Formation Mechanism, Inspection And Safety Assessment Of Electrofusion Joint Containing Cold Welding Defect

The modes of transportation in the world are steping forward in the direction of high speed, large-scale and professionality. As one of the five modes of transportation, the pipeline is the main transportator of key energy media such as oil and natural gas. These pipelines usually play a very important role in the development and stability of national economy. As a result, they are regarded as "lifeline" of national economy.Polyethylene (PE) pipe has been found wide application in gas and water distribution since it was initially introduced to the transport of natural gas in the 1960s. It has satisfactory performances including corrosion resistance, resistance to the effects of gas constituents, ease of installation and cost-effectiveness. It is one of the most recommended products used in the field of building materials in National Long-term Development Planning. The joints between pipes are weak links related to the safety of the whole pipeline system. In order to promote the development of PE pipe industry, it is crucial to study the mechanical performance of the welding joint and then propose a reasonable regulation for the installation, operation and maintenance of PE pipeline system.Electrofusion welding is the most widely used welding method of connecting plastic pipes or plastic composite pipes. In this thesis, the method of defect classification and the corresponding failure mode of electrofusion joints are systematically studied. The cold welding defect, which is the most common defect, is systematically studied in respects of forming reason, inspection method and safety assessment method. The main tasks of this work are listed below:(1) The defects in EF joints were classified into four categories:poor fusion interface, over welding, voids and structural deformity. The forming reasons of these defects were analyzed in detail. The mechanical properties of EF joint containing these defects are investigated by conducting tensile tests, burst tests, peeling tests and sustained hydraulic pressure tests. The results showed that there are three main failure modes of EF joint under inner pressure, i.e., cracking through the fusion interface, cracking through the fitting, and cracking through the copper wire interface. (2) By comprehensively taking the temperature-dependent properties of polyethylene and heating wires into consideration, such as thermal conductivity, heat capacity and density, as well as the power input which changes with the wire temperature, a transient axisymmetric heat transfer model was proposed. Experiments were conducted in order to verify the proposed model. The interface temperature between outer surface of pipe and fitting bore was measured by well-installed thermocouples, and the total input energy and real-time power were recorded by the welding machine. The experimental results were found good match with the numerical results calculated from the proposed model.(3) A method to inspect the cold welding defect in electrofusion joint was proposed for the first time, based on a newly discovered Eigen-line, which exists close to the copper wire in the ultrasonic phased array image. To understand when, where and how the Eigen-line would appear, the forming mechanism was investigated in detail. Three factors may cause the appearance of the Eigen-line, i.e., micro-air-bubbles, difference of acoustic impendence of PE in the melted and un-melted region and small-crowded crystallites in the region of the interface of melted and un-melted region. It was found that the number of Eigen-lines was the same as that of welding times when re-welded with decreasing welding time, but only one Eigen-line could be observed when re-welded with increasing welding time. The result showed that small-crowded crystals in the region of the interface of melted and un-melted region may be the dominating factor. This was then verified by the microscope and differential scanning calorimeter (DSC) tests, and the forming process of the small-crowded crystals was discussed in detail. By applying the proposed temperature model, the moving regulation of Eigen-line was predicted and then verified by experimental measurement.(4) Based on the deformation process and phenomenological theory, two methods to calculate the peeling energy in the tensile-peeling tests were proposed. The electrofusion joints with different degree of cold welding were deliberately made and then used to conduct tensile-peeling tests. On the basis of theoretical analysis and experimental results, the reason of each stage of strength formation during electrofusion process, was studied and discussed in detail. A method to obtain the minimum required welding time of electrofusion process was proposed based on statistical analysis. By combining the method to inspect cold welding defect, a method of evaluating the quality of electrofusion joint was proposed.We would like to acknowledge the financial support from the National Key Technology R&D Program of "Eleventh Five-Year Planning Program" (Project No. 2006BAK02B01), National Key Technology R&D Program of "Twelfth Five-Year Planning Program" (Project No.:2011BAK06B01) and key project of national High Technology research and Development Program (Project No.:2009AA044801).