PSI - Issue 28

M.R. Tyutin et al. / Procedia Structural Integrity 28 (2020) 2148–2156 TyutinM.R./ Structural Integrity Procedia 00 (2020) 000–000

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Fig.4. Microcrack patterns in specimens from low-carbon (а, ε*=0,92), medium-carbon (b, ε*=0,83), austenitic stainless (c, ε*=0,94) and bainitic (d, ε*=0,83) steels.

3.4. The interrelation between damage and magnetic characteristic The approach used in this work allowed us to identify the main stages of the fracture process associated with the movement of the dislocations, the formation, accumulation of microcracks, and their coalescence before fracture. Results obtained were confirmed by the analysis of microcrack patterns. To establish the relationship between damage and magnetic parameters, we have identified a stage at which these parameters change monotonously. It was shown (Fig.2, 3), that only the coercive force grows monotonously at the stage of damage growth. Therefore, the dependencies of the coercive force on the damage were plotted (Fig.5b). As can be seen from Fig.5b, the coercive force increases with increasing damage. The most intense growth is observed at testing of the stainless-steel specimen (Fig.5a), however, the dependence is complex, which may be associated with the deformation induced martensite formation and the corresponding change in the magnetic characteristics. A nonlinear relationship between damage and physical characteristics is also shown in [32], which presents the results of assessing damage in austenitic steel under cyclic loading. It was shown that the damage characteristic proposed by the authors increases with an increase in the volume of the martensite phase and decreases with an increase in damage. This behavior makes it difficult to analyze damage in austenitic steels using magnetic non-destructive methods. For the ferrite-pearlite steels specimens, the obtained dependences are close to linear. The dependence for bainitic steel was described by a polynomial of the second degree: Low-carbon steel: H C =5,0+0,06 ∙ S * Medium-carbon steel: H C =6,9+0,16 ∙ S * Bainitic steel: H C =14,4-0,17∙S* + 0,01 ∙ S * 2 As can be seen from Fig 4b, the highest rate of increase in the coercive force with damage is observed for bainitic steel specimens as compared to low- and medium-carbon steel specimens.