Eddy Current Testing Signals Research Articles

Progress in Three-Dimensional Non-Destructive Assessment of Real Cracks from Eddy Current Testing Signals

Enhancement in three-dimensional non-destructive evaluation of real cracks from two-dimensional simulated eddy current testing signals is reported in the paper. A new uniform eddy current testing probe is employed for the inspection. All three spatial components of the perturbation electromagnetic field are acquired during two-dimensional scan of the probe. The ECT signals due to partially conductive cracks are simulated by a fast forward FEM-BEM solver using a database. Two crack models are proposed for the inversion using the tabu search stochastic method. The former model has a cuboid shape and the latter one reflects a more complex geometry. Numerical results of the three-dimensional reconstruction of partially conductive cracks from simulated two-dimensional signals are presented and discussed in the paper.

A Study on Predicting ECT Signal of Tube Ding from Tube Support Plate Rotation During PWHT of Nuclear Steam Generator

During Post Weld Heat Treatment (PWHT) of nuclear steam generator fabricating process, there may occur permanent deformation on tube from rotation of Tube Support Plate (TSP) due to thermal deformation. The deformation called ding is strictly prohibited for the long-term integrity of steam generator. Non-acceptable tube ding is defined as deformation of which Eddy Current Testing (ECT) signal exceeds 0.5Volt. In this study the relationship between ECT signal and TSP rotation angle was established and the allowable rotation angle of TSP was determined. It will be used for air blowing control system to prevent tube ding damage.

An improved ferromagnetic material pulsed eddy current testing signal processing method based on numerical cumulative integration

A pulsed eddy current (PEC) testing signal processing method for ferromagnetic material is presented. Without any filtering process that may produce signal distortion, the numerical cumulative integration of the noisy PEC signal is calculated and subsequently fitted to the theoretical model. The model parameter estimation is later applied to quantify the specimen thickness and reconstruct a denoised PEC signal. A comparison is made between the presented method and the former direct-fitting method. The results demonstrate that the presented method improves the testing performance in terms of the detectable thickness range, the probe lift-off distance, and the measuring time.

Application of Wavelet Analysis to Eddy Current Testing for the Surface Fatigue Crack of Bolt Holes

Eddy current testing is one of the five major routine nondestructive testing methods and it is convenient, fast and suitable for online detecting of the surface fatigue crack of bolt holes. However, the signals of eddy current testing are so weak that it is difficult to identify the signals. So more effective signal processing method must be adopted to deal with the weak signals. This paper used the wavelet analysis to process signals of the eddy current testing for the surface flaw of bolt holes. It can inhibit the noise and reinforce signal and make qualitative testing possible for quality evaluation of the surface fatigue crack of bolt holes.

증기발생기 전열관 2차측 응력부식균열의 실험실적 모사 방법

In this work, it is aimed to develop the fabrication method of axial stress corrosion cracking (SCC) defects having various sizes, on the outer diameter surface of the steam generator (SG) tubings. To control the length of the artificial SCC defect, the specific area of the SG tubing samples was exposed to an acidic solution after a sensitization heat treatment. During the exposure to an acidic solution, a direct current potential drop (DCPD) method was adopted to monitor the crack depth. The size of the SCC defect was first evaluated by an eddy current test (ECT), and then confirmed by a destructive examination. From the comparison, it was found that the actual crack length was well controlled to be similar to the length of the surface exposed to an acidic solution (5, 10, 20 or 30 mm in this work) with small standard deviation. From in-situ monitoring of the crack depth using the DCPD method, it was possible to distinguish a non-through wall crack from a through wall crack, even though the depth of the non-through wall crack was not able to be precisely controlled. The fabrication method established in this work was useful to simulate the SCC defect having similar size and ECT signals as compared to the field cracks in the SG tubings of the operating Korean PWRs.

Robust and accurate crack reconstruction for eddy current non destructive testing

Fast and accurate crack detection techniques are required for the eddy current non destructive testing, especially in the case where a large amount of environmental noise exists. This paper presents a robust inverse solution for accurate shape reconstruction of natural cracks. A robust formulation is employed to deal with the uncertainties caused by the noise in the eddy current testing signals. The proposed method is capable of producing more robust results than the conventional formulation in terms of the statistical analysis of the reconstruction tests.

Finite element simulation of the probe displacement in eddy current testing

In this paper an approach is proposed for the taking into account of the displacement of the probe in the finite element simulation of eddy current testing. The proposed approach allows to reduce the difficulties associated to the finite element meshing. The computations are performed using two finite element formulations. Two test ECT problems are proposed. Numerical results are compared with experimental data. Eddy current testing (ECT) is widely used for the inspection of conductive industrial components. Simulation techniques are needed for the design of ECT probes and for the analysis of the ECT signals issued from inspections. The finite element method (FEM), with its flexibility and its ability to handle complex geometries, plays an important role in the modelling techniques used in ECT. Nevertheless, an efficient FEM simulation of ECT leads to consider some problems. The first problem is how simulate the complete inspection of a component (probe scan) without remeshing at each ECT probe position relatively to the component. To take into account the differ- ent positions of the probe when the FEM is used, a full or partial remeshing can be used. However from a computational cost point of view, this approach can be unfavourable when, for example, hundreds of probe positions are considered. A solution is then to use a mesh connecting method (i.e. a domain decomposition method) with a mesh related to the probe and another related to the component. The second difficulty is to take into account efficiently the thin regions that can be present in an ECT problem: lift-off (probe-component distance), thin coatings, thin deposits, flat coils.... Indeed, find a suitable mesh with a good compromise between the number of elements and their quality is uneasy. In orderto overcomethe problemscausedby the thin regionsin the FEM the most appropriatesolution is to model these areas with special elements. In fact, with a simplicial mesh, it is difficult to have a good quality of mesh in a thin region with a reasonable number of elements. It is therefore recommended to

Quantitative estimating size of deep defects in multi-layered structures from eddy current NDT signals using improved ant colony algorithm

Detection and quantitative estimation of deep defects in multi-layered structures is an essential task in a range of technological applications, such as maintaining the integrity of structures, enhancing the safety of aging aircraft, and assuring the quality of products. A novel approach to accurately quantify the two-dimensional axisymmetric deep defect size from eddy current nondestructive testing (NDT) signals is presented here. The method uses a finite element forward model to simulate the underlying physical process and an improved ant colony algorithm (IACA) to solve the inverse problem. Experiments are carried out. The performance comparison between the IACA method and the least square method is shown. The comparison results demonstrate the feasibility and validity of the IACA method. Between them, the IACA method gives a better estimation performance than the least square method at present.

Novel electromagnetic modeling approach of carbon fiber-reinforced polymer laminate for calculation of eddy currents and eddy current testing signals

Due to the heterogeneous nature and electric anisotropy, it is challenging to establish a numerical model to analyze the electromagnetic properties of multilayer carbon fiber-reinforced polymer (CFRP) laminate. In this study, we focus on the exploration of an effective electromagnetic modeling approach for calculation of eddy currents in CFRP laminate composite, as well as eddy current testing signals due to surface cracks. In order to prove the feasibility of modeling CFRP laminate with homogeneous anisotropic layers, the electrical parameters in the three directions are measured, and eddy current path in CFRP is investigated according to the measurement results. A finite element solver based on reduced magnetic vector potential ( A r) formulation and edge elements is developed to enable the eddy current simulation of anisotropic CFRP material, which avoids matching the discretization of source coils with the rest of conductor mesh, and can easily solve the field continuity problem in the interface between two adjacent fiber layers of CFRP laminate. The A r formulation and way to calculate the eddy current testing signals are described. To validate of the developed simulation code, a comparison is conducted between the calculated signals and experimental results of thin-opening cracks in a CFRP test piece, which indicates the simulation code can predict eddy current testing signals with good precision.

Coupled approach VIM–BEM for efficient modeling of ECT signal due to narrow cracks and volumetric flaws in planar layered media

Rapid and accurate modeling of Eddy Current Testing (ECT) signal is required in many industrial areas. For example, crack detection via ECT is widely employed in aeronautics and aerospace industry to inspect riveted planar multilayered structures. In these structures, small narrow cracks (e.g. micro-cracks) may initiate at the edge of rivet holes (which can be considered as large volumetric flaws) and propagate through the PMS. This paper proposes a new and efficient model, based on a coupled approach between Volume Integral Method (VIM) and Boundary Element Method (BEM), simulating ECT probe signals due to the presence of both narrow cracks located near volumetric flaws within a given PMS. Simulation results are compared with experimental and simulated signals obtained with a numerical code. The performance of the method, in terms of accuracy and computational time, is discussed and the perspectives opened are presented.

A Bayesian Network Method for Quantitative Evaluation of Defects in Multilayered Structures from Eddy Current NDT Signals

Accurate evaluation and characterization of defects in multilayered structures from eddy current nondestructive testing (NDT) signals are a difficult inverse problem. There is scope for improving the current methods used for solving the inverse problem by incorporating information of uncertainty in the inspection process. Here, we propose to evaluate defects quantitatively from eddy current NDT signals using Bayesian networks (BNs). BNs are a useful method in handling uncertainty in the inspection process, eventually leading to the more accurate results. The domain knowledge and the experimental data are used to generate the BN models. The models are applied to predict the signals corresponding to different defect characteristic parameters or to estimate defect characteristic parameters from eddy current signals in real time. Finally, the estimation results are analyzed. Compared to the least squares regression method, BNs are more robust with higher accuracy and have the advantage of being a bidirectional inferential mechanism. This approach allows results to be obtained in the form of full marginal conditional probability distributions, providing more information on the defect. The feasibility of BNs presented and discussed in this paper has been validated.

Reconstruction of stress corrosion cracks using signals of pulsed eddy current testing

A scheme to apply signals of pulsed eddy current testing (PECT) to reconstruct a deep stress corrosion crack (SCC) is proposed on the basis of a multi-layer and multi-frequency reconstruction strategy. First, a numerical method is introduced to extract conventional eddy current testing (ECT) signals of different frequencies from the PECT responses at different scanning points, which are necessary for multi-frequency ECT inversion. Second, the conventional fast forward solver for ECT signal simulation is upgraded to calculate the single-frequency pickup signal of a magnetic field by introducing a strategy that employs a tiny search coil. Using the multiple-frequency ECT signals and the upgraded fast signal simulator, we reconstructed the shape profiles and conductivity of an SCC at different depths layer-by-layer with a hybrid inversion scheme of the conjugate gradient and particle swarm optimisation. Several modelled SCCs of rectangular or stepwise shape in an SUS304 plate are reconstructed from simulated PECT signals with artificial noise. The reconstruction results show better precision in crack depth than the conventional ECT inversion method, which demonstrates the validity and efficiency of the proposed PECT inversion scheme.

Efficient Modeling of ECT Signals for Realistic Cracks in Layered Half-Space

Efficient modeling of eddy current testing (ECT) signals is needed in many areas of industry. Design of probes may be improved and interpretation of experimental signals better understood by using dedicated numerical simulation tools, if they are computationally effective and accurate, yet remain simple enough to be applied at an end-user level. A boundary element method (BEM), dedicated to the numerical simulation of ECT signals due to complex narrow cracks within a planar multilayered structure (PMS) and presenting arbitrary orientations, is investigated. The theoretical formulation relies on the calculation of the dyadic Green operator, associated to the PMS, via appropriate vector wave function expansions. Then, the use of the discrete complex image method followed by the application of the generalized pencil of function method is proposed for efficient computation of this operator. Results of the validation of the complete model by comparison with the experimental data acquired in the laboratory-controlled conditions and with data computed by a finite element code are discussed.

A Numerical Method for Simulation of Nonlinear Eddy Current Testing Signals Based on Transient <i>A</i><sub>r</sub> Formulation

To explore the principle and to optimize the nonlinear Eddy Current Testing (ECT) system, a forward numerical analysis code for nonlinear ECT signal simulation is indispensable. In this paper, an edge element numerical code of the transient Ar (reduced A) formulation is developed for this purpose. The nonlinear feature of the permeability in each element is taken into account at every time step during step by step numerical integration procedure. The dependence of the third harmonic component of the response signals on the initial permeability is investigated by using the developed code. The qualitative agreement of the simulation and the experimental results demonstrated the feasibility of the proposed simulation method and the corresponding numerical code. [doi:10.2320/matertrans.M2013034]

Decline in ambiguity of partially conductive cracks' depth evaluation from eddy current testing signals

The paper deals with eddy current non-destructive inspection of partially conductive cracks. A non-magnetic conductive plate specimen is examined using numerical simulations. A crack with the rectangular shape is modelled in the middle of plate. Dimensional parameters and the conductivity of the crack are varied in wide ranges to imitate diversity of practical scenarios. A new eddy current probe is designed for the near-side inspection of the plate. An exciting system of the probe drives uniform eddy currents in the plate and all the three spatial components of the perturbation electromagnetic field are sensed with a magnetic sensor. Influences of the crack's partial conductivity as well as its dimensions on the perturbation field's components are studied in order to enhance evaluation of real partially conductive cracks.

Applications of multi-frequency inversion algorithm to quantitative NDE of cracks in welding joints of a metallic lattice sandwich plate

Latticesandwichmetallicplate(LSMP),anewmultifunctionalmaterialwithspecialadvancedmechanicalbehaviours, has potentials to be applied to many important industries. Defects in welding joints of the LSMP may significantly reduce the material strength. Eddy Current Testing (ECT) is considered as a powerful NDT tool for the detection and sizing of defects in LSMP. The welding zone noises in ECT signals swarm the crack signals and may reduce the reliability of the quantitative defect evaluation. To overcome this difficulty, an inversion algorithm based on the strategy of multi-frequency ECT is proposed in this paper for sizing of welding zone crack. Good consistency of the reconstruction results is obtained for both simulated and measured ECT signals, which proves the validity of the proposed scheme.

An inversion scheme for sizing of wall thinning defects from pulsed eddy current testing signals

An inversion scheme for sizing of wall thinning defects from pulsed eddy current testing signals

Numerical analysis of correlation between fibre orientation and eddy current testing signals of carbon-fibre reinforced polymer composites

This paper deals with eddy current testing simulation for carbon-fibre reinforced polymer composites (CFRPs) using an edge - element FEM code. The FEM code is based on the A formulation and it uses the coupled analysis with external fields. The coil impedance variations due to the fibre orientation and cracks in CFRP plates with unidirectional plies are computed. The conductivity of CFRP is characterized as low value and high anisotropy, which cause the distinct difference between CFRP and isotropic metal ECT signals. The correlation of the fibre orientation with coil impedance variations is obtained, but the exact fibre arrangement in a composite plate can't be ascertained because of signal symmetry. Another method for determining the fibre orientation via the edge effect is presented in this study. According to the numerical results, the shape of cracks in a CFRP plate is not easily evaluated from the detection signal curves unless the fibre orientation is perpendicular to the crack path.

Development of a very fast simulator for pulsed eddy current testing signals of local wall thinning

A very fast numerical solver is developed for simulation of pulsed eddy current testing (PECT) signals caused by volumetric defects by introducing a database-type fast eddy current testing (ECT) simulation scheme for single frequency problems to the Fourier-series and interpolation-based PECT signal simulation code. First, the PECT signal simulation method based on a Fourier series scheme and an interpolation approach is briefly described. Then, a database-type fast numerical solver for single frequency ECT problems is introduced to the Fourier-series-based PECT simulation to enhance simulation efficiency. To cope with fast PECT signal simulation of 3D local wall thinning defects, a 2D shifting symmetry scheme is proposed for inspection targets of both plate and straight pipe geometries to reduce the computational burden required to establish databases of the unflawed field, which are necessary for fast ECT signal simulation. Using these strategies, a very fast numerical solver is developed for simulation of PECT signals of local wall thinning, and its validity is verified by comparing its numerical results with measured PECT signals and with those simulated using a conventional numerical code.

159 パルス渦電流探傷試験信号を用いた配管減肉形状の逆解析(材料力学IV,一般講演)

159 パルス渦電流探傷試験信号を用いた配管減肉形状の逆解析(材料力学IV,一般講演)