Critical lifetime of HDPE pipes through damage and reliability models

Abstract

Damage models are not directly applicable on high-density polyethylene (HDPE) pipes. In this paper, static and strain-unified theory damage models are adapted to fit the HDPE case by substituting the dynamic tests’ endurance limits by preloading simulation through notch and stiffness evaluation. Then, tensile and burst tests are following up to evaluate the specimens’ residual life. Compared to virgin specimens, the rupture limit of old HDPE pipes’ specimens had dropped significantly and their elongation decreased from 275 mm to about 26 mm. The degradation of the seven categories of specimens are different. Indeed, the degradation is too noticeable, disappearance of the plastic phase, for the categories 6 and 7, which are in the bottom of the pipe. Then, a reduced plastic phase on the lateral categories 4 and 5 showing an important impact of degradations. Finally, a larger plastic phase for the categories 1 and 2 taken from the top of the pipe, showing a medium impact of degradation. Thus, the use of the stiffness factor, reflecting the variability of degradation of the different categories of specimens, and the thickness reduction as life fractions for both aged and neat HDPE specimens was possible. The developed strains damage model compared to static burst pressures’ one confirmed the damage stages and the critical life fraction of HDPE pipes. By comparing these models, the drastic change of HDPE pipes’ behavior, from a ductile to a brittle one, have been proved. These findings allowed us to find out the critical life fraction of neat and old HDPE pipes, which has been confirmed by comparing the burst pressure curves of a notched and an old pipe. The presented approach is cost effective allowing a deep analysis of HDPE pipes failure and damage quantification through simply made models based on static tensile and burst test instead of tedious and very costly dynamic ones.