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Tetrapole probes, also known as double-core probes, which consist of two pairs of orthogonal electromagnets, apply orthogonal magnetic fields to the surface of a ferrous sample and rotate the field by superposition of these fields. Tetrapole probes have the potential to perform rapid characterization of the magnetic anisotropy of ferromagnetic materials. Conventional understanding dictates that magnetic field superposition is linear. However, superposition in ferromagnetic materials is inherently nonlinear. In this work, an equation for the magnetic flux density generated by superposition of orthogonal fields from a tetrapole probe in a ferromagnetic, anisotropic, single net easy-axis material, is derived, along with an equation for magnetic Barkhausen noise root mean square (rms) and energy, for application to materials with flat surfaces. The model approximates the nonlinear superposition as cubic, inline with analogous optics results, and is accurate for low applied fields. Data, which are accurately fit with the model, are shown. The model facilitates useful analyses of measurements made by tetrapole probes and addresses the inherent nonlinearity of magnetic fields applied to ferromagnetic materials.

The residual magnetization technique is a new method for detecting stress concentration zones (SCZs) in ferromagnetic materials. Local rising of stress at SCZs, such as cracks, results in the development of local elastic and plastic deformation. The two deformation regimes produce different magnetic characteristics, which have competing effects on self-magnetic-flux-leakage signals. In the present research, samples with three different groove depths were subjected to different tensile stress levels. A surface scanning system was used to record the variation of magnetic signals at grooves of varying depth under steadily increasing stress conditions. The results show that elastic deformation increases peak-to-peak values of normal magnetic components and a maximum gradient of the normal magnetic component in the direction of applied stress, while the development of local plastic deformation reduces signal intensity at groove locations. The magnetic object (MO) model is used to explain the mechanism for excess flux leakage field at SCZs as being due to the conversion of magnetoelastic energy—introduced by the application of tensile stress up to the local yield point—into magnetostatic surface poles that are sensed as an increase in the normal component of leakage.

Based on relationships between electromagnetic properties and microstructural changes in steel, the current paper describes the development of an electromagnetic technique to determine steel harden...

Riveted aircraft structures are susceptible to fatigue cracking at fastener locations. Inspection for second layer cracking can be performed by bolt hole eddy current, but requires rivet removal. Pulsed eddy current was adapted for the inspection of aircraft structures in the vicinity of aluminum rivets, without requiring rivet removal. A pulsed eddy current probe was used in combination with modified principle component analysis and cluster analysis to detect notches that simulated cracks. The notches were located in the first and second layer of aluminum structures representative of aircraft skin and underlying structure that was held together by bucked aluminum rivets with universal heads. The technique can detect notches smaller than the overhang of the fastener head. When the aluminum layers were very thin (

Magnetic Barkhausen noise (MBN) can result from abrupt motion of domain walls due to their interaction with pinning sites during magnetization of ferromagnetic steel. However, understanding domain structure interactions with pinning site density is limited. In this article, the effect of density of pinning sites within grains of HY80 steel, on generation of MBN, was investigated. Pinning site density was modified by a heat treatment that produces temper embrittlement. Temper embrittlement refers to the reduction of fracture toughness of alloy steels when heated or slowly cooled through the embrittling temperature range and arises due to migration of impurity elements to grain boundaries. Samples, machined from a casting, were held at a constant temperature (525 °C) for different holding times, to generate varying degrees of temper embrittlement. MBN signal response was observed to decrease exponentially as a function of holding time with decay constants (relaxation time) in the range of 160-190 h. A reduction in the MBN signal along each sample's easy axis was attributed to different densities of impurity elements, which act as pinning sites for domain wall movement within grains. Samples were also characterized using a scanning electron microscope (SEM), hardness testing, and impact toughness testing. Hardness was observed to increase with holding time and MBN energy decreased linearly with increasing hardness. Furthermore, a linear relationship between impact energy and MBN energy was observed. This implies that MBN signal analysis can be used as an indirect measure of the change in material properties of the sample.

The CANDU® nuclear reactor fuel channels consist of a pressure tube (PT) contained within a calandria tube (CT). The gap between the hot (~300° C) PT and cooler (~50° C) CT is required since the contact could lead to hydride blister formation with subsequent cracking. This gap is measured using a drive-receive eddy current technology, providing assurance that the contact is not imminent. The analytical models of probe response to gap use flat plates to approximate the PT and CT geometry. In this paper, a semi-analytical model that approximates the PT within the CT geometry as two concentric tubes with correct PT curvature, but a CT diameter, which is adjusted to change gap, is compared with a flat-plate model of probe response to changing gap against experimental measurements using standard CT and PT samples, with varying wall thickness and resistivity that simulates in-reactor variation of these parameters. The concentric and flat plate models both predict gap values with an average error of 0.1 mm between contact and 5 mm gap at 4.2 kHz and 8.0 kHz. The concentric model has an average relative gap error at the maximum gap of 2% at 4.2 kHz and 8.0 kHz, whereas that of the flat-plate model is 10% and 12%, respectively. The improved accuracy of the concentric model at larger gaps is attributed to the incorporation of tube curvature, which limits the probe’s magnetic field spread when compared to the flat-plate model.

HY80 steel is a low-carbon steel known for embodying high strength and toughness properties. This steel is used in submarine applications. Temper embrittlement, which is the reduction of fracture toughness, occurs in steels when subject to aging and drastic temperature fluctuations. These changes occur in submarines over time while in underwater environments. During temper embrittlement, impurity atoms and carbides migrate to grain boundaries, which make the steel more susceptible to fracture. A non-destructive testing (NDT) method is desirable to assess the temper embrittlement damage in HY80. Magnetic Barkhausen Noise (MBN) is of interest as being a potential NDT method for analyzing HY80. Focusing on microstructural characterization and its effect on MBN could have implications for establishing an MBN based method to detect varied stages of temper embrittlement in HY80 steel. In this research, samples of HY80 were prepared and heat treated for 16–336 hours to mimic various degrees of temper embrittlement. Microstructural changes with heat treatment were characterized and connected to the MBN produced at each holding time. Methods consisted of performing scanning electron microscopy (SEM) and using an MBN measurement system. It was observed that as holding time increases, grain size increases and carbide density within the grains decreases. These carbides, which act as pinning sites, make it more difficult for domain walls to move, consequently affecting MBN energy.

Corrosion of carbon steel rebar in concrete structures, such as highway bridges and buildings, has a direct impact on their structural integrity since the rebar provides the tensile strength within the structure. Rebar strength depends on the remaining effective radius of a given rod. Long-time decay up to 0.1 s, in the transient response of pulsed eddy current (PEC), was examined as a potential method to quantify general corrosion in ferromagnetic rebar. The transient response of a coaxial solenoidal drive–receive coil pair, oriented parallel to the rebar axis, was analyzed over a range of distances into the concrete (liftoff) and rebar radii. At long times, the single exponential decay constant was largely independent of liftoff. A power law relationship for the characteristic decay time, consistent with long-time diffusion of electromagnetic fields into a rod, was observed. The intercept of a best-fit line to measured voltage decay decreased exponentially with liftoff and maintained a measurable response up to 110 mm distance for a 25 mm (1 in.) diameter rebar. This exponential decay was present in 22 mm (7/8 in.), 19 mm (3/4 in.), and 15 mm (5/8 in.) samples as well. Reported results demonstrate the potential for PEC to quantify remaining cross-sectional area of rebar in concrete structures.

A validated analytical model of a transmit–receive coil pair situated above two parallel plates, separated by an air gap, was used as the basis for an inversion algorithm (IA) to extract probe liftoff, second layer plate resistivity, and plate-to-plate gap from multi-frequency eddy current data. The IA was tested over a large range of first layer wall thickness (3.80–4.64 mm), second layer plate resistivity (1.7–174 µΩ cm), second layer wall thickness (1.20–4.85 mm), probe liftoff (2.8–7.9 mm), and plate-to-plate gap (0–13.3 mm). At nominal liftoff (2.8 mm), the IA achieved a gap measurement accuracy of ±0.7 mm and was able to return good estimates of the second layer resistivity within ±1 μΩ cm for low resistivity samples, but with decreasing accuracy for higher resistivities. When the gap was fixed, the IA was able to measure changes in probe liftoff (relative to nominal) to an accuracy of ±0.2 mm. The reported accuracy and a demonstration for the ability to accurately estimate parameters outside of the calibration range provide confidence in the potential utility of the algorithm.

Pipelines are the primary means of land transportation of oil and gas globally, and pipeline integrity is, therefore, of high importance. Failures in pipelines may occur due to internal and external stresses that produce stress concentration zones, which may cause failure by stress corrosion cracking. Early detection of stress concentration zones could facilitate the identification of potential failure sites. Conventional non-destructive testing (NDT) methods, such as magnetic flux leakage, have been used to detect defects in pipelines; however, these methods cannot be effectively used to detect zones of stress concentration. In addition, these methods require direct contact, with access to the buried pipe. Metal magnetic memory (MMM) is an emerging technology, which has the potential to characterize the stress state of underground pipelines from above ground. The present paper describes magnetic measurements performed on steel components, such as bars and tubes, which have undergone changing stress conditions. It was observed that plastic deformation resulted in the modification of measured residual magnetization in steels. In addition, an exponential decrease in signal with the distance of the sensor from the sample was observed. Results are attributed to changes in the local magnetic domain structure in the presence of stress but in the absence of an applied field.

Pulsed eddy current is an emerging technique for measuring wall thinning of steel under insulation. Ideally, it would be desirable to obtain the most localised information of pipe condition possible. The localization of information is constrained by the flow of eddy currents in the pipe. This in turn is heavily influenced by the orientation of the probe. This paper examines three different probe orientations, representative of all probe types for their influence on eddy current flow. All three orientations will have difficulty accurately measuring highly localized thinning, because the eddy currents that give rise to their signal spread out rapidly way from the thin region in the long time signal decay. Of the three orientations, the longitudinal one does the best job of localizing the eddy currents directly under the probe.

The use of eddy current (EC) arrays to detect damage in sandwich panels, such as disbonding of the carbon fiber reinforced polymer (CFRP) face-sheet to the core, is investigated. It is shown that the array is very sensitive to slight core crush and can readily find small dents and disbonds. At the same time, the eddy current array can look much deeper into the honeycomb to detect defects such as tears. The phase map of the EC signal can be used in some cases to distinguish between different types of damage. EC arrays offer the ability to rapidly scan large areas of CFRP panels.

Eddy current (EC) technology is commonly used for detecting flaws, measuring geometric parameters, or determining properties of conducting materials. However, the measurement of a particular parameter can become more challenging if multiple influential parameters vary simultaneously. In particular, eddy current-based measurement of separation (gap) between a pressure tube (PT) and a calandria tube (CT) in the fuel channels of CANDU® reactors is made more difficult by variations in PT wall thickness, resistivity, and probe lift-off. An analytical model of the EC response to changes in PT–CT gap has been developed by approximating the geometry of the PT within the larger diameter CT as a pair of concentric tubes, where gap is varied by changing the CT radius. In this article, this model is used in combination with an error minimization algorithm to construct an inverse algorithm for the extraction of PT–CT gap, PT resistivity (ρ), and PT wall thickness (WT) from measured multi-frequency eddy current signals. Application of a linear regression tool in MATLAB, with fourth-order polynomial fitting of modeled data with varying ρ and WT as a function of PT–CT gap, is used to obtain coefficients that depend on ρ and WT. Output of multidimensional fitting of these coefficients is scaled and rotated to calibration data. Finally, implementation of an error minimization algorithm in MATLAB is used to produce estimates of multiple target parameters from experimental data. Simultaneous extraction of either one, two, or three parameters is examined, using experimental data obtained at frequencies used for in-reactor inspection of 4.2, 8, and 16 kHz, or just two frequencies of 4.2 and 8 kHz. Under full gap variation conditions, the inverse algorithm predicts gap to within 0.1 mm at gaps between 0 and 9 mm and to within 0.4 mm at gaps between 9 and 18 mm. PT resistivity is predicted to within 1 μΩ cm (2% relative error) and PT wall thickness within 0.03 mm (1% relative error) when each is the only extracted parameter. An excellent agreement between actual and predicted values of gap demonstrates the potential of the inverse algorithm for application to in-reactor gap measurement and simultaneous extraction of either PT wall thickness or resistivity when the other parameter is known. The extraction of PT resistivity may be particularly useful, as this parameter cannot otherwise currently be measured in-reactor.

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.

A semi-analytical model of the eddy current response of a drive-receive coil configuration inside two conducting concentric tubes, where the coil axes are perpendicular to the inner tube's surface, is solved using the second-order vector potential formalism. This model determines the voltage response in the receive coil under a constant amplitude alternating drive voltage, accounting for all possible coupling terms in the system. Modeled receive coil voltage responses to changes in outer tube diameter, greater than the drive-receive coil spacing, are compared with experimental measurements and finite element method (FEM) modeling in the impedance plane. Analytical model results are found to be in excellent agreement with both experiment and FEM model results, thereby verifying the solutions.

Flow-induced vibration of loose parts located on steam generator (SG) tube sheets can result in SG tube damage and eventual loss of SG efficiency. Regular inspection of SG tubes at support structures can help extend SG life by detecting and monitoring flaws as well as identifying accumulation of loose parts. Application of conventional eddy-current technologies in this case, however, is limited when magnetite fouling is present or when loose parts are resting directly on the SG tube sheet. Pulsed eddy current (PEC) combined with a modified principal components analysis (MPCA) was investigated as a method of detecting and identifying a variety of loose part materials in close proximity to 15.9-mm outer diameter Alloy-800 SG tubes and surrounding steel support structures. Loose parts were found to be detectable for varying axial positions along the SG tube and when in contact with the tube sheet. When offset radially from the SG tube, 1.6-mm-diameter carbon steel parts with a strong magnetic component allowed detection up to 3.2 mm away, while nonferromagnetic stainless steel parts of the same size could only be detected up to 1.5 mm away. Signal variation in the presence of loose parts was attributed to differences in amplitude of transient response and in relaxation times for diffusion of transient magnetic fields through various materials. The ability to accurately detect and identify loose parts located in close proximity to SG tubes establishes PEC combined with MPCA as a potential candidate for SG inspection.

Aerospace sandwich panels subject to low-velocity impacts can develop damage in both the face sheet and the core that may compromise structural integrity. The standard repair manual for each aircraft specifies allowable damage limits that consider surface dents and the presence of disbonds or delaminations. A strategy for identifying the types of impact damage according to the standard repair manual using a series of non-destructive inspection techniques is presented for honeycomb sandwich panels with composite facesheets and metallic core. Eddy current is first used to identify impact sites, followed by thermography to detect disbonds or delaminations and laser scanning to measure the size of surface dents. Images of damage sites in honeycomb sandwich panels from low-velocity impacts created using a drop tower show that when used together, these inspection methods are able to detect and distinguish between three different damage configurations; core crush, surface dents, and adhesive disbonds or face sheet ply delaminations. While these three methods are commonly used non-destructive evaluation techniques, this paper focuses on their combined use in identifying different types of impact damage in aerospace sandwich panels in-situ according to the requirements of the standard repair manual. The combined methods are shown to address requirements of dent depth, dent area, and the identification of disbonds or delaminations in a more rapid, repeatable and robust manner than is currently employed.

Inspection for corrosion in steam generator (SG) tube support structures is a significant component of monitoring SG condition, since support degradation can lead to SG tube flaws, and thereby further SG deterioration. A pulsed eddy current probe was developed to augment SG inspection at carbon steel trefoil broach supports. The probe demonstrated capability to measure far side wall loss as small as 20% of the original 2.7-mm-thick broach support ligament from within the Alloy 800 SG tube. A power law fit of the data was found to describe signal variation at later times under conditions of varying wall thickness and lift-off. The associated power law exponent varied linearly with wall thickness and was observed to be largely independent of lift-off and angular orientation of the probe, identifying it as a potential parameter for ligament wall thickness monitoring. Voltage response integrated over time also provided a low-noise means of measuring the wall loss when lift-off variation and misalignment were not present.

Recent analytical solutions, that correctly describe transient eddy current signals in voltage-controlled driver-pickup circuits, are applied for the case of a coaxial probe encircling a long ferromagnetic conducting tube. Experimental results, obtained for the case of a square wave excitation, are in excellent agreement with the predicted driver and pickup responses. Using the forward solutions, a novel inverse method, that enables simultaneous and accurate characterization of magnetic permeability and electrical conductivity, has been developed. Specifically, the method considers computed areas under scaled transient eddy current signal curves. In the generalized case, multiple parameters of interest can be extracted from a single transient signal by taking advantage of the frequency domain differentiation property of the Laplace transform. Preliminary experiments show that permeability and conductivity values, calculated for a variety of ferromagnetic and non-ferromagnetic tubes, agree well with published values (permeability) and with values obtained by four point measurement (conductivity). The inverse method introduced in this work may be straightforwardly extended to consider other parameters, such as lift-off and material thickness, and to consider other geometries, such as conducting plates.

Principal components analysis (PCA) involves transforming a set of correlated observations into a set of linearly uncorrelated variables, which can reveal simplified trends in data. Multifrequency eddy current testing contains correlations across different test frequencies. In this paper, PCA is used to extract unique information from multifrequency eddy current data sets, used to measure the pressure tube to calandria tube gap, in CANada Deuterium Uranium fuel channels. Advantages include compressed data acquisition, allowing for increased inspection speed, and monitoring for variation in physical parameters using a reduced number of variables. PCA employing analytical input model data is validated against PCA employing data from physical experiments.

Material electrical resistivity is an important parameter for eddy current based inspections. For example, material resistivity values for the pressure tube (PT) and calandria tube (CT) are required to ensure accurate measurement of the PT-CT gap in CANDU® reactor fuel channels. Currently, gap measurement calibration is performed on a non-heat treated and non-irradiated PT, and the resistivity used during calibration is assumed to reflect that of in-reactor conditions. In contrast, changes in other parameters that affect gap measurement are compensated using in-reactor measurements. To test whether in-reactor conditions, such as exposure to elevated temperatures, could change a PT material's resistivity, this study examined the effect of heat treatment on Zr-2.5% Nb resistivity. Sectioned PT samples were held for varying periods of time at 400 °C and 450 °C, under anaerobic furnace conditions, in order to partially decompose β-Zr and produce varying fractions of ω-Zr. These temperatures were chosen to accelerate the phase transformations, which take place over a longer period of time under reactor operating conditions, where temperatures are typically between 250 °C and 310 °C. The resistivity of the heat-treated PT samples was measured using the four-point method and changes in resistivity with time at temperature were recorded. The magnitude of the resistivity was observed to decrease by up to 10% after approximately 5000 hrs at 400 °C. Reduction of resistivity with heat treatment was associated with changes in the microstructure and supported by empirical resistivity-microstructure models. Transmission electron microscopy (TEM) showed an increase in the HCP (Hexagonal Closed Packed) ω-phase volume fraction, and an associated decrease in the beta-phase network that was reducing contact between higher conductivity α-Zr grains, which resulted in an overall decrease of resistivity. These results have implications for the uncertainty of PT to CT gap measurement, where temperature variations arise along and around the channel during reactor operations.

The application of eddy current (EC) technique in detection of sensitization before the occurrence of intergranular corrosion was investigated. For this purpose, EC experiments were performed on 304 stainless-steels sensitized at 650 °C and 750 °C for 1–20 hours. The double-loop electrochemical potentiodynamic reactivation (DL-EPR) and microstructural investigations were also carried out to evaluate the degree of sensitization (DOS). The DL-EPR results showed that the maximum sensitivity occurred within 4 h at 650 °C. The EC results indicated a reduction in voltage and impedance with increasing DOS. Finally, empirical relationships between DL-EPR and EC test results were developed.

Inspection of bulk wall loss, due to far side surface corrosion in ferromagnetic steel pipe, is a common requirement across multiple industries, including chemical processing, and oil and gas. In the nuclear industry, there is a requirement for inspection of tile holes and balance of plant inspections, where insulated pipe is present. Inspection is typically done using ultrasonic testing, which necessitates a coupling agent and removal of any insulation. Pulsed eddy current (PEC) technique does not require direct contact and has a larger spot size, thereby facilitating more rapid inspection without insulation removal. To further develop the potential of PEC to inspect under these conditions, a reflection type PEC probe was examined for inspection of thickness of steel plate. PEC data was taken for different plate thicknesses so that sensitivity to wall loss due to corrosion could be assessed. The PEC signal was analyzed by fitting the tail of the transient decay with an exponential curve, with curve fitting parameters correlated to wall thickness variation. An analytical model was developed and partially validated in order to examine a wider range of factors, such as probe dimensions, number of turns, and magnetic permeability and conductivity of the steel. The modelled data was also used to perform a sensitivity analysis on probe dimensions in order to realise the largest range of wall thickness with highest spatial resolution. A finite element method model of the reflection probe over steel plate was used to provide insights into the observation from the analytical model that outer pickup coil radius was the most significant optimization parameter.

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.

Degradation and fouling of support structures in nuclear steam generators (SGs) can lead to SG tube damage and loss of SG efficiency. Inspection and monitoring of support structures combined with preventative maintenance programs can alleviate these effects and extend SG life. Conventional eddy current inspection technologies are extensively used for detecting and sizing indications from wall loss, frets at supports, cracks and other degradation modes in the tubes, as well as assessing the condition of support structures. However, these methods have limited capabilities when more than one degradation mode is present simultaneously, or when combined with fouling. Pulsed eddy current combined with principal components analysis (PCA) was examined for inspection of 15.9 mm (5/ $8^{{\prime }{\prime }}$ ) Alloy-800 tubes and surrounding stainless steel (SS410) support structures. Clear separation of PCA scores associated with tubes from those associated with ferromagnetic SS410 supports permitted measurement of tube-to-support gaps, in either the presence of tube fretting or variation of relative position of the tube within SS410 supports. For concentric tubes, frets could be sized independently of SS410 hole diameter variations, which in other materials could represent support corrosion. Capability to clearly separate scores was attributed to large differences in relaxation times for diffusion of transient fields through the tube compared with diffusion into the ferromagnetic support structure.

Procedures employed to non-destructively examine nuclear power plants must undergo inspection qualification to ensure that they meet their respective inspection specification requirements. Modelling is a powerful tool that can be exploited in the inspection qualification process. The gap between pressure tubes (PTs) and calandria tubes (CTs) in CANDU (CANada Deuterium Uranium) fuel channels is periodically measured, as contact can result in localized cooling and potential cracking. This work shows how an analytical model can be employed to characterize the effects of PT wall thickness and resistivity variation on gap measurement, and details its validation against physical experiments.

Fatigue cracks are prone to develop around fasteners found in multilayer aluminum structures on aging aircraft. Probability of detection (POD) studies using eddy current techniques within the bolt holes contribute to risk assessments used in evaluating the serviceability of these aircraft. Signal response to corner and mid-bore cracks by eddy current testing using standard split-D differential probes has been examined for in-service material from the horizontal stabilizer of a Hercules aircraft. Results are compared to data obtained from laboratory samples with drilled holes. Based on a logistic regression analysis, the results show no significant difference for a90/95 (90% of the cracks of size a will be detected 95% of the time) for mid-bore crack length of 0.60 mm in this study compared to 0.62 mm in earlier work on drilled holes. However, there was a substantial loss of sensitivity for corner cracks. In this study on in-service material, the a90/95 for corner cracks was 0.82 mm compared to 0.31 mm obt...

Aging aircraft is susceptible to cyclic fatigue cracks around ferrous fasteners in multilayer aluminum wing lap joints, as found on Canadian Military aircraft CP-140 Aurora and CC-130 Hercules. Current methods of crack detection utilize bolt hole eddy current, requiring fastener removal, a time consuming process that also has the potential to result in collateral damage. In this paper, pulsed eddy currents (PECs) are induced in the aluminum wing structure by a probe centered over a ferrous fastener. The response signals are analyzed using a modified principal components analysis (PCA) and cluster analysis to show separation of groups of PCA scores from fasteners with and without notches in the bore hole. The Mahalanobis distance (MD), a cluster analysis method, is used to obtain a relative distance measurement between scores associated with cracks from those without cracks. The MD evaluation provides confidence intervals for detection of cracks, which range in extent from fastener holes from 0.9 to 5.5 mm, and a means of potentially sizing larger (>3 mm) second layer cracks. In this paper, 100% of the cracks in the sample were detectable. The results demonstrate PEC as a viable technique for second layer crack detection with ferrous fasteners present.

An understanding of how material parameters, especially orientation and misorientation, influence the magnetic properties of non-oriented electrical steel (NOES) is important for improving the efficiency of the material in service. In this study, the local magnetic properties were measured using magnetic Barkhausen noise (MBN) on different test locations on different strips of NOES material. Local variations in magnetic properties, texture, and misorientation were revealed. A new interpretation for misorientation, called the easy axis misorientation (EAM), was created to describe the alignment of the magnetic easy axes between neighboring grains. This new EAM, visualized as a single value parameter or graphed as a distribution, was shown to be more effective at predicting the isotropic magnetic properties than previously used texture parameters based on standard orientation/misorientation definitions. It was found that a larger EAM value, especially when associated with a lower small angle EAM intensity distribution, was associated with a larger MBN energy. A larger MBN energy has been previously associated with lower losses, and therefore a greater material efficiency.

Surface damage on honeycomb aircraft panels is often measured manually, and is therefore subject to variation based on inspection personnel. Eddy current testing (ECT) is sensitive to variations in probe-to-specimen spacing, or lift-off, and is thus promising for high resolution profiling of surface damage on aluminum panels. Lower frequency testing also allows inspection through the face sheet, an advantage over optical 3D scanning methods. This paper presents results from the ECT inspection of surface damage on an approximately flat aluminum honeycomb aircraft panel, and compares the measurements to those taken using optical 3D scanning technology. An ECT C-Scan of the dented panel surface was obtained by attaching the probe to a robotic scanning apparatus. Data was taken simultaneously at four frequencies of 25, 100, 400 and 1,600 kHz. A reference surface was then defined that approximated the original, undamaged panel surface, which also compensated for the effects of specimen tilt and thermal drift within the ECT instrument. Data was converted to lift-off using height calibration curves developed for each probe frequency. A damage region of 22,550 mm2 area with dents ranging in depth from 0.13-1.01 mm was analyzed. The method was accurate at 1,600 kHz to within 0.05 mm (2s) when compared with 231 measurements taken via optical 3D scanning. Testing at 25 kHz revealed a 3.2 mm cell size within the honeycomb core, which was confirmed via destructive evaluation. As a result, ECT demonstrates potential for implementation as a method for rapid in-field aircraft panel surface damage assessment.

A novel strategy, which enables the derivation of exact solutions describing all electromagnetic interactions arising in inductively coupled circuits, is developed. Differential circuit equations are formulated in terms of the magnetic fields arising in inductive systems, using Faraday׳s law and convolution, and solved using the Fourier transform. The approach is valid for systems containing any number of driving and receiving coils, and may be extended to include nearby conducting and ferromagnetic structures. Furthermore, arbitrary excitation waveforms, such as a sinusoid or a square wave for applications in conventional and transient eddy current, respectively, may be considered. In this first work, the general theory is presented and subsequently applied to the simple case of a coaxial driver and receiver coil configuration. Theoretical expressions for the self- and mutual inductance coefficients are shown to fall out of the theory naturally. Experimental results, obtained for a square wave function excitation, are found to be in excellent agreement with the analytical predictions.

Core loss is a significant source of energy loss in electric motor steel laminates. Therefore, there is interest in monitoring the quality and consistency of laminates at various stages of manufacturing. The purpose of this study was to investigate the feasibility of using surface magnetic Barkhausen noise for the evaluation of AC core loss, and further, to examine potential origins of magnetic loss in non-oriented electrical steel. Core loss values were measured by a single sheet tester and Barkhausen noise measurements were performed using pole flux control on eight laminates with various grain size, texture and composition. Magnetocrystalline energy was calculated from X-ray diffraction data to quantify texture. Results demonstrated higher surface Barkhausen emissions for samples with lower core loss. Barkhausen noise analyses were used to examine the interplay among core loss, grain size, magnetocrystalline energy and B–H characteristics. The inverse correlation between core loss and Barkhausen noise emissions was qualitatively explained in terms of the orthogonal vector contribution of microscopic eddy currents to losses associated with bulk currents arising in the sample during magnetization.

Small cracks originating near bolt holes in the inner layers of aircraft lap joints cannot be detected by conventional eddy current techniques with ferrous fasteners present. The current work employs finite element modeling to investigate the effectiveness of pulsed eddy current (PEC) in detecting deep-lying cracks of different sizes and orientations. Differential signals analyzed using a modified Principal Components Analysis (PCA) method reveal the potential success of a PEC-PCA combination to produce scores that can be used to determine the presence of cracks. The method can also identify signals from an off-centered probe. Results are supported by experiment.

A study was conducted to estimate the Probability of Detection (POD) of a screening technique for samples representative of lap joints in wing structure of the P-3 Orion airframe containing EDM notches in the bottom of the top layer and the top of the second layer radiating from holes containing ferrous fasteners. It was assumed that the defects made up only a small fraction of the data (

The pressure tubes (PT) in CANDU (CANada Deuterium Uranium) reactors undergo creep induced deformation due to operating pressure, temperature and radiation conditions. While global deformation of the tube in the form of elongation and diametral creep is well characterized and monitored by station inspection systems, local PT deformation and the presence of inner surface artifacts due to wear are not as directly monitored, but can still provide additional information of fuel channel condition. A surface profiling technique for monitoring local deformation and identification of surface wear using an eddy current probe mounted in a small (50 mm×25 mm) planar probe body is presented. The sensitivity of the eddy current probe to small lift-off variations combined with high density C-Scan information is used to extract information on smoothly varying local deformation as well as monitor more significant wear on the inner surface of pressure tubes. Vector separation of components permits independent identification of axial and circumferential surface features. Analysis of this data can be used to characterize local PT deformation due to constrictions at fuel bundle ends and loaded garter spring spacers, as well as identify areas where shallow mechanical wear has occurred. Examples of the features that may be identified are presented.

The aluminum inner wing spars of F/A-18 Hornet aircraft may undergo stress corrosion cracking along the spar between the fasteners that secure carbon-fiber/epoxy composite skin to the wing. Inspection of the spar through the wing skin is required to avoid wing disassembly. The thickness of the skin varies between 9 and 21 mm (0.36-0.83 in) and fasteners may be either titanium or ferrous. Pulsed eddy currents (PECs), generated by a probe centered over the fastener, demonstrate the capability of detecting simulated cracks within spars with the wing skin present. Comparison of signals from separate pick-up coils, mounted to either side of the excitation coil, is used to detect differences in induced eddy current fields, which arise in the presence of cracks. To overcome variability in PEC signal response due to variation in: 1) skin thickness; 2) fastener material and size; and 3) centering over fasteners, a large calibration data set is acquired. Multidimensional scores from a modified principal components analysis (PCA) of the data are reduced to 1-D using a discriminant analysis method. Under inspection conditions, calibration PCA scores combined with discriminant analysis permit rapid real time go/no-go PEC detection of cracks in F/A-18 inner wing spar.

Stress corrosion cracks may develop between fasteners in the aluminum inner wing spars of F/A-18 Hornet aircraft. These fasteners secure carbon-fibre/epoxy composite wing skin, of nominal 13 mm thickness, to the spar. Inspection of the spar through the wing skin is required in order to avoid wing disassembly. Pulsed eddy currents, generated by a probe design that utilizes the ferrous fastener as a flux conduit, are demonstrated as having the capability of detecting simulated cracks within the spar with the wing skin present. Differentially connected pickup coils are used to sense differences in response due to asymmetries in induced eddy current fields arising in the presence of cracks. To overcome variability in PEC signal response due to variability in magnetic coupling to the fastener, a large measurement set was analyzed using Modified Principal Component Analysis (PCA). The modified PCA minimizes residual sum of squares to extract eigenfunction scores, which are used to detect the presence of cracks ending on one side of the fastener hole. The ability of the method to sense simulated cracks independent of confounding measurement parameters was demonstrated.

Exact solutions for the electromagnetic response of a transmit-receive coil pair situated above two parallel plates separated by a gap, were developed using the recently published general model of Desjardins et al. that accounts for all electromagnetic interactions between the voltage-driven probe and the conducting samples. This model was then compared to the well-known model developed by Dodd and Deeds, which assumes a constant amplitude sinusoidal current and an open-circuit pick-up coil. Both models were compared with experimental results that measured the gap profile for a Grade 2 Titanium plate (54 μΩ·cm) over a SS-316 stainless steel second layer plate (74 μΩ·cm). These materials simulated the electromagnetic properties of a Zr 2.5% Nb pressure tube and Zr-2 calandria tube, respectively, as found in the fuel channels of CANDU® reactors. It was observed that while the Dodd and Deeds’ model as applied to this work achieved a good shape agreement with experimental data for excitation frequencies at 2 kHz, at the higher frequency of 16 kHz good agreement was not achieved. In contrast, the model of Desjardin’s et al., adapted for a transmit-receive probe configuration above two infinite flat plates, achieved an excellent shape agreement at both frequencies.

The sensitivity of magnetic Barkhausen noise to many factors has limited its potential application for basic material characterization and detection of residual stress in carbon steel, a common structural material. The present work investigates Barkhausen noise response in plain carbon steel under conditions of varying carbon content, applied elastic stress and different magnetization level. The surface Barkhausen noise measurement system uses a feedback for controlling the flux waveform, which facilitates reproducibility of measurements and also permits extraction of additional parameters from the B–H loop of magnetic circuit. Barkhausen noise parameters correlate with known material parameters, such as coercivity, which vary with carbon content and stress. These results demonstrate the potential for in-situ characterization of carbon steel.