Time-strain rate superposition for relaxation behavior of polyethylene pressure pipes

Abstract

A new superposition principle based on time and strain rate is suggested as an alternative approach to construct a master curve of relaxation modulus versus time for polyethylene (PE) pressure pipe. The new approach uses results from a series of relaxation tests that reach the same relaxation strain at different strain rates (by varying the crosshead speed). Construction of the master curve for the relaxation modulus is first through horizontal shift using an expression similar to that for the time–temperature superposition principle. Then, a vertical shift is applied to generate a smooth curve profile. Such a time-strain rate superposition principle can serve as an alternative approach to construct a master curve for the long-term behavior of PE. The master curve shows two transitions for the rate of decrease of the relaxation modulus, instead of one transition reported before. The additional transition occurs within a short period after the relaxation strain is reached, and is detectable only if the initial strain rate is sufficiently low. Discovery of the new transition offers an additional perspective for studying mechanisms involved in the PE deformation.