## 2 Experimental Testing
### Experimental Setup
#### 2.1.2 Negative Pressure Test  
In order to investigate the influence of pipe diameter, wall thickness, loading mode and other factors on the lining's anti-buckling ability under negative pressure, five groups of tests were designed (Table 1). The specimens were numbered T1-T5, respectively. In groups T1, T2 and T3, the host pipe diameter's influence was considered, and the inner diameter was set as 800, 1000 and 1200 mm, respectively. In the tests of groups T1 and T4, the lining wall thickness was taken as a variable and set at 1.79 and 1.61 mm. T1 group was loaded with continuous air extraction, while T5 group immediately opened the inlet sealed with the host pipe after the vacuum degree was slightly lower than the measured value  
Figure 2: Sampling rules. of T1, thus forming an instantaneous pressure difference. The specific loading process is described in the following section. The host pipe is made of TPEP (Three layers Polyethylene Epoxy Resin) anticorrosive steel pipe. Before the test, diameter, initial deformation and local defects of the existing pipe were measured by a Laser Displacement Meter. SSL was rolled from a stainless-steel sheet, and the liner was welded into a thin barrel shell by argon arc welding technology, and the lap joint and interface seal of the ring was completed. It should be pointed out that the research shows that the port constraint effect can be ignored when the length to diameter ratio of the thin-walled cylinder exceeds 6 [26]. Thus, the host pipe length in this test was slightly larger than six-folds of the corresponding pipe diameter. After the lining was welded and placed for 24 h, the lining's initial deformation was measured, and the initial ovality and annular gap were calculated as presented in Table 1.  
In each group of tests, three monitoring sections were arranged at 500 mm from the middle to both sides of the pipe, and marked as \(S-1\), S0 and S + 1, respectively, from left to right. Every 10 degrees was marked for monitoring purposes. Strain changes in SSL during vacuum pumping were measured by sticking strain flowers. The strain gauge used was 120-5 AA, with a sensitivity of 2.0 mv/V. Strain monitoring was only carried out at section S0, and two strain gauges were arranged at each measuring point to monitor transverse and longitudinal strains (Figure 3). Particle Image Velocimetry (PIV) technology was used to measure the deformation and displacement of the lining. The targets were made of wood blocks of 50 mm \(\times\) 30 mm, and a unique pattern was pasted on the front surface to distinguish the monitoring targets. An ACA1920-155 UM industrial camera produced in Basler Vision Technology Co., Ltd., Beijing, China was positioned in the middle of the flange cover at the left section with a resolution of 1369 \(\times\) 1216, a pixel size of 5.86 \(\upmu\)m \(\times\) 5.86 \(\upmu\)m, and a maximum frame rate of 155 fps. Simultaneity of multiple system measurements can be maintained by hardware/software triggers/system clocks. Due to the short buckling process under negative pressure, the image recording rate set in the system during the test was 40 frames per second. Two 400 W floodlights were arranged below the Charge Coupled Device (CCD) camera as the light source. A DJI OSMO Action Buff-Proof 4K HD camera was placed on the same side to record the whole test process. The negative pressure value of the internal cavity of the pipeline was obtained by the AIER high frequency absolute pressure sensor of 0-200 kPa, and its comprehensive accuracy was less than or equal to \(\pm\)0.25% FS. The lining temperature change was monitored by a temperature sensor. The data monitored by the strain, pressure and temperature sensors were all collected by the NI system. The data collection cabinet was NI 9188 and NI 9189. The strain was collected by the NI 9237 input module with an accuracy up to \(\pm\)100 ppm, and the pressure and temperature were collected by the NI 9203 input module with an accuracy of \(\pm\)0.18%.