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

125

5

It can be noted that, under static tension, the behavior of the magnetic parameters determined in different gauge length sectors is the same; namely, as the tensile load increases, the parameters vary ambiguously, with the formation of extrema (minima are observed on the dependences of the coercive force, and maxima are observed on the dependences of residual induction and maximum magnetic permeability). The appearance of these extrema is explained, as in the case of the altered shape of the magnetic hysteresis loops in Fig. 3, by the features of the manifestation of the magnetoelastic effect in iron-based alloys. At the same time, it can be noted that the load dependences of the magnetic characteristics in minor cycles (Fig. 5) are in a qualitative agreement with the dependences obtained in the maximum applied field (Fig. 4), except for the load dependence of residual induction b r 0.05 , which has two extrema. The main differences in the magnetic characteristics determined with pickup coil location in different gauge length sectors of the specimen are observed at loads exceeding 25 kN. The difference in the magnetic characteristics is more clearly exemplified in Fig. 6, which demonstrates the effect of pickup coil location at different applied loads on the values of the coercive force obtained in minor magnetization reversal cycles in weak fields.

0.05 measured in minor cycling hysteresis (in weak fields) in different specimen sectors as a function of the distance from the specimen center at different applied loads: curve 1 – 0; 2 – 10; 3 – 20; 4 – 25; 5 – 30; 6 – 37; 7 – 37.5 kN.

Fig. 6. Coercive force h c

As can be seen from the experimental data shown in Fig. 6, at the initial stage of deformation (under loads not exceeding 25 kN), the specimen has a uniform distribution of magnetic characteristics through its length, in sectors 4–9 (the values of h c 0.05 remain practically insensitive to pickup coil location), this being indicative of a fairly uniform deformation process. As soon as a load of 25 kN is reached, a different pattern is observed. The values of h c 0.05 reach their maximum in sectors 4 and 9 and decrease as the specimen center is approached. This indicates that microdiscontinuities appear in the specimen center, thus decreasing the density of dislocations around them, which results in the relaxation of internal stresses near them, see in Nichipuruk et al. (1995). Thus, the analysis of the behavior of h c 0.05 has shown that a plastic strain localization zone is presumably formed in the specimen center corresponding to sectors 6 and 7. This conclusion is supported by the studies conducted with the use of an optical profilometer, which have shown that in sectors 6 and 7 the surface roughness values of the deformed specimen are higher than those of the undeformed part of the specimen and its gauge length outside sectors 6 and 7.

4. Conclusion

The analysis of the obtained dependences of the magnetic characteristics on the pickup coil location on the gauge length of a plastically deformed 09G2S low-carbon steel specimen has shown that a change in the magnetic characteristics occurs near microdiscontinuities. This shows the applicability of magnetic methods to the detection of potential fracture nuclei at the early stages of their formation and development in products made of ferromagnetic materials.