Issue 41

S. K. Kourkoulis et alii, Frattura ed Integrità Strutturale, 41 (2017) 536-551; DOI: 10.3221/IGF-ESIS.41.64 538 are the areas most suspicious for failure to start. The results were very encouraging, revealing interesting data about the succession of failure mechanisms activated during loading the restored epistyle. In the present study advantage is taken of the experimental data of that protocol along a different line of thought, namely that of detecting indicators that could play the role of pre-failure warnings. In this direction, the data recorded by the Acoustic Emissions (AE) and the Pressure Stimulated Currents (PSC) techniques are considered in juxtaposition to each other as well as in comparison to the data recorded by the Digital Image Correlation (DIC) technique. It is concluded that the data provided by both the AE and the PSC techniques include clear signs which precede well the upcoming failure of the structure. Taking into account that the AE is already a more or less well established CSHM technique [13-17] (which means that the respective data can be used as a calibration/validation standard) and also the fact that the PSC technique is characterized by very low application cost, it can be concluded that (after proper calibration) the specific technique could be considered as a flexible alternative tool for effective and economically tolerable CSHM, given that its results are here successfully checked against the respective ones of the AE technique. T HE EXPERIMENTAL PROTOCOL AND THE SENSING TECHNIQUES USED The specimen and the experimental procedure he experimental procedure is analytically described in ref. [12]. For the convenience of the reader, its main features only are here shortly outlined: An accurate copy of an authentic epistyle of the Parthenon Temple was constructed under a scale 1:3, by experienced technicians of the Parthenon worksite. The epistyle was made of Dionysos marble, the material that is nowadays used, almost exclusively, for the restoration project of the Acropolis monuments, given that the quarries of mount Pentelicon are not accessible, for historic and environmental reasons. The characteristics of Dionysos marble are very close to the respective ones of Pentelic marble form mechanical, geological and physico-chemical points of view [18]. During the construction of the copy every effort was paid for the accurate reproduction of the characteristics of the authentic epistyle especially concerning the orientation of the material layers with respect to the longitudinal axis of the member (and therefore to the direction of the load imposed). It is here recalled that Dionysos and Pentelic marbles are orthotropic materials characterized by three distinct anisotropy axes, although along two of them the mechanical properties are quite close to each other and therefore both materials are usually simulated as transversely isotropic ones [19, 20]. Obviously, in case of bending, optimum load-bearing capacity is achieved when the material layers are normal to the loading line. The copy of the epistyle was asymmetrically fractured into two fragments, which were restored with the aid of three pairs of bolted titanium bars driven in pre-drilled holes, as it can be clearly seen in Fig.1a. The holes are filled with a suitable cement paste (binder and water without aggregates), which acts both as adhesion layer and also as matching element between the two extremely incompatible basic constituents of the complex (i.e., the extremely brittle marble and the extremely ductile titanium) protecting the authentic building stone. The angle of the fracture plane with respect to the axis of the member was 70 o . The specific choice is based on observations of fracture planes of epistyles of the Parthenon Temple and it appears to be the maximum one for which the specific arrangement of reinforcing bars could be applied. The bar’s anchoring length (a) (b) Figure 1 : (a) The two fragments of the epistyle’s copy during the restoration phase. The three pairs of reinforcing elements are clearly visible; (b) Schematic representation of the multi-point bending loading scheme. T