## 2 Experimental Analysis
### Results and Discussions  
Figures 5 and 6 show the AE results from the compression test using the Lunitek and Mistras systems, respectively. All plots used the normalized time \(t/t^{*}\), where \(t^{*}=3000\) s is the time when the load reaches 80% of its peak value. The AE data presented after \(t/t^{*}=1\) were caused by the acquisition systems continuing to run for some time after the compression test ended, but such data were not considered in the following analyses. Figures 5a and 6a show the load against the normalized time. Notably, once again, the amplitude threshold changes with the acquisition system in each case. The maximum load \(P_{\text{max}}=4117\) kN occurs at \(t/t^{*}=0.927\), corresponding to a compressive stress of 45.75 MPa. Figure 5b,c and Figure 6b,c illustrate the signal distribution in terms of average frequency (\(AF\)) in (kHz) and signal duration in (\(ms\)). The frequency distribution is similar for both acquisition systems, but the Lunitek system captured longer-duration events after \(t/t^{*}=\)0.8, which did not appear in the Mistras system. The instantaneous and accumulated number of signals are presented in Figures 5d and 6d. The accumulated number of counts appears with the same general tendency in both plots. For the interval roughly between \(t/t^{*}=0.2\) and \(t/t^{*}=0.6\), when the load reaches 70% of the peak load, the number of counts grows with an approximately uniform rate. After that, the count growth becomes pronouncedly nonlinear, which indicates an unstable evolution in the damage process. The expressive differences in total counts for each acquisition system come from the different threshold values. That is also the likely reason for the differences in accumulated AE energy, signal energy distribution, and signal duration time. The evolution of the \(b\)-value during the test appears in Figures 5e and 6e. The test period was divided into 14 equal intervals, and the \(b\)-value was calculated for each of them. The same tendency occurs for both data sets: after presenting relatively low variations for about 60% of the total time, the coefficient suddenly decreases to within the range [1, 1.5], indicating imminent instability in the structure. The behavior indicated through the \(b\)-value is coherent with the natural time (NT) results, Figures 5f and 6f, which also present the following auxiliary horizontal lines: \(\kappa_{1}=0.070\) (dashed red), \(<D>\) limit = \(10^{-2}\) (dashed green), and entropy limit \(S_{n}\approx 0.0966\) (dashed blue). The corresponding analysis was performed according to the same methods given in [49], taking the energy component (\(Q_{k}\)) as equal to \(A_{\text{max}}\)[1.5], where \(A_{\text{max}}\) is the maximum amplitude of the signal (in \(\mu\)V), an approach also commonly found in seismology studies [50]. The calculation of the term \(A_{\text{max}}\)[1.5] was performed hit-by-hit, i.e., every time a new signal was identified, it was included in the NT analysis, leading to the rescaling of the (\(\chi_{k}\),\(Q_{k}\)) time series and the recalculation of \(\kappa_{1}\), \(S\), \(S_{rev}\) and \(<D>\). For this analysis, imminent instability was indicated when all four parameters reached their indicated critical values. In Figures 5f and 6f, this occurs for the region highlighted with a gray background,which is within the interval where the \(b\)-value lies in the [1, 1.5] range. However, the unstable region identified with data from the Mistras system is broader than that identified from the Lunitek system. Note that for \(t/t^{*}\) near 0.1, a narrow critical region was identified with the Mistras system but not with the Lunitek system. Figures 5g and 6g present the information condensed by the parameter \(<D>\), i.e., the comparison between expressions (3) and (4), representing the initial parts of the theoretical spectrum and the real one. The comparison is carried out for three different regions indicated in Figures 5f and 6f with black circles.