PSI - Issue 3

D. Triantis et al. / Procedia Structural Integrity 3 (2017) 346–353 D. Triantis, E.D. Pasiou, I. Stavrakas, I. Dakan li and S.K. Kourko lis / S ructural Integrity Procedia 00 (2017) 000–000 3

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(b)

(a)

(c)

Fig. 1. (a, b) The experimental set-up and the location of the PSC electrodes and the AE sensors; (c) The fracture plane of a typical specimen.

Table 1. Summarized average experimental data and results for the five groups of specimens. Specimen Loading Rate (N/s) Fracture load (kN) PSC max (pA) Q T (pC)

L 0.5 /L f [PSC=0.5PSC max ] m

CMBO1 CMBO2 CMBO3 CMBO4 CMBO5

28 55 88

3.11 3.27 3.31 3.21 3.35

14.7 17.2 22.4 25.9 29.6

281 295 293 288 294

0.88 0.76 0.36 0.27 0.24

0.86 1.08 1.04 0.96 0.98

105 135

3.2. PSC recordings The time variation of the PSC for characteristic specimens of each group is presented in Fig.2a. It is obvious that all five curves exhibit quite similar qualitative behaviour despite the different loading rates: Initially the PSC increases with increasing load, indicating generation of micro-cracks, even at the very early stages of loading. This could be expected for the specific set-up and specimens’ material, given that the reduced tensile strength of cement mortar leads to early cracking at regions of the specimen where tensile stresses are developed, according to the classical Bernoulli Euler bending theory. Suddenly, a short while before macroscopic fracture of the specimen, the value of the PSC drops very abruptly, designating upcoming catastrophic failure. The behaviour just described for the PSC is quite consist ent with previous results, reported for 3PB of pre-notched marble specimens (Stavrakas et al. 2013), for compression tests with cement-based specimens (Kyriazopoulos et al. 2011) and, also, for rocks (Anastasiadis et al. 2007). It is very interesting to note that the maximum PSC values recorded for each group of specimens, PSC max , increase systematic ally with increasing loading rate (Fig.2a, Table 1). Plotting PSC max with respect to the loading rate (Fig.2b), an almost perfect linear correlation is derived, which obviously should be thoroughly and in depth studied in the future. (a) (b)

Fig. 2. (a) The temporal variation of the PSC; (b) The maximum PSC value versus the applied loading rate.