Ultrasonic phased array inspection of electrofusion joints in polyethylene pipes

The use of plastic as a pipe material for the transport of gas and water has increased over the last few years, attributed to the significant advantages of this material such as corrosion resistance, strengthto-weight ratio, lightness, flexibility and cost. Consequently, these pipes have a long predicted service life. One of the pipes’ joining processes is electrofusion welding. As for all kinds of welding process, welding defects can occur due to poor welding practices, in turn affecting the quality of the welded joint. From previously published work and discussion with the water, oil and gas industries, it was clear that both voids and lack of fusion needed to be detected with a rapid simple inspection. For these reasons, an effective non-destructive technique is required to reduce the risk to the environment through leakage of dangerous chemical from PE pipe system. This paper is based on an EC project ‘Polytec’(1), (2) and TWI exploratory work for the in-manufacture quality control of polyethylene electrofusion joints. Polytec is an EC collaborative project lead by TWI with co-industrial partners: Vermont, Isotest, NDT Consultants, Horton Levi, Hessel Ingenieurtechnik, Simplast SpA, Catania Ricerche, Northumbrian Water, Italgaz, North West Water, Egeplast. The object of the project was to develop an online method for determining the quality of electrofusion joints and their fitness for purpose. The industrial interest in Polytec guided the project towards the essential requirements of the inspection methodologies. Two NDT techniques were developed in this project to detect, locate and identify any flaws present. At the same time, a correlation between manufacturing flaws and mechanical properties of the joint has been studied. This paper concentrates on the ultrasonic phased array method developed for the inspection of the electrofusion joints. The technique described in this paper can detect conventional defects and supplies a method for determining whether the weld has been correctly heated by analysing the position of the Heat Affected Zone (HAZ).