Issue 40

I. Stavrakas, Frattura ed Integrità Strutturale, 40 (2017) 32-40; DOI: 10.3221/IGF-ESIS.40.03 33 interpretation of the underlying physical mechanisms of mechanical dynamic processes and the detection of an upcoming event of mechanical failure [2]. Further than the AE, the detection and recording of low-level electric signals provide reliable information, regarding the development of damage processes. Such electric signals are detected due to fracture processes in quasi-brittle materials following microcracks formation and growth. It is known that transient electric phenomena are often appearing when solid materials are subjected to stress and mechanical effects are taking place [3, 4]. Since 2000, the detection of the electric signals that are related to weak electric current emissions is conducted through an innovative experimental technique rendered under the term Pressure Stimulated Currents technique, while the recorded electrical currents are known as Pressure Stimulated Currents (PSC) [5]. The PSC are detected through the recording of a weak (low-level) electric current using a sensitive electrometer, when a pair of electrodes is attached properly on the specimen that is subjected to mechanical tests. The above-described experimental technique was initially applied when marble materials were subjected to axial compressive mechanical stress [6, 7]. The Kaiser effect is an AE phenomenon briefly defined as the absence of detectable acoustic emissions until the previously applied stress level is exceeded. This effect is based on the experimental discovery by Kaiser (1950), that metallic materials had the capability to remember the previous maximum stress level. The existence and the experimental verification of the Kaiser effect was also discovered in rocks and described in other works [8-10] and since then, the Kaiser effect is used to detect and assess the amount of damage that has been developed in rocks [11, 12]. The breakdown of the Kaiser effect can be represented quantitatively by the felicity ratio (FR) that is defined as the ratio of the AE-onset stress to the maximum stress of the previous cycle. It may be taken as a measure of the quality of the rock [13]. A high felicity ratio means that the rock is of good quality. Regarding the PSC emissions, several works in the past demonstrate the ability of a marble specimen to “remember” previous mechanical loadings [5, 6, 14-16]. Specifically, the main properties of the PSC signal, which are affected by the existence of memory, converge to an inertial attitude of the material to the same stimuli and they are quite common with the properties of other fracture induced signals (i.e. AE). Namely, they are the following: (a) The PSC peak evolution over loading cycles is a changing signal property, with respect to the time interval between loadings, (b) The decrease of the dissipated electric energy during cyclic loading tests, (c) The PSC slower relaxation in each loading, quantified by the relaxation process parameters evolution, (d) The PSC signal initiates to show up at higher stress level after each next loading cycle. The aim of this work is to conduct a laboratory experimental investigation and verification of the Kaiser effect on Dionysos marble specimens combining the AE and PSC recordings. The specimens are subjected to compressive loading loops where the first loading is in the region where the material leaves the elastic region and enters the plastic deformation i.e., the stress-strain curve deviates from linearity. T EST FACILITIES AND ARRANGEMENT he marble specimens were subjected to three loading-unloading loops (see Fig. 1). The maximum stress level during the 1st and the 2nd loading was approximately 60 MPa, a value that corresponds to the 70% of the ultimate compressive strength of the material and leads to a Young’s modulus of 72 GPa (see Fig. 2). This stress level, according to preliminary tests on similar specimens, corresponds to the region that the material gradually enters the non- linear region regarding its stress-strain behaviour and initial plasticity effects take place, which is in accordance, also, to previously published data [17-19]. During the unloading processes the stress level was maintained at a value of 15 MPa, approximately. A third loading was attempted during which the specimen failed at a stress level of 82.7 MPa. The compressive stress was applied following load control at a rate of 320 kPa/s. Fig. 1 shows the temporal variation of the loading path until the failure of the specimen and Fig. 2 shows the stress-strain curve during the complete cyclic loading. It must be noted that during the 1st loading the linear region was estimated to last up to a stress level of 58 MPa approximately. The specimens (i.e. Dionysos marble) were prismatic with dimensions 35mm×35mm×75mm. The physical and chemical properties of this kind of marble have already been presented in bibliography [6, 20, 21]. The strain was measured using Kyowa strain gauges attached on the Microlink-770, 120Ω resistor bridge (see Fig. 3). During the tests the AE events were monitored using a Physical Acoustic Corporation (PAC) Mistras Systems. The AEs transducer was the model R6 sensor provided from the Mistras S.A. that obtains a wide frequency range and was attached in the middle of the specimen’s height (see Fig. 3). The AE threshold for detecting acoustic events was set at 40 dB. Concerning the PSC technique, the measuring system consisted of an ultra-sensitive programmable electrometer (Keithley, 6517A) resolving currents ranging from 0.1 fA to 20 mA in 11 ranges. The data of the electrometer were stored T