## 5 Conclusions  
This study proposes an acoustic emission-based method to identify the damage to pipelines using the designed full-scale loading test. The main results are as follows:  
1. Reinforced concrete sewage pipe will produce different AE signal characteristics in the different loading processes. The loading can be divided into three stages. In the online elastic compression stage (OA stage), the concrete undergoes elastic deformation with less structural failure, while the AE signal is relatively calm. In the plastic damage stage (AB stage), the cracks sprout and expand, and the number of AE activities triggered by internal structural damage increases significantly; AE energy increases with the increase of the damage degree. In the residual strength stage (BC stage), the AE signal and cumulative energy rise sharply when the load reaches the damage load, and then gradually decrease.
2. The generation of the AE signal during the loading process is closely related to the changes in the loading state of the pipe; the generation of the AE signal is not related  
Figure 14: Schematic diagram of the principle of AE signal to identify crack types.  
Figure 13: The evolution characteristics of AF/RA parameters during loading.  
to the size of the load it is subjected to, but the change in load (stress) is the direct cause of the generation of the AE signal. In the loading stage (the amount of load/stress change), the AE signal appears, the cumulative AE energy curve begins to rise, and in the load retention stage (load/stress change is 0), the AE activity enters a quiet period, the AE signal energy drops to 0, the cumulative AE signal energy stops rising to maintain a stable state. (3) AE energy and cumulative AE energy can be used as effective monitoring indicators to characterize the loading stage and damage level of full-size pipes, and AE energy and cumulative AE energy vary with the stresses applied to the pipe. The correlation analysis of the load variation \(\Delta\)F, the accumulated AE energy \(\sum\)E, and the single AE energy maximum E\({}_{\max}\) during the plastic damage phase shows that there is a positive correlation between any two indicators, and the correlation coefficients are greater than 0.9, which is a strong correlation. (4) The AF/RA index can effectively characterize the loading state and the damage degree of the pipeline. The value of AF/RA gradually increases when the main damage is approaching, and then starts to decrease after the main damage occurs, showing the characteristics of \"double peaks\". The appearance of the first peak corresponds to the appearance of the crack load, and the second \"peak\" appears when the load reaches the damage load. The appearance of the two peaks corresponds to the change of the loading phase of the pipeline, and the occurrence of the major damage.  
This study is of great significance in evaluating the damage level and instability of the reinforced concrete sewage pipeline. It is found that cumulative AE energy and the AF/RA value can be used as effective indicators for pipeline damage monitoring, but the quantitative relationship between the two indicators and the degree of pipeline damage still needs further study.