## 4 Results and Discussion  
### Joint Evaluation of the Signal Response and Concrete Behavior  
The strain vs. stress behavior of the concrete specimen under compression is presented in Figure 4a. The strain rate was also calculated to correlate the detectability of the cracking events with mechanical findings. As seen, decreases in strain rate were observed at strain values of 0.0008 and 0.0021, indicating a slowing down of the deformation process at these strain values. These reductions in strain rate suggest that the material may undergo internal adjustments to accommodate the applied load. However, beyond 24 MPa of stress, the strain rate continued to increase continuously, and these strain values resulted in the fracture of the specimen, as shown in Figure 4b. Throughout the loading process of the specimen, 72,219 hits were detected and converted into electrical signals by eight AE sensors.  
Figure 4: (**a**) Stress vs. strain curve and strain trend line, (**b**) failure state of the concrete specimen after compression test.  
In this study, the concrete strain was used as an observed damage factor associated with PoD since it directly influences the size and extent of the damage. Thus, to determine the detectability of cracks in concrete using AE, it was necessary to have the measured strain values at the moment when each AE signal was recorded. Consequently, stress vs. strain vs. time values were paired with recorded times of the AE signals. The matrix comprising all recorded AE hits, stress and strain values, and AE parameters formed the dataset for this research. Hence, PoD values based on amplitude- and energy-based approaches were correlated with mechanical observations.  
Figure 5 presents the amplitude and energy distributions of the AE activities, which were used to calculate amplitude-based PoD\({}_{\text{A}}\)s and energy-based PoD\({}_{\text{E}}\)s with respect to concrete strain values during the loading of the concrete specimen. Here, red markers and blue lines indicate the related AE signal parameter and cumulative AE hit origin, respectively. Moving average trends of the AE parameters are also shown with black lines.  
It was noted that there were increments in moving averages of amplitude and energy at the first (TP1) and the second (TP2) turning points for both parameters. These also correspond to changes in the slope of the hit formation curve, indicating accelerated cracking formations. Since these two points are evidently critical moments in the fracture behavior of the specimen, the TP1 and TP2 points were specifically scrutinized to evaluate the performances of PoD\({}_{\text{A}}\) and PoD\({}_{\text{E}}\). Furthermore, considering the alterations at these strain values in the hit formation curve, the influence of strain-dependent AE hit rate on PoD was also investigated in Figure 5.  
Figure 5: AE hit formation curve and distributions of amplitude (**a**) and energy (**b**) with respect to concrete strain.  
As seen from Figure 6, TP1 and TP2, which were marked in the above assessment, are the two most noticeable change points in the AE hit rate trend line, and they are not evident in the stress-strain curve. This demonstrates the importance of highlighting these critical points when evaluating the technique's PoD. This emphasizes the necessity of paying attention to these points.