NDT.net - September 1998,
Table of Contents ECNDT '98
Aircraft Wheel and Fuselage Testing with Eddy Current and SQUID
H.-J. Krause*, R. Hohmann, M. Grüneklee, M. Maus, Y. Zhang, D. Lomparski, H. Soltner,
W. Wolf, M. Banzet, J. Schubert, W. Zander, H. Bousack, and A.I. Braginski
|*Corresponding Author Contact:|
Institut für Schicht- und Ionentechnik, Forschungszentrum Jülich, D-52425 Jülich, Germany ; firstname.lastname@example.org
Aircraft, being exposed to strong forces, moisture and changing temperatures, have to be checked regularly for cracks and corrosion. Nondestructive testing of aging aircraft structures is essential for flight safety and for keeping inspection costs low. Of the many NDT methods being used in aircraft maintenance, eddy-current testing by inductive sensors is a well established technique, especially for layered structures. Nevertheless, some test tasks cannot be assured with conventional eddy current systems because they lack 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 rf SQUID gradiometer . Kreutzbruck et al.  compared a SQUID magnetometer system with a conventional eddy current testing unit (Elotest B1 of Rohmann), using a well defined saw cut in a plate of aircraft aluminum alloy hidden under a stack of flawless aluminum plates. They demonstrated that the SQUID yields an improvement in signal-to-noise ratio of approximately 150, compared to the conventional system.
For practical applications, however, the SQUID systems have to be made mobile and capable to operate without any magnetic shielding in maintenance hangars where the level of electromagnetic disturbances is very high [6,7]. A German research collaboration is introducing HTS SQUIDs into aircraft testing. The project consortium is led by the small company Rohmann GmbH, a manufacturer of standard eddy current equipment. The project partners are ILK Dresden, responsible for cryogenics, University of Gießen for signal analysis, Forschungszentrum Jülich for SQUID development, DASA and Lufthansa for the application requirements of today's aircraft testing. Three testing problems were chosen: aircraft wheel testing, fuselage testing (finding cracks and corrosion next to rivets), and localization of cracks emanating from bolted joints in thick-walled sections of aircraft. Project goal is the development of prototype SQUID systems for the demonstration of the advantages of the new technique.
2. Instrumentation and setup
For the different testing applications, specialized SQUID systems were developed. The systems consist of a SQUID sensor arrangement with readout electronics, a cryogenic apparatus, and an eddy current excitation and data evaluation unit.
2.1. Sensor: Planar HTS SQUID gradiometer
When using extremely sensitive magnetic field sensors such as SQUIDs, the challenge is to resolve small fields in the presence of large ambient fields. These environmental fields require a large dynamic range (maximum field amplitude) and a high slew rate (maximum change of field in time). Gradiometric sensors (measuring only the spatial derivative of the field) greatly facilitate the problem. In order to operate the SQUID in motion, a planar rf SQUID gradiometer was developed  and integrated into a hand-held system. The system can be used during movement in strong ambient fields commonly found in aircraft maintenance facilities. Fig. 1 shows the principle and the layout of the double hole gradiometer. In the case of a homogeneous magnetic field, the currents flowing around the SQUID loops cancel each other at the location of the junction. For an inhomogeneous field, however, the difference of shielding currents provides a signal proportional to the field gradient. A gradiometer with a baseline of 3.7 mm and a gradient-to-flux coefficient of 7 nT/cm 0 yielded a minimum flux noise of 7*10-5 0/Hz, corresponding to a gradient sensitivity of 500 fT/(cmHz) for frequencies above 70 Hz . These values are sufficient for detection of flaws hidden as deep as 20 mm in aluminum. The newly developed readout electronics with a slew rate of more than 2*106 0/s allows fast scanning in strong gradient fields, e.g. from ferromagnetic objects.
Fig. 1. Principle (a) and layout (b) of the planar rf-SQUID gradiometer.
Special care was taken to suppress environmental high-frequency electromagnetic interference. At airports, strong disturbances due to aircraft-ground communication, radar and mobile communication with cellular phones are found. Reliable SQUID operation in this noisy environment is required. A high-frequency shield for the SQUID, consisting of a thin lead foil encapsulation, attenuates radio frequencies by several orders of magnitude while having no effect on the measured signals for frequencies up to 10 kHz. The shielding allows unperturbed SQUID operation in the presence of cellular phones operating 1 m away from the SQUID.
2.2. Cryogenics: Mobile cryostat or Joule-Thomson cryocooler
In order to do the testing directly at the aircraft, the SQUID has to be equipped with mobile cooling. A lightweight nitrogen cryostat [9,10], constructed for operation in any orientation by ILK Dresden, allows portable SQUID operation (Fig. 2). The mobile head weighs less than 2 kg and has an operation time of 12 hours. For a routine, stationary maintenance application
such as aircraft wheel testing, SQUID cooling only needing electricity is advantageous. A commercial Joule-Thomson cryocooler (APD Cryotiger®) was successfully used for liquid-nitrogen-free, low-noise SQUID cooling [7,9]. Flexible plastic gas lines make displacements of the cold head (Fig. 3) possible. However, the bulky compressor restricts the
system to stationary operation and the system cost is higher compared to that of a nitrogen cryostat.
Fig. 2. Mobile liquid nitrogen cryostat ILK 4 by ILK, Dresden, used for mobile SQUID operation. The head flange is removed to show the sapphire cold finger with SQUID.
Fig. 3. Cold head of the Joule-Thomson cryocooler Cryotiger® by APD, used for stationary SQUID cooling requiring only electricity.
2.3. Eddy current: excitation and lock-in detection
The ac current excitation is applied by a differential coil (Fig. 4a). Printed multi-turn double-D coils with a diameter of 25 mm (Fig. 4b) were mounted on the SQUID dewar and operated with excitation currents of typically 200 mArms. This eddy current scheme ensures a minimum primary field at the location of the SQUID gradiometer (Fig. 4c). However, it leads to a quadrupolar signature of a small flaw. Amplitude and phase of the eddy current response field are evaluated by a Stanford Research SR830 digital lock-in amplifier and recorded by a computer.
Fig. 4. (a) Scheme and (b) layout of multi-D excitation coil. (c) Coil arrangement with the planar gradiometer.
3. Experimental results in aircraft parts testing
Fig. 5. Sketch of an Airbus aircraft wheel, with typical crack positions.
Fig. 6. Automated aircraft wheel testing unit, with the SQUID mounted on a robot, during operation.
Fig. 7. In-phase SQUID signal track recorded in one rotation of Airbus wheel with artificial inner flaws.
3.1. Aircraft wheel testing
Aircraft wheels are subject to enormous stress and braking-generated heat during take-off and landing. The brake pulleys are fastened to the rim with ferromagnetic keys. Because of the concentration of mechanical and thermal stress, hidden cracks emanate preferably next to the keys. Fig. 5 indicates typical flaw locations on the inside of the wheel. The cracks are not visible from the inside of the wheels because they are covered by heat shields not sketched here. Today, the wheels are tested from the outside with a circumferential scan measurement, after taking off the tires. Deep flaws are detected with a low frequency eddy current probe. However, the sensitivity is limited to large flaws: flaws with 40% wall penetration from the inside and of length twice the wall thickness can be identified reliably. In order to safely detect small hidden flaws, the wheel has to be disassembled and be tested from the inside.
The prototype SQUID system for wheel testing consists of an automated test stand with the wheel slowly rotating and a robot with the SQUID enclosure scanning stepwise along the wheel axis (Fig. 6). While the wheel is rotating, the robot moves the cryostat along its outer contour. Thus, a two-dimensional eddy current mapping of the outer wheel surface is performed. To simulate different sizes of cracks, the inner wheel surface was cut with a circular saw blade.
Fig. 7 shows a 360° rotation trace of the SQUID gradiometer signal [11,12]. One flaw penetrates about 65% of the wall thickness of 10 mm, with 24 mm length, the other 25% with 10 mm length. Both flaws are easily identifiable in the in-phase signal component. The signal contains a periodicity equivalent to the rotation period, due to slight excentricity of the wheel. This weak lift-off effect can be easily filtered. The nine key peaks are partly due to direct eddy current contribution from the ferromagnetic keys, partly to the increasing wall thickness at the key location. The SQUID method has still a considerable reserve in the signal-to-noise ratio, especially if gradiometers with longer baseline will be used.
Reliable SQUID operation in hostile environment was successfully demonstrated with a mobile prototype SQUID system in the Lufthansa maintenance facility at Frankfurt airport, detecting flaws in aircraft wheels . As a result of the collaborative R&D project, an automated wheel testing unit, with a SQUID cooled by a Joule-Thomson cryocooler and scanned with a robot, has been developed. A realistic demonstration of aircraft wheel testing with SQUID will be presented at the ECNDT'98 exhibition.
3.2. Aircraft fuselage testing
Due to temperature and moisture changes in conjunction with mechanical stress, cracks and corrosion frequently develop in the fuselage, often in hidden layers close to rivets (Fig 8). State of the art with conventional eddy current equipment is the detection of 4.5 mm long second layer cracks underneath 2.2 mm of aluminum next to rivets.
Fig. 8. Sketch of Airbus fuselage section with rivets, with typical crack positions.
For testing of airplane fuselage, a SQUID gradiometer enclosure was mounted on a fuselage surface scanner (Vogt Scanmaster®, with IRT FlexiTrak®). The system is attached to the airplane at selected parts which have to be inspected for either corrosion or cracks. Then the cryostat is moved manually over the surface to locate the flaws on the inner surface of the fuselage (Fig. 9).
Fig. 9. Scan of fuselage with the SQUID. The flexible scanner is affixed at the fuselage with suction cups.
Fig. 10. Scan of a section of Airbus A330/340 fuselage (205×50 mm2) with a row of rivets (marked by dots). Mapped is the in-phase component of the lock-in signal at 144 Hz.
Fig. 10 shows a typical scan of a fuselage section. Using the planar gradiometer sensor and the differential excitation coil, each rivet yields a quadrupolar signal as can clearly be seen in the scan map. The rivet and the rivet hole give a strong signal even if there is no additional fault like a crack (Fig. 6). Because of the insufficient spatial resolution achievable with our 25 mm multi-D-coil, and 8 mm distance between sensor and fuselage surface, the signals of two adjacent rivets are mixing. To find the signal of a crack, which normally runs radially from a rivet in the line of the rivet series, the spatial resolution has to be increased. Improvement is expected after optimizing the eddy current excitation scheme.
The main achievement is the separation of the crack signal from the underlying signals from the structure, e.g. from rivets or stringers. In cooperation with J.P. Wikswo's group at Vanderbilt University , measurements at reference structures were conducted and a signal analysis software was developed. Fig. 11 shows the results on crack separation from rivet signals. In these experiments, a modified cryostat with a distance of only 3 mm between SQUID and sample was used in conjunction with a single circular coil excitation, with the planar gradiometer exactly in the center. Using software spatial integration in the direction of the baseline, the differential signature was transformed into a single peak. A rivet without adjacent crack yields a circular pattern, as depicted in Fig. 11a. A crack in the upper layer of a riveted two-layer structure shows up as a second peak next to the rivet signal when using a high frequency of 1000 Hz (Fig. 11b). In contrast, a crack in the lower layer can be made visible at a lower frequency of 500 Hz (Fig. 11c).
Fig. 11. SQUID measurement of a two-layer laboratory sample riveted together, (a) no crack, (b) crack in the upper layer adjacent to the rivet, (c) crack in the lower layer. The measurements were conducted at eddy current frequencies of 1000 Hz (a,b) and 500 Hz (c). The crack signals show up as small points below the circular rivet signal. |
For selected aircraft NDE tasks, prototype SQUID systems were developed and tested in realistic environments, demonstrating the practical usability of mobile HTS SQUID in conjunction with the eddy current technique in nondestructive evaluation of highly safety relevant objects. The development of planar HTS gradiometers and orientation-independent cryogenics made possible the mobile use of SQUIDs in hostile environments such as airport hangars. The sensors are mounted either in a position-independent miniature cryostat with a liquid nitrogen hold time of up to 12 hours, or on a Joule-Thomson mechanical cooler only requiring electricity. Tested in the Lufthansa wheel inspection facility at Frankfurt airport and at the DASA at Bremen airport, the SQUID gradiometers worked successfully under electromagnetically noisy conditions. An automated test stand for SQUID testing of aircraft wheels, with the wheel slowly rotating and a robot with the SQUID enclosure scanning stepwise along the wheel axis, detects deep cracks smaller than today's limit of conventional eddy current devices. Further development in the spatial resolution and data analysis will improve the performance. Another SQUID system has been successfully operated on a professional fuselage scanner. Using a signal processing software, first and second layer cracks were detected adjacent to rivets. Future work will include the development of longer baseline gradiometer and adapted excitation. Once technology is improved together with system development for specific applications, SQUID systems will have a good chance to be established in aircraft testing.
The authors gratefully acknowledge a very fruitful collaboration with M. Junger and J. Rohmann (Rohmann, Frankenthal), F. Schur (Lufthansa, Hamburg), W.-B. Klemmt (DASA, Bremen), A. Binneberg, H. Buschmann, J. Neubert, and G. Spörl (ILK Dresden), Ch. Heiden, M. Mück, M. v. Kreutzbruck, and U. Baby (University of Gießen). Many thanks to the colleagues from Forschungszentrum Jülich, M. Beylich, A. Bielitza and A. Görtz (ISI-TS), R. Otto and N. Wolters (ISI-TEL), U. Poppe (IFF), G. Brandenburg, U. Clemens, H. Halling and E. Zimmermann (ZEL), for their invaluable support. Funding by the German Federal Ministry of Education, Science, Research and Technology (BMBF) under contract 13N6682 is gratefully acknowledged.
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