Abstract
A high frequency version of the coin-tap method of non-destructive testing has been investigated. The technique involves tapping the structure with a very light striker of mass typically below 1 g and monitoring the sound produced by the tap. The low striker mass increases the frequency range of the excitation compared to that obtained in the conventional version of the test, which means that the membrane resonance of the layer above a defect such as a disbond or delamination is excited. This results in a large increase in the amplitude of motion produced. The method has been tested on aluminium plates with flat-bottomed holes, the results being considerably better than those obtained using the conventional coin-tap method, a defect 50 mm in diameter and 7 mm deep being at the margin of detectability whereas with the conventional method, a 50 mm diameter defect 5 mm deep was not detectable. However, the results on a honeycomb specimen with fibre-reinforced plastic skins were not as good as those with the conventional coin-tap method. This is because the frequency range of the excitation is reduced by the low out of plane stiffness of the fibre reinforced plastic, while the membrane resonance frequency is high due to the large in-plane stiffness and low density. The method may therefore be useful on structures fabricated from metals or un-reinforced plastics.
Published Version
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