Effects of load interactions on the onset of stage two of high pH stress corrosion cracking
Hamid Niazi, Shidong Wang, Lyndon Lamborn, Reg Eadie, Weixing Chen, Hao Zhang
Abstract
The aim of this study was to investigate load interactions that influence crack geometry evolution during the early stages of the High pH Stress Corrosion Cracking (HpHSCC). Variable amplitude loading waveforms were designed to simulate the underload-minor-cycle pressure fluctuations found during the operation of gas pipelines. The role of underload cycles and minor cycles in intergranular crack growth was investigated. It is found that underload cycles form the cyclic plastic zone at the crack tip, and that assists secondary crack initiation. The crack growth rate under the variable amplitude loading condition approximates the crack growth rates caused by low R-ratio cycles, i.e., acts as if all the cycles contained major stress changes. It is argued that small load fluctuations are non-propagating cycles unless they are followed by a major underload cycle. The possible load interactions between minor cycles and underload cycles are studied. It is discussed how an increase in the number of minor cycles between two underload events produces acceleration factors of up to 2.6. These findings imply that avoiding large internal pressure fluctuation delays the onset of stage 2 of HpHSCC and extends the line pipes operating lifetime.