tesi R. Valiante.pdf - EleA@UniSA

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The aim is to obtain a comprehensive structural monitoring methodology
able to overcome drawbacks and exploit advantages of various techniques. In the
low-to-medium frequency range, modal parameters are normally less sensitive to
localized defects, but generally provide indications regarding global changes as a
result of damage. Such changes localize defects and potentially estimate their
severity. In addition to modal monitoring, the Scanning Laser Doppler
Vibrometer is used as an ultrasonic sensor. The two techniques are applied
sequentially: vibration-based monitoring first provides indication of potential
areas of damage, on which the scanning laser Doppler vibrometer then focuses
to perform ultrasonic testing and to obtain detailed damage information. The
technique is demonstrated experimentally on plate-like stringerized structures
affected by artificial delaminations or by wrinkling caused by manufacturing
process and, subsequentially, on numerical models of the previous tested
components. The approach followed in this thesis is divided into five chapters.
The first one presents a description of the SHM, along with the state of the
art on this technique. Moreover a brief introduction to the theory of waves in
solid materials is provided. Then, in the second chapter the description of the
two innovative SHM techniques (i.e. the signal filtering technique in
wavenumber-frequency domain and the comprehensive damage index),
developed by Ruzzene and on which this work relies on, is extensively
described. The third chapter is centered on the innovative experimental analysis
carried out at Alenia Aeronautica laboratories in Pomigliano. An explanation of
the experimental setup is provided along with the obtained results. Then, the
post-processing techniques, explained in the previous chapter, are applied and
the obtained results are reported. In the fourth chapter the two adopted FE
modeling techniques (with CQUAD4 elements and with HEX8 elements) are
extensively described, focusing on the possibility of correctly modeling the wave
propagation in composite strengthened plates through the use of the finite
element method (there are no published works in this field, regarding
strengthened panels). This is no trivial task because of the complex nature of
wave propagation in plates, especially for the composite materials case. Then,
the confrontation between experimental data and the numerical approach is
reported. In the fifth chapter conclusions and future work are reported.