W. Wolf

Eddy current technique with high temperature SQUID for non-destructive evaluation of non-magnetic metallic structures

For the first time, a laboratory test system for non-destructive evaluation of metals using the eddy current technique in conjunction with a high temperature superconductor (HTS) SQUID gradiometer has been developed and set up in a magnetically unshielded laboratory environment. The first preliminary experiments prove the advantages of the technique with respect to low frequency sensitivity (large penetration depths), thus showing the potential of HTS SQUID eddy current systems.

SQUID array for magnetic inspection of prestressed concrete bridges

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.

Defect detection in thick aircraft samples using HTS SQUID magnetometers

Abstract 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.

Operation of HTS SQUIDs with a portable cryostat: a SQUID system in conjunction with eddy current technique for non-destructive evaluation

We present a new design of a portable nitrogen cryostat for operation of moving SQUIDs. A mixture of liquid and gaseous nitrogen fills a reservoir in direct contact with a copper part for the SQUID integration. The temperature at the SQUID position is 77.8 K or 78.8 K depending on orientation, and varies within /spl plusmn/10 mK during lateral movement. The cryostat can operate as a portable system for 7 hours without refilling. Washer rf SQUIDs and dc gradiometers were integrated with the cryostat. We proved the operation of the system as a moving magnetometer in an unshielded laboratory environment. Noise spectra in shielding and outside were independent of orientation. The system was equipped with a differential eddy current excitation. We show the first non-destructive material evaluation results for fatigue crack detection on stationary samples with moving SQUID sensors.

HTS SQUID magnetometer with SQUID vector reference for operation in unshielded environment

Using a HTS rf SQUID vector reference, we demonstrated magnetically unshielded operation of an HTS rf SQUID magnetometer. The reference consists of three magnetometer SQUIDS oriented in x, y and z directions. The sensing SQUID magnetometer is z-oriented and positioned at (baseline) distance of 8 cm from the z-reference. A balance (common mode rejection) better than 4000 was achieved by electronic adjustment of the subtraction coefficients of the reference rf SQUID channels in the homogeneous field of large Helmholtz coils. With 3.5 mm diameter washer rf SQUIDs, the magnetic field sensitivity of the magnetometer was 220 fT//spl radic/Hz at frequencies greater than 100 Hz in unshielded environment. A frequent task in nondestructive evaluation of materials is the determination of the magnitude and size of ferrous inclusions in non-magnetic metal alloys. As a preliminary test, we used the magnetometer for the detection of ferrous particles in unshielded environment. A magnetized 10 mg particle was identified from a distance of 80 mm by a scan of its remanent field. Magnetocardiograms (MCG) measurements are also performed.

Multiplexed SQUID array for non-destructive evaluation of aircraft structures

SQUID sensors offer a significant advantage for eddy-current (EC) testing of aircraft components for material flaws hidden deeply in the tested structure. However, the requirement to take maps of the magnetic field, usually by meander-shaped scans, leads to unacceptably long measurement times. Due to their inductive coupling to a tank circuit, several rf SQUID sensors may be read out sequentially by selectively coupling to their tank circuits, using only one electronics with a multiplexer. The multiplexed operation of three planar HTS rf SQUID gradiometers with one electronics and one cable is shown, demonstrating the advantage of lower liquid nitrogen boil-off. Independent operation and switching is confirmed using local coil excitation of the individual SQUIDs. We report on the implementation of two multiplexed SQUID sensors in conjunction with an EC excitation and lock-in readout at unshielded laboratory environment. Scanning is performed while continuously switching the operating SQUID, thus obtaining two traces simultaneously. The applicability to EC testing of riveted sections of aircraft fuselage is discussed.

Mobile HTS SQUID System for Eddy Current Testing of Aircraft

In Non-Destructive Evaluation (NDE), eddy current techniques are commonly used for the detection of hidden material defects in metallic structures. Conventionally, one works with an excitation coil generating a field at a distinct frequency. The eddy currents are deviated by materials flaws and the resulting distorted field is sensed by a secondary coil. Because of the law of induction, this technique has its limitations in the low frequency range. This leads to a decrease of the Probability of flaw Detection (POD) in larger depths.

Integration of HTS SQUIDs with Portable Cooling Devices for the Detection of Materials Defects in Non-Destructive Evaluation

We have integrated Superconducting Quantum Interference Devices (SQUIDs) made from High Temperature Superconductors (HTS) with a portable cryostat and a Joule-Thomson cooler. With these magnetic field sensing systems, flaws in metallic test samples were detected using an adapted eddy-current technique. The measurements were performed in the absence of any magnetic shielding by moving the sensors below the sample by means of a scanning table. This publication describes in detail the physical parameters of the two cooling devices, the requirements for the integration of the SQUIDs, and its realization. The measurements of test samples demonstrate the applicability of the eddy current technique to SQUID technology. Furthermore, we discuss steps to improve the systems performances.

Non-destructive evaluation of aircraft structures with a multiplexed HTS rf SQUID magnetometer array

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.

HTS SQUID System for Eddy Current Testing of Airplane Wheels and Rivets

Nondestructive testing (NDT) of new and aging aircraft structures is essential for flight safety. Inspection costs can be reduced by using an inspection technique with high sensitivity for small flaws. Of the many NDT methods being used in aircraft maintenance, eddy-current testing is well established, especially for layered structures. Nevertheless, some test tasks cannot be assured with conventional eddy current systems with sufficient sensitivity and dynamic range. Superconducting Quantum Interference Devices (SQUIDs) are the most sensitive magnetic field sensors known to date. With the discovery of High Temperature Superconductors (HTS) ten years ago and the subsequent development of HTS SQUIDs requiring only cooling down to liquid nitrogen temperature, the greatest application barrier appears solvable. SQUID systems offer a high sensitivity at low excitation frequencies, permitting the detection of deeper flaws, and a high linearity, allowing quantitative evaluation of magnetic field maps from the investigated structure [1–3]. The potential of eddy current testing with HTS SQUIDs has previously been demonstrated for up to 5 cm deep-lying defects in stacks of aluminum sheets using a stationary axial SQUID gradiometer [4]. Kreutzbruck et al. [5] performed a direct comparison between a SQUID magnetometer system and a conventional eddy current testing unit (Elotest Bl of Rohmann GmbH), with a well defined saw cut in a plate of aircraft aluminum alloy hidden under a stack of flawless aluminum plates. They demonstrated an improvement in signal-to-noise ratio of approximately 150, when comparing the SQUID signature of the slot with the conventional system.