Issue 40

A. Kyriazopoulos, Frattura ed Integrità Strutturale, 40 (2017) 52-60; DOI: 10.3221/IGF-ESIS.40.05 55     0 t f T Q PSC t dt (2) It is worth mentioning that the total electric charge for the three different loading modes reaches almost the same value (see Tab. 1). In a previous work [13], during compressive stress application on marble specimens it was theoretically supported and experimentally proven [28] that the total electric charge value does not depend on the applied mechanical stress rate. This behavior is verified experimentally herein when cement mortar specimens are subjected to 3PB loading. Figure 2: The time variation of the applied 3PB load and the corresponding time variation of the PSC, during all three experiments conducted. Figure 3: PSC versus normalized load (L/L f ) for all three experiments conducted. Regarding the AE recordings, during the three experiments, a different number of AE hits was recorded (see Tab. 1). The micro-cracks generated during loading are of different sizes and therefore different AE amplitudes are recorded. A common way of obtaining quantitative information from the above statements is by computing the b-value of AE using the methods adopted in seismology [29, 30]. The b-value is defined as the log-linear slope of the frequency–magnitude distribution of AE [31]. In the present study, the b-values were calculated by applying a method that is known as improved b-value (I b ) analysis after Shiotani et al. [32]. The I b -value is defined as:                      1 2 1 2 log log b N N I (3) 0 1 2 3 4 0 10 20 30 40 50 time (s) load (kN) 0 5 10 15 20 25 30 PSC (pA) CB01 CB02 CB03 0 5 10 15 20 25 30 0 0.2 0.4 0.6 0.8 1 L / Lf PSC (pA) CB01 CB02 CB03