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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 uniaxial stress on the self-magnetic-leakage field (SMLF)’s strength. Nevertheless, there is still insufficient research on the combined action of bending and shear. We studied the law of distribution of the magnetic signal, ΔHSF(y), at different stress parts of a steel box girder and the quantitative relationship between the magnetic characteristic parameters and the external load. The results showed that the MMMT could accurately detect the early stress concentration zone (SCZ) and predict the final buckling zone of steel box girders. It could be judged that the corresponding parts of the steel box girder had entered the elastic-plastic working stage by the reverse change of the  ΔHSF(y)-F and |HSF(y)|a -F curve trends, this feature could be used as an early warning sign before the steel box girder was deformed or destroyed. The fitted |HSF(y)|ave -F linear expression could be used as the expression between the magnetic signal and the shear capacity. All the evaluation methods were expected to provide a basis for effectively evaluating the stress state of steel box girders with the MMMT method.

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It is valuable to conduct non-destructive testing of steel box girders in order to evaluate their working status. The metal magnetic memory inspection method can effectively identify early damage and the stress state of ferromagnetic materials. However, applying this technique in the inspection of steel components is difficult due to insufficient research on magnetic memory signals under complex stress states. This study analyses the magnetic memory effect for a steel box girder under four-point bending. It is shown that the normal component Hp(y) of the magnetic signal can effectively locate the stress concentration area. The average absolute value Hm of Hp(y) can identify the yielding state and predict the occurrence of failure. Hm changes roughly quadratically with the average strain ɛm in the elastic stage of the specimen, which is consistent with the theoretical result. The ratio D of Hm to the equivalent stress σeqv changes continuously with the applied load, which can be used to estimate the stress state of the steel box girder.