Ovidiu Mihalache

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.

Nondestructive evaluation of the severity of discontinuities in flat conductive materials by an eddy-current transducer with orthogonal coils

This work presents two methods for nondestructive evaluation of the severity of material discontinuities in nonmagnetic conductive plates by eddy currents generated by a transducer with orthogonal coils. The first method deals with long rectilinear discontinuities located beneath the surface under test and having a negligible width. It is based on the geometrical theory of diffraction of electromagnetic waves generated by the transducer. The conditions under which a real transducer satisfies the diffraction model hypotheses are specified. In this case, the experimental results are in good agreement with the theoretical ones. The second method concerns small volumetric discontinuities whose shape and size are evaluated by an eddy-current holographic procedure. When the discontinuity size is comparable with that of the transducer, this permits reconstruction of the shape and determination of the depth of the discontinuity beneath the surface under test.

Analytical solutions describing the operation of a rotating magnetic field transducer

This work presents the analytical solutions describing the operation of a rotating magnetic field transducer used in the eddy current defectoscopy for detecting the long flaws situated parallelly to the inspected piece generatrix. The method uses the expanding of the real transducers three-phase system into an infinite sequence of axial and longitudinal currents whose intensity is given by a Fourier expansion, estimating the electromotive voltage induced in the transducer. By solving the equation of diffusion for the three media and considering the boundary conditions, the vector magnetic potential is determined for each medium. This work also presents the most important theoretical parameters of the transducer, as well as the experimental graphs obtained for cases of the transducers applications.

Reliability of automatic eddy-current equipment with a rotating magnetic field

Abstract This paper presents a method for estimating the reliability of eddy-current nondestructive inspection by using the rotating magnetic field method. The results of the inspection are considered as memory-less stochastic processes, with the possibility of representation in a bi-dimensional space: ‘good product’ and ‘bad product’. The general formulae are calculated by using the master equation for the time behaviour of the probabilities, the numerical data used for results estimation being obtained by means of an automatic eddy current installation with rotating magnetic field. The samples used had 20 μm, 40 μm and 98 μm deep artificial faults, and the control speed ranged between 0.05 and 1 m s−1.

3D RFEC simulations for the in-service inspection of steam generator tubes in fast breeder reactors

Three-dimensional numeric simulations, based on the finite element model, are conducted to simulate the In-Service Inspection of magnetic steam generator tubes of a Fast Breeder Reactor using remote field eddy current probes. The influen ce of sodium in the gap SP-tube and outer defect detectability is computed for a large SP model with multiple SG tubes using the 3D-RFECT code.

3D RF‐ECT SIMULATIONS OF ISI OF U‐BEND SG TUBES IN FBR

The paper focus on 3‐dimensional finite element simulations of the steam generator inspection using remote field eddy current testing (RF‐ECT) in order to asses the performance of the in‐service inspection in SG tubes. It is evaluated the influences of the SG tubes U‐bend curvature on the eddy current sensor signal by taking into account a 3D model in which also multiple SG tubes are located close to each other, as in the nuclear reactor.

Experimental Measurements of Eddy Current Signal From SG Tubes of Fast Breeder Reactor Covered by a Thin Sodium Layer Using a SG Mock-Up

In Fast Breeder Reactors (FBR) which are sodium cooled, the steam generator (SG) heat exchanger tubes separate the low pressure sodium flowing in the SG vessel with the high pressure water-steam in tubes. During In-Service Inspection (ISI), sodium is first drained and then SG tubes are cooled down to the room temperature. After sodium draining, due to the high temperature (more than 500 °C), sodium adheres to SG tubes and structures around (SG support plates, welds) in a thin layer, filling eventually the gaps between SG support plates and tubes. During ISI, SG tubes are inspected for cracks and corrosions using differential eddy currents (EC) probes. Due to the high electrical conductivity of sodium adhering to the outer SG tube surface, the eddy current testing (ECT) signal modifies, in accord with sodium layer thickness or sodium deposits located on the outer SG tube surface. The sodium wetting properties depends on several factors as: material surface, temperature and sodium wetting time. The effect of sodium adhering to the outer SG tube on ECT signals were measured using a small mock-up tank (2 m high and 0.7 m in diameter) in which were introduced two SG tubes similar with the ones used in the Monju FBR (one tube is ferromagnetic and made of 2.25Cr–1Mo alloy, while the other one is made of SUS321 and is austenitic). Defects, SG support plates (on both helical and straight part of the tube) and welds were added to tubes and the ECT signal was measured before and after sodium draining. Variations in the sodium layer thickness and consequently its effect on ECT signals were measured by filling and draining the tank three times in order to recreate each time new layers of sodium. The paper describes the experimental conditions and the ECT results for both types of SG tubes by comparing the defects, SG support plates and weld signals before and after draining of sodium. Additionally, sodium structures were examined visually using a VideoScope camera, confirming the recorded ECT signals. The paper also presents details about sodium layer thickness measurements in several parts of SG tubes (near defect, SP, weld, bend, helical tube, straight tube) by scratching and collecting the sodium on a small area of 20mm×20mm. The volume of sodium drops is also estimated. The measurement results showed that there are significant differences in the sodium layer thickness depending on the SG tube material.© 2009 ASME

EDDY CURRENT SIMULATIONS AND MEASUREMENTS OF SODIUM EFFECT FOR MAGNETIC AND NON‐MAGNETIC STEAM GENERATOR TUBES OF FBR

In fast breeder reactor (FBR), the steam generator (SG) tube wall is the only barrier between water steam and sodium flow. Eddy current signal (ECT) from outer tube defect is modified by both SG support plates (SP) as well as by sodium layer and unknown sodium drops located on the outer SG tube surface. In the present paper, ECT finite element simulations are conducted to evaluate sodium structures ECT noise and variations of defect and tube support plate signal in the presence of a thin layer covering the SG tube surface. Numerical simulations are validated and calibrated with experimental measurements of artificial outer defect for both magnetic and non‐magnetic SG tubes in the absence or presence of sodium covering the outer surface of SG tubes. The papers presents also details about measurements of sodium structures (drops, layer) formed on the outer SG tube surface when these are soaked in a test tank filled with sodium at high temperatures (500° C) up to two hours.

Development of a hybrid ECT sensor for JSFR SG double-wall tubes

ABSTRACT A new design has been adopted for the steam generator (SG) tubes of the Japan Sodium-cooled Fast Reactor (JSFR) using double-wall tubes. This paper estimates and assesses the effectiveness of detecting defects in SG double-wall tubes of the JSFR by using combined high-frequency eddy current testing (ECT) and low-frequency remote field eddy current sensors. We confirm that the proposed hybrid ECT sensor is highly sensitive to small defects, fatigue cracks, and other defects even when located under support plates of tubes. The parameters of the hybrid ECT sensor are designed and optimized to detect small defects using accurate numerical simulations based on the finite element method, using an in-house developed code. The sensitivity and high performance of the hybrid ECT sensor was validated with experimental measurements.