Papers # 2018 Las-Vegas
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 investigators. A good correlation was found and several positive and negative features of the proposed test method were identified.
The Plastic Pipe Institute (PPI) Hydrostatic Stress Board (HSB) commissioned the Gas Technology Institute (GTI) to perform Phase II of a research effort to develop a method to accelerate the validation of the Long Term Hydrostatic Strength (LTHS) performance of resins using CPF as the accelerating process. Phase I of this effort had shown a strong correlation between the Rate Process Method (RPM) and the CPF method and recommended a second follow on development effort.
The Phase II work has been completed and a final report was submitted to the HSB for review. The research results confirmed the strong correlation between the CPF and RPM methods, but highlighted difficulties in applying the method to Slow Crack Growth evaluation due to mixed mode failures. Detailed analysis of the test method and results supports the assertion that the maximum stress achieved in the stress cycle is the dominant factor accelerating the failure mechanism of the pipe, regardless of whether SCG or mixed-mode fractures are generated. The combination of peak cycle stress and notch-tip stress intensification will determine the exact mix of SCG or ductile tearing at the notch tip.
The CPF method can be used to effectively validate the ductile behavior of resins when used in conjunction with material specific bi-directional shift factors. Furthermore, the method is very useful in extending the cyclic fatigue resistance testing performed on polyethylene pipes over the past three decades. It was shown that the CPF results can extend the cyclic fatigue resistance evaluation of polyethylene materials to 100 million cycles to failure at 20°C. A method for ranking the relative cyclic fatigue performance of resins was suggested.
This paper will discuss the topics mentioned and provide suggestions for potential applications of the results and future work.