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 7

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3.3. Optical techniques The results of the DIC technique were firstly analyzed in the direction of calibrating/validating the technique. Two small areas were isolated on either side (up and down) of the tips of each notch and their displacements along the loading axis were processed in order to determine the opening/closing of the notches. The results obtained for all the specimens were compared against the respective ones provided by the clip gauges. The agreement was very sa tisfactory for all specimens, as it is concluded from Fig.4b in which data for a specimen with a=8 cm are exhibited. As a next step the overall displacement field is determined for a typical specimen. The specimen considered in Fig.4a, is studied here also. The distribution of the u y -displacement component (i.e. the displacement along the loading axis) is shown in Fig.7, after having removed the rigid body displacement and rotation. Three characteristic time instants are chosen: At t~410 s (left photo) only a very slight (if it exists at all) difference of the opening between the two notches is seen. At t~450 s (middle photo) the difference is clearly visible. At the last time instant available, cor responding to the right-most photo of Fig.7, which represents the last image captured by the cameras of the DIC system just before the fracture of the specimen the notch from which the crack is ready to initiate (in this case the “left” one) is clearly distinguished. Moreover, it is seen that the opposite notch is “relieved”, in other words the distance between its lips is reduced. It is here recalled that the cameras of the DIC system monitor the specimen from its rear side and therefore the “left” notch of the photos is in fact the right one if it is observed from the front surface. The data of the high speed camera verified the ones of the DIC: The onset of the crack from the tip of the right notch is clearly seen in Fig.8, where photos of the crack propagation process, as captured by the UHSC, are presented.

Fig. 7. The overall distribution of the u y -displacement component in mm (i.e. the one along the loading axis) for the specimen with a=4 cm mentioned in Fig.4a, for three characteristic time instants: t=410 s (left photo), t=450 s (middle photo) and just before fracture (right photo).

Fig. 8. Two characteristic photos of the crack propagation process. Both the crack initiation tip and also the “wavy” shape of the path are clear.

4. Conclusions and discussion The mechanical response of DENT specimens made of Dionysos marble was studied experimentally with the aid of modern sensing techniques. The response of the specimens in terms of the load-displacement curve is perfectly linear. On the other hand, the response in terms of the load-NMOD curve is linear only up to about 90% of the maximum load attained. From this point on it becomes strongly non-linear and the behaviour of the two notches is diversified.