Issue 40

E. D. Pasiou et alii, Frattura ed Integrità Strutturale, 40 (2017) 41-51; DOI: 10.3221/IGF-ESIS.40.04 46 stress, Ib has high values decreasing from ~2.4 to 1.7. This can be related to the closure of pre-existing micro-cracks of the specimen or/and to rubbing/friction [1]. Afterwards, Ib-value is almost constant with variance around ~2.0 indicating the slow generation of new micro-cracks (until point A 2 in Fig.4(a1) which corresponds to stress level ~80% of the maxi- mum stress and strain ~60% of the maximum strain). This point coincides with the deviation of the stress-strain curve from linearity. Slightly earlier, the first noticeable increase of the electric current was observed. Finally, Ib decreases sys- tematically attaining at the end of the test values close to 1.0 (point A΄ 2 in Fig.4(a2)). This happens at ~95% of the maxi- mum applied stress and ~75% of the maximum strain and it is related to a large number of cracks. During the decrease of Ib-value, PSC increases further. At the final stage (after point A΄ 2 ), Ib slightly decreases further due to the coalescence of cracks which leads the specimen to the final fracture while the electric current increases dramatically reaching its highest value. Figure 4 : The electric current and the Ib-value against the normalized stress for a typical specimen of (a) marble, (b) mortar and (c) glass. In case of mortar, Ib-value (Fig.4b) is relatively high from the beginning of the test until ~35% of both the maximum strain and the maximum stress while a period with almost constant Ib-value (~1.8) follows (until point B 2 , ~60% of the maximum strain and ~70% of the maximum applied stress). Afterwards, Ib-value decreases (while the increase of PSC becomes significant) reaching the value of 1.0 (point B΄ 2 ) when the respective stress-strain curve ceases to be linear (~70% of the maximum strain and ~75% of the maximum stress). Subsequently, the values of Ib become slightly lower than 1.0 remaining almost constant until the fracture of the specimen. During this stage, the electric current attains its peak value indicating the impending failure. The variation of the Ib-values for a typical soda glass specimen is presented in Fig.4c. Ib-value continuously decreases until point C 2 (~80% of both the maximum strain and the maximum stress), i.e. almost during the first two stages of the PSC variation. The rate of Ib-value decrease is higher between ~40% and ~80% of both the maximum strain and the maximum stress. Afterwards, Ib-value remains almost constant until ~90% of the maximum stress. From this point on and up to the end of the test, Ib-value decreases further. It is worth noticing that in case of soda glass, Ib-value attains the 0.00 0.70 1.40 2.10 2.80 0 25 50 75 100 0.00 0.25 0.50 0.75 1.00 Ib value PSC [pA] Normalized stress 0.60 0.80 1.00 1.20 1.40 0 25 50 75 100 0.90 0.95 1.00 Ib value PSC [pA] Normalized stress 0.00 0.40 0.80 1.20 1.60 0 8 16 24 32 0.00 0.25 0.50 0.75 1.00 Ib value PSC [pA] Normalized stress 0.00 0.60 1.20 1.80 2.40 0 250 500 750 1000 0.00 0.25 0.50 0.75 1.00 Ib value PSC [pA] Normalized stress A 2 A΄ 2 C 2 B 2 B΄ 2 (a1) (a2) (b) (c) PSC Ib-value PSC Ib-value PSC Ib-value