Identification of cracks on orthogonally stiffened ship plate panels using on-line monitoring

Budipriyanto, Agung (2006) Identification of cracks on orthogonally stiffened ship plate panels using on-line monitoring. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Due to wave and other loadings, the connections between longitudinals and transverse members of ship's side shell structure become vulnerable to cracking. The main objective of the studies carried out in this thesis was to identify the occurrence of cracks in a stiffened plate model representing the ship's side shell structure. -- Based on analysis using a finite element program package, a one-twentieth aluminum stiffened plate model of a tanker ship's side shell structure was designed. Three models were fabricated; the models were tested using random excitation forces whose dominant frequency was far separated <≈2Hz) from the first natural frequency of the models <≈580Hz) under intact and 8 (eight) damaged conditions. These damages occurred at flange(s) and web(s) of the model's longitudinal(s). In damage cases #1 to #4, the damage occurred at a longitudinal in the connections between the longitudinal and bulkheads while in damage cases #5 to #8, damage the intersections of longitudinals and a web frame. Using the model's responses obtained from finite element analysis the best accelerometer and strain gage locations were determined. Output-only modal analysis was implemented in the experimental investigations; random decrement method and Ibrahim time domain identification technique were employed to extract modal parameters. -- Natural frequencies, root mean square (rms) of acceleration and strain response and amplitudes of a function of the natural frequency, the damping ratio and free response (which was called Dn in this study) obtained from numerical and experimental investigations were compared. Although natural frequencies, the rms and the Dn amplitude values computed from the results of numerical investigations were different from those obtained from the experimental investigations, most of them had similar trends due to increasing crack lengths. -- Damage identification schemes for on-line and off-line identification of cracks, using the rms of acceleration and strain response and the amplitude of Dn function, were presented. Using these values a damage indicator was proposed. It was demonstrated that the schemes could identify the crack locations and lengths. In the first scheme, a damage indicator was computed from the rms of acceleration and strain response obtained from numerical and experimental investigations. To estimate the damage indicator values as cracks increased in length an algebraic function was developed and used. It was observed that the difference between the actual and extrapolated damage indicator values was a maximum of 18.63%. Crack lengths could also be estimated by employing the function along with an iteration procedure using assumed damage indicator value as initial value and defined error tolerance. Very good results were obtained; a maximum error of 3% was observed. Another damage identification scheme using numerically computed rms response to obtain crack length and location was also presented. This scheme can be categorized as a nomogram technique. It is shown that the scheme can be used to identify accurately the extent and the location of cracks occurring in the horizontal's flange and web with maximum error of 2%. -- The amplitude of Dn function was also examined to assess damage in the investigated model. The function was obtained utilizing a neural network technique. While the modal frequencies decreased and modal damping ratios did not have definable trend as the crack lengths grew, the Dn amplitudes changed consistently as a results of cracks occurring at one location. A damage indicator was also computed from the amplitudes of Dn function, for different damage cases. The algebraic function developed earlier was also employed to extrapolate the damage indicator values for subsequent damage cases with assumed crack lengths and locations. In addition, the function was also employed to estimate the crack sizes for known damage indicator values. The scheme produced promising results with a maximum error of about 11%.

Item Type: Thesis (Doctoral (PhD))
URI: http://research.library.mun.ca/id/eprint/10224
Item ID: 10224
Additional Information: Bibliography: leaves 164-170.
Department(s): Engineering and Applied Science, Faculty of
Date: 2006
Date Type: Submission
Library of Congress Subject Heading: Fracture mechanics; Plates (Engineering); Structural stability.

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