NDTnet 1998 Aug, Vol.3 No.8

Full-Text - Abstract

Eddy-current testing occupies an important place in airline maintenance tests. Nevertheless, some test tasks cannot be assured with a sufficient sensitivity and dynamic range. Here, SQUID's (Superconducting Quantum Interference Devices), the most sensitive magnetic field sensors known to date, can offer a significant advantage, if they are made mobile and capable to operate in electromagnetically noisy environment such as aircraft maintenance hangars. Within a collaborative project with Airbus, Lufthansa, Rohmann GmbH (eddy-current test equipment manufacturer), ILK Dresden (cryogenics manufacturer) and University of Giessen, prototype eddy current systems for aircraft testing incorporating SQUID are being developed. To ensure mobile operation in strong disturbance fields, a differential sensor, a so-called planar gradiometer, was developed. The gradiometer has proved a reliable sensor in realistic environments such as the airports in Frankfurt, Bremen and Hamburg. Currently, the 4 mm gradiometer baseline is restricting deep flaw sensitivity. Progress towards long-baseline devices will be shown. The sensors are operated either in position-independent liquid nitrogen cryostats with holding time of up to 8 hours, or on commercial Joule-Thomson machine-coolers.

The three tasks which are addressed are: (1) detection of deep hidden cracks in wheels made of aluminum alloys, (2) detection of cracks and corrosion in hidden fuselage layers, (3) detection of radial cracks at fastening bolts. Wheels, subject to enormous dynamic overloads and high temperatures at landing, have to be tested frequently for cracks. For economic reasons, testing should be performed from the outside without removing the inner ferromagnetic keys which fit the brake. The testing is performed on an automated test stand with the wheel slowly rotating and a robot with the SQUID enclosure scanning stepwise along the wheel axis. Fig. 1 shows the signal for a 360° rotation of an Airbus wheel. Two artificial flaws cut from the inside into the 10 mm thick wall, the first one 10 mm long with 25% maximum penetration, the other 24 mm and 65%, are easily identifiable among the periodic signal due to the presence of the keys. Goal is the detection of 10% flaws. SQUID solutions for the other two tasks will also be presented, e.g. a rotating field scheme for the determination of the orientation of radial cracks. The prototypes proved superior in sensitivity against standard eddy current, thus paving the road towards establishing the SQUID technique.

Fig. 1. SQUID signal recorded during one rotation of an Airbus wheel with articial inner flaws.

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

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