PSI - Issue 65

Yu.V. Khudorozhkova et al. / Procedia Structural Integrity 65 (2024) 121–126 Yu.V. Khudorozhkova, A.M. Povolotskaya / Structural Integrity Procedia 00 (2024) 000–000

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e.g., in Danilov et al. (2009). It is in plastic strain localization zones that material damage accumulates; they are stress concentrators and, consequently, potential fracture nuclei, as in Shveikin and Smirnov (1998). The currently available methods of nondestructive testing are aimed at identifying existing and developing fracture nuclei, whereas the most promising and important task of nondestructive testing is to create methods for detecting locations of pre-fracture at the early stages of fracture formation and development. Currently, magnetic methods are being increasingly used to solve this problem, see, e.g., in Zadvorkin, S.M., and Povolotskaya, A.M. (2022), Khudorozhkova et al. (2023). The application of these methods is based on relationships arising, particularly due to magnetostrictive and magnetoelastic effects, between the magnetic characteristics of the metal and residual stresses induced in it by plastic deformation. As is well known (see, e.g., in Vicena (1955), Bilger and Träuble (1965), Träuble (1966), Mikheev and Gorkunov (1993)), during plastic deformation of steels, a monotonic increase in the density of structural heterogeneities with growing strain results in an increase in the values of the coercive force of ferromagnetic materials and a decrease in their initial and maximum magnetic permeability. The occurrence of significant structural heterogeneities in individual volumes of the material during plastic deformation causes heterogeneity in the distribution of the magnetic characteristics of the product since the processes of magnetization and magnetic reversal become significantly more difficult at sites with increased density of microdefects, see in Gorkunov (2015). Thus, the heterogeneous distribution of the local magnetic characteristics of the test object may indicate the presence of plastic strain localization zones in this object, and this can be used to detect potential fracture nuclei in ferromagnetic products at the early stages of fracture nucleation and development. In this study, in order to test the possibility of detecting strain localization zones by magnetic methods, experiments are performed to determine the magnetic properties of plastically deformed low-carbon steel specimens.

2. Materials and research methods

Flat tensile specimens made of the 09G2S low-carbon steel were tested. The chemical composition (Table 1) was determined by means of a Spectromaxx device. A specimen outline is shown in Figure 1.

Table 1. The chemical composition of the 09G2S steel, wt% Element С Si Mn Cr

Ni

Cu

Al

Fe

Content

0.073

0.680

1.110

0.066

0.061

0.125

0.039

Base

Fig. 1. Specimen outline.

The initial gauge length of the specimen was 80 mm long. The cross-section of the gauge length was 20 × 4 mm. Preliminary mechanical testing revealed that the physical yield strength σ 0.2 of the test specimens was 350 MPa and that their ultimate tensile strength σ U was 500 MPa. The specimen was mentally divided into 12 sectors. The gauge length of the specimen was mentally divided into 8 sectors, each 10 mm wide. Static loading was performed in a Tinius Olsen Super L-60 hydraulic testing machine with the speed of the active gripper equal to 0.5 mm/min. As soon as a certain intermediate value of strain was reached, the loading was suspended, the magnetic characteristics of the specimen were measured without unloading, and the loading was resumed thereafter. The loading was stopped at 5, 10, 15, 20, 25, 30, 35, 36, 37, and 37.5 kN. The tests with the measurement of the main magnetic characteristics, with a monotonic increase in the tensile load on different sectors of the gauge surface, were