Failure of CPVC Pipes - Study of Mechanical Properties

• Author / Creator

  Chen, Bingjun

• Premature failure occurred in the chlorinated polyvinyl chloride (CPVC) pipes from manufacturer A at the solvent welded joints, while pipes from manufacturer B work normally under similar working conditions. Through characterization of mechanical properties and residual stresses in CPVC pipes this thesis seeks to test two hypotheses for pipe failure: (1) primer used in the solvent welding process, and (2) possible differences in mechanical properties between the two brands of CPVC pipes.
  Firstly, the effects of primer on the mechanical properties of CPVC pipe material were studied through D split tensile testing on three types of coupon specimens: round notched pipe ring (NPR) with notch radii ranging from 1 mm to 3.18 mm, flat NPR specimens, and ring specimens. All tests were conducted after 1 min exposure to primer and a half day of drying. Results show that exposure to primer deteriorates the mechanical properties of CPVC pipe material, with the main effect on reducing the material ductility, of which the extent of decrease is related to the specimen geometry and notch radius. To further investigate the changes of the CPVC pipe material in terms of drying time, mechanical properties, cross sectional surface morphology and fracture behavior of ring specimens were examined after 30-min exposure to primer, followed by eight different periods of drying time, ranging from a half-day to 113 days. Results show that the exposure to primer caused swelling of the cross section and generated a core shell structure. The results also show that both the material strength and ductility decreased after exposure to primer. Although the drying process can recover some of the strength, up to 63% of the virgin material, ductility remains low, of at least 57% reduction from the virgin material. Fractography analysis indicates that the permanent loss of ductility is due to multiple sites for crack initiation along the border between the core and shell regions.
  Distribution of residual hoop stress in 2 inch CPVC pipe was characterized by the slit one ring method. The results suggest that the commercial CPVC pipes contain a noticeable level of residual stress, with the maximum tensile residual stress above 2 MPa on the pipe inner surface. In addition, a refined slit one ring method was proposed to estimate the residual stress distribution on the cross section with core shell structure that has different mechanical properties. Finite element simulation, by introducing a temperature field in the model, was conducted to simulate the diametral deformation of the slit ring due to the release of residual stress. Good agreement was found between the analytical and simulation results. The refined slit ring method was then employed to study the effect of primer on residual stress in CPVC pipes that have been dried in air for different lengths of time. The results suggest that residual hoop stress was decreased by the exposure to primer, with the variation of residual stress with drying time mainly occurring within the first 30 days.
  Finally, the mechanical properties and residual hoop stresses of CPVC pipes with different nominal pipe size (NPS), from two manufacturers, were characterized and compared. Though mechanical strength and Young’s modulus are quite consistent among different types of CPVC pipes, material ductility decreases significantly after being in service for a long time, suggesting that strain based criteria should be considered in the specification of CPVC pipes, in addition to results from the stress based standard tests. Higher levels of residual stress were found in CPVC pipes from one specific manufacturer, which is believed to be caused by the cooling rate used in the manufacturing process.
  The thesis concludes that exposure to primer weakens the mechanical properties of CPVC pipe material, with the main effect on the permanent loss of ductility. Therefore, primer should be carefully applied to CPVC pipe to avoid unnecessary contact with pipe surfaces, especially if surfaces will not be covered by cement. On the other hand, the main differences in mechanical properties among commercial CPVC pipes from different manufacturers are material ductility and residual hoop stress. It is suggested that ductility and residual hoop stress in both CPVC pipes and their joints should properly be characterized to ensure their satisfactory long term performance.

• Subjects / Keywords

  • CPVC pipe
  • premature failure
  • mechanical properties
  • residual stress

• Graduation date

  Fall 2018

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3DR2PR21

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.