Slow crack growth and failure induced by manufacturing defects in HDPE-tubes

Medium-diameter pipelines produced from High Density Polyethylene are an economic and generally reliable solution for the transportation of potable water in large infrastructure works. However, in the present case, widespread fracture occurred after a few months of operation. Semi-elliptical cracks ran out from multiple initiation points at the inner radius of tubes, without piercing the outer surface, instead connecting sideward by absorbing other crack initiation zones. Longitudinal crack extension was followed by sudden crack propagation at the moment of catastrophic failure. Cracks were stopped or deviated at the welds between tube sections. Some information on fracture toughness was inferred from compact tensile specimens, complemented by direct observation of the crack surface. The latter indicated excessive brittleness of the tube material, accelerating the process of slow crack growth at low stress intensity. Numerous extrusion defects were found to be responsible for crack initiation; an excessive amount of recycled resin may have increased crack propagation velocity.