Strain Hardening Modulus: An Accurate Measure For Slow Crack Growth Behavior Of HDPE Pipe Materials [PE]

Resistance to slow crack growth is an important material property of polyethylene which determines the application lifetime, especially for utility pipe applications. Usually, the slow crack growth resistance of materials is accessed by time consuming testing methods such as NPT, FNCT, PENT, etc. These methods require often the use of notched samples, the use of specific fluids (e.g. detergents) and elevated temperatures. Within SABIC a sophisticated method – tensile strain hardening measurement – was developed requiring only little material (grams) and delivering excellent reliable and accurate results within only a few hours. The SCG behavior of HDPE is related to the strain hardening modulus as, determined from a tensile test at 80 C. The use of a universal test set up allows for an easy and cost effective implementation at all accredited testing laboratories for HDPE pressure pipe applications. Extensive work was dedicated to fine-tuning the measurement conditions and the method was successfully established in several laboratories and institutes. Correlations between traditional methods (e.g. FNCT) and other accelerated tests (e.g. fatigue testing using CRB specimen) show that the same failure mechanism is probed and that the strain hardening modulus can be used to study and rank the expected SCG behavior of HDPE materials. This method is very suitable in the development of new grades, but also very valuable as a batch release test for both resin suppliers and pipe converters. With the huge benefits achieved for the HDPE pipe grades using raw materials, the logical next step is expansion of the benefits of the strain hardening method to other materials and to explore the possibilities of testing finished products.