## AE Characteristics  
### Effect of Normal Stiffness on AE Characteristics of the Sheared Joints  
During joint shearing, the source of AE signals mainly comes from asperity degradation, including the breaking, rolling, squashing, and moving of asperities on the joint  
Figure 11: Damaged surface area and weight of the damaged asperity of the joint surface under diverse shear rates. Figure 10: Failure modes of joints with different shear rates under CNS conditions. (**a**) Shear rate = 0.5 mm/min; (**b**) Shear rate = 2.5 mm/min; (**c**) Shear rate = 5 mm/min; (**d**) Shear rate = 10 mm/min; (**e**) Shear rate = 20 mm/min. surface. Figures 12 and 13 show the evolution of AE signals and the accumulative AE signals for joints undergoing shear under various normal stiffness, respectively. The AE energy is defined as the area enclosed by the signal envelope (unit of voltage is mv) and axial of abscissas (unit of time is us), which represents the relative intensity of AE signals. In the pre-peak stage (Stage I), the AE signals are relatively quiet at the initial shear stage (approximately 0.7 times the peak shear displacement) due to the elastic deformation of asperities. In this phase, the accumulative AE energy is almost negligible. With the further increase of shear displacement, the AE impact signal increases rapidly and peak at the maximum shear stress. This is because when the shear stress reaches the peak, the surface roughness of the structural plane is chewed off in a large number of moments. Then, as the shear continued, the acoustic emission impact signal began to decline in the II stage. However, the acoustic emission impact signal remained at a high level because there were still rough bodies on the surface of the structural plane at this stage. Finally, in the III stage, there is almost no nibbling failure on the surface of the structural plane, and the AE impact signal is generated by the friction of the structural surface, so the AE impact signal is maintained at a low level. It must be noted that in the post-peak stage, there are still relatively high values of AE impact signals, which may be caused by the secondary crushing of some nibbled bulges. These results are consistent with those of [PERSON] [12] and [PERSON] et al. [33]. The joint acoustic emission energy signal and acoustic emission impact signal have the same development trend, which will not be repeated here. By comparing the AE energy signals at the post-peak stage under different normal stiffness conditions, it can be found that when the normal stiffness is higher, the more AE energy signals the joint is, the denser it will be. This also confirms the phenomenon that the larger the normal stiffness is, the larger the failure range of the joint surface and the greater the weight of the damaged, rough body. In order to further analyze the AE response characteristics in the process of structural plane shear, the AE impact number, and energy at the peak of shear stress, as well as the AE impact number and energy in each stage of the process of structural plane shear, were counted. Meanwhile, the cumulative AE impact number and energy in the whole process of shear were also counted. The percentage of AE signals generated in each stage of the shearing process was obtained, as shown in Table 3. The acoustic impact signal generated in the I stage is less than 12.8% of the total signal, which further indicates that most of the rough structure of the structural plane has elastic deformation, and only a few damages occur in this stage. The AE impact signals at the peak of shear stress are not different, and there is no obvious rule. In the II stage, more than 61.5% of AE signals were detected, indicating that a large number of structural plane failures occurred in this stage. With the rise of normal stiffness, the AE signal ratio of the II stage increases. It is confirmed that more structural plane failure occurs under high normal stiffness conditions.