Anisotropic ductile failure of a high-strength line pipe steel

Fracture properties of a mother plate for API grade X100 line pipe were investigated using tensile notched bars, CT and SENB pre-cracked specimens. The material had an anisotropic plastic behaviour due to the thermo-mechanical control rolling process. In addition, anisotropic rupture properties were also observed. Specimens tested along the rolling direction were more ductile and more crack growth resistant than those tested along the long transverse direction. Unit cell calculations were used to show that this fracture behaviour is not related to plastic anisotropy. Assuming that fracture is controlled by internal necking between anisotropically spaced voids, a model combining GTN and Thomason models is proposed which enables describing rupture anisotropy. A modified phenomenological model is also proposed so as to reduce the computational cost.