Viscoelastic Finite Element Modeling of Bimodal High Density Polyethylene (HDPE) Piping Materials for Nuclear Safety-Related Applications

The U.S. Nuclear Regulatory Commission (USNRC) has recently approved Relief Requests for the use of high density polyethylene (HDPE) piping in safety-related applications. The ASME Boiler and Pressure Vessel Code, meanwhile, has developed Code Case N-755 that defines the design and service life requirements for PE piping in nuclear plants though it has not as yet been approved by the USNRC. One of the issues of concern is premature failure of PE piping due to slow crack growth (SCG) that can initiate due to a combination of sustained loads, elevated temperatures, and a pre-existing defect. Understanding and predicting the SCG behavior is an essential step in developing a methodology for predicting the service life of PE piping. The first step in studying the failure process in a polymer under a constant sustained load is the selection of a suitable constitutive model to represent the time-dependent behavior of the material. In this paper, uniaxial tensile creep tests were performed for a bimodal HDPE (PE4710) piping material. This creep data was used to determine the viscoelastic material constants for this bimodal HDPE using a power-law creep model. These material constants were used in finite element (FE) analyses to study the viscoelastic behavior of the bimodal HDPE. As a first step, the FE model was verified by comparing the results from numerical simulations and experiments for a set of uniaxial tensile creep tests. The FE model was then applied to study the viscoelastic behavior of a SCG specimen. The time dependent stress and strain fields were investigated.