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Sensitization is a process that changes the microstructures of certain metal alloys, making them more susceptible to corrosion. In order to properly maintain and extend the life of structural components made of such materials, it is critical to develop in-situ SHM techniques to monitor the sensitization development on a continues base. This paper explores the influence of the sensitization on the resonant parameters of piezoelectric wafer active sensors (PWASs) bonded on samples underwent sensitization development. To differentiate the influence of sensitization and adhesive property changes, two different aluminum alloys, one sensitizes, and the other does not, were selected as the test and reference materials. The Nitric Acid Mass Loss Tests (NAMLT) of these materials confirmed that heat treatment (HT) at 80oC produced a gradual and approximately linear sensitization rate in the test material. On the other hand, the reference material showed negligible degree of sensitization (DoS). The specimens, with a PWAS bonded at the center, were heat treated and the EMI signatures of the PWASs were acquired at different HT durations. A digital signal processing algorithm was implemented to perform time-frequency analysis of the EMI signals, allowing the identification of the ultrasound propagation modes and the detailed analysis of the resonant phases. Different resonant parameters, such as the maximum amplitude, the rise time, and the frequency, were studied with respect to the HT durations. The experimental results demonstrated that changes of the resonant parameters, including the rise time and the maximum amplitude of the low and high frequency resonances can be correlated linearly to the DoS of the test material. In comparison, the changes of these parameters for the PWASs bonded on the reference material is much smaller. These results suggest that the resonance parameters of the surface bonded PWASs are sensitive to microstructural changes.
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