Relationship between stress and magnetic signal of steel box girder based on magnetic memory method

Metal magnetic memory testing (MMMT) is a nondestructive testing technique that can detect early signs of damage in components. Many scholars have studied the effect of uniform stress on the self-leakage magnetic field (SLMF) strength of ferromagnetic materials. However, there is still insufficient research on the stress concentration caused by uneven stress, under the combined action of bending and shear. In this study, we studied the law of distribution of the magnetic signal, HP(y) and magnetic gradient, K, on a part of the surface of a steel box girder damaged by buckling, using a four-point bending pattern. The results showed that combining the three peak positions, namely, the magnetic signal, its newly added comparing initial magnetic signal, and magnetic gradient distribution curves, could accurately determine all stress concentration positions on the flange of the steel box girder, and reduce the interference of the initial complex residual magnetic field. Furthermore, we studied the quantitative relationship between normal stress, shear stress, and a magnetic signal. It was found that the stress-magnetic relationship conformed to the Jiles–Atherton model. The peak value of HP(y) – 𝜎, |HP(y)|a – 𝜎, and Kmax – 𝜎 relationship curves could be used as the characteristic point of the critical yield stress at the action of normal and shear stress. The degree of stress concentration of the flange and web could be quantitatively evaluated by fitting the Kmax – 𝜎 relationship. This study can provide an experimental reference for the application of MMMT in the evaluation and early warning of the state of damage to a steel box girder.