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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mechanical properties of the material, as well as with the characteristics of non-destructive testing. Thus, the assessment of the stages of the fracture process and the associated stages of changes in mechanical and physical properties becomes the central problem of diagnostics and ensuring the reliability of structures. Many studies have been devoted to this problem. Thus, in the works (Akrabi and Ahmadi, 2010; Ciuplys et al., 2006), carried out on low - carbon steels under tension, the deformation process under static loading is divided into three stages, namely, the microyielding stage (from the beginning of the loading to the appearance of the first sliding lines), the second stage of yielding, and the third stage of deformation hardening. Moreover, the main source of emission at the first stage of the plastic deformation process is considered grain boundaries, at the stage of yielding - the interaction of carbon and nitrogen atoms and the movement of the Lüders bands. The authors connect the decrease in the AE counts at the deformation hardening stage with the reduction of dislocation activity. The effect of microcracks is not considered, although it is noted (Akrabi and Ahmadi, 2010) that the total number of AE signals in notched specimens is less than in smooth ones due to the smaller volume of deformed material, and the total number of AE signals is power-dependent on the fracture toughness of steel. The authors of the study (Murav’ev and Zuev, 2008) identified four stages of changes in the AE activity during the tension of low-carbon steel specimens, or four AE localization stages: elastic stage (I), yield plateau (II), silence zone (III), and stage of parabolic strain hardening, and thus showed the effectiveness of using acoustic emission to identify stages of plastic deformation. A detailed analysis of the stages of changes in the AE parameters in structural materials with different structures was carried out in (Bashkov and Semashko, 2004; Dmitriev et al., 2017; Skalsky and Lyasota, 2014), the stages associated with the achievement of the micro - and macro-yielding limits, the strength limit, as well as transition stages are highlighted. In (Berezin et al., 2004), the interconnection between changes in AE characteristics and the achievement of a knee point on the deformation curve plotted in true coordinates and corresponding to the beginning of fracture localization was studied. Despite numerous assessments of the acoustic emission characteristics of structural materials, the influence of pre cyclic loading on the stages of changing AE parameters is reflected in a few works, including (Skalsky and Lyasota, 2014; Han and Oh, 2006). To fill this gap, a series of studies was conducted to assess the effect of pre-cyclic loading on the mechanical and physical properties of several structural steels widely used in industry (Botvina et al., 2018, 2019, 2020). The purpose of this work was to compare the acoustic properties of low-carbon steel and 15Cr2MnMoV after pre cyclic loading.

Nomenclature dN AE /dt intensity of acoustic emission signals Σ N AE

cumulated number of acoustic emission events cumulative energy of acoustic emission signals

Σ E AE

slope coefficient of the amplitude distributions of the acoustic emission signals relative strain equal to the ratio of the current strain to the fracture strain

b AE

ε * S*

relative area of the damaged surface

relative number of cycles

N/N f

slope coefficient of deformation dependence of Σ N AE

γ

2. Materials and methods The investigated materials were 15Cr2MnMoV steel, used as a material for oil-well sucker rods, and low-carbon steel, which is widespread in industry. The chemical composition and standard mechanical properties of the steels under study are presented in Table 1. The structure of 15Cr2MnMoV steel consists of bainite with highly dispersed carbides along the boundaries of the former austenite grains and along the bainite laths, non-metallic inclusions of manganese sulfide and oxysulfides (Botvina et al., 2020). Low-carbon steel has a ferrite-pearlite structure.