University of Twente Student Theses


Frequency optimisation for damage identification using the Vibro-Acoustic Modulation method

Venterink, M. (2017) Frequency optimisation for damage identification using the Vibro-Acoustic Modulation method.

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Abstract:Composite structures are desired to have an extended service life and a reduction of maintenance costs without compromising safety. The presence and severity of a possible damage has to be monitored to guarantee safety. The steady state Vibro-Acoustic Modulation (VAM-)method has shown to be a promising non-destructive technique for damage identification in a composite structure using two excitation frequencies: a low intense pump frequency (mostly a natural frequency) and a relative weak and high carrier frequency. The influence of the carrier frequency selection, which is mostly done arbitrary, on the response signal modulations is however limited. The main objective of this graduation assignment is to achieve an ideal combination of a pump and carrier frequency for a more efficient damage identification on a composite plate with an artificial damage. The secondary objective is to explain the occurring signal modulations for different carrier frequencies. The pump frequency is chosen as a natural frequency with a clapping behaviour since this is expected to result in the strongest signal modulations. Analytically derived nonlinear dynamic responses of a multi degrees of freedom system subjected to a two-tone forced harmonic, show the relation of amplitudes of the carrier, pump, higher harmonics and sidebands with the natural frequencies. These natural frequencies are separated into the global natural frequencies of the composite plate structure and the local natural frequencies of the delamination. The global and local eigenfrequencies of the composite plate are determined numerically such that the dimensions of the manufactured composite plate and its artificial delamination can be chosen. Experimentally, the natural frequencies of the produced composite plate are also determined and a separation between local and global is made. The pump excitation with the expected clapping behaviour that causes the most signal modulations is selected for the VAM-experiments. Finally the carrier frequency is varied with a constant carrier excitation amplitude. Firstly it is concluded that the VAM-method promises to be more effective in the localisation of the damage when using a local natural frequency as the carrier. The signal modulations will increase more in the damaged location than in the undamaged location compared to using a global natural frequency as carrier. Secondly, the frequency modulation for varying carrier frequencies has shown to be very inconsistent. Therefore it is concluded that damage localisation with amplitude modulation in the time domain is better than using the sidebands in the frequency domain.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
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