Marek Augustyniak

Influence of Plastic Deformation on Stray Magnetic Field Distribution of Soft Magnetic Steel Sample

The effect of various combinations of conditions, i.e., presence of the Earths magnetic field during and after deformation on the distribution of stray magnetic field of S355 steel sample, which is locally deformed, was investigated. Some of the stages of the experiment were carried out in zero magnetic field. Compensation of the Earths magnetic field was obtained by the application of a pair of Helmholtz coils. These coils are able to produce homogeneous field compensating Earths magnetic field over the full length of the sample. Stray magnetic field measurements were carried out along the longest dimension of the sample using the probe consisting of mutually perpendicular sensors allowing the simultaneous measurement of two magnetic field components, i.e., tangential and normal to a surface of the sample. Research has shown that there is a correlation between the level of plastic deformation of the investigated sample and the distribution of stray magnetic field. It was found that the nature of stray magnetic field of the sample depends on presence of external magnetic field during unloading. An attempt was also made, with the finite-element analysis, to verify experimentally obtained stray magnetic field distribution of undeformed sample.

Separation of the Effects of Notch and Macroresidual Stress on the MFL Signal Characteristics

Magnetic flux leakage (MFL) distribution for three configurations of samples has been investigated in order to study the influence of notch and plastic deformation separately as well as together. Samples have been made of S355 steel. The MFL signal measurements were carried out along the longest dimension of the sample over a length of 120 mm. Two components of magnetic field were measured: 1) tangential to the main axis and 2) normal to the largest surface of the sample. Study has shown that geometrical effect, related to the notch presence, as well as plastic deformation with associated compressive macroresidual stresses, together are responsible for the specific MFL distribution above the sample. In order to confirm the assumption that the decrease of the permeability in the central part of the sample and the change of cross section are the main sources of the MFL anomaly, disrupting flux from demagnetization field, a series of simulations has been made. Simulation results coincide with those obtained experimentally and thus confirming the above hypothesis.

The Finite Element Method Simulation of the Space and Time Distribution and Frequency Dependence of the Magnetic Field, and MAE

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

Nonlinear harmonic amplitudes in air coils above and below a steel plate as a function of tensile strength via finite-element simulation

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.

Eddy Current Techniques to Detect Incipient Creep Damage of Stainless Steel Boiler Tubes

As creep damage degradation proceeds in 304, 321, and 347 grade stainless steel boiler tubes, the tubes develop a ferro-magnetic component depending on length of service. Incipient creep damage occurs inside the steel, involving precipitation inside the grains, with precipitates getting larger and forming mostly at or near grain boundaries as degradation continues. This incipient creep damage eventually leads further to the development of cavities at grain boundaries, which in turn lead eventually to microcracking and cracking. The amount of ferromagnetic component has been correlated (albeit in a relatively small number of exploited specimens) to the amount of incipient creep damage. The ferromagnetic component appears to be primarily associated with the formation of ferromagnetic hard oxide scale on the outer surface of the boiler tube. The ferromagnetic part of the scale has been identified, using x-rays, as magnetite. This ferromagnetic oxide surface can be easily inspected in power plants using eddy current techniques. Because of the correlation with incipient creep damage, the suggestion is that measurement of the amount of ferromagnetic component can be used to nondestructively monitor the development of incipient creep damage in austenitic steel at stages of development well before cavitation and microcracking. We have found that a processed signal can be extracted from eddy current measurements, which is directly related to the amount of ferromagnetic component. We have shown mathematically why the signal behaves as it does. This same signal has been simulated using finite element modeling (FEM). Its linear dependence on the amount of ferromagnetic component is verified experimentally, mathematically, and by FEM. In addition, we also demonstrate a way in which frequency dependence of the eddy current signal can be used to separate out effects of conductivity from effects of change in permeability due to the ferromagnetic component, thereby reducing effects of experimental error in evaluating the amount of ferromagnetic component at low amounts of degradation. In the present year, the technique is now being tested via measurements at several power plant sites.Copyright