Issue 40

E. D. Pasiou et alii, Frattura ed Integrità Strutturale, 40 (2017) 41-51; DOI: 10.3221/IGF-ESIS.40.04 47 value of 1.0 when strain is ~65% of the maximum strain and stress is ~65% of the maximum stress, i.e. relatively lower compared to the other two materials. It should be mentioned at this point that b-value is related, among others, to the heterogeneity of the material. As the degree of nonuniformity of the material increases, b-value also increases [33]. This is consistent with the results of the present study. The material layers of marble and the existence of sand in mortar make these structures quite heterogen- eous compared to glass specimens and as a result the Ib-values obtained for marble and mortar are larger than the respect- ive values of glass. In addition, the heterogeneity of both marble and mortar results to the generation of low acoustic activity during the initial load levels leading to an almost constant Ib-value in these first stages of loading. The absence of this constant segment in case of glass makes the variation of the respective Ib-value to be monotonic in contrast to the re- spective variations of both marble and mortar. Approach based on energies Despite the fact that the correlation of PSC and AE techniques became clear in the previous section, an alternative approach based on the released energy was decided to be used in order to quantitatively verify the correlation of the two experimental techniques. The absolute energy (measured in aJ) produced and recorded by the acoustic sensors characterizes each acoustic hit and it is calculated by the software as the integral of the signal voltage at a power of two over the reference resistance (10 kΩ). In the present study, the sum of the energy released during each second of the tests, E AE , was calculated since it is a measure of the size distribution of micro-cracks in such materials [34]. In case of PSC technique, the energy released was calculated by the familiar expression: E PSC =     t t t i i dt t PSC 2 (2) where Δt=1 s and t i = 0, 1, …, n-1 (n is the duration of the experiment). Since PSC is measured in pA, the units of E PSC are (pA) 2 ·s. Finally, the strain energy density, SED, was calculated as the area below the stress-strain curves of the tests. All three quantities were normalized over the respective maximum value and the results are presented in logarithmic scale in Fig.5. For all three materials of the present experimental protocol both curves are formed by two almost straight segments with different slope and a knee point is clearly seen corresponding probably to the formation of more severe cracks. In case of marble (Fig.5a), it is quite interesting that PSC technique seems to detect the internal damage of the specimen earlier than AE technique. In case of marble, the PSC knee point and the AE knee point correspond to about 70% and 80% of the maximum stress, respectively (i.e. about 50% and 60% of the maximum strain, respectively). The same is true for mortar specimens (Fig.5b). The knee point from PSC technique is observed when stress equals about 60% of the maximum stress (~55% of the maximum strain) while AE knee point corresponds to ~65% of the maximum stress and ~60% of the maximum strain. In case of glass specimens (Fig.5c), the knee points formed by the two techniques are observed almost simultaneously (~60% of both the maximum stress and strain). Figure 5 : The energy calculated by both PSC and AE techniques versus the strain energy density for a typical specimen of (a) marble. 1E-6 1E-4 1E-2 1E+0 1E-3 1E-2 1E-1 1E+0 Energy Strain Energy Density knee PSC knee AE (a)