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Fatigue Crack Growth Behaviour in Pipeline Steels under Mean Load Pressure Fluctuation in Near-Neutral pH Environment
- Author / Creator
- Tehinse, Olayinka B
This research aimed to understand the effect of mean load pressure fluctuations on crack growth in near-neutral pH (NNpH) environment. This work contributes to a larger effort to develop a crack growth predictive model based on variable amplitude loading. The motivation for this study is that although pressure fluctuations have been recognized as an important factor in crack propagation in NNpH environments, current predictive models for crack growth are still based on constant amplitude loading. The observed discrepancies between the predicted life of cracks based on such models and the fatigue life observed in the operating pipelines call for a review of such models. It is believed that a predictive model based on realistic pressure fluctuations will provide a more accurate representation of the fatigue lives of cracks in pipelines.
Pressure fluctuations can be divided into underload, mean load and overload cycles. The effect of mean load pressure fluctuation on crack growth in NNpH environment has not been studied previously. Although there is agreement that mean load is less severe compared to underload cycles in terms of crack propagation, it appears that there is disagreement on the effect of load sequence in the mean load on crack propagation. Literature review shows that while some researchers observed retardation of crack growth when Type II mean load cycles (underload+overload+minor cycles, UL+OL+MC) was applied, some other researchers disagree and reported that retardation of crack growth was observed under Type I mean load (overload+undeload+minor cycles, OL+UL+MC).
This research focuses on mean load pressure fluctuations and its effect on crack propagation in the NNpH environment. By studying crack growth behaviour in both Type I and Type II mean load pressure fluctuations, the effect of load sequence in the mean load is highlighted. This study also considers the effect of sensitivity of both Type I and Type II mean loads to increase in the magnitude of overload. The effect of increase in the number of minor cycles was also studied in both mean load types. Crack growth behaviour under mean load pressure fluctuations was compared to constant amplitude cyclic loading as well as underload+minor cycles. The results showed that in the NNpH environment, crack growth is higher under Type II loading than Type I. Type II loading crack growth rate is very sensitive to increase in the magnitude of overload in the mean load compared to Type I. This suggests that Type II mean load could be detrimental to pipeline integrity, similar to underload pressure fluctuations. This is interesting because many cracks and failures have been reported in areas close to the discharge part of the pipe where underload and mean load pressure fluctuation are observed. Overall, the best retardation of crack growth rate was observed in both Type I and Type II mean load with 5% overload. Based on these findings, suggestions were made on actions that can mitigate crack propagation during pipeline operations due to planned or unplanned shutdowns.
- Graduation date
- Fall 2021
- Type of Item
- Doctor of Philosophy
- 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.