Scratches in polyethylene pipes - Sensitivity to scratching and influence on lifetime [PE]

Over the last years, the question has been raised how scratches and indentations effects the strength and lifetime of pressurised pipes. A method for testing the pipe surface’s sensitivity to this kind of damage was devised, and the scratch sensitivity of different polyethylene grades and coating materials was evaluated. Furthermore, the influence of scratches on the technical lifetime of polyethylene pipes was evaluated, and criteria for maximum permissible depths of such damages were developed. There were no obvious differences in scratch sensitivity between different tested pipe surfaces. All tested HDPE materials showed more or less the same linear relationship between actual measured scratch depth and applied scratching force. The same relationship held also for a pipe coated with a nominally harder polypropylene layer. A slightly more pronounced scratch sensitivity was seen for MDPE, and even more so for a pipe with a thick, soft poly-ethylene foam coating. Extensive pressure tests showed that pipes made from modern bimodal materials have a significantly better resistance to surface scratches than pipes manufactured from older materials. Bimodal materials can withstand scratches of up to 10 % of the pipe wall thickness without reduction in their rated pressure. On the other hand, it may be necessary to reduce the permissible pressure of pipes made from older materials even when scratched to less than 10 %. However, a substantial improvement in the strength of the older material can be achieved by planing off the scratch.