PSI - Issue 25

2

Author name / Structural Integrity Procedia 00 (2019) 000–000

Fatima Majid et al. / Procedia Structural Integrity 25 (2020) 430–437

431

Peer-review under responsibility of the VCSI1 organizers

Keywords: Pipes; Maintenance; Ageing degradation impact; HDPE W ӧ hler curve; Burst test; Damage approaches.

1. Introduction The intensive use of HDPE materials pushed many industrial to be suspicious of its quality. Thus, many improvements are conducted through the years to reach the best mechanical characteristics for the produced material. This concern is a good motivation for many researchers to conduct mechanical characterization and damage evaluation, through the necessary tests, to predict the residual lifetime of the used material and improve the characteristics for the newly produced pipes. In this purpose, many studies have been leaded to achieve this goal (Majid and Elghorba, 2016, 2018; Fatima et al. , 2017; Fatima and Mohamed, 2017). The majority of studies have been interested in accelerating the ageing based on total immersion tests in aggressive environments of different site conditions and temperatures. That’s why, the use of real samples from the industrial field remain very important to understand the ageing mechanisms involved and thus correlate those observed on the site to those obtained in laboratory accelerated ageing (Fayolle, Audouin and Verdu, 2000; Devilliers et al. , 2010; Douminge, 2010). In this paper, we have collected old HDPE pipes used for 15 years in a drinking water distribution piping network. The chosen specimen shows many signs of environment effect on the interior and the exterior of the pipes. Some apparent damage and small defects are noticeable over these pipes. To characterize the old material and evaluate its damage, we prepared standard specimens according to the ASTM D638 for tensile test purpose (Materials et al. , 2012). Other standard pipe specimens have been prepared according to the ASTM D1599 for burst test characterization (American and Standard, 2010). Besides, we prepared virgin specimens, taken from a newly produced pipe at the outlet of the production chain, which will serve as a reference in the next steps of this paper We conducted tensile tests. Thereafter, we compared the results of the old pipe stress-train curves to the results of those from the tensile curves of virgin specimens of the same range. Then, we conducted the burst pressure test over the prepared old pipes. The obtained internal pressure-time curves were compared to a virgin pipe curve to show the change which has occurred over the mechanical characteristics of the HDPE material. From the tests results obtained before, we evaluated the damage of a new and the old HDPE pipes through three models using the energy, the stress and the strain parameters. The obtained results have been represented in function of the chosen life fraction, which is the ratio between the elongation at the elastic phase and the one at the rupture phase, commonly called the stiffness index (Devilliers et al. , 2010). The crossing of the damage curves with reliability ones permitted us to establish the three stages of damage and discuss their evolution. Furthermore, a discussion of the evolution of the elastic stress and the rupture stress has been done in this paper. Moreover, we established the lifetime curves for the old HDPE pipes and we estimated the different consumed lifetime for all the specimens using the unified theory formulas. Finally, we showed, that the ageing effect is randomly distributed through the wall of an old pipe because of the variability of degradations.

Nomenclature σa

is the applied stress. is the ultimate stress.

σu σur

is the ultimate residual stress.

γe , γec

is the instantaneous nondimensional endurance limit and endurance limit at failure.

λ ,λe ,λf ,λec are deformation parameters n

is the instantaneous number of cycles under an applied stress.

Nf

is the total number of cycles at the rupture.

wur wu wa

is the ultimate residual energy.

is the ultimate energy corresponding to an undamaged HDPE specimen.

is the energy just before the rupture.