Monitoring fatigue crack propagation of ferromagnetic materials with spontaneous abnormal magnetic signals

Monitoring fatigue crack propagation in industrial fields is very important. Crack can induce spontaneous abnormal magnetic signals in ferromagnetic material. During this research, tension–tension fatigue tests of center-cracked sheet specimens were carried out and the normal component of stray field signals, Hp(y), on the surface of specimens were measured at different fatigue cycles. The results showed that there was a spontaneous abnormal peak which existed on the notch before loading, and reversed after loading. The ΔHp(y) value, i.e. peak-to-peak value of Hp(y), increased with the fatigue crack propagation, and an approximate linear relationship was found between ΔHp(y) and crack length 2a. The cause for this phenomenon was discussed, and the possibility of using ΔHp(y) to predict fatigue crack propagation life was analyzed. This method is simple and reliable for monitoring fatigue crack propagation in ferromagnetic materials.