Giant Magnetoresistance Sensors: A Review on Structures and Non-Destructive Eddy Current Testing Applications
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paper_title: The Electrical Conductivity of Transition Metals

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1— In a recent paper certain property of the transition metals Ni, Pd, and Pt and of their alloys with Cu, Ag, and Au have been discussed from the point of view of the electron theory of metals based on quantum mechanics. In particular, a qualitative explanation was given of the relatively high electrical resistance of the transition metals. It was shown from an examination of the experimental evidence that the conduction electrons in these metals have wave functions derived mainly from s states just as in Cu, Ag, and Au, and that the effective number of conduction electrons is not much less than in the noble metals. On the other hand, the mean free path is much smaller, because under the influence other the lattice vibrations the conduction electrons may make transitions to the unoccupied d states, and the probability of these transitions is several times greater than the probability of ordinary scattering. Since the unoccupied d states are responsible for the ferromagnetism or high paramagnetism of the transition elements, there is a direct connexion between the magnetic properties and the electrical conductivity. The purpose of this paper is as follows: in 2, 3, and 4 we develop a formal theory of conductivity for metals, such as the tradition metals, where two Brillouin zone are of importance for the conductivity; in 5 we apply the theory to show why, at high temperatures, the temperature coefficient of the paramagnetic metals Pd and Pt falls below the normal value; and in 6 we discuss the resistance of ferromagnetic metals, and show in 7 qualitatively why constantan (Cu-Ni) has zero temperature coefficient at room temperature.
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paper_title: Extending the GMR Current Measurement Range with a Counteracting Magnetic Field

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Traditionally, current transformers are often used for current measurement in low voltage (LV) electrical networks. They have a large physical size and are not designed for use with power electronic circuits. Semiconductor-based current sensing devices such as the Hall sensor and Giant Magnetoresistive (GMR) sensor are advantageous in terms of small size, high sensitivity, wide frequency range, low power consumption, and relatively low cost. Nevertheless, the operational characteristics of these devices limit their current measurement range. In this paper, a design based on using counteracting magnetic field is introduced for extending the GMR current measurement range from 9 A (unipolar) to ±45 A. A prototype has been implemented to verify the design and the linear operation of the circuit is demonstrated by experimental results. A microcontroller unit (MCU) is used to provide an automatic scaling function to optimize the performance of the proposed current sensor.
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paper_title: Giant Magnetoresistance of (001)Fe/(001)Cr Magnetic Superlattices

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We have studied the magnetoresistance of (001)Fe/(001)Cr superlattices prepared by molecularbeam epitaxy. A huge magnetoresistance is found in superlattices with thin Cr layers: For example, with ${t}_{\mathrm{Cr}}=9$ \AA{}, at $T=4.2$ K, the resistivity is lowered by almost a factor of 2 in a magnetic field of 2 T. We ascribe this giant magnetoresistance to spin-dependent transmission of the conduction electrons between Fe layers through Cr layers.
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paper_title: Spin-valve effect in soft ferromagnetic sandwiches

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Abstract We demonstrated in a variety of systems that the in-plane resistivity of sandwiches of soft ferromagnetic layers separated by nonmagnetic metallic layers depends on the relative angle between their magnetizations. We observe this phenomenon, which we term the spin-valve effect, in sandwiches where we are able to control the relative angle between the magnetizations of two ferromagnetic layers either by constraining one layer through exchange anisotropy or by fabricating layers with different coercivities. In the first case, for example Si/50A Ta/60A NiFe/25A Cu/40A NiFe/50A FeMn/50A Ta we have seen relative changes in resistance of more than 4% at room temperature in a range of in-plane field of 0 to 15 Oe. In a system where the layers have different coercivities, Si/8 × (30A Fe/60A Ag/30A Co/60A Ag), we observed a relative change of 1.6% at room temperature for fields between 0 and 50 Oe. Since the ferromagnetic layers are essentially decoupled and have high squareness, one can rule out any mechanism requiring scattering by domain walls. The usual anisotropic magnetoresistance in these structures is much smaller than the spin-valve effect. In contrast to noble metals, when using Ta, Al, Cr or Pd spacers of similar thickness (20 to 150A) between layers of permalloy, only the anisotropic magnetoresistance is observed. We believe the spin-valve effect to be related to spin-dependent scattering at the interface and within the ferromagnetic layers, in balance with spin-dependent relaxation within the layers. We also report the observation of a weak exchange-like coupling between the ferromagnetic layers.
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paper_title: Spin valve sensors

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Abstract This paper demonstrates spin valve sensor applications as read elements in storage systems, or, when in a Wheatstone bridge configuration, as rotational speed control devices (for ABS systems), high current monitoring devices for power lines, and positioning control devices in robotic systems. For recording heads, shielded spin valve sensors are adequate for high linear density recording. A tape head was fabricated with output 400 μV per micron of trackwidth, with a D50 value of 100 kfci, and signal loss of −0.34 dB/kfci. For rotational speed measurement, spin valve bridge sensors with flux guides were used, yielding a 400-mVpp amplitude, and square wave output with rise/fall times below 70 μs, when excited by a magnetized wheel. Amplitude is independent of speed (0–3000 rpm), and of sensor to wheel separation (0.5–2.0 mm). For power line applications, currents up to 2100 A (at 50 Hz) could be measured with a sensitivity of 35 μVrms/A and deviations from linearity of ±1.5%. In robot position control, a maximum error of ±9 μm over a ±0.5-mm span was obtained. In the two last cases, bridges without flux guides are used to maximize linear response.
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paper_title: Direct observation of the alignment of ferromagnetic spins by antiferromagnetic spins

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The arrangement of spins at interfaces in a layered magnetic material often has an important effect on the properties of the material. One example of this is the directional coupling between the spins in an antiferromagnet and those in an adjacent ferromagnet, an effect first discovered in 1956 and referred to as exchange bias. Because of its technological importance for the development of advanced devices such as magnetic read heads and magnetic memory cells, this phenomenon has received much attention. Despite extensive studies, however, exchange bias is still poorly understood, largely due to the lack of techniques capable of providing detailed information about the arrangement of magnetic moments near interfaces. Here we present polarization-dependent X-ray magnetic dichroism spectro-microscopy that reveals the micromagnetic structure on both sides of a ferromagnetic-antiferromagnetic interface. Images of thin ferromagnetic Co films grown on antiferromagnetic LaFeO3 show a direct link between the arrangement of spins in each material. Remanent hysteresis loops, recorded for individual ferromagnetic domains, show a local exchange bias. Our results imply that the alignment of the ferromagnetic spins is determined, domain by domain, by the spin directions in the underlying antiferromagnetic layer.
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paper_title: Exchange-biased spin-valves for magnetic storage

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An overview is given of the material properties of exchange-biased spin-valves. More specifically, we discuss the microstructure and magnetic properties of these materials as relevant for (future) industrial application in magnetoresistive read heads for rigid disk and tape recording and Magnetic Random Access Memory (MRAM) devices.
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paper_title: Magnetic field sensors using GMR multilayer

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Wheatstone bridge magnetic field sensors using giant magnetoresistive ratio (GMR) multilayers were designed, fabricated, and evaluated. The GMR ranged from 10% to 20% with saturation fields of 60 Oe to 300 Oe. The multilater resistances decreased linearly with magnetic field and showed little hysteresis. In one sensor configuration, a permanent magnet bias was placed between two pairs of magnetoresistors, each pair representing opposite legs of the bridge. This sensor gave a bipolar bridge output whose output range was approximately GMR times the bridge source voltage. The second sensor configuration used shielding on one resistor pair, and it gave a bridge output dependent on the magnetic field magnitude, but not polarity, and the output range was approximately one half GMR tines the bridge source voltage. Field amplifications of 3 to 6 were accomplished by creating a gap in a low reluctance magnetic path, thus providing the full range of outputs with 1/3 to 1/6 of the intrinsic saturation fields of the GMR multilayers. >
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paper_title: A new GMR sensor based on NiFe/Ag multilayers

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This work describes the fabrication of a giant magnetoresistive field sensor based on NiFe/Ag multilayers. The stacking of the 21 bilayers of NiFe and Ag deposited at liquid nitrogen temperature on a (100) Si substrate has a surface roughness as low as 0.3 nm, which contributes to the good magnetoresistive properties of the material. Owing to a magnetoresistance ratio ΔR/R of 12% under a saturation field of 160 Oe (12.7 kA/m) and a good thermal stability after an annealing at 180°C, this material has been integrated into sensors through microelectronics processing. The particular design of the Wheatstone bridge which constitutes the sensor allows to bias the four active magnetoresistors with only two small magnets stuck along the bridge arms. Such a sensor presents a linearity better than ±2% over a field range of ±35 Oe (±2.8 kA/m) for temperatures ranging from room temperature to 100°C. Hysteresis was evaluated to be smaller than 1 Oe. As a consequence of these good performances without any signal conditioning, this sensor may be considered for applications requiring low fabrication costs such as automotive ones.
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paper_title: Robust giant magnetoresistance sensors

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Abstract The giant magnetoresistance (GMR) effect offers interesting new possibilities for sensor applications. A short overview is given of the GMR effect in relation to its application in (automotive and industrial) field sensors. In the past the thermal and magnetic stability could not fulfil the requirements for use in automotive and industrial environments. Recently, a new, robust GMR material system has been developed that can withstand high temperatures (>200°C) and large magnetic fields (>200 kA/m). Using this material, GMR sensor elements have been fabricated and measured. Moreover, preliminary measurements on the first robust GMR sensor with a full Wheatstone-bridge configuration will be presented.
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paper_title: GIANT MAGNETORESISTANCE IN MAGNETIC NANOSTRUCTURES

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This chapter contains a brief review of the giant magnetoresistance (GMR) effect exhibited by magnetic multilayers, granular alloys, and related materials. Subjects covered include a description of the phenomenon, and the related oscillatory interlayer exchange coupling in magnetic multilayers; a simple model of giant magnetoresistance; the inverse GMR effect in spin-engineered magnetic multilayers; structures that display large changes in resistance in small magnetic fields, possibly for use in magnetic field sensors; and the dependence of GMR on various aspects of the magnetic structures.
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paper_title: Optimized Eddy Current Detection of Small Cracks in Steam Generator Tubing

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A complete, computer based design methodology is described, aiming to develop an eddy current sensor with increased sensitivity to flaws, and reduced sensitivity to probe lift-off. The first part of the paper contains an analysis performed in order to establish detailed criteria for an effective design. Numerical investigations have been carried out and their results are discussed, regarding various problems of detectability and lift-off noise level. Based on these results, in the second part two probe arrangements are proposed, and it is shown how their performance parameters could be further improved.
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paper_title: Recent trends in electromagnetic non-destructive sensing

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The paper deals with material electromagnetic non-destructive testing (eNDT) with emphasize on eddy current testing (ECT). Various modifications of ECT sensing are compared and discussed from the desired detected signal characteristics point of view. Except of the optimization of usual probe coils arrangements for the concrete applications, the new magnetic sensors as giant magneto-resistance (GMR) and spin dependent tunneling (SDT) are presented. The advanced ECT sensors are characterized by their sensitivity, frequency range and sensor dimensions
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paper_title: Bobbin-Type Solid-State Hall Sensor Array With High Spatial Resolution for Cracks Inspection in Small-Bore Piping Systems

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Bobbin coil and bobbin-type Hall sensor arrays are proposed as an alternative for crack inspection inside a small-bore piping system. The cracks can be imaged at high speed without using a scanner since the electromagnetic (EM) field is distorted by the cracks. An array of 32 × 32 Hall sensors with 0.78 mm spatial resolution was set in a cylinder with diameter of 15 mm and length of 24.96 mm. A bobbin coil operating at 5 kHz of alternating current was positioned inside of a piping system and the sensor array outside the cylinder. Distorted EM fields around outside diameter stress corrosion cracking was imaged at 1 frame/s.
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paper_title: A measurement system based on magnetic sensors for nondestructive testing

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The paper deals with a measurement system based on a low-cost eddy current probe for nondestructive testing on conducting materials aimed at reconstructing the shape and position of thin cracks. The magnetic probe is characterized, highlighting good repeatability, linearity, and overall accuracy. A number of different measurement approaches are investigated, in order to choose the more appropriate for NDT applications. A numerical method was then illustrated; it proves to be able to reconstruct cracks also starting from noisy measurement data.
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paper_title: Eddy current probes of inclined coils for increased detectability of circumferential cracks in tubing

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Abstract Conventional bobbin probes, multi-pancake and/or rotating pancake probes, and transmit-receive probes for eddy current tests (ECTs) are currently used to test metal tubing. Each method has its respective strengths and weaknesses considering their characteristics such as test speed, flaw detection sensitivity, and probe structure complexity. This paper proposes a novel eddy current probe with new features. The structure is designed to be sensitive to circumferential cracks, which are not easily detected using conventional bobbin coil probes, as well as longitudinal cracks. The directions of the eddy current around the coils were designed to be not circumferential. The ECT signals of these probes were acquired and analyzed from the artificial defects manufactured for this study. The experimental results show that the proposed probes are more sensitive to circumferential defects than the comparable conventional bobbin probes. In addition, the proposed probes are also sensitive to axial defects. By employing both the new probes and the conventional bobbin probes, ECTs for the metal tubing can be performed more reliably.
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paper_title: Rotating field eddy current (RoFEC)-probe for steam generator inspection

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A novel design of eddy current probe based on rotating magnetic fields is presented for the inspection of steam generator tubes in nuclear power plants. A major advantage of the rotating field probe is that it offers the same functiona lity as that of a rotating probe coil without the need for mechanical rotation, which in turn translates into higher operating sp eed. The probe design is also sensitive to cracks of all orientations in the tube wall.
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paper_title: Techniques for processing remote field eddy current signals from bend regions of steam generator tubes of prototype fast breeder reactor

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Abstract Steam generator (SG) is one of the most critical components of sodium cooled fast breeder reactor. Remote field eddy current (RFEC) technique has been chosen for in-service inspection (ISI) of these ferromagnetic SG tubes made of modified 9Cr–1Mo steel (Grade 91). Expansion bends are provided in the SGs to accommodate differential thermal expansion. During ISI using RFEC technique, in expansion bend regions, exciter–receiver coil misalignment, bending stresses, probe wobble and magnetic permeability variations produce disturbing noise hindering detection of defects. Fourier filtering, cross-correlation and wavelet transform techniques have been studied for noise reduction as well as enhancement of RFEC signals of defects in bend regions, having machined grooves and localized defects. Performance of these three techniques has been compared using signal-to-noise ratio (SNR). Fourier filtering technique has shown better performance for noise reduction while cross-correlation technique has resulted in significant enhancement of signals. Wavelet transform technique has shown the combined capability of noise reduction and signal enhancement and resulted in unambiguous detection of 10% of wall loss grooves and localized defects in the bend regions with SNR better than 7 dB.
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paper_title: Steam generator tube integrity program

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Abstract The degradation of steam generator tubes in pressurized water nuclear reactors continues to be a serious problem, and the US Nuclear Regulatory Commission (NRC) is developing a performance-based rule and regulatory guide for steam generator tube integrity. To support the evaluation of industry-proposed implementation of these performance-based criteria, the NRC is sponsoring a new research program at Argonne National Laboratory on steam generator tubing degradation. The objective of the new program is to provide the necessary experimental data and predictive correlations and models that will permit the NRC to independently evaluate the integrity of steam generator tubes. The technical work in the program is divided into four tasks, (1) assessment of inspection reliability, (2) research on in-service inspection technology, (3) research on degradation modes and integrity, (4) development of methodology and technical assessments for current and emerging regulatory issues. The objectives of and planned research activities under each of these four tasks are described here.
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paper_title: Fatigue Crack Length Sizing Using a Novel Flexible Eddy Current Sensor Array

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The eddy current probe, which is flexible, array typed, highly sensitive and capable of quantitative inspection is one practical requirement in nondestructive testing and also a research hotspot. A novel flexible planar eddy current sensor array for the inspection of microcrack presentation in critical parts of airplanes is developed in this paper. Both exciting and sensing coils are etched on polyimide films using a flexible printed circuit board technique, thus conforming the sensor to complex geometric structures. In order to serve the needs of condition-based maintenance (CBM), the proposed sensor array is comprised of 64 elements. Its spatial resolution is only 0.8 mm, and it is not only sensitive to shallow microcracks, but also capable of sizing the length of fatigue cracks. The details and advantages of our sensor design are introduced. The working principal and the crack responses are analyzed by finite element simulation, with which a crack length sizing algorithm is proposed. Experiments based on standard specimens are implemented to verify the validity of our simulation and the efficiency of the crack length sizing algorithm. Experimental results show that the sensor array is sensitive to microcracks, and is capable of crack length sizing with an accuracy within ±0.2 mm.
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paper_title: Inverse Problem in Nondestructive Testing Using Arrayed Eddy Current Sensors

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A fast crack profile reconstitution model in nondestructive testing is developed using an arrayed eddy current sensor. The inverse problem is based on an iterative solving of the direct problem using genetic algorithms. In the direct problem, assuming a current excitation, the incident field produced by all the coils of the arrayed sensor is obtained by the translation and superposition of the 2D axisymmetric finite element results obtained for one coil; the impedance variation of each coil, due to the crack, is obtained by the reciprocity principle involving the dyadic Green’s function. For the inverse problem, the surface of the crack is subdivided into rectangular cells, and the objective function is expressed only in terms of the depth of each cell. The evaluation of the dyadic Green’s function matrix is made independently of the iterative procedure, making the inversion very fast.
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paper_title: Nondestructive evaluation of a crack on austenitic stainless steel using a sheet type induced current and a Hall sensor array

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Austenitic stainless steels (hereafter A-STS) such as STS304 and STS316 are paramagnetic metals. However, a small amount of partial magnetization is generated in A-STS because of the imperfect final heat treatment and mechanical processing. Surface cracks on paramagnetic metal with a partially magnetized region (hereafter PMR) are difficult to inspect. In this paper, we propose a method for high speed inspection and evaluation of a crack on A-STS. Cracks can be inspected with high speed by using 64 arrayed Hall sensors (HSA) with 3.5 mm spatial resolution and a sheet type induced current (STIC). Then, a crack can be evaluated quantitatively by using the detailed distribution of the magnetic field obtained by using single Hall sensor scanning (SSS) around the inspected crack area. Several cracks on A-STS with partially magnetized areas were examined and the experimental formulas were derived.
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paper_title: Impedance analyzing for planar eddy current probe array

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It is important to measure thickness of slag dynamically in the process of steelmaking in order to produce high quality steel, eddy current testing has lots advantage to measure it, and system design is based on the analyzing of impedance of sensors. In this paper, we take example of measuring the thickness distribution of liquid slag layer in mold of metallurgical industry; Impedance property is work out by figuring out a three-dimensional physical model's electromagnetic fields distribution using finite element method, a unconventional probe array was adopted in the model.
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paper_title: Artificially intelligent 3D industrial inspection system for metal inspection

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Industrial inspection systems have been in use for some time now. However to-date these systems have been built specifically for the application in which it will function. This has lead to such systems becoming obsolete if the manufacturing process changes. Such systems also relied on the programmer's competence in selecting appropriate algorithms to carry out the tasks of image processing and segmentation. This paper presents a system that is adaptable for many inspection tasks and generic in nature. It selects algorithms automatically depending on the task at hand and the domain knowledge given. >
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paper_title: Non-Destructive Testing with Magnetic Sensor Using Rotational Magnetic Flux

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This paper presents a new nondestructive testing (NDT) method which utilizes rotational magnetic flux. In conventional eddy current NDT, the magnetic sensor is driven by an ac or dc current. Consequently, the magnetic field generated in the magnetic material is aligned in a single direction. However, in order to estimate the shape or position of an unknown defect, a two-dimensional alternating magnetic flux density vector is necessary. An NDT system utilizing rotational magnetic flux is proposed. In this system, the magnitude and phase value are measured and used to obtain information about the defect.
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paper_title: Rotational Magnetic Sensor With Neural Network For Non-destructive Testing

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A new non-destructive testing (NDT) method which utilizes rotational magnetic flux is presented. In this system, the magnitude and phase value are measured and used to obtain information about defects. These values include the information about the shape or position of an unknown defect. The neural network technique is employed for estimation of a defect shape. The experimental results show the validity of the authors' method. >
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paper_title: The role of leak-before-break in assessments of flaws detected in CANDU pressure tubes

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Abstract This paper reviews the role of the Leak-Before-Break (LBB) concept in the Fitness for Service Guidelines being developed for cold worked (cw) Zr-2·5Nb pressure tubes in a CANDU reactor. The guidelines complement the rules of Section XI of the ASME Code and the requirements of Canadian Standards Association (CSA) CAN 3-N285.4-M83. The evaluation procedures in the guidelines consist of a flaw growth analysis to determine the maximum size of the flaw at the end of the evaluation period. It must then be demonstrated that the flaw is stable with adequate margins of safety for the various loading conditions. For the delayed hydride cracking failure mode LBB is used as defense in depth against unstable rupture. First the flaw must be shown to be non-susceptible to propagation by delayed hydride cracking during normal operating conditions. In addition, it must then be demonstrated that, if the flaw were to penetrate the tube wall, the leaking coolant would be detected and the reactor shutdown before the postulated crack became unstable. The Guidelines contain criteria for performing both deterministic and probabilistic LBB analyses.
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paper_title: Remote field eddy current technique applied to non-magnetic steam generator tubes

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

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We have observed ‘‘giant magnetoresistance’’ in short‐period Cu/Co‐Ni‐Cu alloy superlattices electrodeposited from a single electrolyte under potentiostatic control. The superlattices were grown on polycrystalline Cu substrates which were removed before transport measurements were made. Room‐temperature magnetoresistances of over 15% in applied magnetic fields of up to 8 kOe were observed in superlattices having Cu layer thicknesses of less than 10 A.
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paper_title: Effect of annealing on the structural and magnetic properties of giant magnetostrictive multilayers

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Abstract Exchange-coupled multilayered thin films which combine giant magnetostriction and soft magnetic properties are of growing interest for applications. TbFe 2 /Fe and TbFe 2 /FeCo nanometric multilayers were grown by RF-sputtering onto glass substrates. A bias field of 250 Oe was applied along the longitudinal axis, inducing an in-plane uniaxial magnetic anisotropy. For es-deposited multilayers, TbFe layers are amorphous while soft layers are nanocrystallized. The effect of annealing was investigated by 57 Fe Mossbauer spectrometry, magnetic and magnetostriction measurements in the temperature ( T ann ) range from 150 to 350 °C. After annealing, the Mossbauer analysis evidences for an increase of the crystallized iron component, probably due to a thinning of the interfaces. A maximum of the magnetoelastic coefficient is found for T ann around 250 °C, with the highest value obtained in the case of TbFe/FeCo. Furthermore, stress relaxation within the film reduces the anisotropy field, leading to a large magnetoelastic susceptibility of about 250 MPa/kOe.
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paper_title: Annealing effects on GMR multilayer films

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Abstract The annealing effects on GMR of electron-beam evaporated [NiFe/Cu] BL multilayer films (10, 12 and 14 bilayers) were studied. The as-deposited multilayer films give a GMR smaller than 0.6% but can be enhanced by further annealing. The multilayer films were annealed in a vacuum at 300°C for 2.5 h with the pressure −7 mbar and the GMR of order of ∼1% was achieved. However, some samples were annealed in the flowing argon at the same temperature and period of time, giving an improved GMR change up to ∼4–4.5% similar to that presented by Smith et al. [N. Smith, A.M. Zeltser, M.R. Parker, GMR Multilayers and Head Design for Ultrahigh Density Magnetic Recording, IEEE Trans. Magn. 32 (1996) 135]. The oxidisation on the surface of the films is thought to cause this critical difference. The periods of annealing time can also affect the GMR which increases dramatically as the film is annealed up to 1 h and tends to be constant at longer times up to 2.5 h. Application of a forming field during annealing is found to induce uniaxial anisotropy.
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paper_title: Design and fabrication of GMR multilayers with enhanced thermal stability

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Abstract Enhanced thermal stability of NiFeCo/Cu multilayers has been observed for multilayer stacks prepared with ‘overdesigned’ magnetic layers. Multilayers such as these show almost no giant magnetoresistance (GMR) in the as-deposited state, but exhibit huge improvements upon annealing. This methodology represents a considerable improvement over conventional methods.
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paper_title: The Lift-Off Effect in Eddy Currents on Thickness Modeling and Measurement

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This paper uses a linear transformer model to investigate the effect of the lift-off on the results of the thickness measurement of nonferromagnetic metallic plates. The transformer model previews that the time derivative of the magnetization curves obtained for different gaps between the excitation coil and the plate should intercept in a single point when low magnetic coupling factors are considered. To assess the validity of the model, results are compared with experimental data obtained with a giant magnetoresistive sensor probe. For comparison, the sensor output voltage time derivative must be performed as well. The similarity of the theoretical model results and those obtain experimentally with pulsed excitation, confirms the correctness of the transformer approach.
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paper_title: Output signal prediction of an open-ended rectangular waveguide probe when scanning cracks at a non-zero lift-off

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Abstract The paper proposes a modeling technique for output signal prediction of a rectangular waveguide probe with finite flange when scanning a surface long crack in a metal at a finite lift-off. The modeling technique approximates the crack–probe interaction with a two-dimensional problem. In this problem, a parallel-plate waveguide with finite flange scans a long crack in a perfect conductor. The method of moments is employed to solve the governing electric field integral equation. The solution provides the reflection coefficient in the parallel-plate waveguide from which the probe output signal is obtained. The main feature of the model is solving the three-dimensional problem in a two-dimensional frame work, thus reducing the degree of complexity and computation time. To validate the accuracy of the model, several simulation results are presented at an operating frequency in the X-band and are compared with their experimental counterparts. To demonstrate the efficiency of the model, we compare our results with those obtained using the well-known HP-HFSS finite element code. It is shown that the proposed model requires less than half of the time taken to solve the same problem running on the same computer.
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paper_title: Nondestructive Inspection Using Rotating Magnetic Field Eddy-Current Probe

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Rotating magnetic field eddy-current (RoFEC) probe for nondestructive evaluation of steam generator tubes in a nuclear power plant offers an alternate method that has compact configuration and higher speed compared to traditional bobbin coil, rotating probe coils, and array probes. This paper investigates the feasibility of the proposed RoFEC eddy-current probe which is composed of three windings excited by three-phase ac current and does not require mechanical rotation of probe. Results of finite-element modeling using reduced magnetic vector potential (RMVP) formulation are presented for modeling the inspection of ferromagnetic and nonferromagnetic tubes. Design parameters of the excitation coils and GMR pick-up sensor are optimized by means of a parametric study.
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paper_title: The Pulsed Eddy Current Differential Probe to Detect a Thickness Variation in an Insulated Stainless Steel

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Non-destructive testing (NDT) plays an important role in the safety and integrity of the large industrial structures such as pipelines in nuclear power plants (NPPs). The pulsed eddy current (PEC) is an electromagnetic NDT approach which is principally developed for the detection of surface and sub surface flaws. In this study a differential probe for the PEC system has been fabricated to detect the wall thinning in insulated steel pipelines. The differential probe contains an excitation coil with two hall-sensors. A stainless steel test sample was prepared with a thickness that varied from 1 mm to 5 mm and was laminated by plastic insulation with uniform thickness to represent the insulated pipelines in the NPPs. Excitation coil in the probe is driven by a rectangular current pulse, the resultant PEC response which is the difference of the two hall sensors is detected. The discriminating features of the detected pulse, peak value and the time to zero were used to describe the wall thinning in the tested sample. A signal processing technique such as power spectrum density (PSD) is devised to infer the PEC response. The results shows that the differential PEC probe has the potential to detect the wall thinning in an insulated pipeline of the nuclear power plants (NPPs).
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paper_title: Measurement and Instrumentation: Theory and Application

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"Measurement and Instrumentation" introduces undergraduate engineering students to the measurement principles and the range of sensors and instruments that are used for measuring physical variables. Based on Morris' "Measurement and Instrumentation Principles", this brand new text has been fully updated with coverage of the latest developments in such measurement technologies as smart sensors, intelligent instruments, microsensors, digital recorders and displays and interfaces. Clearly and comprehensively written, this textbook provides students with the knowledge and tools, including examples in LABVIEW, to design and build measurement systems for virtually any engineering application. The text features chapters on data acquisition and signal processing with LabVIEW from Dr. Reza Langari, Professor of Mechanical Engineering at Texas A&M University. Early coverage of measurement system design provides students with a better framework for understanding the importance of studying measurement and instrumentation. It includes significant material on data acquisition, coverage of sampling theory and linkage to acquisition/processing software, providing students with a more modern approach to the subject matter, in line with actual data acquisition and instrumentation techniques now used in industry. Extensive coverage of uncertainty (inaccuracy) aids students' ability to determine the precision of instruments. Integrated use of LabVIEW examples and problems enhances students' ability to understand and retain content.
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paper_title: Excitation current waveform for eddy current testing on the thickness of ferromagnetic plates

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Abstract Advantages of pulsed excitation current over conventional harmonic excitation for measuring the thickness of a ferromagnetic plate are studied. Compared with the sinusoidal voltage induced by harmonic excitation current, the time-domain voltage induced by pulsed current is highly more sensitive to the thickness. Quantitative proof of this conclusion is provided by solving the normalized derivatives with respect to the thickness. Furthermore, the effects of the time constant of pulsed current on the measuring sensitivity are examined, and an optimal range of the time constant is proposed. Finally, the theoretical model is verified by experimental results.
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paper_title: Detection of the Subsurface Cracks in a Stainless Steel Plate Using Pulsed Eddy Current

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The nondestructive method to detect subsurface defects is limited because conventional eddy current are concentrated near to the surfaces adjacent to the excitation coil. The PEC technique enables detection of cracks buried deeper under the surface with relatively small current density. In the present study, an attempt has been made to investigate detection of subsurface cracks using a specially designed double-D differential probe. The tested sample is a SS304 with a thickness of 5 mm; small EDM notches were machined in the test sample at different depths from the surface to simulate the sub surface cracks in a pipe. The designed PEC probe has two excitation coils and two detecting Hall-sensors. The difference between two sensors is the resultant PEC signal. The cracks under the surface were detected using peak amplitude of the detected pulse; in addition, for a clear understanding of the crack depth, the Fourier transform is applied. In time domain, the peak amplitude of the detected pulse is decreased, and in the frequency domain, the magnitude of the lower frequency component has been increased with an increase in the crack depth. The experimental results have indicated that the proposed differential probe has the potential to detect the sub surface cracks in a stainless steel structure.
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paper_title: A Novel High Sensitivity Sensor for Remote Field Eddy Current Non-Destructive Testing Based on Orthogonal Magnetic Field

paper_content:

Remote field eddy current is an effective non-destructive testing method for ferromagnetic tubular structures. In view of conventional sensors' disadvantages such as low signal-to-noise ratio and poor sensitivity to axial cracks, a novel high sensitivity sensor based on orthogonal magnetic field excitation is proposed. Firstly, through a three-dimensional finite element simulation, the remote field effect under orthogonal magnetic field excitation is determined, and an appropriate configuration which can generate an orthogonal magnetic field for a tubular structure is developed. Secondly, optimized selection of key parameters such as frequency, exciting currents and shielding modes is analyzed in detail, and different types of pick-up coils, including a new self-differential mode pick-up coil, are designed and analyzed. Lastly, the proposed sensor is verified experimentally by various types of defects manufactured on a section of a ferromagnetic tube. Experimental results show that the proposed novel sensor can largely improve the sensitivity of defect detection, especially for axial crack whose depth is less than 40% wall thickness, which are very difficult to detect and identify by conventional sensors. Another noteworthy advantage of the proposed sensor is that it has almost equal sensitivity to various types of defects, when a self-differential mode pick-up coil is adopted.
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paper_title: Noncontact Characterization of Carbon-Fiber-Reinforced Plastics Using Multifrequency Eddy Current Sensors

paper_content:

The characterization of carbon-fiber-reinforced plastics (CFRPs) using multifrequency eddy current sensors is presented in this paper. Three sensors are designed for bulk conductivity measurements, directionality characterization, and fault detection and imaging of unidirection, cross-ply, and impact-damaged CFRP samples. Analytical and finite-element (FE) models describing the interaction of the sensors with the CFRP plate samples are developed to provide an explanation of, and physical insights into, the measured results and observed phenomena. A signal processing method is developed to compensate for the variation in lift-off during the measurements.
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paper_title: Studies to optimize the probe response for velocity induced eddy current testing in aluminium

paper_content:

Abstract Detection and localization of surface and near surface defects in metallic objects using faster and simpler methods is always a matter of great interest in non-destructive testing (NDT). Early defect detection is of utmost importance to maintain safety of the structure and to reduce the maintenance costs. This work proposes a NDT method based on velocity induced eddy currents to detect the surface defects in electrically conductive metal. The approach used is original as it is the resultant magnetic field generated by the eddy currents of the test material induced by the permanent magnet motion, that is measured in order to detect defects. For this purpose new kind of moving magnetic probes were designed and fabricated. Each probe consists of permanent magnets which, due to the movement, induces eddy currents in the sample and a Hall effect sensor able to measure the resultant magnetic field. The total magnetic field has the information of the perturbation of the induced currents produced by the defect. Commercial simulation software was used for the optimization and design of the probe. In order to test the performance and feasibility of the proposed method several experiments were performed on an aluminium plate specimen having linear defects machined with different orientation and depths. The results were obtained by scanning the probe on test specimen at a constant speed. Experimental results confirm that the proposed method with the proposed sensing solution can be an NDT tool to detect the defects in the electrically conductive materials where motion is involved, for example in the inspection of railroads.
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paper_title: GMR array uniform eddy current probe for defect detection in conductive specimens

paper_content:

Abstract The usage of eddy current probes (ECP) with a single magnetic field sensor represents a common solution for defect detection in conductive specimens but it is a time consuming procedure that requires huge amount of scanning steps when large surface specimens are to be inspected. In order to speed-up the nondestructive testing procedure, eddy current probes including a single excitation coil and an array of sensing coils present a good solution. The solution investigated in this paper replaces the sensing coils for giant magneto-resistors (GMRs), due to their high sensitivity and frequency broadband response. Thus, the ECP excitation coil can be driven at lower frequencies than the traditional ones allowing defects to be detected in thicker structures. In this work an optimized uniform eddy current probe architecture including two planar excitation coils, a rectangular magnetic field biasing coil and a GMR magnetometer sensor array is presented. An ac current is applied to the planar spiral rectangular coil of the probe, while a set of GMR magnetometer sensors detects the induced magnetic field in the specimens under test. The rectangular coil provides the DC uniform magnetic field, assuring appropriate biasing of the GMR magnetometers of the probe, setting-up the functioning point on the linear region and at the same branch of the GMR static characteristics. The differences on the images obtained for the same specimen for each GMR are reduced if all sensors are biased on the same working point. Elements of the automated measurement system used to inspect the plate under test using the proposed eddy current probe, including a validation procedure based on a 2D template matching algorithm and the corresponding experimental results are included in the paper.
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paper_title: Giant magnetoresistance-based eddy-current sensor

paper_content:

The purpose of this paper is to introduce a new eddy-current testing technique for surface or near-surface defect detection in nonmagnetic metals using giant magnetoresistive (GMR) sensors. It is shown that GMR-based eddy-current probes are able to accurately detect short surface-breaking cracks in conductive materials. The self-rectifying property of the GMR sensor used in this study leads to a simplified signal conditioning circuit, which can be fully integrated on a silicon chip with the GMR sensor. The ability to manufacture probes having small dimensions and high sensitivity (220 mV/mT) to low magnetic fields over a broad frequency range (from dc up to 1 MHz) enhances the spatial resolution of such an eddy-current testing probe. Experimental results obtained by scanning two different probes over a slotted aluminum specimen are presented. General performance characteristics are demonstrated by measurements of surface and subsurface defects of different sizes and geometries. Dependence of the sensor output on orientation, liftoff distance, and excitation intensity is also investigated.
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paper_title: Eddy Current Technique Based on ${\rm HT}_{\rm c}$-SQUID and GMR Sensors for Non-Destructive Evaluation of Fiber/Metal Laminates

paper_content:

In this work we present non-destructive evaluation measurements on fiber/metal laminate specimen by using eddy current techniques employing HTc SQUID (superconductive quantum interference device) and giant magneto-resistive (GMR) sensors. Our aim is to compare the performance and the capability of HTc SQUID and GMR sensors to detect the presence of damage inside FML composite materials. Experimental results concerning the detection of artificial defects in aeronautical structures with high magnetic sensitivity by using HTc SQUID, and with high spatial resolution using GMR, will be presented and discussed.
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paper_title: Excitation current waveform for eddy current testing on the thickness of ferromagnetic plates

paper_content:

Abstract Advantages of pulsed excitation current over conventional harmonic excitation for measuring the thickness of a ferromagnetic plate are studied. Compared with the sinusoidal voltage induced by harmonic excitation current, the time-domain voltage induced by pulsed current is highly more sensitive to the thickness. Quantitative proof of this conclusion is provided by solving the normalized derivatives with respect to the thickness. Furthermore, the effects of the time constant of pulsed current on the measuring sensitivity are examined, and an optimal range of the time constant is proposed. Finally, the theoretical model is verified by experimental results.
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paper_title: Recent trends in electromagnetic non-destructive sensing

paper_content:

The paper deals with material electromagnetic non-destructive testing (eNDT) with emphasize on eddy current testing (ECT). Various modifications of ECT sensing are compared and discussed from the desired detected signal characteristics point of view. Except of the optimization of usual probe coils arrangements for the concrete applications, the new magnetic sensors as giant magneto-resistance (GMR) and spin dependent tunneling (SDT) are presented. The advanced ECT sensors are characterized by their sensitivity, frequency range and sensor dimensions
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paper_title: Time-domain analytical solutions to pulsed eddy current field excited by a probe coil outside a conducting ferromagnetic pipe

paper_content:

Abstract Using the second-order vector potential formalism and block matrix, the non-axisymmetric eddy current field induced by a probe coil positioned perpendicularly outside a conducting ferromagnetic pipe is solved analytically. Then, the time-domain expressions of induced voltage and eddy current density in the pipe are obtained through the Laplace inverse transformation, which is carried out by calculating the residues of poles. Furthermore, the diffusion process of pulsed eddy current in the pipe is examined. Finally, the analytical solutions are verified through the experiment results of two steel pipes with different wall thickness.
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paper_title: Analysis of the Liftoff Effect of Phase Spectra for Eddy Current Sensors

paper_content:

This paper presents an analytical model that describes the inductance change when a double air-cored coil sensor is placed next to a conducting plate. Analysis of the analytical model reveals that the phase signature of such a sensor is virtually liftoff independent. This finding is verified by numerical evaluations. This paper also finds that the phase signature of a ferrite U-cored sensor can be approximated by that of a double air-cored sensor of similar size and, therefore, possesses a similar liftoff-independent property. Measurements made with a sample U-cored sensor next to plates of nonmagnetic and magnetic materials verified the theoretical results.
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paper_title: Gas Pipeline Corrosion Mapping Using Pulsed Eddy Current Technique

paper_content:

Oil and gas transmission pipelines are critical items of infrastructure in providing energy sources to regions and countries. Steel pipes are commonly used which can be subject to both internal and external corrosion. This paper presents an advanced nondestructive inspection technique for detection of oil-gas pipeline corrosion defect. The Pulsed Eddy Current (PEC) method has been successfully applied in corrosion detection of unburied gas pipeline without removing the insulation. First, the principles of pulse eddy current method is pointed out then, the pulsed eddy current test on a pipe is simulated by Maxwell software to obtain optimum test parameters. To test the new technique, some artificial defects are fabricated on the inner surface of a gas pipe to simulate different corrosions phenomena in practice. Three isolation layers are applied to the pipe in order to show the efficiency of PEC in the detection of wall thinning areas without removing the insulation.
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paper_title: Analytical modeling for transient probe response in pulsed eddy current testing

paper_content:

An improved analytical model by the Fourier method for transient eddy current response is presented. In this work, an alternative approach is considered to solve the harmonic eddy current problem by the reflection and transmission theory of electromagnetic waves, thus a more concise closed-form expression is expected to be obtained. To reduce the inherent Gibbs phenomenon, a harmonic order-dependent decreasing factor is employed to weight the Fourier series (FS) representation. It is shown that the developed model is promising to be used as a fast and accurate analytical solver for the transient probe response and is helpful to gain a deep insight into pulsed eddy current (PEC) testing.
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paper_title: Simulation of Edge Cracks Using Pulsed Eddy Current Stimulated Thermography

paper_content:

Thermography has proven to be one of the most effective approaches to detect cracks in conductive specimens over a relatively large area. Pulsed eddy current stimulated thermography is an emerging integrative nondestructive approach for the detection and characterization of surface and subsurface cracks. In this paper, heating behaviors of edge cracks, excited by pulsed eddy currents, are examined using numerical simulations. The simulations are performed using COMSOL multiphysics finite element method simulation software using the AC/DC module. The simulation results show that in the early heating stage, the temperature increases more quickly at the crack tip compared with other points on the sample. The results indicate that to maximize sensitivity, the response should be analyzed in the early stages of the heating period, no more than 100 ms for samples in which we are interested. The eddy current density distribution is changed with a variation in inductor orientation, but the crack tips remain the "hottest" points during the excitation period, which can be used for robust quantitative defect evaluation. Signal feature selection, transient temperature profile of the sample, and influence of the inductor orientation on the detection sensitivity for edge cracks are investigated. The work shows that positioning of the inductor, perpendicular to the crack line, results in the highest sensitivity for defect detection and characterization. The crack orientation can be estimated through the rotation of the linear inductor near the sample edge and the crack tips.
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paper_title: Interaction of an Eddy-Current Coil With a Right-Angled Conductive Wedge

paper_content:

A fundamental problem in eddy-current nondestructive evaluation is one of finding the quasi-static electromagnetic field of a cylindrical coil in the vicinity of the edge of a metal block. Although the field can be calculated numerically, an effective analytical approach can potentially provide a better understanding of the edge fields and form the basis of a procedure for solving a whole class of related edge problems including edge structures that contain corner cracks. One can represent the metal block as a conductive quarter space in an unbounded region. However, it has been found that the analysis is more straightforward if the problem domain is truncated in two dimensions. With the domain boundaries far from both the coil and the corner, the truncation has a negligible effect on the solution near the edge but the field calculation becomes much easier. A double Fourier series representation of the field is used, as in the case of a rectangular waveguide problem. The field in the conductor is then matched at the interfaces with that in air to determine the expansion coefficients that are used to represent the field in different parts of the domain. In this way we have derived expressions for the magnetic field, the induced eddy-current density and the coil impedance at arbitrary position and orientation of the coil.
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paper_title: Study of Lift-Off Invariance for Pulsed Eddy-Current Signals

paper_content:

Lift-off invariance (LOI) is getting much attention from researchers in the field of electromagnetic nondestructive evaluation (ENDE) because, at the LOI point, eddy-current signals for different lift-offs intersect and the signal amplitude is independent of the lift-off variance. We discuss our ongoing research into LOI, starting from an overview of the state-of-the art of pulsed eddy-current testing (PEC) systems and their use in the elimination of the lift-off effect. We have investigated LOI characteristics with respect to variation in the configuration of the PEC probe in a theoretical study, implemented by extended truncated region eigenfunction expansion (ETREE) modeling. We found that: 1) the LOI occurs when the first-order time derivative of the magnetic field signals are acquired from Hall sensors; 2) an LOI range instead of a single LOI point, when multiple lift-offs are introduced through both experimental and theoretical studies. The LOI range varies with the Hall sensor position in the probe assembly and the conductivity of the samples under inspection, which are important parameters for the design and development of PEC systems. Based on this understanding, we investigated new approaches using theoretical computation and multiple lift-off, or magnetic sensor arrays, for conductivity and lift-off estimation. Our study can be extended for design and development of multipurpose eddy-current sensor systems for surface form measurement and defect detection.
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paper_title: Eddy-current interaction of a long coil with a slot in a conductive plate

paper_content:

We describe a truncated-domain method for calculating eddy currents in a plate with a long flaw. The plate is modeled as a conductive half-space and the flaw is a long slot with a rectangular cross section. A long two-dimensional (2-D) coil carrying an alternating current is aligned parallel to the slot. The coil impedance variation with frequency is determined for an arbitrary coil location. The electromagnetic field due to a long coil above a conductive half-space can be expressed as integrals of trigonometric functions. For a half-space with a long slot, however, additional boundary conditions must be satisfied at the slot walls. The truncated-domain method makes this possible by recasting the problem in a finite domain; as a result, the Fourier integral is replaced by a series. The domain can be made arbitrarily large, thereby yielding results that are numerically as close to the infinite domain solution as desired. We have used the truncated domain approach to study both eddy-current flaw interactions and edge effects in the limiting case of a very wide and deep slot. We confirmed the theoretical predictions by comparing them with results of a 2-D finite element calculation and of experiments.
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paper_title: Assessment of wall thinning in insulated ferromagnetic pipes using the time-to-peak of differential pulsed eddy-current testing signals

paper_content:

Abstract Pulsed eddy current testing (PECT) is a powerful candidate for the detection of wall thinning of insulated ferromagnetic pipes in petrochemical and power generation plants. The main purpose of this study is to find an efficient and easy-to-use signal feature for the assessment of wall thinning. Analytical modeling for a PECT probe over the insulated piping system is performed and its result is verified by experimental test. Two time-related features, the peak value and the time-to-peak, are found in the differential signal obtained by subtracting the test signal from the reference signal. The time-to-peak is superior to the peak value due to its linear variation with wall thickness. Influences of various conditions in practical testing on the PECT signal are investigated. Results show that the time-to-peak is independent of the insulation thickness and the probe lift-off. Robustness of time-to-peak to probe configuration is also validated by employing three probes of different dimensions and structures. To determine the linear range of time-to-peak with amount of wall thinning, differential signals based on different reference thicknesses are examined. Results show that the time-to-peak only keeps linear for the relative wall thinning less than 60%, but still can be effectively used for calibration purpose in periodical in-service inspection of insulated pipeline.
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paper_title: Development of characteristic test system for GMR sensor

paper_content:

In order to test the characteristics of giant magneto-resistance (GMR) sensor comprehensively and accurately, a test system based on 3D Helmholtz coil is designed. The coil can generate sine magnetic field in the range of −10mT to 10mT, 0Hz to 500Hz in three vertical dimensions. The static and low-frequency characteristics of a GMR sensor are tested and analyzed. The dynamic test results reveal that GMR sensor should be applied in unipolar magnetic in its linear range in which case maximal error is still as large as 20% mainly caused by hysteresis. In order to increase the accuracy furthermore in measuring dynamic instantaneous magnetic induction, appropriate hysteresis compensation methods should be analyzed and applied in further research.
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paper_title: Reduction of lift-off effects for Pulsed Eddy Current NDT

paper_content:

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

paper_content:

GMR sensors are widely used in many industrial segments such as information technology, automotive, automation and production, and safety applications. Each area requires an adaption of the sensor arrangement in terms of size adaption and alignment with respect to the field source involved. This paper deals with an analysis of geometric sensor parameters and the arrangement of GMR sensors providing a design roadmap for non-destructive testing (NDT) applications. For this purpose we use an analytical model simulating the magnetic flux leakage (MFL) distribution of surface breaking defects and investigate the flux leakage signal as a function of various sensor parameters. Our calculations show both the influence of sensor length and height and that when detecting the magnetic flux leakage of µm sized defects a gradiometer base line of 250 µm leads to a signal strength loss of less than 10% in comparison with a magnetometer response. To validate the simulation results we finally performed measurements with a GMR magnetometer sensor on a test plate with artificial µm-range cracks. The differences between simulation and measurement are below 6%. We report on the routes for a GMR gradiometer design as a basis for the fabrication of NDT-adapted sensor arrays. The results are also helpful for the use of GMR in other application when it comes to measure positions, lengths, angles or electrical currents.
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paper_title: Removing Eddy-Current probe wobble noise from steam generator tubes testing using Wavelet Transform

paper_content:

One of the most important nondestructive evaluation (NDE) applied to steam generator tubes inspection is the electromagnetic Eddy-Current testing (ECT). The signals generated in this NDE, in general, contain many noises which make difficult the interpretation and analysis of ECT signals. One of the noises present in the signals is the probe wobble noise, which is caused by the existing slack between the probe and the tube walls. In this work, Wavelet Transform (WT) is used in the probe wobble de-noising. WT is a relatively recent mathematical tool, which allows local analysis of non-stationary signals such as ECT signals. This is a great advantage of WT when compared with other analysis tools such as Fourier Transform. However, using WT involves wavelets and coefficients' selection as well as choosing the number of decomposition level needed. ::: ::: This work presents a probe wobble de-noising method when used in conjunction with the traditional ECT evaluation. Comparative results using several WT applied to Eddy-Current signals are presented in a reliable way, in other words, without loss of inherent defect information. ::: ::: A stainless steel tube, with two artificial defects generated by electro-erosion, was inspected by a ZETEC MIZ-17ET ECT equipment. The signals were de-noised through several different WT and the results are presented. The method offers good results and is a promising method because it allows for the removal of Eddy-Current signals' probe wobble effect without loss of essential signal information.
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paper_title: A Novel Triple-Coil Electromagnetic Sensor for Thickness Measurement Immune to Lift-Off Variations

paper_content:

Lift-off variation causes errors in eddy-current measurement of metallic plate thickness. In this paper, we designed a triple-coil sensor operating as two coil pairs and in a multifrequency mode. It is found that the difference in their peak frequencies (the frequency when the imaginary part of the inductance reaches peak) is linearly proportional to the plate thickness but virtually immune to lift-off variations. Mathematical derivation, simulation, and experimental results verified the validity of the methodology.
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paper_title: Liftoff insensitive thickness measurement of aluminum plates using harmonic eddy current excitation and a GMR sensor

paper_content:

Abstract This paper describes the implementation of a device that measures the thickness of metallic plates. A pancake coil for magnetic field sinusoidal excitation is used and detection is performed with a bridge giant magneto-resistor sensor. The paper uses the theory of the linear transformer to explain the liftoff effect with a special attention to the point of interception phenomenon. The transformer model shows that to attain the interception points with instantaneous measured voltages independent of the liftoff gap the excitation coil must be driven with imposed current. This effect was explored to show that a simultaneous process of thickness and conductivity measurement is feasible.
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paper_title: Electrical conductivity measurement of ferromagnetic metallic materials using pulsed eddy current method

paper_content:

Abstract Pulsed eddy current testing (PECT) method for electrical conductivity measurement of ferromagnetic metallic materials is proposed. Based on time-domain analytical solutions to the PECT model of ferromagnetic plates, the conductivity and permeability are determined via an inverse problem established with the calculated and measured values of induced voltage. PECT method for conductivity measurement is verified by the four-point probe method on three carbon steel plates. In addition, the effects of the amplitude of pulsed excitation current and the lift-off of probe coils on measurement results are studied. PECT is an innovative, non-contacting method with good repeatability for electrical conductivity measurement.
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paper_title: New Method for Suppressing Lift-Off Effects Based on Hough Transform

paper_content:

A major problem for crack or electromagnetic property measurement in eddy current detection technique is the lift-off effect. The nonzero value of lift-off distance tends to smear out the discontinuity in the detected signal, causing errors detection. The paper shows the test results of lift-off effect in normalized impedance plane. Curves are transformed to Hough plane by Hough transform and consequently the lift-off effect is suppressed effectively. The method is convenient to distinguish different electromagnetic characteristic of material which can be used to detect crack or other flaws in material.
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paper_title: Probe lift-off compensation method for pulsed eddy current thickness measurement

paper_content:

The pulsed eddy current nondestructive testing signal for ferromagnetic plate thickness testing is sensitive to the plate's thickness and probe lift-off effect simultaneously. This feature leads to the difficulty of eigen-value extraction in thickness quantification. To remove the probe lift-off effect, a method based on calculating the time-varied relative magnetic flux changing rate is presented. Experiment result shows the relative magnetic flux changing rate is approximately lift-off independent and only determined by the plate thickness.
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paper_title: Suppressing sensor lift-off effects on cracks signals in surface magnetic field measurement technique

paper_content:

As in the eddy current technique, a major problem for crack measurement in the surface magnetic field measurement (SMFM) technique is the lift-off distance of the sensor. The nonzero value of lift-off distance tends to smear out the discontinuity in the SMFM crack signal, causing errors in crack characterization. The paper presents a method for sensor lift-off evaluation and crack signal restoration in the SMFM technique. The method employs a deconvolution technique and uses a suitable cost function based on the relations available for electromagnetic field distributions at the metal surface. Simulation results are presented to demonstrate the accuracy of the method.
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paper_title: An approach to reduce lift-off noise in pulsed eddy current nondestructive technology

paper_content:

Abstract The pulsed eddy current (PEC) technique, as an emerging technique of the eddy current technique, has been used in engineering, such as aircrafts, oil/gas pipelines, nuclear steam pipes and high-speed rails, due to its richer information in time domain and frequency domain. However, the lift-off noise, introduced by varying coating thicknesses, irregular sample surface or movement of transducers, has a serious influence on the accuracy of the detection for the defects in these key structures. It greatly limits the application of PEC in quantitative nondestructive testing. In order to reduce the effect of the lift-off, the lift-off effect is analyzed theoretically and experimentally; based on the investigation of the relationship between the peak value of the difference signal and the lift-off, an approach to reduce the lift-off noise for detection the defect depth or width is proposed. In this approach, the defect depth and width are determined by the slope of the linear curve of the peak value of the difference signal and the lift-off. The proposed approach is verified by experiment and the results indicate that it can highly reduce the lift-off noise in the PEC technique. Therefore, it can be applied in characterization of the surface defects in sample with non-ferrous material.
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paper_title: A simplified model for non-destructive thickness measurement immune to the lift-off effect

paper_content:

A simplified model for thickness measurement was introduced in this paper by using an electromagnetic sensor. It uses the phase signature of inductance change to reduce the lift-off effect when air-core coil is placed next to a thin nonmagnetic metallic plate. The phase signature can be extracted through the data at several frequencies. The thickness of the plate can be evaluated non-intrusively and directly. Numerical simulations were carried out for different plate thickness. The results validated the effectiveness of the proposed model when used in eddy-current testing.
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paper_title: Reduction of lift-off effects for Pulsed Eddy Current NDT

paper_content:

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

paper_content:

Pulsed eddy-current (PEC) testing is an electromagnetic nondestructive testing & evaluation (NDT&E) technique and defect classification is one of the most important steps in PEC defect characterization. With pulse excitation, the PEC response signals contain more features in time domain and rich information in frequency domain. This paper investigates feature extraction techniques for PEC defect classification including rising time, differential time to peak and differential time to zero, spectrum amplitude, and differential spectrum amplitude. Experimental study has been undertaken on Al-Mn 3003 alloy samples with artificial surface defects, sub-surface defects, and defects in two-layer structures under different lift-off. Experimental results show that methods are effective to classify the defects both in single-layer structures and two-layer structures. Comparing the results of different methods, it is found that differential process can eliminate the lift-off in defect classification in both time domain and frequency domain. The study can be extended to defect classification in complex structures, where lift-off effects are significant.
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paper_title: Electromagnetic Inspection Technique of Thickness of Nickel-Layer on Steel Plate Without Influence of Lift-Off Between Steel and Inspection Probe

paper_content:

There is a need to inspect the thickness of a nickel layer of nickel-coated steel plate in the production process to guarantee the quality. The conductivity of a nickel layer in the nickel-coated steel is larger than that of mother steel, and its permeability is smaller than that of mother steel. Therefore, the estimation of thickness of the nickel-layer is possible by using differences of these electromagnetic properties. However, the signal is also influenced by the change of distance (lift-off) between the nickel-coated steel and an electromagnetic sensor. In this paper, the inspection method for measuring the thickness of nickel layer taking account of the lift-off is proposed using the 3-D edge-based hexahedral nonlinear FEM.
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paper_title: Sensor-tilt invariance analysis for eddy current signals

paper_content:

In the application of electromagnetic nondestructive evaluation (NDE), the measured signals received by sensors are decided by the flaw associated with operational parameters during inspection. The techniques of invariant pattern recognition have been studied to render NDE signals insensitive to operational variations and preserve or recover crack information. The invariant scheme and algorithms have been addressed to facilitate magnetostatic flux leakage and eddy current NDE, which eliminated operational parameters and lift-off changes corrupt from signal measurements. A novel invariance analysis of eddy current (EC) signals in the inspection of deeply embedded cracks under layered fastener heads has been presented in this paper. A detection system based on uniform EC excitation and giant-magnetoresistive (GMR) pick-up sensors has been developed and shown improved detectability of 2nd and 3rd layer defects around fastener sites in multilayer structures. However, the sensor-tilt due to variation of probe lift-off can generate redundant response as noise effect and obscure the flaw inspection. The variations of GMR sensor-tilt with crack inspection are investigated using an efficient numerical model that simulates the used EC-GMR system. The scheme of invariance transformation is proposed to exclude the sensor-tilt noise and keep the defect inspection identical. The statistical features insensitive to tilt effects are extracted after the invariance processing, which has ensured the probability of flaw detection.
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paper_title: Giant magneto resistance based eddy-current testing system

paper_content:

The purpose of this paper is to introduce a new eddy-current testing system for subsurface crack detection based on giant magneto resistive (GMR) sensors. The optimized probes having small dimensions and high sensitivity to low magnetic fields enhances the spatial resolution of an ECT system. Experimental results of different frequencies and different liftoff distance are presented.
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paper_title: GMR based eddy current system for defect detection

paper_content:

The Giant Magneto Resistance (GMR) sensor is successfully used in eddy current testing because of its good low frequency detection characteristics and high sensitivity. In order to obtain more information about defects, this paper designed a GMR based eddy current system with Field Programmable Gate Array (FPGA) for defect detection. The system can get both the amplitude of the GMR sensor's output signal and the phase related to the exciting current at the same time. Through a series of detection experiments with defects of different widths of 6mm, 8mm and 10mm, it is found that both the amplitude and the phase can clearly reflect the magnetic field changes caused by various defects. By comparing the data curves, it is proved that the phase information matches better with the real size of the defects than the amplitude information. The conclusion is beneficial for further quantitative defect detection.
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paper_title: Defect evaluation using the phase information of an EC-GMR sensor

paper_content:

Phase information is employed to enhance evaluating defects in the metals by eddy current testing using the conventional probes. But for an EC-GMR sensor, the output phase is not used. In this paper, GMR-based eddy current defect detection is studied, in which the phase of the GMR sensor output is taken into account. Based on Biot-Savart law, amplitude and phase for the EC-GMR sensor's output are studied. This paper shows that defect can be better located through the phase analysis of the GMR sensor's output.
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paper_title: Crack detection in steel using a GMR-based MFL probe with radial magnetization

paper_content:

This paper presents the development of a portable probe for detecting cracks in steel plates. The probe consists of a magnet and a giant magneto-resistance (GMR) sensor. The magnet provides a radial magnetization at the surface of the steel plate. The GMR sensor detects the tangential component of the magnetic flux leakage due to a crack in the steel plate. Two steel plates were inspected with six cracks of depths: 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 mm, and widths of 0.25 and 0.5 mm, respectively. The cracks were scanned with the GMR sensitivity axis at 90°, 80°, 70°, 60° and 50°. It is demonstrated that the output voltage of the GMR sensor is sensitive to the orientation of the crack.
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paper_title: Increasing the measurement accuracy of GMR current sensors through hysteresis modeling

paper_content:

A method is presented for increasing the measurement accuracy of GMR (giant magnetoresistive) sensors by numerically eliminating the hysteresis in the output signal. A simplified mathematical model of the hysteresis has been derived from the T(x) hysteresis model for anticipating the measurement values. The model has been implemented in a software simulation environment and compared with real GMR sensor measurements, with very good results. Further, the model has been quantised and implemented on a fixed-point digital signal controller (DSC), connected to the output of the sensors. The linear output characteristic delivered in this case by the DSC confirmed the accuracy of the hysteresis model in the hardware implementation too. An algorithm based on the model has also been developed in order to eliminate error propagation during the measurements.
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paper_title: GMR array uniform eddy current probe for defect detection in conductive specimens

paper_content:

Abstract The usage of eddy current probes (ECP) with a single magnetic field sensor represents a common solution for defect detection in conductive specimens but it is a time consuming procedure that requires huge amount of scanning steps when large surface specimens are to be inspected. In order to speed-up the nondestructive testing procedure, eddy current probes including a single excitation coil and an array of sensing coils present a good solution. The solution investigated in this paper replaces the sensing coils for giant magneto-resistors (GMRs), due to their high sensitivity and frequency broadband response. Thus, the ECP excitation coil can be driven at lower frequencies than the traditional ones allowing defects to be detected in thicker structures. In this work an optimized uniform eddy current probe architecture including two planar excitation coils, a rectangular magnetic field biasing coil and a GMR magnetometer sensor array is presented. An ac current is applied to the planar spiral rectangular coil of the probe, while a set of GMR magnetometer sensors detects the induced magnetic field in the specimens under test. The rectangular coil provides the DC uniform magnetic field, assuring appropriate biasing of the GMR magnetometers of the probe, setting-up the functioning point on the linear region and at the same branch of the GMR static characteristics. The differences on the images obtained for the same specimen for each GMR are reduced if all sensors are biased on the same working point. Elements of the automated measurement system used to inspect the plate under test using the proposed eddy current probe, including a validation procedure based on a 2D template matching algorithm and the corresponding experimental results are included in the paper.
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paper_title: GMR versus differential coils in velocity induced eddy current testing

paper_content:

This paper presents a development on a new nondestructive testing method using eddy currents induced by velocity. This new method uses a constant magnetic field that attached to a moving media induces eddy currents in the conductive material to be tested. By measuring the opposing magnetic field generated by the eddy currents it is possible to obtain information regarding the presence of defects. Two different magnetic field sensors, GMR and differential pick-up coils, were used and compared in the detection of perpendicular components of the magnetic field created by disrupted eddy currents due to linear defects machined on an aluminum plate.
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paper_title: An SVM approach with electromagnetic methods to assess metal plate thickness

paper_content:

Abstract Eddy current testing (ECT) is a non-destructive technique that can be used in the measurement of conductive material thickness. In this work ECT and a machine learning algorithm (support vector machine – SVM) are used to determine accurately the thickness of metallic plates. The study has been made with ECT measurements on real specimens. At a first stage, a few number of plates is considered and SVM is used for a multi-class classification of the conductive plate thicknesses within a finite number of categories. Several figures of merit were tested to investigate the features that lead to “good” separating hyperplanes. Then, based on a SVM regressor, a reliable estimation of the thickness of a large quantity of plates is tested. Eddy currents are induced by imposing a voltage step in an excitation coil (transient eddy currents – TEC), while a giant magnetoresistance (GMR) is the magnetic sensor that measures the transient magnetic field intensity in the sample vicinity. An experimental validation procedure, including machine training with linear and exponential kernels and classification errors, is presented with sets of samples with thicknesses up to 7.5 mm.
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paper_title: Rotating Field EC-GMR Sensor for Crack Detection at Fastener Site in Layered Structures

paper_content:

Eddy current-based techniques have been investigated for the inspection of embedded cracks under fastener heads in riveted structures. However, these techniques are limited in their ability to detect cracks that are not perpendicular to induced current flows. Further, the presence of a steel fastener of high permeability produces a strong signal that masks relatively smaller indication from a crack. In this paper, a rotating electromagnetic field is designed to rotate the applied magnetic fields and related eddy currents electrically so that the sensor shows uniform sensitivity in detecting cracks in all radial directions around fastener sites. Giant magnetoresistive sensors are employed to image the normal component of this rotating field, to detect different crack orientations at aluminum and ferromagnetic fastener sites. Numerical model-based studies and experimental validation are presented.
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paper_title: Open crack depth evaluation using eddy current methods and GMR detection

paper_content:

In this paper the eddy current nondestructive method is used to determine the depth of linear cracks that were machined on aluminum plates. To improve this method a constant field probe was used to produce linear eddy currents launched across the crack. The tangential to plate magnetic field components perpendicular and parallel to the applied field were measured by a giant magneto-resistor sensor at different frequencies to devise one method for crack's depth measurement.
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paper_title: Evaluation of portable ECT instruments with positioning capability

paper_content:

Abstract In this paper two different low-cost eddy current testing (ECT) systems used in detecting and measuring defects in conductive surfaces are evaluated and its performance compared with a commercial equipment. Both developed systems include a probe with a giant magnetoresistance (GMR) as the magnetic field sensor and a computer mouse pointer as the positioning system. This configuration, despite its low cost, allows better performances than commercial equipments because deeper defects can be detected due to the higher sensitivity of the GMR sensor, which is constant in a very large frequency range (10 Hz–1 MHz), but also because a precisely located graphical representation of the defect is delivered to the user due to the incorporated positioning system. Although having the same goal, both developed ECT system implementations differ on their architectures and signal processing algorithms. One system is based on a digital signal processor (DSP) where raw data is digitally processed and the other system uses analog circuits to process the acquired signals. This paper includes a detailed description of each implementation, the obtained results and a performance comparison with commercial equipment.
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paper_title: Routes for GMR-Sensor Design in Non-Destructive Testing

paper_content:

GMR sensors are widely used in many industrial segments such as information technology, automotive, automation and production, and safety applications. Each area requires an adaption of the sensor arrangement in terms of size adaption and alignment with respect to the field source involved. This paper deals with an analysis of geometric sensor parameters and the arrangement of GMR sensors providing a design roadmap for non-destructive testing (NDT) applications. For this purpose we use an analytical model simulating the magnetic flux leakage (MFL) distribution of surface breaking defects and investigate the flux leakage signal as a function of various sensor parameters. Our calculations show both the influence of sensor length and height and that when detecting the magnetic flux leakage of µm sized defects a gradiometer base line of 250 µm leads to a signal strength loss of less than 10% in comparison with a magnetometer response. To validate the simulation results we finally performed measurements with a GMR magnetometer sensor on a test plate with artificial µm-range cracks. The differences between simulation and measurement are below 6%. We report on the routes for a GMR gradiometer design as a basis for the fabrication of NDT-adapted sensor arrays. The results are also helpful for the use of GMR in other application when it comes to measure positions, lengths, angles or electrical currents.
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paper_title: Design and analysis of a GMR eddy current probe for NDT

paper_content:

Defect detection in metallic plates represents an important issue in metal industry, because its potential use in quality control process. Eddy current testing is one of the most extensively used nondestructive techniques for inspecting electrically conductive materials. The purpose of this paper is to present an eddy current testing system for surface defect detection in conducting materials using a giant magnetoresistive (GMR) sensor. An alternate magnetic field is produced by a solenoid and eddy currents are generated in the material under test. The GMR sensor was mounted inside the coil and the arrangement was adapted in the axis of a vertical machining center. In order to validating the measurement device, defects were induced by cracks machined in workpieces made of aluminum. Thus, the parts were scanned with the sensor prototype and a method to estimate the width and depth of the induced defects was proposed after analyzing the output voltage signal.
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paper_title: Detection capabilities evaluation of the advanced sensor types in Eddy Current Testing

paper_content:

The purpose of this paper is to compare performances of various sensing elements in eddy current non-destructive inspection. A new eddy current testing probe is designed to compare detection and resolution capabilities of different sensors. Four magnetic sensors, specifically GMR (1D and 3D- sensor), AMR, Fluxgate, and a standard induction coil are used as a sensing element. For comparison of these sensors the numerical simulations and experimental measurements are performed under the same conditions. The results are presented and discussed in the paper. Streszczenie. W artykule umieszczono wyniki porownania czujnikow wykorzystywanych w defektoskopii wiroprądowej. Autorzy dokonują porownania przy pomocy probnika wlasnego projektu, badając czujniki: GMR, AMR, czujnik pola magnetycznego i standardową cewke indukcyjną. Dodatkowo zostają wykonane symulacje numeryczne odzwierciedlające wykonane pomiary. (Ocena zdolności detekcyjnych czujnikow uzywanych w defektoskopii wiroprądowej)
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paper_title: Crack detection in steel using a GMR-based MFL probe with radial magnetization

paper_content:

This paper presents the development of a portable probe for detecting cracks in steel plates. The probe consists of a magnet and a giant magneto-resistance (GMR) sensor. The magnet provides a radial magnetization at the surface of the steel plate. The GMR sensor detects the tangential component of the magnetic flux leakage due to a crack in the steel plate. Two steel plates were inspected with six cracks of depths: 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 mm, and widths of 0.25 and 0.5 mm, respectively. The cracks were scanned with the GMR sensitivity axis at 90°, 80°, 70°, 60° and 50°. It is demonstrated that the output voltage of the GMR sensor is sensitive to the orientation of the crack.
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paper_title: Current around a crack in an aluminum plate under nondestructive evaluation inspection

paper_content:

This paper presents an inversion problem algorithm applied in eddy current inspection using a sinusoidal magnetic field with fixed-amplitude, in a metallic plate sample with machined crack defects. The uniform magnetic field is obtained in a wide area around the probe due to the building technique of the excitation coil. The magnetic sensor in use is a giant magnetoresistance (GMR) with high sensibility. It measures the amplitude and phase of the magnetic field originated from eddy currents. A perturbation occurs in the measured magnetic field in the proximity of a crack defect. The current density is then determined around the crack defect through an inversion problem algorithm. The applied method consists in determining the transformation kernel and applying a Tikhonov regularization algorithm. With this data it is possible to obtain information about the defect geometrical characteristics.
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paper_title: A GMR–ECT based embedded solution for applications on PCB inspections

paper_content:

Abstract Real-time non-destructive testing and evaluation (NDT/E) of conducting materials using eddy current techniques (ECTs) has gained significance in the last few years. This paper proposes a real-time application of ECT–NDT system exploiting giant magneto-resistive (GMR) sensors for inspection of printed circuit boards (PCBs). Probe design aims to crack inspection over flat surface, especially suitable for micro-defect detection on high density bare PCB. We propose a system based on a GMR sensor able to detect the magnetic field resulting from the interaction between a planar coil exciter and PCBs. The EC signals, detected by the GMR sensor, have been acquired by a high speed analog-to-digital (A/D) converter, for a subsequent application of signal processing based on digital techniques. The achieved results have highlighted the efficient design of the system. The advantages of the proposed models and some possible improvements of the system are also discussed.
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paper_title: Magnetic sensors assessment in velocity induced eddy current testing

paper_content:

Abstract This paper presents an enhancement in the probes to be used on a new nondestructive testing method with eddy currents induced by velocity. In this method, a permanent magnet that is attached to a moving carriage creates eddy currents in the conductive material to be inspected. By measuring the opposing magnetic field generated by the eddy currents, it is possible to obtain information regarding the presence of defects. Different magnetic field sensors, such as, differential pick-up coils, giant magneto resistors (GMR) and Hall sensors have been used and compared. A permanent magnet moving above a plate was studied using a numerical model to allow further improvements to be made in the probe. Depending on each sensor's geometry, sensing axis and range, its position and orientation must be strategically chosen in order to increase defect sensitivity. The best probe's position is the one that guarantees the highest sensibility to the defects’ presence.
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paper_title: The Lift-Off Effect in Eddy Currents on Thickness Modeling and Measurement

paper_content:

This paper uses a linear transformer model to investigate the effect of the lift-off on the results of the thickness measurement of nonferromagnetic metallic plates. The transformer model previews that the time derivative of the magnetization curves obtained for different gaps between the excitation coil and the plate should intercept in a single point when low magnetic coupling factors are considered. To assess the validity of the model, results are compared with experimental data obtained with a giant magnetoresistive sensor probe. For comparison, the sensor output voltage time derivative must be performed as well. The similarity of the theoretical model results and those obtain experimentally with pulsed excitation, confirms the correctness of the transformer approach.
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paper_title: Flexible GMR Sensor Array for Magnetic Flux Leakage Testing of Steel Track Ropes

paper_content:

This paper presents design and development of a flexible GMR sensor array for nondestructive detection of service-induced defects on the outer surface of 64 mm diameter steel track rope. The number of GMR elements and their locations within saddle-type magnetizing coils are optimized using a three dimensional finite element model. The performance of the sensor array has been evaluated by measuring the axial component of leakage flux from localized flaw (LF) and loss of metallic cross-sectional area (LMA) type defects introduced on the track rope. Studies reveal that the GMR sensor array can reliably detect both LF and LMA type defects in the track rope. The sensor array has a fast detection speed along the length of the track rope and does not require circumferential scanning. It is also possible to image defects using the array sensor for obtaining their spatial information.
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paper_title: Pulsed Eddy-Current Based Giant Magnetoresistive System for the Inspection of Aircraft Structures

paper_content:

Research in nondestructive evaluation is constantly increasing the sensitivity of detection of small cracks embedded deep in layered aircraft structures. Pulsed eddy-current (PEC) techniques using coil probes have shown considerable promise in detection and characterization of buried cracks in multilayered structures. In this paper, we describe the design and development of a nondestructive inspection system that uses pulse excitation of a planar multiline coil to generate a transient field that is detected via a giant magnetoresistive (GMR) field sensor. An analysis algorithm using features in time and frequency domain processes the experimentally measured signals for automatic detection of small cracks under fasteners in multilayered structures at a depth of up to 10 mm.
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paper_title: EC-GMR Data Analysis for Inspection of Multilayer Airframe Structures

paper_content:

Eddy-current testing (ECT) is widely used in inspection of multilayer aircraft skin structures for the detection of cracks under fasteners (CUF). Detection of deep hidden CUF poses a major challenge in traditional ECT techniques largely because the weak eddy-current signal due to a subsurface crack is dominated by the strong signal from the aluminum or steel fastener. Giant magnetoresistive (GMR) sensors are finding increasing applications in directly measuring weak magnetic fields associated with induced eddy currents. The measured flux image at a fastener site is in general symmetric and an asymmetry is introduced by the presence of a subsurface crack, which is used for defect detection. This paper presents novel methods that employ the resident phase information, for improving detection probability of GMR signal analysis. Using computational model, the effectiveness of the proposed methods for enhancing detection of CUF is investigated. Results demonstrating the potential of these techniques for detection of second layer CUF are presented.
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Title: Giant Magnetoresistance Sensors: A Review on Structures and Non-Destructive Eddy Current Testing Applications

Section 1: Introduction
Description 1: Introduce the importance of non-destructive testing (NDT) in industries, particularly in the petroleum and gas industry, and highlight the need for efficient defect inspection methods like eddy current testing (ECT).

Section 2: Overview of Giant Magnetoresistance Sensors
Description 2: Discuss the discovery, research, and development of giant magnetoresistance (GMR) sensors, including their advantages and applications in non-destructive testing.

Section 3: Method
Description 3: Explain the foundational principles and studies related to GMR sensors, including structural layers and temperature effects, and introduce different types of GMR sensors.

Section 4: Giant Magnetoresistance Spin Valve Sensor
Description 4: Describe the structure and principles of spin valve GMR sensors, including research findings and different types of spin valves used in various applications.

Section 5: Giant Magnetoresistance Multilayer Sensor
Description 5: Examine the structure and function of multilayer GMR sensors and discuss studies on their behavior, advantages, and specific designs like the Wheatstone bridge GMR sensor.

Section 6: Types of Non-Destructive Eddy Current Testing Probe
Description 6: Detail the various types of ECT probes, including impedance variation probes and excitation-detection probes, and their specific functionalities.

Section 7: Bobbin Probe
Description 7: Discuss the features, applications, and differences between absolute and differential bobbin probes used in tube inspection.

Section 8: Full Saturation Probe
Description 8: Explain the use, advantages, and challenges of full saturation probes in inspecting ferromagnetic or magnetic stainless steel tubes.

Section 9: Rotating Bobbin Probe
Description 9: Describe the functionality and advantages of rotating bobbin probes in detecting circumferential cracks and defects in tube inspection.

Section 10: Array Probe
Description 10: Discuss the structure, design, and application of array probes, including different models like C-Probe, X-Probe, and Smart Array Probe.

Section 11: C-Probe
Description 11: Provide specific details about the design and evolution of the C-Probe and its applications.

Section 12: X-Probe
Description 12: Explain the development, structure, and combined functionalities of the X-Probe in non-destructive testing.

Section 13: Smart Array Probe
Description 13: Detail the improved characteristics of the Smart Array Probe compared to the X-Probe and its simplified control circuits.

Section 14: Intelligent Probe
Description 14: Discuss the design and capabilities of the Intelligent Probe, including field trials and unique coil designs.

Section 15: Rotational Magnetic Flux Sensor
Description 15: Explain the principles, design, and applications of the rotational magnetic flux sensor for flat plate and tube inspections.

Section 16: Rotating Magnetic Field Probe
Description 16: Describe the development and testing of rotating magnetic field probes and their application in small diameter, non-magnetic tubing inspections.

Section 17: The Influence of Various Parameters on the GMR Measurement
Description 17: Identify and discuss key factors that affect GMR measurement, including surface quality, layer thickness, and temperature.

Section 18: Structural Quality of Giant Magnetoresistance Sensor
Description 18: Examine how the surface quality of GMR structures influences magnetic resistance properties and the impact of different fabrication processes.

Section 19: Thickness Structure Layers of Giant Magnetoresistance Sensor
Description 19: Discuss the effects of spacer thickness on the magnetic resistance of GMR structures and optimization techniques for high magnetic resistance properties.

Section 20: Temperature
Description 20: Detail the relationship between temperature and magnetic resistance in GMR structures and methods to enhance thermal stability.

Section 21: Factors Affecting the Eddy Current Testing Inspection
Description 21: Identify and examine factors that influence eddy current inspection results, such as coil frequency, magnetic permeability, and lift-off.

Section 22: Exciting Coil Frequency and Skin Depth Effect
Description 22: Explain the importance of coil frequency in determining defect depth and the effect of skin depth on eddy current density in materials.

Section 23: Material Magnetic Permeability
Description 23: Discuss how material permeability impacts eddy current penetration and defect detection capabilities.

Section 24: Lift-off
Description 24: Examine how lift-off affects probe sensitivity and techniques to mitigate its impact on ECT signal readings.

Section 25: Conductivity of Material
Description 25: Describe how changes in material conductivity influence the magnetic field and sensor output in eddy current probes.

Section 26: Limitations of Coil Sensor in Eddy Current Probe
Description 26: Address the limitations of traditional coil sensors in ECT, including poor sensitivity at low frequencies and the development of hybrid ECT probes.

Section 27: Compensation Techniques in Eddy Current Testing Probes
Description 27: Detail various techniques to compensate for factors like lift-off, temperature, and edge effects in ECT probes to improve inspection accuracy.

Section 28: Application of GMR Sensors in Hybrid Eddy Current Testing Probes
Description 28: Discuss the implementation and benefits of using GMR sensors in hybrid ECT probes for improved defect detection and measurement accuracy.

Section 29: Conclusions
Description 29: Summarize the importance and advancements of GMR sensors in ECT, highlighting their impact on inspection accuracy and defect detection capabilities.