Issue 40

I. Stavrakas, Frattura ed Integrità Strutturale, 40 (2017) 32-40; DOI: 10.3221/IGF-ESIS.40.03 36 The deviation from the optimum value of FR=1, may be justified due to the fact that the maximum stress values of the 1st and 2nd loading loops was selected to lay in the limits of the linear and non-linear region, regarding the stress-strain behaviour of the marble, and additionally to the quality of the used marble. It must be noted that during each loading loop the FR clearly depends on the maximum stress of the previous loop and decreases as the value of the maximum stress increases [13]. This fact clearly is not the case for the described loading protocol of this work since the FR seems to increase rather than decrease during each next loading. Such a behaviour may be attributed to two main reasons. Firstly, the two initial loadings were performed up to the same stress value, a fact that is not typical for the FR development and secondly the selected stress value of 60 MP lays in the limits between the linear and non-linear region of the stress-strain curve, so no further damages may be caused during the second loading leading, as expected at a higher FR value. Figure 4 : The mechanical stress temporal development during the complete experimental procedure (gray line) and the corresponding per second AE event rate (black bars). Figure 5 : The behaviour of the cumulative number of the AE events with respect to the normalized value of the mechanical stress applied on a typical specimen. Pressure Stimulated Current (PSC) recordings Concurrently to the above described observations, PSC emission recordings were conducted. Fig. 6 shows the temporal variation of the recorded PSC with respect to the temporal variation of the mechanical stress. Observing the behaviour of the PSC, it may be clearly seen that during the first loading, where the mechanical stress reached a maximum value of 60MPa approximately, a strong PSC emission was recorded reaching a maximum value of 40pA approximately, while a