Slot depth resolution in vibration signature monitoring of beams using frequency shift

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Abstract
A major issue in defect detection by vibration monitoring is the resolution of defect size. To investigate this issue, a simple structure, namely, a beam, was used. A fixture was designed for experimental testing on beams with several classic end conditions. By allowing for the variation of moment of inertia of a slotted beam, a theoretical model was developed that fits the experimental data closely. It is shown how the relationship between slot size and frequency shift can be used to resolve the question of the minimum detectable crack size. It is concluded that slots of the size of critical cracks should be detectable with commercial frequency counters. Finally, the practicalities that might prevent cracks of such size from being detected are discussed. In the present experiment, the minimum slot depth detected was found to be 10% of the beam depth under the particular conditions of our experiment.

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