## 3 Results and Discussions
### AE Behavior under Tensile Tests  
To elucidate the evolution of damage in the composite under a tensile test, the AE parameters such as energy, counts, AE energy release rate, and AE hit rate are selected and tensile load, accumulative energy, counts, energy release rate, and AE event count rate vs. time for the composite specimen are shown in Figure 8. Generally, the variations of accumulative AE energy and AE counts in terms of the slope and the number of hits are associated with internal damage activity and the damage mechanism, respectively. The value of the accumulative AE count is continuously increasing in progressive damage and there are two inflection points on the AE counts curve. At the first inflection point, AE energy increases sharply and the energy release rate and AE counts are also relatively high, which is attributed to microdamage and matrix cracking in composites. Then, AE energy remains at a high level and AE counts increase gradually, observations that are related to the propagation of microdamage and interfacial failure. Upon reaching the second inflection point, AE energy increases and AE counts and count rate increase sharply. It can be concluded that the fiber/matrix debonding and more serious damage are induced in the composites. The AE energy release rate, AE hit rate, and their average value can achieve noise discrimination and signal recognition. Three points exist with a high release rate with regard to AE energy and count, suggesting the occurrence of damage in the composite specimen. It is not enough to identify the damage mode in the composite due to the simplicity of the description of the progressive damage profile by AE energy and counting alone. Several studies have successfully established a connection between AE parameters (i.e., amplitude and frequency) and various damage modes of the composites. [PERSON] et al. [PERSON] et al. (2018) investigated the distributions of AE signals in amplitude and found that matrix cracking and fiber breakage correspond to 58-65 dB and 89-92 dB, respectively. [PERSON] et al. [PERSON] et al. (2018) studied the failure mechanism of CFRP under tensile tests and concluded that the frequency band of four failure modes such as matrix cracking, delamination, fiber/matrix debonding, and fiber damage is 0-50 kHz, 50-150 kHz, 200-300 kHz, and 400-500 kHz, respectively. In this step, these two AE parameters related to various damage modes in composites were compared and the optimal AE parameters were selected to facilitate the identification of damage patterns. The distinction of AE data with regards to different types of damage mainly reflects different amplitude and peak frequency distribution. By analyzing the distribution characteristics of amplitude and peak frequency, different damage modes can be distinguished and the progressive damage of  
Figure 8: Tensile load, accumulative energy, counts, energy release rate, and AE event count rate vs. time for the composite specimen. composites can be described. The distribution of the AE amplitude and peak frequency at 30%, 60%, and 90% of the failure load is shown in Figure 9. The dominant amplitude is concentrated in the range of 55 to 65 dB and this distribution under the evolutionary processes of the tensile damage is nearly consistent, as shown in Figure 9a. Furthermore, the peak frequency distribution of AE signals at different load levels is shown in Figure 9b, the mean value of frequency intervals is 50 kHz. There is no dominant AE event at 30% of the failure load. Outside the operation frequency range (100-900 kHz), the spectral sensitivities of the sensors drop dramatically. The frequencies below 50 kHz at 60% of the failure load do not appear to correspond to a specific damage mechanism. As the tensile load increases up to a value that is 90% of the failure load, the main frequencies range from 200 to 300 kHz. More importantly, when the load reaches the failure load, AE signals with high peak frequency over 300 kHz are generated, indicating severe damage in the composites.