SLOT DEPTH RESOLUTION IN VIBRATION SIGNATURE MONITORING OF BEAMS USING FREQUENCY-SHIFT

A major issue in defect detection by vibration monitoring is the resol ution of defect size. To investigate this issue, a simple structure, n amely, a beam, was used. A fixture was designed for experimental testi ng on beams with several classic end conditions. By allowing for the v ariation of moment of inertia of a slotted beam, a theoretical model w as 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 concl uded that slots of the size of critical cracks should be detectable wi th commercial frequency counters. Finally, the practicalities that mig ht prevent cracks of such size from being detected are discussed. In t he present experiment, the minimum slot depth detected was found to be 10% of the beam depth under the particular conditions of our experime nt.