Environmental Crack Growth under Variable Amplitude Loading of Pipeline Steel Available to Purchase

An investigation was carried out on API X60 steel specimens with surface cracks to study environmentally assisted crack growth rates in simulated groundwater (NS-4 solution). The environmental conditions were similar to those for buried natural gas pipelines in service, and loading was aggressive. The maximum stress intensity factor at the tip of the crack on the surface ranged from 20.6 MPa√m to 31.4 MPa√m. A periodic underload was applied to pre-cracked cantilever bending specimens. Tests were carried out for a period of 40 days at a frequency of either 1 cycle or 40 cycles per day. Crack growth rates were obtained for two stress ratios (R = minimum/maximum load) of 0 or 0.5 under both constant and variable amplitude loading conditions. Depending on loading conditions, the crack growth varied from 0 μm to 2,640 μm. However, the majority of crack growth measured was <100 μm. The experimental results showed that there was an acceleration in growth following an underload. The growth was higher when the underloads were evenly distributed throughout the load history compared to when they were grouped together in blocks. A model was created based on corrosion fatigue to describe the crack growth behavior that took into account the accelerated crack growth rate.