International Journal of Applied Electromagnetics and Mechanics

Editorial to the special issue on the progress of eddy current based nondestructive testing.

Linear and nonlinear numerical formulations for an efficient solution of forward and inverse problems are summarized. The simulation tool is based on an integral formulation in terms of a two-component current density vector potential expanded over edge-elements. The methods are applied to the numerical simulation of a magnetic field sensor as well as to the crack simulation and detection via eddy current testing (ECT), comparing the results to the experimental measurements. The class of problems treated includes both thin and thick cracks in metallic plates, taking into account the possible presence of leakage in the crack as well as magnetic materials. The inversion of ECT data is carried out by means of genetic algorithms or methods, based on algorithms developed for digital communications, that allow to reduce the computational cost for the solution of the inverse problem.

This paper reports the progress of studies in eddy current inversion in the authors' group and a novel inversion scheme that can deal with conductive cracks using eddy current signals. The novel scheme is based on artificial neural networks with its accuracy enhanced by using a shifting aperture method and signal processing techniques. A four sensor type probe is used in the eddy current testing. Several reconstruction results are introduced in this paper as validation of the proposed new scheme. The scheme is also applied to profile reconstruction of cracks which occurred in an actual steam generator tube of a pressurized water nuclear power plant. Remarkable agreement between reconstructed profiles and those obtained by destructive testing is observed. It can be concluded that the new inverse scheme is promising and will be a powerful tool for the practical applications.

An algorithm is presented for the automated analysis of rotating probe multifrequency eddy current data obtained from nuclear power plant steam generator tubes (SGT). The algorithm consists of four steps, namely, a preprocessing stage for conditioning the data, a decision tree based feature extraction stage for identifying relevant features for analysis, a neural network based classification stage for identifying signals from various defect types and benign structures, and finally a blind deconvolution based characterization stage for accurately estimating the size and orientation of the detected defects. This algorithm is optimized to maximize the probability of detection (POD), while keeping the number of false alarms (PFA) at a minimum. Initial results presented in this paper look very promising and demonstrate the effectiveness of the proposed algorithm.

Non-destructive characterization (NDC) of materials is a technology of increasing application by industrial users for process monitoring and control. The main reason for this fact in materials processing is the need for a better process understanding and mastering in order to produce quality according to a zero-defect-principle and the objective to reduce non-conformities compared with a given quality specification. This means, integration of NDC by use of intelligent sensors into monitoring and control systems to predict mechanic properties and to detect and document their discontinuities (hardness, hardness depth, yield strength, deep drawability, residual stresses, etc.).

NDC-technology, which in the last two decades was mainly pushed by research and development in different national and international program [1] now, is matured for a wider range of applications. The special emphasis of the contribution is to give characteristic examples, which illuminate mainly the gain in economical benefit.

Eddy current and micro-magnetic techniques have a special advantage to be applied for the mentioned objectives, because electrical and micro-magnetic intrinsic properties and mechanic properties are influenced by the same microstructure parameters and their changes during material processing and material degradation. Furthermore they are sensitive for load-induced and residual stresses. However, their application is restricted to ferromagnetic materials and material phases.

A series of the Green's functions describing 2D electro-elastic fields excited in an arbitrary infinite piezoelectric strip by the different combinations of line sources at the surfaces are derived. Four types of sources are considered: a line of forces applied to a free surface, a step of displacements on a clamped surface, a line of electric charge or a line of electric potential discontinuity on a surface. In the absence of electrical sources at the surface the latter is supposed to be adjoined to an isotropic dielectric medium with an arbitrary permittivity. The Green's functions are expressed in terms of eigenvectors and eigenvalues of the generalized Stroh problem in the form of convergent Fourier integrals. These integrals are taken for the particular cases when, due to the symmetry of the material of the strip, the eigenvalues of the 8 × 8 Stroh matrix are purely imaginary. The results of integration are presented as sums of residues at the poles determined by some equations. Finding the roots of these equations needs additional computing.

Electromagnetic EM emissions have been received during and after blasting in quarries and mines. These signals show promise for mine safety monitoring and blast analysis. Results indicate smaller fractures may be detected using EM systems compared to seismic systems. Instantaneous blast analysis may also be possible in the future. EM emissions have also been shown to be produced in concrete.

This paper addresses the modular decoupling methodology (MDM) [1]. It predicts EMC by the simulation of the effects of radiated emissions on an enclosure with a prescribed aperture and some contents. The proposed method could be a tool for preventing some possible EMI problems at the design stage. Computer simulations are developed using the finite element method and the investigation assesses the effectiveness of the shielding and susceptibility predictions. Exploration on an external emission source and its impact to lower stage is also considered.

The global optimisation of Loney's solenoid, aiming at minimizing the level of inhomogeneity of the main field, was proposed and solved as a first benchmark for single criterion optimisation. Accordingly, the TEST(Testing Electromagnetic Synthesis Techniques) environment has been set up. Based on Loney's solenoid even in the unshielded configuration, a multicriteria optimisation problem has been then formulated: to identify the coil geometry producing a main field with the minimum inhomogeneity within the solenoid and, at the same time, a minimum level of stray field around the solenoid. The scope of this contibution is to compare different methodologies for the solution of the multicriteria design problem.

The polarization properties of periodically arranged metallic helices are investigated. The chiral layers have been synthesized by embedding right handed or left handed metallic helices arranged periodically in a dielectric matrix of thermocol (polystyrene foam), a low loss dielectric material. The chiral layers are loaded at the aperture of the transmitting rectangular waveguide horn antenna. The polarization pattern of the transmitted signal (TE _{10} mode) indicates rotation of plane of polarization and partial conversion into either right circular polarization(RCP) or left circular polarization (LCP).

An exact solution of the three-dimensional equations of piezoelectromagnetism is obtained. The solution represents a generalized version of the well-known Bleustein-Gulyaev surface waves in piezoelectric ceramics.

In the present paper we study the effects of initial deformations and electric fields on the behaviour of the {\it electromechanical coupling coefficients} and on the shape of the {\it slowness curves}, in the case of progressive wave propagation in 6mm-type piezoelectric crystals along the symmetry axis, resp. in the plane normal to the previous axis.