## 3 Results and Discussion
### Internal AE Event of FPZ  
Figure 10 shows the positions of 16 AE sensors and the last location distribution of the AE event. The 3 rd to 8 th AE sensors are attached onto the front specimen surface, and the 9 th to 14 th sensors are attached onto the back specimen surface. The first, second and the 15 th and 16 th AE sensor are attached onto the downside and upside of specimen edge respectively. A sufficient number of sensors can effectively suppress the influence on the events localization caused by the individual signals attenuation and sensors position. The definition of AE events is defined by a threshold of 100 millivolts. From Figure 10a, the pencil lead break position is near the 9 th AE sensor. The total 9035 AE events are checked out from the signal waves. A total of 175 AE event positions are located effectively by the proposed method. It can be found that the location distribution of AE event in Figure 10b agreed with the deformation localization positions in Figure 9. Therefore, the location of AE events can be applied to study the micro-cracking events and the evolution of FPZ internal the concrete specimen.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Load Time (t)** & **Load (kN)** & \(\mathbf{K_{\text{I}}}\) **(MPa-m\({}^{1/2}\))** & \(\mathbf{K_{\text{II}}}\) **(MPa-m\({}^{1/2}\))** & \(\mathbf{\Delta\alpha}\) **(mm)** & **Load/Max Load (\%)** \\ \hline
577.34 & 1.58 & 0.206 & 0.018 & 4.65 & 67.2 \\ \hline
712.26 & 2.29 & 2.545 & 0.281 & 8.35 & 97.4 \\ \hline
752.35 & 2.35 & 3.332 & 0.390 & 15.02 & 100 \\ \hline
825.92 & 1.24 & 3.0065 & 0.281 & 24.63 & 52.8 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Key experimental result.  
Figure 8: The curve of mode-II stress intensity factor \(K_{\text{II}}\).  
Figure 9: Horizontal displacement contour maps of the interest area (Unit: mm).  
From Figure 11, the AE events are marked during the loading process. The marked positions on loading curve are the same as what have been mentioned in the Section 3.1. Therefore, it can be analyzed correspondingly for the internal micro-cracking characteristics and the surface deformation localizations of FPZ. The first AE event is caused by the pencil lead break. The following AE event is caused by the micro-cracking during the FPZ evolution process.  
It can be seen that the most AE events are concentrated between the main crack initial and unstable extension period. AE energy is sharply increased as macro crack forms in a FPZ. The emission energy of the event around the peak loading is higher than the other event. The AE energy in this analysis is defined as the product of amplitude and duration of the emission.  
The AE event distribution in the FPZ at loading points \(P_{1}\), \(P_{2}\), \(P_{3}\), and \(P_{4}\) are shown in Figure 12. As it is shown in Figure 12a, the first AE event location is near the up loading roller, but the event is an isolated one. Conversely, the AE events which occurred from the pre-crack tip are a series of event in Figure 12b-d. Meanwhile, these events are distributed in a narrow band, the width of the band is about 10 mm. The band is in accordance with the displacement localization band in Figure 9. Therefore, it can be concluded that the internal AE event band is in accordance with the FPZ ahead of the pre-crack tip. The length of FPZ in Figure 12c,d are in agreement with the crack length shown in Figure 6 observed by the DIC method.  
Corresponding to Figure 12, the surface horizontal strain fields are shown in Figure 13. The surface strain is obviously localized in a narrow band in Figure 13c,d. From Figures 12 and 13, it is observed that the FPZ corresponds to both the surface strain localization zone and the internal AE event zone.  
Figure 11: The curves of loading and AE event energy distribution.  
Figure 10: The AE sensor position and the AE event location.