The butt-fusion process is an important method for connecting PE pipes, especially when the pipes are of large diameter and of thicker section. Since many different environmental factors can cause defects and flaws, the joints become the weak point that should be inspected before the pipes begin service. Especially cold welding caused by insufficient input of heat energy, which can not be detected by nondestructive test methods currently available, is one of the most dangerous defects in the butt-fusion joints of PE pipes. It has been discovered that there are three Eigen-lines in the butt-fusion joint of PE pipes by using the phased array ultrasonic focusing technique. Further research shows that the upper and lower Eigen-line indicates the solid-liquid interface and the middle Eigen-line indicates the initial butt interface. Based on a lot of experiments, it has been discovered that the distance between the upper and lower Eigen-line, is related with the input of heat energy during welding. So a new method called “Eigen-line method” is developed to assess cold welding by measuring the distance between the upper and lower Eigen-line. A set of experiments show that the method can detect cold welding in butt-fusion joints with the accuracy less than 6%, which can meet the testing requirement for welding joints in engineering.

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