## 5 Result Analysis  
### Burst Test Results  
As mentioned above, 16 PVC pipe specimens were subjected to a burst test. A typical sample failure rupture is shown in Figure 13. P630-1, P315-1, and P160-1 exhibited typical ductile failure characteristics; cracks appeared on the pipe wall and propagated to the nearby area, with major cracks extending along the axial direction of the pipe when the internal water pressure increased. When the length of the crack in the P160-1 pipe reached 26.2 mm, the pipe leaked. An obvious shear lip was observed near the main fracture, and there were no fragments on the outer wall in P160-1. Before the destruction of P630-1 and P315-1, significant plastic deformation occurred and no cracks were observed in the pipe wall. The failure showed almost ductile fracture, which is unique to the case of polymer composite pipes with observed brittle failure from other studies [39].  
There was a significant difference between the failure mode of P1200-1 and P315-1. As shown in Figure 13, the P1200-1 specimen showed catastrophic failure without significant expansion and deformation. The pipe broke into pieces, and a through crack appeared in the pipe wall. The surface of the breach was flat and smooth. These failure characteristics indicate that the failure mode of the P1200-1 pipe is a brittle failure.  
When a pipe is cooled at different temperatures, residual stress on the pipe wall will be generated due to the different shrinkage rates of the inner and outer walls of the pipe, which has a considerable impact on the internal pressure bearing performance of the pipe. This feature becomes more pronounced when the pipe diameter and wall thickness increase. A larger wall thickness causes a greater difference between the inner and outer shrinkage. An excessive shrinkage rate leads to dimple fracture and defects, which may reduce the burst pressure when the pipe wall is very thick and the temperature difference between the inner and outer walls is large.  
At the beginning of the test, the acquisition device was activated, and the pressure and deformation data were collected by the sensor and transmitted to the computer during the test pressurization. These monitoring data are drawn as curves such as burst pressure, pressurized time, hoop strain, and radial displacement for 16 pipes as shown by Figures 14-16, respectively.  
Figure 13: Typical specimen failure of short-term burst tests.  
Three stages that can reflect the characteristics of the material are observed clearly from the stress-strain curves shown in Figure 17. In the first stage, the strain of the pipe wall and the internal water pressure increase linearly.  
Three stages were observed from water injection and pressurization to pipeline rupture according to Figure 14. In the first stage, also known as the elastic stage, the strain of the pipe wall and the internal water pressure increase linearly. Then, the nonlinear relationship between internal pressure and strain is observed and the expansion of the pipeline is detected in the second phase. The last stage is the rupture phase. In this phase, the strain increases rapidly while the water pressure remains constant, and the obvious radial expansion occurs on the pipe wall. After a period of time, the pipe ruptures and fails.  
The nominal pressure PN of the P1200-1 pipe was 1.0 MPa and the required minimum burst pressure was 3.2 MPa according to the AWWA M23 standard. In this test, the burst pressure of P1200-1 was 3.65 MPa, which far exceeded the limit specification.  
Figure 14: Internal pressure loading curve.  
Figure 15: Internal water pressure–strain curve.