Ultrasonic and radiographic NDT of butt fusion welds in polyethylene pipe [PE]

The use of polyethylene in the gas, water and chemical process industries has increased dramatically over the past two decades. This is largely due to the impetus provided by the British and American gas industries who have selected polyethylene as a material to replace metallic distribution pipelines. Apart from distribution pipelines, polyethylene is also used for insertion repairs on leaking cast iron mains. 

The popularity of plastic piping can be attributed to its lightness, flexibility and good corrosion resistance, as well as the ease with which it can be joined. Pipes with a diameter <250mm are normally joined using an electrofusion process. For areas where the application is critical or the pipes are of larger diameter (>250mm) and of thicker section, the hot plate butt fusion process is preferred. Both of these heat fusion processes are capable of producing a joint with mechanical properties approximately equivalent to those of the parent material (1). 

The quality of butt fusion joints in polyethylene pipe systems is primarily governed by control of the process parameters during welding. However, as new polyethylene materials are introduced (e.g. PE100) and increased demands are placed on existing polyethylene materials, there is an additional need to monitor quality through reliable non-destructive testing (NDT) methods (2,3). At present there are no nationally accepted standards for the NDT of welds in polyethylene. The most widely used technique is a visual inspection of the outer weld bead. This method is sensitive to those embedded flaws which manifest themselves in the weld bead and to gross flaws, such as pipe misalignment. Attempts have been made to use more quantitative methods of weld examination. For example, X-ray radiography has been used to inspect butt welds in large diameter pipe (4) and an automated ultrasonic inspection tool exists for inspecting welds in smaller diameter (50-100mm) gas distribution pipeline (5). At one stage a guideline standard (ASTM F600-78) for the manual ultrasonic inspection of butt fusion welds in polyethyiene pipe was inrroduced. Unfortunarely, the results obtained using this standard were so heavily dependent upon the skill of the operator that it was withdrawn in 1991. 

This paper summarises work at TWI on NDT of butt fusion welds in polyethylene and compares this work with results from other published literature on this topic. The major objectives of this study were to develop ultrasonic and radiographic testing procedures for the inspection of butt fusion welds in polyethylene pipe systems. The procedures were developed on pipes with diameters greater than l8Omm and thicknesses greater than llmm. The underlying techniques, however, can be used on testpieces other than pipes, and on a range of different polyethylene types and thicknesses.