For two series of API 5L X65 linepipes (Pipes A and B), the critical condition for ductile cracking of the linepipe steel and the applicability of the critical condition to an axially notched linepipe were investigated. Static 3-point bending tests for Charpy V-notch specimens were conducted in order to evaluate the critical condition of ductile cracking from the notch tip by using FE- analyses. At the position of ductile cracking from the notch tip for the Charpy type specimens, the stress triaxiality was approx. 0.6 for both linepipe steels, however the equivalent plastic strain (εp) was different on each linepipe; the εp for the ductile cracking was approx. 0.65 for Pipe A and approx. 1.47 for Pipe B. Hydraulic burst tests were then conducted for internally patched linepipes with an axial through-wall (TW) notch. The results of the FE-analyses for the hydraulic burst tests indicated the following: 1) the position of the ductile cracking at the TW notch tip was not the center of the wall-thickness (WT), but a slightly shifted position to the inner surface from the center of WT, 2) the equivalent plastic strain at the position where a ductile crack was initiated for the TW notched linepipe was almost the same as that obtained from the 3-point bending test result for the Charpy V-notch specimen. The present study revealed that the critical strain for ductile cracking from a notch tip for a Charpy type specimen was in good agreement with that for an axially notched linepipe. It was therefore clarified that the critical condition for ductile cracking for linepipes with an actual flaw could be predicted from the results of a small-scale test and FE-analysis to evaluate the relationship between the stress triaxiality and the equivalent plastic strain at the position of the ductile cracking.
