# Research on Defect Detection of Bare Film in Landfills Based on a Temperature Spectrum Model
## 3 Defect Detection Experiment
### Test Analysis of Different Thicknesses of HDPE Film  
Infrared images of 0.15 cm HDPE film at different heating times are shown in Figure 5; t = 0.0 min means that the HDPE film has just cooled by water flow and the overall temperature of the film surface has dropped. Starting from t = 0.5 min, the defect and complete area begin to show temperature difference characteristics. In the initial cooling stage, the direct contact area of the water flow with the HDPE film complete area is greater than the defect area soil layer, resulting in a decrease in the temperature drop rate of the defect area. Therefore, in Figure 5b, the temperature of the defect area is too high. At t = 10.0 min, the temperature in the complete area is considerably higher than the temperature around the defect area. Due to the convection effect of the heat source and the soil layer in the defect area and the thermal conductivity of the soil layer being greater than the HDPE film, the temperature around the defect area is lower. As shown in Figure 5c, after t = 11 min, under the continuous natural heat source, the temperature around the defect area gradually rises and the defect contour in the infrared image continues to decrease, as shown in Figure 5d-f. Interference areas exist below the square defect and between the square defect and linear defects in Figure 5d,e. This is caused by uneven sprinkling leading to slower temperature rise in interference areas; these areas do not exhibit the purple low-temperature characteristics observed at defects under the same time conditions. Figure 4: Damage diagram of HDPE film. Figure 3: Schematic diagram of infrared temperature difference image acquisition for different defects. The heat capacity is saturated in the t = 20-25 min time period; the temperature of the defective area is similar to that of the intact area, and the infrared edge contour gradually disappears, as shown in Figure 5g-i. Infrared images of 0.20 cm HDPE film at different heating times are shown in Figure 6. Compared with Figure 5, it can be seen that the defect profile change time of the two HDPE films with different thicknesses is about the same under the condition of sufficient natural heat source, but the defect contour of the 0.20 cm thick HDPE film is blurred at the same time, as shown in Figure 6c-f. The 0.20 cm thick HDPE film, being relatively thick and having a high heat capacity, continues to absorb heat and increase in temperature at t = 20 min. Additionally, there are minor differences in the heating rates across the film's surface. This causes the defect contours initially formed by temperature differences in the infrared images to gradually blur, as shown in Figure 6g. As the temperature increases, the heat capacity of the HDPE film approaches saturation, leading to a reduction in regional temperature differences. By t = 25 min, the temperature difference between the defective and intact areas becomes similar and the infrared edge contours almost disappear, as shown in Figure 6i. To further analyze the HDPE film defect status, the temperature data of the defect area and the intact area at different times in Figures 5 and 6 were extracted, as shown in Tables 1 and 2, and temperature curves were plotted for analysis. The surface temperature curve of the defective and intact area for Figure 5 is shown in Figure 7 and the surface temperature curve of the defective and intact area for Figure 6 is shown in Figure 8; from Figures 7 and 8, it can be seen that, in the same role of the environment, the two thicknesses of HDPE film defective area and intact area have a similar temperature trend; HDPE film with a thickness of 0.15 cm has a large relative temperature difference value for a period of 10-20 min. It can be seen in Table 1 that, when t = 13 min, the temperature difference  
Figure 5: Infrared timing diagram for 0.15 cm HDPE film: (**a**) t = 0.0 min; (**b**) t = 0.5 min; (**c**) t = 10.0 min; (**d**) t = 11.0 min; (**e**) t = 12  
is the largest, and the observed results are more obvious.