Issue 41

S. K. Kourkoulis et alii, Frattura ed Integrità Strutturale, 41 (2017) 536-551; DOI: 10.3221/IGF-ESIS.41.64 540 average frequency of the pulse recorded by the sensors and a parameter called RA, which is in fact the ratio of the rise time (i.e., the time required for the pulse to reach its maximum value) over the maximum amplitude of the pulse. Signals of high frequency and low RA parameter are attributed to tensile cracks while signals of low frequency and high RA para- meter are attributed to either shear- or mixed-mode cracks [15-17]. Another approach widely used to evaluate AE signals is the so called b -value analysis [21, 22]. Conventionally, the b -value analysis is based on the Gutenberg-Richter relationship, which is used in seismology and correlates events of high amplitude and lower frequency with events of low amplitude and higher frequency. Due to the fact that the determination of b -value is somehow subjective, an “Improved b-value” ( Ib -value) was proposed in 1994 by Shiotani et al. [23, 24]. The Ib - value uses statistical parameters, as it is the mean and standard deviation of AE amplitude, which vary during the test. It is considered that increased Ib -values indicate that the system approaches a “critical stage”, or in other words failure is impending. The Ib -value is defined as:       10 1 10 2 1 2 log log b N N I         (1) with ω 1 = μ - α 1 σ , ω 2 = μ + α 2 σ , where σ is the standard deviation, μ is the mean value of the amplitude distribution, α 1 is a coef- ficient related to the smaller amplitude and α 2 is a coefficient related to the fracture level. The values of α 1 and α 2 vary in the range 0.5-5.0, however it is proven that changing their value within the specific range does not significantly affect the Ib -value [23, 24]. In this context in the protocol described here it was considered that α 1 = α 2 =1.0. The Pressure Stimulated Currents (PSC) technique The PSC technique is based on the detection of weak electrical signals emitted during the formation and growth of micro cracks within the material’s bulk. The PSC technique has been applied on several materials (marble, amphibolite, cement based materials etc) [25-28] and under several mechanical loading types and so far it is proven that (at least in the laboratory scale) it provides consistent pre-failure indicators. According to the fundamental principles of the technique, the electric charge produced while a material is subjected to an external mechanical load is attributed to several reasons such as moving charged dislocations, the piezoelectric effect and the fracto-emissions. The dislocations are a type of defect in crystals. The dislocations (point, linear, planar, bulk) are areas where the atoms are out of position in the crystal structure and move when a stress field is applied. They are not symmetric with respect to positive and negative charge and when deformation occurs, the dislocations start moving transporting charge. Piezoelec- tric effect is the ability of certain materials to generate electric charge in response to externally applied mechanical stress. In piezoelectric crystals, the unit cell is not symmetrical. Normally, piezoelectric crystals are electrically neutral, the atoms inside them may not be symmetrically arranged, but their electrical charges are perfectly balanced. The deformation of a piezoelectric material leads to push some of the atoms closer together or further apart, upsetting the balance of positive and negative, and causing net electrical charges to appear. Fracto-emission is the emission of particles (e.g., electrons, ions, ground state and excited neutrals, and photons) during and following fracture. The origin of electron and photon emission from fracture has frequently been attributed to either (a) field emission due to electric fields produced by charge separation or to (b) various non adiabatic processes involving fundamental excitations of creation and recombination of point like defects and charge carriers. Dickinson et al. [29] proposed a simple model for systems involving charge separation during fracture already since 1983. E XPERIMENTAL RESULTS The overall mechanical response of the restored epistyle he load-deflection curve, plotted in Fig.3a, exhibits five distinct characteristic regimes, which do not appear in case an intact marble epistyle is subjected to bending [19, 20]. For almost three quarters of the maximum load imposed (i.e. the load that caused catastrophic failure and which was equal to about 375 kN) the epistyle behaves as an intact structure (portion OA of the graph in Fig.3a) and its response is almost perfectly linear, although the overall “stiffness” is well below of that characterizing intact Dionysos marble under bending [20]. It will be indicated later (by taking advantage of data provided by the Acoustic Emissions technique), that point A corresponds (quite possibly) to the time instant at which fracture of the cement layer between the marble fragments starts. The linear portion of the graph is followed by a non-linear response of the restored epistyle (portion AB of the graph), which is in turn followed by a “plateau” BC. The specific behaviour appears similar to that of ductile metallic elements, (like T