Evaluation of the lift-off robustness of eddy current imaging techniques

The development of lift-off invariant strategies is one of the main goals in Eddy Current Non-Destructive Testing research. In the present work, from the analysis of amplitude and phase signals of magnetic field sensors under Multi-Frequency Eddy Current excitation, two imaging procedures are analyzed and compared with respect to their ability to retrieve reliable results even in presence of huge changes of lift-off. A figure of merit based on the Signal to Noise Ratio evaluated on the 2D reconstructed images allows the comparison of the different strategies in terms of the quality of the image to show the defect. The numerical and the experimental tests realized show that the imaging procedure relying on the analysis of the phase-lag is quite insensitive to changes in the lift-off with respect to that based on the amplitude analysis. In detail the former guarantees good results even when the probe lift-off is randomly changed during the test with variations up to 3 mm, while the latter is able to tolerate only lift-off fluctuation lower than 1 mm.     

Difference in the detection limits of flaws in the depths of multi-layered and continuous aluminum plates using low-frequency eddy current testing

We examined the difference in the detection limits of flaws in the depths of multi-layered and continuous aluminum plates using low-frequency eddy current testing. The detection limits were measured by using a magneto-resistive sensor. Comparing the frequency of an applied magnetic field, the detection limit at 50 Hz is deeper than that at 1 kHz. Comparing the sample structure, the detection limit in the multi-layered samples is deeper than that in the continuous samples. These results are likely due to the differences in the skin depth and conductivity of the sample.     

Reduction of lift-off effects for pulsed eddy current NDT

The lift-off effect is commonly known to be one of the main obstacles for effective eddy current NDT testing as it can easily mask defect signals. Pulsed eddy current techniques, which are believed to be potentially rich of information, are also sensitive to the effect. An approach using normalisation and two reference signals to reduce the lift-off problem with pulsed eddy current techniques is proposed. Experimental testing on the proposed technique and results are presented in this report. Results show that significant reduction in the effect has been achieved mainly in metal loss and sub-surface slot inspection. The technique can also be applied for measurement of metal thickness beneath non-conductive coatings, microstructure, strain/stress measurement, where the output is sensitive to the lift-off effect.     

Impedance evaluation of rectangular coils for eddy current testing of planar media

As for any other eddy current probe coil, the analysis of rectangular ones plays a vital role towards understanding their characteristics and performance. In this paper, closed-form expressions are provided both for the impedance of rectangular coils of rectangular cross-section located above a conducting half-space and for the eddy current density induced therein. The formulation is general and can be easily used for any coil shape.     

Remote field eddy current technique applied to non-magnetic steam generator tubes

Unlike the impedance plane analysis form of common eddy current testing (ECT), the remote field eddy current (RFEC) technique is a through-transmission effect that reduces problems such as lift-off normally associated with ECT. In the inspection of steam generator (SG) tubes, the real issue is to detect the minute cracks growing up from the outside. However, using ECT, it is considered infeasible to accurately find them from the inside because of the limitations of penetration of eddy currents. This paper describes a finite-element approach to the solution of time-harmonic electromagnetic fields for the RFEC technique based on a magnetic vector potential and an electric scalar potential. A comparison is made of experimental and finite-element predictions of electromagnetic phenomena under the inspection of non-magnetic tubes. For the cracks outside demanding high sensitive and precise measurements in the SG tube inspection, numerical results are given for parameters to design a RFEC probe.     

Pulsed eddy current testing of ferromagnetic specimen based on variable pulse width excitation

Pulsed eddy current testing (PECT) is a rapidly developing technology which has wide potential applications. For the PECT system which uses detection coils, a no-reference-needed and more efficient method, for quantifying the wall thickness of the ferromagnetic specimen, should be found. In this paper, a kind of variable pulse width excitation is proposed. Based on the excitation, the slope that the relative increment of magnetic flux linear decays with the increase of pulse width in the semi-logarithmic domain is found to be an effective and no-reference-needed feature. First, the analytical expression for the relative increment of magnetic flux is presented, and the validity of the feature is verified by experiments. Then the potential factors affecting the feature are investigated in detail. Results show that when the electromagnetic properties of the specimen are invariant, the feature is independent of pulse width parameters, analysis interval and coatings thickness. At last, a quantitative method is demonstrated. More time could be saved for the narrow pulse comparing it with the existing excitations, and the feature could widely meet engineering applications.     

Numerical modelling and implementation of ferrite cored eddy current probes

This paper focuses on the development of a magnetic moment method of calculating vector field quantities for a highly permeable ferrite cored eddy current probe. Basis functions are used in this method to replace the scattered field caused by the probe core in accordance with the surface equivalence theorem. These functions are further developed and tested for accuracy and convergence. An efficient material profile equation, independent of probe coil and basis function properties, is also designed and verified. Collocation point selection and optimisation is finally undertaken leading to the accurate determination of probe source coil impedance. The accuracy of calculation is verified using an industry standard finite element solver.     

Study on defect classification in multi-layer structures based on Fisher linear discriminate analysis by using pulsed eddy current technique

Pulsed eddy current (PEC) is an emerging non-destructive testing technique with wide application potential. In this study, defect parameter identification in multi-layer structures is examined by using the PEC technique, and a Fisher linear discriminate analysis (FLDA)-based defect classification method is proposed. Defect localization and shape identification are investigated, and defects on the surface and subsurface of the third layer are discriminated. The time domain characterization method is introduced and researched by using the peak time and zero-crossing time of PEC response signals, the principal component analysis algorithm and the FLDA method. The feature extraction results of the three methods are used as the input values of support vector machine for defect classification and feature extraction, and the classification methods are compared. Theoretical analysis and experimental results show that the proposed method can contribute to effective classification for defect parameter identification.     

Detection and measurement of surface cracks in ferromagnetic materials using eddy current testing

Eddy current testing is used to detect and quantify defects in electric conductive materials. Defects investigated are specifically mechanical, chemical and microstructural heterogeneities. In this scope, artificial defects simulating superficial cracks have been achieved in order to establish an eventual relationship between the orientation, the shape and the size of the defect in one hand, and the signature that it generates in the impedance plane on the other hand. We determine in the first stage the operating conditions for the minimization of all signals that may perturb the crack signal such as the background one. We have been able to detect easily a surface crack on ferromagnetic steel in a large range of the frequency. Curves allowing the discrimination between signals corresponding to the presence of cracks and those corresponding to a lift-off have been established. The limit depth has as well been determined and we found that this limit has no relationship with the depth of penetration of the eddy current. The operative parameters such as the diameter and the working mode of the probe, the excitation frequency and the setting of the measurement equipment have been optimized for the testing of the quality and the reliability of critical ferromagnetic steel parts.     

Numerical evaluation of the ill-posedness of eddy current problems to size real cracks

This study evaluates whether or not eddy current testing is applicable to the sizing of cracks that appear in a general structure. Two 10 mm thick specimens with artificial stress corrosion cracking are prepared, eddy current testing is performed to gather eddy current signals that result from cracking, and numerical inversions are performed to evaluate the maximum depths of the cracking. The inversions estimate the depths of the cracks are 0.8 and 1.6 mm. Although the simulated signals agree well with the measured ones, destructive tests reveal that the true depths are 1.27 and 2.58 mm. Another numerical simulation is conducted to discuss the ill-posedness of the inverse problem of sizing crack depths from eddy current signals. The simulation simply models a crack as a rectangular region with a constant length and uniform conductivity inside and calculates the eddy current signals of 1024 cracks having variety of depths, widths, and conductivities. Analyzing the results of the simulation reveal that information contained in conventional single-frequency eddy current tests is not sufficient to size conductive cracks in a general sense.     

Multilayer planar rectangular coils for eddy current testing: Design considerations

Planar rectangular coils provide some interesting features different from those of pancake coils, such as good performance in conductivity measurement at high frequencies, high sensitivity to scratches and other shallow imperfections and the possibility of inspecting complex surfaces. The impedance of these coils has been modeled by different authors. A brief discussion of some analytical approaches in the literature is presented and calculations using the second-order vector potential approximation are introduced, aimed at the study of the influence of coil shape and size on the sensitivity to electrical conductivity of the substrate. Three cases were modeled, (A) coil in free space—inductance L₀ as a function of number of turns (n) and width of the spiral conductive lanes (c) for different coil inner shape factor (rr_i); (B) coil on a conductive substrate (Zircaloy-4)—impedance change, ΔZ, as a function of f·σ, the product of the test frequency (f) and the substrate electrical conductivity (σ) for different rr_i and (C) coil sensitivity to small changes in σ(Δσ).L₀=L₀(c, n) increased as expected with n and/or coil area and decreased with shape factor. Normalized ΔZ depends strongly on inner rr_i, the curves (or surfaces) for smaller values of rr_i enclosing those for larger values. Another shape factor, the outer shape factor (rr_o), was introduced. Strong dependence of the sensitivity to Δσ on n was observed, as well as the existence of an optimal theoretical frequency. Because the sensitivity to surface conditions also increases with coil size, it could be established that for conductivity assessment it is better to use coils with the smallest n and c, compatible with the particular application, and that the resolving power for this type of measurements is not greatly affected by the shape factor.     

Rapid computation of eddy current signals from narrow cracks

An existing boundary element method model for eddy current inspection of ideal or narrow cracks is revised. Using some lately devised concepts on efficiently and rapidly simulating canonical eddy current geometries, we show how this model can be made easier to implement and faster to calculate. The approach is largely based on a novel method for rapidly calculating the Green's function and reducing the matrix fill-time, and also on a similar method for rapidly computing the incident electric field. As a result, the model has been made an order of magnitude faster than the existing one without sacrificing accuracy. We pay attention to numerical analysis details and analyze issues that so far have not been clarified. The validity of our approach is also verified by the experiment. Although we have tested the model against published data, we have also produced our own precision measurements for surface and through-the-thickness slots in plates with the coils performing scans along and across the slots at various frequencies.     

Analysis of eddy current interaction between a parallel coil and a ferromagnetic pipe with remanence

Eddy current field excited by a parallel coil placed next to a ferromagnetic pipe with remanence is studied in this paper. The influence of the remanence is analyzed at first. The analytical solutions to the ferromagnetic pipe eddy current field are deduced by means of a second-order vector potential formalism, and the analytical expression for the impedance change of the excitation coil is constructed. Theoretical analysis of the influence for the remanence and the analytical solutions to the eddy current field are verified by experiments.     

管材涡流探伤最佳频率数值模拟与试验验证

目前广泛采用的确定涡流探伤频率的方法主要是特征频率法和试验法,两种方法得出的频率通常不是最佳频率。采用ANSYS数值计算方法对人工标准缺陷的信号进行数值模拟,从而找出对内外壁缺陷信号的幅度都比较大,并且内外壁缺陷信号的相位差接近极大的激励频率。经过试验验证,计算结果与试验结果完全符合。结论认为,用ANSYS信号模拟方法可以找出最佳的激励频率,既能兼顾内外壁缺陷的检测灵敏度,又能从相位上区分内外壁缺陷的激励频率。     

Automatic measurement and warning of tension force reduction in a PT tendon using eddy current sensing

Post-tensioning (PT) using a bundle of pre-stressed strands is a critical process for assembling pre-fabricated and modularized bridge members. However, the tension force gradually diminishes over time due to such factors as corrosion, creep, and steel relaxation. Such changes compromise the overall safety of such structures. In this study, an eddy-current-based tension-force-loss warning (EC-TFLW) technique is proposed to detect and warn automatically of excessive loss of tension force in a PT tendon. A ring-type eddy-current sensor (ECS) is mounted on the outer surface of a wedge holding a tendon, and the level of eddy current measured by the ECS is related to the tension force of the tendon. The advantages of the proposed technique include: (1) low power consumption, (2) low cost, (3) simple installation, and (4) automated warning. The performance of the proposed EC-TFLW technique was validated experimentally in a full-scale lab test of a 3.3-m long, 15.2-mm diameter, mono-strand tendon that was tensioned using a universal testing machine (from 20 to 180 kN). Statistical hypothesis testing using the chi-square distribution was applied to the measured eddy current signals, and if the decline in tension exceeded a certain level, a warning was sent out automatically.     

An eddy current array instrument for application on ferritic welds

This paper describes a multi-element, eddy current array for the local testing of ferritic specimens including welds. A novel transient method is used to extract coil impedance parameters, and has the advantage of fast measurement, digital compatibility and a minimum of front-end components; a useful feature when large element numbers are involved. The problem of displaying lift-off and flaw information from an array is solved by using a novel ‘volcano display’, which provides a simple two-dimensional image of conditions under the array. However, to produce this display, the burden of interpreting the coil element responses must be left to the instrument. The resident analysis software contains a number of operating modes which provide options for single element and multi-element operation. The array has been applied to the detection of cracks in the HAZ of a weld, where the large surface area of the probe permits a coarser scanning pattern to be used whilst providing a high probability of detection due to the number of elements deployed.     

Rapid prediction of eddy current testing signals using A−φ method and database

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.     

Magnetic field shielding technique for pulsed remote field eddy current inspection of planar conductors

Pulsed remote field eddy current (PRFEC) can be used for detection of deeply buried flaws in non-magnetic flat plates. However, detection sensitivity for buried defect is low due to the great depth and the weak detecting signal. To improve the defect detection sensitivity, the PRFEC probe is optimized with the magnetic field shielding techniques in this work. On one hand, the exciting coil with shielding structure can focus the indirect magnetic field to penetrate through the plate. On the other hand, the detecting coil and the region between these two coils with shielding material can block and minimize the magnetic field energy diffusion along the direct coupling path. In order to investigate the shielding technique which blocks the direct coupling magnetic field, simulation studies using finite element method (FEM) has been first conducted, where three different PRFEC probes have been simulated and compared. Both detection abilities to defects in different directions and to thickness variation have been analyzed by using the second probe with shielding structure. The conclusions derived from the simulation study have been validated through experimental studies. Both simulation and experimental studies have indicated that the shielding technique can improve the detection sensitivity of subsurface defects.