Issue 40

S. K. Kourkoulis et alii, Frattura ed Integrità Strutturale, 40 (2017) 74-84; DOI: 10.3221/IGF-ESIS.40.07 80 are attributed to shear- or mixed-mode cracking [12, 14]. According to this approach, it is concluded that for Alfas stone under 3-point bending the cracks produced before the critical time instant are of tensile nature and are followed by shear- or mixed-mode cracking, which leads eventually to catastrophic macroscopic fracture of the specimen. (a) (b) (c) (d) (e) (f) Figure 10 : (a) , (c) , (e) Time variation of the hits/s and the load applied for three characteristic mortar specimens under 3-point bend-ing and (b) , (d) , (f) the respective average frequency versus the RA parameter. For the specimens made of mortar and cement paste, the load-displacement curves did not provide any sign at all that the material approaches its critical stage and that catastrophic fracture is impending. In this context the acoustic activity appears to be an extremely valuable tool that can potentially provide clear “warning” of upcoming fracture or that the speci- men has entered its critical stage. Along this direction the temporal dependence of the hits/s for three characteristic expe- riments with mortar specimens are plotted in Figs.10(a,c,e), in juxtaposition to the respective variation of the load applied. It is noticed that in all cases a critical time instant exists after which the number of hits/s starts increasing abruptly, indicating increased internal activity (micro-cracking). Following the procedure described for Alfas stone specimens the average values of the RA parameter and the average frequency of the signals recorded are calculated and plotted in Figs.10(b,d,f)