Evaluation of Time Evolution of Mechanical Parameters of Polymeric Pipes by Unsteady Flow Runs

Abstract

The paper presents the results of the calibration, by means of a microgenetic algorithm, of the mechanical parameters of high-density polyethylene pipes, to reproduce experimental unsteady flow runs in an installation consisting primarily of a steel pipe and of an additional polyethylene pipe. The mechanical parameters of viscoelastic models are estimated using both one-dimensional and quasi-two-dimensional models. Two sets of runs carried out at a distance of about 12 years are considered. The calibration of a three-parameter Kelvin-Voigt model provides evidence of the impossibility of obtaining general results. The use of a model with a single Kelvin-Voigt element (two-parameter model) gives good results in terms of comparison between numerical and experimental results for both pressure head and velocity. The calibration for the second series of tests shows increased deformability of the material (the reduction of the elastic modulus is more than 30%). The calibrated values of the retardation time result linearly correlated to the oscillations period, increasing with time due to the reduction of the elastic modulus. The obtained results allow evaluation of the viscoelastic parameters starting only with the knowledge of the equivalent elastic modulus.