H.-J. Krause

Recent developments in SQUID NDE

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.

Substrate resonator for HTS rf SQUID operation

Abstract 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 mm 3 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/ Hz in the white noise range, 38 fT/ Hz at 10 Hz, and 83 fT/ Hz at 1 Hz.

Aircraft wheel testing with machine-cooled HTS SQUID gradiometer system

For eddy current detection of deep-lying flaws in large aircraft wheels, an automated airplane wheel inspection system using a HTS SQUID gradiometer sensor is being developed. Wheel drums made of aluminum alloys have to be tested frequently since they are subject to enormous dynamic loads and very high braking temperatures at landing. For economic reasons, testing should be performed from the outside without removing the inner ferromagnetic keys which fit the brake system. In order to operate the sensor in hostile environments such as airport maintenance hangars, a planar rf double hole SQUID gradiometer was used. SQUID cooling is performed by a closed cycle Joule-Thomson cryocooler, equipped with flexible plastic gas lines. The wheel testing is being performed on an automated test stand with the wheel slowly rotating and a robot with the SQUID enclosure scanning stepwise along the wheel axis. Additional signals due to inner cracks of 10 mm length, penetrating 25 percent of the 10 mm thick wall, are easily identifiable in the periodic signal background due to the presence of ferromagnetic keys. In comparative measurements, the prototype SQUID system clearly exhibited advantages over conventional techniques, with optimization reserve still at hand.

Reproducibility and reliability of fetal cardiac time intervals using magnetocardiography

We investigated several factors which may affect the accuracy of fetal cardiac time intervals (CTI) determined in magnetocardiographic (MCG) recordings: observer differences, the number of available recording sites and the type of sensor used in acquisition. In 253 fetal MCG recordings, acquired using different biomagnetometer devices between the 15th and 42nd weeks of gestation, P-wave, QRS complex and T-wave onsets and ends were identified in signal averaged data sets independently by different observers. Using a defined procedure for setting signal events, interobserver reliability was high. Increasing the number of registration sites led to more accurate identification of the events. The differences in wave morphology between magnetometer and gradiometer configurations led to deviations in timing whereas the differences between low and high temperature devices seemed to be primarily due to noise. Signal-to-noise ratio played an important overall role in the accurate determination of CTI and changes in signal amplitude associated with fetal maturation may largely explain the effects of gestational age on reproducibility. As fetal CTI may be of value in the identification of pathologies such as intrauterine growth retardation or fetal cardiac hypertrophy, their reliable estimation will be enhanced by strategies which take these factors into account.

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.

Planar HTS gradiometers with large baseline

We have designed an HTS single-layer planar rf double-hole gradiometer with long and well defined baseline up to 5 mm and small washer areas. We measured a gradient field resolution of about 1 pT/(cm/spl middot//spl radic/Hz) above 10 Hz in an unshielded environment. The gradiometer could be operated placed on a swinging pendulum in the geomagnetic field. This permitted a simple demonstration of a movable SQUID system. We placed a SQUID system on a scanning table and performed two-dimensional eddy-current nondestructive evaluation measurements on realistic samples.

Pulsed eddy current transient technique with HTS SQUID magnetometer for non-destructive evaluation

Eddy current non-destructive evaluation (NDE) often uses excitation currents at a fixed frequency, which determines the depth of investigation due to the skin effect. We developed a novel pulsed eddy current NDE technique which potentially may cover a broader depth range, by analyzing the decay of the recorded transient. Eddy currents in the inspected aluminium structures were induced by turn-off of the primary magnetic field pulse in a transmitter coil. The secondary magnetic field created by the induced currents was recorded by an HTS superconducting quantum interference device magnetometer. The quickly decaying early time data of the transient signal corresponds to the information from upper layers of the plate, while the late time data delivers information from deep layers. In order to suppress environmental noise and increase the signal-to-noise ratio, repetitive square-wave pulses were used. The transient response of the plate was recorded in off-time between the transmitter pulses and averaged using a commercial LAPTEM data acquisition unit. Measurements of a crack at different depths in a stacked aluminium sample are presented. The data was analyzed by calculating the apparent conductivity of the sample as a function of depth, a technique often used in Geophysical data interpretation. Possible advantages and drawbacks of this new transient NDE technique are discussed.

Defect detection and classification using a SQUID based multiple frequency eddy current NDE system

The probability of detection (POD) of hidden fatigue defects in riveted multilayer joints, e.g. aircraft fuselage, can be improved by using sophisticated eddy-current systems which provide more information than conventional NDE equipment. In order to collect this information, sensor arrays or multi-frequency excitation schemes can be used. We have performed simulations and measurements with an eddy current NDE system based on a SQUID magnetometer. To distinguish between signals caused by material defects and those caused by structures in the sample, such as bolts or rivets, a high signal-to-noise ratio is required. Our system provides a large analog dynamic range of more than 140 dB//spl radic/Hz in unshielded environment, a digital dynamics of the ADC of more than 25 bit (>150 dB) and multiple frequency excitation. A large number of stacked aluminum samples resembling aircraft fuselage were measured, containing titanium rivets and hidden defects in different depths in order to obtain sufficient statistical information for classification of the defect geometry. We report on flaw reconstruction using adapted feature extraction and neural network techniques.

SQUID gradiometry for magnetocardiography using different noise cancellation techniques

Magnetocardiographic (MCG) measurements in unshielded environment require efficient noise cancellation techniques. We have applied two software gradiometry methods to analyze the time series of signal and reference data recorded outside magnetic shielding with high temperature superconducting quantum interference device (HTS SQUID) based gradiometers. One method uses adaptive frequency dependent gradiometer coefficients determined in the Fourier domain to subtract the reference from the signal data. The other method combines recently developed techniques for nonlinear projection with properties of the wavelet transform to extract noise in state space. The analyzed MCG data sets showed improved signal-to-noise ratios for both methods as compared to the data recorded with the electronic gradiometer. In this way, it is possible to increase the bandwidth from 130 Hz for our electronic gradiometer to 250 Hz without using any additional filtering.

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.