Issue 40

A. Kyriazopoulos, Frattura ed Integrità Strutturale, 40 (2017) 52-60; DOI: 10.3221/IGF-ESIS.40.05 58 Finally, the cumulative energy of the AE hits up to the fracture of the specimens for all three experiments versus the normalized load level (L/L f ) is plotted in Fig. 6a. A significant step-up increase of the cumulative AE energy is observed slightly before the failure of the specimens (see the detailed view in Fig. 6b). The corresponding values of the cumulative energy practically start to converge (see Tab. 1). C ONCLUDING REMARKS he application of 3PB loading under various protocols regarding the bending load rate on cement mortar beams causes the emission of electric currents (PSC) that show a clearly deterministic behavior. The total electric charge recorded during each experimental procedure is practically of similar value for all three experiments. It may be concluded that the corresponding behavior of the studied AE parameters is also similar. Micro-cracks generate a sufficient number of weak acoustic emissions leading to a relatively high b-value. When increasing load levels, the fracture process moves from micro- to macro-cracking and the I b -value decreases. A distinct correlation between the PSC and the calculated I b values is observed. Specifically, when I b -values are relatively high and tend to increase, the PSC signals also exhibit a smooth increase. When I b -values progressively decrease, the PSC signals show intense increase, indicating that PSC emissions are mainly attributed to crack formation and propagation. The peak of the PSC signals is another clear indication of the upcoming failure. Finally, the damage initiation is also verified by the study of the cumulative ring down count. The qualitative similarity of the results of the present protocol with recently published ones for natural stones and especially for Dionysos marble [17], which is the material extensively used for the restoration of marble monuments in Greece [34-38], supports further the potential use of the PSC technique for in-situ monitoring the response of restored structural elements of masterpieces of Cultural Heritage. R EFERENCES [1] Rao, M.V.M.S., Lakschmi, P.K.J. Analysis of b -value and improved b-value of acoustic emissions accompanying rock fracture. Current Science, 89 (2005) 1577-1582. [2] Aggelis D.G., Mpalaskas A.C., Matikas T.E., Investigation of different fracture modes in cement-based materials by acoustic emission, Cement and Concrete Research, 48 (2013) 1–8 [3] Stanchits, S., Dresen, G., Vinciguerra, S., Ultrasonic velocities, acoustic emission characteristics and crack damage of basalt and granite, Pure Applied Geophys,163 (2006) 5–6, 975–994 [4] Lockner, D., The role of acoustic emission in the study of rock fracture. Int. J. Rock Mech. Min. Sci. Geomech. Abstr., 30 (1993) 883–899. [5] Stergiopoulos, C., Stavrakas, I., Hloupis, G., Hloupis, D., Triantis, Vallianatos, F., Electrical and acoustic emissions in cement mortar beams subjected to mechanical loading up to fracture, Engineering Failure Analysis, 35 (2013) 454- 461. [6] Stavrakas, Ι., Anastasiadis, C., Triantis, D., Vallianatos, F., Piezo Stimulated currents in marble samples: Precursory and concurrent – with – failure signals, Natural Hazards and Earth System Sciences, 3 (2003) 243-247. [7] Vallianatos, F., Triantis, D., Tzanis, A., Anastasiadis, C., Stavrakas, I., Electric Earthquake Precursors: From Laboratory Results to Field Observations, Physics and Chemistry of the Earth, 29 (2004) 339-351. [8] Varotsos, P.A., Sarlis, N.V., Skordas, E.S., Long-range correlations in the electric signals that precede rupture, Phys. Rev. E, 66 (2002) 011902. [9] Cartwright-Taylor, A., Vallianatos, F., Sammonds, P., Superstatistical view of stress-induced electric current fluctuations in rocks, Physica A: Statistical Mechanics and its Applications, 414 (2014) 368–377. [10] Frid, V., Goldbaum, J., Rabinovitch, A., Bahat, D., Electric polarization induced by mechanical loading of Solnhofen limestone, Phil. Mag. Lett., 89 (7) (2009) 453–463 [11] Varotsos, P.A., The Physics of Seismic Electric Signals, TerraPub (2005) [12] Stavrakas, Ι., Triantis, D., Agioutantis, Z., Maurigiannakis, S., Saltas, V., Vallianatos, F., Pressure Stimulated Currents in rocks and their correlations with mechanical properties, Natural Hazards and Earth System Sciences, 4 (2004) 563- 567. [13] Triantis, D., Stavrakas, I., Anastasiadis, C., Kyriazopoulos, A., Vallianatos, F., An analysis of Pressure Stimulated Currents (PSC), in marble samples under mechanical stress, Physics and Chemistry of the Earth, 31 (2006) 234-239. T