Bending Fatigue Testing of Pipe and Piping Components Fabricated From High Density Polyethylene Materials
Degradation of service water systems is a major issue facing nuclear power plant owners, and many plants will require repair or replacement of existing carbon steel piping components. High Density Polyethylene piping has been used in non-safety service water systems for over nine years and found to perform well, and is now permitted in the ASME Section III Boiler and Pressure Vessel Code, Division 1 for use in nuclear safety-related systems. To assist in this implementation of High Density Polyethylene piping in the ASME Boiler and Pressure Vessel Code, Section III, Division 1 for Safety Class 3 applications, Electric Power Research Institute initiated a testing program that includes tensile and fatigue testing of High Density Polyethylene piping and components and the development of data to evaluate slow crack growth that can emanate from surface scratches. Straight cantilever bending fatigue tests on PE 4710 pipe with a minimum cell classification of 445474C were previously conducted and the results presented at the 2008 PVP Conference in Chicago, Illinois. The tests were designed to comply with the requirements for fatigue testing given in Mandatory Appendix II of the ASME Boiler and Pressure Vessel Code, Section III, Division 1. Based on the straight pipe tests, Stress Intensification Factors can be calculated for other piping components. This paper reports on follow-on testing of PE 4710 cell classification 445574C piping components. The fatigue testing results showed one of the unique characteristics of High Density Polyethylene piping: a significant decrease in material stiffness from the first few test cycles to a lower value that remains almost constant until failure. Thus, Stress at Failure vs cycles at failure curves and Stress Intensification Factors were determined twice: first based on the initial cycle results and again at the midlife of the fatigue tests. This paper provides a description and overview of the test program, testing methods and materials tested. In addition, an overview and summary of the test program results are provided.