Modal testing and analysis of uncracked and cracked plates in air and in water

Budipriyanto, Agung (1993) Modal testing and analysis of uncracked and cracked plates in air and in water. Masters thesis, Memorial University of Newfoundland.

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Abstract

This thesis has investigated the analytical and experimental changes that occur in modal parameters (i.e., natural frequency, damping and peak response magnitude) of a vibrating horizontal cantilever plate due to the presence of a crack and due to immersion in two fluids having different densities. Three plates were investigated, viz., an uncracked plate and two cracked plates with 1/32" and 3/32" crack depths, machined on both sides of the plates. -- From the analycal study, it was found that cracks caused the natural frequencies of plates in air to reduce by a maximum of 3% and also changed the modal vectors slightly. The modal vector changes were mainly around the region of the cracks. It was also observed that the modal vectors changed from "in vacuum" to "in air" condition. -- Experiments were carried out in air and at two levels of water submergence. For the experiment in air, it was observed that due to the presence of a 2x1/32" crack, the natural frequencies of the plate were lowered by a maximum of 0.75% than the uncracked one. When the water level was just about the middle of the plate thickness, the natural frequencies reduced by as high as 26.8% and the damping increased by 5 times. For full submergence (where the water level was about 23 cm above the upper surface of the plate), a maximum natural frequency reduction of 40% and damping increase of about 6 times were observed. -- Experimental and analytical results were compared for plates tested in air. The maximum difference between the measured and the predicted natural frequencies were found to be within 8.6%. The correlation of normalized peak strain response magnitudes were good for modes one, two and four.

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