Influence of applied magnetic field strength and frequency response of pick-up coil on the magnetic barkhausen noise profile M. Vashista a,b , V. Moorthy a,n a Design Unit, School of Mechanical and Systems Engineering, Newcastle University, UK b Department of Mechanical Engineering, Indian Institute of Technology (B.H.U.), India article info Article history: Received 12 March 2013 Received in revised form 4 June 2013 Available online 28 June 2013 Keywords: Magnetic Barkhausen noise Frequency spectrum Gear steel Carburised steel Tempered steel abstract 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 ∼4 kHz to ∼32 kHz and the amplitude of low frequency signals decreases gradually. At lower applied magnetic field strength of 714,000 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 722,000 A/m, the MBN profile showed two peaks for both QT and CT samples for pick- up coils with peak frequency response up to ∼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. & 2013 Elsevier B.V. All rights reserved. 1. Introduction: When a ferromagnetic material is subjected to an external varying magnetic field, a voltage signal is induced in a pick-up coil due to changes in magnetisation of material caused by the discrete movement of magnetic domain walls overcoming various pinning sites in the material [1]. This phenomenon of electromagnetic activity known as Magnetic Barkhausen noise (MBN) signal genera- tion can have a wide frequency spectrum depending on the rate of change of magnetic flux at the micro and macro level of the magnetisation process inside the ferromagnetic material. The frequency of discrete magnetisation changes is believed to range starting from an order of the excitation frequency of the applied magnetic field and spreads beyond 1 MHz in most ferromagnetic materials. Since the MBN signals are the voltage pulses induced by the changing discrete magnetic flux on to a pick-up coil placed on the surface of a test material, it is believed that the detected MBN signal will reflect more of the characteristics of the pick-up coil than that of the actual rate of change of the micro-magnetisation process inside the material which is an unknown factor. However, the strength of induced voltage pulses is expected to decay exponentially as a function of depth under homogeneous excita- tion of magnetic field due to eddy current damping experienced by the propagating electromagnetic fields created by the movement of magnetic domain walls. The extent of damping determines the detection-depth (skin-depth) of MBN signals. The main factors affecting the skin-depth of electromagnetic signals are frequency of the signal, conductivity and permeability of the material [1–3]. Jiles and Suominen [3] while working on the assessment of micro- hardness and residual stress observed that the skin-depth of MBN at the same analysing frequency decreases for materials having higher specific electric conductivity and relative permeability. The determination of skin-depth of MBN signals is more complicated than that of eddy current signals due to the genera- tion of MBN signals in a wide frequency range. It has also been observed that the detection depth of MBN signal also depends on Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2013 Elsevier B.V. All rights reserved. 10.1016/j.jmmm.2013.06.038 n Corresponding author. Tel.: +0044 191 222 6158. E-mail addresses: v.moorthy@ncl.ac.uk, vmoorthy65@gmail.com (V. Moorthy). Journal of Magnetism and Magnetic Materials 345 (2013) 208–214