Correlation between magnetic flux leakage and magnetic Barkhausen noise: Stress dependence in pipeline steel

The tensile-stress dependence of angular-dependent magnetic Barkhausen noise (MBN) was investigated on the inner and outer surfaces of four sections of pipeline steel. Stresses up to 330 MPa or 70% of the yield strength of the steel were applied in either the circumferential or axial pipe direction. An effective MBN energy (MBNENERGY), defined as the time integral of the squared voltage MBN signal, was calculated. The variation of the ratio of the MBNENERGY in the pipe axis direction to that in the circumferential direction was correlated with the stress-dependent variation of the amplitude of radial magnetic flux leakage (MFLpp) signals measured on the outside of the pipe from simulated corrosion pits (13 mm diameter ball-milled pits) in sections of pipeline steel under axial magnetization that was applied from the outer surface of the pipe. The percentage variation of the MFLpp signal with stress was found to agree within uncertainty, with the MFLpp signal variation with stress from defects placed on the pipe wall far surface, at 1.1–1.2 T pipe wall flux density. The MBNENERGY ratio is proposed as a measure of the relative anisotropy in line with the magnetizing field to that perpendicular to it. It is this relative anisotropy that defines the degree to which lines of flux may pass around a high reluctance defect by either remaining within the steel or being forced into the air.