This thesis investigates the development of ultrasonic Structural Health Monitoring (SHM) systems, based on guided waves propagation, for the localization of low-velocity impacts and the detection of damage mechanisms in isotropic and anisotropic structures. For the identi- cation of the impact point, two main passive techniques were developed, an algorithm-based and an imaging-based method. The former approach is based on the dierences of the stress waves measured by a network of piezoelectric transducers surface bonded on plate-like structures. In particular, four piezoelectric sensors were used to measure the antisymmetrical A0 Lamb mode in isotropic materials, whilst six acoustic emission sensors were employed to record the wave packets in composite laminates. A joint time-frequency analysis based on the magnitude of the Continuous Wavelet Transform was used to determine the time of arrivals of the wave packets. Then, a combination of unconstrained optimization technique associated to a local Newton's iterative method was employed to solve a system of non linear equations, in order to assess the impact location coordinates and the wave group speeds. The main advantages of the proposed algorithms are that they do not require an a-priori estimation of the group velocity and the mechanical properties of the isotropic and anisotropic structures. Moreover, these algorithms proved to be very robust since they were able to converge from almost any guess point and required little computational time. In addition, this research provided a comparison between the theoretical and experimental results, showing that the impact source location and the wave velocity were predicted with reasonable accuracy. The passive imaging-based method was developed to detect in realtime the impact source in reverberant complex composite structures using only one passive sensor. This technique is based on the re- ciprocal time reversal approach, applied to a number of waveforms stored in a database containing the impulse responses of the structure. The proposed method allows achieving the optimal focalization of the acoustic emission source (impact event) as it overcomes the limitations of other ultrasonic impact localization techniques. Compared to a simple time reversal process, the robustness of this approach is experimentally demonstrated on a stiened composite plate. This thesis also extended active ultrasonic guided wave methods to the specic case of dissipative structures showing non-classical nonlinear behaviour. Indeed, an imaging method of the nonlinear signature due to impact damage in a reverberant complex anisotropic medium was developed. A novel technique called phase symmetry analysis, together with frequency modulated excitation signals, was used to characterize the third order nonlinearity of the structure by exploiting its invariant properties with the phase angle of the input waveforms. Then, a \virtual" reciprocal time reversal imaging process was employed to focus the elastic waves on the defect, by taking advantage of multiple linear scattering. Finally, the main characteristics of this technique were experimentally validated.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:558884 |
| Date | January 2012 |
| Creators | Ciampa, Francesco |
| Contributors | Meo, Michele |
| Publisher | University of Bath |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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