Research On The Failure Mechanisms Of PE Pipe Electro-fusion Cold And Over Welding Joins

As the international community attaches more importance to the ecologicalenvironment, the widespread use of low carbon and environmentally friendlymaterials becomes a necessary way. Compared to metal pipes used originally,polyethylene pipes have more superiority in both economic and environmentpoints, which is widely used in design and construction of gas, water supply anddrainage pipeline. In order to insure the safety performance of pipelines, thestudy in the connection way of pipes and joints’ performance have attached greatimportance to people.Based on the quality assessment of electro-fusion joins of polyethylenepipes whose welding voltage were different, this article analyzed themicrostructure of the welding joints using X-ray diffractometer and scanningelectron microscopy, discussed the influence of welding voltage on the propertyof electro-fusion joins, sum up the failure mechanism of cold and over weldingelectro-fusion joins.This article chose electro-fusion welding method to welding polyethylenepipes whose nominal diameter is110mm, using five different sets of weldingvoltage while welding time were constant. We assess the quality of the electro-fusion joins through ultrasonic tests and tensile stripping experiment. We can find:cold welding joints didn’t fused completely, the ultrasonic echo came from thesleeve inside surface, and joins brittle fracture in tensile stripping experiment.The ultrasonic echo of over welding joints came from the pipe inside surfacebecause resistance wires ware dislocated, the percentage of friable peel was small,the largest is only11.71%. However the resistance wires in the join which waswelding by reasonable parameter were distributed uniformly, the joins fracture atPE pipe in tensile stripping experiment, and the welding seam didn’t have brittle fracture, so this carrying capacity was the strongest.We use X-ray diffractometer to test the powder which were obtained fromweld fracture, the crystallinity of the electro-fusion joins whose welding voltageis39.5V is the highest, it can reach to75.87%, besides the crystallization degreeof the other welding joints is between the base material and sleeve, thus we canconclude that suitable welding parameters can make the polyethylene fullycrystallized. We observed the size and distribution of the craze to the weldingfracture section through the scanning electron microscopy, the result shows thatthe amount of the cold welding joints’ craze is low and the size of the craze fiberis small. As for the over welding joints，crazes distribute widely but the size isnot uniform and the amount of transverse fiber is small. The electro-fusion joinswhose weld voltage is39.5V had good performance, because the penetration andtangles of the molecular chain was fully, the widespread craze fiber effectivelyrestrains crack propagation.