Lever
# 2004 Milan
It has long been established that the primary long-term failure mode of PE fittings is Slow Crack Growth (SCG) from the area of highest stress concentration. This area of highest stress concentration is usually situated at the edge of the fusion joint, be it a conventional socket, sidewall or electrofusion joint. Pipes and fittings are tested for a given pressure rating by subjecting assemblies to pressure tests at 80°C. The hydrostatic approval tests are designed with reference to the Stress Rupture Curves for the given material class as determined by ISO 9080 or ASTM D2837. These curves are generated using data gathered from the testing of standard pipe specimens. The validity of these curves with respect to more complex stress distributions has been questioned. Indeed, taking stress concentrations in pipe/fitting assemblies into account it is apparent that the fittings in the test assemblies are subjected to stresses, in certain areas, which would lead to ductile failure in pipe specimens. How then, can we explain the SCG failures generated in the fittings? Furthermore, can we claim that a fitting that has met the standard test requirements will indeed last as long as pipe at the same pressure and temperature in service? The author has noted that when true stress in assemblies, as measured using Finite Element Methods, is plotted against time to failure via SCG at 80°C, the failure points lie on the SCG failure line extrapolated above the knee of the ISO 9080 stress rupture curve. This result implies that the current testing approach is indeed valid for fittings and that standard bi-directional shift factors or full knowledge of the ISO 9080 regression coefficients for a particular PE material can be used to predict assembly lifetime. Using this concept it is possible to propose a standardized procedure for specifying a PE material capable of withstanding the stress concentrations inherent in fittings and guarantee an acceptable system lifetime for a given application. Furthermore it should be possible to use SCG data generated from fittings to determine the knee in modern materials where no knee has been found using pipe test specimens.
Related papers
Author(s) : Chris Ampfer, Gene Palermo, Ernest & Oren Lever
This paper looks at the origin of the 10% allowable scratch depth rule for polyethylene (PE) pressure pipe developed in 1971 and research that has been performed over the years since its inception to determine the validity of this rule. The 10% rule has been used by natural gas pipeline operators since the 1970’s to...
Author(s) : Ernest Lever
Six different polyethylene materials were tested using Cyclic Pressure Fatigue (CPF) as the accelerating process to correlate the results to conventional long-term hydrostatic test results that were previously developed as part of the material qualification process. The designation of the materials was unknown to the...
Author(s) : Gerry Groen, Ernest Lever, Oren Lever
A design justification for reinforcement of fabricated Polyethylene elbows is presented. The acceptance criteria is that the average stress in the mitered wall of fabricated elbow shall not exceed the hoop stress in the matching pipe. ASTM and ISO standards have established requirements for the geometry, production...
Author(s) : Ernest Lever, Yongming Liu, Yiming Deng
This collaborative program will provide an integrated set of quantitative tools that will provide a structured approach to reducing operational risk in vintage plastic distribution systems susceptible to Slow Crack Growth failures. A novel endoscopic structured light scanning tool will be developed and prototyped for...
Author(s) : Ernest Lever, Karen Crippen
The objective of this project was to develop methods to be used by any plastic testing laboratory to quantify the effects of hydrocarbon permeation on: 1) the fusibility of plastic pipe; 2) the life expectancy of existing fused joints that have been subjected to hydrocarbon permeation; 3) the Hydrostatic Design Basis...
Author(s) : Ernest Lever
The Popelar bidirectional shift factors have been now been used about a quarter century to great effect. The question has been raised as to whether the shift factors are applicable to modern materials. GTI has been using rheological principles and Dynamic Thermo Mechanical Analysis (DTMA) to routinely develop...
Author(s) : Ernest Lever
Aldyl A piping systems are closing in on fifty years of service in North America. Several North American gas utilities are assessing the remaining lifetime expectancy of their Aldyl A piping systems and preparing replacement programs. GTI has been actively involved in multiple research efforts related to Aldyl A...
Author(s) : Karen Crippen, Ernest Lever
DOT/PHMSA held a Government/Industry Pipeline R&D Forum on July 18-19, 2012. Five working groups met to discuss key pipeline technical challenges facing industry and government. The Design/Materials/Welding-Joining & Valves working group identified the effects of hydrocarbon permeation on plastic pipe strength and...
Author(s) : Lever, Palermo, Wright, Oliphant
The piping industry has a need for a well-founded methodology to predict lifetime of pipeline components subject to stress concentration. Another useful tool would be a method to assess the impact of manufacturing processes on the slow crack growth (SCG) properties of a resin that can be used for process control in a...
Members of the Association
