Application of Ductile Cracking Criterion to the Assessment of Girth Weld Integrity for High Strength Linepipe

Abstract

Fracture behavior of high strength linepipes with weld defects is of great interest for the integrity of pipeline system. Especially, in the seismic or permafrost area, where large ground displacement can be expected, linepipe materials need to have sufficient resistance against brittle and ductile fracture under large deformation. Wide plate tensile test with surface flaw in the girth weld metal of X100 linepipe demonstrated that tensile limit is dominated by ductile crack initiation and its propagation. Conditions for ductile crack initiation for the base materials and girth weld joints of Grade X80 and X100 linepipes were investigated in this study. It was shown that ductile cracking occurs in the notch tip region of the wide plate specimen when notch tip equivalent plastic strain reaches the same critical value as determined by the small-scale tests. Therefore, “the equivalent plastic strain” in the critical regions can be used as a transferable parameter to predict ductile crack initiation behavior. Assessment methodology for tensile limit of high strength linepipe girth weld with respect to preventing ductile cracking was proposed. The effect of strength matching of girth weld and base metal Y/T ratio on limit remote strain as well as allowable defect size was investigated analytically. Increasing strength matching and lowering Y/T ratio of base material can lead to higher limit strain to ductile cracking of girth weld. These effects of material properties were validated by weld wide plate tensile tests. Therefore, careful selection of material properties should be important to improve resistance against ductile cracking of linepipe girth welds under large deformation field.