Issue 40

S. K. Kourkoulis et alii, Frattura ed Integrità Strutturale, 40 (2017) 74-84; DOI: 10.3221/IGF-ESIS.40.07 81 for the time intervals before and after the respective critical time instant. In all cases considered, the earlier signals (0<t< t cr ) are characterized by lower frequency and higher RA values (shear or mixed-mode cracking) while the signals recorded after the critical time instant (t cr <t<t fracture ) are characterized by higher frequency and lower RA values. The respective diagrams for a typical cement paste specimen are given in Figs.11(a,b). The RA values decrease as the load applied approaches its maximum value while the frequency does not exhibit noticeable changes. What is now much differ- ent, however, is that for some specimens (like the one considered in Figs.11(c,d)), after the maximum load is attained the specimen does not disintegrated abruptly but rather a post-peak region follows during which the load decreases gradually and the specimen collapses somehow “smoothly”. The overall duration of the experiments of this class of cement paste specimens is divided into three intervals (instead of two). The respective diagram of the average frequency versus RA is plotted in Fig.11d. It can be seen from this figure that the signals recorded during the last time interval (i.e. after the peak load) have the same qualitative characteristics with those before the peak load. (a) (b) (c) (d) Figure 11 : (a) Time variation of the hits /s and the load applied for a cement paste specimen and (b) the respective average frequency versus RA; (c) Time variation of the hits /s and the load applied for a cement paste specimen with a clearly distinguishable post-peak regime and (d) the respective average frequency versus RA. D ISCUSSION AND CONCLUDING REMARKS pecimens made of either a natural building stone (Alfas stone) or mortar or cement paste were tested under 3-point bending. The main target of the study was to assess the AE signs recorded during the experiments and attempt to classify them according to the nature of the source from which they were emitted. Considering as critical threshold the time instant at which the number of hits/s exhibits an abrupt increase, the duration of the experiments was in general divided into two intervals (excluding the specimens with clear post-peak regime). For each interval the average frequency was plotted against the RA value, in order to apply the classification criterion for the source of the AE emissions [12-14] S