Measurement of Barkhausen emission and magnetoacoustic emission from a fractured steel bar

Barkhausen emission (BE) and magnetoacoustic emission (MAE) measurements have been made on a fractured steel bar. The MAE profile shows a large initial peak and a small final peak, whereas the BE profile contains an initial peak and a large central peak. These observation suggest that the process of domain nucleation involves the enlargement of residual domains that surround the defects, and is a result of the unpinning of the trapped domain walls. In the maximum field, the residual domains confined by the walls are not completely annihilated due to the strong demagnetizing field surrounding the defects, causing the final BE and MAE peaks to be small. Also, in the low field region of the BE profile, the large central peak indicates that the elongated grains have enhanced the motion of the 180° walls. After the bar has been annealed, we found that (i) the differences in magnitude between the initial and final peaks of both BE and MAE profiles are narrowed because of the fewer defects, and (ii) the overall BE and MAE signals are reduced as a result of the coarsening of ferrite and cementite grains