The Use of the Stepped Isothermal Method for Estimating the Long-Term Strain Creep Modulus, Creep Strain and Strength of Polyethylene Pipe Resins

Session 2B
8:30 am

Richard Thomas, TRI/Environmental, Inc.

The Stepped Isothermal Method (SIM) is a special form of Time-Temperature-Superposition (TTS) that has been used to predict long-term creep behavior from very short-term tests (~ 24 hours). A single test specimen undergoes a series of short-term (10,000 seconds) creep tests, each separated by a constant temperature step. At the end of the experiment, one obtains a series of creep curves, each at a different temperature. The results are shifted based on known TTS techniques and a master curve obtained. Creep modulus and creep strain are determined under lower loads, while creep rupture times can be determined under higher loads. It was originally developed in these laboratories on high strength polyester (PET) geotextiles and geogrids for soil reinforcement applications. The results on PET, Kevlar, Nylon, and PP have correlated well to conventional creep results. It has yet to be compared to conventional creep for HDPE.

HDPE is a special case because as the temperature is increased, both the creep rate and the effective stress are changing simultaneously as the material becomes softer. It is basically a stepped-temperature and stepped-stress test. This paper will describe the details of the test and show how Popelar bi-directional shift constants can be used to build confidence in the SIM test for HDPE.

A simple test like SIM would be valuable for the determination of long-term strength, strain, and modulus values for use in design. It may also find use as a quick screening tool or quality control test for pressure rated resins.

Richard Thomas, Principal Scientist
Jarrett Nelson, Special Projects Manager
David Cuttino, Senior Technician