The metal magnetic memory method application for online monitoring of damage development in steel pipes and welded joints specimens

The magnetic memory of metal (MMM) is an after-effect, which occurs in the form of metal residual magnetization in components and welded joints formed in the course of their fabrication and cooling in the weak magnetic field of the earth and in the form of irreversible change of components’ magnetization in zones of stress concentration and damages under working loads (ISO 24497–1:2007(E)). The results of experimental investigation by the metal magnetic memory of steel specimens in the process of their static and cyclic loading are presented. Investigation using the MMM method was carried out online in the process of cyclic loading in the synchronous manner with load frequency. Specific features of self-magnetic field’s (SMLF) intensity variation at maximum load during tensile cyclic load were revealed. The obtained results of specimens investigation experimentally confirmed the earlier drawn conclusion about the possibility of the MMM method and corresponding inspection instruments application for quick testing of mechanical properties and cyclic strength parameters at the physical level. Comparison of σ–ε deformation curves and σ–ΔHx, ΔHx–ε magnetograms, obtained in the static loading mode, of σ–ε cyclic loading profiles and corresponding magnetograms confirms once more the earlier established correlation of magnetomechanical parameters conditioned by the “magnetodislocation hysteresis”. Fatigue curves, plotted by variation of the self-magnetic field of the specimens in the process of cyclic loading, experimentally confirm the possibility of equipment life assessment by parameters of the metal’s magnetic memory. Magnetograms, recorded online in the process of cyclic loading of specimens, allow claiming the possibility of the MMM method and the Tester of Stress Concentration type instruments application for technical state monitoring during operation of equipment under load conditions.