PSI - Issue 2_B

M.R. Tyutin et al. / Procedia Structural Integrity 2 (2016) 2764–2771 M.R. Tyutin/ Structural Integrity Procedia 00 (2016) 000–000

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3. Results 3.1. Estimation of the AE characteristics, intensity of the self-magnetic field and damage parameters of a smooth specimen Using the AE data obtained during tension of smooth specimens, we were able to separate the main stages of deformation with a high AE intensity Ṅ AE . The first AE peak corresponds to reaching macroscopic yield strength σ Y (Fig. 2a). These signals are related to dislocation motion and have a low AE amplitude, which is indicated by relatively high b AE -values (Fig. 2b). Upon further deformation of the specimen, the AE activity and b AE -value decrease sharply. The next AE peak is observed before the stage of localized deformation (Fig. 2a). This stage is also characterized by high b AE -values. Before the third AE peak, corresponding to the stage of localized deformation with necking, b AE – value decreases sharply (from 2,0 to 1,1), which is caused by an increase in the high-amplitude AE signals characterizing the microcrack propagation and coalescence, i.e., by a significant increase in the damage. Thus, an analysis of the b AE -value changes gives additional information on the fracture and signal emission kinetics, which can hardly be analyzed only from the activity of AE signals. Therefore, the conclusion regarding the nature of both deformation and damage accumulation processes at the third AE peak could only be drawn by analyzing the time dependence of b AE -value. Such an approach can be used to formalize and to substantially simplify the analysis of the amplitude distribution of AE signals. The revealed decrease of b AE -value is in agreement with the data obtained by Shiotani et al. (2003) for estimation of the functionality of cement materials. Carpinteri et al. (2009) showed that in building structures, as the load increases, b AE -value decreases from 1.5 at the stage of crack nucleation to 1.0 at the stage of crack development at the load of 0.8 of the ultimate load. Therefore, authors concluded that a decrease in this parameter indicates both the beginning of microcrack coalescence and the load level. Such a change in b AE -value before fracture is also observed for a metallic specimen (Fig. 2b). Thus, it is shown that b AE -value is sensitive to a change in fracture mechanisms and can be used as a forecasting criterion. The joint analysis of stress-time curve and time dependence of the self-magnetic field intensity (Fig. 2c) shows that specific points on the stress-time curve, corresponding to the strength characteristics, are consistent with the specific points on the time dependence of self-magnetic field intensity measured in the process of loading. Thus, point I in Fig. 2c corresponds to the proportionality limit of the specimen, and the point II – to the yield strength. The maximum value of the self-magnetic field intensity responds the stage preceding the localization of deformation and increasing in the AE activity (the second peak in Fig. 2a). At the stage of localized deformation of the specimen, the decrease in self-magnetic field intensity H is observed, which agrees with a decrease in b AE -value (Fig. 2b).

Fig. 2. (a) stress-time diagrams of smooth low-carbon steel specimen combined with the time dependencies of AE activity Ṅ AE ; (b) b AE -value; and (c) the resulting intensity of self-magnetic field H

The microcrack patterns observed on the smooth specimen surface were obtained at different stages of tension (points 1-3 on Fig. 2) using the replicas. The microcrack pattern on Fig. 3a (point 1 on Fig. 2) corresponds to stage of increase in the AE activity before second peak and to maximum intensity of self-magnetic field after which the decrease in H occurs. Point 2 on Fig. 2 corresponds to the ultimate stress and the stage of localized deformation of specimen. Damage pattern at this point is shown in Fig. 3b. Maximum damage was observed at point 3 before the