Issue 50

I. Stavrakas et alii, Frattura ed Integrità Strutturale, 50 (2019) 573-583; DOI: 10.3221/IGF-ESIS.50.48 577 (a) (b) (c) Figure 3 : (a) The applied stress versus the corresponding axial strain during the two loading branches. (b) The degradation of the elastic modulus of Dionysos marble in terms of the non-reversible strain, ε perm [30]. (c) A specimen made of Dionysos marble loaded up to fracture under monotonic axial compression. The lubrication system used (combination of PTFE sheets and steatic acid) was suggested by Vardoulakis et al. [31]. It is clear that Mohr’s cones are almost completely suppressed. the description of the compression test beyond the peak stress), the axial stress is normalized over its peak value, and it is plotted versus the post-peak axial displacement. Describing the experimental results in this way provided very good cor- relations [32]. The main advantage of the “post-peak stress” diffusion theory is that it is totally independent of the strain concept, which is meaningless in the post-peak regime. In addition, it is quite advantageous that this theory permits de- scription of the experimental results with a minimum number of parameters. E XPERIMENTAL RESULTS AND DISCUSSION Spatiotemporal variation of the PSC n previously published studies, in which the PSC technique was employed, a single pair of electrodes was used as sensing tool. In spite of this limitation, it was definitely concluded that, when marble specimens are subjected to compression, the electric activity within the volume of the specimens starts almost simultaneously with the damage processes. It is then gradually intensified with increasing stress level and reaches its maximum value when final fracture is impending [33]. After this maximization, a steep decrease of the PSC is systematically observed, designating exhaustion of the specimen’s load carrying capacity, even though the specimen is not yet macroscopically fragmented. In the present study the five pairs of electrodes attached on the specimens (Fig.1) permitted, in addition, a (more or less) accurate matching of the electrical signals detected to the locus at which they were generated. The main reason for using five pairs instead of a single one is based on the simple argument that an electrical signal that is generated within the mass of the specimen will reach an electrode of the electrometer attenuated, in accordance with the path that it followed, i.e., to the distance from the electrode. Assuming that the specimen is, more or less, homogeneous (and therefore it has the same electric resistance all over its volume) it can be concluded that when a damage event occurs in the vicinity of an electrode, the specific electrode will record higher electrical signal level while the electrodes at longer distances will also receive the signal but they will record it with lower amplitude. In this direction, all the channels of the electrometer are levelled and calibrated with a common reference in order to record electrical signals of the same source amplitude as equal. The time variation of the axial stress and strain as well as the electrical signals received from each one of the five channels is shown in Fig.4, for a typical specimen that failed during the second loading loop (the specimen for which the loading scheme is shown in Fig.2). The experimental results indicated that during both loading branches (i.e., with increasing stress level) a systematic PSC increase is recorded from all the electrometer channels. When the applied stress reaches its maximum value, and it is then kept practically constant, the PSC emission follows a restoration process back to a background level, in agreement with previously published works [34, 35]. The fact that during the loading branch of the first loading loop the PSC emission is initially recorded stronger by channels 1, 2 and 3 (i.e., the ones located closer to the area around the mid-height of the specimen) indicates that for the specific specimen mechanical damage starts in the vicinity of its central cross section and it is gradually developed towards its bases. During the second loading loop, and while the mechanical stress was kept constant for about 100 s (i.e., at the time I