EXPERIMENTAL AND ANALYTICAL ASSESSMENTS ON DAMAGE DETECTION IN CIRCULAR CYLINDRICAL SHELLS USING VIBRATION-BASED METHODS

摘要

Damage detection in an isotropic circular cylindrical shell is investigated in this paper. Beam modal shape functions are chosen to approximate the displacement field of the shell and the Eigen-value problem is formulated by the Rayleigh-Ritz procedure. Damage due a cut or corrosion is modeled as a reduction in the stiffness of the shell and approximated with a torsional spring. For the experimental analysis, a hermetic capsule is considered and corrosion is modeled as a rectangular cut on the shell surface and its effect on the vibration behavior of the structure is considered. The assumed damage or rectangular cut on the shell surface may establish some singularities or discontinuities in the dynamic response of the structure. The magnitudes of the frequency response functions at natural frequencies are considered as the damage index. Continuous wavelet transform which can focuses on the localized structure signals with a zooming procedure and contains both space and scale features is used. Such properties enable them to react to abrupt changes, discontinuities, and singularities in a signal. The two-dimensional continuous wavelet transform at fine scales is applied in order to analyze the response signals of the shell such as mode shape or its variations. The corrosion location is detected by the wavelet transform local maxima. Since the continuous wavelet transform coefficients get large values at the shell boundaries as a result of edge effects, sp-line extrapolation is applied in order to make virtual boundaries around the actual edges of the shell and reduce that effect. The present model predicts the corrosion location on the shell surface by a suitable degree of accuracy, without considering any information regarding the corresponding intact structure.