Development of an accelerated test method to determine the stress crack resistance of high-density polyethylene

Abstract

The Notched Constant Ligament Stress (NCLS) test, ASTM F2136, (1) is a test method that measures the slow crack growth (SCG) in High Density Polyethylene (HDPE). The NCLS test requires a notch on all test specimens to act as a stress concentration at that point so crack propagation can be easily predicted and studied. In addition to the notch, the NCLS test requires a test bath solution of “...10 % non-ylphenoxy poly (ethyleneoxy) ethanol by volume in 90 % deionized water” (1) to be heated and held at 50 deg. C during the test procedure. The detergent present in the aqueous mixture, when held at an elevated temperature, acts as a crack accelerant, leading to relatively short test times (<1000 hours). The shortened test times are beneficial to pipe manufacturers conducting quality assurance checks on their products. There have been ongoing investigations into developing a cyclic fatigue test to emulate the results of the NCLS test in less time. This test method could be developed into an additional quality control check to more quickly assess the stress crack resistance (SCR) of HDPE so that pipe manufacturers can more efficiently develop and qualify various materials. The primary goal of this research is to develop a new accelerated test method for assessing the SCR of HDPE materials correlating with the NCLS test that have considerably shorter failure times, making it more useful for a quality control test. The research investigates a new dynamic accelerated fracture test being conducted on notched specimens similar to those used in the NCLS test. A secondary goal of this research is to produce similar results to the current NCLS test method using only deionized water as the test bath solution, and thereby propose an additional test method to be included in the current NCLS test standard. The absence of the detergent allows for prediction of service life at ambient temperatures. This research indicates a correlation between the current detergent-containing NCLS test method and the proposed additional alternatives, therefore substantiating the possibility of using a purely deionized water bath as an alternative testing option during NCLS stress crack testing. A positive linear relationship can be observed between NCLS hours to failure and Dynamic Accelerated Fracture (DAF) minutes to failure. Images taken of the fracture surfaces of the DAF specimens show brittle cracking, the main failure mechanism of Slow Crack Growth exhibited in traditional SCR creep tests such as the NCLS test. The Dynamic Accelerated Fracture test shows strong potential to be used as a quality control test in conjunction with the NCLS test to determine SCR of HDPE. The heated deionized water bath shows strong potential for the NCLS deionized water test bath to be a substitute for the current Igepal detergent test bath. With additional testing, the test parameters could be further refined to achieve optimal test conditions.