NDTnet 1998 Aug, Vol.3 No.8

Full-Text - Abstract

High Temperature Superconductor (HTS) Superconducting Quantum Interference Devices(SQUIDs) are promising sensors for applications in eddy current NDE. Due to their high field sensitivity at low frequencies, they are especially suitable for applications, where a large penetration depth is required.

Fig.:

We have set up two different prototypes for a SQUID based eddy current NDE system, one of which is optimised for testing felloes of aircraft wheels. It consists of a fixed cryostat(liquid nitrogen, 77K ) mounted close to the outer surface of the rotating felloe. By moving the SQUID vertically while the felloe rotates a two-dimensional scan is obtained.

The second system is used for testing planar structures. It consists of a motorised x-y-stage, which moves the cryostat with the SQUID above the planar samples. In planar structures, we could detect cracks with lengths of 10 mm, having a height of 0.6mm in a depth of 13 mm, and cracks with lengths of 40 mm in a depth of 25.4 mm ( s. Figure).

In an aircraft felloe, we could detect cracks located at the inner surface of the felloe (thickness 8 mm)despite high static background fields of up to 0.5 G caused by ferromagnetic structures inside the felloe. A comparision between conventional eddy current systems and the SQUID based system shows an improvement of the signal to noise ratio of up to 1000 for the same crack when using the SQUID system. We also performed simulations of excitation fields specifically matched to the real geometry of the samples to find an optimum geometry for the excitation coil. We also work on pattern recognition algorithms for better visualisation of cracks.

*This work is supported by the BMBF under Contract No. 13N6677/8.

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:

Publication Contact:
7th ECNDT, Park Alle 345, DK-2605 Broendby, Denmark, Fax: +45 46 26 70 11, Email: 101373.3414@compuserve.com