Nondestructive inspection of a fractured nickel bar by Barkhausen and magnetoacoustic emissions

Barkhausen emission (BE) and magnetoacoustic emission (MAE) measurements have been made on a nickel bar before and after it is tom apart by a tensile stress. The BE and MAE profiles obtained are all single-peaked. The peaks obtained after fracture are found at the lower field regions of the profiles, and this suggests that the field required for the domain wall motion in the fractured bar has been lowered. In the BE measurement, the signals show an increase of 44% (away from fracture end) to 144% (at fracture end);on the contrary, the MAE signals show a reduction of 43% (away from fracture end) to 14% (at fracture end). As the nickel bar is fractured, we find that in the broken pieces, the grains have been elongated in the stressed direction, and there is an increase of defects such as dislocations, voids and microcracks, especially near the fracture end of the bar. BE which is sensitive to these factors, has therefore been intensified. On the other hand, the plastically elongated grains, which have a lower absolute magnetostriction, enhance the reduction of MAE activity. The results measured from the nickel bar will be compared with those obtained from a fractured mild steel bar, which has different magnetostriction and other magnetic properties. (C) 1996 American Institute of Physics.