Citation: Tan, N.; Lin, L.; Deng, T.;
Dong, Y. Evaluating the Residual
Stress and Its Effect on the
Quasi-Static Stress in Polyethylene
Pipes. Polymers 2022, 14, 1458.
https://doi.org/10.3390/
polym14071458
Academic Editors: Alberto
Campagnolo and Alberto Sapora
Received: 12 March 2022
Accepted: 30 March 2022
Published: 3 April 2022
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Article
Evaluating the Residual Stress and Its Effect on the Quasi-Static
Stress in Polyethylene Pipes
Na Tan
1,2,3,
* , Liyang Lin
1,2
, Tao Deng
1,2,3,4
and Yongwu Dong
1,2
1
School of Aeronautics, Chongqing Jiaotong University, Chongqing 400074, China;
jack_linliyang@cqjtu.edu.cn (L.L.); d82t722@cqjtu.edu.cn (T.D.); dongyw234@hotmail.com (Y.D.)
2
The Green Aerotechnics Research Institute, Chongqing Jiaotong University, Chongqing 401120, China
3
Department of Mechanical Engineering, University of Alberta, Edmonton, AB T6G 1H9, Canada
4
Chongqing Key Laboratory of Green Aviation Energy and Power, Chongqing 401120, China
* Correspondence: ntan2@ualberta.ca
Abstract:
Residual stress is generated during the production process. It can significantly affect
the mechanical performance of pressurized polymer pipes. In this paper, six polyethylene (PE)
pipes, including three high-density PEs (HDPE) and three medium-density PEs (MDPE) provided by
different suppliers, were tested using a one-slit-ring method to measure the residual stress distribution
along the hoop direction. Finite element (FE) simulation and mechanical testing were also employed
in an iteration process to obtain the mechanical parameters of the six PE pipes. For the same PE pipe
code from different suppliers, the results show that the magnitude of the residual hoop stress can
be very different, resulting in different mechanical behaviors. In addition, the results are proposed
to explain the scenario that was reported previously, i.e., the different critical quasi-static stress (the
time-independent stress) levels of the PE pipes with the same pipe code. Since the quasi-static stress
is expected to dominate the long-term behavior of the PE pipes, it is of great importance to carefully
consider the effect of the residual stress on the determination of the quasi-static stress.
Keywords: residual stress; mechanical testing; polyethylene; long-term performance
1. Introduction
Residual stresses within a pipe are generated as a consequence of a temperature
gradient developing during the cooling phase in the extrusion or molding process [
1
–
5
].
The outer surface of the pipe is usually quenched in water while the inner surface of the
pipe is exposed to static air [
6
]. Therefore, the outer surface solidifies faster, resulting
in a thermal gradient distribution along the pipe wall. The nonuniform solidification
process generates residual stress by causing different crystallinity distribution along the
pipe wall [
6
]. Generally, the inner surface is under tensile residual stress while the outer
surface is under compressive residual stress [
7
]. The compressive residual stress within
the outer wall of the pipes can benefit their stretching resistance [
7
]. However, as pointed
out by many investigators [
8
–
10
], the presence of tensile residual stress within the pipes
can accelerate the fracture process when conducting a creep rupture test, resulting in the
premature failure of the PE pipes. Although the magnitude of the residual stress is not
large, it can significantly affect the long-term performance of the pipes, even though the
applied stress levels are always very low, typically lower than 5 MPa (hoop direction) [8].
During the last decades, different analytical and computational techniques have been
developed to measure the residual stress distribution within pressure pipes.
Williams et al
. [
5
]
determined the residual stress in a high-density polyethylene (HDPE) pipe by a tube slitting
(layer removal) method and evaluated the role of residual stress on fracture. The results
were coupled with a semi-elliptical flaw to estimate the stress intensity factor at flaws in
the pipe wall. Turnbull et al. [
11
] compared the residual stress in polycarbonate, filled
and unfilled acrylonitrile butadiene styrene (ABS), and nylon using different techniques,
Polymers 2022, 14, 1458. https://doi.org/10.3390/polym14071458 https://www.mdpi.com/journal/polymers
