Issue 50

I. Stavrakas et alii, Frattura ed Integrità Strutturale, 50 (2019) 573-583; DOI: 10.3221/IGF-ESIS.50.48 580     Δ 2 t t t E t PSC t dt    (1) In order to compare the PSC energy for each one of the electrode pairs and estimate the source location of the electrical emissions, as well as their time variation, Matlab scripts were prepared. The results of this procedure are presented (in juxtaposition to the respective time variation of the axial stress and axial strain, which are plotted again in Figs.6(a,d) for convenience) for both loading loops in Figs.6(b,e). The colour variation in these figures corresponds to the level of the PSC energy (according to the scale shown in the figure embedded between Figs.6b and 6e) while the y-axis represents the x-location on the specimen. From Fig.6b it is clearly seen that during the loading branch of the first loading loop and for stress values which do not exceed about 50% of the respective maximum value attained, electrical signals of relatively low energy are recorded. These signals may be well attributed to the pore closing process. Increasing further the stress level, high energy PSC emissions are recorded approximately at the mid-height of the specimen spreading gradually towards its bases. While the axial stress is then kept constant at its maximum level, the PSC energy is relaxed back to a background level although the strain recorded does not exhibit significant variations. During the second loading loop the maximum values of the PSC energy were lower compared to the corresponding ones of the first loop (Fig.6e), as it could be perhaps expected according to the Kaiser effect [36]. It is to be noticed here that during this second loading loop, and while the stress is kept constant at its maximum level, the corresponding strain keeps increasing though at a relatively low rate. However, what is astonishingly interesting in Fig.6e is that, well before the final fracture of the Figure 6 : Time variation of the axial stress and the axial strain (a, d), the spatial distribution of the PSC energy (b, e), and the I b -value (c, f), for the first (left column) and the second (right column) loading loop. (a) 0 50 100 150 200 t [s] 1.6 0.8 0.0 1 st loading (g) 80 40 0 Stress [MPa] Strain [mstrain] (a) 75 50 25 0 Stress [MPa] 2.0 1.0 0.0 Strain [mstrain] 0 40 80 120 160 t [s] 2 nd loading (d) 0 50 100 150 200 t [s] 10 50 90 x [mm] 5000 4000 3000 2000 1000 0 PSC Energy [A 2 ] (b) 10 50 90 x [mm] 0 40 80 120 160 t [s] (e) 0 50 100 150 200 t [s] 3.0 2.0 1.0 0.0 Improved b-value (c) 0 40 80 120 160 t [s] 3.0 2.0 1.0 0.0 Improved b-value (f) Stress Strain Stress Strain ꞏs]