PSI - Issue 3

S.K. Kourkoulis et al. / Procedia Structural Integrity 3 (2017) 326–333 S.K. Kourkoulis, D. Triantis, I. Stavrakas, E.D. Pasiou and I. Dakanali / Structural Integrity Procedia 00 (2017) 000–000 3

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(d)

(a)

(b)

(c)

metallic screws

holes (for the loading)

Fig. 1. (a) A sketch of the specimens of the experimental protocol; (b) A typical specimen before being loaded; (c) The plexiglas plates and the screws used for the reinforcement of the load transferring area of the specimens; (d) The “transparency” of Dionysos marble.

plexiglas plates was enhanced by means of two metallic screws which passed through additional holes of smaller diameter, drilled on both the marble and the plexiglas plates (Fig.1c). To avoid premature cracking around the holes, the ones drilled on the plexiglas plates were of slightly smaller diameter (compared to the respective ones of marble). In this way the pins and the screws did not come in direct contact to the marble. Therefore, the load was transferred from the pins to the plexiglas plates and through them to the marble plates in the form of a quite uniform distribution of surface shear stresses. Thus, any parasitic stress concentration around the holes was completely eliminated. 2.2. Experimental set-up For the application of the AE technique eight acoustic sensors were mounted on the specimen (seven on the front- and one at its rear-side) as it is seen in Figs.2a,b. Six of them were placed in the immediate vicinity of the two notches. The remaining two (numbered 6 and 7 in Fig.2a) were attached near the metallic pins in order to assist in the direction of detecting the parasitic signals that must be excluded from the analysis of the data provided by the AE technique. It is emphasized, that the number of sensors used permits accurate spatiotemporal location of the acoustic events’ source during loading. The threshold of the acoustic signals was set equal to 40 dB based on preliminary pencil brakes. For the technique of PSC to be applied, i.e. for the detection of the electric signals produced during loading, two pairs of electrical contacts made of copper were formed in cyclic shape. Before attaching the electrodes around the notches (and as close as possible to the tips of them - see the orange coloured ellipses in Fig.2a) they were covered with patches to assist in keeping them in place. The areas of marble, on which the electrodes were to be attached, were coated with a thin layer of liquid silver in order for the contact between the specimen and the electrodes to be enhanced. As a next step, the rear side of the specimens was properly prepared for the application of the DIC technique by spraying a random black dots’ pattern (Fig.2b). The two cameras of the DIC system were focused at the center of a predefined rectangular area of the specimen (dimensions 20 cm x 15 cm), which included the line of symmetry, i.e. the line determined by the axes of the notches. The sampling rate of the cameras was set equal to one photo every 5 s. Finally, in order to detect the notch from which the crack initiates and follow the propagation of the crack, the front side of the specimen was monitored by an ultra high speed camera (UHSC). The camera was tuned to start snapping (50000 photos per second) when the integrity of the silver line painted in front of the notches’ tips (Fig.2a) was inter rupted. It is here mentioned that both optical techniques (DIC and UHSC) used, require intense lighting (especially the high speed camera), obstructing one another, because Dionysos marble plates are “transparent” for small thicknesses. As a result, the light used for the high speed camera is transmitted through the specimen (Fig.1d). To cope with this, the front side of the specimen was initially painted black and afterwards it was sprayed using white colour (Fig.2a). The experiments were carried out with the aid of an INSTRON 1126 servo-hydraulic loading frame of capacity 250 kN (Fig.2c) under displacement-control mode and quasi-static loading conditions at a rate equal to 0.2 mm/min. Two clip gauges (INSTRON 2670-120) were attached at the “mouths” of the notches (Fig.2b) with the aid of two