Ultrasonic Testing And Safety Assessment Of Defects In Electro-fusion Joint Of Polyethylene Fuel Gas Pipe

Electro-fusion(EF) is widely used in joint of Polyethylene(PE) fuel gas pipe, of which quality is very important for piping system. However, there is no non-destructive testing(NDT) method for EF joint, and no safety assessment system for EF joint of PE pipe. These problems must be solved or the safety usage of PE gas pipeline can never be ensured.Funded by the National Science and Technology Support Project (2006BAK02B01-03), researches are performed on ultrasonic testing(UT) method and safety assessment of EF joints with and without defects. The main research results are as follows:(1) Research on defects classification of EF joint. After dissecting a great mount of EF joints with typical defects, the defects are classified and named as lack of fusion, holes, wire dislocation and cold welding for the first time, of which the cause of formation are analyzed and corresponding preventive methods are brought forward as well.(2) Research on UT using phased arrays for EF joint. The result of theoratical analysis shows UT has a good testing ability for defects of for lack of fusion, holes and wire dislocation. A great number of EF joints with different defects were prepared and tested using phased arrays as well as B-scan real time image technology, of which results shows the method detectable rate can reach 100% when defects≥1.0mm above wires or between wire signal, or≥ 2.0mm below wire signal; the method quantification and localization accuracy can reach 0.5mm for lack of fusion, holes above wires and wire dislocation, or 1.0mm for holes below wires.(3) Research on UT method for cold welding of EF joint. Based on contrast tests, an "cold welding eigen line" that formed by air bubbles is discoved, which indicates the interface of solid PE and liquid PE during welding. As testing result shows the location of "cold welding eigen line" changes regularly with the change of welding time or power, a brand new testing method called "colding welding eigen line method" is brought forward to test and characterize cold welding.(4) Research using experiments on mechanical properties of EF joints with and without defects. As shown in the experimental results, defects of lack of fusion with size≤60%L(L, fusion zone length,mm;) has little influence on joint failure mode of tensile and short-time bursting, joints always necking at the pipe in or near the outside cold fusion zone in tensile testing while always have ductile ruptures on pipes in short-time bursting testing; peeling energy of joint increases with the increase of welding energy(increase of welding time or power) while too big welding energy may cause resin degradation; the bigger the defect is, the lower peeling energy the joint has; when defects size are the same, joint with middle fusion zone defect has the shortest peeling time, the lowest peeling force and peeling energy; cracking in fitting and cracking along fusion interface are the failure modes of EF joint depends on different defects, which are lack of fusion that connected with inside fusion zone and heavy cold welding, respectively; soil in fusion interface, unscratched pipe, and too big gap between pipe and fitting will do great harm to EF joints.(5) Research on safety assessment of defects in EF joint. Based on UT and joint testing, a preliminary characterization and safety assessment method for EF joint defects are established. Lack of fusion that do not connects with inside fusion zone and with X/L(X, size of defect)≤15% is acceptable; with no sharp corners and do not connects with inside fusion zone, single hole with X/L≤10% and multiple holes with X/L≤20% is acceptable; joint with 4.0%≤l_e/D≤5.0%(l_e,vertical distance between wires and cold welding eigen line,mm; D, external diameter of pipe,mm;) is acceptable.