Issue 40

A. Kyriazopoulos, Frattura ed Integrità Strutturale, 40 (2017) 52-60; DOI: 10.3221/IGF-ESIS.40.05 53 an increase in the AE activity rate, as a result of the deterioration of the mechanical properties. Particularly in cement based materials and concrete structures the AE testing is one of the most widely used methods for monitoring crack growth [1]. Another phenomenon related to the micro-crack growth in quasi-brittle nonmetallic materials is the production of electric charges that shape electric dipoles forming a rather complicated charge system [6-8]. Such electric dipoles produce an electric potential across a crack resulting in the appearance of an electric current [9]. Such currents have been measured in both laboratory [7, 10] and at a geodynamic scale [11] and their detection may be useful as a precursor of a fracture. These electrical signals (weak electrical current emissions) are detected using a novel experimental technique, called Pressure-Stimulated Currents Technique, and the recorded electrical currents are described by the term Pressure Stimulated Currents (PSC) [12]. The PSC’s are weak electric currents detected with sensitive electrometers when a pair of electrodes is attached at proper locations on the specimen that is subjected to mechanical stress. Initially, the PSC technique was applied when rock specimens like marble [6, 13] and amphibolite [14] were subjected to compressive axial stress increasing up to failure. Consequently, it was successfully applied to cement-based materials [15, 16]. The PSC technique was also tested during laboratory experiments of three-point bending (3PB) tests on marble [17] and cement- based specimens [5]. The PSC technique has been adopted by several researchers [9, 18], while others use similar techniques [19-21]. Both AE and PSC signals provide important information about the damage processes occurring in specimens under compression or under bending tests. In particular, the PSCs show a considerable increase when the applied load reaches the vicinity of failure and attain their peak value shortly before failure [12, 13, 15, 16]. In Acoustic Emissions one of the statistical parameters, which is often used to estimate a critical situation, is the b-value [1, 22, 23], which exhibits systematic variations during the different stages of fracture processes. The AE based b-value analysis and the variation of the b-value, have attracted researchers working in the engineering field [23, 24]. Other AE statistical parameters that have been used include the event and energy release rates, the cumulative energy and the ring down counts. In this paper attention is focused on the parallel presentation of AE and PSC detected during 3PB of cement mortar specimens. The main difference between the three experiments is the loading mode. Specifically, one test was conducted under a constant loading rate (i.e. linear load increase) up to fracture while at the other two experiments the load was increasing according to a non-linear mode. E XPERIMENTAL DETAILS he specimens used for the experiments were prismatic cement mortar beams with dimensions 250x50x50 mm 3 . Their bending strength (L f ) varied from 3.5kN to 4.0kN. Details regarding the specimens, the preparation process and the experimental apparatus can be found in a previous work [25]. Contrary to previous publications [25], the electrodes that were used to capture the PSC emissions were placed as shown in Fig. 1, i.e. on the lower side of the beam (tension zone) and at the left and right sides, symmetrically with respect to the specimen’s central cross section where the load is applied. This topology was decided after several experiments conducted in order to estimate the best installation process that ensures the recording of strong PSCs and limits the influence of electric noise. The best electrode distance (  ) was also investigated and it was empirically found that for the specific type of experiments it should be    / 5 , where  is the distance between the two rigid metallic cylindrical rods used to support the cement beam. The PSC was captured by the electrodes and measured using a high sensitivity electrometer (Keithley, model 6517). The data were recorded in real time and stored on a hard disk through a GPIB interface. The mechanical load applied was recorded with the use of an analog-to-digital (A/D DAQ) data acquisition device (Keithley model KUSB-3108). The whole setup was placed in a Faraday shield in order to avoid interference from external electrical noise. The system that was used to detect and record the AE is the 2-channel PCI-2 AE acquisition system (Physical Acoustics Corp). The R15a sensor (manufactured by PAC, resonant at 75 kHz) was placed in the middle of the beam (see Fig. 1) in order to focus on the region of the crack development processes that take place due to the externally applied bending load. The sensor was coupled to the test specimen using vacuum grease. Preamplifier was used along with the sensor with gain set at 40dB. The signals were band-pass filtered between 20-400 kHz using the software control of the data acquisition system. To set the threshold value for recording and to ensure that sensors were correctly performing, pencil lead breaks (5mm, HB leads) were carried out near the crack tip and the recorded signals were observed. The value of 40dB was selected since it was found to prevent the recording of lower amplitude reflected signals from the pencil lead break tests. For the detected AE data processing the Physical Acoustics Corp. Noesis software was used. T