Magnetic Barkhausen Noise Energy Research Articles

Influence of uniaxial plastic deformation on magnetic Barkhausen noise in steel

Magnetic Barkhausen noise (MBN) measurements were made on hot-rolled mild steel samples uniaxially deformed to differing magnitudes of plastic strain, to study the dependence of MBN activity on plastic strain. The results indicated an initial increase in MBN energy with increasing plastic strain followed by a decrease at higher plastic deformations. At still higher plastic deformations, the MBN energy was found to be almost independent of plastic strain. The results are explained in terms of different mechanisms of interaction of domain walls with dislocations, with increasing plastic strain. The behavior of MBN energy with plastic strain was found to be anisotropic and the angular MBN measurements indicated that the deformation-induced easy axis of magnetization changed direction with increasing plastic strain. At higher deformations, the MBN activity was largely controlled by the deformation-induced anisotropy, due to residual stress.

Plastic versus elastic deformation effects on magnetic Barkhausen noise in steel

A study was performed to differentiate the effects of elastic and plastic deformation on magnetic Barkhausen noise (MBN) signals. Linear and angular MBN measurements were made on a number of mild steel plate samples subjected to varying degrees of uniaxial elastic and plastic deformation up to ∼40% strain. Elastic strain effects on the MBN energy were determined to be far more significant than plastic strain effects. MBN energy increases in the early plastic deformation region were attributed to a slight increase in elastic strain due to work hardening. Magnetic anisotropy studies indicated that elastic strain significantly alters the magnetic anisotropy ( α) in the sample, but changes the isotropic signal ( β) very little. Plastic deformation has a smaller, but apparently opposite effect, in that it appears to change β but has little influence on α. As plastic deformation levels become large, however, behaviour becomes more complex and is less well understood.

Measurement of residual stress in steel using the magnetic Barkhausen noise technique

Surface residual stresses on a structural beam steel sample were evaluated using a non-destructive technique based on the measurement of surface magnetic Barkhausen noise (MBN). MBN measurements were performed using a high resolution probe consisting of a small magnetic read head mounted between the poles of a ferrite U-core magnet. Applied stress magnitudes were correlated to MBN energy levels for calibration purposes. MBN energy measurements were done at various locations on the steel sample. The magnitude of the residual stress component along the sample beam axis was evaluated across the width of the beam using these calibration curves. The range of sensitivity of the MBN signal to stress as defined by the calibration curves was limited by the direction of easy magnetization. The effect of stress on the MBN signal was interpreted in terms of the active 180° domain wall population. The validity of the residual stress results was confirmed experimentally using conventional methods: cutting and sectioning and hole drilling. Possible sources of residual stresses and their distribution on the structural steel specimen are discussed.

MAGNETIC BARKHAUSEN NOISE INDICATORS OF CRACKS IN STEEL

High resolution magnetic Barkhausen noise (MBN) measurements were performed on cracks that were present in a section of 390 mm diameter and 29 mm thick steel pipe. The MBN indicators considered were the MBN energy signal, the number of events, and the variation of pulse height distributions (PHD) in the crack vicinity. Results were compared with magnetic particle inspection and ultrasonic depth sizing methods. A sensitivity to crack depth was observed; deeper cracks and/or portions of cracks generating a greater reduction in the MBN energy signal. Variations in the number of events and a narrowing of the PHD in the crack vicinity were also observed. Results were attributed to the generation of nonuniform magnetization in the crack vicinity due to modified eddy current field interactions with the crack as well as reduced magnetic coupling across the crack.

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.

Correlation of magnetic Barkhausen noise with core loss in oriented 3% Si–Fe steel laminates

The angular dependence of surface magnetic Barkhausen noise (MBN) with flux densities up to 1.7 T on eight different samples of oriented 3% Si–Fe steel laminate was investigated. On two of the samples encircle MBN measurements were performed. MBN energy values were obtained by integrating the square of the MBN voltage signal with respect to time. The angular variation of the MBN energy signal was modeled by considering anisotropic internal fields that moderate 180° domain wall motion and the local eddy current field interactions between spatially correlated Barkhausen events. The eddy current field interactions arising between 180° domain walls that lie along the sample’s primary easy axis direction were represented by a parameter, α′. α′ had a cos θ angular dependence where θ is the angle of the applied sweep field with respect to the sample rolling direction. The angular averaged MBN energy signal, 〈Energy〉, was evaluated and compared with core losses measured using the standard Epstein technique at 1.5 T. Surface MBN measurements on samples with higher 〈Energy〉 demonstrated lower core loss. This was associated with a larger proportion of microscopic eddy currents generated parallel to the sample plane which were, therefore, uncoupled from macroscopic eddy currents generated by the bulk magnetization of the sample. Larger surface 〈Energy〉 signals were identified with a finer domain wall structure. For samples with a nonzero α′, the core loss was observed to decrease with increasing α′ magnitude. This was interpreted as a result of a reduction of microscopic eddy currents arising from an increase in interactions between simultaneously moving 180° domain walls.

Magnetic Barkhausen noise: stress-dependent mechanisms in steel

Angular dependent magnetic Barkhausen noise energy signal measurements were performed on two similar pipeline steel samples up to tensile stresses of 330 MPa. Variations between the stress dependence of the energy signals at five investigated positions were interpreted in terms of the relative proportion of three stress-dependent mechanisms for modifying the magnetic properties of steel. These were; the change in the number and in the orientation of 180/spl deg/ domain walls and the modification of pinning barriers.

Characterization of the magnetic easy axis in pipeline steel using magnetic Barkhausen noise

The angular dependence of magnetic Barkhausen noise (MBN) on eight surfaces through the thickness of a 2% Mn steel pipeline sample was investigated. The MBN signal was analyzed by integrating the square of the MBN voltage signal with respect to the time axis. The resulting value, referred to as the MBN energy signal, was modeled by considering the irreversible motion of 180° domain walls, under the influence of an oriented magnetic field. An expression for the angular dependence of the MBN energy signal was derived and was given by energy=α cos2 θ+β, where α and β are the fitting parameters and θ is the angle between the maximum MBN signal and the applied sweep field. The α parameter was associated with the irreversible motion of 180° walls that contributed to the net macroscopic easy axis near the surface of the sample, while the β parameter was associated with the isotropic background MBN signal. The energy equation could be used to fit the data for all sweep field amplitudes in which the MBN spectrum was observed. A dependence of the α and β parameters on the applied sweep field amplitude was observed.