PSI - Issue 41

G. Agalianos et al. / Procedia Structural Integrity 41 (2022) 452–460 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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low acoustic activity follows, during which the PSC decreases in accordance with the acoustic hits, while just before the fracture of the specimen, hits with amplitude equal to 99 dB were recorded as long as the PSC increases abruptly. Taking into account the small number of the recorded hits, the cumulative acoustic energy and the cumulative number of counts were also studied. Their variation versus time is presented in semi logarithmic plots in Figs.3(c,d), respectively. The results obtained were similar to the ones obtained by the distribution of the amplitudes of the hits. Both quantities smoothly increase during the initial stage of loading and afterwards they increase significantly during the period from 600 s to 800 s, where the PSC fluctuates reaching its maximum value. Then, an al most “silent” period follows, i.e., the cumulative acoustic energy and the cumulative number of counts remain almost constant. Finally, just before the fracture of the specimen, both quantities increase remarkably, just like the electric signal does.

Fig. 3. The temporal variation of the PSC in juxtaposition to that of (a) the applied load; (b) the AE amplitudes; (c) the cumulative AE energy and (d) the cumulative AE counts, for the case of Class A specimen. The respective graphs of Fig.3, for a specimen of Class B (porosity: 5.85%) are shown in Fig.4. The fracture load for this specimen was 18.83 kN (Fig.4a) which is higher than the fracture loads recorded for the Class A specimens due to the lower porosity of the Class B specimens. Contrary to the load-time curve of the Class A specimen, the respective plot of Fig.4a seems to be divided into three linear segments: the first one from the beginning of the experiment until the time instant ~300 s, the second one between ~300 s and ~500 s, and the last one from the time instant ~500 s until the fracture of the specimen. Concerning the electrical activity, it seems that it is favoured by the low porosity of the specimen, or in other words the absence of massive capillary volumes in the bulk of the specimen. As it can be seen from Fig.4a, the PSC increases gradually from the beginning of the experiment until the time instant ~530 s (load equal to about 12 kN, or in other words load level equal to ~65% of the maximum load attained) where the maximum PSC value was recorded (equal to about 14.5 pA). Before the maximum value, a plateau is observed, between ~435 s and ~480 s. After the maximum value is attained, the PSC slightly decreases and remains almost constant (plateau) until the fracture of the specimen. In this specimen, the number of the acoustic hits recorded was 326 and the temporal distribution of their amplitudes is presented in Fig.4b. Based on this distribution, three regions can be seen where the amplitudes exceed the previously recorded values (shadowed areas in Fig.4b). Ignoring the first region (at time instant ~250 s), since it is observed in