Issue 40

S. K. Kourkoulis et alii, Frattura ed Integrità Strutturale, 40 (2017) 74-84; DOI: 10.3221/IGF-ESIS.40.07 77 production quality control etc. The major difference between AE and other non-destructive techniques is that AE records signals due to the external application of load to the material. AE relies on energy produced by the material only under stress. The acoustic emission frequencies are in the range of 150-300 kHz, which is above the frequency of audible sound. Usually the sensors used for monitoring the response of structural materials have a recording ability in the 20 kHz -1 MHz range [7]. The basic advantages of the AE method are its high sensitivity, the early detection of defects and cracks and the real time monitoring at a relatively low cost. Several models are proposed to analyze acoustic emissions signals, such as the b-value and the improved b-value, the intensity analysis [11], the Ledeczi et. al. method, etc. Nowadays an alternative approach is widely used for the classification of the acoustic emissions’ source based on the relation between the signals’ average frequency with the RA (Rise Time/Amplitude) parameter (Fig.4). The results have proven very encouraging [12, 13]. The latter analysis model is adopted in the present study. Figure 4 : Cracking modes and typical acoustic emission signals [12]. The specimens and the experimental procedure The specimens made of Alfas stone were of orthogonal parallelepiped shape of dimensions equal 22.5x10x2.2 cm 3 . The artificial specimens made of mortar and cement paste were also of orthogonal parallelepiped shape and their dimensions were equal to 4 cm x 4 cm x 16 cm according to the ASTM C348 standard. The specimens were subjected to 3-point bending (3PB), under displacement-control mode and quasi-static loading conditions, using an INSTRON (300 kN) servo- hydraulic loading frame, with a 50 kN calibrated load cell. The load was applied monotonically up to the fracture of the spe- cimens. A long series of preliminary experiments highlighted the crucial role of the loading rate on the results and indicated that for the specific materials a rate equal to 0.02 mm/s was the one allowing optimum sampling of the AE data. For the Alfas stone specimens four acoustic sensors (R15α) were properly arranged around the critical region (Fig.5). For the mortar and cement paste specimens a single acoustic sensor (R15α) was attached in the middle of the specimens’ span at their lowest side. The sensors were mounted on the specimen by means of proper silicone paste (Figs.5, 6 and 7). Figure 5 : (a) - (d) Experimental set up for the Alfas stone specimens and the position of the AE sensors; (e) typical fractured specimens. (a) (b) (c) (d) (e)