NDTnet 1998 Aug, Vol.3 No.8

Full-Text - Abstract

The reliable detection of small and deep lying flaws in aircraft structures at an early stage requires a sensitive NDE system with a high dynamic range.
For this purpose we have replaced the conventional detection coil by a SQUID, which measures the z-component of the magnetic field. In contrast to the conventional coil systems the SQUID detects the magnetic Flux instead of the induced time variation ^' in case of an conventional induction coil. This allows one to use low excitation frequencies of the order of a few 100Hz with a negligible decrease in field sensitivity of the SQUID sensor and a dynamic range of more than l40dB/Hz in a noisy magnetic environment.

We have performed several numerical simulations for a HT_c - SQUID based eddy current system for the optimisation of several parameters, like coil geometry and excitation frequency. We have focussed here on obtaining a maximum field in larger depths.
We have simulated eddy current distortions caused by cracks in both stacked samples of several sheets and massive samples by means of the 3D-Finite Element Method. The results are in a good agreement with our measurements.

Besides the feed forward calculations we report of first developments of a pattern recognition algorithm for better crack visualisation. Therefore the primary results of measurements are used as input for an iteration procedure which converges to a volume of interest in the sample and indicates the location of the crack. An additional correlation algorithm gives some information about the depth of the crack. Further efforts are directed towards the direct inversion of the magnetic field data. For simple crack geometry and low frequencies we found linear functions for the direct determination of the depth of the crack.

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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