"Identification of sub-surface cracks in mechanical structures by nonlinear features of electro-mechanical impedance measurements"
Identification of sub-surface cracks in mechanical structures by nonlinear features of electro-mechanical impedance measurements
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The present thesis deals with the identification of damages in mechanical structures using the
electro-mechanical (E/M) impedance method. The E/M impedance method is a vibration based structural health monitoring (SHM) method. It uses for both the vibration excitation as well as for the measurement of the vibration response piezoelectric wafer active sensors (PWAS).
In particular, damages of a mechanical structure are investigated, which produce a nonlinear response to a harmonic excitation. Examples therefore are delamination in laminates or facelayer debonding in sandwich panels. For a crack-opening small enough the crack surfaces get for certain frequencies of the harmonic excitation into mechanical contact, and thus nonlinear response results.
The effects of such a structural nonlinearity on the vibration response is initially investigated by means of a simplified two-dimensional model of a beam on a nonlinear elastic foundation. An analytical investigation and a numerical investigation based on the finite element method (FEM) are conducted.
Further investigations are carried out on a second and more realistic model, an aluminum beam with rectangular cross-section and a sub-surface crack in the beam?s lengthwise center. In contrast to the first model, the vibration excitation and the sensing of the response are realized by piezoelectric wafer active sensors (PWAS). The focus for this investigation is mainly on the numerical and experimental investigation. A manufacturing process of a sub-surface crack in metallic beams, capable to produce structural nonlinearity is presented.
For both models the evaluation and assessment of the effects of the sub-surface crack are carried by comparing the dynamic responses of the pristine and damaged structure and the by evaluation of nonlinear features in the dynamic response of the damaged structure. The discussed nonlinear features are sub- and higher harmonic oscillations with respect to the excitation frequency.