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Crack Growth Behaviour of Pipeline Steels under Variable Pressure Fluctuations in a High pH Environment
- Author / Creator
- Niazi, Hamid
High pH Stress Corrosion Cracking (HpHSCC) in the pipeline steel has been a threat for buried pipeline integrity since the first occurrence in the 1960s. Much research has been devoted to understanding the causes of this phenomenon and remedies for mitigation of HpHSCC failures. In addition to developing the methods and strategies to mitigate such failures, the knowledge of HpHSCC cracks growth behaviour has been at the forefront of pipeline industries’ attention. This knowledge helps pipeline operators to estimate the reliable lifespan of the cracked pipe. Parkins proposed the bathtub model to show the time-dependent behaviour of HpHSCC crack growth. Accordingly, a pipe susceptible to HpHSCC may go through the five sequential stages viz. incubation stage, crack nucleation and early stage of crack development (stage 1a), crack development to provide the mechanical driving force for sustainable crack growth (stage 1b), mechanically derived crack propagation (stage 2), and rapid crack growth to failure (stage3). There are two significant imperfections associated with the current model. First, stage 1b, the lifetime determining stage in HpHSCC crack growth, has been less studied. Parkins assumed random crack initiation and crack coalescence causes stochastic crack propagation. Second, Parkins employed either monotonic loading or constant amplitude loading conditions in the early experiments. However, pipelines experience variable amplitude loading conditions during their operations. The load interaction effects remain to be studied.
This research attempts to study the HpHSCC crack growth Behaviour under both constant amplitude and variable amplitude loading conditions at stage 1b and stage 2 to address the bathtub model’s problems, as mentioned earlier. In this study, compact tension specimens made from X65 pipeline steel were used, and the environmental conditions were 0.5 M Na2CO3 and 1 M NaHCO3 at 40 °C and applied cathodic protection of -590 mVSCE.
For stage 1b, it is shown that the crack growth rate is highly sensitive to loading characteristics, so that the highest crack growth rate was obtained under constant amplitude cyclic loading with the stress ratio (R-ratio=minimum stress/maximum stress) of 0.2 and frequency of 10-2 Hz. On the other hand, high R-ratio cycles showed the lowest crack propagation rate. Variable amplitude loading waveforms composed of a number of high R-ratio cycles flowed by an underload cycle (low R-ratio cycle) showed an intermediate crack propagation rate. It was observed large load fluctuations (i.e. low R-ratio cycles) form a cyclic plastic zone at the preexisting crack tip. The cyclic plastic zone is a breeding ground for secondary cracks initiation close to the existing crack tip. If these cracks merge to the main crack, the crack length on the free surface increases, followed by increasing the mechanical driving force for crack propagation in depth. Crack initiation and crack coalescence is the primary mechanism for crack propagation during stage 1b. Increasing the number of minor cycles between two underload cycles enhances the crack growth rate by increasing the chance for crack coalescence and more available time for low-temperature creep strain to be exhausted.
The dominant mechanism for HpHSCC crack growth in Stage 2 is anodic dissolution or repeated rupture and passive film formation at the crack tip. The loading condition at this stage is potent enough to provide sustainable crack growth. There is a power-law relationship between stage 2 HpHSCC crack growth and strain rate. The strain rate is a function of several parameters, including mean stress intensity factor, amplitude as well as the frequency of load fluctuation. Similar to crack propagation in stage 1b, low R-ratio cycles, particularly high-frequency ones, assist secondary cracks initiation on the free surface.
The implication of this study’s results for the pipeline operators is to control the pressure fluctuations and minimize underload cycles, particularly high-frequency ones. Such cycles increase the chance for secondary crack initiation on the pipe’s free surface, followed by crack coalescence. Additionally, such cycles generate higher strain rates and increase the crack propagation rate at stage 2. Ergo, avoiding underload cycles increases the pipe’s reliable lifetime.
- Graduation date
- Spring 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.