Development of techniques for lifetime prediction of Polyethylene resins for the water and gas industry

Abstract

Being able to accurately predict the lifetime of polyolefins is very important in industry for water and gas distribution systems. Having reliable knowledge of the lifespan and maintenance requirements of such networks is crucial to be able to schedule maintenance and avoid emergency repairs which are very costly and disruptive especially when pipes are run under busy streets of towns, cities and other thoroughfares in highly populated areas. There is much research into improving the reliability and longevity of polyolefins. To do this, there needs to be an understanding of the failure mechanisms brought about by ageing. Newer resins are increasingly resistant to onset of brittle failure and as such current test methods are struggling to keep up with their assessment. Therefore, more discriminating laboratory experiments are required to measure the improved resistance of brittle fracture of these newer resins. When investigating Slow Crack Growth (SCG) in polyethylene (PE) it is important to understand how a crack is initiated and the mechanisms by which it propagates. Several of the main tensile, constant stress tests that are currently used to evaluate polymer life expectancies that have been proven by many researchers of SCG over the years are described, as well as several notching methods associated with the tensile tests. For the new experimental procedure the research partner required the following conditions:  To be a tension test with plane strain stress conditions,  For the sample to be in a bath at elevated temperature and with an accelerating agent (Arkopal N100).  Failure times to be faster than the current setup using the Full Notch Creep Test (FNCT) and the samples used must have certain dimensions.  The evaluative procedure to be able to simply rank PE grades by failure time and then if possible correlate the failure times to life expectancy. For development of the new experimental procedures the starting point was the standard FNCT which has a square ligament cross section. By modifying ligament cross section, applied stress, notch depth, notch angle and notch geometry a more discriminating experimental procedure was developed which reduced the time to failure by roughly half when compared to the FNCT. This failure time can then be correlated to an approximation of the pipes ESCR, i.e. its lifetime whist in use. Results clearly demonstrated the transition for ductile to brittle behaviour with ageing of the polymer resins. A valuable acceleration of the failure process has been achieved which allows an increased number of assessments to be made on new resins. Failure processes have been observed optically and by Scanning Electron Microscope (SEM) to identify the micro-mechanisms of fracture.