Theoretical studies of metal magnetic memory technique on magnetic flux leakage signals

Compared to traditional nondestructive testing methods, the technique of metal magnetic memory (MMM) is a more effective way in evaluating early damages of ferromagnets due to the existence of material stresses. In practical engineering, this technique has been extensively applied in different fields. However, very limited quantitative research has been carried out on quantitatively studying the relations between the stress state and self-magnetic flux leakage (SMFL) signal. In this paper, the distribution of SMFL is investigated based on the theory of magnetic charges. A linear description of the magnetic charge and the stress state is given. The theory can capture some basic characteristics of the SMFL distribution in the stress concentration of ferromagnets, e.g. the tangential SMFL component HP(x) reaching a maximum value and the normal component HP(y) changing positive–negative sign in the maximum stress concentration zone (SCZ). Moreover, the effects of the stress-concentration range and lift-off value on SMFL signals are discussed as well.