This paper proposes a strategy for the fast simulation of eddy current testing signals due to a conductive crack of arbitrary width. To cope with a crack of width less than that selected for establishing the database, which is necessary in the fast-forward analysis scheme proposed by authors, a new finite element is introduced to treat the case when the crack boundary is contained in the element. By using such a new element, the fast-forward analysis scheme becomes suitable for the reconstruction of both the shape and width of a planar crack. It is verified that such a multiple material element is efficient for an ECT sensor inducing eddy current parallel with the crack surface. For the case with perpendicular eddy current component, however, the approach is not valid because of a scalar potential jump at the crack surface. Finally, the reason of such a singularity is investigated through numerical simulations.

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 ofECT 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 type (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.

This study enhances the applicability of eddy current testing to the evaluation of deep surface-breaking cracks in the front face of thick structures, while keeping the advantages of higher frequency inspection. The key idea is to suppress eddy currents on the surface of a testpiece, and thus realize the deeper penetration of eddy currents. Based upon this idea, a mutual induction eddy current testing probe, consisting of four coaxial rectangular tangential exciter coils and a pancake pick-up coil, has been designed. The exciters are driven by AC currents with different phases and amplitudes, which contrive to emulate the desired behavior. Numerical simulations and consecutive experimental verifications are presented in the paper. It is revealed that when the new probe is used for inspections under a frequency of 10kHz there is a 20% difference between signals due to an electro-discharge machined (EDM) notch of 15mm depth and those due to one of 20mm depth. On the other hand, a conventional probe driven with the same frequency gives only 1% difference between the signals of the two EDM notches.

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 cunent 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 sho~ high perfolTnance in practical applications are proposed finally. In addition, the ring curent model and the simplified detectability analysis method are validated further for a conducting tube by comparing the magmetic field perturbation due to a crack find 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.

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 impe- dance 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 nmnerically from the electric field in the crack region. The scheme was implemented for the shape reconstruction of EDM cracks from the mea- sured 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. Takagi et al. Good recon-struction results were obtained within a short CPU time. Consequently, the validity, of the new approach was erified.