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SQUID technology has recently evolved to the point that it can be used for industrial applications in Non-Destructive Evaluation (NDE). We present the implementation of an HTS SQUID magnetometer in an eddy current testing system to measure very thick structures in large aircraft. We measured a 62 mm-thick, bolted aluminum sample from EADS-Airbus, similar to the three-layered outer wing splice that is being proposed for the Airbus A-380. The combination of field sensitivities of a few pT/ √Hz and a large dynamic range of about 140 dB/ √Hz enabled us to detect defects at a depth of up to 40 mm. However a problem was presented by the fact that deep-lying defects which caused small field variations were superimposed on field changes, in their turn caused by current distortions in the vicinity of the titanium bolts. Separation of these two contributions was achieved through parameter optimization based on FEM simulations and signal processing. We report on the possibilities for flaw detection using adapted eddy current excitation. [ABSTRACT FROM AUTHOR]

Eddy current (EC) testing of aircraft components for material flaws hidden deeply in the tested structure is facilitated by using an array of HTS rf superconducting quantum interference device (SQUID) sensors. Recently, the multiplexed operation of three planar HTS rf gradiometers with one electronics and one cable was shown [IEEE Trans. Appl. Supercond. 11 (2001) 1168]. In this paper, a multiplexing setup with three magnetometers is presented. SQUID magnetometers have proven advantageous over short-baseline gradiometers for EC measurements of deep flaws. Using three standard HTS rf washer SQUID magnetometers with step-edge junctions, we implemented a multiplexed SQUID array. In conjunction with EC excitation and lock-in readout, measurements of aluminium aircraft samples were carried out in an unshielded laboratory environment. Newly developed software controls the continuous switching of the SQUIDs during the scan of the samples. The quasi-simultaneously obtained traces of the magnetometers are lock-in demodulated to yield in-phase and quadrature components, respectively. For EC excitation, a double-D excitation coil was selected. We performed measurements to localize an artificial crack of 20 mm length in an aluminium sheet with 0.6 mm thickness uncovered and covered by unflawed aluminium plates. In addition, the angle between crack and scanning direction was varied. Recorded signals of the SQUID array are presented and compared to signals received by scanning with just one and two magnetometers. [Copyright &y& Elsevier]

For detection of tendon ruptures in prestressed members of bridges, a four-channel SQUID system was developed. The tendons are magnetized by scanning a yoke electromagnet over the concrete surface along the hidden member. Four HTS dc-SQUID magnetometers with ramp-type junctions, optimized for high-field performance, are mounted in an orientation-independent liquid nitrogen cryostat. The SQUIDs are integrated as a linear array within the yoke and operated in magnetic fields up to 15 mT, recording the stray field during magnetization as well as the remanent field after switching off the excitation. Unwanted signals from stirrups of the mild steel reinforcement are suppressed with two types of techniques: either the comparison of remanent field signals after changing the magnetization direction of the stirrups, or a best fit of typical stirrup signals to the stray field signal and their subtraction. Subsequent correlation analysis with the dipolar signal of a typical void yields rupture signal amplitudes. A finite element program was written to simulate stray field and remanent field traces of typical steel configurations. Excellent agreement with measured data was found. Results of measurements on a prestressed highway bridge are presented. Signal amplitudes above the threshold values were verified as originating from ruptures of the steel tendon by opening the bridge deck. [Copyright &y& Elsevier]

By presenting brief summaries of recent application highlights, an overview of NDE methods using SQUIDs is given. Bridge inspection with a SQUID array integrated with a yoke magnet excitation was shown by scanning along the pre-stressed steel of bridges and verified by opening the bridge deck. As the construction of the megaliner Airbus aircraft progresses, testing procedures for extremely thick-walled structures are needed. Defects at a depth of up to 40 mm were measured in a bolted three-layer aluminum sample with a total thickness of 62 mm. For the investigation of aircraft wheels, a remote eddy current (EC) excitation scheme yields better depth selectivity. Defects with an inside penetration of only 10% could be detected. SQUID magnetometers are well suited for pulsed EC techniques which cover a broader depth range than standard single frequency EC. An inversion procedure is presented providing a tomographic-like conductivity image of stacked aluminum samples. A recent SQUID application is nondestructive testing of niobium sheets used for superconducting cavities of particle accelerators. The detection of tantalum inclusions and other impurities which lower the cavity performance is based on the measurement of local current inhomogeneities caused by EC excitation or thermal gradients. Alternate techniques using SQUID sensors, such as modulated excitation arrays, rotating field schemes, sensor multiplexing, magnetic moment detection, and microscopy setups, are discussed. [Copyright &y& Elsevier]

Examines step-edge junction radio frequency-superconducting quantum interference device (SQUID) magnetometers and gradiometers fabricated using yttrium-barium-copper-oxide films. Effects of step structures and film properties on yield, optimal operating temperature and SQUID low-frequency noise; Sensitivity to the step structure of devices fabricated on different substrates.

Analyzes the magnetic field behavior of yttrium barium copper oxide step-edge Josephson junctions in radio frequency-washer superconducting quantum interference devices (SQUID). Effects of critical current suppression on SQUID operation; Requirements for operation of the sensors in the Earth's magnetic field.

Describes an approach for fatigue defect detection and classification using a superconducting quantum interference device-based multiple frequency eddy current systems. Ways of distinguishing signals caused by material defects; Analog dynamic range; Flaw construction using adapted feature extraction and neural network techniques.

Analyzes the time series of signal and reference data recorded outside magnetic shielding with high temperature superconducting quantum interference device-based gradiometers using two software gradiometery methods for magnetocardiography. Signal-to-noise ratios for both methods; Signal sensor requirements; Possibility of increasing the signals' bandwidth.

Focuses on high temperature superconducting quantum interference device (SQUID) magnetometer with SQUID vector reference for magnetically shielded operations. Low-frequency crosstalk between axial SQUID; Magnetocardiogram measurement results.

Presents information on an eddy current system in cojunction with a planar Superconducting Quantum Interference Device (SQUID) gradiometer, for detection of deep-lying flaws in aircraft structures. Importance of a mobile system; Results of the operation of high temperature superconductor (HTS) SQUIDs.

By employing high-temperature superconducting quantum interference device (SQUID) magnetometers, we have assembled a second-order gradiometer for magnetocardiography (MCG) in unshielded environment. With this high-temperature superconductor (HTS) SQUID system, we demonstrated its diagnostic relevance for MCG in terms of signal-to-noise ratio, spatial resolution, frequency bandwidth, rejection of environmental disturbances, and long-term stability. The electronically balanced gradiometer consists of three HTS radio-frequency SQUIDs with superconducting coplanar resonators, mounted in axial gradiometric arrangement with a baseline of 7.5 cm. The system achieves a common mode rejection for axial homogeneous fields of about 10[sup 4] without any mechanical balancing, and a white noise about 130 fT/&sqrt;Hz at 77 K, with an 8x8 mm[sup 2] flux pickup area. MCG maps above volunteers' chests have been recorded in unshielded environment in a bandwidth of about 130 Hz. We showed the influence of several notch filters (suppressing the power line frequency) on the quality of the MCG signals. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Examines the operation of a radio frequency (rf) superconducting quantum interference device (SQUID) with a lumped-element tank circuit. Development of a matching scheme; Characterization of the noise of high-temperature rf SQUIDs; Stabilization against high frequency disturbances.

Abstract: High-resolution low-field nuclear magnetic resonance (LF-NMR) investigations of liquid samples, recorded using a HTS radio frequency (rf) superconducting quantum interference device (SQUID) are overviewed in this paper. The measurements were performed either in a magnetically shielded room (MSR) or in the Earth’s Magnetic Field (EMF). In MSR, measurements with Larmor frequencies (f L) ranging from 2Hz to 40kHz were demonstrated. The natural spectral linewidth of water and the scalar coupling spectra of 2,2,2-trifluoroethanol and of fluorobenzene were determined. An additional nitrogen-cooled resonant LC input circuit was also introduced in higher f L (>10kHz) region to enhance the signal-to-noise ratio up to an order of magnitude. The influence of the speed of decay of the polarizing magnetic field (B P) on the free-induction-decay (FID) signal was analyzed quantitatively. In EMF, NMR spectra of different liquid samples were recorded. To compensate the line broadening due to EMF fluctuations we used frequency-adjusted averaging. [Copyright &y& Elsevier]

We describe the use of single substrates as resonators for radio-frequency (rf) superconducting quantum interference devices (SQUID) operation. A standard strontium-titanate substrate with dimensions 10×10×1 mm3 serves as a tank circuit (resonator), a YBCO thin film SQUID washer structure is patterned on it. On the resonator substrate, a small rf washer SQUID with a step-edge junction is positioned in flip-chip geometry, thus forming a magnetometer sensor. This resonator is inductively coupled to the readout electronics. At 77 K, the field sensitivity of this SQUID magnetometer achieved 24 fT/√ of Hz in the white noise range, 38 fT/Hz at 10 Hz, and 83 fT/Hz in the white noise range, 38 fT/√ of Hz at 10 Hz, and 83 fT/Hz at 10 Hz, and 83 fT/√ of Hz at 1 Hz. [Copyright &y& Elsevier]

Step edge junction (SEJ) rf-SQUIDs were made of 200 nm thick YBCO films on LaAlO3(1 0 0) substrates using pulsed laser deposition technique. The steps on the substrates were developed using a combination of stationary and rotating angled argon ion beams with different beam energies and intensities. While sharp clean steps with heights up to 300 nm were obtained on the substrates using the combinatorial ion beam etching (IBE) process, very shallow ramp-type surfaces were found developing on the bottom of the trench, close to the steps. The ramp-type surfaces were found to be a source of hole-type defects in the films grown at the step edges. High quality films could be obtained on the flat regions away from the steps. Higher defect densities in the films close to the SEJs resulted in devices with higher 1/f noise and wider spread of the junction parameters. The 1/f noise of such devices increased with decreasing temperature. High quality films on sharp clean steps with flat substrate surfaces, developed using optimized combinatorial IBE process, resulted in higher yield of low 1/f noise SQUIDs. The Ic of the junctions and hence the working temperature of the SQUID could also be controlled by the junction width and the step height. [Copyright &y& Elsevier]

The in-phase current noise of the two junctions leads to additional 1/f noise in dc SQUIDs, which is reduced by the well-known bias reversal technique (ac bias reversal). In this paper, a new idea for dc SQUID operation with ac bias reversal non-constant bias current is described. In most conventional readout electronics with constant dc bias, the dc SQUID is connected to the preamplifier by means of a transformer in order to enhance the SQUID voltage signal and thus reduce the preamplifier noise contribution. We analyse this case in ac bias condition and find that the current through the SQUID is not constant after the bias reversal. In order to solve this problem, we derived the differential equation governing the network and solved it for a constant current through the SQUID. The transient of the applied bias current is chosen to satisfy this differential equation. The time constant of the transient does not depend on the dynamic resistance of the SQUID. The bias reversal frequency could be increased without introducing additional noise to the SQUID. [Copyright &y& Elsevier]

Although the sensitivity of the magnetic field sensor is important for many applications in electromagnetic testing, SQUID sensors are usually employed for other reasons. For successful defect detection, properties such as high linearity, large dynamic range, and good spatial resolution are required. We present the implementation of a SQUID magnetometer in an eddy current testing system for the measurement of very thick structures of large aircrafts. A three-layer aluminium sample from EADS Airbus was measured, with a total thickness of 62 mm, resembling the projected outer wing splice of the Airbus A-380. The sample has bolted joints and second-layer cracks adjacent to the titanium bolts. The combination of field sensitivities of a few pT/√Hz and a large dynamic range of about 140 dB/√Hz at low frequencies enables us to detect defects at a depth of up to 40 mm in aluminium. For sufficient current penetration into the layered aluminium sample, remarkably low excitation frequencies in the range of 10–40 Hz are required. The small field variations caused by the defects are superimposed on the current distortions and the corresponding field changes in the vicinity of the titanium bolts. Separation of these two contributions requires additional efforts in signal processing and simulations. The measurements were complemented by 3D-FEM calculations in order to find proper excitation frequencies, thus providing an easier separation of flaw signatures from structural background signals. [Copyright &y& Elsevier]