Issue 50

I. Stavrakas et alii, Frattura ed Integrità Strutturale, 50 (2019) 573-583; DOI: 10.3221/IGF-ESIS.50.48 581 specimen (almost 10 s earlier), the electrodes closer to the specimen’s upper and lower bases start suddenly recording PSC signals of high energy. This observation is in accordance with what was concluded from Figs.4(h- l ). During this last time interval only minor emissions are recorded from the area around the specimen’s mid-height. This sudden electric activity, which starts almost 10 s before the specimen’s collapse could be considered as an interesting pre-failure indicator, especially due to the time distance from the specimen’s collapse. Concerning the amplitudes of the AE events, they were processed in the direction of conducting b-value analysis for both loading loops. In this context, the improved b-value (I b ) was calculated for each one of the two loading loops according to well-known formalisms [5, 19, 40, 41]. The time variation of the I b -value calculated in this way during the two loading loops is plotted in Figs.6(c,f). Concerning the first loading loop, the I b -value (after a slight increase) decreases with increasing axial stress. This decrease is abruptly terminated when the stress attains its maximum value and an abrupt increase is observed. From this point on, the I b -value remains practically constant at a relatively high level (equal to about 2.0), implying that no strong AE events are recorded. During the second loading loop, the I b -values exhibit a similar behaviour. The most important observation during the second loading loop is that about 70 s after the stress has reached its maximum value (and is kept constant) the I b -value starts decreasing gradually implying that new AE events of higher amplitudes occur. Then a characteristic drop of the I b -value at levels approaching I b =1 is observed, slightly before the specimen’s collapse, in excellent accordance with the respective behaviour of the PSC energy. C ONCLUDING REMARKS he Pressure Stimulated Currents (PSC) and the Acoustic Emission (AE) experimental techniques were employed ac- cording to a combined manner, in order to study the mechanical response of marble specimens subjected to successive compressive loading-unloading-reloading loops at stress levels very closely approaching the compressive strength of the specific marble variety. The two techniques were here combined for the first time (to the authors’ best know- ledge) in order to assess the spatiotemporal damage evolution and at the same time to explore the applicability of their data as estimators of proximity to fracture. In this direction, the PSC technique was here developed further by using a grid of electrode pairs, aiming to the collection of PSC emissions from multiple sensors. It was clearly shown that the updated PSC technique provides valuable information regarding the location of the PSC source and the spatiotemporal evolution of damage within the specimens. The correlation of the two techniques was here implemented in terms of the PSC energy and the improved b-value (I b ) of the AE events. The comparison indicated very good qualitative and quantitative agreement between the results obtained by the two techniques. Moreover, it was proven that both the as above mentioned quantities (PSC energy and I b -value) provide clear indicators of the proximity to fracture: The PSC energy starts increasing abruptly well before the specimens’ collapse while at the same time the I b -value exhibits a characteristic steep drop towards the I b =1 critical limit. R EFERENCES [1] Chu, T.C., Ranson, W.F. and Sutton, M.A. (1985). Applications of digital-image-correlation techniques to experimental mechanics, Exp. Mech., 25(3), pp. 232–244. [2] Iliopoulos, A. and Michopoulos, J.G. (2012). Direct strain imaging for full field measurements, ASME 2012 Interna- tional Design Engineering Technical Conferences & Computers and Information in Engineering Conference IDETC/ CIE, August 13-15, 2012, Chicago, IL, USA. [3] Lavrov, A. (2005). Fracture-induced physical phenomena and memory effects in rocks: a review, Strain, 41, pp. 135– 149. [4] Lockner, D. (1993). The role of acoustic emission in the study of rock fracture, Int. J. Rock Mech. Min. Sci. Geomech. Abstr., 3, pp. 883–899. [5] Rao, M.V.M.S. and Lakschmi, P.K.J. (2005). Analysis of b-value and improved b-value of acoustic emissions ac- companying rock fracture, Curr. Sci. India, 89, pp. 1577–1582. [6] Stanchits, S., Dresen, G. and Vinciguerra, S. (2006). Ultrasonic velocities, acoustic emission characteristics and crack damage of basalt and granite, Pure Appl. Geophys, 163(5–6), pp. 975–994. [7] Yamada, I., Masuda, K. and Mizutani, H. (1989). Electromagnetic and acoustic emission associated with rock fracture, Phys. Earth. Planet. In., 57, pp. 157–168. T