Measurement of stress in an elastically bent steel bar by magnetoacoustic emission

Magnetoacoustic emission (MAE) measurements are made on a 6-mm thick mild steel bar subjected to a slowly varying magnetic field with a small excitation field superimposed. After the signals are normalized to take the eddy current shielding into account, it is found that the MAE activity depends strongly on the frequency of the excitation field. When the frequency is below 30 Hz, the MAE signals increase linearly with frequency. Above this frequency, the signals increase with frequency as f^0.57. When the bar is bent, and the excitation field frequency is varied in order to control the field penetration, it is observed that the overall MAE signal on the tension side of the bar decreases but the fractional change of MAE signals increases with the field frequency and decreased with depth