Issue 41

S. K. Kourkoulis et alii, Frattura ed Integrità Strutturale, 41 (2017) 536-551; DOI: 10.3221/IGF-ESIS.41.64 539 for both fragments was equal to 25 cm, symmetrically with respect to the fracture plane. The surfaces of the fragments that were to come in contact were coated with the same cement paste (used to fill the holes) to ensure optimum matching of the fragments rather than to increase the element’s bearing capacity (the tensile strength of the cement paste is negligible). After curing for 28 days the specimen was placed on two marble cubes (simulating the in-situ supporting conditions, i.e. the capitals or abacuses of the Temple) on the platform of a very stiff (capacity 6 MN) servo-hydraulic loading frame. For the load transferred to the epistyle to be as uniformly distributed as possible, a three-level steel construction was placed above the specimen, as it is shown schematically in Fig.1b. The first layer included eight equidistant metallic rods in direct contact to the marble. Above these rods four wide flange H beams (HEB 140) were placed. The second layer consisted of four equidistant rods and two wide flange H beams (HEB 200). The beam of the last layer was supported on the previous layer through two metallic rods and it was loaded with the aid of another rod, in contact to the frame’s traverse. Eight Linear Variable Differential Transformers (LVDTs) were used to measure deflections and two clip gauges were used to measure the relative displacement of two knife-edges attached on the fault’s either side. The overall specimen’s deform- ation was also monitored with the aid of innovative sensing techniques gaining data from suitable sensors, mounted on the specimen’s surface. A proper arrangement of sensors of various types was designed, aiming to the most efficient pumping of data both from the external surface and the interior of the epistyle: Eight acoustic sensors were attached around the fault’s area for the spatiotemporal determination of the location of sources of Acoustic Emissions and four electric con- tacts which permitted recording of the Pressure Stimulated Currents. In addition, the three dimensional displacement field of the epistyle’s area in the immediate vicinity of the fracture plane was recorded using the Digital Image Correlation (DIC) technique. DIC is a full-field image analysis method, based on a sequence of digital images, which permits the de- termination of the contour and the displacements of mechanically loaded bodies. The epistyle was loaded monotonically under displacement-control mode at a rate ensuring quasi-static loading conditions. A general view of the overall experimental set-up and the sensors attached is shown in Fig.2. Further details, concerning the exact position of each sensor, the position of the reinforcing bars, the dimensions of the epistyle’s copy and the loading scheme can be found in ref. [11, 12]. Figure 2 : An overview of the experimental arrangement. The Acoustic Emissions (AE) technique When a rigid body is under mechanical loading and the stress field developed exceeds certain limits (depending on the body’s material) elastic waves are emitted due to the nucleation of free surfaces and the subsequent release of stored elastic strain energy. These waves can be detected by proper acoustic sensors suitably attached on the material’s surface. Monitoring AE provides useful information related to the spatiotemporal evolution of internal damage. Proper analysis of AE data is useful, among others, for the prediction of upcoming failure since they are strongly correlated with crack initiation and propagation [13]. The characteristics of the acoustic events recorded differ, in accordance to the nature of the sources of the events. It is nowadays accepted that, the signals caused by cracking are “explosive” with low duration while, on the other hand, signals related to shear phenomena (for example friction) are characterized by higher duration [13, 14]. In concrete, the classification of the type of cracks is standardized and implemented using the combination of the