Issue 50

D. Triantis et alii, Frattura ed Integrità Strutturale, 50 (2019) 537-547; DOI: 10.3221/IGF-ESIS.50.45 546 C ONCLUSIONS he experimental protocol described here was designed in order to gather data regarding the response of rock-like materials under stepwise uniaxial compression, at load levels closely approaching those causing fracture. As a first step, the acoustic activity was analyzed in terms of the improved b-value (I b -value) and the cumulative energy of the acoustic signals. It was demonstrated that, when the acoustic activity is weak, the I b -value varies in the 2<I b <3 interval. However, as the acoustic activity is intensified, the I b -value drops rapidly, attaining values close to the critical value of 1, i.e., to the limit designating the onset of formation of macro-cracks. As far as it concerns the cumulative energy of the acoustic signals, it was observed that during the stages of constant stress level, its time evolution is governed by a power law with two distinct branches. However, during the last seconds before fracture it exhibits an abrupt increasing trend, in spite of the fact that the stress is constant. The detection of signals that could play the role of pre-failure indicators was also considered. This was attempted it terms of the time evolution of the I b -value, the cumulative energy of the acoustic hits and the F-function. It was definitely in- dicated that all three parameters just mentioned provide interesting and clear indices which could be considered as timely warning signals designating entrance of the system (specimen) into the stage of impending failure. Especially the F-function, well before macroscopic fracture is visible, exhibits an increasing trend which is governed by a power law. Then, a few seconds (or even tenths of a second) before the onset of macroscopically visible fracture, it reaches a maximum value and starts decreasing. Concerning the stages at which the specimen is under constant externally applied stress, the mean frequency of the AE hits, as it is expressed by the F-function, exhibits an exponential decrease (during the first one or two seconds of the constant stress stage). Then, it shows a stabilization tendency at fairly low values. An exception is the stage at which the stress is stabilized at levels closely approaching its ultimate value (i.e., the compressive strength of the material). In this stage, after a time interval of about 40 seconds (almost 25 s before fracture), the F-function exhibits a sharp increase (under constant stress), thus presaging the final macroscopic collapse of the specimen. Finally, the standardized approach for the classification of the damage process in terms of the RA-AF pair (which in fact determines the ratio of each type of damage, i.e., Mode-I or Mode-II micro-cracking), based on the RILEM TC 212-ACD recommendation, was applied. The classification showed that, for the phases with weak acoustic activity, Mode-I crackingh type appears as the dominant one, while, when the acoustic activity is intense, the majority of the acoustic signals corresponds to the Mode-II type of cracking. R EFERENCES [1] Rao, M.V.M.S. and Lakschmi, P.K.J. (2005). Analysis of b-value and improved b-value of acoustic emissions accompanying rock fracture, Curr. Sci. India, 89, pp. 1577–1582. [2] Colombo, I.S., Main, I. and Forde, M.C. (2003). Assessing damage of reinforced concrete beam using ‘‘b-value’’ analysis of Acoustic Emission signals, J. Mater. Civil. Eng., 15, pp. 280–286. [3] Cox, S.J.D. and Meredith, P.G. (1993). 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