Issue 50

E. D. Pasiou, Frattura ed Integrità Strutturale, 50 (2019) 560-572; DOI: 10.3221/IGF-ESIS.50.47 570 Figure 12 : The released energy measured by AE and PSC techniques versus the mechanical energy for a typical specimen of Group A. C ONCLUSIONS AND DISCUSSION he majority of the monuments of the classical Greek antiquity are mainly built using natural building stones, which are anisotropic and brittle materials. The mechanical response of such materials is usually characterized by the “size effect” [11-13], dictating the use of relatively large specimens with high cost, in case their behaviour is to be studied experimentally. The problem becomes more difficult when complex specimens, i.e., specimens made of more than one material, simulating structural members of monuments are studied. For such cases, standards/instructions concerning the geometry of the specimens or the experimental set-up do not exist. As a result, proper design of both the specimens’ geometry and the experimental set-ups (according to the loading mode that is to be reproduced and the specific structural member that is going to be studied) becomes an one-way road. Taking the above into account, it is easily understood that many restrictions are imposed concerning the basic equipment (especially the loading frame) that can be used for the implementation of such structural experiments. For example, the specimens must be smaller than the available space offered by the loading frame. At the same time, the loading frame must have the required capacity and also to be able to apply the load according to the required loading mode. In addition, proper equipment must be available by the frame otherwise a series of custom-made devices must be constructed. As a second step and after the first class of difficulties (geometry, experimental set-up and equipment) is properly solved, the need for accurate measurements of various quantities arises. The raw data usually obtained during an experiment are the force and the displacement imposed by the loading frame on the specimen. For example, even in the case of a typical experiment (e.g., a standardized tensile test), the question concerning the accuracy of the displacement measured (or in other words of the deformation of the specimen) arises. For this reason, the elongation/strain of the specimen is usually obtained using external measuring instruments (like for example extensometers or strain gauges) and it is proven to be significantly different compared to the elongation/strain determined in terms of the displacement provided by the traverse of the loading frame. In addition, the measurements obtained by these traditional instruments refer to a specific point or a small area of the specimen. Moreover, the location at which they are attached is decided before the experiment and the researcher cannot obtain data from any other point/points of the specimen after the end of the experiment. Therefore, it is easily understood that in case of large specimens much more sophisticated sensing systems must be improvised. In this study the above difficulties were overpassed using the three dimensional DIC technique, which provides the actual displa- cement of the volume and permits the determination of the full-field displacement components. In any case, the typical force-displacement curve (or stress-strain curve) accompanying every experiment is obtained. In case of a typical specimen subjected to a standardized test, the aforementioned curves are adequate to understand the mechanical behaviour of the specimen and their interpretation is quite simple. On the other hand, these typical curves can only roughly describe the mechanical behaviour of complex specimens and they are definitely not adequate for under- standing the response of the specimen. In the present study, the combined use of the AE and the PSC techniques was catalytic since both of them not only monitored the internal damage developed in the specimen but they also located the area of the expecting fracture well before the fracture of the specimen occurs. Summarizing the conclusions drawn during both the preliminary and the main stages of the experimental protocol (as well as during similar protocols carried out using the specific experimental techniques, i.e., 3D-DIC, AE and PSC), it is to be 0E+0 3E+5 6E+5 9E+5 0E+0 1E+7 2E+7 3E+7 0 15 30 45 Cumulative PSC energy [pA 2 ·s] Cumulative absolute energy [aJ] Mechanical energy [J] AE PSC T