Title page for ETD etd-02282017-153621


Type of Document Master's Thesis
Author Kalhor, Roozbeh
Author's Email Address rkalhor@vt.edu
URN etd-02282017-153621
Title Accelerated Testing Method to Estimate the Lifetime of Polyethylene Pipes
Degree Master of Science
Department Macromolecular and Science Engineering
Advisory Committee
Advisor Name Title
Scott W. Case Committee Chair
Donald G. Baird Committee Member
Michael Bortner Committee Member
Keywords
  • small angle X-ray scattering (SAXS)
  • digital image correlation (DIC)
  • time-temperature superposition
  • ductile failure
  • plastic pipe
  • long-term hydrostatic strength (LTHS)
  • creep characterization
  • wide angle X-ray diffraction (WAXD)
Date of Defense 2017-02-14
Availability restricted
Abstract
The ability to quickly develop predictions of the time-to-failure under different loading levels allows designers to choose the best polymeric material for a specific application. Additionally, it helps material producers to design, manufacture, test, and modify a polymeric material more rapidly. In the case of polymeric pipes, previous studies have shown that there are two possible time-dependent failure mechanisms corresponding to ductile and brittle failure. The ductile mechanism is evident at shorter times-to-failure and results from the stretching of the amorphous region under loading and the subsequent plastic deformation. Empirical results show that many high-performance polyethylene (PE) materials do not exhibit the brittle failure mechanism. Hence, it is critical to understand the ductile mechanism and find an approach to predict the corresponding times-to-failure using accelerated means. The aim of this study is to develop an innovative rupture lifetime acceleration protocol for PE pipes which is sensitive to the structure, orientation, and morphology changes introduced by changing processing conditions. To accomplish this task, custom fixtures are developed to admit tensile and hoop burst tests on PE pipes. A pressure modified Eyring flow equation is used to predict the rupture lifetime of PE pipes using the measured mechanical properties under axial tensile and hydrostatic pressure loading in different temperatures and strain rates. In total, the experimental method takes approximately one week to be completed and allows the prediction of pipe lifetimes for service lifetime in excess of 50 years.
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