PSI - Issue 10

A. Kyriazopoulos et al. / Procedia Structural Integrity 10 (2018) 97–103 A. Kyriazopoulos et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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Fig. 1. The experimental setup used showing the specimen installation, the loading system, the PSC recording electrical contacts and the cor responding instrumentation.

loading frame, under load-control mode at loading rates 100 N/s approximately. The specimens were supported by two metallic rods, each one positioned at a distance of about 8 0 mm from the specimen’s central cross -section. The load was applied at the central cross-section by means of a third metallic rod. Thin teflon sheets were placed between the specimen and the three rods to electrically isolate the specimen (Fig.1). A pair of gold plated copper electrical contacts were created at the lower surface of each specimen (Fig.1) on either side of the central cross section at a distance l=2 cm from each other in order to capture the PSC. An electrometer of very high sensitivity (Keithley, model 6517) was used to record the PSCs and the data were recorded in real time and stored on a hard disk through a GPIB interface. The load applied was recorded using an analog-to-digital (A/D DAQ) data acquisition device (Keithley model KUSB-3108). Due to the fact that the expected and recorded PSCs are of the order of pA the whole experimental set-up was enclosed in a Faraday shield to blockade any external electrical noise. The specimens were subjected to a specific loading scheme: An initial relatively low load equal to 1.8 kN was applied and it was kept constant for a long time interval in order for the PSC to relax. Afterwards, the load was increased from 1.8 kN to 4.2 kN approximately (i.e., in the immediate vicinity of 3PB strength) at a rate of 0.1 kN/s. Then, the mechanical load was again kept practically constant for a relatively long time interval in order for the PSC to relax back to a background value. Finally, the mechanical load was reduced back to the level of 1.8 kN. The above pattern was repeated 5 times and the PSC behavior was concurrently recorded. Fig.3 shows a typical curve of the applied mechanical load during the above patterns. The corresponding behavior of the PSC emissions is also, for all five repetitions of the loading pattern. The time scale was synchronized according to the time (tm) where the mechanical load reached the value of 4.2 kN and then it was kept constant. Observing the PSC emission curves it can be easily seen that during each next loading cycle the values of the recorded PSC become lower, a fact that is in full agreement with the expected PSC behaviour. The application of each new compressive stress cycle of the same characteristics causes the removal of the micro-crack edges to new excitation positions, resulting in a smaller number of new micro-cracks, to which the smaller peak values of PSC may be attributed. At the same time, some of the existing dislocations within the material sample are replaced by partially grown neighbouring dislocations corresponding to smaller energy values, so as to meet the requirements of thermodynamics. The relaxation of the PSC emissions when the applied mechanical load is kept constant at the high level of 4.2 kN is studied under the frame of NESP. Specifically, Tsallis’ entropy modeling (Tsallis (2009)), that was previously used to interpret experimental results with marble and amphibolite specimens (Vallianatos and Triantis (2012)), is also adopted herein. For all five PSC relaxation processes, the function: 3. Results and discussion

   



PSC t

PSC

  t

b





(7)

PSC tm PSC 

b