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Research Papers: Materials and Fabrication

# Effects of Crosshead Speed on the Quasi-Static Stress–Strain Relationship of Polyethylene Pipes

[+] Author and Article Information
Yi Zhang

Department of Mechanical Engineering,
University of Alberta,
for Engineering,
9211-116 Street NW,
e-mail: yz4@ualberta.ca

P.-Y. Ben Jar

Department of Mechanical Engineering,
University of Alberta,
for Engineering,
9211-116 Street NW,
e-mail: ben.jar@ualberta.ca

1Corresponding author.

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received March 9, 2016; final manuscript received May 29, 2016; published online September 27, 2016. Assoc. Editor: Kunio Hasegawa.

J. Pressure Vessel Technol 139(2), 021402 (Sep 27, 2016) (6 pages) Paper No: PVT-16-1040; doi: 10.1115/1.4033777 History: Received March 09, 2016; Revised May 29, 2016

## Abstract

Quasi-static stress–strain relationship of polyethylene (PE) pressure pipe that plays an important role on its long-term performance has been established by removing the viscous stress component from the experimentally measured total stress. Work reported here is focused on the influence of crosshead speed on the notched pipe ring (NPR) specimens that are prepared from PE pressure pipe of 2 in. in diameter. Viscous component of the stress–strain relationship was determined using a spring–damper–plastic element model, calibrated using results from stress relaxation tests. Crosshead speeds considered for the initial stretch of the stress relaxation tests are 0.01, 1, and 10 mm/min which due to the relatively uniform deformation in the gauge section generate the same order of difference in the strain rates. Results from the study suggest that the quasi-static stress–strain relationship is affected by the crosshead speed used to generate the deformation, and the trend of change is opposite to the total stress counterpart that includes the viscous component.

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## Figures

Fig. 1

Information on specimens used in the study: (a) a pipe section (left) and a NPR specimen (right), and (b) specimen dimensions

Fig. 2

Information for the stress relaxation tests: (a) test setup (also used for the monotonic tensile tests) and (b) schematic diagram for the stroke and area strain as functions of time

Fig. 3

Schematic diagram of the viscous model, modified from Ref. [25]

Fig. 4

Plots of experimentally determined true stress-area strain curves under monotonic tension at three crosshead speeds

Fig. 5

Stress decay (Δσ) as a function of relaxation time at different relaxation strains which were introduced at the crosshead speeds of (a) 0.01 mm/min, (b) 1 mm/min, and (c) 10 mm/min (larger the relaxation strain, higher the stress decay)

Fig. 6

Comparison between curves generated from Eq. (6) (presented by markers) and curves obtained from stress relaxation tests (lines) at the relaxation strain of 20%, with the initial stretch introduced at the crosshead speeds of 0.01, 1, and 10 mm/min

Fig. 7

Quasi-static stress–strain curves for the three crosshead speeds used in the study

Fig. 8

Variation of reference stresses σ0 with the applied relaxation strain

Fig. 9

Variation of the quasi-static stress–strain curve by assuming σ0 being independent of crosshead speed. Curves in dashed line are generated using Eq. (6) with σ0 being equivalent to those for the crosshead speed of 0.01 mm/min in Table 1.

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