Issue 50

E. D. Pasiou, Frattura ed Integrità Strutturale, 50 (2019) 560-572; DOI: 10.3221/IGF-ESIS.50.47 569 surface of the moving volume of the specimen). In Fig.11a the electric current recorded by both pairs during the loading of a specimen of Group B is plotted versus the applied load [24]. It is clear that the electric current detected in the moving volume (both on the front and on the rear surface of the specimen) is almost the same until a load-level of about 16 kN. After this load, the electric current on the rear part of the moving volume increases faster compared to what happens on the front part of the volume (which also increases but very smoothly) indicating the rear part of this marble volume as the more “active” one, i.e., with more severe damage accumulation. In general, it can be stated that even one pair of electrodes records the electric activity (and therefore the damage evolution within the specimen [18]) according to a more or less satisfactory manner, assuming that it is attached close enough to the area of the expected failure. Unfortunately, this is not always known a priori, rendering the use of a second (or even more) pair of electrodes indispensable demand. In such a case (like the one discussed here) the different electric measurements re- corded by each pair allows the determination of the area which is going to fail, even when complex specimens are loaded. The results are compatible with previously published ones [25] obtained when double edge notched marble plates were sub- jected to tensile loading. In those experiments, a pair of electrodes was attached close to each notch of the specimens and the electric current produced indicated the notch from which the crack was going to start. Very encouraging results were, also, obtained attaching a network of pairs of electrodes on prismatic marble specimens under compression [26]. The electric activity can be, also, studied in terms of the respective PSC energy using the equation:      2 i i t t PSC t E PSC t dt (1) In Fig.11b, the temporal variation of the electric energy, recorded by the electrodes during an experiment of a Group A specimen, is plotted. The respective load-time curve is also shown in the same figure. The abrupt increase of the electric energy observed at t~450 s and t~700 s corresponds to the two slope changes of the load-time curve. Figure 11 : (a) The electric current recorded at the front and rear surfaces of a Group B specimen versus the applied load; (b) The temporal variation of both load and cumulative PSC energy of a specimen of Group A. Plotting the PSC energy versus the respective mechanical energy, calculated as the area below the load-displacement curve (square points in Fig.12) for a typical specimen of Group A, a clear critical point is detected (black oval), i.e., a point at which a significant increase of the released energy is recorded. The same point is detected by the AE technique. It is known that in brittle materials the released energy recorded by the acoustic sensors is a measure of the size distribution of micro-cracks [27]. In Fig.12, the energy recorded by an acoustic sensor attached on the moving marble volume is also pre- sented by triangles. The critical point is observed when the energy density equals about 26-29 J and the respective load level is equal to about 85% of the fracture load. This point corresponds to the last slope change observed in the load-dis- placement curve. The same procedure was followed by Pasiou and Triantis [28] in marble specimens under uniaxial com- pression and the critical point was determined at about 80% of the respective maximum stress, very close to the one obtained in the present study, although the specimens of the present study are complex (three materials exist, however marble is the one that is finally fractured) and the loading mode is different. 0E+00 1E+11 2E+11 3E+11 0 10 20 30 0 300 600 900 Cumulative PSC energy [pA 2 ·s] Load [kN] Time [s] Load PSC energy (a) (b) 0 18 36 54 0 10 20 30 PSC [pA] Load [kN] Front BackRear