Eddy current

Determining Residual Austenite with the Eddy Current Method. 

The eddy current method is used for the quantitative evaluation of residual austenite contents in the martensite struaure. 

The principle physical phenomenon of applying the eddy current method for evaluating the amount of residual austenite in the structure of quenched steel is magnetic induction, involving the influence of the changeable magnetic field on the studied area, found under the probe. 

In effect of such activity eddy currents in the studied area are induced, producing own magnetic fields which following Lenz s rule are directed adversely to the induction field this decreasing its intensity (Fig. 1). 

Fig. 1. A diagram of the principle of the contact probe during the study of the metal matenal structure Ho - field produced by the probe, Hp - field ensuring the measuring signal, Hw - field produced by eddy currents.

The intensity of the magnetic field produced by eddy current is depended on electrical conductivity and magnetic permeability of the studied area. In case of a uniform structure, when the conductivity of the material is high, the intensity of the induced magnetic field is big and signal received by probe Hp is small. 

Structure defects decrease conductivity of the studied material, and then the intensity of the induced magnetic field is small and the signal received by the probe Hp is big (Fig.2). Low conductivity of austenite is a defects of the structure in case of residual austenite in the martensite structure, which with regard to the magnesite structure is as 1 5. Eddy currents produced in the studied area are subject to excitation in effect of small conductivity of austenite grains in the structure of the studied material. 

Eddy current devices for detecting residual austenite comprise ... 

The detection of residual austenite in fact requires average frequency, however for comparison reasons (reference) with a different recognized method, it is recommended to use high frequency, as with high frequency of eddy currents the penetration depth is comparable in the diffraction method and eddy current method. 

Attention should be given in the fact, that penetration of eddy currents in residual austenite will be slightly deeper than in the martensite structure of steel, as austenite shows low electrical conductivity. The signal originatimg from the austenite structure will be amplified in effect of the influence of the structure found at greater depth. There will be no error as the method of measurement is compartable and the samples made for reference purposes will have the same structure as the studied part. 

Fig. 3. A diagram of the sample with residual austenite tor calibrating eddy current devices...

The effectiveness of the approach is demonstrated on two rqjresentative NDT techniques intapretation of data acquired with an ultrasonic rail inspection system and interpretation of eddy-current data from heat exchangers in (petro-)chemical industry. The results show that it is possible to provide a high level of automation in combination with efficient operator support for highly variable NDT measurements where up to now use of automated interpretation was only limited. 

The first system called LiSSA has been developed for interpretation of data from eddy-current inspection of heat exchangers. The data that has to be interpreted consists of a complex impedance signal which can be absolute and/or differential and may be acquired in several frequencies. The interpretation of data is done on the basis of the plot of the signal in the impedance plane the type of defect and/or construction is inferred from the signal shape, the depth from the phase, and the volume is roughly proportional to the signal amplitude. 

Modelling of Eddy-Current Interaction with Cracks in the Thin-Skin Regime Two Approaches. 

Eddy-current non-destructive evaluation is widely used in the aerospace and nuclear power industries for the detection and characterisation of defects in metal components. The ability to predict the probe response to various types of defect is highly valuable since it enables the influence of particular parameters to be studied without recourse to costly and time consuming experiments. The solution of forward problems is also essential in the process of inverting experimental data. 

At sufficiently high frequency, the electromagnetic skin depth is several times smaller than a typical defect and induced currents flow in a thin skin at the conductor surface and the crack faces. It is profitable to develop a theoretical model dedicated to this regime. Making certain assumptions, a boundary value problem can be defined and solved relatively simply leading to rapid numerical calculation of eddy-current probe impedance changes due to a variety of surface cracks. 

Once y/" has been determined, the impedance change in an eddy-current coil due to the crack can be calculated using the following integral over the crack mouth [3] ... 

The comparison between measured data and simulated data are good for both real and imaginary parts. The measured signal has a low resolution due to the low interaction between the eddy current and the slot. 

NDT inspection in the thin-skin regime is well adapted to finding the length of a surface slot or crack from eddy current data. With the electromagnetic skin depth smaller than the slot depth, the interaction between the eddy current induced in the test block and tbe slot is strong at the surface of the block. 

When tbe skin depth is of the same magnitude as the slot depth, the eddy current interaction with slot can lead to a deviation of currents that are able to pass under an inner defect, deeper in the block. In that case the interaction is not total, and the signal is smoothed. 

The analysis of the curves obtained in the thin-skin regime ean lead to a simple determination of slot length depending on the dimension of the probe chosen for the inspection. If the size of the probe (outer diameter) is smaller than the defect length we can notice 5 zones relative to the relationship between the position of the probe, the interaction of the induced eddy current and the slot, and the impedance change for the probe. 

N. Flarfield and J. R. Bowler, Theory of thin-skin eddy-current interaction with surface cracks, J. Appl. Phys., 82(9), 4590 - 4603, 1997. 

B. A. Auld, F. G. Muennemann and M. Riaziat, Quantitative modelling of flaw responses in eddy current testing. In R. S. Sharpe (ed.), Research techniques in nondestructive testing, Vol. 7. Academic, London, 1984. 

Numerical Modeling of eddy current steam generator inspection Comparison with experimental data, P.O. Gros, Review of Progress in Quantitative Nondestructive Evaluation, Vol 16 A, D.O. Thompson D. Chimenti, Eds (Plenium, New York 1997) pp 257-261. 

Simulations about Eddy Current Distributions and Crack Detection Algorithms for a SQUID Based NDE System. 

In contrast to a direct injection of dc or ac currents in the sample to be tested, the induction of eddy currents by an external excitation coil generates a locally limited current distribution. Since no electrical connection to the sample is required, eddy current NDE is easier to use from a practical point of view, however, the choice of the optimum measurement parameters, like e.g. the excitation frequency, is more critical. Furthermore, the calculation of the current flow in the sample from the measured field distribution tends to be more difficult than in case of a direct current injection. A homogenous field distribution produced by e.g. direct current injection or a sheet inducer [1] allows one to estimate more easily the defect geometry. However, for the detection of technically relevant cracks, these methods do not seem to be easily applicable and sensitive enough, especially in the case of deep lying and small cracks. 

In this paper we present simulations and measurements of several types of excitation coils, which match the special requirements for a SQUID based eddy current NDE system. We note however that all calculations presented here on penetration depths, current distributions and crack-detecting algorithms are also useful for conventional eddy current testing systems. 

This correction function was calculated for different kinds of excitation coils, like circular coils without ferrite core, spiral coils, double-D coils and a sheet inducer. For this purpose the eddy current density was determined for frequencies between 10 and 1000 Hz and for depths between 0 and 30 mm. 

For each frequency 100 points were taken along a line running from the surface of the conductor into a depth of 30 mm in that region below the coil, where the maximum eddy currents are located (dashed vertical lines in the sketch). These data are fitted by appropriate polynomials to obtain an analytical expression for s (to, z) in the frequency and depth interval mentioned above. 

First, the eddy current density is damped while penetrating into the conductor (penetration effect). Here the frequency dependence of the penetration depth implies that for deep lying cracks low frequencies must be used for obtaining a sufficient current density in the vicinity of the crack. Secondly, due to the induction law the induced current density at the surface jco is diminished when using lower frequencies. Therefore, in total, there is a certain excitation frequency which results in a maximum response field from the crack. 

The simulation of the actual distortion of the eddy current flow caused by a crack turns out to be too time consuming with present means. We therefore have developed a simple model for calculating the optimum excitation frequencies for cracks in different depths of arbitrary test sarriples Using Equ. (2.5), we are able to calculate the decrease in eddy current density with increasing depth in the conductor for a given excitation method, taking into account the dependence of the penetration depth c on coil geometry and excitation frequency. 

For this calculation we have to consider the following facts First the induced eddy currents at the defect site have to have a sufficient amplitude, controlled by ac. 

We have perfomied some simulations of the eddy current distribution in a test object for a spiral coil and a circular one (see Fig. 4.1). Both coils had 9 turns and the excitation current was 6 mA. Figs. 4.1 show the cross section of the sample at the location of the crack and the amplitude of the eddy current density. One observes a 1.5 higher current density at the sides of the crack for the case of the circular coil. 

Left Fig. 4.1 Excitation with spiral (top) or circular (bottom) coil. While color corresponds to a high eddy current density in the sample (9turns, 6mA). 

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