Lift-off Variations Research Articles

Monitoring of LISS Nozzle Proximity to CANDU® Fuel Channels Using the Eddy Current Gap Probe

Fuel channels in CANDU® (CANadian Deuterium Uranium) nuclear reactors consist of two non-concentric tubes; an inner pressure tube (PT) and a larger diameter calandria tube (CT). The gas annulus space between the 6 m long tubes, known as the gap, is monitored from within the PT using a transmit-receive eddy current probe that can detect changes in the environment using electromagnetic induction according to Faraday’s Law. Accurate measurements are required to ensure that contact between PT and CT is not imminent. Fuel channels may also pass over perpendicularly oriented tubes (nozzles) that are part of the liquid injection shutdown system (LISS). The proximity of LISS nozzles not only compromises gap measurement, but flow-induced vibrations from within the moderator could lead to CT deformation and/or fretting at LISS-CT contact. Investigation of the eddy current based gap probe as a tool to measure proximity of LISS nozzles was carried out experimentally. Eddy current response as a function of LISS-PT proximity, the distance between the PT outer diameter and LISS nozzle outer diameter, was determined. When PT wall thickness, PT resistivity, probe lift-off, and PT-CT gap variations spanned typical values found in-reactor this dependence could be used to determine the LISS-PT proximity up to 20 mm with sub-millimeter accuracy. This method has the potential to provide LISS-CT proximity using existing gap measurement data. Obtaining LISS nozzle proximity at multiple inspection intervals could be used to provide an estimate of the time to LISS-CT contact, and thereby, provide a means of optimizing maintenance schedules.

A new high-frequency eddy current technique for detection and imaging of flaws in carbon fibre-reinforced polymer materials

Using a high-frequency (50 kHz-4 MHz) alternating current field measurement (ACFM)-style inspection, non-destructive imaging of carbon fibre-reinforced polymers (CFRPs) was performed for the detection and evaluation of flaws developed during their manufacture or in-service use. This was achieved using quantum well Hall-effect (QWHE) sensors, which have proven suitability for use in different non-destructive testing and evaluation (NDT&E) applications based on their high sensitivity (they require lower strength applied fields and can detect smaller perturbations in the magnetic field), high linearity (high contrast and imaging evaluation capabilities), wide dynamic range (making them less sensitive to offset and lift-off variations), wide frequency operating range (DC to MHz) and compact size (5-70 microns depending on the application). Their advanced III-V semiconductor materials and design enable these characteristics. Their low capacitance allows them to be operated at significantly higher frequencies than coils of comparable sensitivity or size. As such, the inherent advantages of QWHE sensors have been used in conjunction with a high-frequency ACFM-style magnetic imaging inspection technique, which is referred to as quantum well eddy current field measurement (QW-ECFM) in this paper. Here, the fundamentals of this new technique are outlined, as well as the outcomes of such a technique for evaluating CFRP materials, where individual fibre bundles have been resolved in high detail with high contrast. In addition, the ability to detect fibre misalignment has been shown, suggesting technique sensitivity to 3D orientations of fibre for better material qualification and the detection of delamination down to 2 mm in diameter. Therefore, this paper aims to provide an overview of this new QW-ECFM technique and summarise its performance for the detection and evaluation of various CFRP material flaws that are commonly found during manufacture and service.

Semi-Precise Analytical Method for Investigating the Liftoff Variation on the Hall Sensor in Metal Defect Sensing.

Hall-effect sensors are used to detect metal surface defects both experimentally and numerically. The gap between the specimen and the sensor, called the liftoff, is assumed to remain constant, while a slight misplacement of a sample may lead to incorrect measurements by the Hall-effect sensor. This paper proposes a numerical simulation method to mitigate the liftoff issue. Owing to the complexity of conducting precise finite-element analysis, rather than obtaining the induced current in the Hall sensor, only the magnetic flux leakage is obtained. Thus, to achieve a better approximation, a numerical method capable of obtaining the induced current density in the circumferential direction in terms of the inspection direction is also proposed. Signals of the conventional and proposed approximate numerical methods affected by the sensor liftoff variation were obtained and compared. For small liftoffs, both conventional and proposed numerical methods could identify notch defects, while as the liftoff increased, no defect could be identified using the conventional numerical method. Furthermore, experiments were performed using a variety of liftoff configurations. Based on the results, considering the threshold of the conventional numerical method, defects were detected for greater liftoffs, but misdetection did not occur.

Estimating Al 1230 clad thicknesses over Al 2024 alloys using apparent eddy current conductivity measurements

The recent developments in apparent eddy current conductivity (AECC) spectroscopy have made it a viable candidate for clad thickness estimation where both clad and substrate are nonmagnetic metals. It negates the previous requirement of establishing a probe characteristic curve with a consistent lift-off distance between the measured sample and the calibration blocks. This eliminates the adverse effects in measurement uncertainty that occurred earlier due to liftoff variations. This study presents a practical approach for clad thickness estimation using AECC spectroscopy targeting clad thicknesses relevant to the industry. The technique once validated both numerically and experimentally has shown to deliver a measurement uncertainty of 3% over the entire coating thickness range of interest at a ±25.4μm lift-off range. Furthermore, it also reduces the coil diameter requirement to as low as 3 mm without the need of any correction factors.

Evaluation of Coating Thickness Using Lift-Off Insensitivity of Eddy Current Sensor.

Defect detection in ferromagnetic substrates is often hampered by nonmagnetic coating thickness variation when using conventional eddy current testing technique. The lift-off distance between the sample and the sensor is one of the main obstacles for the thickness measurement of nonmagnetic coatings on ferromagnetic substrates when using the eddy current testing technique. Based on the eddy current thin-skin effect and the lift-off insensitive inductance (LII), a simplified iterative algorithm is proposed for reducing the lift-off variation effect using a multifrequency sensor. Compared to the previous techniques on compensating the lift-off error (e.g., the lift-off point of intersection) while retrieving the thickness, the simplified inductance algorithms avoid the computation burden of integration, which are used as embedded algorithms for the online retrieval of lift-offs via each frequency channel. The LII is determined by the dimension and geometry of the sensor, thus eliminating the need for empirical calibration. The method is validated by means of experimental measurements of the inductance of coatings with different materials and thicknesses on ferrous substrates (dual-phase alloy). The error of the calculated coating thickness has been controlled to within 3% for an extended lift-off range of up to 10 mm.

Measuring Lift-Off Distance and Electromagnetic Property of Metal Using Dual-Frequency Linearity Feature

Lift-offs of the sensor could significantly affect the measurement signal and reconstruction of material properties when using the electromagnetic (inductive) eddy current (EC) sensor. Previously, various methods (including novel sensor designs, and features like zero-crossing frequency, lift-off point of intercept) have been used for eliminating the measurement error caused by the lift-off distance effect of the sensor. However, these approaches can only be applied for a small range of lift-off variations. In this article, a linear relationship has been found between the sensor lift-off and ratio of dual-frequency EC signals, particularly under the high working dual frequencies. Based on this linear relationship, the lift-off variation can be reconstructed first with a small error of 2.5% when its actual value is up to 10 mm (10.1% for 20 mm). The reconstructed lift-off is used to further get the property of the material under a low single frequency. Experiments on different ferrous metals have been carried out for the testing of the reconstruction scheme. Since the inductance is more sensitive to the material property (and less sensitive to the lift-off) under low frequencies, the reconstruction error of the material property is much smaller than that of the lift-off, with 1.4% under 12 mm (and 4.5% under 20 mm).

Measurement of Ferromagnetic Slabs Permeability Based on a Novel Planar Triple-Coil Sensor

Eddy current methods are wildly used for measuringthe electromagnetic property of ferromagneticsamples due to its high sensitivities and portability (when using the remote electromagnetic sensor). However, in diverse applications, the eddy current measurement is always hampered by the lift-off variation – the gap between the sensing probe and the sample. In this paper, the measurement error of the permeability caused by the lift-off variation could be significantly reduced when using a developed planar triple-coil sensor. An inverse algorithm, the single-frequency eddy current algorithm is established for the evaluation of the permeability by referring to the phase of themeasured impedance or inductance. Based on the experiments of several ferromagnetic samples when using the developed sensor and inverse algorithm, the measured permeability is proved can be almost immune to the lift-off variations.

Identification of Tiny Surface Cracks in a Rugged Weld by Signal Gradient Algorithm using the ACFM Technique.

It is still a big challenge to identify tiny surface cracks in a rugged weld due to the lift-off variations using the nondestructive testing (NDT) method. In this paper, the signal gradient algorithm is presented to identify the tiny surface crack in the rugged weld using the alternating current field measurement (ACFM) technique. The ACFM simulation model and testing system was set up to obtain the insensitive signal to the lift-off variations. The signal gradient algorithm was presented to process the insensitive signal for the identification of the tiny surface crack in the rugged weld. The results show that the Bz signal is the insensitive signal to lift-off variations caused by the rugged weld. The signal to noise ratio (SNR) of the crack identification signal was greatly improved by the signal gradient algorithm, and a tiny surface crack can be identified effectively in the weld and the heat affected zone (HAZ).

Investigation on Time-to-Peak Feature Insensitive to Liftoff Effect for Pulsed Eddy Current Evaluation

The influence of liftoff variation on the peak value and time-to-peak feature of pulsed eddy current (PEC) signals is investigated in this article by simulations and experiments. First, PEC response is analytically modeled via Fourier transform for simulations. In an alternative way, PEC response is formulated with Laplace transform. Using an approximation of Laplace transform-based PEC formulation, the physics that liftoff distance only changes the amplitude of PEC responses rather than shifting the position of peak heights is discovered. Second, simulations were implemented, and the results demonstrate that the time-to-peak feature is insensitive to liftoff variation, i.e., around one-fourth of the peak value. Finally, the experimental results agree well with those from the simulations, which validates the physics that the time-to-peak feature is insensitive to the liftoff effect.

Accurate measurements of plate thickness with variable lift-off using a combined inductive and capacitive sensor

In eddy current testing, lift-off variations between sensor and sample plate affect sensor inductance and consequently inferred thickness. In this paper, a novel combined inductive and capacitive sensor is designed in which the output of the capacitive sensor can be used to infer the lift-off. An algorithm is proposed to combine the inferred lift-off and the inductance measurement for predicting thickness. The effect of lift-off variations is significantly reduced using this combined sensor approach. Compared to the multi-frequency thickness measurements in our previous work, this only requires a single frequency measurement for each of the capacitive and inductive modalities, leading to a relaxation of the instrument and measurement requirements.

Non-destructive testing for carbon-fiber-reinforced plastic (CFRP) using a novel eddy current probe

Carbon-fiber-reinforced plastic (CFRP) is of low conductivity and has a layered structure. High-frequency transmitter-receiver (T-R) probes are widely chosen to inspect CFRPs using eddy current testing (ECT). However, in these works, the variation in the distance between the probe and test sample can cause a larger signal than that caused by defects and may cover up the defect. The detection sensitivity was also reduced by random noise resulting from lift-off change. To address these issues, it is meaningful to design a probe which can overcome the effect of lift-off variation and meanwhile offer high sensitivity to defects in CFRPs. In this study, a T-R probe with a special structure for detection of CFRPs was developed. The probe contains an 8-shaped transmitter coil (TX coil) and a circular receiver coil (RX coil), which is placed on a line equidistant from the two parts of the transmitter coil. Theoretically, regardless of how the lift-off changes, the output signal is always 0 if the azimuth of the probe agrees with one of the fiber orientations of an intact CFRP. Experimental studies demonstrate that the proposed probe is insensitive to lift-off compared with a traditional T-R probe and offers high sensitivity to defects. For defect detection, in-plane waviness can be detected with the proposed probe. Quantitative experiments for crack detection were performed. The cracks were clearly visualized in the scanning images. The length and location of the cracks can also be estimated from the scanning images.

Comparison of time and frequency domain features’ immunity against lift-off in pulse-compression eddy current imaging

This paper reports a comparison between imaging procedures based on time- and frequency-domain features applied to eddy current data collected on an Aluminium alloy benchmark sample. Both the defect detection capability and the immunity against lift-off are analysed. Historically, the pulsed eddy current method was introduced to improve the performance of eddy current testing both in terms of increased penetration depth and lift-off invariance by exploiting time-domain features analysis, commonly precluded when using single- or multi-tone excitations. In this work, such analysis is instead accomplished by using a swept-frequency excitation signal along with an optimized pulse-compression procedure. This approach allows the user to easily tune the energy delivered to the system by increasing the sweep duration to improve on demand the resultant Signal-to-Noise Ratio (SNR) without losing the capability of time-domain feature extraction. The imaging procedure makes use of amplitude and phase features of both frequency- and time-domain data where time-phase is defined through the Hilbert transform of the pulse-compression output. The detection capability of various imaging strategies, namely A-, B- and C-scans, are compared in terms of inspection depth and lift-off robustness by using the SNR merit factor. It is shown that time-domain features outperform frequency-based ones in terms of SNR for the case of deeper defects and that phase features are robust against lift-off variations for both time and frequency domains. In addition, the analysis of time-amplitude images clearly shows the presence of lift-off invariant points. To our knowledge, this is the first experimental evidence of the lift-off invariance points retrieved after applying pulse-compression in combination with coded excitation instead of using directly pulsed, multi-tone or single-tone sinusoidal signals. This not only confirms previous results achieved by the authors but also demonstrates that pulse-compression eddy current can represent a solution to combine the advantages of pulsed and sinusoidal excitation strategies.

Thickness measurement of non-magnetic steel plates using a novel planar triple-coil sensor

In the eddy current testing, the impedance tested by the sensor is influenced by the lift-off between the sensor and the sample plate. Consequently, the thickness inferred from the measured impedance is also affected by the lift-off. In this paper, a novel planar triple-coil air-core sensor is designed, in which three coils (two excitation coils and one receiving coil) with different radii are co-axially arranged in the same plane. Based on this sensor setup, an algorithm for the thickness measurement of the non-magnetic metallic plate is proposed. By using the transmitter-receiver combinations and the proposed algorithm, the measured thickness is almost immune to the lift-off variation. Moreover, previously the measurement of thickness requires a complex frequency feature – peak frequency for the real part of the impedance, which is obtained from the multi-frequency impedance or inductance spectra (frequency-sweeping mode), while with the method proposed in this paper, only single frequency measurement is needed, hence relaxing the instrument and measurement requirements. This method has a potential of on-line real-time measurement of the thickness for non-magnetic steel plates.

Extraction Method of Crack Signal for Inspection of Complicated Steel Structures Using A Dual-Channel Magnetic Sensor.

Conventional eddy current testing (ECT) using a pickup coil probe is widely employed for the detection of structural cracks. However, the inspection of conventional ECT for steel structures is difficult because of the magnetic noise caused by the nonuniform permeability of steel. To combat this challenge, we have developed a small magnetic sensor probe with a dual-channel tunneling magnetoresistance sensor that is capable of reducing magnetic noise. Applying this probe to a complicated component of steel structures—such as the welds joining a U-shaped rib and deck plate together—requires the reduction of signal fluctuation caused by the distance (liftoff) variations between the sensor probe and the subject. In this study, the fundamental crack signal and the liftoff signal were investigated with the dual-channel sensor. The results showed that the liftoff signals could be reduced and differentiated from the crack signals by the differential parameters of the dual-channel sensor. In addition, we proposed an extraction technique for the crack signal using the Lissajous curve of the differential parameters. The extraction technique could be applied to the inspections not only for flat plates but also for welded angles to detect cracks without the influence of the liftoff signal.

Microwave open-ended waveguide for detection and characterisation of FBHs in coated GFRP pipes

Accessing to glass fibre reinforcement plastic (GFRP) pipes in the on-line process is often limited to one side. To detect an internal defect of coated GFRP pipe, common non-destructive testing (NDT) methods such as eddy current testing and thermography are not effective because GFRP pipes are not conductive and their coating prevents heat transfer to the inner section of the material. Moreover, the curvature of GFRP pipes introduces lift-off variation, which is the primary challenge for NDT methods using C-scan technique; e.g., ultrasonic. This work proposes a novel microwave NDT application to detect and characterise internal defect; i.e., flat-bottom hole (FBH) in coated GFRP pipes. Initially, responses of four FBH areas obtained by microwave open-ended waveguide probe are analysed using principal component analysis (PCA) to reveal FBH defect and eliminate the pipe curvature influence. Following that, synthetic aperture radar (SAR) tomography images are produced to select the corresponding principal component and verify the location of FBHs. Finally, FBHs are evaluated using PC feature and SAR tomography including time-of-flight feature. The results show that the proposed method can clearly reveal the area of FBHs. Moreover, depths of FBHs are characterised by analysing the time-of-flight feature of background-subtracted responses.

A decay time approach for linear measurement of electrical conductivity

Nitrogen contamination of weld material affects mechanical properties and results in small change of the electric conductivity. This small difference of resistivity needs to be measured since the material property is affected by the nitrogen contamination level. A major challenge in determining the contamination level is the accurate measurement of resistivity in the presence of lift-off variation, top oxide layer and surface irregularities. In this paper, a novel approach based on Decay Time (DT) using Pulsed Eddy Current (PEC) and a high sensitive magnetoresistive (MR) sensor is proposed to measure small changes in electrical conductivity of materials. The DT parameter defined herewith is linearly correlated with material conductivity allowing it to be used for estimating the conductivity from transient PEC measurements. Moreover, this method is less sensitive to lift-off variations. The operation principle of this method is demonstrated using finite element model (FEM) simulation and validated using a prototype probe comprising an excitation coil and MR sensor. Weld metal samples with varying levels of nitrogen contamination are tested using the prototype probe demonstrating the feasibility of the method.

Eddy current evaluation of metallic coating thicknesses over nonmagnetic metals

The use of titanium and nickel alloys in metal coating technologies have been gaining rapid popularity due to their high strength characteristics and excellent corrosion resistance behaviour operating at different temperature ranges. Phase-sensitive eddy current technique set by the International Organization for Standardization to assess metal coating thicknesses over nonmagnetic metals delivers ±30% uncertainty in ±25.4 μm lift-off variation from that used over metal-coated calibration blocks. This study takes the first step towards adopting AECC spectroscopy as an alternative forward measurement technique, which offers a reduced sensitivity to lift-off and sample conductivity deviations from those used over the calibration blocks in a broad range of inspection frequencies. To map the depth-dependent conductivity profile from the frequency-dependent AECC measurement, a new AECC-based inversion algorithm is developed to estimate metallic coating thicknesses over nonmagnetic metals. This work is validated following the plane-wave approximation and COMSOL simulations over different coating thicknesses relevant to the industry. Results indicate that AECC spectroscopy can potentially deliver ±3% uncertainty in metallic coating estimation in ±25.4 μm lift-off range. That is a significant improvement over existing eddy current measurement methods.

Solution for a transmit-receive eddy current probe above a layered planar conductive structure

Exact solutions for the electromagnetic response of a transmit-receive coil pair situated above two parallel plates that are separated by a gap, have been developed. The analytical model was used to characterize the effect of variations in probe liftoff, conductor resistivity, plate wall thickness, and plate-to-plate gap. All electromagnetic coupling coefficients arising from the probe and layered plate conductors were determined and substituted into Kirchhoff's circuit equations to calculate the induced voltage in the pickup coil. Excellent agreement with experiment was observed for differing first layer plate resisitivities (174 μΩ cm and 53.9 μΩ cm), first layer plate thicknesses (3.80 mm and 3.18 mm), second layer plate resistivity (1.72–174 μΩ cm), second layer plate wall thickness (1.20 mm–4.85 mm) and probe liftoff (2.81 mm and greater), providing confidence in the general validity of the model.

Reducing the Lift-Off Effect on Permeability Measurement for Magnetic Plates From Multifrequency Induction Data

Liftoff variation causes errors in eddy current measurement of nonmagnetic plates as well as magnetic plates. For nonmagnetic plates, previous work has been carried out to address the issue. In this paper, we follow a similar strategy, but try to reduce the lift-off effect on another index, zerocrossing frequency for magnetic plates. This modified index, termed as the compensated zero-crossing frequency, can be obtained from the measured multifrequency inductance spectral data using the algorithm we developed in this paper. Since the zero-crossing frequency can be compensated, the permeability of magnetic plates can finally be predicted by deriving the relation between the permeability and zero-crossing frequency from Dodd and Deeds method. We have derived the method through mathematical manipulation and verified it by both simulation and experimental data. The permeability error caused by liftoff can be reduced within 7.5%.

Pulsed magnetic flux leakage method for hairline crack detection and characterization

The Magnetic Flux leakage (MFL) method is a well-established branch of electromagnetic Non-Destructive Testing (NDT), extensively used for evaluating defects both on the surface and far-surface of pipeline structures. However the conventional techniques are not capable of estimating their approximate size, location and orientation, hence an additional transducer is required to provide the extra information needed. This research is aimed at solving the inevitable problem of granular bond separation which occurs during manufacturing, leaving pipeline structures with miniature cracks. It reports on a quantitative approach based on the Pulsed Magnetic Flux Leakage (PMFL) method, for the detection and characterization of the signals produced by tangentially oriented rectangular surface and far-surface hairline cracks. This was achieved through visualization and 3D imaging of the leakage field. The investigation compared finite element numerical simulation with experimental data. Experiments were carried out using a 10mm thick low carbon steel plate containing artificial hairline cracks with various depth sizes, and different features were extracted from the transient signal. The influence of sensor lift-off and pulse width variation on the magnetic field distribution which affects the detection capability of various hairline cracks located at different depths in the specimen is explored. The findings show that the proposed technique can be used to classify both surface and far-surface hairline cracks and can form the basis for an enhanced hairline crack detection and characterization for pipeline health monitoring.