Materials Science and Engineering A 386 (2004) 74–80 Effect of deformation in bending on magnetic Barkhausen noise in low alloy steel M. Blaow, J.T Evans ∗ , B. Shaw DesignUnit,SchoolofMechanicalandSystemsEngineering,NewcastleUniversity,NewcastleuponTyne,NE17RU,UK Received 5 January 2004; received in revised form 1 July 2004 Abstract The influence of bending deformation on magnetic Barkhausen noise (MBN) was studied in low alloy steel. The peak height of the rectified MBN profile was observed to increase systematically with increasing tensile strain until the onset of plastic deformation. Increasing compressive strain initially produced a reduction in peak height, but more complex behaviour was observed at larger values. In particular, broadened profiles were produced under compression, which may indicate the presence of two or more overlapping peaks. As confirmed by X-ray diffraction measurements, one effect of plastic deformation is to leave a residual compressive stress on the tension side of the specimen and a residual tensile stress on the compression side. Measurements of residual stress were correlated with MBN peak height, to the extent that both quantities show functional forms similar to the theoretical relationship between residual stress and the bending moment applied prior to unloading. © 2004 Elsevier B.V. All rights reserved. Keywords: Magnetic Barkhausen noise; Bending deformation; Residual stress 1. Introduction Techniques based on the phenomenon of Barkhausen noise are potentially useful for non-destructive evaluation of ferromagnetic materials. For instance, residual tensile stress, microstructural deterioration and composition gradients can be detected in ground-finished surfaces using measurements of magnetic Barkhausen noise (MBN) [1,2]. Because of the large number of influential variables, the technique produces only relative comparisons between different material states. For any alloy, measurements have to be calibrated against a standard condition for that particular alloy. It was the aim of the present work to investigate the effect of deformation on MBN emission in quenched and tempered low alloy steel with a well-defined microstructure. Barkhausen noise is produced by the irreversible move- ment of domain walls in a magnetisation cycle. Domain walls are pinned temporarily by inhomogeneities and then released ∗ Corresponding author. Tel.: +44 191 222 7911; fax: +44 191 222 8600. E-mailaddress: j.t.evans@ncl.ac.uk (J.T Evans). in the increasing magnetic field. The discrete changes in mag- netisation can be detected as voltage pulses in a search coil or magnetic read head. Precipitation and dislocations act as effective barriers to domain wall motion so that MBN is sen- sitive to microstructure and plastic deformation in the mate- rial. Because of the magnetostrictive interaction, long-range stresses (applied or residual) also affect the intensity of the MBN emission. The effect of applied stress on MBN emission has been presented in a number of studies (e.g. in [3–8]). However, stress and microstructure interact to affect MBN. Accord- ingly, it is important to consider the effect of applied or resid- ual stress on MBN across a range of microstructures and alloy types. Lindgren and Lepisto [6] found that the MBN intensity–stress curve of mild steel saturated at low tensile stress values (approximately 100 MPa). Hyde et al. [9] pro- duced calibration curves for a range of case hardened steels, which showed increasing root mean square MBN output with increasing tensile stress. However, the sensitivity of MBN emission to applied stress was found to vary significantly with the composition and heat treatment of the alloy. Most 0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.08.007