Influence of applied magnetic field strength and frequency response of pick-up coil on the magnetic barkhausen noise profile

The influence of applied magnetic field strength and frequency response of the pick-up coil on the shape of Magnetic Barkhausen Noise (MBN) profile have been studied. The low frequency MBN measurements have been carried out using 5 different MBN pick-up coils at two different ranges of applied magnetic field strengths on quenched and tempered (QT) and case-carburised and tempered (CT) 18CrNiMo7 steel bar samples. The MBN pick-up coils have been designed to obtain different frequency response such that the peak frequency response varies from similar to 4 kHz to similar to 32 kHz and the amplitude of low frequency signals decreases gradually. At lower applied magnetic field strength of +/- 14,000 A/m, all the pick-up coils produced a single peak MBN profile for both QT and CT sample. However, at higher applied magnetic field strength of +/- 22,000 A/m, the MBN profile showed two peaks for both QT and CT samples for pick-up coils with peak frequency response up to similar to 17 kHz. Also, there is systematic reduction in peak 2 for QT sample and asymmetric reduction in the heights of peak 1 and peak 2 for CT sample with increase in peak frequency response of the pick-up coils. The decreasing sensitivity of pick-up coils with increasing peak frequency response to MBN signal generation is indicated by the gradual reduction in width of MBN profile and height of peak 2 in the QT sample. The drastic reduction in peak 1 as compared to peak 2 in the CT sample shows the effect of decreasing low frequency response of the pick-up coils on lowering skin-depth of MBN signal detection. This study clearly suggests that it is essential to optimise both maximum applied magnetic field strength and frequency response of the MBN pick-up coil for maximising the shape of the MBN profile for appropriate correlation with the magnetisation process and hence the material properties. (C) 2013 Elsevier B.V. All rights reserved.