Graduation Semester and Year

2017

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Civil Engineering

Department

Civil Engineering

First Advisor

Mohammad Najafi

Abstract

During a pipe’s life cycle, failures occur due to numerous factors such as age, loading conditions, environmental conditions, installation quality, manufacturing procedures, operation and maintenance strategies and so on. Frequent pressure surges or fluctuations in a piping system may cause a fatigue failure as fatigue is one of the most important issues in water pipes due to water hammer impacts. Application of buried High Density PolyEthylene (HDPE) water pipelines has significantly increased in recent years, and PE4710 is a new member of HDPE family. So, there is not yet enough information about PE4710 long-term performance. Due to lack of enough information on resistance of large diameter HDPE pipes under transient pressures, this research focuses on post-fatigue mechanical properties of a large diameter DR17 (16in.) High Density PolyEthylene (HDPE) 4710 pipe under cyclic loading. This work consists of two parts: First, experimental work which included the fatigue tests on an HDPE pipe sample for 2,000,000 cycles. Tensile tests on the dogbone specimens cut from the fatigue tested HDPE pipe and a new pipe sample from the same manufacturer were performed to compare material properties. Scanning Electron Microscopy (SEM) tests were done to visually show the impacts of fatigue on the molecular structure of the HDPE pipe. Second, Finite Element Modeling (FEM) were used to investigate the effects of different stress amplitudes on the HDPE pipe. The findings of this experimental study showed that after two million cycles of internal water pressure of 125 psi to 188 psi, rupture strain and tensile strength of PE4710 increased about 15% and 2% respectively, while yield strain reduced by approximately 20%. The circumferential location of dogbone specimens from the pipe sample did not have any effects on the results, but longitudinal location of specimens had impacts on fatigue mechanical properties of samples. Sections near butt-fused joint showed more reduction in yield strain and less increase in tensile strength than end sections near the end supports. The SEM test results showed initiation of micro-cracks in longitudinal direction of the pipe. Using finite element analysis, a new equation using Stress Amplitude vs. No. of Cycles (S-N) curve was developed for PE4710. It was concluded that current equations to predict fatigue life of PE4710 pipes may overestimate its fatigue life, by approximately 15%.

Keywords

Plastic pipes, Fatigue, HDPE, Experiments, FEM

Disciplines

Civil and Environmental Engineering | Civil Engineering | Engineering

Comments

Degree granted by The University of Texas at Arlington

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