**3. Test Results**

The results are presented using the example of a quasi-brittle cement composite of slurry type with dispersed reinforcement in the form of structural polypropylene fibers (Figures 3 and 4).

In the top part of Figure 3, the recording of AE effects is presented. The figure shows charts from four-point bending tensile tests, force-deflection, and force–time; in addition, it shows the recorded events as a function of time (0–160 s) and the AE events total.

**Figure 3.** Results of tests on the composite in the range of 0–160 s: force-deflection, force-time curve, total acoustic emission AE, events, and AE recording.

**Figure 4.** Results of tests on the composite Figure 1 in the range of 0–40 s: total AE, events, and AE recording.

The characteristic points and ranges of AE events are highlighted in the presented figure. In addition, the chart of total AE is shown. The red color marks the event fcr (the end of Hooke's law), corresponding to the critical point fcr on the force-deflection curve. Point f1 corresponds to the sudden drop in stress in the strengthening area, and point ftb corresponds to maximum stress.

In order to analyze the destruction process in the proportionality area shown in Figure 3, the effects obtained in that range of deflections (in the range of 0–40 s) were enlarged and presented in Figure 4.

The spectrogram of the AE signal of the tested composite is presented in Figure 5. Events are visible as vertical lines with marked characteristic points fcr, f1, ftb, and image of events occurring in quasi-brittle composites, with highlighted areas of proportionality AE, strengthening control AS and deflection control AD. The area with a dominating impact of event noise as well as the areas with the concentration of events EII and EIII are marked. The spectrum is presented in the range of low, medium, and high frequency from 0–4000 Hz. The red intermittent line in the spectrogram marks an area that is linked to the possible occurrence of dispersed reinforcement breaking. Figure 5 enables the observation of the concentration of micro-events preceding fcr (EII and EIII).

**Figure 5.** Spectogram of the AE signal.

Figure 6 presents the 2D sound spectra for different AE effects obtained at characteristic points or ranges of events by means of the Audacity program. Event E0 corresponds to the background spectrum. Three characteristic areas were identified: EI, EII, and EIII. Area EI groups single events from E1 to E4, area EII includes events from E5 to E8, and EIII refers to events from E9 to E15. The spectra are presented in the wide range of low, medium, and high frequency from 0–45,000 Hz.

**Figure 6.** Sound spectra for characteristic events: sound background E0 and ranges of events: EI, EII, EIII.

Figure 7a,b presents the 3D sound spectra from the Hooke's law range. Figure 7a shows the sound background spectrum and spectra of single AE events (groups of sounds EII), with spectra in the time range of 5.4–33 s. Figure 7b presents the background spectra, and events spectra EII and EIII in the time range 12.5–39.7 s. Figure 7c displays the background spectra and spectra of multicracking between points fcr and f1 in the time range 44.9–70.3 s. Relative amplitudes (of sound intensity components) were analyzed between 50–110 dB. The range of sound intensity components for background spectra and spectra of events EII was from −110 to −85 dB, while the group of events EIII and multicracking corresponded to the level of −80 dB.

Figure 8a is an image of the sound spectra of the composite: its background, events EII, EIII, and the critical point fcr. Figure 8b presents the spectra of the area of sudden drops in stress in points f1 and f2 in the time range 74.8–102.3 s. Figure 8c shows the spectra of event in point ftb and spectra of the deflection control area. The image is presented in the range 106.3–133.8 s. Relative amplitudes (sound intensity components) were analyzed in the range between 60–120 dB. The range of sound intensity components for background spectra and spectra of events EII was from −110 to −85 dB, while the group of events EIII and multicracking corresponded to the level of −80 dB.

**Figure 7.** Composite sound spectra: (**a**) background, micro-events EII, (**b**) micro-events EII and EIII, (**c**) multicracking between points f1 and f2.

**Figure 8.** Composite sound spectra: (**a**) background, micro-events EII, EIII, and fc, (**b**) event f1, (**c**) event ftb and deflection control area AD.
