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Research Papers: Design and Analysis

# Development of Maximum Secondary Creep Strain Method for Lifetime of HDPE Pipes

[+] Author and Article Information
Cunjiang Cheng

Bjorksten Research Laboratory, BIT 7, Inc., Madison, WI 53718

G. E. Otto Widera

Center for Joining and Manufacturing Assembly, Marquette University, Milwaukee, WI 53233

J. Pressure Vessel Technol 131(2), 021208 (Jan 13, 2009) (9 pages) doi:10.1115/1.3066913 History: Received February 11, 2008; Revised May 12, 2008; Published January 13, 2009

## Abstract

Considering only the primary and secondary creep stages of high-density polyethylene (HDPE) material, a maximum secondary creep strain method (MSCS) and formula to calculate the maximum secondary creep strain has been developed based on existing test data and the creep law. Since the tertiary creep stage has a very short lifetime compared with the primary and secondary stages, it is reasonable to assume that rupture will occur once the creep strain reaches the maximum secondary creep strain. The implicit method of the finite element analysis software, LS-DYNA (1998, LS-DYNA Theoretical Manual, Livermore Software Technology Corporation), is employed to determine the HDPE pipe primary creep process using an effective numeric algorithm (Whirley and Henshall, 1992, “Creep Deformation Structural Analysis Using an Efficient Numerical Algorithm  ,” Int. J. Numer. Methods Eng., 35, pp. 1427–1442) followed by a formula to determine the secondary creep process. The expression for the overall lifetime of the HDPE pipes is then derived. Based on the agreement between the numerical and test results, it is concluded that the MSCS method can accurately predict the long-term hydrostatic strength of a HDPE pipe.

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## Figures

Figure 1

Creep strain of HDPE at load level of 1,2,4,…,18 MPa (8)

Figure 2

Creep strain rate of HDPE at load level of 1,2,4,…,18 MPa (8)

Figure 3

Strain-time curves of HDPE at constant stress at temperature of 23°C(9)

Figure 4

Typical creep rupture curve for HDPE pipe (9)

Figure 5

Stress versus secondary creep starting time (HDPE 7058Z)

Figure 6

Stress versus secondary creep strain rate (HDPE 7058Z)

Figure 7

Formula to fit stress versus secondary creep starting time

Figure 8

Formula to fit stress versus secondary creep rate

Figure 9

Using effective numerical algorithm to fit the HDPE pipe material creep data

Figure 10

Tensile creep modulus versus time and stress intensity (9)

Figure 11

Creep rupture curves and fitting formula at 20°C and 23°C of HDPE pipe

Figure 12

Geometry and elements of HDPE pipe

Figure 13

von Mises stress distribution of the pipe at 280 psi internal pressure

Figure 14

von Mises strain distribution of the pipe at 280 psi internal pressure

Figure 15

Number 136 element on the outer layer of the pipe

Figure 16

Element (No. 136) primary creep strain versus time of the pipe

Figure 17

Element (number 136) primary creep strain rate versus time of the pipe

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