Issue 40

I. Stavrakas, Frattura ed Integrità Strutturale, 40 (2017) 32-40; DOI: 10.3221/IGF-ESIS.40.03 35 Figure 3 : The schematic diagram of the experimental setup showing the physical locations of the sensors on the specimen, the geometry as well as the data communication interfaces. V ERIFICATION OF THE K AISER E FFECT Acoustic Emission (AE) recordings he AE activity during the complete experimental procedure is illustrated in Fig. 4. Specifically, the AE events per second are shown with respect to time. In the same figure, the temporal variation of the normalized applied mechanical stress is also presented. It may be clearly seen that AE were continuously detected during the entire process. Specifically, high AE rate is observed during the loading procedures while low AE rate is observed during the unloadings. It must be noted that despite the fact that the experimental apparatus is well isolated from external AE noise and the configuration of the AE hardware system significantly reduces the probability to record noise as AE events (i. preamplifier filters were applied and ii. background noise was continuously recorded and subtracted) the low values of AE rate may be attributed to such an external influence. During the first loading loop, a total number of 978 AE events is recorded while a significantly smaller number of AE events (289 AE events) were recorded during the second loading loop. It is noticeable that during the second loading of the specimen, practically no AE events were recorded until the load reached the 68% of the compressive strength (i.e. only 24 low amplitude AE events were recorded). The most of the AE events (i.e. 90% of the detected AE events) were recorded in the region between 68% and 73% of the compressive strength. This region corresponds to the upper limit of the linear behaviour of the material regarding its stress-strain curve. During the 3rd loading and before the stress reached the level of the previously applied stress, only 45 AE events were recorded while the 36 of them were recorded when the applied stress was in the region between 71% and 73% of the compressive strength. When the stress exceeded these values, 1237 AE events were recorded until the failure of the specimen. The AE records of the entire experimental procedure are plotted in a cumulative form, as shown in Fig. 5. These experimental results are in good agreement with the existing literature and specifically works published in [8-12]. During the 2nd and the 3rd loading-unloading loops the Felicity Ratio (FR) can be calculated to be equal to 0.66 and 0.8 approximately, correspondingly (see Fig. 5). The FR values were calculated according to the following formula: load at onset of significant AE event FR previous maximumload  T