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Effect of unloading rate on surface crack growth during underload cycles in X-65 pipeline steel exposed to near-neutral pH environments
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- Author / Creator
- Ang, Jerome M
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This thesis has studied the crack growth behaviour of fully-coated, partially-coated, and fully-exposed semi-elliptical cracks immersed in a near-neutral pH environment with an applied cathodic potential. The effects of adding a one-hour monotonic hold and waveform shape were explored. The addition of a one-hour hold was shown to cause crack arrest at the surface of the cracks for fully-coated specimens due to room-temperature creep. Similarly, crack arrest was observed in the fully-exposed specimen due to room-temperature creep, anodic dissolution, and hydrogen-enhanced localized plasticity. Enhanced crack growth from increased hydrogen concentrations at the crack tip due to crack morphology and applied cathodic potentials may overcome retardation effects expected from a static hold. Rapid unloading was found to enhance crack growth due to enhanced hydrogen segregation associated with reduced screw dislocation mobility and reduced time for hydrogen diffusion out of the fracture process zone during elastic lattice relaxation. Greater hydrogen effects were observed in the depth compared with the surface of the cracks due to differences higher crack tip stresses. Sufficiently high unloading rates may allow for cracks to enter an active growth state. Pipeline operators can optimize their hydrostatic test programs to minimize the amount of potential crack growth during operations and fitness-for-service testing. A static hold at maximum operating pressure can retard subsequent crack growth in the length direction, especially following a significant depressurization-repressurization event. However, crack growth may still occur in the depth direction. Further, the lifetime of the pipelines can be extended by reducing the number of rapid depressurization events.
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- Graduation date
- Spring 2019
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- Type of Item
- Thesis
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- Degree
- Master of Science
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- 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.