## 4 Experimental Results and Discussion
The time evolution of the \(F\)- and \(P\)-functions, combined with the time evolution of the recorded axial deformation, is presented in Figure 4. It is evident that the temporal variations of the \(F\)- and \(P\)-functions throughout the experiment show qualitative similarities to those of the PSC signal variations. The increase in \(F\)- and \(P\)-functions, after exiting the non-linear elasticity region and until the stabilization of the uniaxial stress, is intense. After the stabilization of the applied load, the occurrence rate of hits and the energy release rate show an attenuation as a consequence of no further increase in the applied load, which has been systematically reported in tests of cement-mortar beams under a constant flexural load regime [51]. Similar variations regarding the \(F\)-function attenuation have been observed in marble specimens under uniaxial compression [51; 52; 53]. This behavior is disturbed when, in the time period between 195 s and 220 s, the axial deformation rate shows a transient increase, as compared with the neighboring areas. As a consequence, in this time period, both the \(F\)- and \(P\)-functions show a peak which is also compatible with the corresponding peak presented by the PSC signal (see Figure 2). After the 300 s, the occurrence rate of hits and the energy release rate increase quite strongly with the axial deformation rate, showing a progressive increase, and this time, the final fracture occurs. The temporal evolution of the \(b\)-values, in combination with the recorded mechanical axial deformation, is shown in Figure 5. Correspondingly to the \(F\)- and \(P\)-functions, each \(b\)-value was calculated using a group of 100 consecutive amplitudes of AE hits with the next value calculated by shifting the hits window by 50 hits. Once again, each \(b\)-value is associated with a time instance \(\tau\), which corresponds to the mean value of the occurrence times of the AE hits of each group that was used to calculate the \(b\)-value. From observing Figure 4, it becomes clear that the \(b\)-value shows three sharp drops. During the initial stages of the loading protocol, and while \(\tau<120\) s, the \(b\)-value attains values around 2.2, which can be considered as high. Sequentially, the \(b\)-values gradually lower and drop rapidly while exiting the linear elasticity region, and internal damages occur in the bulk of the specimens (see Figure 5). During this period of time and until the maximum level of the applied stress was reached, the critical \(b\)-value of 1.0, which is indicative severe damage has occurred, is approached for the very first time. Then, and while the axial deformation rate, the \(b\)-value of the stress remains constant, the b-value tends to rebound, approaching higher values close to 2.0. Similar behavior in the variation of the \(b\)-values has been identified in previous work in marble specimens under uniaxial compression [50]. But, in the time interval from 195 s \(<\tau<\) 120 s, the observed increase in the axial deformation rate again sinks the \(b\)-value close to 1.0. Finally, in the last 20 s when the specimen is in the final fracture stage, the \(b\)-value drops to its lowest value of 0.9. Commenting on the behaviors of PSC, \(F\)-, \(P\)-function and \(b\)-value, the following is noted: in the initial stages of loading, and while the deformation of the specimen is within the linear elasticity region, any pre-existing failures and microcracks remain unexcited; therefore, low acoustic activity and background values in the case of the PSC signal are observed. When reaching the linear elasticity limit, and as the axial stress increases linearly up to the maximum value of 54.5 MPa, the microcracks in the material are generated, grow and propagate within the bulk of the specimen. This is because of both the observed PSC signal and the acoustic activity excitations, but also in the corresponding emitted acoustic power.