Bergstroem, Nilsson, Lindqvist
# 2004 Milan
When making creep tensile tests on butt fusion joints in PE pipes almost all failures emerge from the outside of the pipe. Results indicate an influence from the outside surface on the long term behaviour of butt fusion joints. It is a well established practice to scrape the outside surface of PE pipes when making electro-fusion joints. This study stress the question if scraping or bead removal is a positive thing also when making butt fusion joints. Using PE100 pipes butt fusion joints were made using different levels of surface scraping before the butt fusion and with and without removing the weld beads before testing. PE-pipes with an outside PP or PE-skin were also included with and without the skin. Tensile test pieces were cut parallel to the pipe axis centered over the butt fusion joint. The long term strength evaluation were made using a standard accelerated test procedure with the test pieces exposed to a 5 MPa tensile loading immersed in 80 °C water with 2 % Arkopal N100 surfactant. With outside weld beads left intact failure times 200 h to 400 h are normal. Leaving the inner beads but removing the outer beads increase failure times to 1000 h to 2000 h. Failure times for test pieces without weld beads approach 10000 h. Scraping of the pipe surface to a depth of 0.2 mm did not substantially change the failure-time above that for an unscraped surface. Removing 0.7 mm from the pipe surfaceincrease failure times to 2000 h to 3000 h. Outside weld beads and outside pipe surface clearly interact to lower the time to failure in accelerated creep tensile tests. Weld bead removal and scraping of outside pipe surface significantly increase failure times. The exact mechanism is not clear and the implication on butt fusion joint performance in service is not demonstrated but the results indicate possible ways to enhance safety margins when making butt fusion joints. The topic will be given further attention in coming studies.
Author(s) : Janson, Bergstroem, Baeckman, Blomster
When performing strength quality control of PE pressure pipes made of new grades it is important to take the creep vs. time into consideration and not only the common stress vs.time behaviour. Thus, it is just the relation between linear strain and logarithmic time, which will within a reasonable testing time help us...