Monitoring yield failure of ferromagnetic materials with spontaneous abnormal magnetic signals

Yield failure is a common failure mode of low carbon steels in practical engineering application, so detecting it timely and accurately is of great significance. Tensile tests were carried out under different loads which covered tensile elastic loads up to plastic deformation. Meanwhile, the normal components of spontaneous abnormal magnetic field, the Hp(y) signals, were measured offline using the TSC-1M-4 type instrument. The correlation between the magnetic gradient K and the tension load F is discussed. The results show that the Hp(y) signals, which change gradually from negative to positive, approach a straight line and have a unique zero-crossing point in the elastic stage, and the magnetic gradient K fluctuates with the load increasing. However, the distribution curve of the Hp(y) signals has a peak and a trough in the yield stage, and the Lüders bands appear and expand as strain increases. The phenomenon of magnetic polarity reversal appears so that the magnetic gradient K changes from positive to negative, thus the mechanical state of the specimens can be judged from the magnetic gradient K.