A new nondestructive eddy current technique is described for testing for incipient creep damage in austenitic steel boiler tubing in power plants. It is used because as incipient creep damage is formed, a magnetic oxide scale forms on the outside of the boiler tube in concentration proportional to the incipient creep damage; simultaneously in the base metal under the scale, a magnetic ferrite phase also forms in the grains and grain boundaries, which is in smaller concentration, but which is also in concentration proportional to the incipient creep damage. The eddy current signal can be processed in such a way that it varies monotonically and nearly linearly with the magnetic phase concentration and monotonically and nearly linearly with the incipient creep damage. Various aspects of the measurement are analysed and discussedfor example, liftoff, wall thickness, and diameter dependence. Using a zero point value to assess oxide layer permeability and dependence on conductivity is also discussed. Comparison is made between measurements and finite element modelling results.

In this paper, the magnetic material characteristics are reconstructed for magnetic circuits with non-uniform electromagnetic field patterns, including excitation winding and/or air gaps, as in the case of rotating electrical machines. The identification process is done using a set of well chosen global and/or local magnetic measurements. Moreover, numerical inverse techniques are implemented in order to reconstruct the material characteristics from a limited number of global and/or local measurements.

The paper presents the results of a complementary study including magnetic hysteresis loops B(H), magnetic Barkhausen noise (MBN) and magnetoacoustic emission (MAE) signals measurements for plastically deformed Fe–2%Si samples. The investigated samples had been plastically deformed with plastic strain level (εp) up to 8%. The properties of B(H) loops are quantified using the coercivity HC and maximum differential permeability μrmax as parameters. The MBN and MAE voltage signals were analysed by means of rms-like voltage (Ub and Ua, respectively) envelopes, plotted as a function of applied field strength. Integrals of the Ub and Ua voltages over half of a period of magnetization were then calculated. It has been found that HC and integrals of Ub increase, while μrmax decreases monotonically with increasing εp. The MAE (Ua) peak voltage at first decreases, then peaks at εp≈1.5% and finally decreases again. The integral of the Ua voltage at first increases for low εp and then decreases for εp>1.5%. All those various dependence types suggest the possibility of detection of various stages of microstructure change. The above-mentioned results are discussed qualitatively in the paper. Some modelling of the discussed dependency is also presented.

The phenomenon of magnetoacoustic emission (MAE) has been ascribed usually to one of two origins: either (1) motion of non-180° domain walls or (2) creation or annihilation of domains. In this paper, we present strong evidence for the argument that the only origin for MAE is motion of non-180° domain walls. The proof is evident as a result of measurements of zero MAE for a wide range of stress in the isotropic zero magnetostrictive polycrystalline alloy of iron with 6.5% silicon. We also explain why it was that the alternative origin was proposed and how the data in that same experiment can be reinterpreted to be consistent with the non-180° wall motion origin.

Cold-rolled (0-19% of reduction) 0.5% Si electrical steel sheets were studied in detail, including macro and micro residual stress measurements, crystallographic texture, dc-hysteresis curves and iron losses. Even for the smallest deformation, losses increase significantly, with large increase of the hysteresis losses, whereas the anomalous losses reduce slightly. The residual microstresses are similar to 150-350 MPa, whereas residual macrostresses are compressive, similar to 50 MPa. The large increase of the hysteresis losses is attributed to the residual microstresses. The dislocation density estimated by X-ray diffraction is in reasonable agreement with that predicted from the Sablik et al. model for effect of plastic deformation on hysteresis. The intensity of the texture fibers {1 1 1}< u v w > and < 110 >//RD (RD = rolling direction) increases with the reduction. (C) 2008 Elsevier B.V. All rights reserved.

In the past several years, there has been some interest in modelling hysteresis in giant magnetoresistance (GMR) as a function of magnetic field in magnetic multilayer films. The purpose of this paper is to review work published a decade ago on such modelling and on application of such modelling to the design of a GMR field sensor. The computed GMR curves qualitatively reproduce the GMR hysteresis seen experimentally. In particular, two GMR peaks are found to be symmetrically placed about

The study concerns a flat large construction steel plate of varying thickness magnetized with a C-core magnet. Modeling of the time- and spacefield distribution inside the plate is carried out. The novelty of the approach consists in carrying out a transient solution when a driving saw-tooth voltage is used. Also new is a refined magnetoacoustic emission (MAE) signal modeling, numerically deduced for various frequencies, and compared with experimental data. The distributions of fields are indirectly compared with stray field measurements and a close agreement is found

We report measurements and modelling of magnetic effects due to plastic deformation in 2.2% Si steel, emphasizing new tensile deformation data. The modelling approach is to take the Ludwik law for the strain-hardening stress and use it to compute the dislocation density, which is then used in the computation of magnetic hysteresis. A nonlinear extrapolation is used across the discontinuous yield region to obtain the value of stress at the yield point that is used in fitting Ludwik's law to the mechanical data. The computed magnetic hysteresis exhibits sharp shearing of the loops at small deformation, in agreement with experimental behavior. Magnetic hysteresis loss is shown to follow a Ludwik-like dependence on the residual strain, but with a smaller Ludwik exponent than applies for the mechanical behavior.

Modeling was used to assist with the design of eddy current GMR probes for detecting small cracks. The approach was to (1) compute the eddy current flow generated by a planar coil and (2) then compute the perturbation flux density B' as the current flows around a surface crack. In Step 1, the Dodd and Deeds analytic solution was adapted to the geometry of an N-turn horizontal coil. The maximum induced current density from Step 1 was then used to compute B' for a crack with half-penny cross-section and oblate spheroidal 3D shape.

In 2.2% Si electrical steel, the magnetic hysteresis behavior is sharply sheared by a rather small plastic deformation (0.5%). A modification to the Jiles–Atherton hysteresis model makes it possible to model magnetic effects of plastic deformation. In this paper, with this model, it is shown how a narrow hysteresis with an almost steplike hysteresis curve for an undeformed specimen is sharply sheared by plastic deformation. Computed coercivity and hysteresis loss show a sharp step to higher values at small strain due to an n=1∕2 power law dependence on residual strain. The step is seen experimentally.

The paper analyzes the impact of frequency on magnetoacoustic emission (MAE) and Barkhausen effect (BE) signals in terms of the time and spatial distribution of magnetic induction inside the magnetized plate. Phase relationships between the signals are examined experimentally and with ANSYS software. The time-spatial distribution of magnetic induction induced by eddy currents inside the plate accounts for a shift of the Barkhausen signal relative to the MAE signal.

A model is presented for magnetoacoustic emission (MAE) and magnetic Barkhausen emission (HBN) during incipient creep damage stages in Cr–Mo steel. The model qualitatively reproduces the peak in HBN intensity as one goes through the early incipient stages, and also qualitatively reproduces the strong decrease in MAE intensity through all the incipient stages, leading to over a 50% decrease.

One way to measure nonlinear harmonic amplitudes associated with a steel plate is to measure the secondary electromotive forces (EMFs) in air coils above and below the plate when the field is generated by ac currents in primary coils also above and below the plate. The flux density inside the plate is not being measured in this case, but rather the flux density of the stray field outside the material. The harmonic behavior of this stray field is different from the harmonic behavior of the flux density inside the material itself. Indeed, as a demonstration of this different behavior, we present finite-element modeling (FEM) for the given configuration using ANSYS EMAG 2D in the transient mode (with eddy currents turned on in the plate). It is found, after Fourier transforming the computed EMF in the secondary coils, that the third harmonic actually increases with increasing dislocation density and decreasing grain size (or, equivalently, with increasing tensile strength). This behavior is opposite in trend to what is found for the flux density inside the material from prior modeling or from experiment. An explanation for this is provided in the paper. The increase in the third harmonic is not only predicted by the FEM, but also by measurements with air coils, as cited in this paper.

Plastic deformation affects the hysteretic magnetic properties of steels because it changes the dislocation density, which affects domain-wall movement and pinning, and also because it places the specimen under residual strain. An earlier paper proposed a model for computing hysteresis loops on the basis of the effect of grain size d and dislocation density /spl zeta//sub d/. In that paper, hysteresis loops were compared that all had the same maximum flux density B/sub max/. The result was that coercivity H/sub c/ exhibited a linear relationship with inverse grain size (1/d) and /spl zeta//sub d//sup 1/2/. The same was true of hysteresis loss W/sub H/. If one compared hysteresis loops all with the same H/sub max/, these linear dependences were only approximately found. Because the relationships are simpler for loops of constant B/sub max/, core loss experimenters compare loops that all have the same B/sub max/. In this paper, we modify the model to study the effect of plastic tensile deformation on hysteresis loops with the same B/sub max/. We found linear relationships between H/sub c/ and residual plastic strain /spl epsiv//sub r/ and between W/sub H/ and /spl epsiv//sub r/. With increasing residual tensile strain, H/sub c/ increases (whereas with increasing elastic tensile strain, H/sub c/ decreases). Also, with increasing residual tensile strain, the slope of the hysteresis loop decreases (whereas with increasing elastic tensile strain, the slope increases). We also consider the effect of compressive plastic deformation.

Using finite element modeling and building in the effect of grain size and dislocation density on magnetic properties, it is shown how a magnetic test can be used to determine if a weld is annealed or not. In a weld, the grain size increases as one goes through the heat-affected zone from the base metal to the fusion zone. In an unannealed weld, dislocation density increases very sharply from the base metal to the fusion zone, so much so that its effect dominates the magnetic properties, causing permeability to decrease from the base metal to the fusion zone. In the annealed weld, this very sharp increase in dislocation density from base metal to fusion zone is not present, and the grain size effect dominates, which causes the permeability to slightly or moderately increase (instead of decrease) from the base metal to the fusion zone. This change in behavior of the permeability from the base metal to the fusion zone determines whether the weld is annealed or not.

In this paper, the relationship between microstructural properties of steels and the material parameters in the Preisach model and in the Jiles-Atherton (JA) model is discussed, in the instance where both models describe quasi-static hysteretic magnetic behaviour. It is shown how the material parameters in both hysteresis models should be modified to reflect their dependence on dislocation density and grain size. The dependence of the Preisach material parameters on these microstructural features is identified starting from hysteresis loops calculated by the microstructurally dependent modified JA model. For the Preisach model, a Lorentzian distribution function is used for the distribution function. This makes it possible to compare predictions here to results of an earlier paper in which the Lorentzian distribution was used for Preisach fits to experimental data for steels of different grain sizes. Also, in a different earlier paper, it was shown how the Lorentzian distribution can be formulated so that it connects with salient features of the JA model. The procedure in this paper enables one to examine and predict microstructural variations of Preisach parameters in steels not only for the case of grain size variation but also for the case of variation in dislocation density.

This paper describes experiments with 2Cr-1Mograde steel at various stages of creep damage. Properties of magnetoacoustic emission (MAE) were tested and compared to other magnetic properties such as hysteresis loop B(H), Barkhausen effect (HBN), and magnetostriction lambda. The creep-damage stage was evaluated from microstructural analysis and mechanical properties tests. It was revealed that incipient creep damage leads to decrease of MAE and magnetostriction as well to slight decrease of coercivity Hc and to nonmonotonic behavior of HBN intensity. These features are discussed qualitatively using models of magnetic hysteresis and of MAE.