Issue 40

E. D. Pasiou et alii, Frattura ed Integrità Strutturale, 40 (2017) 41-51; DOI: 10.3221/IGF-ESIS.40.04 45 deviates from linearity. Afterwards, the electric current starts increasing with higher rate, indicating the onset of micro- cracking. When stress equals about 97% of the maximum applied stress (point A΄ 1 in Fig.3(a2) where the stress-strain curve tends to become horizontal) and strain equals about 80% of the maximum strain the increase of PSC is remarkable, severe cracking takes place (not yet visible with naked eyes) and the specimen is just before collapse. In case of mortar specimens, the first noticeable change of the PSC values is observed when strain equals ~55% of the maximum strain and stress is ~60% of the maximum stress, point B 1 in Fig.3b) although the material is still below its pro- portional limit. The increase of the electric signal starts becoming significantly larger (point B΄ 1 in Fig.3b) when the respective stress-strain curve deviates from linearity (~75% of the maximum stress and ~70% of the fracture strain, point B in Fig.2b). PSC gets its maximum value slightly before the final fracture (~90% of the maximum strain and ~95% of the maximum stress) indicating the impending failure. Concerning soda glass, the variation of PSC could be clearly divided in three stages. In the beginning of the test, a weak electric signal is detected until point C 1 in Fig.3c where strain is equal to ~60% of the maximum strain and stress is ~60% of the maximum stress. During the second stage (between points C 1 and C΄ 1 in Fig.3c), the increase of PSC is considerable while the increase rate becomes even higher during the last stage where the applied stress is ~85% of the maximum stress and strain equals ~80% of the maximum strain. The maximum value of PSC is attained ~96% of both the maximum stress and the maximum strain. Figure 3 : The electric current and the normalized axial strain versus the normalized stress for a typical specimen of (a) marble, (b) mortar and (c) glass. PCS values in comparison with Ib-values In all cases, groups of 70 hits were used for the calculation of the Ib-value. The first group included the first 70 hits while each successive group contained 35 hits of the previous one and the next 35 hits. Each Ib-value corresponds to the instant which was calculated as the mean time of the respective hits. The variation of Ib-value in conjunction with the electric current is shown versus the normalized stress in Fig.4(a1) for a typical marble specimen. In the beginning of the test, until ~30% of the maximum strain and ~35% of the maximum 0.00 0.25 0.50 0.75 1.00 0 250 500 750 1000 0.00 0.25 0.50 0.75 1.00 Normalized strain PSC [pA] Normalized stress 0.00 0.25 0.50 0.75 1.00 0 8 16 24 32 0.00 0.25 0.50 0.75 1.00 Normalized strain PSC [pA] Normalized stress 0.00 0.25 0.50 0.75 1.00 0 25 50 75 100 0.00 0.25 0.50 0.75 1.00 Normalized strain PSC [pA] Normalized stress 0.60 0.80 1.00 0 50 100 0.90 0.95 1.00 Normalized strain PSC [pA] Normalized stress (a1) (a2) (b) (c) A 1 A΄ 1 C 1 B 1 B΄ 1 C΄ 1 Normalized strain PSC Normalized strain PSC Normalized strain PSC