Issue 50

D. Triantis et alii, Frattura ed Integrità Strutturale, 50 (2019) 537-547; DOI: 10.3221/IGF-ESIS.50.45 538 been long ago proposed and are being continually developed since then. Among them, the Acoustic Emission (AE) tech- nique is perhaps the one most widely applied worldwide. Although the AE technique is introduced many decades ago, it is still developed and refined and today it is a flexible tool for monitoring and understanding the dynamic processes taking place within the mass of mechanically loaded structural elements. Moreover, the AE technique is considered as a unique tool providing warning signals about impending failure, especially for brittle materials [1-3]. Acoustic Emissions (AEs) are detected as series of short impulse energy packets, sequentially released when damage accumulation exceeds some critical limits leading to micro-cracking within the mass of materials subjected to mechanical loads. The detection of AEs is now- adays widely used for the spatiotemporal damage localization as well as for the assessment of the damage level (or in other words of the remaining load carrying capacity) in brittle materials such as concrete and rocks [4-7]. It is known that mechanical loading of rocks and rock-like materials generates a stress field, which is responsible for the activation of various micro-mechanisms, such as dislocation activity and formation of micro-cracks, which in turn are res- ponsible for the release of elastic energy, traveling in the form of elastic waves and detected in the form of acoustic signals. The fracture process starts by the nucleation of micro-cracks, which grow and coalescence forming cracks leading to cata- strophic fracture. Micro-cracking is the most important source of AEs at least for brittle materials, such as rocks [8]. Initially, tiny cracks are produced, which are gradually growing, reaching eventually a critical size, beyond which coalescence of the micro-cracks starts generating macro-cracks. In general, AEs are generated at different spatial and temporal scales. It should be noted that the release of seismic waves related to an earthquake and the AEs detected in specimens of solids during their fracture process have remarkable similarities. The initial correlation of AEs and seismology was attempted by Ono and Ohtsu [9-11], who employed techniques used for earthquake data processing to elaborate laboratory AEs data. As a result, quite a few techniques applied in seismology from the civil engineering point of view are nowadays employed in the field of analysis of AEs data. The number of incoming signals is a basic parameter when monitoring the acoustic activity. As the stress level increases ap- proaching that causing failure of specimens or structural members, the number of AE hits recorded starts increasing at an increasing rate, depending on the material and the loading mode. Therefore, the rate of AE hits (dn/dt), can provide direct information on the frequency of micro-cracking activity in rocks. Graphical representation of dn/dt versus time provides information about the various stages or phases of micro-cracking activity in rocks. For further analysis of the failure mode, two additional AE parameters are widely used: the average frequency (AF) of the acoustic signals, defined as the number of counts divided by their duration, and the RA parameter (Rise Time/Amplitude) [12, 13]. It is accepted that cracks generating signals of relatively high AF and low RA are related to tensile cracking (Mode-I) while shear cracking (Mode-II) corresponds to lower AF and higher RA. Moreover, researchers in the field of Rock Mechanics use the so-called improved b-value analysis to quantify damage and describe the respective fracture processes [1, 3, 14]. In this work, rock samples (marble) are subjected to uniaxial compression until fracture. The analysis of the AEs data focuses on how the acoustic activity evolves, especially in phases near the catastrophic fracture of the specimens. For this purpose, a special loading protocol of four successive steps is implemented, as it is described in next section. For the re- presentation and analysis of the acoustic activity, besides the above mentioned parameters, a recently proposed F-function is used [7], which reflects the mean occurrence of AE hits, in a predefined time window of N consecutive hits. Τ HE MATERIAL AND THE EXPERIMENTAL PROTOCOL rismatic specimens made of Dionysos marble, of dimensions 40 x 40 x 100 mm 3 were used. Dionysos marble is exclusively used for the restoration of the Athenian Acropolis monuments. Its composition is 98% calcite, 0.5% muscovite, 0.3% sericite, 0.2% quartz and 0.1% chlorite. Detailed characteristics of the material are given by Exa- daktylos et al. [15]. During a preliminary experimental protocol with standardized specimens, the strength of the specific material’s batch under monotonic uniaxial compression was determined, and it was found varying between 62.5 MPa and 65.1 MPa, with an average of about 63.8 MPa. An Instron DX 300 loading frame was used for the main experimental protocol. The strain developed was measured using Kyowa strain gauges attached on the Microlink-770, 120 Ω resistor bridge. An acoustic sensor was attached in the central cross section of the specimens with the aid of vacuum grease. Preamplifier with 40 dB gain and analogue band-pass filters in the 20-400 kHz range was also used. The AE equipment and software were by Mistras Group, Inc. The specimens were subjected to uniaxial compression until fracture, following a specially designed protocol of four discrete consecutive stages [16]: During the first stage (A), the load was applied at a constant rate, resulting to a stress rate of 0.44 MPa/s, up to a stress level of 60.5 MPa. During the second stage (B), the duration of which was equal to about 120 s, the stress was kept constant at 60.5 MPa. At the end of stage B the stress level was increased slightly at 63.5 MPa of P