Crack identification procedures in beams using experimental modal analysis

Owolabi, Gbadebo Moses (2001) Crack identification procedures in beams using experimental modal analysis. Masters thesis, Memorial University of Newfoundland.

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Abstract

Identification of defects in structures and components is an important aspect in decision making about their repair and retirement. Failure to detect the faults has various consequences, and they vary based on the application, and importance of the structures, and components. -- In recent years, significant efforts have been devoted in developing non-destructive techniques for damage identification in structures. The work reported in this thesis was on the experimental investigations of the effects of a crack on beams with fixed and simply supported end conditions, with a view to detecting, quantifying and determining its extent and location. Two sets of aluminum beams were used for this experimental study. Each set consisted of seven beams, the first set had fixed ends, and the second set was simply supported. Cracks were initiated at seven different locations from one end to the other end along the length of the beam for each set, with crack depths ratio ranging from 0.1d to 0.7d (d was the beam depth) in steps of 0.1, at each crack location. Measurements of the acceleration frequency responses at seven different points on each beam model were taken using a dual channel frequency analyzer. The measured responses were curve-fitted using a STAR Structural Analysis Software package running on a PC. The results obtained were tabulated, plotted, and discussed in this thesis. -- The damage detection schemes used in this study depended on changes in the first three natural frequencies, the abrupt changes in slopes of the mode shapes near the crack location and changes in the peak amplitudes of the mode shapes. From the observations made, a method was suggested for locating the extent and position of the crack. The fundamental principle of this technique is the fact that the dynamic response of a structure changes when it is damaged. Measurements of these changes can be used as a diagnostic tool when the relationships between the dynamic properties of a structure change, and the damage is established analytically or experimentally. -- The experimental results obtained were compared with theoretical results obtained in an earlier study and found to compare very well.

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