Guided Waves for the Detection of Defects in Welds in Plastic Pipes

In recent years, a joining method known as electrofusion welding has been introduced into the water industry to join together plastic pipes. The welding procedure is fairly straightforward: the ends of two pipes are inserted into a coupler and an electric current is supplied to a coil moulded within the coupler. The heat generated in the coils acts as a source of the energy needed to fuse the coupler and pipes together. A graphical representation of a joint is shown in Fig. 1. As the welding is performed on-site, often in unforgiving conditions, it is inevitable that some joints will be defective. The result is that the life of the joint can be greatly reduced. It is therefore important that defective welds are identified as soon as possible (i.e. immediately after welding). For such a problem, nondestructive evaluation could offer a solution. Probably the most obvious technique to use is normal incidence C-scanning. However, this technique tends to be time consuming as virtually the entire area covered by the coupler has to be scanned. An alternative technique which is potentially much quicker is via the use of Lamb waves. The potential of Lamb waves in NDE has been demonstrated by various researchers [1–3]. The original theory was developed for plate structures [4], but in more recent times has been extended to cylindrical bodies such as pipes [5–8]. Lamb waves have also been investigated with regards to defect detection [see for example 3, 9–12]