Monitoring of Fatigue Crack Growth in Metastable Fe-20 % Mn Steel by Nondestructive Evaluation Methods

Gorkunov, Eduard; Institute of Engineering Science, Ural Branch of the Russian Academy of Sciences; Russian Federation

Gladkovsky, S.V.; Institute of Engineering Science UB of RAS; Russia
Gorkunov, E.S.; Institute of Engineering Science UB of RAS; Russia
Kamantsev, I.S.; Institute of Engineering Science UB of RAS; Russia
Veselova, V.E.; Institute of Engineering Science UB of RAS; Russia

ID: ECNDT-0062-2018
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The work was aimed at the study of fatigue cracks growth process in dual (γ + ε) Fe-20% Mn metastable steel belonging to the class of TRIP-steels by non-destructive evaluation methods. Previously it was shown that plastic deformation induced γ→ε→α’ martensitic transformation in these steels have been developing during mechanical loading. Fracture mechanics cyclic tests of compact tension (CT) V-notched specimens with thickness of 2.6 mm were performed at room temperature using resonant testing machine. Monitoring of fatigue crack was carried out directly in the process of cyclic loading by non-destructive evaluation methods such as magnetic Barkhausen noise measurement, dynamic speckle interferometry and thermal imaging. The use of these methods was based on the evaluation of changes in physical properties of metastable Fe-20% Mn steel related to the formation of ferromagnetic α’-martensite in the local plastic zone ahead of crack, as well as on evolution of the of cyclically deformed specimens surface. As informative parameters when measuring the magnetic Barkhausen noise RMS voltage and the coercive force Hc were selected. Revealed significant changes in RMS and Hc magnetic parameters measured along the direction of growing fatigue crack are associated with the formation of α’-martensite in plastic zone. The usage of thermal imager makes it possible to observe markedly changes of temperature field on the specimens surface with fatigue crack growth after reaching 3.3×105 of loading cycles at the appropriate range of the stress intensity factor ΔK = 33 МРа×m1/2. Possibilities of fatigue crack monitoring under cyclic loading by non-destructive evaluation methods and establishing the relationship between stages of crack growth and the detected changes of Fe-20% Mn metastable steel physical properties in the local area of plastic deformation ahead of crack are highlighted.