A Tensile Strain Hardening Test Indicator of Environmental Stress Cracking Resistance

Environmental stress cracking resistance (ESCR) is an important indicator of performance for high density polyethylene (HDPE) in structural and polymer pipe applications. The commonly used test for determining ESCR of HDPE can be time consuming and rather imprecise. A tensile strain hardening test was recently proposed to offer a faster way to characterize ESCR of polyethylene. In this paper, a practical approach is adopted whereby the test is extended to room temperature and shown to relate reliably to the ESCR of HDPE. Several HDPE resins (including pipe‐grade resins) are analyzed at strain rates of 0.5 mm/min and 7 mm/min to compare the effect of strain rate. Comparisons between the conventional ESCR test method and the strain hardening test show that strain hardening can be used to rank ESCR of HDPE in a reliable fashion. In our study the more direct measure of “hardening stiffness” is used to compare resins instead of strain hardening modulus. Because no true stress‐strain measurement is needed, this is a much simpler test method than other methods previously suggested. In addition, the use of the natural drawing ratio (NDR) as ESCR ranking indicator is examined. Results show that NDR can also be employed as a strain rate‐independent indicator of ESCR of HDPE. The test proposed herein is practical, simple and precise, and hence a more reliable indicator of ESCR performance of HDPE.

Keywords:

• environmental stress cracking resistance
• strain hardening test
• high density polyethylene
• natural drawing ratio
• tensile test
• polyethylene resin comparison

6 Acknowledgments

The authors would like to thank the Natural Sciences and Engineering Research Council (NSERC) of Canada, and the Canada Research Chair (CRC) program of NSERC, for financial support. Many thanks go to Ronald Cooke, Blair Graham and Jeff Nicholas, of Exxon Mobil Chemical Canada, for useful discussions. In addition, the authors are grateful to ExxonMobil Chemical Canada (Imperial Oil Canada), Nova Chemicals (Canada) Ltd., and Repsol YPF, Spain, for supplying resins.