Effect of crack depth on fatigue crack growth rates for a C–Mn pipeline steel in a sour environment

Setting conditions for the avoidance of in-service crack growth in aggressive corroding environments has long been a major challenge owing to the number of variables that have a significant effect on material behaviour. The fatigue behaviour of API 5L X65 pipeline steel parent material tested in a sour environment has been investigated. Fatigue crack growth rate (FCGR) tests have been performed to evaluate the influence of crack depth on crack growth rate (da/dN), over the range 2–10 mm. The results obtained showed crack growth rates for deep flaws to be a factor of 5–30 higher in the sour environment than in air, dependent on the applied stress intensity factor range (ΔK). Shallow flaws have been shown to grow up to an order of magnitude faster than deep flaws in a sour environment at the same value of ΔK. The results highlight a potential non-conservatism associated with using deep-crack data to predict the behaviour of shallow flaws. The observed behaviour is attributed to the uptake of hydrogen at specimen surfaces exposed to the sour environment.