Issue 40

A. Kyriazopoulos, Frattura ed Integrità Strutturale, 40 (2017) 52-60; DOI: 10.3221/IGF-ESIS.40.05 57 It is obvious that during the early stages (L<0.6L f approximately), the ܫ ௕ -values show an almost constant increase indicating the prevalence of micro-cracking [33]. In these stages the PSC signal is low, showing a slight increase (see Fig. 4). Then, at the following stages (L>0.6L f ) ܫ ௕ -values show an intense reduction combined with a sharp increase of the PSC. This behavioral change in the variability of the ܫ ௕ -value is observed at a critical normalized load value [L/L f ] C ≈ 0.6, for all three experiments. As it is marked in Fig. 4 (dotted arrows), the exact value [L/L f ] C is 0.55, 0.62 and 0.70 for specimens CB01, CB02 and CB03, respectively. It seems that increasing the rate of the applied bending load the increase of the PSC signal and the corresponding reduction of ܫ ௕ -values are recorded earlier. It should be pointed out that the ܫ ௕ - value fluctuation, which is observed in all three experiments at the early stage, is related to the fact that the micro-cracks begin to form randomly. Moreover, in all three experiments, ܫ ௕ attains values around 1 during the last seconds before the failure of the specimens, which is indicative of the final formation of a localized macro-crack. Figure 5 : The cumulative ring down count of ΑΕ hits vs. normalized load (L/L f ) for the three conducted experiments. The ring down count (rdc) data was also used, since it is a measure of the number of AE waveform oscillations through a preset voltage threshold [1]. In order to highlight the identification and assessment of important stages of micro-crack damage in a cement mortar material, a common practice has been followed to examine the cumulative ring down count Σ(rdc) and energy (ΣE). These results are illustrated in Figs. 5 and 6 with respect to the normalized load (L/L f ). In all three experiments an almost constant increase in Σ(rdc) was initially observed, related to the initiation of new micro- cracks. For L/L f > [L/L f ] C the Σ(rdc) increases at a rate indicating the development of severe damage in the specimen. Concurrently, an abrupt increase of the PSC signal is observed. Figure 6: (a) The cumulative AE energy vs. normalized load (L/L f ) for the three experiments and (b) a detailed view close to the fracture load. 0.E+00 2.E+03 4.E+03 6.E+03 8.E+03 1.E+04 0 0.2 0.4 0.6 0.8 1 L/Lf Σ (rdc) CB02 CB01 CB03