Dr. Love Pallon, Dr. Karin Jacobsson, Daniel Ejdeholm, Marie Allvar, Rikard Kärrbrant
Papers # 2018 Las-Vegas
This work presents the development of the Full Length Tensile Test (FLTT), a method that enables an objective judgement of welding quality of electrofusion welds by measuring the energy to break (J/mm2 ). In this way a brittle, ductile and transitional mode failure can be differentiated without subjective visual inspection.
A large number of leaks in the joints between large diameter pipes have occurred in the Swedish distribution network after only a few years in service. The problems have mostly happened in cases were the pipes have been joined by electrofusion. One of the actions taken to improve the quality is that the amount of procedure tests on welded PEpipes has increased. A procedure test is performed before the installation starts and a passed test indicates that products, welders, equipment, and procedure works and quality welds are produced. Following the increase in numbers of tests, the test methods are being investigated, as it is not uncommon that they produce misleading results. For electrofusion welds the standardized testing methods used today are all based on peeling apart the fitting from the pipe and visually inspecting the fracture surface. The weld approval is based on a maximum percentage of brittle fracture. The most notable disadvantage with this evaluation technique is that the assessment is subjective and that the distinction between ductile and brittle can be somewhat unclear. Therefore Swerea KIMAB together with 4S Ledningsnät develops a new testing method for electrofusion welds that offers an easy and objective evaluation of the quality.
The procedure for the new Full Length Tensile Test (FLTT) is to take specimens from the welded joint and pull them apart using a fixture designed to reduce the risk of deviating fractures. The sample is pulled apart perpendicular to the weld and the resulting force-elongation curves can then be used to calculate the fracture energy of the weld. The initial study of the fracture energy correlated very well to the ductility of the weld. With a quantified parameter as a criterion for approval the assessment is significantly simplified and no longer subjective and dependent on who conducts the test.