PSI - Issue 26

Andronikos Loukidis et al. / Procedia Structural Integrity 26 (2020) 277–284 Loukidis et al. / Structural Integrity Procedia 00 (2019) 000 – 000

283

7

a)

b)

marble amphibolite

1 1.1 1.2 1.3 1.4 1.5 0.00 0.20 0.40 0.60 0.80 1.00 q - index σ i / σ f marble amphibolite

5

4

3

τ q (s)

2

1

0

0.00 0.20 0.40 0.60 0.80 1.00

σ i / σ f

c)

marble amphibolite

3

2

1 I B (pA)

0

0.00 0.20 0.40 0.60 0.80 1.00

σ i / σ f

Fig. 5. The variability of a) the entropic index q b) the parameter τ q and c) the PSC background against the normalized applied stress for both cases of specimens. This behaviour of high background PSC values has also been observed in earlier works (Anastasiadis et al., 2004; Triantis et al., 2012;), since at high constant stress values we have a gradual increase in strain with subsequent maintenance of the PSC even at low values. 5. Conclusions In this work abrupt mechanical loading steps were applied on cylindrical amphibolite and prismatic marble specimens that were subjected to uniaxial stress. For both cases of specimens, PSC signals were recorded and analysed during the relaxation under the concept of NESP based on Tsallis entropy eq uation. Tsallis’ entropic index q and q  parameter along with background level of the PSC ( B I ) were studied in order to conclude whether these parameters can be used as a pre- failure indicator of the upcoming specimens’ fracture predicting the upcoming fracture of a cement mortar beam. Results showed that the value of the q-parameter increases as the level of the applied mechanical stress becomes higher. Specifically, the entropic index q appears to increase progressively, until the applied mechanical stress approaches around 0.85 of the specimens’ total strength, where it reaches a maximum value close to 1.4. But when the applied mechanical stress reaches 0.90 or higher values of the total applied the applied mechanical stress, the value of the q -parameter slightly decreases. The above findings advocate the use of the entropic index q as a pre-failure indicator of the upcoming specimen fracture.

References

Anastasiadis, C., Triantis, D., Stavrakas, I., Vallianatos F., 2004. Pressure stimulated currents (PSC) in marble samples after the application of various stress modes before fracture, Annals of Geophysics, 47, pp 21 ‐ 28. Benson, P.M., Vinciguerra, S., Meredith, P.G., Young, R.P., 2008. Laboratory simulation of volcano seismicity. Science 322, 249 – 252. doi:10.1126/science.1161927. Chang, C., Haimson, B., 2000. True triaxial strength and deformability of the German continental deep drilling program (KTB) deep hole amphibolite. Journal of Geophysical Research: Solid Earth 105, 18999 – 19013. doi:10.1029/2000JB900184.