Constitutive Modelling of the Time and Temperature Dependent Behavior of HDPE Applied to Piping

Abstract

HDPE pipes are used extensively in both industrial and societal infrastructure. However, even though HDPE pipes are designed as per standard procedures, they still suffer from leakage especially HDPE pipes with large diameters. Many factors affect leakage including HDPE stress relaxation and temperature. Thus, this study investigated the effect of temperature on the leakage of HDPE gasketless flanged connection after 1 year of service using Finite element analysis (FEA). To build the FEA model, the temperature dependent time independent and time dependent mechanical properties of HDPE were experimentally obtained. The experimental results showed that HDPE generally relaxes less as temperature increase for both tensile and compressive relaxation. Moreover, as temperature increases, the difference between final tensile relaxation stress level and final compression relaxation stress level becomes negligible, indicating that at higher temperatures, the deformation mode has less effect on stress relaxation. Two attempts to model the mechanical behavior were carried out in this study. Firstly, linear viscoelasticity was used to model HDPE behavior, which proved to be unsuccessful. Then the three network model (TNM) was used, which predicted the mechanical behavior of HDPE quite well. The calibrated TNM was then used to simulate a three point bending relaxation test at elevated temperatures to validate the material model using FEA, which accurately predicted the behavior of HDPE in the validation test. Finally, the calibrated TNM was used in an FEA study to investigate leakage at 23 Co and the effect of heating to 40, 60, and 80 Co on leakage during 1 year of service through examining average contact stress. It was concluded that leakage occurs earlier as temperature increases at a given torque (690 N.m) and operating pressure (6 bars). The FEA results were used to find an expression for average contact stress as a function of logarithmic time and temperature.

Date of Award	May 2020
Original language	American English