known (Krasovskii' et.al. 1976), with the dislocation movement and is accompanied by a sharp change in the self magnetic field intensity H r . At the stage of strain hardening, the activity of AE signals practically does not change, while the parameter H r increases and reaches a maximum value at the stage of nucleation and accumulation of microcracks at ε* = 0.3. Figure 1b shows that at the strain hardening stage the magnetic field intensity H EC , estimated by the eddy current method, and the coercive force H C increases. At ε* = 0.3, maximum is observed on the deformation dependence of H EC (Fig. 1b). The next peak of AE activity is associated with the development of multiple fracture before reaching the ultimate strength. It is accompanied by a local drop in the b AE -value at ε* = 0.45. The change in the H EC and H C values in this deformation range ( ε* = 0.3 – 0.45) is slowed down (Fig. 1b). With further deformation, a significant increase in damage is observed, while the parameters of acoustic emission and the self-magnetic field intensity change only slightly. The coercive force increases monotonically, and the eddy current parameter decreases. Before the final fracture of the specimen, there is a drop in the b AE -value and H r (fig. 1, a). At testing stainless steel specimens, AE activity is less localized (fig. 1c). Unlike steel 20, the AE activity peak at the yield point is not observed, but self-magnetic field intensity H r changes dramatically at this stage like changing this parameter during deformation of low-carbon steel specimens (fig. 1, a). Maximum AE activity and H r value corresponds to the strain hardening stage at ε* ~ 0.2. It should be noted that in the initial state, stainless steel specimens are non-magnetic, therefore the characteristics of H EC and H C practically do not change at this stage (fig. 1d). When loading specimen, deformation-induced martensitic transformation takes place, and the magnetic properties of this steel appear, as can be seen from the graph on fig. 1d. The local drop in the b AE -value corresponds to a deformation of ε* = 0.2 – 0.3. When deformation of ε* = 0.5 is reached, a sharp increase in the coercive force and martensite content is observed. It should be noted that the growth of the eddy current parameter begins earlier – at ε* = 0.3. When the ultimate strength is reached, the values of b AE , H r , H C sharply decrease (fig. 1c, d). Analysis of the b AE -values determined by processing the entire set of AE amplitudes for each steel showed that this parameter has similar values ( b AE = 1.41 for low-carbon steel and b AE = 1.40 for stainless steel) and coincide with previously obtained results ( b AE = 1.4 ± 0.15) when testing low -carbon steel specimens with a notch (Botvina et al. 2013). It follows that, in this case, the influence of the notch on b AE -value is not significant. In the work (Botvina et al. 2017), we showed that in a more durable medium carbon steel, the b AE -value is significantly lower ( b AE = 1.13). The beginning of the intensive process of microcracks nucleation and coalescence before fracture can be determined by the change in b AE -value with the deformation. Sharp decrease in b AE precedes the stage of strain 3. Results