Fatigue Crack Growth at Electrical Resistance Welding Seam of API 5L X-70 Steel Line Pipe at Varied Orientations

Very few studies have been conducted concerning fatigue in steel line pipe and fewer using full-scale testing. Further, at the time of this study, no research on full-scale testing was available in open literature regarding fatigue behavior of line pipe with longitudinal cracks, despite being considered more critical than the line pipe with cracks oriented in the circumferential direction. In the current research work, fatigue crack growth was investigated in NPS 20, API 5L X-70 grade, electrical resistance welding (ERW) straight-seam steel line pipes in the base metal and at the weld seam for various orientations. It was found that there was no significant difference between fatigue crack growth in the base metal and at the weld seam for the tested stress ratio. Increasing the angle of inclination of the crack with respect to the weld line was found to decrease the rate of fatigue crack growth due to a decrease in the mode I stress component. Finally, it was observed that despite the difference in fatigue crack growth rates, the crack aspect ratios were nearly identical for all cracks at the same crack depth.