Crack detection in slender cantilever plates using modal analysis

Perchard, Derek Rex (1993) Crack detection in slender cantilever plates using modal analysis. Masters thesis, Memorial University of Newfoundland.

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Abstract

Research over the past decade or so, has found a new application for modal analysis theory and experimentation. It involves non-destructive evaluation, with the primary intent of detecting and locating structural faults (cracks, voids, etc.) in structures such as off-shore oil platforms which are vulnerable to fatigue and other forms of inconspicuous damage. -- This current research program was conducted in an attempt to add to the existing body of knowledge on this subject. It illustrates through analytical and experimental modelling, the effect that a simulated crack has on the modal characteristics of a simple structure, ie., a cantilever plate, with 5 levels of damage. -- The assessment involved creating a finite element model of the structure using an integrated software package. SDRC - I-DEAS, to estimate the changes in various modal parameters, caused by a notch type crack. Additionally, a series of tests were carried out using response measurement equipment to experimentally determine the effect of the imposed defect. This was accomplished via measurement of dynamic response, using both accelerometers and strain gauges, and subsequent test data analysis using I-DEAS. -- Results obtained from both phases of analysts showed that the crack could be detected, primarily through observation of reduction in natural frequency for a number of modes. The analytical model, in particular, provided a useful technique of crack location via observed inconsistencies in the plots derived from differences in both displacement and rotation mode shapes for cracked plates, with respect to a reference plate. The experimentation performed verified the analytical results, and provided additional insight into changes in damping, residue, and amplitude of dynamic response for frequency values at, and offset from those associated with the natural modes. This latter phenomena proved to be very sensitive and correlated well with observed changes in frequency. The measurement of strain response also proved to be a good indicator of crack existence with major increases in response magnitude, far exceeding the corresponding acceleration response.

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