## 4 AE Signal Analysis and Application
### AE Energy Parameters for Pipeline Damage Identification
Firstly, it increases and then decreases with the increase of the load, and then starts to increase gradually when the load is close to the damage load of the pipe; the first peak corresponds to the critical point of the pipe loading from the linear elastic compression stage to the plastic damage stage, and the second \"peak\" corresponds to the load reaching the damage load. The appearance of the two peaks indicates the critical point of the transition of the pipeline loading stage and the appearance of the main damage. Therefore, the evolution law of the AF/RA index can effectively characterize and identify the load stage and damage degree of the pipeline, and then realize the monitoring and early warning of the service condition of the pipeline. Many previous experimental studies have proved that when the value of AF/RA is small, the material mainly undergoes shear damage, and when the value of AF/RA is large, the material mainly undergoes tensile damage, schematic diagram of the principle of AE signal to identify crack types is shown in Figure 14. During the pipeline loading process, transverse cracking occurs mainly due to the extrusion deformation of structural units of the pipeline by external stresses, and shear damage occurs at the weak structural surface. The longitudinal cracks appear in the time scale after the transverse cracks sprout; in terms of spatial location distribution, the two ends of the longitudinal cracks must be connected with the transverse cracks in the neighboring areas, and the new cracks expand in the vertical direction from the weak position of the transverse cracks under the action of tensile stress. In terms of the number of cracks, there are significantly more transverse cracks than longitudinal cracks, and the expansion rate of transverse cracks is faster than that of longitudinal cracks. In the plastic damage stage, the development and expansion of transverse cracks mainly occurred in the pipe, and the number of new cracks was significantly reduced. In the online elastic compression stage, the elastic deformation of the concrete material mainly occurs in the pipe. The stress concentration area is formed due to the uneven structure of the pipe surface. The extrusion deformation of each stress unit finally occurs in the weak structural surface with shear damage forming transverse cracks, and the AF/RA value of the AE signal is generally small; with the increase of the load, the resulting fine-scale transverse cracks are subjected to tensile stress and start to expand and crack continuously. The maximum value of AF/RA corresponds to the same time as the crack load; after entering the plastic damage stage, a large number of new transverse cracks are generated in the early stage, and the value of AF/RA( the ratio of AF to RA) starts to increase. After entering the plastic damage stage, a large number of new transverse cracks are generated in the early stage, and the AF/RA value starts to decline. When the load reaches the failure load of the pipe, the crack  
\[\text{RA}=\text{RT}/\text{A} \tag{1}\]0 The evolution characteristics of RA and AF parameters during loading. completely penetrates under the action of shear stress, making the pipe lose its load-bearing capacity. Through the observation of the crack category, number, and sequence of cracks in the foot pipe loading experiment, it is shown that AF/RA can effectively characterize the damage mechanism of the whole process of pipe loading in real-time, and the \"double peaks\" presented in the loading process correspond to the crack load and failure load of the pipe loading.