Temperature-Dependent Microwave Responses of Polyethylene Pipe Butt-Fusion Joints

Welding temperature affects the structure and performance of polyethylene (PE) butt-fusion joints. PE100 DN200 SDR11 joints were prepared at six temperatures (150–300 °C), with pressure, heating time, and changeover time held constant. Microwave line-scan signals over 6–14 GHz were acquired, and the normalized weld-to-background amplitude contrast (NWBCA), the complex spectral difference root mean square (CSD-RMS), and the background-corrected CSD-RMS (BG-CSD-RMS) were calculated. Macroscopic bead measurements, scanning electron microscopy, and tensile fracture observations provided complementary evidence. Relative to the 230 °C reference, both complex spectral metrics increased toward the low- and high-temperature conditions. NWBCA rose from 0.037 at 150 °C to 0.143 at 230 °C and fell to 0.094 at 300 °C. The 150 °C specimens fractured in a brittle manner in the weld, whereas the 300 °C specimens fractured in a ductile manner in the heat-affected region. Similar spectral deviations therefore accompanied different fracture behaviors. Overall, the three metrics described temperature-dependent joint responses within this specimen series and could serve as a basis for screening deviations from the nominal welding-temperature window.