Zhenmao Chen

Rapid prediction of eddy current testing signals using A-φ Method and database

Abstract This paper describes a new scheme for the fast evaluation of eddy current testing (ECT) signals by using databases and formulation of the A−φ method. As the calculation of flaw signals is localized around the flaw region, substantial computational work can be reduced comparing with the conventional FEM–BEM code even for a conductor with complex geometry. Unlike the approach proposed by Badics et al. (Rapid flaw reconstruction scheme for 3-d inverse in eddy current NDE. In Studies in Applied Electromagnetics and Mechanics, Vol. 12, eds T. Takagi et al., IOS Press, 1997, pp. 303–309), the new scheme can predict ECT signals due to a large and complex-shaped crack rapidly because of its higher order interpolation and the newly introduced special element, say, an element with different media. The efficiency of the new method is verified by comparing its numerical results with the measured impedance for several bench-mark problems. The fast and accurate features make this new forward approach especially feasible in the reconstruction of flaw shapes by combining with the conventional optimization method.

Large-scale parallel computation for the reconstruction of natural stress corrosion cracks from eddy current testing signals

An inversion algorithm for the reconstruction of cracks from eddy current signals is developed in this study and applied to the profile evaluation of natural stress corrosion cracks that were found in steam generator tubes of a nuclear power plant. A crack is modeled as an assembly of small regions having conductivities inside so that eddy currents that flow across the cracks are considered. The conductivity of each region, which is assumed to be a discrete value, is reconstructed by means of the algorithm. Since the algorithm is based upon a tabu search that usually requires a large number of evaluating solution candidates, simulations are carried out on a supercomputer with the use of parallel computation using up to 128 CPUs so as to reconstruct the crack profiles within a reasonable computational time. It is demonstrated that the algorithm can estimate the profiles of the natural cracks with sufficient accuracy. The simulations also show that the algorithm is highly compatible with parallel computation. Additional simulations using other models of natural cracks are performed. Reconstructed profiles of the natural cracks, as a notch with zero conductivity, are very different from the true profiles, even though the reconstructed signals agree well with the measured values. This reveals that it is necessary to take the internal conductivity into consideration when dealing with natural cracks.

Reconstruction of cracks with physical closure from signals of eddy current testing

In this paper, a method for reconstruction of cracks with faces in physical contact (closures) is proposed by using signals of eddy current testing (ECT). Based on an idealized crack model, a database used forward approach is extended at first to enable the rapid simulation of ECT signals due to the closed crack. An algorithm in the conjugate gradient method category is introduced then for tackling inversion of crack parameters in different types (e.g. coordinates and conductivity). A code involving reconstruction of square cracks is developed based on the proposed scheme and be adopted to the reconstruction of several typical problems. The acceptable accuracy and high robustness shown in these examples verified the validity of the proposed strategy.

ECT Inversion Using a Knowledge-Based Forward Solver

In this paper, a new approach was proposed for crack reconstruction based on ECT technique and optimization theory. A newly developed fast forward solver was applied for predicting the impedance signals during the inversion procedure. Comparing with the conventional approaches, the scanning signal due to an arbitrary shaped crack can be calculated in a significantly reduced CPU time but with higher accuracy. The conjugate gradient algorithm was employed in the optimization of the crack parameters, with the gradient calculated numerically from the electric field in the crack region. The scheme was implemented for the shape reconstruction of EDM cracks from the measured impedance data having in mind possible applications in the inspection of steam generator tubing. Several cracks embedded in conducting tube or plate were reconstructed from the observed impedance data by concerning the benchmark models proposed by T. Takagiet al. Good reconstruction results were obtained within a short CPU time. Consequently, the validity of the new approach was verified.

Efficient Numerical Solver for Simulation of Pulsed Eddy-Current Testing Signals

The pulsed eddy-current testing (PECT) method has the promising capabilities for detecting defects and evaluating material properties. It achieves this through its rich variety of frequency components and large driving electric current. Efficient numerical simulation of PECT signals plays an important role in probe optimization and quantitative signal processing. This study primarily focuses on the development of an efficient numerical solver for PECT signals, and its validation via the consideration of the nondestructive testing problems of wall thinning defects in pipes of nuclear power plants. A frequency domain summation method combined with an interpolation strategy was proposed and implemented. It is based on the finite element method with edge elements. The number of total frequencies used in signal summation and the number of selected frequencies for interpolation were thoroughly discussed. In addition, a code using the time domain integration method was also developed for the signal prediction of a transient PECT problem. It was used for comparison with the frequency domain summation method. A comparison of numerical results of the two proposed simulation methods and experimental results indicates that both of these simulation methods can model PECT signals with high precision. However, the frequency domain summation method combined with an interpolation strategy is much more efficient in its use of simulation time.

Shape reconstruction of multiple cracks from ECT signals by means of a stochastic method

This paper presents an approach to reconstruct multiple cracks from eddy current testing signals by means of a stochastic method such as tabu search. The size, number, and locations of cracks are considered unknown in the inversion process, whereas only the orientation of cracks is supposed to be revealed in advance. Eddy current inspections of a stainless steel plate with closely located EDM slits are performed, and the inversions of measured signals are carried out. The results agree well with the true ones, which validates the proposed algorithm

Eddy current inspection of closed fatigue and stress corrosion cracks

This study evaluates the effect of loading on closed crack openings on eddy current signals caused by the crack. Three plate specimens having a fatigue crack and another three specimens having stress corrosion cracking are prepared. Four-point bending of the specimens is carried out to introduce compressive stress to close the opening of the cracks. Then eddy current signals due to the crack are gathered using a plus point type probe. The bending was conducted with a variable load; the eddy current signals remain almost unchanged regardless of the loading.

Reconstruction of crack shapes from the MFLT signals by using a rapid forward solver and an optimization approach

In this paper, the reconstruction of crack shapes from magnetic flux leakage testing (MFLT) signals is realized by introducing a rapid forward simulator and applying a deterministic optimization approach. The MFLT signals due to cracks of different shape are calculated with an FEM-BEM code employing the A method and polarization algorithm, which is accelerated by the new rapid forward scheme. For reconstructing the crack shape, the conjugate gradient method is applied with the gradients predicted by using the difference technique. Both inner and outer cracks are successfully reconstructed from simulated MFLT signals that verified both the efficiency of the fast-forward scheme and the feasibility of the deterministic inverse approach.

Nonlinear FEM-BEM formulation and model-free inversion procedure for reconstruction of cracks using pulse eddy currents

Pulse eddy currents are proposed as a nondestructive testing (NDT) technique to detect flaws in conductive structures with large thickness. The harmonic component of a pulse is rich, so that the pick-up signal containing the amount of information corresponds to a multifrequency analysis. Due to the short time length of the pulse, the amplitude of the excitation increases up to 100 times of the amplitude for an AC signal. Both direct simulation of pulse eddy-currents phenomena using an A-/spl phi/ FEM-BEM code and neural network-based inversion techniques are performed. Numerical results for the inversion of signals due to outer defects are shown.

A new approach for optimal design of Eddy Current Testing probes

The optimal design of ECT probes for advanced nondestructive inspection of steam generator tubing in nuclear power plants is studied in this paper. A new approach for probe design is proposed, on basis of both a simplified detectability analysis method and a ring current model newly developed by the authors. The new approach is incorporated in the optimization of pick-up arrangements as well as the corresponding excitation coils. Two designs of a potential ECT probe which may show high performance in practical applications are proposed finally. In addition, the ring current model and the simplified detectability analysis method are validated further for a conducting tube by comparing the magnetic field perturbation due to a crack and the S/N ratios evaluated by the present method and an FEM-BEM hybrid code respectively. Consequently, the validity of the new design approach and the high performance of the new probe designs are assured.