Magnetic memory signals of ferromagnetic weldment induced by dynamic bending load

Metal magnetic memory testing is a novel method for evaluating the fatigue damage of ferromagnetic weldments. To promote a further understanding of the impact of fatigue loads on magnetic memory signals of weldments, the normal component of magnetic signals, Hp(y), on the centre-notched welding specimens were measured during dynamic bending fatigue tests. The variation of peaks–troughs differential value ΔHp(y) of magnetic signals and its maximum gradient Kmax at the welded joint were also investigated. Results indicated that the ΔHp(y) on scanning line near the notch increased sharply when stress concentration and crack initiation appeared in the specimens. The microstructures on scanning lines were also examined. It shows that there was an increase in intragranular slips with the increase in loading cycles, and the change in crystallographic textures and movement of dislocations contributed to the magnetic signals varying significantly. The maximum gradient Kmax was found to increase exponentially with the increase in crack length, which is well coincident with the analytic result of the magnetic dipole model. The research provides a potential possibility of quantitative evaluation on stress concentration and crack in weldments under dynamic bending loads.