PSI - Issue 37

Claudia Barile et al. / Procedia Structural Integrity 37 (2022) 307–313 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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3. Results and Discussion 3.1. Frequency Characteristics of the Acousto-Ultrasonics and Corrosion Behaviour

As mentioned in Section 2.2, the first structural response as a result of the incident stress wave is considered as the input signal. The signal in its time series and its frequency characteristics in Fast Fourier Transform (FFT) is presented in Figure 3. It is clear from Figure 3 that the incident signal is characterized by a single frequency of about 325 kHz.

Fig. 3. (a) Time-Series representation of the input signal (b) FFT of the Input Signal

The acoustic waves are propagated through each of the specimens for a distance of 30 mm and recorded by the sensor. The FFT of the recorded signal in each of the specimens A, B and C are presented in Figures 4(a), 4(b) and 4(c), respectively. The power spectral density of all the signals in Figure 4 clearly show that there is a large signal dispersion and energy loss during propagation compared to the input signal, which is quite well-known. However, the attenuation in terms of frequency characteristics is intriguing. The signal propagated in Specimen A energy distributed in two frequency bands; 120 kHz and 300 kHz. The signals propagated in Specimens B and C have the spectral energy distributed in the same frequency band; around 300 kHz. A comparison between the frequency characteristics of these 3 signals can quantify the attenuation due to geometrical variations and the corrosion behaviour.

Fig. 4. FFT of the signals propagated in (a) Specimen A (b) Specimen B and (c) Specimen C

Specimens A and B are both coated with oxide activator and stable corrosion formation is achieved. The only characteristic difference between these specimens is their thickness. But this geometrical difference made the propagated signal to attenuate in the frequency spectrum with a frequency peak observed around 120 kHz in the thinner Specimen A (Figure 4(a)). Specimen B and Specimen C have the same geometrical features but the Specimen C is tested in virgin conditions. Since they both show the same frequency characteristics of 300 kHz, and by comparing these results with the signal propagated in Specimen A, it can be inferred that the geometrical variations leads to the attenuation in terms of