PSI - Issue 41

G. Agalianos et al. / Procedia Structural Integrity 41 (2022) 452–460 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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specimen, a relatively small number of hits (20 hits) were selected for the calculation of the Ib-value. Each Ib-value was calculated following the sliding window method, i.e., shifting the group of the first 20 hits by one hit. For the specimen of Class B, 50 hits were initially selected and the shift was realized by five hits. Each Ib-value was associated to the time instant of the last hit of the respective group of hits. The temporal variation of the Ib-value in juxtaposition to the PSC is plotted in Figs.5(a,b) for the two characteristic specimens of Class A and Class B, respectively. Due to the small number of the recorded hits in the case of the Class A specimen, the first Ib-value corresponds to the time instant of about 670 s. It is clear that the Ib-value decreases and remains almost constant and equal to about 1.2 as long as the PSC exhibits the fluctuations before it reaches its maximum value (time instant about 800 s). Right afterwards, the Ib-value decreases further attaining a value equal to about 0.5. This drop coincides with the decreasing branch of the PSC. Finally, just before the fracture of the specimen, where the PSC increases abruptly, the last drop of the Ib-value is observed. Correlation between the drops of the Ib-values and the PSC is also observed for the Class B specimen. Now, the decreasing branches of the Ib-value are detected just before the PSC plateaus. Similarly to other materials, that have been studied under three-point bending, the evolution of the Ib-value and the PSC recording of the Alfas stone, exhibit a similar behavior (Rao and Lakshmi, 2005; Loukidis et al., 2020; Li et al., 2021).

(a) (b) Fig. 5. The temporal variation of the Ib-value in juxtaposition to the PSC for a typical specimen of (a) Class A and (b) Class B. 4. Conclusions The electric and acoustic signals, recorded during three-point bending experiments of beam-shaped Alfas-stone specimens, were considered in juxtaposition to each other and, also, to the load applied. The specimens were classified into two categories, a low and a high porosity one. The analysis of the experimental data suggests that both signals depend at least qualitatively on the density of the specimens tested. Indeed, both the acoustic and electrical activities recorded are quite weaker in the case of high porosity specimens (with finer internal structure) in relation to the low porosity ones (with coarser internal structure). Moreover, a potential qualitive correlation between the electric and acoustic activities is indicated, in the sense that the latter is strongly intensified when the PSC evolution exhibits local disturbances. Finally, the time variation of the Ib-values, was studied in juxtaposition to the respective one of the PSC signals recorded and that of the load applied. A relatively good correlation (at least qualitative) between the Ib-value, the PSC signal recorded and the load applied was revealed. References Aggelis, D.G., Mpalaskas, A.C., Matikas, T.E., 2013 Investigation of different fracture modes in cement-based materials by acoustic emission. Cem. Concr. Res . 48, 1-8. Archer, J.W., Dobbs, M.R., Aydin, A., Reeves, H.J., Prance, R.J., 2016. Measurement and Correlation of Acoustic Emissions and Pressure Stimulated Voltages in Rock Using an Electric Potential Sensor. J. Rock Mech. Min. 89, 26-33. Armstrong, B.H., 1969. Acoustic emission prior to rockbursts and earthquakes. Bull. Seismol. Soc. Am. 59 (3), 1259-1279. ASTM,1982, Standard Terminology Relating to Acoustic Emission, E610-82. Cartwright-Taylor, A., Vallianatos, F., Sammonds, P., 2014. Superstatistical view of stress-induced electric current fluctuations in rocks. Phys. A: Stat. Mech. Appl. 414, 368-377.