PSI - Issue 3

S.K. Kourkoulis et al. / Procedia Structural Integrity 3 (2017) 326–333 2 S.K. Kourkoulis, D. Triantis, I. Stavrakas, E.D. Pasiou and I. Dakanali / Structural Integrity Procedia 00 (2017) 000–000

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A typical example of a field, where the need for data regarding the behavior of notched or cracked structural ele ments is demanding is the restoration of structural elements of marble monuments. In this field, additional difficulties appear due to the anisotropy (either orthotropy or transverse isotropy) that characterizes rock-like materials. General ized analytical solutions become then prohibitively difficult even for the simplest case, i.e. that of transverse isotropy. During the last decades a solution to the problem was offered by the rapid development of numerical techniques that became a useful and flexible tool in hands of design engineers, at least for qualitative analyses. Nevertheless, as it was clearly pointed out by Filippi and Lazzarin (2004), the numerical approximations may “… lead to a typical sparse data output which is much less manageable than analytical results and, moreover, make uneasy to understand the role played by all geometrical parameters involved ”. Especially for anisotropic materials pure numerical studies are of limited efficiency because the mechanical properties depend on the exact inclination of the material layers with respect to the loading axis and are usually known as only average values. In this case experimental studies appear of unique importance for both engineers working in the field and also researchers working in laboratory research projects. In this direction, an experimental protocol is presented here with Double Edged Notched Tensile (DENT) speci mens made of marble quarried from Dionysos Mountain in Attica, Greece. It is the material used, almost exclusively, to cover the needs of the restoration project in progress of the Parthenon Temple on the Acropolis of Athens, since its properties are similar to the respective ones of the Pentelik marble, the authentic building material of the monument. The need to study the mechanical response of Dionysos marble under tension in presence of notches is due to the fact that most elements of the Temple’s upper structure are cracked. Moreover, Theocaris & Koroneos (1979) indicated that “... the critical stresses in case of seismic loading ... are the tensile ones ... at the upper part of the columns …”. The mechanical response and fracture of the specimens were monitored using simultaneously the Acoustic Emission (AE) and the Pressure Stimulated Currents (PSC) techniques. In addition, the displacement field developed is de termined with the aid of the Digital Image Correlation (DIC) technique while the onset and propagation of the crack is captured with the aid of an Ultra High Speed Camera (UHSC). In parallel, two clip gauges are used to measure the Notch Mouth Opening Displacement (NMOD), mainly for comparison and calibration reasons. The main advantage of the complex sensing system used here is that it provides data both from the surface and also from the interior of the specimens tested. This aspect is crucial, especially for restored structural members, where the co-existence of marble with metallic reinforcing elements creates non-visible and non-accessible interfaces (Pasiou 2014), which are potential origins of fracture onset. The present protocol is part of a wider project aiming to assess various sensing techniques, in the direction of developing a flexible and low cost monitoring system, which could be used for in-situ continuous Structural Health Monitoring (SHM) of fragmented and restored structural members of marble monuments. The out comes of both the traditional and innovative sensing techniques are in good mutual agreement while the data of the AE and PSC techniques provide excellent pre-failure indicators, suggesting their use as reliable health monitoring tools. The composition of Dionysos marble (98% calcite with very small amounts of muscovite, sericite, chlorite and quartz) was analytically described by Tassogiannopoulos (1986). It is a fine marble of white colour with some thin parallel ash green veins along the schistosity. Due to the existence of chlorite and muscovite some silver areas may be visible. From a purely engineering point of view, it is a very stiff brittle material of orthotropic nature, characterized by three distinct anisotropy directions. In a first approximation however, its mechanical properties along two of the anisotropy directions are quite close to each other and therefore it can be considered as transversely isotropic, described in a satisfactory manner with the aid of only five elastic constants (Kourkoulis et al. 1999, Vardoulakis et al. 2002). The specimens used in the present experimental protocol were plates of dog-bone shape. Their thickness, d, was equal to about 1.2 cm. Their geometry and the remaining dimensions (in cm) are shown in Fig.1a. Two symmetrical notches were mechanically machined at both sides of the specimens. Four classes of specimens were prepared with different notch lengths, a, equal to a=2, 4, 6 and 8 cm. Two circular holes of 20 mm diameter were drilled on their wider areas (“ears”) as it is shown in Fig.1, through which the load was applied by means of two rigid metallic pins. For the classes with the shorter notches (i.e. a=2, 4 cm), both “ears” of the specimens were reinforced using a pair of plexiglas plates which were glued on both sides of the specimens (Figs.1c). The adhesion between the marble and the 2. Experimental protocol 2.1. Material and specimens