AC Field Measurement Research Articles

An Approximate Analytical Model of Surface Cracks in AC Field Measurement Technique

An appropriate modeling technique is critical for the quick analysis of alternating current (ac) field measurements destined for qualitative or quantitative studies of surface defects in conductive materials. Traditional modeling techniques often use the finite-difference method to solve the governing Laplace equation in the crack region, which gives a closed-form solution in the Fourier domain for regions outside the crack. However, these models are complex and only useful for cracks with extremely narrow openings. This article proposes an approximate analytical model that is based on current shunting of parallel circuits and the Biot–Savart law. We assume that the current induced by a high-frequency current-carrying coil is evenly distributed within a very thin layer at the surface of a conductor. In the presence of a defect, the current flows only along the edges and the bottom of the defect. Under these assumptions, the model simplifies the induced current by treating it as due to many current sources of equal amplitude. The magnetic field above the defect can be mathematically described by the linear superposition of these current sources. The proposed model can thus be used to analyze cracks with wider openings. We compare the magnetic-field signals of flat-bottomed cracks of various sizes as obtained by the proposed model and the finite-element method and also explore cracks with complex bottom profiles. The results show that the proposed model produces accurate quantitative and qualitative results for a wider range of defects.

Measurement of Magnetic Nanoparticles by Small HTS SQUID Array

We propose a method to investigate the brain activity of a small animal by measuring blood flow with magnetic nanoparticles (MNPs) using a high-Tc Superconducting Quantum Interference Device (SQUID) array. For the preliminary study, we fabricated a three channel high-Tc SQUID array and demonstrated the detection of a small volume of water diluted MNPs in motion. Two detection methods were applied; one was dc field measurement and the other was dc with ac field measurement. In the latter case, a second harmonic response of the nonlinear M-H characteristics of MNPs was detected. In the dc magnetic field measurement, it was demonstrated that the signal from each SQUID magnetometer was dependent on its position and the movement of the MNP sample. The detectable minimum iron content was compared between the dc and dc with ac field measurement. As a result, it was found that the minimal detectable iron content by ac with dc measurement was 12.2 ng with an SNR of 2.3 and was 1/2 times smaller than that detected by the DC field measurement.

Magnetic Field Measurement and Analysis of the CSNS/RCS Quadrupole Magnets

The China Spallation Neutron Source/rapid-cycling synchrotron (CSNS/RCS) quadrupole magnets excited by both dc current and 25 Hz ac biased dc current were tested and measured in the past two years. All the magnets had been installed in the tunnel by the end of December, 2015. The challenges of the field measurements included the measurement repeatability of the dc field and the time harmonic measurement of the dc+ac field. The paper will summarize the results of the dc field measurement and the dc+ac field measurement, the key techniques of the rotating coil fabrication and the measurement stability for the CSNS/RCS quadrupole magnets.

Controllable synthesis, crystal structure and magnetic properties of Monomer-Dimer Cocrystallized MnIII Salen-type composite material

By the reaction of manganese-Schiff-base complexes with penta-anionic Anderson heteropolyanion, a new supramolecular architecture [Mn2(Salen)2(H2O)2][Mn(Salen)(H2O)2]2Na[IMo6O24]·8H2O (1) (salen = N,N′-ethylene-bis (salicylideneiminate) has been isolated. Compound 1 was characterized by the single-crystal X-ray diffraction, elemental, IR and thermal gravimetric analyses. Structural analysis reveals that the unit cell simultaneously contains MnIII-Salen dimer and monomer cation fragments, for which the Anderson-type polyanions serve as counter anions. In the packing arrangement, all the MnIII dimers are well separated by polyoxometalate units and form tertiary structure together with MnIII monomers. Interestingly, different from the previous work, in the exact same reaction conditions, we are able to template MnIII-Salen complexes into different configurations by varying the charge state of polyanions. Besides, the magnetic properties of 1 were also examined by using both dc and ac magnetic field of the superconducting quantum interference devices. Most importantly, our fitting of the experimental data to a Heisenberg-type spin model shows that there exists a ferromagnetic exchange interaction ∼5 K between the spins (S = 2) on MnIII in the dimer, while antiferromagnetic ones exist among monomers and dimer (∼2 K). This meta-magnetic state could induce a slight spin frustration at low temperature, which would in turn affect the magnetic behavior. In addition, our ac field measurement of the susceptibilities suggests a typical signature for a single-molecule magnet.

An analysis in metal barcode label design for reference

We employ nondestructive evaluation involving AC field measurement in detecting and identifying metal barcode labels, providing a reference for design. Using the magnetic scalar potential boundary condition at notches in thin-skin field theory and 2D Fourier transform, we introduce an analytical model for the magnetic scalar potential induced by the interaction of a high-frequency inducer with a metal barcode label containing multiple narrow saw-cut notches, and then calculate the magnetic field in the free space above the metal barcode label. With the simulations of the magnetic field, qualitative analysis is given for the effects on detecting and identifying metal barcode labels, which are caused by metal material, notch characteristics, exciting inducer properties, and other factors that can be used in metal barcode label design as reference. Simulation results are in good accordance with experiment results.

An Efficient Modeling Technique for Analysis of AC Field Measurement Probe Output Signals to Improve Crack Detection and Sizing in Cylindrical Metallic Structures

The paper proposes an efficient modeling technique for predicting the output signal of an AC field measurement (ACFM) probe when encountering a fatigue crack in a cylindrical metallic structure. The proposed technique is used to determine the electromagnetic fields around an arbitrary-shape surface crack in a cylindrical metal, induced by a high frequency alternating-current-carrying coil of arbitrary shape. The modeling technique involves two steps. First, the finite-difference method is used to solve the governing Laplace equation inside the crack region. Then, the normal component of magnetic field at the crack opening on the metal is obtained to derive a closed-form solution in the Fourier domain for the regions outside the crack. Experimental and theoretical results associated with prototype ACFM probes confirm the validity of the proposed technique while demonstrating its remarkable computation efficiency. The latter is an important feature of the proposed modeling technique as a fast “forward” solver that results in a remarkable improvement in solving the “inverse” problem required for the sizing of fatigue cracks in cylindrical metallic structures, particularly in real-time applications.

Self-powered, hybrid antenna-magnetoresistive sensor for magnetic field detection

A self-powered hybrid sensor integrating a resonant antenna with a magnetoresistive spin valve sensor was microfabricated. The device is activated by a radiofrequency (rf) external electromagnetic source. This hybrid sensor is capable of detecting dc and ac external magnetic fields in the thermal noise regime. For an excitation rf field of 17 dB m at 130 MHz, the sensitivity to a transverse magnetic field normalized by the current induced in the device was 812 V (T A)−1 for dc magnetic field detection and 918 V (T A)−1 for the ac field measurement.

A fuzzy alignment approach to sizing surface cracks by the AC field measurement technique

In real world applications, one major issue with most of non-phenomenological methods is the need for a large and complete databanks. In most practical cases, it is impossible to obtain a complete and representative database that includes sufficient number of representative examples. This fact renders most of available defects and cracks databases useless. In this paper, an aligning method is formalized by a fuzzy recursive least square algorithm as a learning methodology for electromagnetic alternative current field measurement (ACFM) probe signals of a crack (data). This method along with a set of fuzzy linguistic rules, including adequate adaptation of different crack shapes (knowledge) for combining knowledge and data whenever the superposition theory can be utilized, provide a means to compensate for the lack of sufficient samples in available crack databases. We have shown that the combination of this fuzzy inference method and the method of the adaptation for different crack shapes provides sufficient means as a priori empirical knowledge for the training system. This approach significantly reduces the need for a large and complete crack databases. The validity of the proposed methodology is demonstrated by examining the sizing errors in the case of several surface cracks with elliptical depth profile when inverting their respective ACFM signals.

Removal of Probe Liftoff Effects on Crack Detection and Sizing in Metals by the AC Field Measurement Technique

In the alternative current field measurement (ACFM) technique, the nonzero value of liftoff distance for the magnetic sensor acts as a low-pass filter on surface crack signals, causing errors in crack detection and sizing. We present a blind deconvolution algorithm for liftoff evaluation and surface crack signal restoration. The algorithm employs the available closed-form expressions for the distribution of electromagnetic fields at the metal surface in the vicinity of a crack. To examine the accuracy of the algorithm, we use the original and the restored signals for crack sizing by a wavelet network inversion method. We present simulated and experimental results to demonstrate the role of the proposed algorithm in improving the inversion process.

Using AC field measurement data at an arbitrary liftoff distance to size long surface-breaking cracks in ferrous metals

We propose a model-based inversion method to size long surface-breaking cracks in ferrous metals using alternative current field measurement (ACFM) data at an arbitrary liftoff distance. This method employs conjugate gradients optimization to invert measured surface ACFM signal to crack depth. To alleviate the adverse effect of sensor liftoff uncertainty on crack sizing, we propose a blind de-convolution algorithm for reconstructing respective surface ACFM crack signal. In this algorithm, the partially known filter function associated with the sensor liftoff is estimated from which the surface crack signal can be restored. The validity of the proposed inversion method is demonstrated by comparing the actual and predicted depths of several simulated and machine-made long cracks in mild steel test blocks.

Using a wavelet network for reconstruction of fatigue crack depth profile from AC field measurement signals

This paper presents a wavelet-network-based technique for reconstructing the crack depth profile of a fatigue crack in a metal from the output signal of an alternating current field measurement (ACFM) probe. The main feature of this technique is that it requires only the ACFM probe output signals along the crack opening. The database for training the network is established by developing a random crack-depth generator, using a fast pseudo-analytic ACFM probe output simulator. The validity of the proposed technique is demonstrated by comparing the actual and reconstructed depth profiles of several simulated and machine-made cracks with no predetermined geometries.

Overview of scientific payloads onboard the KSR-III rocket

The Korea Sounding Rocket-III (KSR-III) was successfully launched on November 28, 2002 from the west coast of the Korean Peninsula. The science payload onboard the KSR-III included an ozone detector and two magnetometers along with other various sensors installed to measure physical characteristics such as temperature, pressure, strain, and acceleration. The main objective of KSR-III was to evaluate the liquid propulsion engine system which has been newly adopted in the KSR series. In addition to this main objective, the science payload conducted atmospheric soundings. The payload data were transmitted to the ground station in real time by an onboard telemetry system. The UV radiometer measured the direct solar UV radiation and during the ascending phase the vertical ozone density profile was obtained. This result was compared with coincident measurements taken by other satellites, a ground station, and an ozonesonde. A fluxgate-type magnetometer was onboard the KSR-III to observe the Earth's DC magnetic field and for AC field measurements, a search-coil magnetometer was installed. This was the first Korean mission to use magnetometers on a rocket-borne platform to measure the Earth's magnetic field. Using the telemetry magnetometer data, a study on the rocket attitude was carried out. This paper will give an overview of the design, calibration, and test results of the science payload onboard the KSR-III.

Thin-skin analysis technique for interaction of arbitrary-shape inducer field with long cracks in ferromagnetic metals

In using the AC field measurement (ACFM) technique for non-destructive evaluation (NDE) of metals, a current-carrying wire structure is used to induce eddy current within a thin layer of the metal and a magnetic field sensor to measure the field perturbations in the vicinity of the metal. The sensitivity of ACFM crack detection and sizing relies on an appropriate design of the wire structure geometry together with a dully placement of the sensor. This paper presents an analytical modeling technique for evaluating the electromagnetic field interaction of an ACFM probe with a long uniform crack in a ferromagnetic metallic slab. The probe in the proposed model can have an arbitrary-shape wire inducer with no restrictions on its relative sensor position. The technique is accurate and very efficient computationally. It first uses the two-dimensional Fourier transform to obtain the field distribution at the metal surface. The Laplacian field distribution above the metal is then determined by satisfying the so-obtained boundary condition at air–metal interface. To demonstrate the accuracy of the model, we consider the special case of a rhombic wire inducer. The comparison of our results with those obtained using the conventional algorithm in the literature validates the accuracy of the model introduced in this paper. To show the generality of the model, we also present theoretical and experimental results associated with a solenoid inducer with a three-dimensional geometry for which no analytical solution is available in the literature. The theoretical prediction of crack signal supported by experimental results is used to develop a model-based method for inverting crack signal into crack depth.

Detecting and measuring flaws using electric potential techniques

The electric potential techniques are of two types: the direct current potential drop method (DCPD) and the alternating current potential drop method (ACPD). While the latter can be used mainly to detect surface defects, the first is more appropriate for detecting the initiation of cracks and monitoring their growth. One of the advantages of the ACPD is that it can be easily employed as a non‐destructive inspection tool. The DCPD has been used mainly in the laboratory environments under various conditions of loading including high gross inelastic deformations where subsurface flaws are present. Both these techniques have high accuracy and can be used as tools to detect defects in manufactured parts such as flaws in welds. Their findings are very useful in preventive maintenance; the inspectors and engineers use them to take decisions for scheduling maintenance. The present paper presents a review of the evolution in the design of ACPD and DCPD systems, with their advantages, disadvantages and fields of application. It is shown that ACPD and DCPD have comparable sensitivity and are widely used for surface crack measurement. The relatively new AC field measurement technique will be described. Its performance will be compared to that of ACPD. The use of DCPD in applications involving high temperature and gross inelastic strains will be stressed. The results obtained in low cycle fatigue conditions show that by including a special reference potential ratio, the DCPD yields a good estimation of the average surface and subsurface crack lengths. The method also allows an accurate detection of crack initiation in these conditions.

Effects of probe and inducer on saturation of crack signal in high-sensitivity AC field measurement technique

As in the eddy-current technique, a major problem in separating and sizing deep surface cracks in the AC field measurement (ACFM) technique is the saturation of the crack signal. It is essential that the operator ensures the crack signal does not reach saturation for the deepest possible crack. The paper explains how the parameters of the probe and inducer in the high-sensitivity ACFM technique with a rhombic inducer control the sensitivity and saturation level. Theoretical results are presented on the effects of the inducer size, probe position and probe length on the crack signal. It is shown that for an inspection task where there is an indication of the maximum crack depth, the saturation problem can be avoided by choosing the right size inducer, the right position for the probe and the correct probe length.

ACFM — a new NDT technique : Lugg, M.C. Metals and Materials, Vol. 6, No. 3, pp. 142–144 (Mar. 1990)

Recent progress in the theoretical modelling of magnetic fields has allowed the development of the ac potential drop (ACPD) crack sizing technique into a non-contacting form, known as ac field measurement (ACFM). This new NDT technique aims to combine the sizing capability of ACPD with the practical benefits that arise when electrical contact does not have to be maintained with the material being inspected. The technique is already being used for cracks in mild steel and development of a rapid scanning system using an array sensor is also under way. Other applications in hostile, corrosive or high temperature environments for materials testing are envisaged in the near future

Simple electromagnetic detection of variations in properties of metal surfaces

Recently, a significant improvement in the sensitivity of the nonuniform AC field measurement technique for the detection of cracks was reported. In this paper, the results of the application of the improved technique in the detection of heat-effected zones and ground welds are presented and briefly discussed. In such applications, the use of a long probe is essential.

Analysis technique for interaction of high-frequency rhombic inducer field with cracks in metals

In the nondestructive evaluation (NDE) involving the ac field measurement (ACFM), a current carrying structure is required to induce the eddy current in the work-piece and a probe to sample the field. Due to its flat profile, slender shape, and other advantages, the rhombic wire loop is a suitable inducer for developing linear flexible arrays for the ACFM inspection of large surfaces of ferrous and nonferrous metals. This paper introduces an analysis technique for the evaluation of the interaction of the field of the rhombic inducer carrying a high-frequency current, with long surface cracks of uniform depth in flat metal plates. The technique is accurate and very efficient computationally. It uses the two-dimensional Fourier transform together with a special boundary condition at the metal surface. The boundary condition takes into account the thin-skin nature of the eddy current in the metal as well as the flux leakage at the crack mouth. The analysis technique benefits from the use of scalar potential functions and can be extended to simply or multiply connected wire loops. Also, it is applicable to high-frequency (thin-skin) eddy current problems. Using the analysis technique, the tangential field below a rhombic inducer along its long diagonal when the inducer is located above the surface of aluminum and steel is given in the presence and absence of a crack. This field was found to have a nonuniform phase distribution. Near a crack, the phase change is significant, even for shallow cracks. The role of the nonuniform phase in the detection sensitivity is addressed. Also, simulated ACFM crack responses using an inducer with a linear probe attached along the long diagonal are presented and discussed. To support the validity of the analysis technique, experimental results obtained for some of the simulations are also reported. In addition to its application in predicting crack responses, the technique can be used for model-based inversion of crack signals.

Simultaneous AC and DC magnetic field measurements in residential areas: implications for resonance theories of biological effects

The goal of this study was to obtain data that could be used to evaluate the applicability of resonance theories of biological effects in residential settings. We first describe a measurement system which allows the study of AC and DC magnetic fields simultaneously in space and in time. Sample measurements were taken near two power lines, two objects and in two residential homes. The results show that the Earth's (DC) magnetic field was unaffected near power lines. The compass orientation of the power line influenced the relative values of the AC components parallel and perpendicular to the DC field. The electric heating system greatly affected the AC field levels in the home, causing the levels to increase from less than 1 mG to a maximum of 7.5 mG during heating. The magnitudes of the DC field in the two homes varied from about 380 to 650 mG, with the larger variations near metallic or magnetic objects such as the refrigerator or a metallic air duct. The Earth's field was elevated above its natural level within a distance of 8 feet from a subcompact passenger car, e.g., the level changed from about 540 to 1100 mG beside the headlight. A steel chair changed the Earth's field by up to 60 mG within a distance of one foot. These results suggest that some of the narrow resonances described in laboratory studies may be difficult to observe against the variations in DC field amplitude and direction resulting from the presence of everyday metallic objects. >

Measurement of crack-profiles using AC field measurement method

The alternating current field measurement technique has been employed in the present study to predict cracks of different profiles. Profiles used included surface thumb-nail cracks of semi-circular, semi-elliptical, asymmetrical and rectangular shape with aspect ratios ranging from 2 to 10. From the potential ratios measured experimentally, a numerical approach was employed to compute the crack profiles. Crack profiles were also computed from theoretical potential profiles obtained using boundary element method. The present study shows that with high aspect ratios, the crack profiles predicted using theoretical data bore great resemblance to that of the actual. The underpredictions at the centre-line position of the cracks were small. Using experimentally measured potentials with 1-dimensional interpretation, all predicted profiles showed an under estimation of the actual. The errors appeared generally lower for the narrower plate and decreased with increase in aspect ratio. Pseudo-random errors were introduced to the theoretical potentials to simulate measurement errors that may occur in practice, to improve the error handling capability of the computer programme designed for the study. In addition, a smoothing technique was also applied to improve the accuracy of the crack profile prediction. By “freezing” some of the distant potential field, a significant computer processing time reduction of 25–35% has been achieved.