R. Hohmann

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.

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.

Aircraft wheel testing with remote eddy current technique using a HTS SQUID magnetometer

An aircraft wheel testing system using a planar HTS SQUID gradiometer with Joule-Thomson machine cooling in conjunction with differential eddy current (EC) excitation has recently been developed. From a routine performance test in the wheel testing facility at the Lufthansa Base, Frankfurt/M. Airport, we learned that quadrupolar flaw signatures complicate signal interpretation considerably. In order to overcome these difficulties, the system was equipped with a HTS rf magnetometer SQUID sensor and an absolute EC excitation coil. The coil was mounted with a lateral displacement with respect to the SQUID. The geometry was chosen similar to the remote EC technique: a given point on the rotating wheel first passes underneath the excitation coil and then underneath the sensor. We analyzed the dependence of the response field of an inside crack on excitation coil displacement, EC frequency and lock-in phase angle and found an optimum rotation velocity for deep lying defects. The depth selectivity of the technique is discussed.

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.

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.

HTS-SQUID Magnetometer with Digital Feedback Control for NDE Applications

Superconducting Quantum Interference Devices (SQUIDs) are extremely sensitive detectors for the measurement of magnetic flux. Especially the current High Temperature Superconductor (HTS) SQUIDs, which can be operated at liquid nitrogen temperature with easy cryogenic requirement, are well suited for the practical use [1]. Nowadays this HTS-SQUIDs are used for applications like the detection and localization of currents within the human heart, for nondestructructive evaluation of materials and for geological exploration.

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.

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.