PSI - Issue 18

Danilo D’ Angela et al. / Procedia Structural Integrity 18 (2019) 570–576 Danilo D’Angela et al. / Structural Integrity Procedia 00 (2019) 000–000

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3. Results and discussion The results related to the traditional AE analysis are not reported in the paper for the sake of brevity. However, the traditional analysis is confirmed as qualitative and inefficient for reliable fracture damage assessment. Historical and comparison plots exhibit qualitative damage correlations in some cases; they are not robust enough for real-time monitoring. The b-value analysis does not show clear and univocal damage correlations; in only two cases out of four, b-value is correlated to the fracture damage (i.e., it decreases along with the cracking process). The traditional AE analysis methods do not address the chaotic nature of the AE phenomena, as well as the mechanical noise disturbance. Therefore, the evaluation of acoustic Entropy is even more motivated. Both logarithmic cumulative Shannon Entropy ( H S ) and relative Entropy ( H R ) are plotted in Fig. 3 for all considered cases; the times related to crack initiation/propagation and failure are also shown. The use of the logarithmic scale was already found to be effective for the representation of the cumulative AE Entropy: this allows to identify better the potential damage correlations (Kahirdeh et al., 2017b). Fig. 3 shows that Shannon Entropy curves related to the different cases have a very similar pattern: (1) short-duration sub-vertical-tangent branch, (2) smooth knee , (3) decreasing-tangent branch, and (4) long-duration sub-horizontal-tangent branch ( plateau ). The relative Entropy curves also have a similar trend (1) short-duration sub-vertical-tangent branch, (2) abrupt knee, and (3) long-duration sub horizontal-tangent branch (plateau); the A 1 case also presents a more irregular increasing branch just before the plateau occurring, as well as it shows an increasing stage after the plateau. Overall, the Shannon and relative Entropy curves present a very similar trend.

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Fig. 3. Logarithmic cumulative Shannon Entropy (log Σ H S ) and relative Entropy (log Σ H R ) for (a) A 1 , (b) A 2 , (c) S 1 , and (d) S 2 cases. The crack initiation and fracture failure times are shown, as well as the crack propagation range is shaded.