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Research Papers: Materials and Fabrication

Formation Mechanism of the Eigen-line in Electrofusion Joints of Polyethylene Pipes

[+] Author and Article Information
Jianfeng Shi

Institute of Process Equipment, Zhejiang University, Hangzhou, P. R. China, 310012

Jinyang Zheng1

Institute of Process Equipment, Zhejiang University, Hangzhou, P. R. China, 310012jyzh@zju.edu.cn

Weican Guo

Institute of Process Equipment, Zhejiang University, P. R. China; Zhejiang Inspection Center of Special Equipment, Hangzhou, P. R. China, 310020

1

Corresponding author.

J. Pressure Vessel Technol 133(5), 051403 (Aug 24, 2011) (6 pages) doi:10.1115/1.4003471 History: Received September 24, 2010; Revised November 22, 2010; Published August 24, 2011; Online August 24, 2011

Cold welding is the most dangerous defect in the electrofusion joint of polyethylene (PE) pipes. A proprietary method was developed to detect the degree of cold welding by using an Eigen-line, which was discovered in our previous study. To understand when, where, and how the Eigen-line would appear, the forming mechanism was investigated in this paper. Three factors, i.e., micro-air-bubbles, difference of acoustic impendence of PE in the melted and unmelted region, and small-crowded crystals in the region of the interface of melted and unmelted region, may cause the appearance of the Eigen-line. It was found that the number of Eigen-lines was the same as that of welding times when rewelded with decreasing welding time, and only one Eigen-line could be observed when rewelded with increasing welding time. The result showed that small-crowded crystals in the region of the interface of melted and unmelted region may be the dominating factor. This was then verified by the temperature analysis and differential DSC tests, and the forming process of the small-crowded crystals was discussed in detail.

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Copyright © 2011 by American Society of Mechanical Engineers
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References

Figures

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Figure 4

Ultrasonic image of section after first welding: (a) 110 s and (b) 55 s

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Figure 9

DSC result of annealing procedure

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Figure 10

DSC results of multi-annealing process

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Figure 11

Sketch of evolution process of crystallization in EF welding: (a) melted region, (b) semimelting region, and (c) unmelted region

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Figure 1

Welded EF joint: (a) ultrasonic image and (b) section view

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Figure 2

(a) Sketch of the location of the Eigen-line. (b) Magnified view of the interspaces of heating wire and surrounding PE.

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Figure 3

Section view of EF fitting. (a) Grooves in the inner surface of EF fitting. (b) Magnified view of the interspaces of heating wire and surrounding PE.

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Figure 5

Ultrasonic image of section after second welding: (a) 77 s and (b) 77 s

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Figure 6

Ultrasonic image of section after third welding: (a) 44 s and (b) 110 s

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Figure 7

Relationship between temperature and time

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Figure 8

Three regions of the cross section of EF joint

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