PSI - Issue 3

D. Triantis et al. / Procedia Structural Integrity 3 (2017) 346–353 D. Triantis, E.D. Pasiou, I. Stavrakas, I. Dakanali and S.K. Kourkoulis / Structural Integrity Procedia 00 (2017) 000–000 5

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Fig. 4. (a) The temporal variation of the acoustic hit rate; (b) The dependence of t cr / t f versus the loading rate.       1 2 b 1 2 log N log N I               

(2)

In Eq.2 μ is the mean amplitude of the acoustic emissions under consideration, σ is the standard deviation and α 1 , α 2 are constants, which are usually assumed equal to 1 (Rao & Lakschmi 2005). It is here emphasized that the number of events/hits forming the groups required to calculate the Ib-value influences the results, and this was already pointed out in previous studies (Colombo et al. 2003, Shiotani et al. 2001). It is generally accepted that a number of at least 50 events/hits should be considered for reliable results to be obtained. In the present study, the groups consisted of 100 hits, i.e. the first 100 hits form the first group, the second group consists of hit number 2 to hit number 101, the third group consists of hit number 3 to hit number 102 etc. Each Ib-value was assigned to the instant of the last hit. The variations of both Ib-value and PSC are plotted in Fig.5 versus t f -t (where t f is the duration of the experiment) in logarithmic scale. The use of (t f -t) in the horizontal axis of the graphs is an indirect way to “magnify” the last stage of the experiments where the fracture occurs (the horizontal axis is read from the right to the left). The Ib-value attains relatively high values at the beginning of the experiments, something that could be attributed to the heterogeneity of the mortar specimens due to the presence of sand (Mogi 1967). In addition, the heterogeneity of the specimens results

Fig. 5. The temporal variation of both PSC and Ib-value (the graphs are read from right to left).