The dependence of magnetoacoustic emission on magnetic induction and specimen thickness

Magnetoacoustic emission (MAE) measurements have been made on mild steel bars of various thicknesses h, subjected to a slowly varying magnetic field. The MAE signal was found to decrease with increasing h. For smaller h, B as well as dB/dt are large, and the MAE signals are therefore strong. For larger h, the MAE signals are weaker because dB/dt is smaller. It is found that the peak height is proportional to h^-0.73±0.02. This result is in good agreement with predictions by two theoretical approaches, namely (i) calculations based on considering the magnetic circuit formed by the c-core magnet, the nonmagnetic spacer and the specimen in series, and (ii) calculation of the amount of magnetic field penetrating into the specimen. In this study, the lateral dispersion of magnetic induction penetrating inside the bar was also estimated. The result shows that B decreased in the lateral direction in the same manner as its dependence on h. The sensitivity of MAE to specimen thickness and width enables this technique to be used for size determination in cases where more direct methods are not applicable