Issue 50

I. Stavrakas et alii, Frattura ed Integrità Strutturale, 50 (2019) 573-583; DOI: 10.3221/IGF-ESIS.50.48 579 instant with t=140 s in Fig.4) a gradual increase of the PSC was detected from all electrodes, designating the upcoming col- lapse of the specimen, which occurred about 30 s later, under constant external load. It is worth noting that, contrary to what happened during the loading branch of the first loop, the PSC increase was now stronger at the edge electrodes (i.e., channel-0 and channel-4). It is thus indicated, that catastrophic cracking started at points closer to the bases rather than at the mid-height area of the specimen (although at the initial loading steps damage was more intense around the specimen’s mid-height). The time variation of the AE data The time variation of (i) the amplitudes of the AE hits recorded from both the AE sensors used (Fig.1a), and of (ii) the rate (per second) of the AE hits is shown in Figs.5(a,b), respectively, for the first loading loop, for the as above mentioned specimen. The corresponding plots for the second loading loop are shown in Figs.5(c,d). Comparing the behaviour of the amplitudes of the AEs recorded during the two loading loops, it is clearly seen that the loading branch of the second loop is accompanied by AEs of significantly lower amplitudes. This observation clearly supports the dominance of the Kaiser effect [36]. In addition, it becomes clear that, during the first loading loop and after the maximum stress level is attained, the AEs recorded are of lower amplitude. On the other hand, during the second loading cycle, and after reaching the maximum stress level, the number of AE hits increases and it is kept almost constant for about 100 s (Fig. 5d). From this point on, the number of AE hits rate starts increasing and their amplitudes become gradually higher until the final fracture of the specific specimen, which occurred at a time instant around t=170 s. Recapitulating, the AE activity is much weaker during the loading branch of the second loading loop, as it is well established in literature [37-39]. Figure 5 : Time variation of (a, c) the axial stress and the corresponding amplitudes of the AE hits, and, (b, d) the corresponding behaviour of the rate of the AE hits (s -1 ), for the first (left column) and the second (right column) loading loop. Similar conclusions can be drawn for the time variation of the rate of the AE hits during the two loading loops, which is plotted in Figs.5(b,d). Indeed, during the loading branch of the first loop the rate of the AE hits is significantly higher than the respective rate during the second loop, reaching a maximum value of about 600 AE hits/s. The corresponding value for the loading branch of the second loop is limited to only 200 AE hits/s, demonstrating again the existence of the Kaiser effect. It is interesting to notice the sudden increase of the AE hits rate only a short while after the maximum stress value is attained during the second loading loop (although the stress level is kept constant). This is a clear sign that some kind of dynamic process is activated, which will soon lead to the final fracture of the specimen. The sudden increase of the rate of the AE hits, a little before the final fracture, is another pre-failure indicator that is worth to be highlighted. Correlation between PSC and AE data The correlation between the data regarding electric and acoustic emissions is here attempted in terms of the PSC energy and the corresponding time variation of the AE I b -value. More specifically, the PSC energy is calculated according to Eq.(1), after applying proper de-noise and background elimination filters on the signals recorded: