Plastic Pipes Conference Association # 2023 Lake Buena Vista
Roland Valk
To gain an insight into the residual quality of first-generation polyethylene (PE) pipes used in the Dutch gas distribution grid, pipe segments from all over the Netherlands were excavated by the Dutch Distribution System Operators (DSOs) and tested by Kiwa Technology. The residual quality was determined by performing the Pennsylvania Edge Notch Tensile (PENT) test in accordance with ASTM F1473/ISO 16241. The test specimens were taken directly from the pipe segments. However, this test is not practical for the latest generation of polyethylene grades (including PE 100-RC and PE 4710 PLUS), as the testing time would be impractically long [1]. The Cyclic Round Bar (CRB) test method carried out in accordance with ISO 18489 can solve this problem. A correlation with the PENT test has also been shown [2]. The wall thickness of many pipes is too thin to produce suitable test specimens. In this situation, the Cyclic PENT (CPENT) test is a practical alternative. This test method combines the PENT specimen shape with the cyclic loading used in the CRB test. This method was introduced by Deakin University in Australia in 2018 [3] [4].
This study compared the PENT test with the CPENT test. Multiple (old) pipes were tested using the PENT test and CPENT test. The failure time for the PENT test was then compared to the cycles to failure for the CPENT test.
Pipe material was also artificially aged using hydrostatic pressure at an elevated temperature and subsequently tested using the PENT and CPENT tests. The failure time and cycles to failure decreased due to artificial aging (hydrostatic pressure at 3 MPa and 80 °C for 800 hours). This confirms that both tests can be used to assess the reduction in residual quality.
Microscopic images of the fracture surfaces were taken to allow the results to be evaluated in greater detail. Optical microscopy was used to distinguish different zones, including a surface cut with a knife, which was the initial crack from the notching, a smooth region, which represented the slow crack growth (SCG) during the test, and a rough zone where the final (rapid) fracture occurred. The stress increased as the surface area decreased during the test. This resulted in a ductile fracture. These zones could be found on the specimens from both the PENT and CPENT tests. The elongation of the test specimens during both the CPENT tests was also studied.
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