Issue 50

I. Stavrakas et alii, Frattura ed Integrità Strutturale, 50 (2019) 573-583; DOI: 10.3221/IGF-ESIS.50.48 575 In addition, these experiments indicated that Dionysos marble is slightly non-linear (both under tension and compression) and, also, slightly bimodular (the elastic modulus under compression exceeds that under tension by about 15%). The specimens prepared were prismatic of square cross section 40x40 mm 2 . Their length was equal to L=100 mm. Special attention was paid during the preparation of the specimens for their longitudinal axis to be normal to the material layers, in an attempt to avoid scattering of the experimental results due to the anisotropy of Dionysos marble. The experimental set-up The main experimental protocol consists of a series of uniaxial compression tests with a specially designed loading scheme. The specific loading model was chosen due to its increased practical importance for quasi-brittle rock and rock-like materials. An additional factor taken into account, in order to choose compression tests, is the fact that the orientation of the fracture planes (along which increased acoustic activity is expected after a load threshold) is more or less clearly predefined. The force was applied normally to the material layers. The experiments were carried out using an INSTRON-DX 300 load- ing frame of capacity equal to 300 kN under quasi-static loading conditions. Preliminary tests indicated that the compressive strength, σ f , of the specific block of Dionysos was equal to about σ f =63.8 MPa. Concerning the PSC technique, the sensing system consisted of an ultra-sensitive programmable electrometer (Keithley, 6517A) resolving currents ranging from 0.1 fA to 20 mA in 11 ranges. The system was further supported by an electrical cur- rent 10-channel scanner card (Keithley 6521). The data of the electrometer were stored in a computer using a GPIB inter- face. The sensing system consisted of five pairs of electrodes, installed along the loading axis, enabling the collection of electric emissions as close as possible to any potential source of electric current (or, equivalently, to any point where damage occurs). The distance of the uppermost and the lowest pairs of electrodes from the upper and lower bases of the specimens, respectively, was 10 mm and the distance between two successive pairs of electrodes was 20 mm (Fig.1). For the detection of acoustic emissions the 2-channel PCI-2 AE detection system was used (Physical Acoustics Corp). Two R15α acoustic emission sensors (150 kHz resonant frequency) were placed at the positions shown in Fig.1a. The AE sensors were properly attached on the specimens by means of vacuum grease. Preamplifiers with 40 dB gain were used. The thres- hold of the AE amplitudes was set at 40 dB. In addition to the electrodes and the AE sensors, an electric strain gauge was glued at the center of one of the free lateral surfaces at the mid-height of the specimens, in order to measure the axial strain developed. Kyowa strain gauges were used, together with the Microlink 770 resistor bridge. Both the force exerted and the strain developed were digitized and stored to a computer using the KUSB-3100 (manufactured by Keithley) A/D data acquisition module. Special attention was paid in all tests for the minimization of friction. The specific factor is of crucial importance, as it was pointed out by Labuz and Bridell [25]. In addition, Drescher and Vardoulakis [26] and Read and Hegemier [27] have defi- nitely indicated that little can be inferred from non-lubricated tests. In this direction, two sheets of Polytetrafluoroethylene (PTFE) were interposed between each loading platen and the respective base of the specimen. In addition, steatic acid was spread between both the specimen's surface and the ‘external’ PTFE sheet and also between the two PTFE sheets. A similar (a) (b) Figure 1 : (a) Sketch of the experimental set-up (1: Specimen, 2,3: Lower and upper acoustic sensors, 4: Strain gauge, 5(+) to 9(+): Positive edge of the five successive channels (chan0-chan4) of the electrometer, 5(-) to 9(-): Negative edge of the five successive channels, 10: Load cell, 11: Loading platens). (b) Photo of a typical specimen and the position of the sensors of the PSC technique.