| Since the introduction of polyethylene (PE) pipe for use in natural gas distribution about twenty years ago, nearly one billion feet of various grades have been installed in North America. Now more than 80% of all pipe installed for natural gas distribution is made of PE, and this percentage is increasing annually. The material now used in the natural gas industry is a PE copolymer made with ethylene and an {dollar}alpha{dollar}-olefin, generally either a hexene or octene, which causes side branches of 4 or 6 carbons respectively. The amount of branches is kept fairly low, somewhere in the range of 3-10 branches per 1000 backbone carbons, such that the resultant density is between 0.926 and.940 gm/cc.; Due to the relatively new entry of PE material in the gas industry, only limited information on the long term failure behavior of PE pipe is currently available. Better understanding of the failure process in commercial piping materials was the purpose of this research with the long range goal of being able to rank various pipe resins as to their expected lifetime by use of a theoretically and experimentally sound accelerated test. Three major areas of research have been studied and are covered in this thesis individually in the chapters: (1) crack growth in commercial gas pipe resins and how molecular mobility affects the process of crack growth, (2) characterization of processing induced microstructure and its effect on failure mechanisms and (3) clarification of the effects of environmental stress cracking (ESC) agents on the deformation process and crack growth of various commercial PE resins. The results of this experimental work were used in simultaneous theoretical studies by Dr. Fuh-Gwo Yuan and Professor Su Su Wang investigating the basic mechanisms and mechanics of flaw initiation and growth behavior in PE piping materials.; The fundamental mechanisms of deformation and failure, including crack growth, crack initiation and creep were examined, both with and without an environmental stress cracking (ESC) agent present. X-ray and electron microscopy studies (SEM and TEM) were conducted to examine the structure of commercial gas pipe resins (Marlex 400 and 8600) and production pipe samples. Molecular mobility was characterized by dynamic mechanical spectroscopy and by small angle x-ray scattering measurements of change in long period with annealing time and temperature. Results of these studies suggest that present commercial gas pipe resins are superior to linear or low density PE in the areas of toughness and resistance to ESC agents. Greatly reduced molecular mobility is believed to be partially responsible for the increased resistance to ESC and slow crack growth of these gas pipe resins. |
