Damage in Polyethylene and its Pressure Pipe - Characterization and Detection Methods

  • Author / Creator
    Zhang, Yi
  • Approaches for accurate prediction of ductile failure of polyethylene (PE) pipe have been explored using the experimentally determined, material-specific damage parameter D. Although many methods are available for quantifying the D values, they are mainly suitable for metallic materials. For PE, damage characterization using these existing methods is difficult because of the insignificant effects of damage on the short-term mechanical properties for PE. In addition, deformation of PE is a nonlinear viscous behavior that further increases the challenge for the damage characterization. The main objective of this thesis is through characterizing the deformation-induced damage in PE to find a reliable method for its damage characterization. Initially, mechanical properties of PE pipe materials under various strain rates were systematically studied. It was observed that mechanical behavior of PE is strongly dependent on strain rate. In addition, it was found that strain rate can serve as an additional factor for time-temperature superposition principle to predict long-term properties of PE. Based on the above findings, time-strain rate superposition principle is proposed to construct relaxation master curve for PE pipe. Furthermore, damage evolution in PE pipe materials under tensile loading condition was investigated, using the proposed two-stage test method and phenomenological finite element (FE) modelling. Mechanical properties of two most popular PE pipe materials (PE80 and PE100) were also investigated and compared. The above two-stage test method was then applied to examine the influence of squeeze-off on the degradation of mechanical properties for PE pipe. Three squeezing speeds of 0.01, 1 and 50mm/min were used to cover the possible scenarios that may be encountered during the pipe repair or maintenance. Results show that squeeze-off of PE pipe causes significant degradation in elastic modulus and yield strength, with the maximum reduction of 82% for elastic modulus and 27% for yield strength. Furthermore, it was found that reducing the squeezing speed has no effect on the extent of property degradation. In view of those findings, a study was conducted using mechanical testing and FE simulation to elucidate the damage evolution in PE pipe. Results show that both tensile and compressive loading modes can cause severe degradation in elastic modulus and yield strength. The results also show that under a single loading mode, the extent of damage at a given prestrain level is indeed a function of loading rate. However, in a squeezed pipe which is subjected to both tension and compression through the pipe wall thickness, our analysis based on damage mechanics suggests that degradation of elastic modulus and yield strength can become insensitive to the loading rate. A new methodology based on the effective stress concept in continuum damage mechanics (CDM) was developed to characterize ductile damage in PE pipe. Quasi-static stress-strain relationships as a function of strain rate and ligament width in the notched pipe ring (NPR) samples were first determined by conducting stress relaxation tests and applying a viscous model to remove viscous stress from the total true stress-strain relationships. By fitting the experimentally determined variation of quasi-static stress with strain rate which was then extrapolated to zero strain rate, an estimate was made for the effective stress at an undamaged configuration. Finally, the damage parameter was determined using the proposed method and showed good correlation with the method based on the degradation of elastic modulus. Finally, a non-destructive ultrasonic test method was proposed to characterize damage in high-density polyethylene (HDPE). Various damage levels were first introduced to the HDPE specimens through stretching the specimens to different prestrain levels at a constant crosshead speed of 1mm/min. Ultrasonic speeds in virgin and damaged HDPE specimens were then measured using time-of-flight in the through-transmission mode. The results show that the ultrasonic wave speed, normalized by the speed in the virgin plate of the same thickness, decreases with the increase of prestrain introduced to the specimens. The study also shows that with the correction of density change by the prestrain, the normalized ultrasonic wave speed can be used to determine the dependence of damage level on the prestrain, which for specimens with long gauge length, is consistent with the damage determined from the mechanical testing. The study concludes that ultrasonic testing can be used as a non-destructive means to quantify deformation-induced damage evolution in PE.

  • Subjects / Keywords
  • Graduation date
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.
  • Language
  • Institution
    University of Alberta
  • Degree level
  • Department
    • Department of Mechanical Engineering
  • Supervisor / co-supervisor and their department(s)
    • Jar, Ben(Mechanical Engineering)
  • Examining committee members and their departments
    • Zuo, Ming(Mechanical Engineering)
    • Le, Lawrence(Radiology & Diagnostic Imaging)
    • Polak, Marianna(Civil and Environmental Engineering)
    • Jar, Ben(Mechanical Engineering)
    • Wang, Xiaodong(Mechanical Engineering)