PSI - Issue 40

A.M. Povolotskaya et al. / Procedia Structural Integrity 40 (2022) 359–364 A.M. Povolotskaya, A.N. Mushnikov / Structural Integrity Procedia 00 (2019) 000 – 000

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performed by means of the above-described Remagraph C-500 device. The internal magnetic field reached 500 A/cm. 3. Results and discussion Figure 2 presents the relative changes in the magnetic characteristics (coercive force, residual induction, and maximum magnetic permeability) of the specimens after plastic deformation by uniaxial tension ε. With an increase in plastic strain, a monotonic change in the magnetic characteristics is observed. At low plastic strains, at ε ranging from 0 to about 2 % there are a significant increase in the coercive force and a sharp drop in the values of residual induction and maximum magnetic permeability. The change in the magnetic parameters is more than 40 % from the initial value (without loading). The reason for the significant changes in the values of these magnetic parameters is that the removal of the load that caused plastic deformation indices, in a significant part of the grains, large residual compressive stresses oriented along the tension axis. These stresses cause magnetic anisotropy of the “easy magnetization plane” type, in which it is energetically more favorable for the spontaneous magnetization vectors to line up perpendicular to the tension axis, and, accordingly, to the magnetizing and switching fields. In this case the processes of magnetization reversal are hindered, the coercive force increases, and the values of residual induction and the maximum magnetic permeability decrease. With a further increase in plastic strain ε (in the range between about 2 % and 10 %), the growth of the coercive force slows down, and so does the decrease in the values of residual induction and maximum magnetic permeability. The monotonic change of the coercive force with increasing plastic strain in its entire range under study makes it possible to use this parameter to evaluate the strain state of structural components made of the 20GN steel.

Fig. 2. Relative variation of magnetic parameters (coercive force ( a ), residual induction ( b ), and maximum magnetic permeability ( c )), measured in a closed magnetic circuit, as dependent on the value of plastic strain ε .

Figure 3 shows the field dependences of the differential magnetic permeability µ d ( H ) for different levels of plastic strain. As the level of plastic strain increases, the amplitude of the peak observed in negative fields on the field dependences  d max of the differential magnetic permeability decreases, and its location shifts towards higher fields. At the same time, on the  d ( H ) curves for plastically loaded specimens at ε ≥ 1.96 % (curves 4 – 7 in Fig. 3), another peak begins to form in positive fields. With increasing ε, the peak becomes more distinct, and its location also shifts towards higher fields. On the major hysteresis loops, the characteristic features on the µ d ( H ) dependences observed in positive fields for plastically deformed specimens are manifested as bends in the range of fields from saturation to residual induction (Fig. 4). Note in addition that, with increasing plastic strain, the shape of the magnetic hysteresis loop becomes flatter.