## 4 Experimental Results and Discussion
### Notched Cement Mortar Beams under Three-Point Bending  
#### 4.2.1 Unreinforced Specimen (R0)  
During this test N = 929 acoustic hits were recorded until the instant of macroscopic fracture. The temporal evolution of the load applied is plotted in Figure 6a, in juxtaposition to the evolution of the amplitude of the acoustic hits recorded. (It is recalled that the specimen is now loaded under displacement-, rather than load-, controlled conditions; therefore, the load-time curve is not linear.) The overall duration of the specific test was equal to t\({}_{\text{f}}\) = 990 s, and the macroscopic fracture was rapid and almost instantaneous. Indeed, it was only 2 s before the rapid load drop that the load exhibited a slight decreasing trend.  
The respective evolution of the load and that of the amplitude of the acoustic hits in the Natural Time domain is plotted in Figure 6b. Each hit is located at a \"distance\" equal to \(\Delta\chi\approx 0.0011\) from its two neighbor acoustic hits. Considering that the maximum load attained was equal to L\({}_{\text{max}}\) = 11.7 kN and it corresponds to a natural time \(\chi\) = 0.53, it is concluded that the acoustic hits recorded until the load attained its peak value are only equal to 50% of the overall number of acoustic hits recorded. The remaining 50% of the acoustic hits are recorded during the very short interval of gradual load decrease, the duration of which is equal to only 2 s before macroscopic fracture.  
The evolution of the F-function, in terms of the Natural Time X, is plotted in Figure 7, again in juxtaposition to that of the load applied L(X). For the specific test, the values of the F-function were calculated using a sliding window of n = 50 successive interevent times. The thick, blue, linear segment, corresponding to the last value of the F-function, denotes, as previously, the last \"window\", i.e., the last n = 50 values of the Natural Time (X).  
Figure 6: The temporal evolution of the load applied, in juxtaposition to that of the amplitude of the acoustic hits in (**a**) the conventional- and (**b**) the Natural-Time domains for the R0 test.  
In Figure 7 it is observed that the F(X)-function starts increasing rapidly at X = 0.55 and keeps increasing until X = 0.69 (see the bright green rectangle in Figure 7). The power law of Equation (5) governs again, excellently, the evolution of the F-function in this interval of rapid increase, and the respective numerical values of the exponent m is equal to m = 12.5. It is worth observing that, in the above interval of X-values, the load applied has attained its peak value and has, already, started decreasing, although according to an almost imperceptible manner. Therefore, this rapid increase of the F-function, in the specific time interval, designates the upcoming fracture of the specimen or, in other words, its entrance into the critical stage (that of impending fracture).  
Finally, it is worth indicating the existence of a local maximum of the F(X) at the 0.29 < X < 0.36 interval, which is related to a premature load decrease (as it is seen in the figure embedded in Figure 7). This observation may be related to a local non-critical fracture (perhaps a local exfoliation observed in the vicinity of the one of the two supporting metallic rollers). Although the percentage of acoustic hits in this X-interval is not significant, it is observed that acoustic signals with low interevent time intervals are detected, which results in increased values of the F-function.