L. Clapham

Plastic versus elastic deformation effects on magnetic Barkhausen noise in steel

Abstract 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.

The influence of bacteria on struvite crystal habit and its importance in urinary stone formation

Infection-induced urinary stones form as a result of a urinary tract infection by urease-producing bacteria. These stones are not totally crystalline in nature but rather consist of an agglomeration of bacteria, organic matrix, and crystal of struvite (MgNH4PO4· 6H2O). Crystal formation is related to the ability of the bacteria to effect an increase in the urine pH. Another equally important bacterial role lies in their formation of a ‘biofilm’ which later becomes the organic matrix constituent of the stone. Results of the present in vitro study indicate that crystals are formed more readily if produced within the bacterial biofilm than in the surrounding urine. It is proposed that supersaturation, due in part to a bacterial-induced pH increase and in part to the metal binding tendency of the biofilm, leads to crystal formation via a gel growth mechanism within the biofilm itself. In time further bacterial cell division, microcolony.

Investigation of the stress‐dependent magnetic easy axis in steel using magnetic Barkhausen noise

Angular‐dependent magnetic Barkhausen noise (MBN) measurements were performed on a pipeline steel sample for various values of applied uniaxial stress at three angles with respect to the sample’s zero stress magnetic easy axis direction. It was observed that the response of the MBN signal to stress was dependent upon the direction of the stress with respect to the zero stress easy axis. The stress response of the MBN signal was greatest for (i) tensile stresses oriented perpendicular to the zero stress easy axis direction and (ii) compressive stresses applied parallel to the easy axis direction. The modification of the MBN signal under an applied stress was attributed primarily to a change in the 180° domain wall population in the material investigated. Results were described by a model that considered regions of locally correlated domain behavior, termed ‘‘interaction regions,’’ that were typically the size of grains within the steel material. A basic result of the model was the stress required to modify...

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 w...

A model for magnetic flux leakage signal predictions

An analytical model was developed to determine the length of a surface-breaking defect along the direction of the applied field when using the magnetic flux leakage (MFL) non-destructive technique. The theoretical model fits the experimental MFL results from simulated defects. The extreme positions of the normal magnetic leakage field component were subsequently used for a quantitative evaluation of the defect length. Permeability variations were neglected by employing a flux density close to sample saturation. Four different defect geometries were experimentally investigated and the validity of the analytical model was verified. Good agreement between theoretical and experimental results suggests that this method can be used as an inverse MFL data interpretation technique.

The influence of pearlite on barkhausen noise generation in plain carbon steels

Abstract Barkhausen noise refers to the abrupt, discontinuous changes in magnetization which result from domain walls overcoming barriers to their movement. Barkhausen noise is sensitive to a number of parameters, notably stress level (for which it is being examined as a basis for non-destructive evaluation) and microstructure. The present study investigates the influence of pearlite content on the pulse height distribution of the magnetic Barkhausen noise signal in plain carbon steels. The responses of purely pearlitic and purely ferritic microstructures are determined to be quite different. Samples containing no pearlite display a narrow, symmetrical distribution of relatively small pulse heights. The behaviour of the pearlitic steel is quite different, however, exhibiting an asymmetrical pulse height distribution with a significant tail extending to comparatively large pulse heights (approximately four times that of the maximum for the ferrite signal). Steels containing both ferrite and pearlite produce signal distributions between the two extremes, but contain no significant tail and also appear to tend toward larger pulse heights than what would be expected for a purely “composite” distribution. Results are explained in terms of differences in domain wall pinning behaviour between the various microstructures.

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.

Influence of uniaxial elastic stress on power spectrum and pulse height distribution of surface Barkhausen noise in pipeline steel

Barkhausen noise (BN) measurements were made on a half-section of a pipe under tensile and compressive isostress. The RMS BN voltage was found to increase with increasing tensile stress and to decrease with increasing compressive stress. Pulse-height distribution analysis and power density spectral analyses were performed on the BN data. These analyses revealed that the BN events with large amplitudes occur in the presence of tensile stress, while events with small amplitude occur under compressive stress. The power of the BN spectrum increased or decreased, respectively, with increasing tension and compression. While the exact mechanism responsible for the shift in BN pulse height distribution is not yet clear, the authors suggest that it may be related to stress-induced changes in domain size, magnetization distribution, and/or the clustering behavior of domain walls. >

Influence of Chondroitin Sulfate, Heparin Sulfate, and Citrate on Proteus Mirabilis-Induced Struvite Crystallization in Vitro

Struvite crystals were produced by Proteus mirabilis growth in artificial urine, in the presence of a number of naturally occurring crystallization inhibitors. The use of phase contrast light microscopy enabled the effects of added chondroitin sulfate A, chondroitin sulfate C, heparin sulfate, or sodium citrate, on struvite crystal growth rates to be rapidly monitored as changes in crystal habit. Struvite crystals formed as a consequence of the urease activity of P. mirabilis under all chemical conditions. In the absence of inhibitor, early crystal development was marked by large quantities of amorphous precipitate, followed immediately by the appearance of rapidly growing X-shaped or planar crystals. Addition of the glycosaminoglycans, chondroitin sulfate A, chondroitin sulfate C, or heparin sulfate to the artificial urine mixture had no effect on the rate of crystal growth or appearance. When sodium citrate was present in elevated concentrations, crystal appearance was generally slowed, and the crystals assumed an octahedral, slow growing appearance. None of the added compounds had any influence on bacterial viability, pH, or urease activity. It is therefore likely that the inhibitory activity displayed by sodium citrate might be related to its ability to complex magnesium or to interfere with the crystal structure during struvite formation. From these experiments it would appear that citrate may be a factor in the natural resistance of whole urine to struvite crystallization.

Characterization of texture and residual stress in a section of 610 mm pipeline steel

Abstract Gas pipelines are inspected for defects such as corrosion. The most commonly used nondestructive inspection tool uses the magnetic flux leakage (MFL) technique. The MFL signals depend on the magnetic behaviour of the pipe, which is sensitive to its microstructure and crystallographic texture as well as both residual and applied stresses. Here a section of commercial X70 pipeline is characterized using microstructural examination, X-ray diffraction (to determine crystallographic texture) and neutron diffraction (for residual stress measurement). The results correlate well with the manufacturing steps used for this type of pipe. Magnetic characterization is also performed using magnetic Barkhausen noise measurements, which reflect the magnetic anisotropy in the pipe and thus the MFL signal. These results do not correlate simply with crystallographic texture and residual strain results, but this is not unexpected given the complex nature of the material and its stress state.