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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 (

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.

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.

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.

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.

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.

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...

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.

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.

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.

The development of magnetic Barkhausen noise methods for rapid detection of residual stress concentrations has implications for integrity assessment of submarine pressure hulls. However, the stress-response of Barkhausen noise in submarine hull steel, HY-80, is complicated by the influence of the material’s martensitic microstructure. The present work sheds light on the stress-dependent behavior of Barkhausen noise in HY-80 by comparing its signal characteristics with those of more common ferrite/pearlite steels. HY-80 and various ferrite/pearlite steel plates were uni-axially stressed up to and beyond the level for plastic deformation. Barkhausen noise measurements, performed using the same sensor under reproducible flux-controlled magnetization conditions, facilitated a direct comparison of material responses. Results showed that with the application of tensile stress, the Barkhausen noise signal of ferrite/pearlite steels linearly increased, reached a peak value and saturated in the elastic region. By contrast, HY-80 demonstrated a linear increase with tensile stress characterized by a transition from a lower to a seven times higher rate of change for stresses above 200 MPa up to its yield point. The different stress-response of HY-80 was attributed to its martensitic microstructure, which modifies the response of the domain structure under tensile stress conditions.

Fatigue cracks are prone to develop around fasteners found in multi-layer 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. The signal response to corner and mid-bore cracks by eddy current testing using standard split-D differential probes has been examined. The data indicate that split-D probes are primarily sensitive to cracks at the corners of the Ds and that this gives rise to a double maximum for small mid-bore cracks as the corners pass over the crack. The double signal is not seen for corner cracks because the excitation coil, which is almost completely out of the hole, excites almost no eddy currents when the second corner of the D is over the crack. However, the eddy current response is more sensitive to corner cracks. The a90/95 for corner cracks was measured to be 0.22 mm (length) compared to 0.34 mm (depth) and 0.62 mm (length) for mid-bore cracks. The enhanced detectability of corner cracks is attributed to the presence of the edge, which restricts passage of eddy currents around the crack.

Fatigue cracks are prone to develop around fasteners found in multi-layer aluminum structures on ageing aircraft such as the CC-130 Hercules and CP-140 Aurora (P-3 Orion). 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. Improving POD by optimizing the inspection system can reduce the required frequency of inspections, since assurance of detection of smaller crack sizes extends the interval for which growth of cracks to a critical size may occur. In this work signal analysis and POD of laboratory grown fatigue cracks in the corners of bolt holes of 7075-T6 aluminum is examined. A number of parameters that enhance crack detection are identified, including the use of intimate contact probes versus steel sheath (non-contact), higher frequencies and the use of C-Scan display. Results demonstrate better detectability at 1600 kHz, than at the normally used 400 kHz, and enhanced recognition and assurance of identification of peak crack signal for data recorded and displayed in a C-Scan format. Results are compared with a previous POD study, which used current field techniques for detection.

Fatigue cracks are prone to develop around the fasteners found in multilayer aluminum structures such as ageing aircraft. The determination of their probability of detection (POD) using eddy current techniques is critical for risk assessments used in evaluating serviceability. Normally, a large number of samples of both fatigue cracks and electric discharge machined notches are required for such a study. In this study, the use of quantitative fractography (QF) to efficiently gather POD data for bolt hole eddy current is demonstrated. The technique can be readily automated, significantly reducing man-hour requirements. The method is applied to mid-bore cracks, grown without starter notches, in order to simulate naturally occurring fatigue cracks. POD of the mid-bore cracks is examined using a probit analysis. An a 90/95 is the probability that 90% of the cracks of size a or larger will be detected 95% of the time. An a 90/95 of 0.34 mm was obtained for mid-bore crack depth. This value is consistent with sk...

Probability of detection (POD) of fatigue cracks and electric discharge machined (EDM) notches in 7075-T6 aluminum was evaluated for a bolt-hole eddy current inspection system. The POD study involved simulated bolt holes in coupons representative of wing areas on CC-130 and CP-140 aircraft. The data were obtained from 24 inspectors who inspected 468 coupons that contained a set of coupons with 45 EDM notches and 72 laboratory-grown fatigue cracks located at the inner surface corner of the bi-layer structures. A single point calibration based upon a 0.76 mm (0.030 inches) deep corner notch set to 20% of screen height (SH) was utilized. Large variability in detection capability between inspectors was observed. A portion of this variability was attributed to significant background noise on the order of 5% screen height (SH). Normalization of each inspector's data to signal amplitudes from notches at twice the depth (1.5 to 1.6 mm (0.060 inches)), close to 40% SH, reduced the relative variations and resulted ...