Li, Yulan (1996) Nearfield acoustics of a submerged cantilever plate for fatigue crack detection application. Masters thesis, Memorial University of Newfoundland.
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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 using a finite element program called ABAQUS. 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. It was observed that a maximum lowering in the structure's natural frequency of 33% occurred in the first bending mode. The damping ratios of the submerged cantilever increased one to six times in accordance with the different modes. When the ratio of immersion depth to the span length of the cantilever exceeded 0.4, the modal properties of the submerged structure tended to be independent of the depth of the submergence. -- 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. Modal parameters and normalized acoustic intensities (NAI) were monitored as fatigue cycles accumulated. It was found that those modal frequencies (bending frequencies in this study), with wave propagation directions perpendicular to the crack propagation direction were sensitive indicators of structural cracking. When the ratio of crack area to the total cross sectional area of the plate (normalized crack area) assumed 33%, the crack was detectable by a maximum natural frequency change of 3-4% in both the air and the water media. It was about 17% when the normalized crack area was 57%. 1.5 to 2 times increase in damping ratios and 3 to 6.7 dB decreases in NAI magnitudes were observed when the normalized crack area exceeded 47%. 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. There were discrepancies of modal damping ratios for the 2nd and the 6th bending modes of the dry cantilever. -- The frequency response functions (FRF) and the nearfield acoustical pressure transfer functions (PTF) of the uncracked cantilever in air were simulated by using ABAQUS computer program and compared with the experimental results. Measured and simulated natural frequencies of the dry cantilever plate have good agreements. The maximum difference was 2.67%. Good agreements were also observed between measured and simulated FRFs for the first four bending modes and PTFs for the 3rd and 4th bending modes. The maximum discrepancy of PTF magnitudes was found to be 5.9 dB for the PTF magnitude of the first bending mode.
|Item Type:||Thesis (Masters)|
|Additional Information:||Bibliography: leaves 136-140.|
|Department(s):||Engineering and Applied Science, Faculty of|
|Library of Congress Subject Heading:||Plates (Engineering)--Fatigue--Testing; Underwater acoustics; Modal analysis|
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