244 L. Zhou et al. 200 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 50000 100000 100000 200000 250000 300000 100000 200000 300000 400000 500000 600000 500 0 1000 2000 3000 4000 1000 1000 2000 2500 3000 3500 400 600 800 t (minutes) 1000 1200 200 400 600 800 1000 1200 200 400 600 800 1000 1200 200 400 600 800 1000 1200 200 400 600 800 1000 1200 200 400 600 800 1000 1200 b-value A (mV) f (kHz) AE/min cum AE bt a b c d e f Fig. 31.5 Basalt specimen: Last phase. Cumulated AE (a), AE/minutes (b), Ae signal Frequency (c), AE Amplitudes (d), “t exponent (e), b-value (f) On the other hand, the increase of AE rate were accompanied by a ˇt close to 1 denouncing a metastable condition. This evidence, together with the b-value showing a drastic decreasing down to 1, denoted that after 1.32 106 cycles a evident damage took place into the monitored specimen. It is interesting to observe that after this first increment of AE activity no signals were observed for about 500 min. A second appreciable increase in AE events was observed only after about 8 h from the beginning of the test and precisely after 610 min. In this case a considerable AE increasing was observed in AE events with greater magnitude respect to their smaller counterparts (Fig. 31.4). The increment in the number of AE with greater magnitude was accompanied byb-values changing between 1.8 and 1.5 indicating little variability in signal amplitudes and a damage still distributed in a volume. In Fig. 31.5 the last phase for the basalt specimen is reported and a stronger increase of AE with values between 3,000 and 4,000 for the AE number per unit time were detected. In this phase maximum cumulated AE counts about 20 times greater than cumulated AE counts observed during the first phase (see Fig. 31.4). The trend of ˇt shows a progressive increase up to values between 1 and 1.3. As far as the b-value is concerned it decreased drastically to values smaller than 1 just in correspondence of the final failure occurred after about 31 106 cycles. Similar results in AE activity were observed in the case of granite and magnetite specimens, in particular the b-value trends for the final stages are reported in Fig. 31.6. Also in this case it is possible to observe that the b-value changes from higher value down to 1 in correspondence to the final failure of the specimen when the final collapse took place.
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