| This thesis presents an investigation which examines experimentally the change in modal parameters of a submerged cantilever due to the existence of fatigue cracks, through vibrational and near field acoustical measurements. The modal behaviour of an uncracked cantilever, in air and in various water submergences was also studied and compared with numerical analysis.;The modal properties of an uncracked cantilever plate in air, and in an infinite water medium were first estimated by solid structure modelling and underwater shock analysis. Ten modes were investigated for the dry cantilever and three bending modes for the submerged one. The effect of the depth of submergence of the cantilever on its modal properties was determined experimentally.;In the fatigue test, a cantilever plate was subjected to a cyclic loading with constant magnitude in air. Vibrational and near field acoustical tests were conducted at various fatigue cycles both in air and in water. It was found that modal frequencies (bending frequencies in this study), with wave propagation directions perpendicular to the crack propagation direction were sensitive indicators of structural cracking. Fatigue results of the submerged cantilever agreed well with that of the dry cantilever for all modal frequencies and the lower modal damping ratios. Modal parameters extracted from the acoustical measurements agreed well with those from the vibrational measurements made on the submerged cantilever, and with most of the parameters noted for the dry cantilever. (Abstract shortened by UMI.). |
