Fracture Mechanics Based Lifetime Assessment and Long-term Failure Behavior of Polyethylene Pressure Pipes

Andreas Frank

Research output: ThesisDoctoral Thesis

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Abstract

For gas and water distribution, Polyethylene (PE) has become one of the most important materials for pipe applications, which results in required service times of at least 50 years. With the knowledge of crack growth laws under static loading conditions, a fracture mechanics lifetime prediction of pressure pipes is possible using linear elastic fracture mechanics methods. One of the main objectives of this Dissertation was to develop a test program for a lifetime prediction of pressurized PE pipes at application near temperatures. Therefore, a fracture mechanics concept was applied. To determine the creep crack growth kinetics under static loads, the fatigue crack growth kinetics of cyclic tests at different loading ratios (R-ratios) was extrapolated to static loading conditions. To address the issue of technical feasible testing times especially for fracture mechanics testing of modern PE pipe grades cracked round bar (CRB) specimens were used. For a direct measurement of the crack growth kinetics in the CRB specimens a system of three extensometers was applied, which detects the crack length dependency on the specimen compliance. After having created a compliance-calibration curve it was possible to measure the crack growth kinetics in a single cyclic CRB test. Beside the determination of material parameters, also different K calculations were compared to each other and in a sensitivity study several boundary conditions were investigated. The lifetime predictions for different PE pipe grades show meaningful results, which indicate failure times of at least 50 years. An investigation of two old PE pipes from real field installation point out, that even after a service period of several decades, a sufficient residual lifetime can be expected to reach the overall service time. The required times for material testing of a PE 100 type were not longer than 6 months, what considers an enormous advantage compared to standardized tests, which do not give any information on quasi-brittle failure, even after more than one year. A further aspect in the long-term application of pipes is the repair and rehabilitation of existing pipe systems. One of the most important trenchless technologies is the pipe rehabilitation with a Close-Fit-Liner (CFL). Within the scope of this method a folded PE pipe has to be inserted into an old pipe and reforms during a temperature controlled installation procedure. Due to the fact that there are no scientific studies on material changes available yet, a further objective of this Dissertation was a comprehensive investigation of the influence of deformation processes on relevant material properties. The morphological characterization of the CFL improves the understanding of the interaction between the pipe deformation with the inserted molecular orientation and the memory effect which is essential for the inherent redeformation. Although the material is exposed to considerable thermal treatment during the installation, it could be shown that there is no significant thermo-oxidative aging taking place. However, a significant improvement of the finally installed CFL was detected concerning the residual stresses, which were nearly completely reduced due to the additional heating step during installation. This result seems to cause a positive effect on the crack growth resistance of the material and thus is also said to positively affect the structural reliability of CFL pipes.
Translated title of the contributionBruchmechanische Lebensdauerabschätzung und Langzeit-Versagensverhalten von Druckrohren aus Polyethylen
Original languageEnglish
QualificationDr.mont.
Supervisors/Advisors
  • Aust, Nicolai, Assessor B (internal)
  • Lang, Reinhold, Assessor A (internal)
Publication statusPublished - 2010

Bibliographical note

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Keywords

  • polyethylene fracture mechanics
  • linear
  • elastic lifetime
  • prediction slow crack growth
  • brittle
  • quasi-brittle pipe
  • pressurized rehabilitation
  • trenchless fatigue aging specimen
  • cracked
  • round
  • CRB

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