Research of PE Pipe's SCG Characterization under Varying Temperature Conditions

Gas utilities are required to accurately understand the lifetime characteristics of gas polyethylene (PE) pipe in detail. Understanding the effects of varying temperature conditions is one of our goals, because the actual environmental temperature of PE gas pipe is not constant but varies. This is particularly true for exposed (non-buried) PE pipe. To determine the expected lifetime of PE pipes under varying temperature conditions, the cumulative damage calculation method (Miner’s rule) can be used according to ISO 13760. But although Miner’s rule is a well-known method, there have been few studies that experimentally examined its consistency for thermoplastic pipe. One such study was Studvik’s long-term internal hydrostatic pressure test with temperature cycling. They stated that Miner’s rule is valid, but more work is needed for absolute confirmation. We therefore started to examine the possibility of applying Miner’s rule to the characterization of PE’s slow crack growth (SCG). As the traditional internal hydrostatic pressure test is time-consuming and needs much space, we used the Full Notch Creep Test (FNCT), which is currently standardized in ISO 16770. We developed a unique FNCT testing device with not only an electric heater but also a cooler for the water chamber. Temperature of water chamber, which PE test samples were immersed into, was program-controlled in cycles. By measuring the failure time at different stress levels, the SCG behaviors of PE were drawn.