Eddy Current Testing Problems Research Articles

A two-step algorithm for finite-element solution of electromagnetic-testing problems: Eddy-current testing

An algorithm for finite-element simulation of eddy-current testing problems is described. This algorithm allows calculation of transducer signals from any 3D flaw in a metallic item. In practice, the studied flaws are often characterized by small dimensions. Therefore, the measured increment of the voltage induced in the transducer’s output winding owing to the presence of such a flaw is only several percent of the voltage’s average value. If a standard calculation scheme based on the finite-element method is applied, significant computational resources are needed to solve this problem. The two-step algorithm allows a significant reduction in the required resources. The essence of the algorithm is that the source of the field of a flaw’s effect is determined from the distribution of eddy currents in metal free of flaws. Such an approach allows the calculations of the “unperturbed field” to be simplified significantly through consideration of the geometric symmetry of the problem solved and thus, through reduction of its dimensionality. The efficiency of the two-step algorithm is demonstrated during the solution of a typical problem: the use of a through differential transducer for eddy-current testing of a segment of a heat-exchange pipe in the steam generator of a nuclear power plant.

Diagnostics of stress corrosion and fatigue cracks using benchmark signals

On the basis of the background that most studies for quasi-static, low-frequency electromagnetic nondestructive methods consider artificial notches that are, in reality, not always similar to real cracks such as stress corrosion and fatigue cracks, this paper introduces original eddy current inspection data in order to make it available to other researchers in the field. The authors expect this data to be helpful to those who are interested in the application of quasi-static low-frequency electromagnetic field simulations to practical eddy current testing problems. Stress corrosion and fatigue cracks are fabricated into nickel–chromium alloy plates, and eddy current inspections of the plates are performed using four eddy current probes. Signals of four specimens with stress corrosion cracking and another four with fatigue cracks are presented together with profiles of the cracks revealed through destructive testing.

Multiresolution Analysis for Reconstruction of Conductivity Profiles in Eddy Current Nondestructive Evaluation Using Probe Impedance Data

This paper presents a wavelet-based multiresolution analysis method for solving the inverse problem in eddy current testing (ECT). Using the probe impedance signals as the measurement data, the reconstruction of conductivity profiles can be performed by a minimization scheme. The method allows to identify the regions where the perturbation may be localized and to retrieve the unknown parameters only in those regions. Some numerical simulation results show the feasibility of the technique.

Crack Shape Reconstruction in Ferromagnetic Materials Using a Novel Fast Numerical Simulation Method

This paper describes research on crack shape reconstruction in ferromagnetic materials using a novel fast numerical simulation method. The fast numerical method developed here, which can treat ferromagnetic materials, is an extension of a precomputed database approach based on the reduced magnetic vector potential method. It provides a fast forward simulator that is about 80 times faster than a conventional one, even in the case of ferromagnetic materials, without losing accuracy. The fast simulator is applied to the inverse problem of eddy current testing (ECT), crack shape reconstruction, and results of some electric discharge machining (EDM) cracks on a ferromagnetic plate are shown.

Numerical techniques in finite element analysis of eddy current testing problem

Several types of finite element methods have been developed and shown the validity and the effectiveness for eddy current testing (ECT) problem. However, it is difficult to analyze the finite element model including two or more objects whose positions relatively change, such as conductor plate and ECT probe. Furthermore, it takes a large amount of computer resources to compute an accurate finite element model for a natural crack, because of the complex shapes and the narrow gap. In this paper, a superposition method to avoid the finite element mesh regeneration for the relative position change is presented, and the method is applied to the ECT probe with a ferrite core. Furthermore, the finite element model for the natural crack is shown and the model is applied to the ECT problem with the fatigue crack.

Finite element model of natural crack in eddy current testing problem

Eddy current testing (ECT) is one of the nondestructive inspection techniques to detect cracks and eddy current analysis is important for ECT. However, it is difficult to model a natural crack in finite element method (FEM), because of the narrow gap and the complex shape. In this paper, a finite element model for the natural crack is proposed. The conductivity for the finite element model is expressed by a tensor. In the proposed finite element, the change in crack shape and contact condition are simulated by changing the coefficients of the conductivity tensor. The proposed model is applied to a conductor plate model with a crack and a coil. The computation results are compared with the computation results of an artificial crack model. The validity of the proposed method is shown by the computation results.

A Computing Method of Functional Derivatives with respect to System Parameters of Steady-state Electromagnetic Field Equations

The paper presents a computing method of functional derivatives with respect to system parameters such as magnetic resistivity, electric conductivity and source current in steady-state electromagetic fields. The method is based on Gataux differential, namely a kind of sensitivity analysis, and the functional derivatives are proved to be calculated by explicit forms of the solutions of the electromagnetic field equation and its adjoint equation. The methodis applied in success to a simple eddy current testing problem

Analytical solutions to eddy-current testing problems for a layered medium with varying properties

A one-parameter family of analytical solutions is found for the case where a single-turn coil is located above a two-layer conducting medium. The conductivity /spl sigma/ and the relative magnetic permeability /spl mu/ of the upper conducting layer are constants, while for the lower conducting half-space, these are of the form /spl sigma/(z)=Az/sup b/ and /spl mu/(z)=Bz/sup a/, where A and B are constants and a+b=-2 or a+b=0. The change in impedance is computed, and the results of numerical simulation are presented.

Application of a perturbation method to the solution of eddy current testing problems

This paper presents an analytical solution for the change of the impedance of a double conductor line due to eddy currents induced in presence of an infinitely long horizontal cylindrical flaw with vertically symmetric cross-section. The solution, found by a perturbation method, is applicable in the case the conductivities of the flaw and of the surrounding material do not differ by much. Numerical results for flaws of circular and elliptic cross-sections are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

On the application of moment methods to electromagnetic nondestructive evaluation

The application of moment methods (MMs) to eddy-current testing problems for nondestructive evaluation (NDE) is considered. A general formulation for the MM that can be used to analyze NDE problems is derived, and calculated results and experimental data obtained from eddy-current testing of an artificially made sample are presented. Good agreement between the calculated results and the experimental data confirms the validity of the method and shows that the MM can be used as an alternative to the finite-element method (FEM) and the boundary-element method (BEM) in NDE. >