Study of the fatigue failure process of steel specimens using the metal magnetic memory method

Dubov, Anatoly; Energodiagnostika Co. Ltd; Russian Federation

Dubov, An.A.; Energodiagnostika Co. Ltd; Russia
Kolokolnikov, S.M.; Energodiagnostika Co. Ltd; Russia

ID: ECNDT-0021-2018
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Session: Material Characterisation Metal Magnetic Memory
Room: R2
Date: 2018-06-12
Time: 09:20 - 09:40

Studies in the field of metals service life at low-cycle fatigue are a challenging issue in the residual life assessment. The goal of the paper is to investigate the effects associated with variations of the specimens’ self-magnetic stray field (SMSF) gradients that characterize their resistance to strain under conditions of cyclic loads application. Experimental studies are based on inextricable relationship between the dislocations density and the magnitude of SMSF intensity in local zones of the metal, which allows to monitor remotely, using the metal magnetic memory method, the dislocation processes kinetics and the staged accumulation of fatigue damages in the specimens metal with the increase of load cycles. As a result of the carried out studies, it was established that when the samples are subjected to both static and cyclic tensile loads not exceeding the stress proportional limit, variation of the specimen’s self-magnetic stray field is reversible. When the load amplitude is increased above the proportional limit, appearance of the plastic (relaxation) component, which increases with load increase, was recorded. It was also established that as the number of load cycles increases, the growth of the specimen’s metal resistivity increases. The metal resistance to strain will grow until the internal energy of the specimen becomes equal to the external effect energy. A point is identified on the fatigue curve, which can be considered as a point of stability loss corresponding to the limiting strain state of the specimen under given cyclic loading parameters, after which irreversible fatigue failure with a macrocrack formation occurs. The performed tests on cyclic tensile loading of specimens made of various grades of steels allow stating the possibility to estimate the equipment life and to monitor variation of its state by metal magnetic memory parameters in stress concentration zones.