CORRELATION BETWEEN MAGNETIC-FLUX LEAKAGE AND MAGNETIC BARKHAUSEN NOISE - STRESS DEPENDENCE IN PIPELINE STEEL

The tensile-stress dependence of angular-dependent magnetic Barkhausen noise (MEN) 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 a xial pipe direction. An effective MBN energy (MENENERGY), defined as t he time integral of the squared voltage MBN signal, was calculated. Th e variation of the ratio of the MBNENERGY in the pipe axis direction t o 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 cor rosion pits (13 mm diameter ball-milled pits) in sections of pipeline steel under axial magnetization that was applied from the outer surfac e of the pipe. The percentage variation of the MFLPP signal with stres s was found to agree within uncertainty, with the MFLPP signal variati on with stress from defects placed on the pipe wall far surface, at 1. 1-1.2 T pipe wall flux density. The MENENERGY ratio is proposed as a m easure of the relative anisotropy in line with the magnetizing field t o that perpendicular to it. It is this relative anisotropy that define s 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 a ir.