Issue 41

S. K. Kourkoulis et alii, Frattura ed Integrità Strutturale, 41 (2017) 536-551; DOI: 10.3221/IGF-ESIS.41.64 544 0.0 0.3 0.5 0.8 1.0 1.3 1.5 1.8 2.0 0 100 200 300 400 0 1 2 3 4 PSC [nA] Load [kN] ζ [mm] ζ at y =0.07m Load PSC 0.18 1.04 2.26 3.05 0.57 Concerning the overall response of the restored epistyle it was concluded by the data of the DIC system that (in excellent agreement with the conclusions drawn by the data of the remaining systems that will be analyzed in next sections) the epistyle behaves as an intact structure up to about t=400s. Then the fragments start separating from each other and their distance increases relatively smoothly. Then at a time instant equal to about t=1100 s the separation tendency is accelerated denoting entrance of the system to its “critical stage” which will lead the structure to final collapse. Data provided by the Acoustic Emission and the Pressure Stimulated Currents techniques The variation of the PSC recordings against the opening of the fault is plotted in Fig.7 in juxtaposition to the respective variation of the load imposed. It is observed from this figure that the electric current produced during loading follows, according to a quite satisfactory manner, the respective variation of the load imposed. The PSC increases almost linearly during the very first steps of the loading procedure and then it is almost stabilized in the period of non-linear mechanical response of the restored epistyle. At the time instant of the sudden load drop the PSC starts increasing smoothly attaining a maximum value equal to about 2 nA. After this maximum its value starts decreasing more or less smoothly. A sudden drop is observed when the load level is equal to about 320 kN, i.e., the load level at which the crack opening starts increasing (see Fig.3b) designating approach to the final “critical stage” of the structure. Figure 7 : The variation of the load and the PSC versus the opening, ζ, of the fault. It is quite interesting to note that the variation of PSC against the opening of the fault exhibits characteristic slope changes at points where the respective load-opening of the fault graph exhibits similar changes. These points, indicated by blue lines in Fig.7, are detected for values of the fault’s opening equal to ζ =0.18, 0.57, 1.04, 2.26 and 3.05 mm. According to the previous discussion the first increasing part of the PSC graph (i.e., that until ζ =0.18) can be attributed to the fracturing of the cement paste between the two fragments, the second one (i.e., that from ζ =1.04) to cracking of the cement-paste surrounding the bars, while the last one (i.e., that after ζ =3.05 mm) corresponds to the entrance of the “system” (restored epistyle) in its “critical stage” (i.e., the epistyle approaches its catastrophic failure) and can be safely considered as a relatively early “pre-failure” indicator. In general, the two phases of PSC increase (i.e., from ζ =0 το ζ =0.18 mm and from ζ =1.04 mm to ζ =2.26 mm) can be attributed to production of electric charge due to micro-cracking in the cement paste (and perhaps in the marble’s body), while the drops of its value are attributed to the appearance of cracks of larger size which interrupt conductive paths. As a next step, in the direction of gaining an overview of the acoustic activity within the structure, the time rate of cumulative hits is plotted against time in Fig.8. In the same figure the time variation of the deflection, δ , and that of the fault’s opening, ζ , is also plotted for comparison. It is quite encouraging to observe that the time variation of the cumulative hits per second is quite similar (from a qualitative point of view) to the respective one of the PSC: It exhibits, also, characteristics slope changes at exactly the same time instants (or equivalently at the same values of the fault’s opening) as it was observed for the PSC. These changes are indicated by dotted lines in Fig.8, together with the respective values of the fault’s opening (mm).