Issue 40

S. K. Kourkoulis et alii, Frattura ed Integrità Strutturale, 40 (2017) 74-84; DOI: 10.3221/IGF-ESIS.40.07 82 according to which the emissions are due to microcracking or friction depending on the combination of frequency and RA values. It was concluded that a clear difference between specimens made from artificial materials and specimens made from natural stone exists: Indeed, for Alfas stone specimens the failure mechanisms during the primary loading stages are governed by tensile micro-cracking while the last stages before macroscopic fracture are governed by shear- or mixed- mode cracking. On the contrary, for the specimens made of artificial materials the succession of mechanisms is inverted: Shear- or mixed-mode cracking prevails during the primary loading stages and it is then followed by tensile micro-cracking at the loading stages before the final disintegration of the specimens. Differences appear, also, between the mortar and the cement paste specimens concerning mainly the existence or not of a clearly distinguishable post-peak regime which characterized some specimens made of cement paste. This difference can be attributed to their different microstructure, which is clearly visible in Fig.12. (a) (b) Figure 12 : Cross section of typical specimens made of (a) mortar and (b) cement paste. (a) (b) Figure 13 : (a) A typical prismatic specimen made of Alfas stone subjected to uniaxial compression and (b) the respective time variation of load, cumulative hits/s and Pressure Stimulated Currents [15]. Another critical conclusion that can be drawn from the present experimental protocol is that the signals recorded by the AE sensors are characterized by some indicators that can be considered as warnings that the specimens change their state and enter critical stage, well before the peak load is attained. Due to its significance the specific conclusions must be further studied with specimens made of different materials and subjected to other loading modes. In this direction, an experimental protocol is in progress, with Alfas stone specimens subjected to uniaxial compression (Fig.13a) [15]. The results of this protocol are very encouraging. In Fig.13b the data concerning the cumulative hits/s for a typical compres- sion experiment of that protocol are plotted versus time in juxtaposition to the respective variation of the load imposed. It is seen from this figure that at about 85% of the peak-load finally attained the cumulative hits/s exhibit an abrupt jump