Issue 40

S. K. Kourkoulis et alii, Frattura ed Integrità Strutturale, 40 (2017) 74-84; DOI: 10.3221/IGF-ESIS.40.07 79 E XPERIMENTAL RESULTS he time variation of the load imposed in conjunction with the respective variation of the cumulative number of hits/s recorded by the AE sensors are plotted in Fig.8 for three characteristic specimens made of Alfas stone. It is worth noticing that at about 65% and 95% of the maximum load abrupt increases of the hits/s appear systematically, in very good correlation with changes (either slope changes or slope discontinuities) of the respective load- time curve. (a) (b) (c) (d) Figure 9 : (a) The time variation of the hits/s and the load applied; (b) Average frequency versus the RA parameter (c) and (d) Average frequency versus the RA parameter for two additional characteristic experiments with Alfas stone specimens. In an attempt to gain a deeper insight of the damage mechanisms activated within the mass of the Alfas stone specimens during loading, the data of sensor 01 (i.e., the one attached at the mid-section of the specimens’ supporting length, or in other words the one closest to the critical area where the onset of macroscopic cracking is expected) are here analyzed further: The hits/s recorded by this sensor, are considered for two time intervals, i.e., before and after a critical time instant which, for example, for the specimen studied in Fig.9a is selected equal to t cr ≈1570s, for obvious reasons. For these two time intervals average values of the frequency and the RA parameter (Rise Time/Amplitude (μs/V)) are calculated. The mutual dependence of these two quantities is plotted in Fig.9b. A clear qualitative distinction of the acoustic signals re- corded before and after the critical time instant is clearly visible. The signals recorded during the first time interval (0<t< t cr ) are characterized by very high frequency and relatively low RA, while the signals recorded during the second time interval (t cr <t<t fracture ) are characterized by much lower frequency and extremely higher RA values. This classification of the recorded signals has been a concern of the scientific community for a long time and it is nowadays attributed to different modes of cracking producing elastic waves of different characteristics. According to the dominant explanation, AE with higher frequency and lower RA values are attributed to tensile cracking, while signals with lower frequency and higher RA T