PSI - Issue 54

Claudia Barile et al. / Procedia Structural Integrity 54 (2024) 225–232 C. Barile et al. / Structural Integrity Procedia 00 (2023) 000–000

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where, w is the range of the signal data points in the identified segment k and N is the total length of the segment. The point at which AIC returns the minimum value is the ToA of the signal. Few sample signals are taken for analysis and their ToA are estimated and presented in Figure 3.

Fig. 3. Estimation of ToA by the Proposed Approach for Sample Signals

3. Experimental Methods

In order to experimentally validate the application of ToA, the following experiment is carried out. Cenosphere reinforced unsaturated polyester resin composites (syntactic foams) are prepared using the resin cast moulding pro cess. In the author’s previous works, syntactic foams are prepared from vinyl ester resin systems and cenosphere composites [Vimalathithan and Vijayakumar (2018)]. The same procedure is followed in this work to prepare the un saturated polyester syntactic foams while keeping the catalyst and accelerator for the polymerization process the same as in the previous work. Tensile specimens with rectangular cross-sections are prepared according to ASTM D3039 - “Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials”. To record the acoustic waves generated during the tensile tests, a wide-band piezoelectric sensor is attached to the surface of the test specimen in the gauge length. The piezoelectric sensor has a minimum acquisition frequency of 200 kHz and resonant frequencies at 250 kHz and 550 kHz. The signals which cross the detection threshold of 35 dB are acquired and amplified by 40 dB before recording them at a sampling frequency of 2 MHz. The length of the signals acquired are 1024 samples and the ToA is estimated by using the procedure explained in Section 2 by setting k = 32 segments with each segment having an equal length of 32 samples.

4. Results and Discussions

Three cenosphere-reinforced unsaturated polyester syntactic foams are tested in this study. Before going to the AE results based on the ToA, the mechanical test results are explained in this section. Understanding the mechanical behaviour of these test specimens may help in comparing them with the AE results.