A shaft displacement indicator

Abstract

The sensitivity of a commercial instrument based on the membrane resonance method of nondestructive testing has been investigated. Tests have been carried out on aluminium plates with flat-bottomed holes and on structures with CFRP skins having aluminium and Nomex honeycomb cores with deliberately introduced skin-core disbonds. The results indicate that on conducting structures, using eddy current excitation, the instrument has greater sensitivity to deep defects than either the mechanical impedance or coin-tap methods, 50 mm diameter defects 7 mm deep being detectable in an aluminium structure, whereas with the other techniques, the margin of detectability is at depths between 3 and 5 mm. The results indicate that the sensitivity of the method to near-surface defects is comparable to that of the coin-tap technique, and that like the coin-tap method its reliability is maintained on flexible structures whereas that of the mechanical impedance method is severely reduced. The eddy current excitation system used in the tests reported here cannot be used on weakly conducting materials such as CFRP-skinned Nomex honeycomb. If an alternative piezoelectric exciter were to be used, the advantages of a non-contacting system would be lost.