Pulsed eddy current thickness measurement using phase features immune to liftoff effect

Conventionally, peak value and peak time are extracted from pulsed eddy current (PEC) response as features for thickness measurement. However, they suffer from liftoff variations. In this work, the phase of spectral PEC response from a Hall sensor are proposed to serve as robust features for thickness evaluation. The presented novel features are immune to liftoff effect, because phase signals remain nearly constant against liftoff variations. An analytical model was formulated, and simulations were performed to uncover the physics of the characteristics of the phase feature and establish the relationship between the phase feature and sample thickness. Experiments were carried out to validate the proposed phase feature. Eventually, the proposed phase feature was evaluated for accurate thickness measurement and some key factors were discussed.     

Improvement of sensitivity of eddy current sensors for nano-scale thickness measurement of Cu films

Measurement of nano-scale copper film thickness is of great importance in the semiconductor industry. The eddy current method is used for the purpose due to its non-destructive and fast dynamic response features. In this paper, an equivalent circuit model is used to get the relationship between the measurement sensitivity and sensor parameters. It is found that the internal resistance of an eddy current sensor plays a primary role in the improvement of the measurement sensitivity beside of the Q factor of the sensor. A simple experimental setup is established and a series of Cu films with the thicknesses ranging from 20 nm to 350 nm are prepared as test samples. Test results indicate that the sensitivity of an optimized sensor made of a lower resistant multi-wire Cu line has better sensitivity than that wound with a higher resistant single Cu wire under large lift-off.     

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.     

Characterization of ductile cast iron by eddy current method

Eddy current method was applied for evaluation of material properties of ductile cast iron. Measurements of eddy current response signals from FCD 450–600 grades of cast iron demonstrated good correlation with their hardness and tensile properties. Alternative ultrasonic method was explored for assessment of hardness of the studied samples and showed its inadequacy for specimens with similar graphite nodularity.     

Pearlite determination in plain carbon steel by eddy current method

Eddy current method has been used for the evaluation of pearlitic microstructure in low to high plain carbon steels. Plain carbon steel samples were heat treated in a furnace. Half of these were slowly cooled in the furnace and half in the air. This resulted in different pearlitic microstructures in the samples. Eddy current readings for all the samples were recorded and studied. It was found that eddy current measurement can be used for obtaining reliable and quick detection of pearlitic percentage in plain carbon steels.     

A detecting method for spherical fuel elements in pebble-bed HTGR using eddy current detection

In pebble-bed high temperature gas-cooled reactors (HTGRs), spherical fuel elements move inside pipelines of a handling system, which should be controlled precisely. A detecting method for these elements is proposed in this paper, which is achieved by a detecting system with self-diagnosis function. Detecting signals are obtained by sensors installed outside pipes. A signal identification algorithm was designed for graphite ball detection. Electromagnetic simulations and detecting experiments were performed for system optimization and development of the method. The results show that the proposed method is capable for spherical fuel elements detection, and can be successfully used in practical application.     

Lift-off effect in high-frequency eddy current conductivity spectroscopy

Precision eddy current measurements have been shown to be capable of characterizing the near-surface residual stress and cold work profiles in surface-treated components. To capture the peak compressive residual stress in moderately shot-peened (Almen 4–8 A) nickel-base superalloys, the eddy current inspection frequency has to be as high as 50–80 MHz. Unfortunately, spurious self- and stray-capacitance effects render the complex eddy current coil impedance variation with lift-off, the so-called lift-off curve, highly nonlinear, which makes it difficult to achieve accurate eddy current conductivity measurements beyond 25 MHz in the presence of even the slightest lift-off uncertainties. As opposed to the well-known inductive lift-off effect that decreases with increasing probe size, the capacitive lift-off effect increases with probe size. Both effects increase with frequency with the inductive effect being initially stronger, but then taken over at high frequencies by the faster growing capacitive effect. Since the two effects produce opposite curvature in the lift-off curve, in the frequency range where they are approximately equal to the lift-off curve becomes essentially linear and fairly accurate, conductivity measurements can be conducted even in the presence of lift-off variations.     

Thickness measurement of non-magnetic plates using multi-frequency eddy current sensors

A robust feature in multi-frequency eddy current (MEC) testing has been found that can be directly linked to the thickness of the plate under test. It is shown mathematically that the peak frequency of the imaginary part of the inductance change when an air-cored coil is placed next to a non-magnetic metallic plate is inversely proportional to the thickness of the plate for a given material. Experimental results indicate that this relationship also holds for a ferrite-cored U-shaped coil. In addition, this peak frequency has been shown to be relatively independent of lift-off variations. Use of this new feature provides a fast and accurate method to gauge the thickness of plates. Measurements made for a sample air-cored and ferrite U-cored coil next to copper and aluminium plates of various thicknesses verified the proposed method.     

Application of eddy current nondestructive method for determination of surface carbon content in carburized steels

Apart from traditional application of nondestructive eddy current technique for detection of discontinuities, the method has been recently used to determine physical and metallurgical properties of steel parts. In the present research the application of eddy current method for determination of surface carbon of steel parts in gas carburizing process has been studied. The relation between the surface carbon content and various parameters such as impedance, phase angle and voltage has been established. Besides the effect of carbon content of the impedance plain has been investigated. The study shows that the best relation (R²=0.91) can be achieved using phase angle.     

Discrimination method of through-wall cracks in steam generator tubes using eddy current signals

A unique non-destructive technique to discriminate a through-wall crack and to measure the through-wall length of a through-wall crack was developed using the EDM notch specimens and verified by using real fatigue cracks. It is based on the distribution profile of the eddy current signal amplitude along the crack length direction. It was also verified that the internal pressurization of the in situ pressure test induced a plastic deformation of the crack, resulting in a remarkable increase of the signal amplitude.     

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.     

Iterative inversion method for eddy current profiling of near-surface residual stress in surface-treated metals

Because of their frequency-dependent penetration depth, eddy current measurements are capable of mapping the near-surface depth profile of the electric conductivity. This technique can be used to nondestructively characterize the subsurface residual stress distribution in certain types of shot-peened metals, e.g., in nickel-base superalloys. In this paper, a highly convergent iterative inversion procedure is presented to predict the frequency-independent intrinsic electric conductivity depth profile from the frequency-dependent apparent eddy current conductivity (AECC) spectrum. The proposed technique exploits three specific features of the subsurface electric conductivity variation caused by near-surface residual stresses in shot-peened metals. First, compressive residual stresses are limited to a shallow surface region of depth much less than typical probe coil diameters. Second, the change in electric conductivity due to residual stresses is always very small, typically less than 1%. Third, the electric conductivity depth profile is continuous and fairly smooth. The accuracy of the proposed iterative inversion procedure is one order of magnitude better than that of the previously developed simpler method (J Appl Phys 2004;96:1257).     

Investigation into eddy current pulsed thermography for rolling contact fatigue detection and characterization

This paper reports on the use of eddy current pulsed thermography (ECPT) for detection and characterization of rolling contact fatigue (RCF). Detection mechanisms with eddy currents and heat propagation effects were discussed with RCF modeled as a simple angled defect. Two different angled defects were studied through numerical simulations and experimentally by using uniform magnetic field (UMF) excited by Helmholtz coils. Finally, a rail sample with RCF defects was inspected using UMF excitation. It is shown that ECPT with UMF excitation provides an efficient and robust method to detect angled defects, compared with nonuniform magnetic field (NUMF) excitation.     

Inspection of defects in conductive multi-layered structures by an eddy current scanning technique: Simulation and experiments

The forward problem of eddy current detection of defects by scanning conductive multi-layered structures is investigated and the change of the probe coil impedance is modeled by using finite element analysis method. Based on the ANSYS software a fast simulating program is developed and then the coil impedance changes due to the existing of defects in different lengths, shapes and at different locations in conductive multi-layers are calculated. An experimental eddy current testing system combined with a scanner is established and scanning testing experiments are carried out. The simulation and experimental results are compared. The agreement of them shows that the technique studied is promising and can help us to understand the probe responses and can be applied to the inversion model to determine the defect parameters in many important fields ranging from aerospace to energy and transportation industries.     

Transient response of a driver-pickup coil probe in transient eddy current testing

Novel solutions that correctly incorporate all electromagnetic interactions arising in inductively coupled circuits are presented for the case of a coaxial driver and pickup coil probe encircling a long ferromagnetic conducting rod. The differential circuit equations are formulated in terms of the rod׳s impulse response using convolution theory, and solved by Fourier transform. The solutions presented here are the first to account for feedback between a ferromagnetic conductor and the driver and pickup coils, providing correct voltage response of the coils. Experimental results, obtained for the case of square wave excitation, are in excellent agreement with the analytical equations.     

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.     

Structural analysis of CFRP using eddy current methods

Nondestructive inspection of carbon fibre-reinforced plastics (CFRP) using eddy current methods is not only based on the conductivity of the fibres. High frequencies (up to 10 MHz) enable the exploitation of capacitive effects reflecting the properties of the matrix. The paper presents a method using rotating eddy current probes to measure the anisotropic electrical properties. Potential applications of the method are structural identification of laminates and estimation of their degradation. For example, the strength properties of constructions based on unidirectional CFRP are very sensitive to small differences in fibre direction angle. Best detection of these angles could be obtained above 7.5 MHz. In addition, a signal perpendicular to the fibre orientation was found and termed the plateau effect. It is a result of capacitive connections between the fibres and presents new opportunities for the characterization of matrix properties.     

Evaluation of the surface of alloy tool steel treated by cavitation shotless peening using an eddy current method

Cavitation shotless peening (CSP) of hot forging die made of alloy tool steel is an effective peening method because it increases the fatigue strength of the material, and both introduces compressive residual stress into the material and also work hardens it. In order to evaluate the peening intensity of the die, a quick non destructive method is needed. In this paper, the dependence of the electromagnetic properties of an alloy tool steel, Japanese Industrial Standard (JIS) SKD61, on CSP processing time was investigated using an eddy current method. The induced stress and changes in the microstructure depend on CSP processing time, and these affect the electromagnetic properties of the material, such as the electrical conductivity and the permeability. The peening intensity was evaluated by measuring these electromagnetic properties using the eddy current method.     

Transient and harmonic eddy currents: Lift-off point of intersection

It has been previously determined experimentally that the transient responses of pulsed eddy current excitation of a conductive semi-infinite plane pass through a common point, regardless of the lift-off value. The origin of this intersection point is investigated and related to the harmonic components of the Fourier series representation of the pulse. Discrete sinusoidal signals, when individually sent through the same inspection system, also exhibit this common point feature when only the lift-off is varied. Moreover, the transient responses can be reconstructed from the harmonic outputs by using truncated Fourier series.