Effects of defect size on failure of butt fusion welded MDPE pipe under tension

Since medium density polyethylene (MDPE) pipes are increasingly used in the gas industry, welding defects are becoming a safety concern. In this study, butt fusion welded MDPE pipe joints with spherical and planar defects of various sizes were studied by experimental tensile tests and finite element analysis. These defects were considered for simulating lack of bonding during the welding. The tensile test results showed that pipe strength was not reduced by defects that were up to 15% of the pipe's thickness in size. On the other hand, when the size of the defect was increased to 30% and 45% of the thickness, the failure strain decreased considerably. Finite element analysis verified that the effects of steel defects inserted intentionally before welding are comparable to those of air defects, which are close to actual weld defect conditions. Hence, the experimental results obtained in this study can represent the actual situation. It is argued that a single welding defect with a maximum size smaller than 15% of the pipe's thickness can be accepted without compromising the structural integrity of the pipe during short term of usage not considering long term creep or slow crack growth.