Issue 40

E. D. Pasiou et alii, Frattura ed Integrità Strutturale, 40 (2017) 41-51; DOI: 10.3221/IGF-ESIS.40.04 42 mechanical properties of the material. The improved b-value (Ib-value) is one of the acoustic characteristics which is related to the impending failure [1-2] since its value changes during the failure process. Ib-value, firstly introduced by Shiotani et al. [3], is defined as:                      2 1 2 1 N N Ib log log (1) where μ is the mean amplitude, σ the standard deviation and α 1 , α 2 constants (which are usually equal to 1 [1]). Damage process is also well detected by the Pressure Stimulated Currents (PSC) technique [4] and more specific by the weak electric signals emitted during the generation of micro-cracks in brittle materials when they are subjected to mechanical loading [5-6]. More than ten years ago, electric signals were recorded by Stavrakas et al. [6] during mechanical loading of marble. A few years later, electric signals were also recorded when specimens of rock materials and cement based materials were tested [7-10]. The signal was captured by a pair of gold plated electrodes and it was recorded using a sensitive electrometer. In all cases, when the applied stress was above 80% of the maximum stress, PSC increased rapidly reaching a maximum value just before the specimen’s fracture [4, 10]. PSC technique is also used by other researchers [11- 14] while similar techniques are used [15-16] to detect cracking of rocks and concrete specimens. Some of the advantages of the PSC technique are the low cost of the sensors and the easiness of sensors’ production as well as the fact that electrodes don’t affect the specimens’ structure or the stress and strain fields. The qualitative correlation of the aforementioned techniques has already been mentioned in previous studies during trivial tests [17-18] as well as in more complex specimens (i.e. made of more than one material [19]). In the present experimental protocol, specimens of three brittle materials (marble, mortar and glass) are subjected to uniaxial compression tests in order to confirm and quantify the correlation of PSC and AE techniques. M ATERIALS AND EXPERIMENTAL SET UP Materials ionysos marble is the stone exclusively used for the restoration of the monuments of the Athenian Acropolis. Its chemical composition is 98% of calcite, 0.5% of muscovite, 0.3% of sericite, 0.2% of quartz and 0.1% of chlorite. Its grain size varies from 100 μm to 400 μm and its specific and apparent densities are equal to 2730 kg/m 3 and 2717 kg/m 3 , respectively. The absorption coefficient by weight of Dionysos marble is about 0.11% and its thermal expansion coefficient is 9x10 -6 / o C between 15 o C and 100 o C. Its very low porosity varies between 0.3% (virgin state) to 0.7% (superficial porosity) [20]. The mechanical properties of Dionysos marble vary between broad limits [21]. The specific marble is of rather orthotro- pic nature, i.e. it is characterized by three different anisotropy directions. However, it can be approximately considered as a transversely isotropic material described adequately with the aid of five elastic constants as it was definitely concluded by detailed experimental protocols including direct tension and compression tests as well as three-point bending and Brazilian Disc tests [22-29]. The as above experimental protocols revealed also that Dionysos marble is slightly non-linear (both in the tension and in the compression regime) and slightly bimodular, i.e. the elastic modulus in compression is about 15% higher than the respective one in tension [24, 25]. Three marble specimens were used in the present study of prismatic shape with dimensions 40 mm x 40 mm x 100 mm. The load was applied normal to the material layers, i.e. along the strong direction of Dionysos marble’s anisotropy. The second material tested was a mortar which consisted of three parts of fine sand, one part of ordinary Portland cement and half part of water. The grain size of the sand varied between 3 mm to 6 mm and its fineness modulus was equal to 2.8. In addition, its specific gravity was found equal to 2.6, its density was 2200 kg/m 3 and its porosity was evaluated at ap- proximately 8% [10]. The constituents of the mortar were mixed at a low speed to enable better moisturizing of the cement grains while at the end of the production process the mixture was agitated very fast for 1 min. Mortar was formed in three prismatic blocks using metallic moulds of dimensions 50 mm x 50 mm x 70 mm the inner surfaces of which were oiled. The moulds were mounted on a desktop vibrator in order to enable compaction. The mortar prismatic blocks were demoulded after 24 h and they were cured in a room with constant ambient temperature of 22 °C and 75-80% humidity. The specimens were stored for 100 days to reach 90-95% of their strength [30]. The third material studied in the present experimental protocol was one of the most common types of glass produced, i.e. the soda-lime-silica one (or simply soda glass). One of its most important advantages is that it is nearly chemically inert, D