Issue 41

S. K. Kourkoulis et alii, Frattura ed Integrità Strutturale, 41 (2017) 536-551; DOI: 10.3221/IGF-ESIS.41.64 542 0.0 1.5 3.0 4.5 0 500 1000 1500 δ, ζ [mm] Time [s] δ ζ 0.5 1.5 2.5 3.5 1110 1130 1150 1170 0 1 2 3 0.0 1.5 3.0 4.5 Deflection, δ [mm] Opening of the fault, ζ [mm] C, D ΄ Β, Β ΄ C ΄ (a) (b) Figure 5 : (a) The time evolution of deflection, δ , of the epistyle’s central section and of the opening of the fault, ζ , at the lowest edge of the epistyle. (b) The dependence of δ on ζ . maximum value equal to about ζ =3.5 mm. The letters designating these characteristic intervals in Fig.3b are in accordance to those of Fig.3a. Attention should be paid, however, to the fact that their succession is not identical. In order to follow the time evolution of δ and ζ , the time dependence of them is plotted in Fig.5a. The similarity of the two graphs is quite interesting. Both quantities exhibit an almost perfectly linear portion, followed by a non-linear one which is terminated by a characteristic “plateau” until about t=1130 s and then the deformation (either in terms of δ or ζ ) starts increasing quite abruptly. From Fig.5a it could be concluded that the two “plateaus” do not appear simultaneously. As it is more clearly seen in the plot embedded in Fig.5a, a time shift between them appears. In the authors’ opinion this should be attributed to differences in the sampling rate between the devises recording deflection (LVDTs) and those re- cording the fault’s opening (clip-gauges) as well as to inevitable time-delays. This opinion can be supported by Fig.5b where δ and ζ are plotted against each other. It is reasonable to assume that neither δ can increase with ζ remaining constant (portion ΒC of the graph) nor can ζ increase with δ remaining constant (portion D΄C΄). Given that the number of experi- mental points between (B, B΄) and C΄ is very small it could be perhaps more wise to ignore the BCC΄ (or B΄D΄C΄) path and consider a virtual linear segment BC΄ instead. In spite of the above discussed criticism, it is to be emphasized that, in general, the data provided by the clip-gauges for the opening of the fault are quite reliable. This was verified since they were found in excellent agreement with the respect- ive data obtained from the Digital Image Correlation technique [32, 33]. The displacement field of both fragments was monitored by the cameras of the system during the whole duration of the loading procedure [11, 12]. A typical view of the epistyle as it is seen by the two cameras is shown in Figs.6(a,b). The specific images correspond to the very last loading steps. As a next step the axial (horizontal) component of the displacement vector was determined for both fragments. Typical colour-scale images of the axial displacement of the two fragments are exhibited in Figs.6(c,d,e) for three char- acteristic time instants. Fig.6c corresponds to a very early loading stage (t=10 s), Fig.6d corresponds to the load-level at which separation of the fragments starts (t=405 s) and Fig.6e to a load level relatively close to the entrance to the “critical stage” of the loaded system (t=1085s). For the quantitative correlation of the data provided by the DIC and the clip-gauges, two pairs of elementary areas of the fragments on either side of the fault were isolated in the immediate vicinity of the clips and the distance between them was determined as a function of time. The data concerning the opening of the fault as obtained from the DIC system are plotted in Fig.6f, in juxtaposition to those of the clip-gauges. It can be seen that the agreement between DIC and clip- gauges is excellent and the differences recorded do not exceed 1% for the whole duration of the loading procedure.