Issue 40

A. Kyriazopoulos, Frattura ed Integrità Strutturale, 40 (2017) 52-60; DOI: 10.3221/IGF-ESIS.40.05 54 Figure 1: The experimental installation and the location of the AE and PSC sensors. R ESULTS AND DISCUSSION ab. 1 includes all the details regarding the loading protocol of each specimen for the three experiments implemented, i.e. load (L) vs. time (t), considering that the function is best described as:       2 1 2 L t C t C t (1) where 1 C and 2 C are constants. During the first experiment (i.e. specimen CB01) a linear increase of the load was conducted at a rate of 100N/s while the two following experiments were conducted following a non-linear increase of the mechanical load (see Tab. 1). Specimen       2 1 2 L t C t C t L f (kN) PSC peak (pA) Q T (pC) ΑΕ hits Cum. Energy AE CB01 ܥ ଵ =0, ܥ ଶ =+100N/s 3.6 28.8 294 1062 5.17x10 7 aJ CB02 ܥ ଵ =-7.3N/s 2 , ܥ ଶ =+333N/s 3.8 27.2 298 543 4.50x10 7 aJ CB03 ܥ ଵ =+0.7N/s 2 , ܥ ଶ =35.4N/s 3.5 29.9 291 893 4.98x10 7 aJ Table 1. The characteristics of the loading protocol followed during each of the three experiments conducted and the main quantities studied regarding the PSC and AE recordings. Fig. 2 shows the temporal variation of the emitted PSC from the three experiments, as well as the corresponding behavior of the mechanical load. Shortly before failure of the specimens the emitted PSC tends to show a peak. The recorded peak value (PSC peak ) is similar in all three experimental loading protocols (see Tab. 1). The fact that the PSC is maximized before the failure of the cement mortar beam has been systematically observed during compressive stress tests on both cement based [15] and rock [26, 27] materials. This behavior is best demonstrated by the specimen CB02. This is attributed to the fact that the specimen approaches failure at the lowest load rate (see blue lines in Fig. 2). The level of the bending load, for which the PSC signal begins to show an intense and continuous increase, irrespectively of the loading mode, is estimated to be at approximately 60% of the ultimate 3PB strength (L f ) (see Fig. 3), in all three experiments. As a next step, it is interesting to consider the total electric charge ( T Q ), released during the three experiments up to failure. The T Q was calculated using the relation ( f t is the failure time). T