Issue 50

E. D. Pasiou, Frattura ed Integrità Strutturale, 50 (2019) 560-572; DOI: 10.3221/IGF-ESIS.50.47

started moving apart from each other) at a load-level equal to about 10 kN. This time instant corresponds to a load drop observed at the respective load-time curve, confirming the failure of the mortar. In addition, during the preliminary stage of the protocol, the DIC technique permitted quantification of the parasitic bending tendency of the moving block of the specimens as it can be seen in Fig.4b. In this figure the horizontal displacement recorded at the lower level of the moving block is presented for some characteristic load levels. The latter was the catalytic reason for the design and construction of specimens with different geometry, used during the main stage of the present experimental protocol. In all experiments of this stage, the moving “Γ”-shaped volume was displaced almost perfectly parallel to the fixed one (i.e., along their interface), as it was confirmed by the DIC technique. The horizontal displacements (along the length of the connector) on the “front” surface of a typical specimen of Group A, just before its fracture, are presented in Fig.3(b1). The blank areas of the figure are the areas of the specimen on which various sensors were attached and also the areas of the specimen which are hidden by the metallic rods used for the fixation of the prismatic volume on the table of the loading frame (see Fig.2b). It is seen from Fig.3(b1) that the field of horizontal displacements is almost uniform all over the moving volume. The respective maximum values do not exceed 0.3 mm, as it is concluded by the variation of the opening/closing (displacement along the x-axis) of the blocks during the whole duration of the experiment (Fig.5a). It is to be noted that the displacements presented in Fig.5a were measured at the upper and lower level of the blocks’ interface. It could be anticipated that a three dimensional motion cannot be fully described in terms of the displacements of a single plane (in this case the front surface of the specimen). In this context, additional data are required concerning the displace ments of a second material plane. For this reason, two clip gauges were attached on the rear surface of the specimen (at the aforementioned levels of the volumes’ interface). However, it is to be mentioned that clip gauges provide data concerning exclusively the displacement which is parallel to the knife edges (on which the clip gauges are attached). In other words, they only record the “normal” displacement of the volumes (i.e., perpendicularly to the blocks’ interface), which is plotted in Fig.5b for a typical specimen of Group A (recall that the respective displacement on the front surface of the same specimen were presented in Fig.3(b1) and Fig.5a).

Figure 4 : (a1) Enlarged view of the marble volumes’ interface around the mortar (at the center of a typical specimen of the preliminary protocol); (a2) The horizontal displacements of the two isolated mortar surfaces (shown in (a1)) versus the applied load; (b) The horizontal displacement of the lower level of the moving block for various loads. The measurements are referred to the point of the red circle presented in the embedded figure.

D ATA OBTAINED FROM THE INTERIOR OF THE CONNECTION USING A SERIES OF SENSORS / TECHNIQUES Strain gauges, attached on the outer surface of the specimens, are the sensors most widely used for measuring strain. How ever, in some of the experiments of the present study, strain gauges were glued inside the specimens, taking advantage of the A lthough the importance of the raw data measured from the external surface of the specimens is not to be disputed, it is nowadays accepted that data must be, also, obtained from the interior of the specimens given that failure mechanisms leading to final fracture are firstly activated within the mass of the specimens. Therefore, the need of sensing systems that can detect the internal damages and monitor its evolution becomes imperative. Using strain gauges “inside” the specimens

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