Issue 50

I. Stavrakas et alii, Frattura ed Integrità Strutturale, 50 (2019) 573-583; DOI: 10.3221/IGF-ESIS.50.48 574 The quest for pre-failure indicators attracts long ago the interest of engineers and researchers from quite a few scientific fields, ranging from Civil Engineering to the Management of Natural Hazards and Natural Disasters. The key concept in studies dealing with pre-failure indicators is to understand the mechanisms leading to damage evolution within the volume of loaded structural elements. It is obvious that traditional strain/displacement sensing techniques used in Mechanics of Materials (like, for example, electric strain gauges, caustics, photoelasticity, Moiré fringes, interferometry etc.) are not suitable for such applications. Indeed, such techniques can only provide data from the external surface of the loaded members while it is definitely known that, well before any kind of damage is detected at the member’s outer surface, internal damage mech- anisms have been already activated at the member’s interior, guiding in fact the phenomena observed on the member’s sur- face. The same is true for some modern (and flexible for many other purposes) sensing techniques like, for example, Digital Image Correlation [1] and Direct Strain Imaging [2]. In this context, description of the mechanical response of materials is usually achieved with the aid of analytic and numerical models, based on experimental data concerning the macroscopic mechanical properties of the material (or of its constituent materials in case of composite structures). In the direction of overcoming the above difficulties, various experimental techniques have been developed during the last decades, aiming to the quantitative description of the structural changes, which take place at the interior of mechanically loaded structures (both before and during the initiation and propagation of micro-fracturing processes). Among them, the Acoustic Emission (AE) technique (i.e., the detection and analysis of the characteristics of transient elastic waves caused by the rapid release of strain energy during mechanical loading when micro-fracture phenomena take place) is, perhaps, the most mature and widely used one. It is based on firm and well understood physical principles [3-7]. Recently, another promising experimental technique, permitting insight at the interior of loaded materials, has been intro- duced [8, 9], based on the detection of weak electrical current emissions, usually called Pressure Stimulated Currents (PSCs). These currents (the magnitude of which is of the order of pA), produced during mechanical loading, are detected using extremely sensitive electrometers. This technique, known as PSC technique, has been successfully used for both natural building stones [10, 11] and artificial materials, like cement-based ones [12, 13]. It is already adopted by many researchers [14-16] worldwide while other researchers employ techniques [17, 18] based on the same natural principles. The two sensing techniques mentioned above (AE, PSC) are here employed in a combined manner in order to monitor damage evolution within Dionysos marble specimens subjected to uniaxial compression until fracture. The novelty of the study (besides the combined use of the two techniques and the comparative analysis of the respective data) is that the PSC technique is applied, for the first time, using a grid of sensors rather than a single pair of electrodes. The specific system permits gathering data from multiple electrical current channels. Valuable information is, thus, gained, concerning the spatial and the temporal distribution of micro-cracking processes, which precede final catastrophic crack propagation. The combined analysis of the experimental results indicates that the specific arrangement of electrical sensors provides a clear overview of the spatiotemporal evolution of damage at the interior of the loaded specimen. Moreover, it is indicated that the time evolution of the improved b-value (I b -value) [15, 19] of the AE hits and the energy of the PSCs provide reliable indicators of of upcoming catastrophic failure. E XPERIMENTAL PROTOCOL The material and the specimens he specimens of the present experimental protocol are made of Dionysos marble, an extremely fine-graded white marble quarried from Mount Dionysos in Attica region. It is extensively used, among others, by the scientific per- sonnel of the restoration project of the Athenian Acropolis monuments for the construction of copies of lost or destroyed members. Dionysos marble is composed by about 98% of calcite, 0.5% of muscovite, 0.3% of sericite, 0.2% of quartz and 0.1% of chlorite [20]. It is emphasized here, that the extremely low quartz content is crucial for the present protocol, since it ensures that the electrical currents recorded are by no means related to the familiar piezoelectric effect. The specific and apparent densities of Dionysos marble are equal to about 2730 kg/m 3 and 2717 kg/m 3 , respectively. The thermal expansion coefficient between 15 o C and 100 o C is 9x10 -6 o C -1 . The coefficient of absorption by weight is about 0.11%. Its porosity is extremely low, ranging between 0.3% (virgin state) and 0.7% (superficial porosity) [20]. Its low porosity is another crucial aspect for the present study, since it blocks any electrokinetic effects (due to the existence of natural moisture) that would affect the behaviour of the electrical currents detected and recorded during the experiments. From the Mechanics of Materials point of view, Dionysos marble is of orthotropic nature. Data for its mechanical properties, reported in literature, vary between very broad limits [21-24]. Based on a long series of experiments Vardoulakis et al. [21, 24] drew the conclusion that the mechanical properties along two of the anisotropy directions are close to each other and therefore the material is usually considered as transversely isotropic, described with the aid of only five elastic constants. T