Ashish M. Sukhadia, Mark J. Lamborn, Pam L. Maeger
Papers # 2014 Chicago
Over the past several years, we have conducted extensive investigations of the Rapid Crack Propagation (RCP) behavior of Polyethylene (PE) pipes. While the typical data of interest is the Critical Pressure (Pc) and/or Critical Temperature (Tc), we chose to take our investigations further by exploring and generating the full Pc – Tc maps of three PE resins with differing polymer architectures. This fuller picture of the RCP behavior then revealed some interesting behaviors. We also report on some new observations relating the S4,Pc and S4,Tc data. Lastly, we show results that highlight the effects of Residual Stresses on RCP performance of PE pipes.
The rapid crack propagation (RCP) resistance of polyethylene (PE) pipes is an important consideration in the design of pipes for use in sustained pressure applications. New applications calling for the use of PE pipes at colder temperatures, higher pressures, and larger diameters strengthen the importance of understanding the RCP behavior of these systems. To date, PE pipes exhibit an outstanding record of evading RCP failures. Despite the fact that RCP failures in PE pipe are rare, or perhaps even nonexistent, it remains important for resin and pipe manufacturers to understand the factors affecting RCP performance, to ensure the continued production of PE pipe with excellent RCP resistance. In 2004, Chevron Phillips Chemical (CPChem) acquired the ability to measure RCP performance of PE pipes with the purchase, installation and commissioning of a Small-Scale Steady-State (S4) test unit. Since this time, ongoing research in the area of high performance PE resins used in pipe systems has provided the opportunity to study the RCP performance of a large number of PE resins and pipes. Since 2004, CPChem has performed well in excess of 250 S4 Critical Pressure (S4,Pc) and S4 Critical Temperature (S4,Tc) tests in efforts to characterize and better understand the RCP performance of PE pipe systems. It is perhaps noteworthy that the large body of PE pipe resins tested by us represents a mix of both experimental and commercial resins, having widely varying unimodal or bimodal architecture and of different classes (PE80 vs. PE100). Furthermore, this database also represents a mix of pipes fabricated under both laboratory and commercial extrusion conditions. It is the purpose of this report is to share some of the key findings from this work, with an emphasis on S4,Pc and S4,Tc behaviors and relationships.
Two standard test methods are available to measure the RCP performance of PE pipe systems; specifically, the Full Scale (FS)  and Small-Scale Steady-State (S4)  tests. The FS test acts as the primary or reference method for characterizing the RCP performance of pipe systems. The FS test, however, is very difficult to perform, is very costly and only very few laboratories around the world offer this test. By comparison, the S4 test setup has a much smaller footprint making it suitable for general laboratory environments, is relatively easier to run compared to the FS test, is lower cost and for all these reasons is a test that is offered by several laboratories around the world. A good overview of the two tests may be found in Greig et al.