Low Crystallinity Molten Zone Inducing Tensile Ductility Reduction in Polyethylene Pipe Butt Fusion Joints

Abstract The cold welding defects in polyethylene pipe butt fusion joints led to a degradation of their mechanical properties. However, the underlying mechanism by which cold welding defects affected the mechanical behavior of the butt fusion joints remained unclear, hindering the development of mature non-destructive testing methods for such defects. In this study, microstructural characterization and uniaxial tensile tests were conducted on polyethylene pipe butt fusion joints with varying degrees of cold welding. The interrelationships among heat soak time during the butt fusion process, tensile mechanical behavior, and crystallinity of the butt fusion joints were analyzed. As the heat soak time diminished, the bead-up dimension within the butt fusion joint progressively decreased due to a reduction in the volume of molten polyethylene extruded into the fusion bead, while the molten zone width increased. Simultaneously, with the escalating severity of cold welding, there was a pronounced decline in the total elongation at high tensile rate, accompanied by a corresponding decrease in tensile strength at lower tensile rate. Through the utilization of X-ray diffraction to evaluate the crystallinity of both the molten zone and the parent material, coupled with industrial computed tomography for investigating the morphological characteristics of the tensile fracture regions, it was established that the substantial reduction in crystallinity within the molten zone, relative to the parent material, was responsible for the diminished tensile ductility observed in the polyethylene pipe butt fusion joints. This study provided theoretical guidance for the selection of non-destructive testing methods for cold welding defects in polyethylene pipe butt fusion joints, with crystallinity emerging as a potential breakthrough to address the non-destructive testing challenges.