Plastic Pipes Conference Association # 2010 Vancouver
R.Thomas
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 7ºC. 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 to create a master curve. Creep modulus and creep strain are determined under lower loads, while creep rupture times can be determined under higher loads. SIM 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. Polyethylene 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 describes the details of the test, presents some results on recycled-containing HDPE resins, and shows how SIM relates to sustained burst tests on crosslinked polyethylene (PEX) and a PE 3408, pressure rated pipe. The results suggest that SIM might be useful for selecting stresses in sustained burst test programs, act as a screening tool for formulation designs or preliminary assessment of resins, or provide some long-term property information for situations where it is not possible to test pipe. It may also prove to be useful for quality assurance.
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